Cone-beam CT is almost certainly going to revolutionize dental radiology and impact on almost all aspects of dental practice. CBCT is an emerging technical advancement in CT imaging that uses cone beam acquisition geometry to provide relatively low-dose imaging with high isotropic spatial resolution acquired with a single gantry revolution. Efficient use of the x-ray beam in CBCT imaging produces a relatively low x-ray tube power requirement, which, along with flat panel detection and limited anatomic coverage, has facilitated the production of compact CBCT scanners suitable for use in an office-based setting. The development and rapid commercialization of CBCT technology dedicated for use in the maxillofacial region will undoubtedly increase both general and specialist practitioner access to this imaging modality. CBCT is capable of providing accurate, sub-millimetre resolution images in formats enabling 3D visualization of the complexity of the maxillofacial region. Increasing availability of this technology provides the practitioner with a modality that is extending maxillofacial imaging from diagnosis to image guidance of operative and surgical procedures.
CONTENTS
1 INTRODUCTION
2 PRINCIPLES OF CONE BEAM IMAGING
3 CONVENTIONAL VERSUS CONE BEAM CT
4 CONE BEAM CT SYSTEM CONFIGURATION
5 CBCT IMAGE PRODUCTION
6 ADVANTAGES OF CBCT
7 LIMITATIONS OF CBCT IMAGING
8 APPLICATIONS
9 CONCLUSION
10 BIBLIOGRAPHY
ABBERIVATION
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INTRODUCTION
Wilhelm Conrad Roentgen, a German Physicist, discovered X - rays on November 8, 1895. X rays belong to a group of radiations called electromagnetic radiation. Electromagnetic radiation is the transport of energy through space as a combination of electric and magnetic fields. Initially X rays produced a two-dimensional image of any given object.1
As early as the 1920s, manufacturers attempted to overcome the inherent problems of 2 -D imaging by devising movement of the receptor and source in opposite directions to produce tomographic “slices” of oral and maxillofacial anatomy this process is termed “linear” or “multidirectional tomography.” Within the last 20 years, diagnostic digital imaging modalities in dentistry, including periapical, bitewing, panoramic and cephalometric imaging, have been replacing conventional (film-based) radiography.2 In 1967, Sir Godfrey Hounsfield developed the first computed tomography (CT) devices for medical use Medical CT scanners consist of an x -ray emitter and a series of detectors on the opposite side which rotate around the patient’s body capturing 2 -D images at many angles. The technique was further improvised subsequently over the past decades and now the seventh generation scanner wherein the x-ray source is an integral part of the system design is available. Conventional CT devices image patients in a series of axial plane slices that are captured as individual stacked slices or from a continuous spiral motion over the axial plane. Conversely, CBCT presently uses one or two rotation sweeps of the patient similar to that for panoramic radiography. Image data can be collected for a complete dental/maxillofacial volume or limited regional area of interest. 3
Within the past decade, technology termed “Cone Beam Computed Tomography” (CBCT) has created a revolution in maxillofacial imaging facilitating transition of dental imaging from 2D to 3D imaging from diagnosis to image guidance of operative and surgical procedure via third party application software. CBCT was first adapted for potential clinical use in 1982 at the Mayo Clinic Biodynamics Research Laboratory. The first CBCT system became commercially available for dentomaxillofacial imaging in 2001. This imaging modality eliminates the shortcomings of 2 -D imaging, produces a smaller radiation dose than that produced by medical CT and enables clinicians to make more accurate treatment planning decisions.2
Cone beam CT is a developing revolutionary imaging technique designed to provide relatively low-dose high-spatial-resolution visualization of high-contrast structures in the head and neck and other anatomic areas. It is a relatively recent installment in the growing inventory of clinical CT technologies. CBCT uses radiation in a similar manner as does conventional diagnostic imaging and reformats the raw data into Digital Imaging and Communications in Medicine (DICOM) data. CBCT is capable of providing sub -millimeter resolution in images of high diagnostic quality, with short scanning times (10–70 seconds) and radiation dosages reportedly up to 15 times lower than those of conventional CT scans. 4
For most dental practitioners, the use of advanced imaging h as been limited because of cost, availability and radiation dose considerations; however, the introduction of cone-beam computed tomography for the maxillofacial region provides opportunities for dental practitioners to request multiplanar imaging. 4
Dental 3D is an extraoral radiology modality with application which is an umbrella for all dental specialties like implantology, maxillofacial surgery, oral pathology, orthodontics, endodontics etc.
As CBCT exposure incorporates the entire FOV, only one rotational sequence of the gantry is necessary to acquire enough data for image reconstruction. Cone beam geometry therefore has inherent quickness in volumetric data acquisition and uses a comparatively less expensive radiation detector.
Most dental practitioners are familiar with the thin-slice images produced in the axial plane by conventional helical fan -beam CT. CBCT allows the creation in “real time” of images not only in the axial plane but also 2 -dimensional (2D) images in the coronal, sagittal and even oblique or curved image planes. In addition, CBCT data are amenable to reformation in a volume, rather than a slice, providing 3 - dimensional (3D) information. 114 Additionally, the radiation dose delivered to the patient as a result of one CBCT scan may be as little as 3% to 20% that of a conventional CT scan, depending on the equipment used and the area scanned. 4
PRINCIPLES OF CONE BEAM IMAGING
Imaging is a very important adjunct to the clinical assessment of the dental patient. Cone-Beam Computed Tomography (CBCT) uses a divergent or “cone” – shaped source of ionizing radiation and a two- dimensional area detector fixed on a rotating gantry to acquire multiple sequential projection images in one complete scan around the area of interest. It is only since the late 1990s that it has become possible to produce clinical systems that are both inexpensive and small enough to be used in the dental office shown in Figure 1 .12
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Figure 1: Cone Beam Configuration
The cone-beam technique involves a single 360° scan in which the x-ray source and a reciprocating area detector synchronously move around the patient’s head, which is stabilized with a head holder. 4
The principal feature of CBCT is that multiple planar projections are acquired by rotational scan to produce a volumetric dataset from which inter-relational images can be generated. Four technologic factors have converged to make this possible:
1. Development of compact high quality flat-panel detector arrays
2. Reductions in the cost of computers capable of image reconstruction
3. Development of inexpensive x-ray tubes capable of continuous exposure
4. Limited volume scanning (e.g., head and neck), eliminating the need for sub second gantry rotation speeds
This technique has gained broad acceptance in dentistry only in the past 10 years, though its roots go back to about 2 decades. This technique is a major breakthrough as it provides high quality, thin slice images. Cone beam machines emit an x ray beam shaped like a cone, which is quite different from that conventional fan that is seen in CT machines.
All CT scanners consist of an x-ray source and detector mounted on a rotating gantry. During rotation of the gantry, the receptor detects x rays attenuated by the patient. These recordings constitute “raw data” that is reconstructed by a computer algorithm to generate cross sectional images whose component picture element (pixel) values correspond to linear attenuation coefficients. 12
Cone-beam scanners use a two-dimensional digital array providing an area detector digital array rather than a linear detector as CT does. This is combined with a three-dimensional (3D) x-ray beam with circular collimation so that the resultant beam is in the shape of a cone, hence the name “cone beam”. Because the exposure incorporates the entire region of interest (ROI), only one rotational scan of the gantry is necessary to acquire enough data for image reconstruction.
Cone-beam geometry has inherent quickness in volumetric data acquisition and therefore the potential for significant cost savings compared with CT. CBCT produces an entire volumetric dataset from which the voxels are extracted. Voxel dimensions are depende nt on the pixel size on the area detector. Therefore CBCT units in general provide voxel resolutions that are isotropic-equal in all three dimensions shown in Figure 2 .5
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Figure 2: Cone-Beam Geometry
CBCT scanners are based on volumetric tomography, using a 2D extended digital array providing an area detector. This is combined with a 3D x-ray beam. The cone-beam technique involves a single 360° scan in which the x-ray source and a reciprocating area detector synchronously move around the patient’s head, which is stabilized with a head holder. At certain degree intervals, single projection images, known as “basis” images, are acquired. These are similar to lateral cephalometric radiographic images, each slightly offset from one another. This series of basis projection images is referred to as the projection data. Software programs incorporating sophisticated algorithms including back-filtered projection are applied to these image data to generate a 3D volumetric data set, which can be used to provide primary reconstruction images in 3 orthogonal planes (axial, sagittal and coronal). 4
As the beam covers the entire region of interest, it is only necessary for the x-ray source to make one pass or less around the patient’s head, when acquiring images. The beam exiting the patient is acquired on a 2D planar detector, usually an amorphous si licon flat panel or sometimes an image intensifier/CCD detector. The beam diameter ranges from 4cm to 30 cm. As the x ray source goes around the patients head, the sensor captures from 160 to 599 basis images. These images are used to compute a spherical or cylindrical volume including, all or a portion of the face. In this volume, the densities at all locations are calculated from the basis images. Voxels are cuboids and can be as small as 0.125mm. Serial cross sectional views are made in the sagittal, axial and coronal planes. The operator can also extract thick or thin planar or curved reconstructions in any orientation. True 3 D images of bone or soft tissue surfaces can also be viewed. Third party software is being developed continuously that uses the d ata generated by cone beam units and typically provides display and measuring tools for specific purposes.
When a dentist uses software, the processed volumetric data are exported from the CBCT manufacturer’s software as a Digital Imaging and Communications in Medicine (DICOM) data set. The data may then be imported into the third party software for analysis. The software has been developed for assisting in implants, treatment planning and for orthodontics, for display of relationships between hard and soft tissues, and for making measures of true distance and angles. The data set generated by cone beam imaging can also be used to produce rapid prototyping models for treatment planning like in orthognathic cases, forensic uses and to build surgical guides f or implant placement.
Cone beam machines may be broadly classified as providing large or limited imaging volumes. They are:
- Large volume machines
- Limited volume machines
The large volume machines have image fields with diameters of 6 inch to 12inch.24 The limited volume machines generate images 4cm or 6cm in diameter but have higher spatial resolution. The large volume machines are more appropriate for orthodontics, orthognathic surgery and full arch implants treatment planning. The limited volume machines are most appropriate for examining individual teeth for fracture or periapical disease, the osseous components of the TMJs, evaluating the relationship of third molars to the mandibular canal and single site implant. 12
CONVENTIONAL VERSUS CONE BEAM CT
CT can be divided into two categories on the basis of acquisition x-ray beam geometry, namely, fan beam and cone beam.
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Figure 3: Difference between CT and CBCT
The difference between conventional multi-detector CT (fan- beam) and CBCT is that CBCT acquires information using a high- resolution two-dimensional detector instead of multiple one- dimensional (1D) detector elements shown in Figure 3.5 In standard multi-detector CT, a series of detector element rows is used. In multi - detector spiral CT, the patient is scanned in a helical fashion with gantry speeds on the order of 0.4 seconds for current state-of-the-art 64-slice scanners. With a detector row width of 0.5 –0.6 mm, coverage of approximately 4 cm in the z-axis allows large anatomical regions to be imaged in several seconds. For C-arm CBCT systems, current detector arrays are 40 x 30 cm 2, allowing 25 x 25 x 18 cm3 volumetric datasets to be generated in a single rotation of the source and detector. However, C-arm CBCT systems currently available require 5–20 seconds for image acquisition. Therefore, despite the fact that multi - detector CT systems require multiple rotations of the CT gantry to cover the same region of interest covered by the C-arm CBCT system in a single rotation, the multi-detector CT image acquisition is actually more rapid. Furthermore, due to the mathematical complexity of the cone-beam reconstruction algorithm, which is a modification of the algorithm initially described by Feldkamp, state -of-the-art C-arm CBCT systems require 1 minute of post-processing time for image reconstruction, compared to essentially realtime image reconstruction for multidetector CT. 5
With 2048 x 1538 detector elements, isotropic voxel sizes of under 200 x 200 x 200 _m3 are achievable with current C-arm CBCT systems. Assuming an isotropic voxel size of 600 x 600 x 600 µm3 for current state-of-the-art 64-slice scanners, the flat-panel detector system can theoretically achieve a volumetric resolution reduction factor of approximately 25. However, patient dose considerations make utilization of this high resolution impractical (and unnecessary) for most imaging applications. Furthermore, previous investigations suggest that spatial resolution and noise for the flat -panel detector- based system is governed primarily by blur in the x-ray converter (CsI:Tl) (and reconstruction filter), rather than pixel size, limiting the practical voxel size of current C-arm CBCT systems. Therefore, although the pixel size in flat-panel detectors theoretically allows for voxel sizes of 0.008 mm3, blur caused by the x-ray converter and reconstruction filter, protracted reconstruction times, patient dose considerations, and general lack of clinical necessity result in practical C-arm CBCT spatial resolutions similar to, if not slig htly larger than, those of multidetector CT.
The most significant difference between 3D tomographic datasets generated via cone-beam geometry versus fan-beam geometry is the significant increase in scattered radiation with CBCT. In fact, as multidetector CT systems employ increasing numbers of detectors, the geometry changes from fan-beam to cone-beam. The exact point at which this transition occurs is not well defined. Investigations of 256 - detector multidetector CT systems, employing cone-beam geometry, demonstrate image quality degradation secondary to scatter similar to that seen with CBCT systems. Multi-detector CT scanners employ anti-scatter septae between the individual detector channels. Anti - scatter septae of this nature cannot be used with flat -panel detectors. The important point is that increased scatter radiation due to wider x - ray beam collimation in CBCT leads to a significant degradation of image quality. To account for the increased scatter, multiple antiscatter techniques have been investigated for use with CBCT systems including, anti-scatter grids, software correction algorithms, beam-stop scatter mapping, and adjustment of object-to-detector distance (air-gap).5
Conventional CT uses a fan shaped x-ray beam rotating in a helical fashion around the patient, with the data being acquired by solid state detectors located around the gantry. In most modern scanners, the detectors are arranged in parallel arrays, allowing up to 64 slices to be obtained simultaneously with each rotation. This considerably reduces the scanning time compared to the older single slice acquisition formats. The images obtained are typically ‘axial’ cross-sections through the region of interest, but with associated computer algorithms these can be reformatted to be vie wed in coronal and sagittal planes, as well as 3 -dimensionally shown in Figure 4 .3
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Figure 4: 3-Dimensional View
Conversely, cone-beam scanners are based on a cone-shaped beam of x-rays rotating around the object of interest giving a volume of data, using a 2-dimensional extended digital array as an area detector shown in Figure 5 .3
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Figure 5: 2-Dimensional View
The technique involves a single 360 degree scan in which the x - ray source and reciprocating area detector synchronously move around the patient’s head, which is stabilized with a head holder. At certain degree intervals, single projection images, known as ‘basis’ images, are acquired. These are similar to lateral cephalometric radiographic images, each slightly offset from one another. This series of basis projection images is referred to as the projection data. Software programmes incorporating sophisticated algorithms, including back- filtered projection, are applied to these image data to generate a 3D volumetric dataset, and can be used to provide primary reconstruction images in three orthogonal planes (axial, sagittal and coronal), as well as 3-dimensionally.3
Conebeam CT indicates an effective radiation dose between 0.035 and 0.10 mSv, which is up to a 98% reduction compared to conventional CT.
- CT (both jaws) - 0.6 mSv
- CBCT (both jaws) - 0.068 mSv
CBCT provides an equivalent patient radiation dose of 5 to 14 times that of a single film-based panoramic radiograph, 1.3% to 22.7% of a comparable conventional CT exposure or 7 to 116 days of background radiation.
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Figure 6: Isotropic and Anisotropic
In CBCT imaging, voxel dimensions are primarily dependent on the pixel size on the area detector, not as with conventional CT, on slice thickness. Therefore in contrast to conventional CT, CBCT units provide voxel resolutions that are isotropic - i.e. equal in all three dimensions. In contrast to conventional CT, cone beam data reconstruction is performed by personal computer rather than workstation platforms shown in Figure 6.12
CONE BEAM CT SYSTEM CONFIGURATION
“Cone-beam” computed tomography (CBCT) is accomplished using a rotation in which a pyramidal - or cone-shaped x-ray beam is directed towards an area x-ray detector on the other side of the patient’s head. Multiple 2D projection images are acquired for a field of view (FOV) selected according to the region of interest (ROI). This varies from a traditional medical CT which uses a fan shaped X -ray beam in a helical progression acquiring individual image slices of the FOV and then stacks the slices to obtain a 3D representation. Each slice requires a separate scan and separate 2D reconstruction. 4
Because CBCT exposure incorporates the entire FOV, only one rotational sequence of the gantry is necessary to acquire enough data for image reconstruction. Cone-beam geometry therefore has inherent quickness in volumetric data acquisition and uses a comparatively less expensive radiation detector. Herein lies its potential for significant cost savings.
The current cone beam machines scan patients in 3 possible positions:
1. Sitting
2. Standing
3. Supine
Equipment that requires the patient to lie supine physically occupies a larger surface area or physical footprint and may not be accessible for patients with physical disabilities. Standing units may not be able to be adjusted to a height to accommodate wheel chair bound patients. Seated units are the most comfortable, however fixed seats may not allow scanning or physically disabled or wheel chair bound patients. As scan times are often greater than those required for panoramic imaging, perhaps more important than patient orientation in the head restraint mechanism used. 12
Main Components are shown in Figure 7:12
a. X-ray tube
b. Image intensifier
c. Head rest
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Figure 7: Components of CBCT system
CBCT system consists of a U-arm with an X-ray source and a 900 or 1200 image intensifier, plus some additional equipment. Equipment that requires the patient to lie supine physically occupies a larger surface area or physical footprint and may be accessibl e for patients with some physical disabilities. Standing units may not be able to be adjusted to a height to accommodate wheelchair -bound patients. Seated units are the most comfortable; however, fixed seats may also not allow scanning of physically disable or wheelchair-bound patients. Because scan times are often greater than that used with panoramic imaging, perhaps more important than patient orientation is the head restraint mechanism used. With all systems it is important to immobilize the patient’s head because any movement degrades the final image. The rotation axis-to-detector distance is 290 mm and the focal spot to rotation axis distance is 820 mm. During an examination, the patient sits in the rotation centre. Tube voltage is selectable from 60 kV, 80 kV, 100 kV to 120 kV and tube current is also selectable at 10 mA or 15 mA. Exposure time is 9.6 s. The unit acquires 288 projected images, each of which consists of 512 £ 512 matrices and 12-bits grey depth. Following this, 3D (volume) data are reconstructed using a parallel processing system designed for this system. 10
Other Components are shown in Figure 8:10
- Collimation
- Source Filtration
- Compensating Filtration
- Anti-scatter Grids
X-ray filtration at the source, beam collimation, and compensating filtration constitute direct methods of scatter reduction.
Source Filtration:
Filtration at the source can be achieved by applying an aluminium filter to remove low-energy photons uniformly from the x- ray beam.
Collimation:
Beam collimation eliminates photons outside the intended FOVz, reducing the contribution of peripheral scatter to the SPR in the FOV.
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Figure 8: Components of CBCT
Compensating Filtration:
The x-ray path length through tissue at the edges of the FOV is typically shortened in relation to the structure of the scanned object. This result in less attenuation of peripheral scatters and, thus, disproportionately increased peripheral scatter contri bution to image degradation. Peripheral scatter not only constitutes the largest contribution to total scatter but forms the basis of the cupping artifact, the effect of which can be mitigated by compensating filtration. The bow tie or wedge filter is the prototypical compensating filter used in CBCT systems. It modulates the beam profile by increasing photon density at the center of the cone and decrementally reducing density at the periphery. In the radiation therapy CBCT literature, Graham et al were able to demonstrate a 50% reduction in scatter with the implementation of copper bow tie filters. Image-quality improvement has been described with bow tie filters in a CBCT system integrated into the gantry of a conventional CT scanner as well.8 Compensating filtration is not without criticism, however, because beam hardness has been shown to negatively impact detector efficiency, as demonstrated by a decrease in the ratio of the output signal intensity –to-noise ratio (SNR) to the entrance exposure (SNR/entrance exposure). The kilovolt (peak) (kVp), which is related to the beam hardness, has also been shown to produce optimal low-contrast detectability when it is kept at lower settings. Thus, although scatter and cupping artifacts may be reduced with bow tie filters, this reduction may come at the expense of detector efficiency and low-contrast detectability. Compensating filtration and the other direct scatter-reduction methods at the source side of the apparatus have the added appeal of reducing patient dose and can, of course, be used in series. 1
Anti-scatter Grids:
Anti-scatter grids represent an alternative method of direct scatter reduction that has been used with FPDs in digital radiographic and fluoroscopic imaging for some time. Rather than modulating the beam properties at the source, a grid of lead leaves is fitted over the detector to preferentially absorb off-axis radiation not contributing to primary photon fluence. In CBCT systems, the lead leaves are arranged in a radial pattern centered on the focal spot of the FPD.
Anti-scatter grids have been evaluated in several experimental CBCT systems with mixed results. A reduction in both cupping artifact and overall scatter has been observed, though there may be insufficient improvement in contrast and observed image quality to warrant use except in situations of high scatter. Siewerdsen et al evaluated anti - scatter grids in a linear accelerator-coupled CBCT system and found that image quality and CNR improved only in situations of high scatter—such as with a large FOVz covering a large anatomic site— or in input quantum-limited situations such as with high dose or low spatial resolution. To the extent that anti-scatter grids improve soft- tissue contrast and artifacts, they also increase noise, which leads to a degradation in overall image quality. An escalation in dose or reduction in spatial resolution is needed to offset the increased noise with the implementation of grids. For a relatively small FOVz, such as that used in a targeted head and neck scan, anti-scatter grids may improve image contrast and reduce cupping artifacts, but the increased noise requires that the dose be increased or spatial resolution be decreased to produce a high-quality image with a favorable CNR.1
Imaging Protocol:
An imaging protocol is a set of technical exposure parameters for CBCT dependent on the specified purpose of the examination. An imaging protocol is developed to produce images of optimal quality with the least amount radiation exposure to the patient. For specific cone beam units, manufacturer- provided imaging protocols are usually available. Most commonly they involve modifications in imaging field, number basis projections, and voxel resolution. Operators should be aware of the effects of all parameters on image quality and patient does when choosing imaging protocols. 12
Image Elements:
Most of us are familiar with pixels (picture element), which are unit measurements used in computer screens, digital cameras, and 2D intraoral sensors. With three dimensions, the unit of measurement is a voxel (a blending of the words volumetric and pixel), representing a value on a regular grid in 3D space. With CBCT technology, the voxels are isotropic, meaning that they are equal in all dimensions. This feature is important for implant planning and execution16 because of the 1:1 relationship in CBCT images in all three orthogonal planes. Measurement is generally precise, so standardized data (usually Digital Imaging and Communications in Medicine [DICOM] files) can be exported for use in fabricating stereolithographic surgical guides to assist in the positioning of implant placement. 10
Voxel Size:
The voxel size with which projection images are acquired varies from manufacturer principally on the basis of the matrix si ze of the detector and projection geometry. In addition, CBCT units may offer a selection of voxel sizes. For these choices the image detector collects information over a series of pixels in the horizontal and vertical directions and averages the data. This collection or pixel binning results in a substantial reduction in data processing, reducing secondary reconstruction times. Therefore voxel size should be specified as either acquisition or reconstruction. Generally, decreasing voxel size increase spatial resolution, but because of the pixel fill factor of a particular flat panel, a higher radiation does may be required.12
Projected Data:
The projection data are corrected using calibration data generated from the detector’s response to a scan of air. Geometrical distortion of the image intensifier is then corrected using a correction table for distortion of the image intensifier. Based on t hese corrected projection data, CT images are reconstructed according to the cone beam back projection method proposed by Feldkamp et al. 73 A total of 512 axial images are produced and saved in the original format but they can also be exported in the DICOM format. Reconstruction of images from projected images takes approximately 5 mins. The unit has three field of view (FOV) modes for each image intensifier size. The 1200 system has facial (F), panoramic (P) and implant (I) modes. The 900 system has P, I and dental (D) modes. Images produced by this system consist of 512 x 512 x512 isotropic voxels. Sizes of the FOV and voxels for each mode are, respectively, 192.5 mm and 0.376 mm in F mode, 150 mm and 0.293 mm in P mode, 102 mm and 0.200 mm in I mode, and 51.2 mm and 0.100 mm in D mode shown in Figure 9.10
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Figure 9: Technical Data of CBCT
CBCT uses a cone-shaped X-ray beam that goes around the object under observation. This CBCT system allows the physician to acquire 3D volume data in one rotation. Compared with conventional CT, CBCT yields high resolution images, especially in the longitudinal direction. CBCT system will be useful for examining peri-tooth lesions or implants, both of which require high resolution images, particularly in the longitudinal direction. 38 It takes approximately 5 mins to produce 512 axial images with this system. Increases in computer processing speed will further decrease the time necessary to produce these images. The variety of image displays possible with this system, such as axial images, 3D volume rendering and dental mode, will be useful for diagnosing maxillofacial lesions and as well as performing pre-implant examinations. After reconstruction of the 512 axial images is completed, any type of image can be displayed in close to real-time. One of the characteristics of our new CBCT system is its choice of three FOVs, which can be chosen to suit the particular type of examination being performed shown in Figure 10 .10
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Figure 10: Three FOVs CBCT system.
The D mode has a small FOV and a high resolution, which is suitable for examination of lesions involving two or three teeth. The I mode has a lower resolution than the D mode but has a FOV that can cover an area stretching from the inferior border of the m andible to the lower half of the maxillary sinus. The I mode is suitable for pre - implant examination and examination of mandibular lesions such as odontogenic cysts and tumors. The P mode and F mode have large FOVs that are suitable for examining deformations of the mandible, such as mandibular protrusions. We must also point out that the image noise of this CBCT is higher than that of conventional helical CT. This is likely to result from noise of the image intensifier and high scattered radiation of this system. The influence of this high noise on the diagnostic capability of this system needs further clarification. 5
Scatter Correction Algorithms:
Some sort of scatter subtraction or homogenization preprocessing algorithm is used in most clinical CBCT systems. Several approaches have been studied, including Monte Carlo simulations, blocker -based or beam-stop techniques, analytic calculations, and collimator shadow estimation. Perhaps the most theoretically robust algorithm is that based on the Monte Carlo simulation, which predicts scatter on the basis of a voxel density model of the entire acquired tissue volume during preprocessing. The predicted scatter contribution at each detector element is then subtracted before reconstruction. Monte Carlo simulations still require significant computation time, however, which has fueled continued research in other algorithmic approaches. Methodologically, algorithms do not reduce the additional patient dose attributable to scatter, but they have been able to achieve significant improvements in image uniformity, CNR, and CT number accuracy. They can, of course, be implemented in conjunction with other direct methods of scatter reduction. 1
CBCT IMAGE PRODUCTION
Cone-beam imaging, sometimes referred to as digital volume tomography, is one of the most exciting developments in dental and maxillofacial radiology and, owing to its versatility, will almost certainly become an increasingly popular form of imaging availa ble in dental practice. For each image acquisition there are procedural steps and numerous operator-controlled exposure parameters that must be specified. Consistent and methodical imaging technique minimizes patient radiation exposure and optimizes the resultant image quality. 12
Patient Selection Criteria:
Cone-beam exposure provides a radiation does to the patient higher than those of other dental radiographic producers. Accordingly, the principal of ALARA must be applied there should be justification of the exposure to the patient so that the total potential diagnostic benefits are greater than the individual detriment radiation exposure might cause. Currently CBCT is most commonly used in the assessment of pathologic conditions and structural maxillofa cial deformity, the preoperative assessment of orthodontics, and in the assessment of available bone for implant placement. It is advisable that the indication for the CBCT examination be documented by entry in the patient’s chart or on the written request or prescriptive order for the CBCT examination. 12
Patient Preparation:
Patients should be escorted into the scanner unit and before head stabilization provided with appropriate personal radiation barrier protection. Although the mandatory use of these devices is regulated by regional (state) or federal legislation, it is recommended that at least a leaded torso apron be applied correctly (above the collar) to the patient. This is particularly advisable for pregnant patients and for children. It is highly recommended that a lead thyroid collar also be used, provided that this will not interfere with the scan, to reduce thyroid exposure.
Each CBCT unit has a unique method of head stabilization, varying from chin cups to posterior or lateral head supports to head restraints. Patient motion can be minimized by application of one or more methods simultaneously. Image quality is severely degraded by head movement, so it is important to obtain patient compliance. Alignment of the area of interest with the x-ray beam is critical in imaging the appropriate field, thereby reducing patient radiation exposure and optimizing image quality by reducing scattered radiation. Often facial topographic reference planes (e.g., the mid sagittal plane, palatal plane) are adjusted to coincide or be aligned with external laser lights to position the patient correctly. 12
Immediately before the scan, the patient should be asked to remove all metallic objects from the head and neck areas. This includes eyeglasses, jewellery (including earrings and piercings), and metallic partial dentures. It is not necessary to remove plastic completely in removable prostheses. Unless specifically indicated otherwise (e.g., closed temporomandibular [TMJ] views or orthodontic views), it is desirable that the dentition be separated but held together firmly during the scan. This can be performed with a tongue depressor or cotton rolls. Separation of the teeth is particularly useful in single arch scans where scatter from metallic restorations in the oppos ing arch can be reduced. The patient should be directed to remain as still as possible before exposure, to breathe slowly through the nose, and to close the eyes. This latter suggestion reduces the possibility of the patient moving as a result of following the detector as it passes in front of the face.12
Acquisition Configuration:
The geometric configuration and acquisition mechanics for the cone beam technique are theoretically simple. A single partial or full rotational scan from an x ray source takes place while a reciprocating area detector moves synchronously with the scan around a fixed fulcrum within the patients head. There are four components of CBCT image acquisition. They are:
1. X-ray generation
2. Image detection system
3. Image reconstruction
4. Image display
X- ray Generation:
During the scan rotation, each projection image is made by sequential, single image capture of attenuated x-ray beams by the detector. Technically the easiest method of exposing the patient is to use a constant beam of radiation during the rotation and allow the x ray detector to sample the attenuated beam in its trajectory. However, continuous radiation emission does not contribute to the formation of the image and results in greater radiation exposure to the patient. Alternatively the x ray beam may be pulsed to coincide with the detector sampling, which means that the actual exposure time is markedly less than scanning time. This technique reduces patient radiation dose considerably. Currently 4 units (Accuitomo, CB MercuRay, Iluma Ultra Cone and PreXion 3D) provides continuous radiation exposure. Pulsed x-ray beam exposure is a major reason for considerable variation in reported cone beam unit dosimetry. 12
Field of View:
The dimensions of the FOV or scan volume able to be covered depend primarily on the detector size and shape, the beam projection geometry, and the ability to collimate the beam. The shape of the scan volume can be either cylindrical or spherical (e.g. New Tom 3G). Collimation of the primary x ray beam limits the x ray radiation exposure to the region of interest. Field size limitation therefore ensures that an optimal FOV can be selected for each patient, based on disease presentation and the region designated to be imaged. CBCT systems can be categorized according to the available FOV or selected scan volume height as follows:
Localized region:
- Approximately 5 cm or less (e.g. dentoalveolar, temperomandibular joint)
- Single arch: 5 cm to 7 cm (e.g. maxilla or mandible)
- Interarch: 7cm to 10 cm. (e.g. Mandible and superiorly to include the inferior concha)
- Maxillofacial: 10 cm to 15cm (e.g. Mandible and extending to nasion)
- Craniofacial: greater than 15cm (e.g. from the lower border of the mandible to the vertex of the head)
Extended FOV scanning incorporating the craniofacial region is difficult to incorporate into cone beam design because of the high cost of large – area detectors. The expansion of the scan volume height has been accomplished by one unit (iCAT Extended field of View Model) by the software addition of two rotational scans to produce a single volume with a 22cm height. Another novel method for increasing the width of the FOV while using a smaller area detector, thereby reducing manufacturing costs, is to offset the position of the d etector, collimate the beam asymmetrically and scan only half the patient. (e.g. Scanora 3D, SOREDEX, Helsinki, Finland) shown in Figure 11. 12
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Figure 11: Difference in Field of view
During the scan single exposures are made at a certain degree intervals, providing individual 2D projection images, known as “basis”, “frame” or “raw” images. These images are similar to lateral and posterior anterior “cephalometric” radiographic images, e ach slightly offset from one another. The complete series of images is referred to as the “projection data”. The number of images comprising the projection data throughout the scan is determined by the frame rate (number of images acquired per second), the completeness of the trajectory arc and the speed of the rotation. The number of projection scans comprising a single scan may be fixed (e.g., New Tom 3G, Iluma, Galileos, or Promax 3D) or variable (e.g. i -CAT, PreXion 3D). More projection data provide more information to reconstruct the image allows for greater spatial and contrast resolution; increase the signal to noise ratio, producing smoother images and reduce metallic artifacts. However, more projection data usually necessitates a longer scan time, a higher patient dose, and longer primary reconstruction time. In accordance with the “as low as reasonably achievable” (ALARA) principle, the number of basis images should be minimized to produce an image of diagnostic quality. 4
Frame Rate and Speed of Rotation:
Higher frame rates provide images with fewer artifacts and better image quality. However the greater the number of projections proportionately increases the amount of radiation a patient receives. Detector pixels must be sensitive enough to capture radiation adequate to register a high signal to noise output and to transmit the voltage to the analog and the digital converter, all within a short arc of exposure. Within the limitations of solid state detector readout speed and the need of short scanning time in a clinical setting, the total number of available view angles is normally limited to a several hundred.
Completeness of the Trajectory Arc:
Most CBCT imaging systems use a complete circular trajectory or a scan arc of 360d to acquire projection data. This physical requirement is usually necessary to produce projection data adequate for 3D reconstruction using the FDK algorithm. However it is theoretically possible to reduce the completeness of the scanning trajectory and still reconstruct a volumetric data set. This approach potentially reduces the scan time and is mechanically easier to perform. However the images produced by this method may have greater noise and suffer from reconstruction interpolation artifacts. Currently this technique is being used by at least two units (Galileos and Promax 3D).
X-ray Generator:
During the scan rotation, each projection images is made by sequential single-image capture of the remnant x-ray beam by the detector. Technically, the easiest method of exposing the patient is to use a constant beam of radiation during the rotation and al low the x- ray detector to sample the attenuated beam in its trajectory. However, this results in a continuous radiation exposure to the patient, much of which does not contribute to the formation of the image. It is preferable to pulse the x-ray beam to coincide with the detector sampling. This means that actual exposure time is markedly less than scanning time. This technique reduces patient radiation dose considerably.
The ALARA (As low As Reasonably Achievable) principle of does optimization necessitates that CBCT exposure factors should be adjusted on the basis of patient size. This can be achieved by appropriate selection of either tube current (milliamperes [mA]), tube voltage (kilovolts peak [kVp]), or both. On some CBCT units both kVp and mA are automatically modulated in near real time by a feedback mechanism detecting the intensity of the transmitted beam, a process known generically as automatic exposure control. On others, exposure settings are automatically determined by the initial scout exposure. This feature is highly desirable because it is operator independent. The variation in exposure parameters together with the presence of pulsed x-ray beam and size of the image field are the primary determinants of patient exposure. 3
Scan Factors:
The speed with which individual images are acquired is called the frame rate and is measured in frames, projected images, per second. The maximum frame rate of the detectors and rotational speed determines the number of projections that may be acquired. The number of projection images comprising a single scan may be fixed or variable. With a higher frame rate, more information is available to reconstruct the image; therefore, primary reconstruction time is increased. However, higher frame rates increase the sig nal-to-noise ratio, producing images with less noise.
In the maxillofacial region, another advantage of a higher frame rate is that it reduces metallic artifact. Note that higher frame rates are usually accomplished with a longer scan time and hence higher patient dose.
Most CBCT imaging systems use a complete circular trajectory or a scan arc of 360 degrees to acquire projection data. This physical requirement is usually necessary to produce adequate projection data for 3D reconstruction. However, it is theoretically possible to reduce the completeness of the scanning trajectory to less than a full circle and still reconstruct a volumetric data set. This approach potentially reduces the scan time and is mechanically easier to perform. It is desirable to reduce CBCT scan times to as short as possible to reduce motion artifact resulting from subject movement. This can be substantial and may be limiting factor in voxel resolution. Decreased scanning times may be achieved by increasing the detector frame rate, reducing the number of projections, or reducing the scan arc. 12
Image Detection:
Currently CBCT units can be divided into two groups, based on detector types. They are:
- Image intensifier tube/charge couple device (IIT/CCD) combination or
- Flat panel imager
IIT/CCD Configuration:
The IIT/CCD configuration comprises an x ray IIT coupled to a CCD by way of a fibre optic coupling. The former configuration comprises an x-ray image intensifier tube coupled to a charge-coupled device with a fibre optic coupling.
Flat Panel Imager:
Flat panel imaging consists of detection of x-rays with an “indirect” detector that is based on a large area solid -state sensor panel coupled to an x-ray scintillator layer. The most common flat- panel configuration consists of a cesium iodide scintillator applied to a thin film transistor made of amorphous silicon. Flat panel imaging consists of detection of x rays using an “indirect” detector based on a large area solid state sensor panel coupled to an x ray scintillator layer. Flat panel detector arrays provide a greater dynamic ran ge and greater performance than the II/CCD technology.
Image intensifiers may create geometric distortions that must be addressed in the data processing software, whereas flat panel detectors do not suffer from this problem. This disadvantage could poten tially reduce the measurement accuracy of CBCT units using this configuration. II/CCD systems also introduce additional artifacts. 12
Voxel Size:
The principal determinants of nominal voxel size in CBCT are the x-ray tube focal spot size, x-ray geometric configuration, and the matrix and pixel size of the solid state detector. Both the focal spot size and the geometric configuration of the x-ray source determine the degree of geometric unsharpness, a limiting factor in spatial resolution. However, the cost of x-ray tubes, and therefore of the CBCT the detector position is limited because it must be located far enough from the patient’s head so that it freely rotates and clears the patient’s shoulders. Limitations also exist in extending the source to - object distance because this increases the size of the CBCT unit. However, reducing source-to-object distance produces a magnified projected image on the detector, increasing potential spatial resolution.12
Gray Scale:
The ability of CBCT to display differences in attenuation is related to the ability of the detector to detect subtle contrast differences. This parameter is called the bit depth of the system and determines the number of shades of gray available to display the attenuation. At the time of writing, all available CBCT units used detectors capable of recording gray scale differences of 12 bits or higher. If a 12-bit detector (212) is used to define the scale 4096 shades are available to display contrast. Although higher bit -depth images in CBCT are possible, this added information comes at the expense of increased computational time and substantially larger file sizes.
CBCT systems that use flat panel detectors also have limitations in their performance that are related to linearity of response to the radiation spectrum, uniformity of response throughout the area of detector and bad pixels. The effects of these limitatio ns on image quality are most noticeable at lower and higher exposures. To overcome this problem, detectors are linearized piecewise and exposures that cause non uniformity are identified and calibrated. In addition pixel by pixel standard deviation assessment is used in correcting non uniformity. Bad pixels are also examined and most often replaced by the average of the neigh boring pixels. A reduction in image matrix size is desirable to increase spatial resolution and therefore provide greater image detail. However, detector panels comprise an array of individual pixels with two components, photodiodes that actually record the image and thin film transistors that act as collators and carriers of signal information.
Therefore not all of the area of an image is taken up by the photodiode. In fact the percentage area of detector that actually registers information within an individual pixel is referred to as the “fill factor”. So although a pixel may have a nominal area, the fill factor may be of the order of 35%. Therefore, smaller pixels capture fewer x ray photons and result in more image noise. Consequently, CBCT imaging using smaller matrix sizes usually requires greater radiation and higher patient dose exposure. The resolution and therefore the detail of CBCT imaging is determined by the individual volume elements or voxels produced from the volumetric data set. In CBCT imaging, voxel dimensions primarily depend on the pixel size on the area detector, unlike those in conventional CT, which depends on slice thickness. The resolution of the area detector is sub millimeter (range .09mm to .4mm) which primarily determines the size of the voxel. Therefore CBCT units in general provide voxel resolutions that are isotropic (equal in all three dimensions). 1
Image Reconstruction:
Once the basis projection frames have been acquired, data must be processed to create the volumetric data set. This process is called primary reconstruction. The number of individual projection frames may be from 100 to more than 600, each with more than one million pixels, with 12 to 16 bits of data assigned to each pixel. The reconstruction of data is therefore computationally complex.
To facilitate data handling, data are usually acquired by one computer (acquisition computer) and transferred by the way of an Ethernet connection to a processing computer (workstation). Reconstruction times vary, depending on the acquisition parameters (voxel size, FOV, number of projections), hardware (processing speed, data through out from acquisition to workstation computer), and software (reconstruction algorithms) used. Reconstruction should be accomplished in an acceptable time (less than 3 minutes for standard resolution scans) to complement patient flow. The reconstruction process consists of two stages shown in Flow chart Figure 12 , each composed of numerous steps. 12 Two stages are:
- Acquisition Stage
- Reconstruction Stage
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Figure 12: Flow Chart
Acquisition Stage:
For each image acquisition there are procedural steps and numerous operator-controlled exposure parameters that must be specified. Consistent and methodic imaging technique minimizes patient radiation exposure and optimizes the resultant image quality. The spatially varying physical properties of the photodiodes and the switching elements in that flat panel, and also variations in the x ray sensitivity of the scintillator layer, raw images from CBCT detectors show spatial variations of dark image offset and pixel gain.
The dark image offset (i.e the detector output signal without any x-ray exposure) and it’s spatial variations are mainly caused by the varying dark currents of the photodiodes. Gain variations are caused by the varying sensitivity of the photodiodes and by variations in the local conversion efficiency of the scintillator material caused by, for example, thickness or density variations. In addition to offset and gain variations, even high quality detectors exhibit inherent pixel imperfections or a certain amount of defect pixels. To compensate for these in homogeneities, raw images require systemic offset and gain calibration and a correction of defect pixels. The sequence of the required calibration steps is referred to as “detector pre -processing” and the calibration requires the acquisition of additional image sequences.
Reconstruction Stage:
Once images are corrected, they must be related to each other and assembled. One method involves constructing a sinogram: a composite image relating each row of each projection image. The final step in the reconstruction stage is processing the corrected sinograms. A reconstruction filter algorithm is applied to the sinogram and converts it into a complete 2D CT slice. The most widely used filtered back projection algorithm for cone beam acquired volumetric data is the FDK algorithm. Once all the slices have been reconstructed, they can be recombined into a single volume for reconstruction shown in Figure 13.12
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Figure 13: Reconstruction Stage
Image Display:
The availability of CBCT technology provides the dental clinician with a great choice of image display formats. The volumetric data set is a compilation of available voxels and for most CBCT devices; it is presented to the clinician on scree n as secondary reconstructed images in three orthogonal planes (axial, sagittal, and coronal) usually at a thickness defaulted to the native resolution shown in Figure 14. Optimum visualization of orthogonal reconstructed images depends on the adjustment of window level and window width to favour bone and application of specific filters. 10
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Figure 14: Reconstruction, Axial, Sagittal and Coronal view
During CBCT rotation, single exposures are made at certain degree intervals, providing projection images, known as basis or projection images. These are similar to lateral, oblique, A-P and P-A “cephalometric” radiographic images, each slightly offset from one another. The complete series of images is referred to as the projection data. The number of images comprising the projection data, called the frame rate is variable, depending on the system and settings applied.
The greater the frame rate for a given scan time, the more data that can be collected to construct the image. Unfortunately, while high frame rates improve image quality they increase radiation dose to the patient proportionately. Finally reconstruction software programs incorporating sophisticated algorithms including back-filtered projection are applied to these projection data to generate a three dimensional (3D) volumetric data set, which can be used to provide secondary reconstruction images in three orthogonal planes (axial, sagittal and coronal). The resolution and therefore detail of CBCT imaging is determined by the individual volume elements or voxels produced from the volumetric dataset. In CBCT imaging, voxel dimensions are primarily dependent on the pixel size on the area detector, not as with conventional CT, on slice thickness. As the resolution of the area detector is sub-millimeter (range: 0.09mm to 0.4mm), this principally determines the size of the voxels. Therefore CBCT units, in general, provide voxel resolutions that are isotropic - i.e. equal in all three dimensions.
Since the introduction of the first CBCT scanner, the NewTom® QR DVT 9000 (Quantitative Radiology s.r.l ., Verona, Italy), several additional systems have been marketed. Moreover additional systems are in development and are likely to be launched shortly. All current generations of CBCT systems provide useful diagnostic images - future enhancements will most likely be directed towards reducing scan time, providing multi-modal imaging (capabilities of conventional panoramic and cephalometric as well as CBCT images), improving image fidelity including soft tissue contrast and, incorporating task specific protocols (e.g. high resolution, small FOV for dento-alveolar imaging or medium resolution, large FOV for dento-facial orthopedic imaging) to minimise patient dose. 10
Image Optimization:
Most programs offer the user means to adjust brightness, contrast, and edge sharpening. To optimize image presentation and facilitate diagnosis, it is necessary to adjust contrast (window) and brightness (level) parameters to favour bony structures. Great variability exists in cone beam imaging between CBCT units and within the same unit depending on the number of scans performed. Although CBCT proprietary software may provide for window/level presents, it is advisable that this be adjusted for each scan. After these parameters are set, further enhancements can be performed by th e application of sharpening, filtering, and edge algorithms. The use of these functions must be weight against the visual effects of increased noise in the image. 12
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Reports:
Cone-bean imaging comprises not only the technical component of patient exposure but a responsibility for interpreting the resultant volumetric data set. Documentation of an imaging examination is an important part of a patient’s medical record. The mechanics of image reporting include the development of a series of images formatted t o display the condition/region appropriately (image report) and a cognitive interpretation of the significance of the imaging findings (interpretive report). 12
Archiving, Export and Distribution:
The process of CBCT imaging produces two data products, the volumetric image data from the scan and the image report generated by the operator. Both sets of data must be archived and distributed. Scan data backup is usually performed in its native or proprietary image format. However, export of image data is usual ly in the DICOM (Digital/Imaging and Communications in Medicine) file format standard for use in specialized software. 12
ADVANTAGES OF CBCT
Cone beam CT (CBCT) is advancement in CT imaging that has begun to emerge as a potentially low-dose cross-sectional technique for visualizing bony structures in the head and neck. The impact of CBCT technology on maxillofacial imaging since its introduction cannot be underestimated. This does not imply that CBCT is appropriate as an imaging modality of first choice in dental practice. However there are no published specific patient selection criteria for the use of CBCT in maxillofacial imaging - guidelines as to the when, where, why, what, how and on whom. As cone beam exposure provides a radiation dose to the patient higher than any other imaging procedure in dentistry it is paramount that practitioners abide by the Code of Practice: Radiation Protection in Dentistry in that a responsible person keep radiation exposure ALARA (As Low As Reasonably Achievable). The basis of this is that there should be justification of the exposure to the patient such that the total potential diagnostic benefits are greater than the individual detriment radiation exposure might cause. CBCT should not be considered a replacement for standard digital radiographic applications that, ironically, also use a cone beam of radiation, but without computed integration of basis projection s.4
CBCT equipment has a greatly reduced physical footprint and is approximately 20-25% of the cost of conventional CT. CBCT provides images of high contrasting structures and is therefore particularly well-suited towards the imaging of osseous structures of t he craniofacial area. The use of CBCT technology in clinical dental practice provides a number of advantages for maxillofacial imaging.
These include:
Size and Cost:
CBCT equipment has a greatly reduced size and physical footprint compared with conventional CT and it is approximately one fourth to one fifth the cost. Cost these features make it available for the dental office.12
High Speed Scanning:
Compared with conventional CT, the time for CBCT scanning is substantially reduced and, for most equipment, is less than 30 seconds. This is because the CBCT requires only a single scan to capture the necessary data compared with conventional CT scanners, where several fan beam rotations are required to complete the imaging of an object.12
Rapid Scan Time:
Because CBCT acquires all projection images in a single rotation, scan time is comparable to panoramic radiography. This is desirable because artefact due to subject movement is reduced. Computer time for dataset reconstruction however is substantially longer and varies depending on FOV, the number of basis images acquired, resolution and reconstruction algorithm and may range from approximately 1 to 20 minutes.4
X- ray Beam Limitation:
Collimation of the CBCT primary x-ray beam enables limitation of the x-radiation to the area of interest. Therefore an optimum FOV can be selected for each patient based on suspected disease presentation and region of interest. While not available on all C BCT systems, this functionality is highly desirable as it provides dose savings by limiting the irradiated field to fit the FOV. 5
Sub Millimeter Resolution:
Currently all CBCT units use mega pixel solid-state devices for x-ray detection. These devices provide sub millimeter pixel resolution of component basis projection images. The size of these voxels determines the resolution of the image. CBCT produces images with sub millimeter voxel resolution ranging from 0.4mm to as low as 0.125mm. Because of this characteristic, coronal and subsequent MPR of CBCT data has the same resolution as axial data. This level of spatial resolution is applicable for maxillofacial applications. 12
Low Patient Radiation Dose:
Published reports indicate that the effective does (2005 International Committee on Radiation Protection) for various CBCT devices ranges from 52 to 1025 microsieverts (μSv) depending on the type and model of CBCT equipment and imaging protocol used. These values are approximately 13.3 μSv) or 5 to 103 days equipment per capita back ground does (approximately 3600 μSv in the United States). Patient radiation does can be lowered by collimating the beam, elevating the chin, and using thyroid and cervical spin e shielding. CBCT provides a range of dose reductions of between 96% and 51% compared with conventional head CT (range 1400 to 2100 μSv).5
Interactive Analysis:
CBCT data reconstruction and viewing is performed natively by use of a personal computer. In addition, some manufactures provide software with extended functionality for specific applications such as implant placement or orthodontic analysis. Finally, the availability of cursor-driven measurement algorithms provides the practitioner w ith an interactive capability for real-time dimensional assessment, annotation, and measurements.
Image Accuracy:
CBCT imaging produces images with sub-millimeter isotropic voxel resolution ranging from 0.4 mm to as low as 0.09 mm. Because of this characteristic, subsequent secondary (axial, coronal and sagittal) and MPR images achieve a level of spatial resolution that is accurate enough for measurement in maxillofacial applications where precision in all dimensions is important such as implant site assessment and orthodontic analysis. 4
Reduced Patient Radiation Dose Compared to Conventional CT :
Published reports indicate that the effective dose (E) varies for various full fields of view CBCT devices from 29 -477 μSv depending on the type and model of CBCT equipment and FOV selected. Patient positioning modifications (tilting the chin) and use of additional personal protection (thyroid collar) can substantially reduce dose by up to 40%. These doses can be compared more meaningfully to dose from a single digital panoramic exposure, equivalent CT dose, or the average natural background radiation exposure for Australia (1,500 μSv) in terms of background equivalent radiation time (BERT). CBCT provides an equivalent patient radiation dose of 4 to 15 times that of a single film-based panoramic radiograph, 1.3% to 22.7% of a comparable conventional CT exposure or 7 to 116 days of background radiation.4
Interactive Display Modes Unique to Maxillofacial Imaging :
Perhaps the most important advantage of CBCT is that it provides unique images demonstrating features in 3D that intraoral, panoramic and cephalometric images cannot. CBCT units reconstruct the projection data to provide inter-relational images in three orthogonal planes (axial, sagittal and coronal). In addition, because reconstruction of CBCT data is performed natively using a personal computer, data can be reoriented such that the patient’s anatomic features are realigned. Basic enhancements include zoom or magnification, window/level and the capability to add annotation. Cursor driven measurement algorithms provide the clinician with an interactive capability for real-time dimensional assessment. Onscreen measurements provide dimensions free from distortion and magnification. Because of the isotropic nature of the volumetric dataset, data sets can be sectioned non-orthogonally, referred to as multiplanar reformation (MPR). Such MPR modes include oblique, curved planar reformation (providing “simulated” distortion free panoramic images) and, serial trans-planar reformation (providing cross sections), all of which can be used to highlight specific anatomic regions and diagnostic tasks.
Multiplanar Reformation:
Because of the isotropic nature of the volumetric data sets, they can be sectioned non-orthogonally. Most software provides for various non axial 2D images, referred to as MPR. Such MPR modes include oblique, curved planar reformation (providing simulated distortion free panoramic images) and serial transplanar reformation (providing cross sections), all of which can be used to highlight specific anatomic regions and diagnostic tasks which is important, given the complex structures, two methods have been developed to visualize adjacent voxels shown in Figure 15 .12
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Figure 15: Multiplanar Reformation
This is important given the complex structure of the maxillofacial region. Finally techniques are available that provide true 3D visualization of the dataset including ray sum, maximum intensity projection and 3D computer generated models shown in Figure 16 .3
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Figure 16: 3D Skull view Ray Sum or Ray Casting:
Any multiplanar image can be “thickened” by increasing the number of adjacent voxels included in the display, which creates an image slab that represents a specific volume of the patient, referred to as a ray sum. Full thickness perpendicular ray sum images can be used to generate simulated projections such as lateral cephalometric images. Unlike conventional x rays, these ray sum images are without magnification and are undistorted. However this technique uses the entire volume data set and interpretation suffers from th e problems of “anatomic noise” the superimposition of multiple structures. 12
Three Dimensional Volume Rendering:
Volume rendering refers to techniques that allow the visualization of 3D data through integration of large volumes of adjacent voxels and selective display, two specific techniques available.
Indirect Volume Rendering:
Indirect volume rendering is a complex process, requiring selecting the intensity or density of the gray scale level of the voxels to be displayed within an entire data set (called segmentation) this technique is technically demanding and computationally difficult requiring specific software, however it provides a volumetric surface reconstruction with depth.
Direct Volume Rendering:
Clinically and technically, direct volume rendering is a much more simple process. The most common direct volume rendering technique is maximum intensity projection(MIP) MIP visualizations are achieved by evaluating each voxel value along an imaginary projection ray from the observers eyes within a particular volume of interest and then representing only the highest value as the display value. Voxel intensities that are below an arbitrary threshold are eliminated.12
Reduced Image Artifact:
With manufacturers’ artifact suppression algorithms and increasing number of projections, our clinical experience has shown that CBCT images can result in a low level of metal artefact, particularly in secondary reconstructions designed for viewing the teeth and jaws.4
LIMITATIONS OF CBCT IMAGING
While there has been enormous interest, current CBCT technology has limitations related to the “cone beam” projection geometry, detector sensitivity and contrast resolution. These parameters create an inherent image “noise” that reduces image clarity such that current systems are unable to record soft tissue contrast at the relatively low dosages applied for maxillofacial imaging. Another factor that impairs CBCT image quality is image artefact such as streaking, shading, rings and distortion. Streaking and shading artifacts due to high areas of attenuation (such as metallic restorations) and inherent spatial resolution may limit adequate visualization of structures in the dento-alveolar region.3
Large and limited volume cone beam imaging and conventional CT have a major limitation of presenting metallic artifacts that are caused by metallic restorations and also due to root canal filling material and implants. The artifacts appear either as bright or dark streaks in the image plane containing these structures and degrade image quality. The artifacts may also be seen as dark bands around amalgam restorations simulating recurrent caries or as dark zones or streaks around endodontic materials simulating root fractures, depending on the region involved and the type of material.
The principal limitation of large volume cone beam imaging is the moderate resolution provided by the images compared with intra oral radiographs or the limited volume CBCT machines. In broad terms, though the image quality is comparable to that of panoramic imaging, sufficient for a broad range of tasks but not adequate enough for high detail tasks, like non displaced fractures in teeth or small root canals. Unfortunately, the contrast resolution is limited to the densities of calcified structures such as bone unlike in conventional CT. 3
CBCT technology has limitations related to the cone beam projection geometry, detector sensitivity, and contrast resolution that produce image that lack the clarity and utility of conventional CT images. The present limitations are related to the “cone beam” projection geometry, detector sensitivity, and contrast resolution that produces images that lack the clarity and usefulness of conventional CT images.12
The clarity of CBCT images is affected by:
- Artifacts
- Noise
- Poor soft tissue contrast
Artifacts:
An artifact is any distortion or error in the image that is unrelated to the subject being studied. Artifacts can be classified according to their cause.
They are:
- Acquisition Artifacts
- X-ray Beam Artifacts
- Patient – Related Artifacts
- Scanner Related Artifacts
- Cone Beam Related Artifacts
Acquisition Artifacts:
Artifacts can arise from limitations in the physical processes involved in the acquisition of CBCT data. As an x -ray beam passes through an object, lower energy photons are absorbed in preference to higher energy photons. This phenomenon, called beam hardening, results in two types of artifact:
1. Distortion of metallic structures as a result of differential absorption, known as a cupping artefact.
2. Streaks and dark bands that can appear between two dense objects.
In clinical practice it is advised to reduce the field size, modify patient position, or separate the dental arches to avoid scanning regions susceptible to beam hardening (e.g., metallic restorations, detail implants).
X- ray Beam Artifacts:
CT image artifacts arise from the inherent polychromatic nature of the projection x ray beam that results in what is known as beam hardening (i.e. its mean energy increases as lower energy photons are absorbed in presence to the higher energy photons) this beam hardening results in two types of artifacts:
1. Distortion of metallic structures due to differential absorption known as a cupping artefact.
2. Streaks and dark bands that can appear between two dense objects.
Because the CBCT x-ray beam is heterochromatic and has lower mean kilovolt (peak) energy compared with the conventional CT, this artifact is more pronounced on CBCT images. In clinical practice it is advisable to reduce the FOV to avoid scanning regions susceptible to beam hardening (e.g.: metallic restorations, dental implants) which can be achieved by collimation, modification of patient positioning or separation of the dental arches. Recently the CBCT manufacturers have introduced artifact reduction technique algorithms within the reconstruction process (e.g., Scanora 3D, SOREDEX, Helsinki, Finland). These algorithms reduce image, metal and motion related artifacts and require fewer projection images and therefore may allow for a lower acquisition dose. However, they are computationally demanding and require increased reconstruction times 12.
Patient – Related Artifacts:
Patient motion can cause mis-registration of data, which appears as unsharpness in the reconstructed image. This unsharpness can be minimized by using a head restraint and as short a scan time as possible. The presence of dental restorations in the FOV can lead to severe streaking artifacts. They occur because of extreme beam hardening or photon starvation due to insufficient photons reaching the detector, resulting in horizontal streaks in the image and noisy projection reconstructions. This problem can be reduced by removing metallic objects such as jewellery before scanning commences.
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Scanner Related Artifacts:
Typically the scanner related artifacts present as circular or ring shaped, resulting from imperfections in scanner detection or poor calibration. Either of these two problems will result in a consistent and repetitive reading at each angular projection of the detector, resulting in a circular artifact.
Cone Beam Related Artifacts:
The beam projection geometry of the CBCT and the image reconstruction method produces three types of cone beam related artifacts.
1. Partial volume averaging
2. Undersampling
3. Cone beam effect
Partial Volume Averaging:
This is a feature of conventional fan and CBCT imaging. It occurs when the selected voxel resolution of the scan is greater than the spatial or contrast resolution of the object to be imaged. In this case the pixel is not representative of the tissue or boundary; however it becomes a weighted average of the different CT values. Boundaries in the resultant image may present with a ‘step” appearance or homogeneity of pixel intensity levels. Partial volume averaging artifacts occur in regions where surfaces are rapidly changing in the z direction (e.g. in the temporal bone). Selection of the smallest acquisition voxel can reduce the presence of these effects.
Under sampling:
This can occur when too few basis projections are provided for the reconstruction. A reduced data sample leads to mis-registration and sharp edges and noisier images of aliasing, where fine striations appear in the image. This effect may not degrade the image severely, however when resolution of fine detail is important, undersampling artifacts need to be avoided as far as possible by maintaining the number of basis projections images.
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Cone Beam effect:
The cone beam effect is a potential source of artifacts, especially in the peripheral portions of the scan volume. Because of the divergence of the x-ray beam as it rotates around the patient in a horizontal plane, projection data are collected by each detector pixel. The amount of data corresponds to the total amount of recorded attenuation along a specific beam projection angle as the scanner completes an arc. The total amount of information for peripheral structures is reduced because the outer row detector pixels record less attenuation, whereas more information is recorded for objects projected onto the more central detector pixels, which results in image distortion, streaking artifacts and greater peripheral noise. This effect is minimized by manufacturers incorporating various forms of cone beam reconstruction. Clinically it can be reduced by positioning the region of interest adjacent to the horizontal plane of the x-ray beam and collimation of the beam to an appropriate FOV. 12
Image Noise:
The cone beam projection acquisition geometry results in a large volume being irradiated with every basis image projection. As a result, a large portion of the photons engage in interactions by the way of attenuation. Most of these occur by Compton Scattering producing scattered radiation. Most of the scattered radiation is produced omni directionally and is recorded by pixels on the cone beam area detector, which does not reflect the actual attenuation of the object within a specific path of the x ray beam. This additional recorded x -ray beam attenuation, reflecting nonlinear attenuation is called noise. Because of the use of an area detector, much of this nonlinear attenuation is recorded and contributes to image degradation or noise. The scatter - to-primary ratios are about .01 for single-ray CT and .05 to .15 for fan beam and spiral CT and may be as large as .4 to 2.0 in CBCT. Problems also exist with detector and algorithms.
Additional sources of image noise in CBCT are variations in the homogeneity of the incident x-ray beam (quantum mottle) and added noise of the detector system (electronic). The in homogeneity of x -ray photons depends on the number of the primary and scattered x-ray absorbed, the primary and scattered x-ray spectra incident on the detector and the number of views (projections). Electronic noise is due to the inherent degradations of the detector system related to the x -ray absorption efficiency of energy at the detector.
In addition, because of the increased divergence of the x -ray beam over the area detector, there is a pronounced heel effect. This produces a large variation or non-uniformity of the incident x-ray beam on the patient and resultant non-uniformity in absorption with greater signal-to-noise ratio on the cathode of the image relative to the anode side.12
Poor Soft Tissue Contrast:
Contrast is the spatial variation of the x-ray photon intensities that are transmitted through the patient; contrast thus gives a measure of difference between regions in an image. The variation in transmitted intensities is a result of differential attenuation of x rays by tissues that differ in density, atomic number, and thicknes s. Two principal factors limit the contrast resolution of CBCT. Although scattered radiation contributes to increased noise of the image, it is also a significant factor in reducing the contrast of the cone beam system. X-ray scatter reduces subject contrast by adding background signals that are not representative of the anatomy, thereby reducing image quality.
Second there are numerous inherent flat panel detector -based artifacts that effect its linearity or response to x radiation. Saturation (nonlinear pixel effects above a certain exposure), dark current (charge that accumulates over time with or without expo sure), and bad pixels (pixel that do not react to exposure) contribute to nonlinearity. In addition, the sensitivity of different regions of the panel to radiation (pixel-to-pixel gain variation) may not be uniform over the entire region.12
Three factors limit the contrast resolution of CBCT. Although scattered radiation contributes to increased image noise, it is also significant factor in reducing the contrast of the cone beam system. In addition, the divergence of the x ray beam over the area detector causes a pronounced heel effect. This effect produces a large variation in, or non uniformity of the incident x-ray beam on the patient and resultant non uniformity in absorption with greater signal to noise ratio (noise) on the cathode side of the image relative to the anode side. Finally, numerous inherent flat panel detector based artifacts affect its linearity or response to x radiation. Although these conditions limit the application of current CBCT imaging to the assessment of the osseous structures, several techniques and devices are currently being investigated to suppress this effect.
Cone-beam imaging is not in itself a panacea in radiological terms. In fact, its exact role in head and neck imaging has yet to be critically evaluated. The quest to accumulate an evidence-based approach is meeting a number of obstacles. For example, who should collate the evidence and make recommendations for best practice.
Research into cone-beam imaging has to meet the challenge of rapid changes in both hard- and soft-ware technology, which can render publications outdated before they even get to press. 3
The equipment itself is changing in order to meet the clinical requirements reported to manufacturers, who in turn have markets to consider. However, it should not be forgotten that there are intrinsic limitations in the technique which mean, in some circumstances, other forms of dental imaging would be more appropriate. Caries and teeth adjacent to amalgam and other dense prosthetic restorations are not well imaged by cone-beam technology owing to beam hardening and streak artefact. Some units combat this anomaly better than others. Even gutta percha may give rise to streak artefact and appear as dense as amalgam might on conventional CT. This should be borne in min d when assessing a potential site for implants adjacent to root filled teeth.3
If the clinical question is about lamina dura configuration or bony detail, then the periapical image may provide the answer with a fraction of the radiation dose, both lamina dura and bony detail being superior on periapical radiographs compared to cone-beam.
In order to acquire an undistorted image with cone-beam imaging, it is essential that the patient’s head is kept still during the gantry rotation. Obviously, quicker scan times help facilitate this and most machines come with a head positioning and stabilizing device but, as with dental panoramic tomography, patient movement can limit the technique for very young children, those unable to stay still or with movement disorders.
Interestingly, to those not used to working with 3 -D volumes, radiological interpretation can be difficult when using a smaller field of view, as it is easy to become disoriented when scrolling through the images, as points of reference such as normal dental landmarks, or anomalous anatomy can make orientation difficult. 3
Cone-beam technology based on an image intensifier may allow the periphery of the image to be distorted.
To date, cone-beam technology gives little in the way of soft tissue detail and, although newer algorithms have been developed to improve this aspect, it in no way compares to those capable of conventional CT. This, obviously, precludes the technique in the assessment of head and neck malignancy where evaluating the soft tissue extent of the lesion is crucial. Cancer staging will continue to be performed with conventional CT and/or MRI supplemented with newer imaging offered by CT/PET scanning in the near future. 3
Causes of Scatter in CBCT:
Multidetector helical CT scanners employ multiple rows of detector elements, 64 channels for current systems, with recently introduced models employing 256 and even 320 channels. For current commercial 64-slice systems, the total slab thickness in the z-direction is 4 cm, compared to up to 18 cm for C-arm CBCT systems. For C-arm CBCT systems, this significantly increased imaging volume results in a marked increase in scattered radiation, described by the scatter -to primary ratio, which is the ratio of scattered to prima ry radiation incident on the detector. The scatter-to-primary ratio, which is typically around 0.2 for multidetector CT, may increase to greater than 3 for large volume CBCT. This result in increased cup and streak artifacts as well as inaccuracies in calculated CT numbers. For tomographic images obtained during interventional procedures, the importance of inaccurate CT numbers is not expected to be great. Streak and cupping artifacts, on the other hand, directly impact image quality.
For C-arm CBCT systems, scatter is determined primarily by field-of-view in the z-direction, imaging geometry (which determines the air-gap size), and object size. Object size is not an adjustable variable, and while important as larger objects generate greater scatter, will not be discussed further. Air-gaps, which are further discussed below with anti-scatter grids, are not an operator variable and depend on the region of interest and patient size, typically varying from 25 to 35 cm. Field-of-view in the z-direction is the most important adjustable variable for determining scatter magnitude. 5
As the cone angle is increased to allow larger regions of interest to be imaged, the scatter-to-primary ratio increases significantly. This effect was shown experimentally to increase the scatter-to-primary ratio from 14% for a cone angle of _0.5° to greater than 120% for cone angles greater than 7°. Furthermore, as the cone angle is increased, the scatter fluence at the center of the image plane increases rel ative to the periphery due to increasing scatter contribution from out -of-plane. The cone angle has also been shown experimentally to affect the presence and magnitude of cupping artifact, which is due to a combination of scattered radiation and beam hardening. The cupping artifact, or concave-downward shape of the scatter fluence profile, is decreased with decreasing cone angle and can, for small cone angles, actually be reversed, resulting in a concave upward shape (capping). 5
The cupping artifact (reduced voxel values near the center of an image), was shown experimentally to increase from approximately 2% for scatter-to primary ratios of _10% to almost 20% for scatter -to- primary ratios 100%. Similar inaccuracies were demonstrated for CT numbers, which are underestimated by more than 30% for scatter-to- primary ratios of _100%. Scatter-to-primary ratio values of 100% are expected for abdominal and pelvic imaging. For geometries resulting in high scatter-to-primary ratios, detector exposure will increase, thereby reducing voxel noise. However, the decrease in voxel noise is more than offset by the decrease in contrast resulting from the scattered radiation, the end result of which is a decrease in contrast - to-noise with increasing scatter-to-primary ratios. Siewerdsen et al. showed a factor-of two CNR decrease for scatter-to-primary ratios increased from 0% to 100%. As, is the case for all radiographic imaging applications, increasing radiation dose or decreasing spatial resolution will increase CNR. 5
APPLICATIONS
There are numerous applications for CBCT in dentistry because this technology far surpasses film or 2D digital radiology in giving diagnostic information of the dental or maxillofacial regions. The amount of information one can obtain from a 3D image over a 2D image is considerable. However, the diagnosis comes from the assessment of clinical and imaging evaluation, and the clinician must be ever mindful of the risks vs benefits to the patient in the use of radiation, regardless of the amount. Use guidelines always must be in line with the “as Low as Reasonably Achievable” (ALARA) principle, and if a definitive diagnosis can be made with a single intraoral radiograph, there is no justification for using CBCT. 12
CBCT applications that have been reported in t he literature include:
- Dental implants
- Impactions
- Inferior alveolar nerve location
- Airway studies for sleep apnea
- Temporomandibular joint (TMJ) structure visualization
- Pre and postoperative assessment of craniofacial fractures
- Surgical assessment of pathology
- Rapid prototyping
- Orthodontics
- Periodontics
- Endodontics
Cone beam imaging allows images to be displayed in a variety of formats, the interpretation of volumetric datasets, particularly when it involves large areas, means more than simply the generation of 3 -D images. Interpretation demands an understanding of the spatial relations of bony anatomical elements and a comprehensive pathological knowledge of the various maxillofacial structures involved. CBCT units initially provide correlated axial, coronal and sagittal perpendicular MPR (multiplanar reformation) images. Basic enhancements include zoom or magnification and visual adjustments to narrow the range of displayed grey-scales (window) and contrast level within this window, the capability to add annotation and cursor- driven measurement. Perhaps the greatest practical advantage of CBCT in maxillofacial imaging is the ability it provides to interact with the data and generate images replicating those commonly used in clinical practice. All proprietary software is capable of various real - time advanced image display techniques, easily derived from the volumetric dataset. These techniques and their specific clinical applications include: 2
Oblique planar reformation: This technique creates non-axial 2D images by transecting a set or “stack” of axial images. This mode is particularly useful for evaluating specific structures (e.g., TMJ, impacted third molars) as certain features may not be readily apparent on perpendicular MPR images4.
Curved planar reformation: This is a type of MPR accomplished by aligning the long axis of the imaging plane with a specific anatomic structure. This mode is useful in displaying the dental arch, providing familiar panorama like thin-slice images. Images are undistorted so that measurements and angulations made from them have minimal error.4
Serial transplanar reformation: This technique produces a series of stacked sequential cross-sectional images orthogonal to the oblique or curved planar reformation. Images are usually thin slices (e.g., 1 mm thick) of known separation (e.g., 1 mm apart). Resultant images are useful in the assessment of specific morphologic features such as alveolar bone height and width for implant site assessment, the inferior alveolar canal in relation to impacted mandibular molars, condylar surface and shape in the symptomatic TMJ or evaluation of pathological conditions affecting the jaws. 4
Multiplanar volume reformations: Any multiplanar image can be “thickened” by increasing the number of adjacent voxels included in the slice. This creates an image that represents a specific volume of the patient. The simplest technique is adding the absorption values of adjacent voxels, to produce a “ray sum” image. This mode can be used to generate simulated panoramic images by increasing the slice thickness of curved planar reformatted images along the dental arch to 25–30 mm, comparable to the in focus image layer of panoramic radiographs. Alternatively, plain projection images suc h as lateral cephalometric images can be created from full thickness (130 –150 mm) perpendicular MPR images. In this case, such images can be exported and analyzed using third-party proprietary cephalometric software. Unlike conventional radiographs, these ray sum images are without magnification and are undistorted. Another thickening technique is maximum intensity projection (MIP).
MIP images are achieved by displaying only the highest voxel value within a particular thickness. This mode produces a “pseudo” 3D structure and is particularly useful in representing the surface morphology of the maxillofacial region. More complicated shaded surface displays and volume rendering algorithms can be applied to the entire thickness of the volumetric data set to provide 3D reconstruction and presentation of data that can be interactively enhanced.
The availability of CBCT is also expanding the use of additional diagnostic and treatment software applications – all directed towards 3D visualization. This is because CBCT data can be exported in the non-proprietary DICOM (Digital Imaging and Communications in Medicine) file format standard. CBCT permits more than diagnosis, it facilitates imaged guided surgery. Diagnostic and planning software is available to assist in orthodontic assessment and analysis (e.g. Dolphin 3D, Dolphin Imaging, Chatsworth, CA, USA) and implant planning to fabricate surgical models (e.g. Biomedical Modeling Inc., Boston, MA, USA), facilitate virtual implant placement, create diagnostic and surgical implant guidance stents (e.g. Virtual Implant Placement (VIP); Implant Logic Systems, Cedarhurst, NY, USA; Simplant; Materialise, Leuven, Belgium), and even assist in the computer aided design and manufacture of implant prosthetics (NobelGuide/Procera software; Nobel Care AG, Göteborg, Sweden).
Software is also available to provide surgical simulations for osteomies and distraction osteogenesis (Maxilim; Medicim NV, Mechelen, Belgium). This is a blossoming field which provides opportunities for practitioners to combine CBCT diagnosis, 3D simulations, with virtual surgery and computer assisted design and manufacturer. Image-guidance is an exciting advance that will undoubtedly have substantial impact on dentistry. 4
CBCT Applications:
Implantology:
CBCT provides cross sectional images of the alveolar bone height, width and angulation and accurately depicts vital structures such as the inferior alveolar dental nerve canal in the mandible or the sinus in the maxilla. The most useful series of images fo r implant site transplanar images at the specific location in many instance a diagnostic stent is made with radiographic markers and inserted at the time of the scan. It provides a precise reference of the location of the proposed implants or teeth. DICOM data can be imported into third party software application that provide many useful tools that can be used to asses and plan both the surgical and prosthetic components of implant therapy.12
3D CT scans allow the surgeon and restorative dentist to optimally plan and place dental implants. Their uses and benefits are present throughout the continuum of care from diagnosis to treatment to post-op examinations and includes:
- Locate and determine the distance to vital anatomic structures
- Measure alveolar bone width and visualize bone contours
- Determine if a bone graft or sinus lift is needed
- Select the most suitable implant size and type
- Optimize the implant location and angulation
- Increased case acceptance
- Reduced surgery time
- Build patient confidence
And with the use of guided implant placement based on 3D CT scans shown in Figure 17 , all the above benefits are enhanced to the point that the surgeon can approach each case with the confidence that comes from knowing that the best available image data and technology have been used to ensure success. 12
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Figure17: 3D CT View of Implant Placement
Cross-sectional imaging techniques can be an invaluable tool during preoperative planning for complicated endosseous dental implantation procedures. Conventional linear tomography and CT have traditionally been used in presurgical imaging, though the forme r has overlain ghosting artifacts and the latter has relatively high radiation exposure and cost. Practitioners have begun using office-based CBCT scanners in preoperative imaging for implant procedures, capitalizing on availability and low dosing requirements. A review by Guerrero et al outlines the clinical and technical aspects of CBCT, which have popularized this new technique. Preliminary evidence addresses the ability of CBCT images to characterize mandibular and alveolar bone morphology, as well as to visualize the maxillary sinuses, incisive canal, mandibular canal, and mental foramina, all structures particularly important in surgical planning for dental implantation. Several studies have described the 3D geometric accuracy of CBCT imaging in the maxillodental and mandibular regions as well shown in Figure 18.12
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Figure18: 3d View of Implant Assessment.
Localization of Inferior Alveolar Canal:
The relationship of the inferior alveolar canal to the roots of mandibular third molar teeth is of importance when attempting to minimize the likelihood of nerve damage that may lead to permanent loss of sensation to one side to the lower lip. Thus accurate assessment of the position of the canal in relation to the impacted third molar may reduce injuries to this nerve. Traditional panoramic imaging may be adequate when the third molar is clear of the canal, but in the case of radiographic superimposition it is advised to use a 3D imaging approach shown in Figure 19a and b. This can be achieved at comparatively low radiation dose with CBCT combined either with the proprietary software accompanying the imaging device or with third party diagnostic software. 12
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Figure 19: Inferior Alveolar Canal Tracing
Impactions:
Cone Beam CT scans can provide a more accurate and 3 - dimensional assessment to provide more predictable treatment results while reducing the risks associated with any impacted tooth.
- Visualize an impacted tooth’s position in relation to surrounding vital structures and nearby teeth and their roots.
- Better assess the risk of treatment or non-treatment based on more accurate 3-dimensional analysis shown in Figure 20a and b.20
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Figure 20a and b: Impacted 3rd Molar Assessment.
Sinus and Airway Studies:
Comparatively low dosing requirements, high-quality bony definition, and the compact design afforded by CBCT scanners have made them attractive for office-based and intraoperative scanning of the paranasal sinuses. To date, there have been few studies comp aring image quality in paranasal sinus CBCT scans with that in MDCT. Alspaugh et al did directly compare the spatial resolution obtained with CBCT scans of the paranasal sinuses with that of 16 - and 64- section MDCT scanners. They concluded that 12 line pai rs per centimetre (lp/cm) isotropic spatial resolution could be obtained with an effective dose of 0.17 mSv compared with a dose requirement of 0.87 mSv for 11-lp/cm spatial resolution in a 64 -section MDCT scanner. To a large degree, evidence supporting sinus CBCT imaging has emerged from exploration of intraoperative CBCT applications in endoscopic sinus surgery (ESS). In preclinical cadaver studies, Rafferty et al provided proof of principle for the application of C -arm CBCT imaging to ESS, concluding that both spatial and soft-tissue contrast was sufficient to aid surgical navigation in the frontal recess. More recent clinical studies have also provided qualitative evidence that intraoperative CBCT provides high-quality definition of bony anatomy, which can lead to refinement of surgical strategy. In a series of 25 patients undergoing ESS, Batra et al found that residual bony partitions and stent locations could be visualized with intraoperative CBCT scans, leading to surgical revision.
CBCT has also been used recently to evaluate contrast delivery during sinus irrigation after ESS. Preliminary evidence suggests that CBCT may be suited for specific imaging tasks in the context of intraoperative and perioperative bony structural evaluations, enabling low-dose assessment of individualized paranasal sinus anatomy, surgical outcomes, and stent placements. To our knowledge, there is no current evidence, however, supporting CBCT use in general diagnostic sinus imaging owing to lack of soft-tissue contrast resolution. Furthermore, significant complications of ESS, including encephalocele, subarachnoid hemorrhage, and meningitis are unlikely to be evaluated adequately with current CBCT image quality. 14
Volumetric data obtained from a CB3D survey can be used to visualize the sinuses shown in Figure 21 and the entire airway path from the nasal and oral entrances to the laryngeal spaces for:
- Identification of anatomical borders
- Determination of degree of infection and presence of polyps
- Assistance in airway studies for diagnosis and treatment of obstructive sleep apnea
- Calculation of actual volume of airway space
- Determination of the point of airway constriction
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Figure 21: Sinus and Airway Analysis
TMJ:
CBCT provides multiplanar and potentially 3D images of the condyle and surrounding structures to facilitates analysis and diagnosis of bone morphologic features joint space and dynamic function, critical keys to providing appropriate treatment outcomes in patients with TMJ signs and symptoms. Imaging can depict the features of degenerative joint disease, developmental anomalies of the condyle, ankylosis and rheumatoid arthritic disease. Appropriate imaging protocols should include reformatted panoramic and axial reference images, corrected parasagittal and paracoronal and axial reference images, corrected parasagittal and paracoronal transerial slices and for those cases in which asymmetry or surgery is contemplated, 3D reconstructions.
Accurate evaluation of the temporomandibular joint (TMJ) has been difficult due to the superimposition of other structures in conventional radiographs shown in Figure 22 and Figure 23 . With Cone Beam CT imaging, it is now possible to 21:
- Assess the condylar anatomy of the TMJ without superimposition and distortion of the image
- Obtain a true 1:1 imaging of the condylar structures for more accurate assessments.
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Figure 21: TMJ View on CBCT
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Figure 22: TMJ Analysis on CBCT
Morphologic changes of the temporomandibular joint (TMJ) as depicted with conventional MR imaging, CT, and radiographic imaging are often useful in diagnosing pathologic processes such as degenerative changes and ankylosis, joint remodeling after diskectomy, malocclusion, and congenital and developmental malformations.
CBCT is a technique that has recently inspired research in TMJ imaging, though preliminary experiments have yet to translate into clinical studies. Several cadaveric series have explored th e use of TMJ CBCT to assess periarticular bony defects, flattenings, osteophytes, and sclerotic changes.
Preliminary studies have also directly compared CBCT with radiography, multidetector row CT (MDCT), and linear tomography for detection of osseous abnormalities of the TMJ. Although early results are promising, more research is needed before CBCT should be used clinically to assess the TMJ.
A recent systematic review by Hussain et al suggests that axially corrected sagittal tomography is still the method of choice in the detection of periarticular erosions and osteophytes. 21
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Craniofacial Fractures:
Imaging of complex high-contrast bony structural pathology such as craniofacial fractures is a logical application for CBCT. Terakado et al reported a case series in 2000, which included 2 patients with facial trauma for whom CBCT was used to characterize a mandibular head fracture, dental root fractures, and the displacement of anter ior maxillary teeth. Since that time, several additional reports have extolled the low-dose high-resolution properties of CBCT imaging in preoperative characterization of mandibular and orbital floor fractures. In orbital floor fractures, although CBCT can demonstrate orbital content herniation, it lacks the contrast resolution to differentiate the tissue composition of the herniated materials.
The intraoperative uses of C-arm CBCT systems have been evaluated for fractures of the zygomaticomaxillary compl ex (ZMC), demonstrating the feasibility of CBCT use in surgical navigation, localization of bony fragments, and evaluation of screw anchorage and plate fittings with low levels of metal artifact.
These results have been corroborated in a study of postoperative patients with ZMC fractures, though investigators noted that poorly aerated ethmoidal air cells limit the ability of CBCT to visualize the medial orbital wall. Low bone density in older patients also reduced bony structural definition in their series. Intraoperative efficacy has been evaluated in mandibular fracture fixation as well. 16
Conditions of the maxillofacial complex:
CBCT can assist in the assessment of many conditions of the jaws, most notably dental condition such as impacted canines and supernumerary teeth, fractured or split teeth, periapical lesions and periodontal disease. Benign calcifications can also be identi fied by location and differentiated from potentially significant calcifications of the arteries such as carotid artery calcifications or veins. Although CBCT does not provide suitable soft tissue contrast to distinguish the contents of paranasal opacifications, the morphologic characteristics and extent of these lesions are particularly well seen.
Most important the location, size, shape, extent and full involvement of jaw conditions can be visualized with a combination of two and three dimensional images. CBCT has been found to be particularly useful for trauma and for visualizing the extent and degree of involvement of osteomyelitis.
Head and Neck:
As CBCT imaging systems have become more widely available, interest in the intraoperative and diagnostic CBCT applications in the extracranial head and neck regions has intensified. The reported high isotropic spatial resolution and relatively low dose req uirements of CBCT are characteristics that have made it particularly attractive. In the head and neck region, a premium is placed on discriminating fine anatomic detail in territories where the vascular and bony structural anatomy is particularly complex. Potential applications in sinus, temporal bone, and skull base imaging have been explored, as discussed below.
Temporal Bone/Lateral Skull Base:
The temporal bone was one of the earliest targets for head and neck CBCT imaging. Specific applications have been explored, including post procedural middle and inner ear implant evaluation, visualization of the reuniting duct in the inner ear, and intraop erative temporal bone surgical guidance Preliminary evaluation of an experimental CBCT system for general temporal bone diagnostic imaging was performed by Gupta et al on a small series of partially manipulated cadaveric specimens. They found that observer scores of the quality of structural visualization with CBCT were significantly higher compared with scores for MDCT. Particularly well -visualized structures included the ossicular chain, bony labyrinth of the inner ear, internal cochlear anatomy, and the facial nerve. They also noted reduced metal artifacts with cochlear implant imaging as well as improved detection of small laser-induced lesions in the ossicular chain.
Gupta et al suggest that lack of soft-tissue contrast in their evaluations did not interfere with diagnostic accuracy due to the abundance of high-contrast structures housed in the temporal bone and the positive effect of higher spatial resolution on resolving some low - contrast structures such as the facial nerve.
Peltonen et al compared a commercially available CBCT scanner with MDCT in a study on unoperated temporal bone specimens by using a modified Likert scale (scored by 2 otologists and 1 radiologist) to assess visualization of important structures in the lateral skull base. They concluded that CBCT was at least as accurate as MDCT in defining surgically relevant middle ear structures. The inner ear was incompletely visualized with CBCT in their study. Perhaps the most well-studied use of temporal bone CBCT is for the evaluation of middle and inner ear implants. Early preclinical studies in temporal bone specimens fitted with cochlear implants demonstrated that an adapted CT angiography CBCT system could noninvasively depict the electrode modiolus relationship post implantation. These results were later corroborated in another cadaver study comparing single- and multisection CT with CBCT. 23
When compared with single- or multisection CT, a reduction in metal artifacts was observed with CBCT, which allowed more precise determination of electrode-array positioning within the scala tympani or scala vestibuli. Reduced metal artifacts with implant imaging using CBCT compared with conventional CT were also demonstrated by Offergeld et al in a study evaluating middle ear implants in postsurgical temporal bone specimens. Preclinical studies have been followed by studies of patients with inner and middle ear implants, suggesting that the combination of high spatial resolution and reduced metal artifacts with CBCT imaging may facilitate the posts urgical evaluation of reconstructed middle and inner ears. A recent study has also explored the utility of CBCT in evaluating progressive hearing loss. Dalchow et al submitted 25 patients with audiometry -confirmed conductive hearing loss to preoperative CBCT and concluded that CBCT could be accurate both in predicting the continuity of the ossicular chain and in detecting ossicular erosions.
Multiple commercial CBCT systems have temporal bone - acquisition protocols. The miniCAT acquires temporal bone image s at 125 kilovolt (peak) (kVp) and 58.8 mA with a 20 -second scanning time using a sharp kernel (manufacturer’s data). This protocol delivers 4.62 and 4.18 mGy at the center and periphery, respectively, of a 100 - mm ion chamber, achieving spatial resolution in the range of 14–16 lp/cm (manufacturer’s data). In their study of limited -FOV temporal bone imaging described above, Peltonen et al14 noted a 60 -fold effective dose reduction with CBCT compared with MDCT, though they attributed much of this dramatic reduction to significantly smaller FOVs and shorter scanning times for their CBCT images. They noted that low-dose MDCT settings can acquire images with effective doses like those in CBCT if the FOVs and scanning times are, in fact, comparable with those of CBCT. These data suggest that CBCT might be useful for select imaging tasks in temporal bone imaging, including evaluation of inner and middle ear implant positioning, as well as definition of high-contrast postsurgical change and structural anatomy within the lateral skull base. Possible applications in evaluation of bony pathology, such as ossicular chain erosions, may also be emerging. Currently, further research is required to characterize the ability of CBCT to define temporal bone structures and bony pathology reliably, especially given the technologic and scan - parameter variability of commercial CBCT scanners. Lack of soft - tissue contrast resolution also continues to limit the use of CBCT in general diagnostic imaging of the temporal bone. 9
Skull Base:
The particularly complex bony and neurovascular anatomy of the skull base makes it an attractive target for high -spatial resolution imaging. Current practices in oncologic imaging of the skull base rely on MDCT and MR imaging for combined osseous and soft-tissue definitions.83 Several preclinical reports have begun to explore the potential uses of CBCT during surgeries at the skull base, suggesting high 3D localization accuracy and low target-registration error with effective doses in the range of 0.1 – 0.35 mSv. The xCAT intraoperative CBCT scanner (Xoran Technologies), a cousin of the MiniCat, has been evaluated in clinical scenarios at the skull base as well, with favourable preliminary results. 23
Pathology:
Cone Beam 3D scans provide a superior means of visualizing and studying pathological processes in the maxilla and mandible. This information is invaluable when planning any surgical efforts for biopsy or resection. The data can be used to:
- Render three-dimensional images of hard tissue abnormalities
- Provide more accurate information related to size, extent, location, and the relation to an effect on nearby anatomical structures.
- Monitor the progression of the pathology as well as the success of treatment with the use of multiple scans.
Cone Beam CT scan is an invaluable diagnostic and treatment planning tool for the oral surgeon for:
- Determine the precise three-dimensional position of a tooth within the alveolar bone and how this position relates to vital structures for extractions and impactions.
- Visualize hard and soft tissues on the computer in three dimensions for planning maxillofacial surgeries.
- Generate life-size CAD-CAM stereolithic (STL) models for surgical planning and preparation.
- Monitor skeletal changes, airway changes, and healing responses.9
Rapid Prototyping:
Rapid prototyping is broad term used to describe a group of related processes and techniques that are used to fabricate physical scale models directly from 3D computer – assisted design data. The purpose of rapid prototyping in maxillofacial imaging is to create a life size dimensionally accurate model of an anatomic structure. These models are also referred to as biomodels. DICOM data imported to proprietary software can be used to compute 3D images generated by thresholding the intensity of the voxel values to be displayed and segmenting these from the background. The models produced are used for presurgical planning of a number of complex maxillofacial surgical cases, including craniofacial reconstruction for correction of deformity caused by trauma, tumor resection, distraction osteogenesis and more widely dental implants. The models provide the practitioner with a higher level of confidence before he or she performs a surgical procedure and may reduce surgical and anesthetic time. 12
Orthodontics:
Orthodontics has traditionally relied on 2 -dimensional x-rays in evaluating 3-dimensional structures. With Cone Beam CT imaging, a more comprehensive orthodontic diagnosis and more accurate treatment planning is possible by allowing for:
- Assessment of growth and development
- 3D views of vital structures
- 3D evaluation of impacted tooth position and anatomy
- TMJ assessments of condylar anatomy in three dimensions
- Orthognathic surgery treatment planning and growth assessments in true 1:1 imaging
- Airway assessments
- Planning for placement of dental implants for tooth restoration or orthodontic anchorage and for placement of temporary anchorage devices (TADs)
- Assessment skeletal symmetry or asymmetry
Cross-sectional imaging affords overlay-free visualization of structural and anatomic relationships important for addressing many radiologic questions in orthodontics. The current standard of care for overlay-free imaging in orthodontics is conventional CT. Low-cost office-based CBCT imaging has recently been explored for orthodontic applications, including assessment of palatal bone thickness, skeletal growth patterns, dental age estimation, upper airway evaluation, and visualization of impacted teeth. Although preliminary results are encouraging, established cross-sectional techniques such as conventional CT provide superior image quality of dental and surrounding structures for advanced orthodontic treatment planning. Low dosing requirements appear to remain a benefit of CBCT when compared with conventional CT, with a routine orthodontic CBCT study delivering an effective dose of 61.1 Sv compared with 429.7 Sv for multisection CT. Lateral cephalograms deliver 10.4 µSv in comparison, though without the benefit of 3D structural visualization.
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Figure 23: CBCT imaging in Orthodontics
CBCT imaging is being used in the diagnosis, assessment and analysis of maxillofacial orthodontic and orthopedic anomalies shown in Figure 23. CBCT provides display of the position of impacted and supernumerary teeth and their relationships to adjacent roots or other anatomic structures. This facilitates surgical exposure and planning of subsequent movement. Also, information regarding palatal morphologic features and dimensions, tooth inclination and torque, root resorption and available alveolar bone width for buccolingual movement of teeth can be obtained. 2
Perhaps the greatest potential use of CBCT in orthodontics is that it is capable of providing both conventional two and three dimensional cephalometric images in one acquisition. CBCT data sets can be manipulated by the ray sum technique to generate simulated panoramic, lateral, submentovertex and posteroanterior cephalometric images. Alternatively, it is possible to extract the topographic feature of the skull and air/soft tissue interfaces in high detail by using a variety of orthodontic centred products. There are numerous potential benefits to 3D cephalometry including accuracy of line ar measurements, visual demonstration of dentosketal relationships and facial esthetics, and the demonstrastion of dentoskeltal relationships and the potential for assessment of growth and development 2
Endodontics
CBCT has been explored for applications in endodontics, including periradicular surgical planning, assessment of periapical pathology, and dentoalveolar trauma evaluation. The diagnostic properties of CBCT at the root apices and perira- dicular region have been reported in several studies. In retrospective cohorts and case reports, CBCT has been suggested as superior to periapical radiographs in the characterization of periapical lucent lesions, reliably demonstrating lesion proximity to the maxillary sinus , sinus membrane involvement, and lesion location relative to the mandibular canal. There may also eventually be a role for CBCT in early detection of periapical disease, which could lead to better endodontic treatment outcomes. Promising results have been demonstrated in studies characterizing CBCT images for endodontic surgical planning purposes as well shown in Figure 24 .6
Although conventional radiography is more practical and better suited for everyday endodontic procedures, volumetric data from CB3D scans can provide serial axial, coronal, and sagittal views that are not possible to obtain from conventional radiography. 6
The ability to reduce or eliminated superimposition of surrounding structures makes it easier to visualze areas of interest in three- dimensions. This provides much clinically relevant diagnositic information and has many potential applications for endodontics including:
- More accurate identification and diagnosis of periapical endodontic pathogenesis than conventional radiography
- Visualization of obscure internal pulpal anatomy and root canal systems
- Assessment of internal and external root resorptive processes
- Identification of root fractures and other areas of trauma
- Volumetric and density comparisons of periradicular bone following endodontic treatment in order to assess the degree of success or failure
- Pre-surgical planning.6
Abbildung in dieser Leseprobe nicht enthalten
Figure 24: Endodontic Assessment of CBCT
Periodontics
The first reported applications of CBCT in periodontology were for diagnostic and treatment-outcome evaluations of periodontitis. Ex vivo studies later characterized the ability of CBCT to accurately reconstruct periodontal intrabony and fenestration defec ts, dehiscences, and root furcation involvements in comparison with radiography, MDCT, and histologic measurements. CBCT 3D geometric accuracy has been suggested to be equal to radiography and MDCT but with better observer-rated image quality than MDCT as well as superior periodontal-defect detection than radiography shown in Figure 25. Although periodontal bony defects are well visualized with CBCT, conventional radiography still affords higher quality bony contrast and delineation of the lamina dura. CBCT ex vivo visualization of the periodontal ligament and periodontal ligament space has been evaluated in comparison with radiography with mixed results, a more recent study suggesting that CBCT visualization is still inferior to that of radiography. 24
The disadvantages of conventional 2 -dimensional x-rays for accurate periodontal assessment is avoided by 3 -dimensional and cross-sectional analyses helping to avoid surprises often encountered during periodontal surgery.
- Analyze periodontal bone defects on all sides of every tooth.
- Assess the extent of every furcation involvement.
- Track the progression of advancing periodontal bone loss.
- Treatment plan dental implants by evaluating bone parameters such as bone width, depth, and density.
- Visualize vital structures such as the maxillary sinus, mental foramen and mandibular nerve prior to periodontal or implant surgeries.24
Abbildung in dieser Leseprobe nicht enthalten
Figure25: Bone Assessment in CBCT Scan
CONCLUSION
CBCT imaging systems have been recently been introduced for imaging hard tissues of the maxillofacial region. CBCT is capable of providing accurate submillimeter resolution images at shorter scan times, lower dose and lower costs compared with medical fan beam CT. Increasing availability of this technology provides the practitioner with an imaging modality capable of providing a 3D representation that is extending maxillofacial imaging from diagnosis to image guidance of operative and surgical procedures. 12
CBCT is an emerging technical advancement in CT imaging that uses cone beam acquisition geometry to provide relatively low -dose imaging with high isotropic spatial resolution acquired with a single gantry revolution. Efficient use of the x-ray beam in CBCT imaging produces a relatively low x-ray tube power requirement, which, along with flat panel detection and limited anatomic coverage, has facilitated the production of compact CBCT scanners suitable for use in an office-based setting. CBCT acquisition parameters can be optimized to produce isometric voxels as small as a 150 x150x150 µm 3 at the isocentre. Limited contrast resolution, however, continues to impair low-contrast detectability in CBCT images. Several factors contribute to this limited contrast resolution, including the increased x-ray scatter in cone beam acquisition. Improvements in scatter subtraction methods continue to be the subject of research aimed at improving image quality in CBCT systems. Dedicated CBCT scanning of restricted anatomic volumes in the maxillofacial region can be obtained with effective patient dosing in the approximate range of 30 – 80 µSv, and imaging of the paranasal sinuses is possible with delivery of approximately 0.2 mSv. Research on patient dose, however, has been conducted with largely variable exposure parameters and still requires further research and adoption of an appropriate dose metric for comparison with MDCT scanning. 6
Cone-beam CT is almost certainly going to revolutionize dental radiology and impact on almost all aspects of dental practice. As a consequence, many dental practitioners will be considering purchasing this type of imaging equipment, but should take into ac count the potential implications of imaging a large area of craniofacial anatomy. If considering obtaining a unit with an extended field of view, ie including more than purely the dento-alveolar region, liaison with a specialist in oral and maxillofacial imaging would be recommended.
Currently, any dental practitioner can purchase and operate a CBCT unit. There is mounting concern among oral and maxillofacial radiologists, based on issues of quality and patient safety, that interpretation of extended field of view diagnostic imaging studies using CBCT should not be performed by dentists with inadequate training and experience. The AAOMR has indicated that, to use CT in implant imaging, the interpreting practitioner should either be a board certified oral and maxillofacial radiologist or a dentist with adequate training and experience. Perhaps, as has occurred in medical imaging where the use and costs of imaging have increased at double -digit rates, third-party payers and federal policymakers will also become involved in setting standards for providers who bill the government for obtaining and interpreting diagnostic images.
Non-radiologist dentists should not be excluded from performing CBCT imaging provided they have appropriate and documented training and experience. Given that A single CBCT scan uses ionizing radiation at levels exceeding any current dental imaging protocol series, it is timely to recommend the development of rigorous training standards in maxillofacial CBCT imaging in the interests of o ur patients who deserve to have imaging performed by competent clinicians. The development and rapid commercialization of CBCT technology dedicated for use in the maxillofacial region will undoubtedly increase both general and specialist practitioner acces s to this imaging modality. CBCT is capable of providing accurate, sub - millimetre resolution images in formats enabling 3D visualization of the complexity of the maxillofacial region. Increasing availability of this technology provides the practitioner with a modality that is extending maxillofacial imaging from diagnosis to image guidance of operative and surgical procedures. 15
No matter how exciting a new technology is, the dentist still has to look at its use in his or her practice as a business decisi on. There has to be a return on investment, and if this technology truly improves the ability to better care for patients, its incorporation should yield a return. For multispecialty group practices or group surgical specialists, it makes a great deal of sense to incorporate the technology at this time, either in-office or by referral to an imaging center. For general practices that are invested heavily in implant placement or complex reconstructive dentistry, there is great value in expediting more definitive care. However, for most general practitioners, it would be a significant financial investment that may not deliver a return. For these clinicians, there are thousands of units sold already, and all that is needed is access to one for the more complex cases or for implant placement. Everyone should be able to refer patients for a CBCT when necessary or desired, without having to own a system.
Rather, CBCT is a complementary modality for specific applications. While deceptively relatively simple, the tec hnical component of patient exposure is only one half of cone beam imaging. Based on the medical model of imaging, there is also a moral, ethical and legal responsibility of interpretation of the resultant volumetric dataset. The mechanics of interpretation involve image reporting with the development of a series of images formatted to display the condition/region appropriately (image report) and a cognitive interpretation of the significance of the imaging findings (interpretive report). These skills are not within the domain of most general and specialist practitioners however act as the de facto standard of care in providing CBCT services. It would behove those contemplating or currently using CBCT imaging to develop and maintain these skills by self study of journal articles and participation in study groups or attendance at continuing education courses.
This Library Dissertation is presented to update t he entities comprising the past, current and future trends in Cone Beam Computed Tomography and its clinical significance in dentistry.
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[...]
- Quote paper
- Dr. Mohammed Anser (Author), Dr. Shams Ul Nisa (Author), Dr. B. Karthika (Author), 2022, Cone Beam Computed Tomography and its applications in Dentistry, Munich, GRIN Verlag, https://www.grin.com/document/1278103
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