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CBCT and MRI Data Acquisition as a Basis for Computer-Assisted Maxillofacial…
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13
2 Invention ofCone Beam Computed Tomography (CBCT)
The scope of dental imaging and digital dentistry has been greatly expanded with
the invention of CBCT. This 3D modality, also known as cone beam volumetric
imaging (CBVI) and cone beam volumetric tomography (CBVT), is the most signicant technologic advancement in maxillofacial imaging. After justifying any
radiographic image, one should peruse the rule of optimization consisting “ALARA”
(As Low As Reasonably Achievable) to reduce any unnecessary patient exposure.
CBCT benets from lower dose and cost compared to MDCT imaging. The radiation dose varies based on the applied imaging protocol (exposure parameters, e.g.,
mA, kVp), eld of view (FOV), and resolution preference (standard or high); however, even in extended or craniofacial FOVs, the dosage is substantially lower than
MDCT (Table1) [5]. In addition, ultralow-dose CBCT scan manufacturers use a
radiation dose comparable with a single plain radiograph.
Currently, there are 279 CBCT models from 47 manufactures available commercially with applications not limited to the skull, such as cardiac imaging, radiotherapy, extremities, and peripheral bone imaging (Table2). Maxillofacial CBCT units
can be classied according to the orientation of patient during image acquisition:
standing, seated, and supine (Fig.2) [6]. Standing units, like a panoramic machine,
are the most common type; however, these units are prone to motion artifacts and
cannot be adjusted for lower heights especially in disabled patients in wheelchair.
Supine units, such as NewTom 7G (Verona, Italy), provide higher patient stability
and greatly reduce patient motion artifacts. In addition, these types of units allow
accurate assessment of airway, especially in micro-gnathic patients with obstructive
sleep apnea. However, supine units are physically large with a bigger footprint consuming more area. In all settings, the patient’s head should be completely
immobilized.
Table 1 Comparison of effective dose from CT examinations. Note the higher effective dose of
MDCT compared to CBCT
Imaging modality
CBCT Small FOV
MDCT Maxillofacial
CBCT cone beam computed tomography, d days, FOV eld of view, M month, MDCT multidetector computed tomography, msv millisievert, μSv micro-sievert
Examination Median effective dose
Medium FOV
Large FOV
Head 2msv 8M
50µSv
100µSv
120µSv
650µSv
Equivalent background
exposure (d, M)
6 d
12 d
15 d
2M
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M. Ghazizadeh Ahsaie
Table 2 Selected maxillofacial CBCT machines available with large detector size, providing
large FOV for craniofacial assessments. Note that large FOVs generally have lower spatial resolutions compared to small FOVs
Manufac-
Model
Viso G7 Planmeca
NewTom
7G wide
vision
NewTom
VGi EVO
3D Accuitomo 170
i-CAT
FLX17
CS 9600 Carestream
SCANORA
3Dx
i3D- Premium
Green 21
NewTom
VGi
GALILEOS Comfort PLUS
KaVo OP
3D Pro
Vision
turer
(Helsinki,
Finland)
Quantitative radiology
(Verona,
Italy)
Quantitative radiology
(Verona,
Italy)
J.Morita
(Kyoto,
Japan)
Kavo
imaging
(Hateld,
PA)
dental,
(Atlanta,
GA)
Soredex
(Tuusula,
Finland)
VATECH
(Gyeonggido, Korea)
Quantitative radiology
(Verona,
Italy)
Dentsply,
Sirona
(Bensheim,
Germany
KaVo
imaging
(Hateld,
PA)
X-ray
Unit
type
Stand upFixed
Supine Rotat-
Stand upRotat-
Seated Fixed
Seated Fixed
Seated
or
stand
up
Seated Fixed
Seated Fixed
Stand upRotat-
Stand upFixed
Stand upFixed
gen-
erator
anode
ing
anode
ing
anode
anode
anode
Fixed
anode
anode
anode
ing
anode
anode
anode
Focal
spot
size
(min–
max)
(mm)
0.5 1–36 200–
0.3–
0.6
0.3 15–25360 100–
0.5 5.4–
0.5 4.8–
0.3–
0.7
0.5 18–34360 100–
0.5 18 360 200–
0.3 18–26360 75–
0.5 14 204 125–
0.5 11–42360 85–
Scan
Scan
technol-
time
ogy
min–
(degree
max)
of rota-
(s)
tion)
360
7.2–26Partial
or complete
360
180–
17.5
360
360 125–
26.9
5.5–40360 75 4×4–16×17
Voxel
size
(min–
max)
(𝞵m)
75–
600
90–
500
300
80–
250
400
500
400
300
250
420
FOV D×H
(min– max)
(cm)
3×3–30×30
4×4–29×56
5×5–24×19
4×4–17×12
8×5–17×23
5×5–25×16.5
8×8–21×19
6×6–15×15
15×15
5×5–13×15
D diameter, FOV eld of view, H height, max maximum, min minimum
abc
CBCT and MRI Data Acquisition as a Basis for Computer-Assisted Maxillofacial…
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Fig. 2 Various types of CBCT machines. NewTom 7G wide vision (Verona, Italy) supine CBCT
unit (a). The patient is reclined on back and further inserted into the gantry. The supine position
reduces patient motion artifact and enables accurate assessment of the airway in patients with sleep
apnea disorder. Planmeca Viso 7G (Helsinki, Finland) standing CBCT unit (b). Soredex
(SCANORA 3Dx, Tuusula, Finland) seated CBCT unit (c). The machine turns around the patient’s
head like a panoramic imaging system (b, c)
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3 Principles ofImage Production
CBCT image production consist of three major steps: 1- X-ray generation, 2- X-ray
detection, and 3- image reconstruction. A divergent cone-like or pyramidal X-ray
source is directed to the specic region of interest (ROI) and the remaining beam
(attenuated, scattered, and with no interaction) reaches the detector on the other
side. Different image detectors could be used in CBCT machines, the most common
is cesium iodide/amorphous silicon at panels (CsI/a-si FPD). A series of raw data
(also known as basis frame) are produced by a single 180° to 720° rotation of gantry
around the patient’s head [7]. The number of raw images vary from 100 to 1000.
Each frame is similar to a 2D cephalometric radiography, each slightly offsets from
the next. The projection data is primarily reconstructed to three orthogonal planes
of the axial, sagittal, and coronal planes. A secondary reconstruction is further
applied to provide multi-planar reformat (MPR) and volumetric rendering (VR)
images (Figs.3 and 4) [8].
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M. Ghazizadeh Ahsaie
MPR
Reformatted Panoramic
Axial
Cross-sectional
Coronal
Orthogonal projections
Sagittal
Ray sum
VR
IVR
MIPSurface renderin
DVR
Fig. 3 Cone beam computed tomographic volumetric data reconstruction. Primary reconstruction
of raw data resulting in orthogonal planes (axial, coronal, and sagittal). Display modes can further
be divided into two categories: multi-planar reformation (MPR), such as reformatted panoramic
and cross-sectional views, and volumetric rendering (VR), which can be further divided into two
categories of direct (DVR) and indirect VR (IVR)
CBCT and MRI Data Acquisition as a Basis for Computer-Assisted Maxillofacial…
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abc
de f
gh i
17
Fig. 4 Various CBCT 3D volumetric renderings. (a) Maximum intensity projection (MIP) which
is a “pseudo”-three-dimensional image representing the highest pixel value along an imaginary
projection ray. (b) A full-thickness ray sum simulating lateral cephalometric image. (c) Surface
rendering of bone as a solid surface or shaded surface display. (d) Surface rendering of soft tissue,
bone, and airway. (e) Paranasal sinuses and airway view transparent bone. (f) Soft tissue prole
view. (g) Color shaded surface bone rendering; note that the patient has a lesion in anterior mandible. (h) Spectral colored 3D view; higher X-ray attenuating structures, such as titanium implant
xture, crown, and enamel, are provided in blue hue. Lower attenuating structures such as bone
and roots are shown in green hue. (i) MIP image with indication of the major nerves: inferior
alveolar, nasopalatine, infraorbital, and pterygopalatine nerves
4 CBCT Resolution andVoxel Size
CBCT resolution can be divided into two categories, contrast resolution and spatial
resolution. The ability to show difference in photon attenuation in gray values is
called the contrast resolution of system. CBCT has poor soft tissue contrast compared to MDCT and therefore cannot assess soft tissue of maxillofacial region in
detail. This limitation is mainly due to presence of artifacts and inherent noise of
FPD detectors.
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M. Ghazizadeh Ahsaie
The volumetric data in CBCT are in the form of isotropic voxels. The smaller the
voxel size, the higher detail is presented in the imaging, which directly effects spatial resolution. These result in a greater spatial resolution (0.4–0.076mm), often
exceeding some of the highest grade MDCT scans. Most CBCT machines have the
ability of adjusting to at least two voxel sizes, one for the standard resolution and
one for high-resolution scans. The focal spot size also affects the spatial resolution
with smaller focal spots, higher resolution is obtained. High-resolution scans are
specially needed in 3D reconstruction of delicate anatomic structures, such as the
orbital oor, lamina papyracea, and anterior wall of maxillary sinus [9].
5 Field ofView (FOV)
The anatomical volume being assessed is controlled by FOV, also known as scan
volume, which contains the region or regions of interest the clinician needs to
assess. The dimensions of FOV depends on detector size, beam projection geometry, and collimation. This dimension should be selected based on patient’s anatomical size and prescription (Table 3). The correct choice of FOV size reduces
unnecessary patient dose and minimizes scatter radiation, consequently improving
the image quality. Most CBCT units are classied based on the largest dimension of
FOV (Tables 2 and 3).
Table 3 FOV option size and applications provided by various CBCT machines
FOV size (Cm)
FOV Type
Extended 23×17 • Full skull
Extra large 15×15 • Bimaxillary orthognathic
Large 15×12 • Multiple dental implants in both
3D surface rendering
(D×H)
Applications
• Craniofacial
• Cervical vertebrae
• Paranasal sinus
• Airway
surgeries
• Maxillofacial trauma
• Paranasal sinus
• Bilateral TMJ assessment
jaws
• Bimaxillary orthognathic
surgeries
• Maxillofacial trauma
• Bilateral TMJ assessment
CBCT and MRI Data Acquisition as a Basis for Computer-Assisted Maxillofacial…
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Table 3 (Continued)
FOV size (Cm)
(D×H)
8×12 • Multiple dental implants in both
8×8 • Dentoalveolar assessment of one
Applications
jaws
• Full mouth assessment
• Assessment of impacted third
molars
jaw (maxilla or mandible)
• Assessment of multiple impacted
teeth, multiple implants in both
left and right side in a single jaw
FOV Type
Medium
Small
3D surface rendering
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Extra small
Cm centimeters, D diameter, 3D three-dimensional, FOV eld of view, H height
6×6 • Localized region of maxilla or
mandible
• Limited dentoalveolar trauma
• Root fractures, luxation, and/or
displacement of teeth
• Endodontic assessment, potential
for extra canals, complex
morphology root or crown, and
dental abnormalities
• Impacted tooth
• Single dental implant
For large FOVs, larger detector size is required; due to the higher cost of large
FPD detectors, some machines apply stitching or bio-image registration, in which
two or more FOVs are integrated vertically or horizontally to provide the needed
anatomy.
Larger FOVs provide information on craniofacial anatomy in cases in need of
orthognathic surgeries or reconstruction prosthesis and are generally applied with
standard resolution protocols to further control patient radiation dose. To enable
scanning ROI larger than FOV of the detector, data can be obtained from two or
more scans and further superimposition or fusion of volumetric data using ducial
markers (bio-image registration). In addition, some software can fuse adjacent volumetric scans to provide larger volumetric data.
Small FOVs are usually applied in high resolution to present higher detail especially in endodontic cases in search of a missing or obliterated root canals and vertical fractures. Although each FOV can be cropped or segmented into a smaller FOV,
to adjust patient dose, FOV should be compatible with clinical indications and
patient size.
Segmentation, meaning separation of area of interest, can be performed in any
FOV using three methods: 1- manual, 2- semiautomatic, and 3- fully automatic
segmentation. Segmenting an outline of desired anatomy is an essential step in generating 3D models especially in treatment planning of patients with craniofacial
deformities [10] (Fig. 5) (See chap “Data Storing and Conversion in ComputerAssisted Oral and Maxillofacial Treatments”).
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Fig. 5 Automatic segmentation. (a) Segmentation of paranasal sinuses and airways using
OnDemand application (Cybermed, Seoul, Korea). (b) Segmentation of maxillofacial area to maxilla (yellow), mandible (green), teeth (white), and airway (teal) using Diagnocat application
(Diagnocat Inc., USA). This masking process results in better visualization of the maxillofacial
area and can provide ready-made models for printing on a 3D printer
M. Ghazizadeh Ahsaie
Fig. 6 Selected CBCT artifacts. (a) Cone beam artifact at the superior aspect of image due to
inadequate data in reconstruction, (b) aliasing artifact causing ne alternating hypodense and
hyperdense stripes in the posterior periphery of volumetric mage, (c) motion artifact resulting in
double cortical border in mandible, and (d) beam hardening and metallic artifact
CBCT and MRI Data Acquisition as a Basis for Computer-Assisted Maxillofacial…
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21
6 CBCT andArtifacts
CBCT artifact is the fundamental factor impairing image quality. CBCT inherently
has higher artifacts than MDCT, due to a lower X-ray energy spectrum and cone
beam geometry. However, metallic artifacts are lower in CBCT.Various types of
artifacts such as inherent artifacts, procedure-related artifacts, introduced artifacts,
and patient motion artifacts may be present in the image (Fig.6). Artifacts may
interfere with the diagnostic process; therefore, every clinician should be aware of
their presence.
7 Clinical Reflections
CBCT imaging is now applied in all areas of dentistry such as diagnosis and management of impacted teeth, implantology, temporomandibular disorders, traumatology, pathological lesions (e.g., inammatory conditions, cysts, benign or malignant
tumors, paranasal sinus disorders, and soft tissue calcications and ossications),
orthognathic surgery, cleft palate deformities, obstructive sleep apnea patients, and
surgical navigation (Fig.7) [11]. It is assumed that CBCT can replace MDCT in
most maxillofacial diagnostic, surgical planning, and follow-up needs. The greatest
impact of CBCT is in dental implant surgeries. Cross-sectional images provide
information on alveolar bone height and width and accurately provides distance to
inferior alveolar nerve in the mandible, nasal, and sinus oor in maxilla (Fig.8).
CBCT has a major role in computer-assisted surgeries (CAS) and additive manufacturing (AM). The obtained volumetric data can be used in the two main categories: computer-assisted presurgical planning and navigation (See chap “Data Storing
and Conversion in Computer-Assisted Oral and Maxillofacial Treatments”) [12].
Computer-assisted presurgical planning includes preoperative surgical simulation
with 3D images (Fig.9) or models (Fig.10). Preoperative surgical simulations with
3D images are used to determine the appropriate position, angulation, and size of
dental implants noting the ridge anatomy, quality, and landmarks prior to insertion [13].
Recent advances in the elds of computer-assisted orthognathic surgery planning
and resection surgeries, due to the presence of intrabony pathologic lesions, use
software incorporating the DICOM (Digital Imaging and Communications in
Medicine) data from CBCT and have provided a valuable tool assisting in the diagnosis, treatment planning, and evaluation of treatment outcomes of maxillofacial
deformities [1]. CBCT is by far the most used modality for digitizing CAD models
for AM purposes for 3D bone imaging, as bone provides an excellent contrast and
allows for high spatial accuracy to be pursued by utilizing “sharp” reconstruction
kernels [14]. (Fig.10).
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M. Ghazizadeh Ahsaie
a
d
ef
g
h
Fig. 7 Selected applications of CBCT in the eld of dentistry. (a) MIP view of a patient with his-
tory of bimaxillary orthognathic surgery shows the locations of the pin and plates and sites of
osteotomies. (b) Coronal MPR follow-up image of patient with history of complete maxillectomy
due to the presence of a malignant lesion and reconstruction of left orbital oor with mesh prosthesis. (c) Axial CBCT accurately locates the position of the impacted supernumerary premolar. (d)
Cross-sectional image indicates the presence of a cystic radiolucent pathologic lesion at the apical
region of upper lateral incisor. (e) Cross-sectional view showing implant xture fracture. (f) Axial
view indicates complete root canal calcication of left central incisor due to the history of trauma.
(g) Reformatted panoramic view shows relation of impacted mandibular third molar to the inferior
alveolar nerve canal. (h) Reformatted panoramic of a patient captured with dentures in occlusion
with radiopaque markers for further scan registration and implant guide reconstruction
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