Cone beam computed tomography
Encyclopedia
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 voxel
s of anatomical data that can then be manipulated and visualized with specialized software.
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.
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. 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 (in that they do not correctly display HU), 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 underway to perfect this method in vivo
.
Dental implant
A dental implant is a "root" device, usually made of titanium, used in dentistry to support restorations that resemble a tooth or group of teeth to replace missing teeth....
, 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
Endodontics
Endodontics is one of the dental specialties recognized by the American Dental Association, Royal College of Dentists of Canada, and Royal Australasian College of Dental Surgeons, and deals with the tooth pulp and the tissues surrounding the root of a tooth...
and orthodontics
Orthodontics
Orthodontics, orthodontia, or orthodonture is the first specialty of dentistry that is concerned with the study and treatment of malocclusions , which may be a result of tooth irregularity, disproportionate jaw relationships, or both...
, 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 voxel
Voxel
A voxel is a volume element, representing a value on a regular grid in three dimensional space. This is analogous to a pixel, which represents 2D image data in a bitmap...
s of anatomical data that can then be manipulated and visualized with specialized software.
History
Cone beam technology was first introduced in the European market in 1998 and into the US market in 2001.CBCT use in orthodontics
As a 3DThree-dimensional space
Three-dimensional space is a geometric 3-parameters model of the physical universe in which we live. These three dimensions are commonly called length, width, and depth , although any three directions can be chosen, provided that they do not lie in the same plane.In physics and mathematics, a...
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.
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.Bone density and the Hounsfield scale
The Hounsfield scaleHounsfield scale
The Hounsfield scale, named after Sir Godfrey Newbold Hounsfield, is a quantitative scale for describing radiodensity.-Definition:The Hounsfield unit scale is a linear transformation of the original linear attenuation coefficient measurement into one in which the radiodensity of distilled water at...
is used to measure radiodensity
Radiodensity
Radiodensity refers to the relative inability of electromagnetic radiation, particularly X-rays, to pass through a particular material. Radiolucency indicates greater transparency or "transradiancy" to X-ray photons...
and, in reference to medical-grade CT scans, can provide an accurate absolute density for the type of tissue
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
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
Voxel
A voxel is a volume element, representing a value on a regular grid in three dimensional space. This is analogous to a pixel, which represents 2D image data in a bitmap...
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 (in that they do not correctly display HU), 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 underway to perfect this method in vivo
In vivo
In vivo is experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...
.