|Year : 2014 | Volume
| Issue : 2 | Page : 162-166
Cone-beam computed tomography: Hype and facts
Vivek Velayudhan Nair1, Bindu J Nair2
1 Department of Oral Medicine and Radiology, PMS College of Dental Science and Research, Trivandrum, Kerala, India
2 Department of Oral Pathology & Microbiology, PMS College of Dental Science and Research, Trivandrum, Kerala, India
|Date of Submission||24-Jun-2014|
|Date of Acceptance||17-Sep-2014|
|Date of Web Publication||30-Oct-2014|
Vivek Velayudhan Nair
Prof and Head Department of Oral Medicine and Radiology, PMS College of Dental Science and Research, Golden Hills, Vattapara, P.O.- Venkode, Trivandrum - 695 028, Kerala
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Cone beam computed tomography (CBCT) is a relatively new gadget in the field of clinical CT technology. CBCT has been projected as a low-dose alternative to medical CT, with a wide range of applications, and it is said to have paved the way for a paradigm shift in the role of imaging in dentistry from diagnosis to image guidance. However, several questions remain regarding the applicability of CBCT in dentistry, as some of the applications suggested appear to be applications for the sake of an application. This article attempts to look into the hype and facts of cone beam CT.
Keywords: Applications, cone beam CT, limitations
|How to cite this article:|
Nair VV, Nair BJ. Cone-beam computed tomography: Hype and facts
. J Indian Acad Oral Med Radiol 2014;26:162-6
| Introduction|| |
Dentomaxillofacial Radiology has come a long way since its initial application, hardly weeks after the discovery of x-rays, in 1895.  Maxillofacial radiology that remained confined to the photographic emulsion in a two-dimensional format for a long time, expanded its horizons after the discovery of digital radiography and computerized tomography (CT). After the introduction of the CT scanner in 1967, by Sir Godfrey N Hounsfield, CT imaging has evolved considerably in the data acquisition geometries, as well as in the design of x-ray sources and detectors.  Cone beam x-ray CT (CBCT), the prototype of which was put to clinical application in 1982, is a relatively new gadget in the field of clinical CT technology. The commercial availability of such scanners became a reality only in the last decade.  Ever since its introduction commercially, the scientific world is being flooded with information regarding the applicability and the advantages of the cone-beam CT. The possibility of three-dimensional image reconstruction further enhanced its applicability. Cone-beam CT is said to scan, acquire information, and generate three-dimensional data, using radiation doses much lower than the medical CT counterparts. In short CBCT has generated interest in all fields of dentistry and has paved the way for a paradigm shift in the role of imaging in dentistry from diagnosis to image guidance of operative and surgical procedures, using third party application software.  However, is CBCT the embodiment of all the goodness in radiology? Is it the ultimate wonder machine that is without faults? Several questions remain regarding the applicability of CBCT in dentistry, as some of the applications suggested appear to be applications for the sake of an application and not applications on the basis of requirement. Moreover, can we justify the price we pay for such applications, both monetary and radiation risk? This article attempts to look into the hype and facts of the cone-beam CT.
| How it Works|| |
The commercial availability of CBCT became a reality only after the development of computers that were capable of highly complex computations; x-ray tubes with continuous exposure capability; high quality two-dimensional detector arrays; and refinement of cone-beam algorithms.
The CBCT utilizes an x-ray beam that is shaped like a cone or a pyramid, with the x-ray source forming the apex, the detector the base, and a beam diameter ranging from 4 cm to 30 cm (hence, the name cone-beam CT). ,, The x-ray source and the detector are fixed in a gantry with the rotation fulcrum fixed within the center of the region of interest (the patient's head).  During the scan the gantry rotates around the patient's head in a synchronous manner capturing 150-600 distinct sequential planar images. ,, The sequential planar images are then processed to generate an entire volumetric data set that covers the entire area of interest/FOV (Field Of View) in one pass or less. ,,
The image receptors in earlier CBCT models were made up of a combination of a scintillation screen, image intensifier, and a charge-coupled device (CCD). , The image intensifier CCD system, however, was bulky and suffered from peripheral truncation effects (volumetric cone cuts) in addition to sensitivity issues. Introduction of flat panel detectors (FPD) has enabled direct conversion of x-ray energy into a digital signal,with improved spatial resolution. , A flat planar detector is made of a screen of scintillator crystals (terbium-activated gadolinium oxysulfide or thallium-doped cesium iodide) grown on a large pixel array of hydrogenated amorphous silicon (aSi:H) thin film transistors.  The scintillation crystals convert the x-rays into visible light, which in turn is registered in the photodiode array, where the signal intensity charge is stored. The thin film transistors in the array relay a signal with an intensity proportional to the charge stored in the array, which in turn is proportional to the incident x-ray photons. Such new FPDs, in spite of the requirement for slightly more radiation exposure, are less complicated and offer greater spatial resolution, in comparison to the image intensifier CCD combination. ,
Image reconstruction is the name given to the process of obtaining a volumetric data set from a set of sequential planar images. The sequential planar images are obtained with more than one million pixels per image, with 12-16 bits of data assigned to each pixel.  As computing this data to reconstruct an image is very complex, reconstruction in CBCT is usually done using the acquisition computer, which acquires image data from the work station or the processing computer. The greatest advantage of CBCT is that the data can be reconstructed using a personal computer, unlike the CT where workstation platforms are required.  The uni-dimensional projection data obtained during the scan or a single pass around the head of the patient is converted into multi-dimensional images using reconstruction algorithms. The modified Feldkamp algorithm, a 3D adaptation of the filtered back projection method used in conventional fan beam 2D reconstructions is used for image reconstruction in CBCT. , The advantage of the Feldkamp algorithm was that it could address the inversion problem associated with image acquisition involving full circular rotation of the x-ray source and detector about the fulcrum point.  Some other algorithms have also been developed for short circular arc trajectories of the x-ray source keeping in view the requirement for reducing the exposure time. ,
The CBCT with FPD technology is characterized by the isotropic nature of acquisition and reconstruction that allows image reconstruction in any plane with high fidelity and improved spatial resolution of high-contrast structures, in any chosen viewing plane.  However, CBCT is limited in the detectability of low-contrast structures, probably due to the x-ray scatter, low DQE (DQE is the metric for characterizing the overall efficiency of an x-ray imaging detector), and limited temporal range of the flat panel detectors. ,
| Applications in Dentistry|| |
CBCT has been suggested to be useful in:
- Assessment of developing dentition, facial bone shape, position and inter-relationship of both erupted and unerupted teeth, age estimation, and congenital and developing skeletal abnormalities. 
- Differentiation of pathosis from the normal anatomy. 
- Assessment of the cleft palate, the palatal bone thickness, and also for the assessment of bone graft quality following alveolar surgery. 
- 3D visualization and prediction of growth patterns of the craniofacial skeleton. 
- Assessment of orthodontic-associated paresthesia. 
- Planning for surgical removal of impacted teeth, implant placement, and orthognathic surgery. ,
- Assessment of bony pathosis, trauma, and temporomandibular joint diseases. 
- Even though not recommended as a routine technique, CBCT has been suggested for detection of proximal caries, periodontal assessment, assessment of periapical diseases, endodontics, root resorption (both external and internal), root perforation by posts, accessory canal identification, and estimation of root fractures. ,
| Drawbacks/Limitations|| |
The CBCT technology, in spite of its applicability, is not without drawbacks. The major drawbacks of the CBCT are inappropriate bone density determination, artifacts (x-ray beam related, movement-related, patient-related, scanner-related, and cone beam-related artifacts), image noise, and poor soft tissue contrast. ,,
Errors in the estimation of bone density
Even though several authors have supported the use of CBCT to evaluate bone density, the reliability is questionable. ,, During a scan, the image value of a voxel of an organ depends on the position of the image volume; hence, different areas in the scan present with different gray scale values, depending on their positions, in spite of these areas possessing identical densities. This means that the radiodensity of organs measured in Hounsfield units (CT number) using CBCT and the medical grade CT scanner differ. The radiodensity values of CBCT are often higher than the corresponding medical CT values.  Where a medical grade CT provides the accurate absolute density for the quality of bone, there is lack of data to relate the CBCT Hounsfield unit (HU) values to the bone quality. , Moreover, dental CBCT systems, between themselves, do not employ a standardized system for scaling the gray levels that represent the reconstructed density values, which makes comparison of values from different machines difficult.  Attenuation coefficients that may help in deriving the actual Hounsfield units (HU) from CBCT are just being developed and evaluated. , Attempts to use the Misch CT scale from D1-D5, incorporating the Hounsfield scale and development of phantoms to calibrate grayscale readings are currently underway. 
Errors or distortions in the image not related to the subject of interest are called artifacts. , Artifacts can often complicate image interpretation, especially in CBCT where data is lacking. ,, The artifacts can be due to:
- X-ray beam artifacts , : The x-ray beam artifacts are produced due to beam hardening (increase in the mean energy of the x-ray beam, as the lower energy photons are absorbed in preference to the high energy photons) and scattered radiation. Beam hardening leads to the formation of a series of streaks or dark bands leading to loss of information for reconstruction. In severe cases the photons may not reach the detector-forming gaps or voids in the image termed as 'photon starvation'. Scattered radiation artifacts appear as white bands at the edges and star-shaped structures. These artifacts make the image interpretation complicated especially in areas near the occlusal plane, alveolar bone (especially near the teeth with endodontic filling material), implants and areas near the metallic restorations. Moreover the, photon starvation in axial images can present as pseudo-fractures or peri-implant defects, the diagnosis of which becomes critical during CBCT evaluation of the alveolar bone between the implants.
- Patient-related artifacts , : Unsharpness or double contours in the reconstructed image are produced due to the movement of the patient during exposure. Minor movement unsharpness can be prevented by using head restraints and a short scan time. The reconstructed images with movement unsharpness can further be corrected by using specific image-enhancement programs.
- Scanner-related artifacts , : A poorly calibrated or an imperfect scanner can lead to circular and ring-shaped artifacts, which can often be differentiated by consistent repetitive readings. Such artifacts can spoil the quality of a reconstructed image.
- Cone beam-related artifacts [1,4]: Three types of cone beam-related defects can be produced related to the cone beam projection geometry. They are:
- Partial volume averaging, which occurs when the selected voxel resolution of the scan is greater than the spatial or the contrast resolution of the object to be imaged, resulting in a situation where the pixel is not representative of the tissue or the boundary, instead it becomes a weighted average of the different CT values. The net result is a washed-out image or areas in the image. It often becomes very critical in the evaluation of CBCT images of the maxillary posterior region, where the bone surface thins out, or in the temporal bone, where the bone surface rapidly changes directions.
- Under-sampling occurs when the number of images available for reconstruction is less, leading to noisy images. Even though this may not degrade the image, it can impair the quality of the image and can present problems when fine details are necessary.
- Cone beam effects are produced due to the divergence of the x-ray beam as it rotates around the patient. On account of the divergence, there is a reduction in the amount of information available for reconstruction of the peripheral structures in comparison to the amount of information available for the central areas, resulting in image distortion, streaking artifacts, and greater peripheral noise.
Poor soft tissue contrast ,
The soft tissue contrast of the CBCT system is poor in comparison to medical-grade CT. The contrast resolution of CBCT is limited by:
- Scattered radiation: Increases the image noise and reduces the contrast of the CBCT system.
- Divergence of the x-ray beam: Produces an enhanced heel effect, resulting in nonuniformity in the incident x-ray beam, with resultant absorption, leading to increased signal-to-noise ratio on the cathode side of the image.
- Flat panel detector-based artifacts.
| How the Limitations of CAPS Systems are Managed|| |
The above-mentioned limitations of the CBCT systems, irrespective of the source of the limitations or artifacts, are presently managed by development of third party application software and reconstruction algorithms capable of eliminating or ignoring the shortcomings of the CBCT system. ,, The efficacy and reliability of these techniques are yet to be evaluated and established. The amount of information lost during algebraic reconstruction techniques is still under study. Hence, at present, the reliability of CBCT imaging for assessment and/or monitoring of minor osseous changes, critical in making radiological diagnosis, is limited.
| Radiation dose in Cone Beam Computed Tomography|| |
The radiation dose parameter in CBCT imaging is foremost to patient safety and image quality. CBCT has been projected as a low-dose alternative to medical CT.  Even though studies have shown CBCT scan doses (30-80 μSv for restricted anatomical volumes in the maxillofacial region and 0.2 mSv for imaging of the paranasal sinus) to be less than those used in the medical-grade CT, several questions remain, regarding the variables considered in these studies, which affect the radiation dose.  Conventional dosimetry metrics used for evaluation of a medical CT, cannot be directly adapted for CBCT imaging because of the characteristic beam geometry and scatter radiation profile of the CBCT systems. A standardized, universally applicable technique, which generates absorbed dose metrics comparable with those used in conventional CT is yet to be adopted.  In this context, it has been suggested that the dose differences between the present-day CBCT systems and medical CT may not be significantly different, if the field of view (FOV) and image quality parameters are approximated. ,, Therefore, further research is required for the development of common exposure protocols and measurement methodologies prior to the authentic comparison of the CBCT systems with a medical CT.
| Conclusion|| |
Cone Beam Computed Tomography is still an emerging technical advancement in the field of radiology. Even though CBCT is capable of providing useful high-resolution diagnostic images, with 3D visualization of the maxillofacial region, with a wide range of applications in the field of dentistry; the machine is not without limitations. It has been suggested that CBCT technology should not be used as a 'fishing excursion' for non-symptomatic diseases.  Moreover, it is the ethical duty of the practitioner to evaluate the 'radiation dose to benefit value' for the patient, before advising CBCT imaging. Several aspects, such as, reducing scan time, multimodal imaging, improving image fidelity, improving soft tissue contrast, and development of task-specific protocols for minimizing patient dose, have to be addressed in the future, to bring the standard of CBCT up to its present day hyped image.
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