Cone beam computed tomography

Cone beam computed tomography
Diagnostics
MeSH	D054894

Cone beam computed tomography (commonly referred to by the acronym CBCT) is a medical imaging technique that has become increasingly important in treatment planning and diagnosis in implant dentistry, among other things. Perhaps because of the increased access to such technology, CBCT scanners are now finding many uses in dentistry, such as in the fields of endodontics and orthodontics, as well.

During a CBCT scan, the scanner rotates around the patient's head, obtaining up to nearly 600 distinct images. The scanning software collects the data and reconstructs it, producing what is termed a digital volume composed of three dimensional voxels of anatomical data that can then be manipulated and visualized with specialized software.^[1]

History

Cone beam technology was first introduced in the European market in 1998 and into the US market in 2001.^[1]

CBCT use in orthodontics

As a 3D rendition, CBCT offers an undistorted view of the dentition that can be used to accurately visualize both erupted and non-erupted teeth, tooth root orientation and anomalous structures that conventional 2D radiography cannot.^[2]

Disadvantages of CBCT technology

There are a number of drawbacks of CBCT technology over that of medical-grade CT scans, such as increased susceptibility to movement artifacts (in first generation machines) and to the lack of appropriate bone density determination.^[3]

Bone density and the Hounsfield scale

The Hounsfield scale is used to measure radiodensity and, in reference to medical-grade CT scans, can provide an accurate absolute density for the type of tissue depicted. The radiodensity, measured in Hounsfield Units (HU, also known as CT number) is inaccurate in CBCT scans because different areas in the scan appear with different greyscale values depending on their relative positions in the organ being scanned, despite possessing identical densities, because the image value of a voxel of an organ depends on the position in the image volume.^[4] HU measured from the same anatomical area with both CBCT and medical-grade CT scanners are not identical^[5] and are thus unreliable for determination of site-specific, radiographically-identified bone density for purposes such as the placement of dental implants, as there is "no good data to relate the CBCT HU values to bone quality."^[6]

Although some authors have supported the use of CBCT technology to evaluate bone density by measuring HU,^[7][8] such support is provided erroneously because scanned regions of the same density in the skull can have a different grayscale value in the reconstructed CBCT dataset.^[9]

X-ray attenuation of CBCT acquisition systems currently produces different HU values for similar bony and soft tissue structures in different areas of the scanned volume (e.g. dense bone has a specific image value at the level of the menton, but the same bone has a significantly different image value at the level of the cranial base).^[3]

Dental CBCT systems do not employ a standardized system for scaling the grey levels that represent the reconstructed density values and, as such, they are arbitrary and do not allow for assessment of bone quality.^[10] In the absence of such a standardization, it is difficult to interpret the grey levels or impossible to compare the values resulting from different machines. While there is a general acknowledgment that this deficiency exists with CBCT systems (in that they do not correctly display HU), there has been little research conducted to attempt to correct this deficiency.^[11]

With time, further advancements in CBCT reconstruction algorithms will allow for improved area detectors,^[12] and this, together with enhanced postprocessing, will likely solve or reduce this problem.^[4] A method for establishing attenuation coefficients with which actual HU values can be derived from CBCT HU values was published in 2010 and further research is currently underway to perfect this method in vivo.^[11]

References

1. ^ ^{a b} Hatcher, DC. Operational principles for cone-beam computed tomography. JADA 2010;141(10S):3S-6S
2. ^ Mah, JK; et al. Practical applications of cone-beam computed tomography in orthodontics. JADA 2010;141(3S):7S-13S
3. ^ ^{a b} De Vos, W; et al. Cone-beam computerized tomography (CBCT) imaging of the oral and maxillofacial region: A systematic review of the literature. Int J Oral Maxillofac Surg 2009;38:609–625.
4. ^ ^{a b} Swennen, GRJ; Schutyser, F. Three-dimensional cephalometry: spiral multislice vs cone-beam computed tomography. Am J Orthod Dentofacial Orthop 2006:130:410–416.
5. ^ Armstrong RT. Acceptability of cone beam ct vs. multi-detector CT for 3D Anatomic model construction. AAOMS 2006;64:37.
6. ^ Miles DA, Danforth RA. "A clinician’s guide to understanding cone beam volumetric imaging (CBVI)." 2007. Available from: www.ineedce.com/.
7. ^ Ganz SD. Conventional CT and cone beam CT for improved dental diagnostics and implant planning. Dent Implantol Update 2005:16:89–95.
8. ^ Lee S; et al. Bone density assessments of dental sets. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2007:104:814–820.
9. ^ Katsumata A; et al. Image artifact in dental cone-beam CT. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2006:101:652–657.
10. ^ Norton, MR; Gamble, C. Bone classification: an objective scale of bone density using the computerized tomography scan. Clin Oral Implants Res 2001;12:228–234.
11. ^ ^{a b} Mah, P; et al. Deriving Hounsfield units using grey levels in cone beam computed tomography. Dentomaxillofacial Radiology 2010;39:323–335.
12. ^ Vannier, MW. Craniofacial computed tomography scanning: technology, applications and future trends. Orthod Craniofac Res 2003:6(Suppl 1):23–30 discussion 179–182.