Cone beam computed tomography


Cone beam computed tomography is a medical imaging technique consisting of X-ray computed tomography where the X-rays are divergent, forming a cone.
CBCT has become increasingly important in treatment planning and diagnosis in implant dentistry, ENT, orthopedics, and interventional radiology, 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 oral surgery, endodontics and orthodontics. Integrated CBCT is also an important tool for patient positioning and verification in image-guided radiation therapy.
During dental/orthodontic imaging, the CBCT scanner rotates around the patient's head, obtaining up to nearly 600 distinct images. For interventional radiology, the patient is positioned offset to the table so that the region of interest is centered in the field of view for the cone beam. A single 200 degree rotation over the region of interest acquires a volumetric data set. 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. CBCT shares many similarities with traditional CT however there are important differences, particularly for reconstruction. CBCT has been described as the gold standard for imaging the oral and maxillofacial area.

History

Cone beam technology was first introduced in the European market in 1996 by QR s.r.l. and into the US market in 2001.
In 2013, during the Festival della Scienza in Genova, Italy, the original members of the research group received an award for the invention of cone-beam CT.

Applications

Endodontics

According to the American Association of Endodontics, there are many specific situations where 3D images produced by CBCT enhances diagnosis and influences treatment and its use can not be disputed over conventional intraoral radiology following the principles of ALARA.

Implantology

A dental cone beam scan offers useful information when it comes to the assessment and planning of surgical implants. The American Academy of Oral and Maxillofacial Radiology suggests cone-beam CT as the preferred method for presurgical assessment of dental implant sites.

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.
Processing example using x-ray data from a tooth model:

Orthopedics

The CBCT scanner offers undistorted views of the extremities. One advantage of orthopedic CBCT is the ability to take weight bearing images of the lower extremities. In the realm of the foot and ankle particularly, weight bearing CBCT is gaining momentum due to its ability to combine 3 dimensional and weight bearing information which are of the utmost importance in diagnosis and surgical planning. The preferred term used for CBCT in the lower limb is thus WBCT for Weight Bearing CT following the first scientific publications on the subject.

Image-guided radiation therapy

is a form of external beam radiotherapy where the patient is positioned with the organs to be treated accurately matched in position to the treatment field, to reduce the dose to nearby organs which are not being treated. Many organs inside the body move by millimeters relative to the external skin surfaces, and a CBCT scanner mounted on the head of the radiotherapy unit is used immediately before treatment to ensure the patient's organs are in exactly the right position to match the treatment field, and to adjust the position of the treatment table if necessary. The images may also be used to check for other requirements of some types of treatment, such as full or empty bladder, empty rectum, etc. The same cone beam beam source and detector can alternatively be used to take simple X-ray positioning images if the organ shows particularly well on X-ray or if Fiducial markers have been inserted into the organ.

Interventional radiology

The CBCT scanner is mounted on a C-arm fluoroscopy unit in the interventional radiology suite, which offers real time imaging with a stationary patient. This eliminates the time needed to transfer a patient from the angiography suite to a conventional computed tomography scanner and facilitates a broad spectrum of applications of CBCT during IR procedures. The clinical applications of CBCT in IR include treatment planning, device or implant positioning and assessment, intra-procedural localization, and assessment of procedure endpoints. CBCT is useful as a primary and supplemental form of imaging. It is an excellent adjunct to DSA and fluoroscopy for soft tissue and vascular visibility during complex procedures. The use of CBCT before fluoroscopy potentially reduces patient radiation exposure.

Clinical applications

While the practicality of CBCT fosters its increasing application in IR, technical limitations hinder its integration into the field. The two most significant factors that affect successful integration are image quality and time. Compared to multidetector computed tomography, the wider collimation in CBCT leads to increased scatter radiation and degradation of image quality as demonstrated by artifacts and decreased contrast-to-noise ratio. The temporal resolution of cesium iodide detectors in CBCT slows data acquisition time to approximately 5 to 20 seconds, which increases motion artifacts. The time required for image reconstruction takes longer for CBCT compared to MDCT due to the computationally demanding cone beam reconstruction algorithms.

Reconstruction

Cone beam reconstruction algorithms are similar to typical tomographic reconstruction algorithms, and methods such as filtered backprojection or iterative reconstruction may be used. However, since the reconstruction is three-dimensional, modifications such as the FDK algorithm may be needed.

Risks

Total radiation doses from 3D dental CBCT exams are 96% lower than conventional CT exams, but deliver more radiation than standard dental 2D x-ray. The time of exposure in CBCT is also comparatively less when compared to conventional CT.
CBCT use is only lightly regulated in the US. The recommended standard of care is to use the smallest possible field of view, the smallest voxel size, the lowest mA setting and the shortest exposure time in conjunction with a pulsed exposure mode of acquisition. International organisations such as the World Health Organization and ICRP, as well as many local bodies and legislation, encourage the idea of justification for all medical exposures, where risks and benefits must be weighed up before a procedure goes ahead.

Disadvantages

There are a number of drawbacks of CBCT technology over that of CT scans, such as increased susceptibility to movement artifacts and to the lack of appropriate bone density determination.

Bone density and the Hounsfield scale

The Hounsfield scale is used to measure radiodensity and, in reference to CT scans, can provide an accurate absolute density for the type of tissue depicted. The radiodensity, measured in Hounsfield Units 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. HU measured from the same anatomical area with both CBCT and medical-grade CT scanners are not identical 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."
Although some authors have supported the use of CBCT technology to evaluate bone density by measuring HU, 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.
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. 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, there has been little research conducted to attempt to correct this deficiency.
With time, further advancements in CBCT reconstruction algorithms will allow for improved area detectors, and this, together with enhanced postprocessing, will likely solve or reduce this problem. 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 under way to perfect this method in vivo.

Literature