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Clinical Considerations for Cone Beam Imaging in Dentistry
Author Acknowledgements
William C. Scarfe BDS, FRACDS, MS
Allan G. Farman BDS, PhD, DSc
School of Dentistry
University of Louisville
Louisville, KY
metlife Quality Resource Guide
Educational Objectives
Topic: Clinical Considerations For Cone
Beam Imaging In Dentistry
Following this unit of instruction, the practitioner
should be able to:
1) Understand the principles of CBCT imaging.
2) Describe the technical factors involved in
performing a CBCT examination.
3) Introduce the concept of task specific imaging
to reduce patient radiation dose and optimize
image quality.
4) Appreciate the relative patient radiation dose
provided by CBCT imaging.
5) Recognize the appropriate use of CBCT as a
supplemental diagnostic imaging modality
with specific clinical applications.
The following commentary highlights fundamental
and commonly accepted practices on the subject
matter. The information is intended as a general
overview and is for educational purposes only. This
information does not constitute legal advice, which
can only be provided by an attorney.
Introduction
Principles of CBCT Imaging
The introduction of Cone Beam Computerized
Tomography (CBCT) to dentistry has created an unprecedented revolution in oral and
maxillofacial imaging, eclipsing the introduction of panoramic radiography in the 1960’s.
Beginning with the commercial introduction
of CBCT in Europe in 1999, the adoption of
this technology in dentistry has expanded
globally both in terms of its manufacturing
centers (units are now made in Japan, France,
the United States, Finland and Korea) and
clinical applications in dentistry. More than
a dozen CBCT systems are currently commercially available, all providing useful diagnostic
images. The purpose of this guide is to outline
the concepts of CBCT technology, introduce
practitioners to technical considerations in
obtaining and viewing the image, and provide
clinical guidance on the appropriate use of this
modality in dental practice.
The mechanics of CBCT imaging comprise two
phases 1 (Figure 1):
1) Acquisition phase. A pyramidal or cone
shaped x-ray beam is directed towards an
area x-ray detector on the opposite side
of the patient’s head and multiple exposures are made during a single full or partial synchronous rotation. The resultant
series of two-dimensional (2D) projections or basis images form a set referred
to as the projection data.
2) Reconstruction phase. Software programs
are applied to projection data to generate
a three-dimensional (3D) volumetric data
set composed of cuboidal volume elements
(voxels). The default presentation of the
data set is usually as a series of contiguous
images in three right angle planes (axial,
sagittal and coronal).
Figure 1: The Mechanics of CBCT Acquisition
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Published March 2009. The content of this Guide
is subject to change as new scientific information
becomes available.
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Multiple basis projections form the projection data from which orthogonal
planar images are secondarily reconstructed in cone beam geometry.
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Table 1. Comparison of Imaging Procedures:
Panoramic Radiography vs. CBCT.
StageSimilaritiesDifferences for CBCT
Set technique
factors
Made before exposure;
controls image quality and
patient radiation dose
CBCT offers more choices than
panoramic radiography, which
usually has only kVp; Scan factors
for CBCT should be adjusted to be
task specific.
Prepare patient
Patient standing or seated,
head stabilized, position
critical to resultant image.
Patient may also be supine and no
bite block used for CBCT .
Protect patient
Lead torso shield
Lead torso shield; thyroid shield is
desirable if possible.
Expose
Patient informed to keep still
Scan time varies from 5s to greater
than 30s; motion artifacts are
more likely to occur; frequent image calibration necessary.
View image
Image viewed immediately
Image must be reconstructed
before viewing (30s – 20 min),
secondary orthogonal images must
be reformatted; data is interactive
(contrast, brightness, image mode);
resultant data can be re-oriented
to compensate for head position.
CBCT Technique vs. Panoramic
Radiology Technique
Procedural similarities exist between
CBCT and panoramic radiography (Table
1). However, there are also a number of
important distinctions between the two techniques, the most important being the greater
number of technique parameters available
for CBCT imaging.
1) T
echnical parameters. Two parameters
need to be adjusted when performing
a CBCT scan; the tube current and the
tube voltage. These two factors control
the quantity and the quality of the
x-ray photons generated by the tube
head. They have a direct influence on
the quality of the image and the dose
of radiation received by the patient.
Adjustment of these parameters, if
possible, can provide significant dose
reduction without compromising the
image quality.2,3
In contrast with panoramic radiography, CBCT
units also allow additional modifications of
scan parameters that may influence image
quality and affect patient dose. These include:
A. Field of View (FOV). The tissue volume
of the patient’s head exposed during
imaging is referred to as the FOV. An
adjustable FOV, particularly in large
units, is desirable as x-ray exposure
should be limited to cover only the
region of interest. This provides marked
reduction in patient radiation exposure
compared to panoramic radiology. 4
B. Projection data. The total number of
basis images comprising the projection
data of a single scan may be fixed or
variable. This is usually reflected in the
selection of scan time. While increasing
scan time provides more basis images, and
produces “smoother”, less grainy images,
this is usually accomplished at a higher
patient radiation dose.
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C. Spatial resolution. While nominal resolution for CBCT units is equipment specific
(range: 0.076mm to 0.4mm), the resolution of some CBCT units can be varied
at the reconstruction phase using a process of pixel binning (the gathering and
combining of information from adjacent
regions). This can substantially reduce file
size and therefore reconstruction time.
Resultant images have reduced resolution but improved image contrast. Higher
resolution settings may not be clinically
important as patient motion may be the
limiting factor in CBCT resolution.5
D. Scan arc. Many CBCT imaging systems
employ a complete circular (3600) trajectory, however some use a limited, or
even a variable, scan arc. A limited or
variable arc reduces the scan time and is
mechanically easier to perform, however
data must be extrapolated to provide a
full volumetric dataset. The effect, if any,
on diagnostic image quality or radiation
dose is currently unreported.
Scan parameter choice should be based on
the requirements of the imaging task – a
concept referred to as task specific imaging.
For example a secondary TMJ scan to determine the degree of translation of the condyle
with jaw opening should be performed at the
lowest resolution, shortest scan time and
reduced FOV. This provides optimal imaging
with a nominal radiation dose.
2) Patient Positioning. The patient’s head
must be firmly stabilized, whether the
they are standing, lying or seated, during the entire scan when obtaining a
CBCT image. This reduces the potential
for motion during the scan, a significant source of reduced image quality.5
Stabilization can be accomplished using
equipment such as chin rests and/or head
holders and providing adequate instructions to the patient prior to exposure to
remain still during the procedure and to
keep the teeth closed either together or
on a bite block.
3) Patient Protection. The patient should
be draped in a lead torso apron to reduce
exposure to scatter radiation during the
obtainment of both CBCT and panoramic
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images. Use of a thyroid collar should be
considered when it does not interfere
with the area to be imaged as this substantially reduces patient radiation by
shielding exposure to the hyoid, esophagus and cervical spine.6
4) Exposure Adjustment. Because CBCT
exposes the head in one rotational scan,
acquisition time is comparable to that
of panoramic radiography. However
CBCT imaging also incorporates correction and further computational processes on the original projection images. The time for data set reconstruction
can be much longer than the scan time
and may range from 30 seconds up
to several minutes. Image correction
necessitates routine calibration of the
digital detector, referred to as image
calibration, to prevent untoward artifacts affecting image quality.
5) Image Viewing. Unlike panoramic radiography, CBCT units provide a sequential “stacked” set of coronal, sagittal and
axial orthogonal images. These images
are not inherently easy to interpret.
Viewing CBCT images, unlike panoramic images, is performed as an interactive process – windowing and leveling,
changing the brightness and contrast,
reorienting, rotating or reslicing the
volume in all directions may be used by
the operator to adapt the final image to
his/her diagnostic objectives.
Relative Radiation Exposure
The type and model of CBCT device as well
as the scan parameters used (particularly
FOV) markedly influence the radiation
dose to the patient. CBCT provides an
equivalent dose of 3 to 44 times that of a
single panoramic radiograph, or between
8 to 131 days of background radiation.4
While smaller FOV units should reduce
radiation exposure, some actually produce
far greater patient exposure because they
have tube heads producing continuous
radiation exposure.4
Figure 2: CBCT Display Formats
Display modes can be divided into 3 categories:
A) M
ultiplanar Reformatted (MPR) consisting of linear, curved oblique and serial trans-axial images,
B) R
ay Sum comprising images of increased section thickness and,
C) Volumetric Images, consisting of Indirect Volume Rendering (IVR), the most common of
which being Maximum Intensity Projection (MIP), and Direct Volume Rendering (DVR).
Imaging Techniques
Personal computer based OEM (original equipment manufacturer), or third party, software
facilitates dynamic interaction with the clinician to provide task specific display modes
useful in dentistry (Figure 2). Strategies that
are useful in OMF imaging include:
1) Multi-planar Reformation (MPR). This
technique creates non-axial 2D images
by transecting a set or “stack” of axial
images. Linear or curved oblique MPR
provides useful sectioning with respect
to specific maxillofacial anatomy such
as the TMJ or dental arch. Subsequent
serial trans-axial cross-sectional imaging
provides sequential multiple thin-slice
images, at right angles to the MPR.
2) Increasing Slice Thickness. The addition
of the grayscale values of adjacent voxels
of orthogonal or MPR sections is known
as “ray sum” and enables the production
of simulated, but undistorted, projection
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images such as lateral cephalometric and
panoramic images.
3) 3D Volume Rendering. These techniques allow the visualization of 3D data
by selective display of voxels. This can
be achieved by direct volume rendering
(DVR) providing a volumetric surface
reconstruction with depth, or indirect
volume rendering (IVR), most commonly as a maximum intensity projection
(MIP). MIP is used to demonstrate high
intensity structures by providing a “pseudo” 3D reconstruction.
Appropriate Use Of CBCT Imaging
Evidence-based clinical efficacy studies and
consensus-derived specific patient selection
criteria are currently not available for CBCT
use. Generally accepted guidelines7 state that
CBCT should be used as an adjunctive diagnostic tool to existing dental imaging techniques for
specific clinical applications, not as a screening
procedure for oral pathology, dental caries
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detection and/or assessment of periodontal Figure 3: CBCT Artifacts
destruction. Parameters should be adjusted to
deliver the minimum exposure to the patient
and to provide the image quality necessary for
adequate diagnostic information.
CBCT imaging provides excellent detail of
osseous structures, however cone beam projection geometry and limitations in detector sensitivity provide images with reduced contrast
resolution and higher ”noise” (more grain)
compared to medical computed tomography.
Image quality can be further compromised
by image artifacts due to acquisition (beam
hardening producing scatter streaks and dark
bands) (Figure 3), patient related artifacts
(patient motion leading to unsharpness), the
scanner itself (ring artifacts) or the cone beam
technique (distorted periphery). Contrast, resolution and artifacts currently make CBCT imaging unsuitable for dental caries diagnosis.8
The presence of metallic restorations in the mouth can create streaks and
dark band effects which present horizontally on axial (left), coronal (lower right)
and sagittal (upper right) orthogonal images.
The American Academy of Oral and Maxillofacial
Radiology7 recently published their executive
committee opinion statement on performing Figure 4: CBCT for Impacted Tooth Assessment
and interpreting diagnostic CBCT in dentistry
<http://www.aaomr.org/carter_2008_Oral_
Surgery.pdf?PHPSESSID=20c5ccfe9f28707a7
558a934ad109817>. This document provides
guidance on the appropriate use and prescription of CBCT, details the responsibilities of
practitioners and licensed operators in performing the examination, outlines the appropriate documentation and radiation safety considerations and provides recommendations for
quality control and patient education.
Specific Clinical Applications
CBCT has been applied to adjunctive diagnosis in all areas of dentistry including:
1) Implant Sites. The most common use for
CBCT imaging is for the assessment of
potential implant sites by providing crosssectional images of the alveolar bone and
accurately depicting important anatomic
features (the mandibular canal in the mandible or maxillary sinus in the maxilla).
2) Orthodontics. Large FOV imaging of facial
asymmetry craniofacial syndromes9 and
maxilla/mandibular disparities can be demonstrated using 2- or 3-D formats allowing
Reformatted panoramic image (upper) provides a reference and undistorted conventional image
demonstrating relative angulation of multiple impacted teeth. Serial cross-sectional images (lower)
show bucco-lingual orientation and relationship of maxillary canine to existing teeth.
precise measurements of the skull and
facial bones.10 Small regions can also be
imaged to determine the exact position of
impacted11 and/or supernumerary teeth
and their relationships to adjacent roots or
other anatomical structures.
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3) TMJ. CBCT facilitates the visualization
of bone morphology, joint space and
dynamic function as compared to conventional imaging12, a critical key to providing
appropriate treatment in patients with
signs and symptoms of TMJ pathology.
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4) Oral Pathology. CBCT demonstrates
the location, size, shape, extent and full
involvement of pathology of the jaws.
Various dental conditions including additional teeth (supernumeraries), impacted canines (Figure 4) and third molars
are clearly identified.
Table 2. Technique Variables Involved In CBCT Imaging
FactorVariables
Exposure Setting
Generator Type (constant or pulsed)
Scan Parameter
5) Extragnathic Conditions. Diagnostically
important soft tissue such as the pharyngeal airway space13 and sinus conditions
can also be visualized on CBCT images.
CBCT data can be exported in the non proprietary Digital Imaging and Communications in
Medicine (DICOM) file format standard and
imported into task specific third party diagnostic and planning software to: assist in orthodontic assessment and analysis; facilitate
virtual implant placement and/or create diagnostic and surgical implant guidance stents;
and assist in the computer-aided design and
manufacture of implant prosthetics.
Variable (Ma, kVp) or fixed
Field of View (variable or fixed)
Number of Projection Images (fixed or variable)
Resolution (fixed or variable)
Completeness of Trajectory (full, partial or limited)
Conclusion
Use of CBCT imaging by both general and specialist practitioner will undoubtedly increase. This technology provides the clinician with an imaging modality with
increased precision, acceptable patient dose, and the capability of visualizing the
third dimension. CBCT also extends dental imaging from diagnosis to image guidance
for operative and surgical procedures. Practitioners using CBCT should be aware that
guidance documents by relevant organizations7 will be periodically updated to ensure
optimize image quality and minimize patient radiation exposure.
References
1.Scarfe WC, Farman AG. What is Cone-Beam CT and How Does it Work? Dent Clin North Am. 2008;52(4):709-730.
2.Kwong JC, Palomo JM, Landers MA, Figueroa A, Hans MG. Image quality produced by different cone-beam computed tomography
settings. Am J Orthod Dentofacial Orthop. 2008;133:317-27.
3.Palomo JM, Rao PS, Hans MG. Influence of CBCT exposure conditions on radiation dose. Oral Surg Oral Med Oral Pathol Oral Radiol
Endod. 2008;105:773-82.
4.Ludlow JB, Ivanovic M. Comparative dosimetry of dental CBCT devices and 64-slice CT for oral and maxillofacial radiology. Oral Surg
Oral Med Oral Pathol Oral Radiol Endod. 2008;106:106-14.
5.Bontempi M, Bettuzzi M, Casali F, Pasini A, Rossi A, Ariu M. Relevance of head motion in dental cone-beam CT scanner images
depending on patient positioning. Int. J of Computer Assisted Radiology and Surgery, 2008; 3:249-255.
6.Tsiklakis K, Donta C, Gavala S, Karayianni K, Kamenopoulou V, Hourdakis CJ. Dose reduction in maxillofacial imaging using low dose
Cone Beam CT. Eur J Radiol. 2005;56:413-7.
7.Carter L. Farman A. Geist J. Scarfe W. Angelopoulos C. Nair M. Hildebolt C. Tyndall D. Shrout M. American Academy of Oral and
Maxillofacial Radiology executive opinion statement on performing and interpreting diagnostic cone beam computed tomography.
Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2008;106:561-562.
8.Haiter-Neto F, Wenzel A, Gotfredsen E. Diagnostic accuracy of cone beam computed tomography scans compared with intraoral image
modalities for detection of caries lesions. Dentomaxillofac Radiol. 2008;37:18-22.
9.Korbmacher H, Kahl-Nieke B, Schöllchen M, Heiland M. Value of two cone-beam computed tomography systems from an orthodontic
point of view. J Orofac Orthop. 2007;68:278-289.
10.Kau CH, Richmond S, Palomo JM, Hans MG. Three-dimensional cone beam computerized tomography in orthodontics. J Orthod.
2005;32:282-923.
11.Liu DG, Zhang WL, Zhang ZY, Wu YT, Ma XC. Localization of impacted maxillary canines and observation of adjacent incisor resorption
with cone-beam computed tomography. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2008;105:91-8.
12.Honey OB, Scarfe WC, Hilgers MJ, Klueber K, Silveira AM, Haskell BS, Farman AG. Accuracy of cone-beam computed tomography
imaging of the temporomandibular joint: comparisons with panoramic radiology and linear tomography. Am J Orthod Dentofacial
Orthop. 2007;132:429-438.
13.Aboudara CA, Hatcher D, Nielsen IL, Miller A. A three dimensional evaluation of the upper airway in adolescents. Orthod Craniofac
Res 2003;6(Suppl 1):173–5.
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Post TesT: Clinical Considerations For Cone Beam Imaging In Dentistry
Internet Users: This page is intended to assist you in fast and accurate testing when completing the “Online Exam.”
We suggest reviewing the questions and then circling your answers on this page prior to completing the online exam.
(1.0 CE Credit Contact Hour) Please circle the correct answer. 70% equals passing grade.
1. Which of the following image types form the projection data for image reconstruction in cone beam imaging?
a. basis
c. coronal
b. axial
d. sagittal
2. What is the range of radiation exposures a patient could receive for a full head cone beam imaging procedure (in units of panoramic exposure)?
a. 1-3 x panoramic
c. 3-44x panoramic
b. 3-5x panoramic
d. 10-80x panoramic
3. How can radiation exposure to the patient be minimized during a cone beam imaging procedure?
a. Use of a lead apron
c. Reducing the area to be irradiated to a region of interest (ROI)
b. Use of a thyroid collar
d. All of the above
4. Which of the following statements regarding cone beam imaging is incorrect?
a. Image spatial resolution is potentially limited by patient motion.
c. Exposure and technical imaging parameters for CBCT are similar to
panoramic radiography.
b. The procedural stages for performing cone beam imaging are similar to
panoramic radiography.
d. Currently there are more than a dozen FDA approved cone beam units
for sale in the United States.
5. Which of the following must be performed to adequately view CBCT images?
a. Adjustment of the value of the voxels (e.g. brightness, contrast)
c. Reorientation of the entire dataset, allowing realignment of the
patient’s anatomic features.
b. Reformatting of the data for display.
d. All of the above
6. Which of the following statements correctly describe task specific imaging?
a. Fees for CBCT imaging should be determined based on the reason for the scan.
b. Exposure and technical parameters for CBCT imaging should be adjusted according to the reason for imaging.
c. CBCT imaging reimbursement is based on the difficulty of the scan.
d. CBCT imaging should be performed without changes in exposure and technical parameters, irrespective of task, to ensure uniformity.
7. What is the non-proprietary file format for export of CBCT images?
a. DICOM
b. JPEG
c. TIFF
d. PDF
8. Which of the following currently make CBCT imaging unsuitable for dental caries diagnosis?
a. Limited contrast resolution
c. Artifacts
b. Limited spatial resolution
d. All of the above
9. For which of the following diagnostic imaging tasks is conventional intraoral radiography superior to cone beam imaging?
a. Interproximal dental caries detection
c. Determination of alveolar bone height
b. Determination of the extent of maxillofacial pathology
d. Both a. and c.
10. Which of the following is not an indication for use of CBCT?
a. Impacted mandibular third molars adjacent the mandibular canal.
b. As a screening imaging modality to replace panoramic imaging to investigate occult pathology.
c. Investigation of TMJ articulation of patients with clinical signs and symptoms of a temporomandibular disorder.
d. Alveolar bone assessment in edentulous area for potential endosseous implant placement.
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