basic principles of cbct

1. BASIC PRINCIPLES OF CONE BEAM COMPUTED
TOMOG RAPHY
Kenneth Abramovitch, DDS, MS*, Dwight D. Rice, DDS
BY
SURYA K R
DEPT OF CONSERVATIVE DENTISTRY
AND ENDODONTICS
Dent Clin N Am 58 (2014) 463–484
Loma Linda University School of Dentistry, 11092 Anderson Street, Loma Linda,
USA

2. KEY POINTS
The use of cone beam computed tomography (CBCT) imaging in the dental
profession has blossomed since its inception 15 years ago. CBCT unit design has
undergone many changes that enhance CBCT access and practical utility in dentistry.
The scanners have become smaller, scan patients in an upright position, use primarily
flat panel detectors, and readily convert projection data to DICOM file formats.
Units themselves have various scanning options that include the size of the area to be
scanned (field of view [FOV]), voxel size (spatial resolution), bit depth (contrast
resolution), and scan times (frame rate)

3. CBCT manufacturers have incorporated various aspects of imaging technology
in a cost effective, efficient, and practical manner. There are now numerous
CBCT applications in many software formats that are helpful in a multitude of
dental disciplines including but not limited to dentoalveolar disease and
anomalies, vertical root and dentin fractures, jaw tumors, prosthodontic
evaluations, and advances in orthodontic/orthognathic and implant patient
evaluations
The latter also include mechanisms for surgical and prosthodontic splint
design and the capability of CBCT scan data to bridge with computer-aided
design/manufacturing image files for the fabrication of various dental
restorations.

4. • Streaking and beam hardening remain as ominous imaging artifact that
compromise CBCT utility in various case situations. However, because of the
popularity of CBCT, computer hardware and software developers, machine
manufacturers and dental researchers will continue to improve the applications
of this imaging modality for the betterment of patient care
• KEYWORDS
Cone beam computed tomography ,Flat-panel silicon detector ,DICOM viewer
software , Beam-hardening artifacts

5. INTRODUCTION
• Imaging with cone beam technology has rapidly become a popular and
frequently used imaging modality to assist dentists and other health care
professionals in a multitude of diagnostic tasks to improve patient care.
Cone beam imaging technology is most commonly referred to as cone
beam computed tomography (CBCT).
• The terminology “cone beam” refers to the conical shape of the beam that
scans the patient in a circular path around the vertical axis of the head, in
contrast to the fan-shaped beam and more complex scanning movement of
multidetector-row computed tomography (MDCT) commonly used in
medical imaging

6. • Two factors played a big part in the rapid incorporation of CBCT technology into
dentistry, the first of which was the availability of improved, rapid, and cost
effective computer technology. The second was the ability of software
engineers to develop multiple dental imaging applications for CBCT with broad
diagnostic capability

7. CBCTVERSUS COMPUTEDTOMOGRAPHY
• CBCT, by virtue of the terminology, is a form of computed tomography (CT). In a
single rotation, the region of interest (ROI) is scanned by a cone-shaped x-ray
beam around the vertical axis of the patient’s head. Digitized information of
objects in the ROI such as shape and density is acquired from multiple angles.
• These imaging data are then processed by specialty software that ultimately
constructs tomographic images of the ROI in multiple anatomic planes, namely
the standard coronal, axial, and sagittal anatomic planes and their various
paraplanar derivatives, the parasagittal, paracoronal and para-axial planes.

8. • Historically standard and more sophisticated form of CT, present since the
1970s, was developed in part by British engineer and Nobel Prize winner Sir
Godfrey Hounsfield.
• The technology of Hounsfield’s first scanner was followed by the development
of a larger body scanner by a group of researchers in the United States headed
by American dentist and physicist Robert S. Ledley. This more advanced form of
CT is known as MDCT, although other terms such as multislice CT and multirow
CT are used. Because MDCT is more commonly used in medicine, it is often
referred to as medical CT
• A more appropriate term for MDCT might be “conventional CT.” Differences
between CBCT and MDCT have been widely reporte

10. HISTORICAL DEVELOPMENT OF CBCT UNITS
• During the early development of CBCT, the technology was being advanced
primarily for the dental office . Early panoramic units were mainly sit-down, but
there was also a lay-down unit. Several other sit-down machines were
manufactured, but eventually units were made whereby the patient could stand
upright for the panoramic exposure. Upright machines became preferable, as it
is more convenient and takes less time to transfer patients into and out of these
stand-up panoramic units.
• One of the very first commercially available cone beam machines, the NewTom
9000 (QR srl, Verona, Italy), was a large unit that scanned the patient lying in a
supine position. It was followed by the NewTom 3G . These early NewTom units
eventually lost favor to smaller, sit-down chair units or to stand-up units. These
smaller units with better scanner quality more readily fit into dental office space
and overhead budgets

11. • CBCT units that scan patients in a supine position have made a comeback; the
NewTom 5G (QR srl) and the SkyView (MyRay, Imola, Italy) are currently
available. These units, has upright patient loading and supine position for patient
scanning. NewTom is also producing standing machines such as theVGi.

12. NewTom 3G.This supine CBCT scanner was
one of the first commercially available
units in North America
The Accuitomo 170 (J. Morita USA, Irvine, CA).
The Scanora 3Dx (Soredex, Milwaukee, WI).
The CS 9300 (Carestream Health, Rochester, NY)

13. The Orthophos XG
3D (Sirona USA,
Charlotte, NC)
The i-CAT FLX (Imaging
Sciences International,
Hatfield, PA)
The NewTom 5G in patient
entry (left) and patient scan
(right) positions.
The SkyView CBCT
scanner (MyRay,
Imola, Italy)

14. EFFECT OF FIELD OFVIEW ON SCANNERTYPE
• The size of the scanned object volume is called the field of view, commonly
abbreviated as FOV. The CBCT scanning controls are programmed to scan an FOV
of sizes and areas that are built into the scanner by the manufacturer. Other factors
that affect the FOV are the size and type of the detector and the degree of beam
collimation on the x-ray tubehead.
• The FOV for units with a flat-panel detector (see later description) is a cylindrical
shape in the center of the scanner between the detector and the x-ray source. The
range of commercially available FOVs for flat-panel detectors can be from 3.0 cm
(H) 3.0 cm (D) to 24 cm (H) 16.5 cm (D)

15. The FOV for image-intensifier detectors is shaped differently, not as a cylinder but
rather as a sphere.The dimensions are usually measured by the diameter of the
circular shape in inches (eg, 6”, 9”, 12”)
The size of the FOV significantly affected the evolution of the CBCT scanner. Early
CBCT units were restricted to a single-size FOV that was either large or small,
which limited the usefulness of the scanner.The general rule was the larger the
FOV, the greater the cost of the scanner.The FOV most typically included the
jaws, midface, and skull base
The FOV most typically included the jaws, midface, and skull base. Some had
options that included a more extended part of the skull toward the vertex, that is,
40 cm (H).

16. • Smaller FOV units large enough to image 2 to 4 teeth of a jaw (either maxilla or
mandible) was another earlier scanner option. The area covered in these smaller
volumes is adequate for a thorough 3-dimensional (3D) periapical evaluation of
selected teeth, alveolar bone, and a limited amount of maxillary or mandibular
basal bone
• Contemporary scanners are now capable of a range of FOVs from the smaller 3.0
cm (H) 3.0 cm (D), to the midrange FOVs for coverage of one or both jaws, to the
larger FOVs that include the cervical spine, jaws, more of the paranasal sinuses,
skull base, and parts of the cranium. Larger FOVs that include superior areas of
the skull are not usually indicated for most dental applications.

17. FEATURES OFTHE IMAGING PROCESS
• IMAGE CAPTURE
During a rotational scan of an object, multiple exposures are taken at fixed intervals
(angles) of the rotation. Each of these exposures is referred to as a basis image.
The images are standard radiographic images captured on the detector, and the signal
of each projection is unique for each of the different angles in the rotational arc.
The images are standard radiographic images captured on the detector, and the signal
of each projection is unique for each of the different angles in the rotational arc
Instantaneously the image data for each basis image are sent to a data-storage area so
that the detector can be cleared to capture the next basis image at a position interval
further along the rotational arc.

18. • Once the rotation is complete and all the basis images are made, the
complete set of basis images forms the “projection data.” The total number
of basis images taken depends on the radiographer’s preferences and the
scanner’s capability.
• This total ranges from 100 to 600 basis images per scan.The greater the
number of basis images, the longer the scan time, the greater the radiation
dose, and the better the quality of the constructed images.

19. Basis-image capture for a hypothetical CBCT rotational
scan of the cervical spine. Two basis-image capture
sequences are depicted in this diagram as the machine
rotates counterclockwise from Position 1 to Position 2.
An arrow depicts the counter-clockwise rotation.
(Modified from Zhen X, Yan H, Zhou L, et al. Deformable
image registration of CT and truncated cone beam CT
for adaptive radiation therapy.

20. IMAGING SOFTWARE AND DATA FILE
MANAGEMENT
• Image reconstruction software programs, usually proprietary to each machine
manufacturer, then manage the projection data and construct a 3D volumetric
data set
• These processed data are then accessed to construct various types of images for
display. The choice of images constructed depends on the power of the imaging
software and the needs and preferences of the clinician
• Depending on the capability of the software, there are multiple options of image
construction from the 3D volumetric data set. Most scanner programs display a
primary image reconstruction of the object in the 3 anatomic planes of imaging:
the axial, sagittal, and coronal planes

21. • These primary reconstruction displays are also referred to more typically as
the multiplane or multiplanar images.
• The same volumetric data set can be used to also construct multiple kinds of
secondary reconstructions. The choice of secondary reconstruction is often
task specific, and is also related to the reconstruction options within the
scanner’s proprietary software.
• At present, a variety of independent third-party imaging software is
commercially available for image reconstruction of CBCT volumetric data
sets. Third-party imaging software is software not associated with the
capture and proprietary software of the CBCT scanner

22. • If third-party software is being used, the file format of the volume set must be
converted from the proprietary file format or file language to a more universal
or common digital file format. This common format must be conformant with
the Digital Imaging and Communications in Medicine standard (DICOM
09v11dif); that is, the current DICOM standardized file format.
• This digital format is the International Organization for Standardization (ISO)
referenced standardized digital file format for medical images and related
information, namely ISO 12052. To facilitate access to health care, multiple
imaging modalities (x-ray, visible light, ultrasound, and so forth) used in
medicine and dentistry must be compliant with ISO 12052.
• Types of task-specific reconstruction capabilities of viewing software include,
but are not limited to, panoramic reconstructions, implant planning
reconstructions with 2-dimensional and 3D windows, temporomandibular joint
reconstructions, airway reconstructions, and so forth.

23. examples of multiplanar reconstructions.The upper example (A) is constructed by OneVolume viewer
software (J. Morita USA).The lower (B) reconstruction is by CS 3D Imaging Software (Carestream)

25. (B) Implant planning with 2D reconstructions
and a tracing of the mandibular nerve
(A) Two-dimensional (2D) panoramic reconstruction.
Although a CBCT scan is not indicated solely for
panoramic imaging, many imaging software
packages can reconstruct panoramic images from
the storage data.

26. (C) Implant planning with 2D/3-dimensional (3D)
reconstructions.
(D) Bilateral reconstructions of the
temporomandibular joints in coronal and sagittal
sections.

27. Sagittal reconstruction without (top) and with (bottom) Airway
Measurement tool from InVivo 5.2 imaging software
(Anatomage, San Jose, CA). When the airway is traced in the
airway measurement window, the program wizard computes
the volume of the airway space. Threshold values for
compromised airway volumes have not yet been determined
for this software

28. X-RAYTUBE AND GENERATOR SYSTEMS
• Although the time of exposure is usually an exposure control for an x-ray system, in
CBCT the time of the exposure is actually dependent on the number of basis images
and the degree of spatial resolution requested in the voxel size. The smaller the
voxel size and the greater the number of basis images, the longer the exposure.
• The major difference in a CBCT exposure compared with the exposure of intraoral
and panoramic imaging is that the CBCT exposure consists of capturing the series of
multiple basis images. Because of the process of basis-image projection, the x-rays
are not generated during the entire rotational path
• In most units, the exposure is pulsed at intervals so that there is time between basis-
image acquisition for the signal to be transmitted from the detector area to the
data-storage area and the detector to rotate to the next site or angle of exposure.

29. • These intervals may inherently reduce patient exposure during the time interval that
the detector is not ready to receive x rays. These intervals are also beneficial for the x-
ray duty cycle, reducing heat buildup during an exposure cycle.
• Time for the acquisition of basis images is known as the frame rate. For a shorter
exposure, the rotational arc remains the same but the frame rate is reduced. In this
scenario where less basis images are taken, the radiation exposure is less, the
rotational arc takes less time, and the scanner parts rotate faster. The clinician can
actually observe the slower or longer scan times necessary for longer exposures with
higher frame rates
• A data set with fewer basis images may be undersampled. Undersampled data sets
have a lower signal-to-noise ratio, and thus lack the range of image contrast capable
with a more complete volumetric data set.

30. • This shorter feature reduces patient exposure and is particularly helpful in
reducing motion artifact in scans of younger patients, geriatric patients, or those
with disabilities, during which motion artifact is more difficult for the patient to
control. These smaller data sets also have less computational and construction
time. With fewer data, they require less storage space
Sagittal temporomandibular
joint reconstruction from
projection data processed from a
full quota of basis images in the
projection data set
Sagittal temporomandibular joint reconstruction
from a shorter exposure scan that has fewer basis
images in the projection data and resulting
volumetric data set. There is less detail and contrast
resolution in the resulting image display than with
projection data from a full quota of basis images
used for construction of the volumetric data set.

31. • Exposure factors for a CBCT scan can be preset from an exposure selection guide, or
can be determined by automated features in the image-acquisition software from a
scout exposure.
• Some units may have a direct automated exposure feedback feature in the detector
that determines the exposure factor for more optimal signal detection
• CBCT units that scan larger object areas (larger FOV) generally need higher kV
potentials. The higher kV is necessary for adequate penetration of denser and larger
anatomic structures in the maxilla, midface, and skull base. Consequently, higher kV
is often necessary for adequate diagnostic quality of the larger FOV data sets.

32. IMAGE SENSOR SYSTEMS
• Two types of image detectors are used as the sensors in contemporary CBCT units. A
scanner will have either (1) a charge-coupled device with a fiber-optic image
intensifier, or (2) an amorphous silicon flat-panel detector
• During the initial introduction of CBCT, most units were constructed with the large,
bulky image-intensifier detectors. In the latter half of the first decade of commercial
CBCT development, CBCT scanners have nearly all transitioned to the smaller, flat
panel linear array detectors. Sirona and MyRay still manufacture scanner units with
this type of detector.

33. The Galileos has a charge-coupled image
intensifier detector
The Orthophos XG 3D unit (bottom) has the
smaller flat-panel detector.

34. • The image-intensifier detectors are larger and make the scanners’ overall
dimensions larger, which may be critical for certain office designs. They are more
sensitive and susceptible to distortion from magnetic fields.
• In addition, the phosphors in image intensifiers lose their sensitivity over time and
use, and the entire image-intensification unit may need to be replaced to maintain
image quality. This process is very expensive. Despite the drawbacks, in certain
cases the data sets from these detectors are more compatible with “bridging” to
some of the data sets used in computer-aided design and manufacturing
(CAD/CAM) technology, and thus remain useful.

35. • The flat-panel detectors are thin, amorphous silicon transistor panels with a cesium
iodide scintillator. The scintillator is the part of the detector used to amplify the
electrical signal from the x-ray attenuation.
• Besides being smaller and less bulky, the flat panels have minimal distortion of the
image dimensions at the periphery of an image display ; hence, these units are
considered to generate better data sets. Because these detectors are smaller than
their image-intensifier predecessors, CBCT units with the flat-panel detectors have
smaller footprints. This feature alone had made the flat-panel detector more
popular.
flat-panel detector
coupled device image
intensifier

36. Distortion patterns produced by image detectors.
(A) Grid type X is the type of grid distortion pattern
produced by the image-intensifier detector that affects
the image construction and is subsequently noted in the
image display. There is distortion of the image grid when
moving away from the center.
(B) With flat-panel detectors (ie, grid type Y) the image
receptor area receiving the signal from the flat-panel
detector’s scintillator is flat. Therefore, even at more
distant areas from the center of the grid, there is
minimal to no distortion of the grid pattern.

37. • Another property of the image detector is the bit depth, an exponential binary
property expressing the total number of gray shades. A 14-bit detector (ie, 214) can
display 16,384 shades of gray. The range of bit depth of commercial CBCT units
ranges between 12 and 16 bits , indicating the wide range of contrast discrimination
capability
• Although the detector is capable of this degree of gray-scale discrimination,
limiting features to the contrast resolution include the lower bit depth of the
imaging software and the monitor display, and the visual perception of the viewing
clinician.
• Even though bit depth is important for contrast resolution, the American College of
Radiology has concluded that there is no added benefit to diagnostic
interpretations by the use of higher than 8-bit depth in the workstation’s operating
system

38. SCATTER AND BEAM-HARDENING ARTIFACT
• Scatter and beam-hardening artifact occurs in CT imaging where image
reconstructions of a data set are necessary for review of the data volume.
• Dense metal structures frequently in the FOV for dental applications present
metal artifact on CBCT reconstructions. Silver amalgam, precious and
semiprecious metal alloys used in coronal restorations, dental implants, silver-
point endodontic fillings, and, to a lesser extent, gutta percha endodontic fillings,
all create these artifacts in image reconstructions.
• The artifact presents as light or dark streaks, or as a dark periphery adjacent to
metallic borders.

39. • Scatter artifact is seen as radiopaque lines and patterns of metallic density that
“scatter” on image reconstructions. The main types of beam hardening are the dark
streaks or dark bands that show up in the image reconstructions.
• The latter often simulate disease such as recurrent caries or fractures in
endodontically treated teeth. The light streaks often superimpose regular anatomy,
and may also significantly degrade image quality
• These artifacts are prominent problems for dental applications with CBCT, as
metallic restorations are often within the FOV of most CBCT scans of dental
patients.
• The metallic restorations then cause the resultant beam hardening and streak
artifact, which then compromises the image quality with the various areas of dark
and light artifact

40. Beam hardening and streak artifact in CBCT
image reconstructions.
(A) Axial section with dental implant in #18 region
highlighted by black arrow.
(B) Beam-hardening artifact is indicated by red
arrows.The green arrows depict streak artifact.
(C) The locations of cross sectional and parasagittal
reconstructions are shown.
(D) The effect of beam hardening simulating peri-
implantitis and alveolar bone defects in the cross-
sectional and parasagittal reconstructions.

41. (E) The effect of streak artifact creating the outline of a
“ghost” implant (as well as other radiopaque streak outlines) in
the cross-sectional reconstruction. The streak artifact makes it
more difficult to discern the validity of the cortical bone
outlines.

42. • Recent attempts via software correction algorithms have been reported that have
the potential to control these visible artifacts on image reconstruction. However, the
application of software correction modes to reduce these artifacts have been inferior
to non corrected software viewing programs when evaluating peri-implant and
periodontal diseases as well as root fractures.
• Consequently, there are no immediate methods to correct or minimize these
prominent artifacts. The best way to avoid streaking and beam hardening is to try to
keep the FOV as small as possible in an attempt to minimize or keep these metals
outside the FOV. In so doing, one may be able to minimize their impact on image
reconstructions.

47. Criteria for use of CBCT in Endodontics
A CBCT examination should only be considered after a detailed clinical
examination, including conventional radiographs, has been performed
(Kruse et al. 2015, Patel et al. 2019a). The potential benefits as well as
potential risks must be discussed with the patient beforehand. Even
though the effective dose is relatively low, CBCT must be used judiciously

48. • In those cases in which lower dose conventional radiography does not provide
sufficient information for confident diagnosis a small FOV CBCT examination
should be considered if the additional information from reconstructed three-
dimensional images is likely to aid diagnosis and treatment planning and/or
enhance clinical management examples include the following:
• detection of radiographic signs of periapical pathosis when the signs and/or
symptoms are nonspecific and plain film imaging is inconclusive;
• assessment and/or management of dento-alveolar trauma, which may not be
fully appreciated with conventional radiographs;
• appreciation of anatomically complex root canal systems prior to endodontic
management (e.g dens invaginatus);

49. • nonsurgical re-treatment of cases with possible untreated canals and/or
previous treatment complications (e.g. perforations);
• assessment and/or management of root resorption, which clinically appears
to be potentially amenable to treatment;
• presurgical assessment prior to complex periradicular surgery (e.g. large
periapical lesions in posterior teeth, and the evaluation of their proximity to
adjacent relevant anatomical structures);
• identification of the spatial location of extensively obliterated canals, also
taking into account the possibilities of guided endodontics;
• detection of periradicular bone (secondary) changes indicative of root
fractures, when clinical examination and conventional imaging modalities are
not conclusive

50. SUMMARY
• CBCT is now a well-accepted diagnostic tool for the care of dental patients. Design
changes in the evolution of contemporary CBCT scanners include making the units
smaller, and making changes whereby instead of needing to be scanned in a supine
position, the patient either sits or stands upright during the scan.
• Along with these design changes, better stabilization devices for the patient’s head and
chin were produced. Mechanical changes included the switch to smaller, flat-panel
silicon detectors with better image quality compared with the bulkier, cumbersome,
and eventually more costly image-intensifier detectors.
• Variable kV and multiple options for voxel size, FOV dimensions, scan times, and so
forth, then followed, alongside more powerful software applications for the care of
dental patients.

51. • The ability of CBCT manufacturers to use various aspects of imaging technology in a
cost-effective, efficient, and practical manner means that there are now numerous
CBCT applications that are helpful in a multitude of dental disciplines. These
applications include, but are not limited to, dentoalveolar abnormality, vertical root
fractures, jaw tumors, prosthodontic evaluations, and advances in orthodontic/
orthognathic and implant patient evaluations.
• The latter also include mechanisms for surgical and prosthodontic splint design and
the capability of CBCT scan data to bridge with CAD/CAM image files for fabrication
of various dental restorations. This approach facilitates implant and prosthodontic
rehabilitation by synchronously planning and subsequently milling coronal
restorations for teeth and root form implants. As the demand for CBCT technology
continues to increase, so will the number of new applications for improved
diagnostic techniques

52. REFERENCES

55. CRITICAL APPRAISAL
• TITLE: BASIC PRINCIPLES OF CONE BEAM COMPUTEDTOMOG RAPHY
evokes interest in reader
• AUTHORS: Kenneth Abramovitch, DDS, MS*, Dwight D. Rice, DD
• NAME OF JOURNAL: Dental clinics of North America
• TYPE OF ARTICLE : Review article
• ABSTRACT: not mentioned
• INTRODUCTION: Meaningful and explanatory
• KEYWORDS : Mentioned
• CONCLUSION: Mentioned