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Comparison between tooth mesiodistal angulation measurements from constructed panoramic images and three-dimensional volumetric images

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Comparison between tooth mesiodistal angulation measurements from constructed panoramic images and three-dimensional volumetric images

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Content COMPARISON BETWEEN TOOTH MESIODISTAL ANGULATION MEASUREMENTS
FROM CONSTRUCTED PANORAMIC IMAGES AND THREE-DIMENSIONAL
VOLUMETRIC IMAGES

by

Nicole Sakai

A Thesis Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(CRANIOFACIAL BIOLOGY)

May 2011

Copyright 2011 Nicole Sakai

ii
Dedication

This thesis is dedicated to my family. Without their continued love and support, I would
not have been able to accomplish the success that I have had in my life and career.

iii
Acknowledgements

Dr. Hongsheng Tong: To my research advisor, thank you for your guidance with this
project. Your extensive knowledge and enthusiasm on this topic was extremely helpful.

iv
Table of Contents

Dedication ii

Acknowledgements iii

List of Tables iv

List of Figures v

Abstract vii

Chapter One: Introduction 1

Chapter Two: Literature review 3
Importance of mesiodistal inclination of teeth 3
Panoramic radiography 8
Overview 8
Accuracy and distortions 11
CBCT 14
How CBCT works 16
Image resolution: pixel and voxel size 18
Radiation dosage 19
Accuracy 19
Limitation 21
DICOM 22

Chapter Three: Hypothesis 24

Chapter Four: Materials and methods 25

Chapter Five: Results 41

Chapter Six: Discussion 49

Chapter Seven: Conclusion 59

References 60

v
List of Tables
Table 1: Average age of the sample 38

Table 2: Sex of the sample 38

Table 3: Ethnicity of the sample 38
Table 4: Overall ICC 41
Table 5: ICC by tooth 41

Table 6: Right vs. left measurements 43
Table 7: Maxillary measurements 43

Table 8: Mandibular measurements 45

Table 9: Maxillary measurements above 2.5° 46

Table 10: Single vs. separately constructed panoramic images 47

Table 11: Mandibular measurements above 2.5° 48

vi

List of Figures

Figure 1: Skull with normal occlusion 3
Figure 2: Definition of Andrew’s FA point and FACC 4
Figure 3: Example of Andrew’s FACC for different teeth 4
Figure 4: Long axes of teeth 5
Figure 5: ABO Panoramic image 6
Figure 6: ABO root parallelism 7
Figure 7: ABO root parallelism 7
Figure 8: Movement of the film and objects 9
Figure 9: Movement of the film and x-ray source 9
Figure 10: Movement of the film and x-ray source 10

Figure 11: Focal trough 10
Figure 12: Movement of the x-ray source and beam 11
Figure 13: Traditional CT vs. CBCT 16
Figure 14: Conventional CT vs. CBCT 16
Figure 15: Isotropic voxels vs. Anisotropic voxels 18
Figure 16: Image quality 23
Figure 17: Image of all 3 planes 27
Figure 18: 3D- Point placed at center of the crown 28
Figure 19: 3D- Point placed at center of the root 29
Figure 20: 3D- Digitized points 29
Figure 21: 3D- Determining the tip 31
vii
Figure 22: 3D- Determining the torque 32
Figure 23: 3D- Determining the mandibular arch form 33
Figure 24: 3D- Determining the maxillary arch form 33
Figure 25: 2D- Constructing the panoramic image 35
Figure 26: 2D- The transverse plane is set to the cervical margin 35
Figure 27: 2D- Plotting the points 36
Figure 28: 2D- Setting the thickness 36
Figure 29: 2D- 3-point measuring system 37
Figure 30: 3D vs. 2D for the maxillary teeth 44
Figure 31: 3D vs. 2D for the mandibular teeth 45

viii
Abstract
The purpose of this study was to evaluate if there are differences between
mesiodistal angulations measurements for all the teeth in constructed 2D panoramic
images as compared to 3D measurements. Pre-treatment 3D DICOM data was obtained
from the University of Southern California’s patient database and mesiodistal angulation
measurements from the constructed 2D panoramic images were compared to 3D
coordinate system measurements (gold standard). Thirty three cases were chosen based
on the specified inclusion and exclusion criteria. Results indicated that there was a
relative trend in the differences in the tooth mesiodistal angulation measurements
between the 3D and 2D measurements (p<.007). The maxillary central incisors, canine,
and first premolar showed significant differences in the measurements, while the lateral
incisors, second premolars, and molars did not show significant differences. On the other
hand, the mandibular incisors, canine, and premolars did not show significant differences
in the tooth mesiodistal angulations, while the molars did show a significant difference in
the measurements. From this study, it can be concluded that there are significant
differences for some teeth between angulation measurements obtained from the 2D
constructed panoramic images and those directly obtained from 3D measurements.

1
Chapter One: Introduction
Orthodontics is a specialty of dentistry that is concerned with the study and
treatment of malocclusions, which may be a result of tooth position irregularity,
disproportionate jaw relationships, or both. The specialty of orthodontics has continued to
evolve since its advent in the early 20
th
century. In the 1890’s Edward H. Angle
published his classification of malocclusion based on the occlusal relationships of the
first molars. This was a major step toward the development of orthodontics because his
classification defined “normal occlusion”. Angle then helped to pioneer the means to
treat malocclusions by developing orthodontic appliances. Following Angle, orthodontic
treatment, mechanics, and appliances have continued to evolve with the development of
principles from Charles Tweed and Lawrence Andrews.
Dr. Andrews studied 120 people with optimal tooth alignment and occlusion and
found that the positions of the same type of teeth among different individuals fall within a
narrow range. He also found six common features or “six keys” that were shared by those
optimal natural occlusion cases. Based on these six keys, he developed and designed the
Straight-Wire Appliance (SWA), which has built-in dimensional and angulation features
for each tooth. Following that, many orthodontists have adopted various types of fully-
programmed appliances and have developed their own bracket systems with specific
prescriptions, treatment philosophies, and mechanics.
Although there have been constant changes in diagnosis, treatment philosophy,
mechanics, and appliances, orthodontic treatment objectives have generally remained the
same. The main treatment objective can be stated as obtaining ideal functional occlusion,
2
esthetics, and stability. One of the criteria for obtaining a functional occlusion is to have
ideal inclinations of all teeth in all three planes of space at the end of active treatment.
For a long time, the only way to see the whole tooth including the root to judge the
inclination of teeth was to take two-dimensional panoramic radiographs.
In 1948, Paatero developed panoramic radiography from the medical process of
laminagraphy, or bodysection radiography. The principal advantages of this radiographic
technique are the broad anatomic region imaged, the relatively low patient radiation dose,
and the convenience, ease, and speed of the procedure. However, the disadvantages of
panoramic radiography include its lack of fine detail compared with intraoral films and
the variable magnification and geometric distortion that are inherent in image generation.
However, with the advent of the cone beam computed tomography (CBCT) and 3D
software, the oral facial complex can accurately be reproduced on to a computer screen,
and unlike a 2D image, distortion, magnification and superimpositions can be completely
avoided.

3
Chapter Two: Literature Review
Importance of Mesiodistal Angulation of Teeth
A fundamental goal in comprehensive orthodontic treatment is the proper
angulation of all teeth in three planes of space. As illustrated by Andrews in 1972, ideal
occlusion and proper articulation are difficult to obtain without adequate mesiodistal
angulation of all teeth.
1-3
Proper mesiodistal angulations are necessary for distributing
occlusal forces through tight interproximal contacts and are an important factor in
maintaining a stable treatment result (Figure 1).
4-6
This has special significance in
orthodontically closed extraction sites, which are prone to open if the adjacent teeth are
not parallel.
7-9
Furthermore, if roots are well positioned, there will be sufficient bone
between adjacent teeth.
10-13

Figure l. Normal occlusion illustrating varying degrees of mesiodistal angulation of
teeth.
6

4
From his historic study of 120 casts of nonorthodontic patients with normal
occlusions, Andrews stated that the proper mesiodistal angulation (tip) and facial-lingual
inclination (torque) are required for ideally positioned teeth.
1-3
Andrews used the long
axis of the central developmental lobe of each crown, defined as the facial axis of clinical
crown (FACC) and 2) the center point of the FACC, or the FA point (Figure 2-3).

Figure 2. The FACC drawn through the central developmental lobe of the clinical crown.
The FA point is determined as the center of the FACC. Examples of positive and negative
crown angulation are shown relative to a line drawn perpendicular to the occlusal plane.
1

Figure 3. The FACC drawn for maxillary and mandibular incisors, canines, bicuspids,
and molars.
1

5
However, there is inherent difficulty in determining the location of FACC. This
difficulty may be due to 1) the excessive wear of the surface structure, making it hard to
identify the central developmental lobe used to define FACC; 2) the buccal surface of
some of the teeth taking the round shape rather than spear or shovel shape, making it hard
to visualize the long axis especially if the tooth is angulated; and 3) the crowns partially
being covered by the gums. In addition, the clinical crown can be short relative to the
root, and a slight error in FACC angulation will send the root far off its proper position.
Lastly, the root long axis does not often follow the direction of the long axis of the
crown, and proper angulation of the crown does not guarantee proper angulation of the
root (Figure 4).
14

Figure 4. Long axes of roots are not parallel with long axes of crowns.
6

Since the FACC and FA points are hard to locate and may not necessarily
represent the long axis of the whole tooth, the alignment of the whole tooth, including the
roots, should be used. Root position has traditionally been assessed by using conventional
6
panoramic radiographs taken before, during, and after treatment. In a 2002 survey of
orthodontic diagnosis and treatment procedures of American orthodontists, 57.9% and
79.1% of the respondents reported taking progress and posttreatment panoramic
radiographs, respectively.
15
Furthermore, panoramic radiography is recommended by the
American Board of Orthodontists to asses root angulation and parallelism as one part of
their objective grading system for cases to become a board certified orthodontist. Based
on the 2002 criteria, according to the American Board of Orthodontists, the roots of the
maxillary and mandibular teeth should be parallel to one another and oriented
perpendicular to the occlusal plane (Figure 5).

Figure 5. Panoramic radiograph showing root parallelism.
16

If this situation exists, or if a deviation of the apex is 1 mm or less, then no points are
subtracted. If a root is angled to the mesial or distal on the panoramic radiograph, and if
the discrepancy is mild with the apex of the affected tooth greater than 1 mm but less
7
than 2 mm from its ideal relationship (Figure 6), then 1 point is subtracted for that tooth.
If the discrepancy is greater than 2 mm (Figure 7), then 2 points are subtracted for that
tooth.
16

Figure 6. Section of panoramic radiograph showing deviation of the apex more than
1mm.
16

Figure 7. Section of panoramic radiograph showing deviation of the apex more than
2mm.
16

8
Panoramic Radiographs
Overview
Panoramic imaging is a technique for producing a single tomographic image of
the facial structures that includes both the maxillary and mandibular dental arches and
their supporting structures. In 1948, Paatero developed panoramic radiography from the
medical process of laminagraphy, or bodysection radiography.
17
It is a curvilinear variant
of conventional tomography and is also based on the principle of the reciprocal
movement of an x-ray source and an image receptor around a central point or plane,
called the image layer, in which the object of interest is located. (Figure 8-10) The image
layer is a three-dimensional curved zone, or “focal trough,” where the structures lying
within this layer are reasonably well defined on the final panoramic image.
19
(Figure 11).
Most panoramic machines now use a continuously moving center of rotation rather than
multiple fixed locations (Figure 12). This feature optimizes the shape of the image layer
to reveal the teeth and supporting bone.
Aside from its many other uses in dentistry, including pathology, root resorption
assessment, eruption sequence, impacted canines, and other adjuncts, panoramic
radiographs are used to judge root parallelism and mesiodistal angulations of teeth in
orthodontics. Many orthodontists use panoramic radiographs either at the start of
treatment or while treatment is in progress to reposition brackets to align roots.
18,20

9

Figure 8. Movement of the film and objects (A,B,C, and D) about two fixed centers of
rotation. Pb, Lead collimator.
19

Figure 9. Movement of the film and x-ray source about one fixed center of rotation. Pb,
Lead collimator.
19

10

Figure 10. Movement of the film and xray source about a shifting center of rotation. Pb,
Lead collimator.
19

Figure 11. Focal trough. The closer to the center of the trough (dark zone) an anatomic
structure is positioned, the more clearly it is imaged on the resulting radiograph.
19

11

Figure 12. Movement of the x-ray source and beam. The dark line shows a continuously
moving center of rotation. As the source moves behind the patient’s neck and anterior
teeth are imaged, the center of rotation moved forward along the arc (dark line) toward
the sagittal plane. The x-ray source continues to move around the patient to image the
opposite side.
19

Accuracy and Distortions
The principal advantages of this radiographic technique are the broad anatomic
region imaged, the relatively low patient radiation dose, and the convenience, ease, and
speed of the procedure.
19
However, the disadvantages of panoramic radiography include
its lack of fine detail compared with intraoral films and the variable magnification and
geometric distortion that are inherent in image generation.
19
The actual image from most
panoramic systems is enlarged by about 20%.
20

Various investigators have studied image layer (or focal trough), projection angle,
horizontal and vertical magnification, angular distortion, and patient positioning and their
12
effects on the dimensional accuracy of panoramic images.
26-44
Distortion on panoramic
films of the angle between inclined teeth is the result of the combined distortions in the
vertical and horizontal dimensions.
28
Considering the inherent dimensional inaccuracy of
panoramic images, it has been demonstrated that this radiographic technique also has
limitations in assessing angular measurements of tooth inclinations.
4,521-26

Many of the studies in the literature regarding angular distortion on panoramic
images have the following limitations.
5,21,23
Some studies were restricted to either the
maxilla or the mandible and to specific areas of the jaws such as the anterior segments.
5,24

Consequently, the results applied only to the locations where the test films were exposed.
Furthermore, some studies were carried out on nonanatomical devices with unrealistic
tooth orientations that did not depict true human dentition. Most studies were based on
wire meshes and pins representing the dentition and supporting structures.
21,23
The
authors also referred to the angular distortion as changes in root parallelism and not
individual structures representing teeth.
McKee et al studied the accuracy of four panoramic systems with regard to
mesiodistal tooth angulations. It was concluded that statistically significant differences
were noted for the majority (74%) of maxillary and mandibular image mesiodistal
angulations as compared with the true mesiodistal angulations found from the Coordinate
Measuring Machine (CMM). The CMM machine is a device for measuring the physical
geometrical characteristics of an object. Measurements are defined by a probe attached to
the third moving axis of this machine. The CMM was reported by the manufacturer to be
accurate to within 0.013 mm and was found to be accurate to within 0.031°. For the
13
maxilla, the image angle typically underestimated the central and lateral incisors and the
canine, and overestimated the premolars and the first molar. For the mandible, almost all
image angles underestimated the true angles, with the canine and the first premolar the
most severely underestimated. The discrepancies between the image angles and the true
angles were larger for mandibular teeth than they were for maxillary teeth. Applying
clinically significant tolerance limits of ± 2.5° in the mesiodistal angulation of teeth to the
reference archwire still resulted in the majority (61%) of maxillary and mandibular image
angles being significantly different from the true angle measurements. The clinical
assessment of mesiodistal tooth angulation with panoramic radiography should be
approached with extreme caution and with an understanding of the inherent image
distortions.
4

Garcia Figueroa et al studied the effect of buccolingual inclination on mesiodistal
angulation on panoramic radiographs and found that when the buccolingual inclination
changes, the largest angular differences between adjacent teeth occurred in the canine-
premolar area. These discrepancies were larger for the maxillary arch than for the
mandibular arch. Furthermore, buccolingual inclination changes in the incisor area do not
seem to affect the expression of root parallelism in panoramic images.
45

Part of the reason why traditional panoramic radiographs are inaccurate in
capturing the angulations of teeth may be attributed to the inorthogonal nature of the X-
ray beams as the X-ray tube and the sensor move around the target, as well as the large
variations in the size and shape of the dental arches. To overcome these problems,
panoramic-like images constructed from three-dimensional volumetric images have been
14
recommended. Three-dimensional volumetric images have been shown to capture the
target at a 1:1 ratio with very little distortion; furthermore, the trough used to generate the
pan-like images can be customized to closely follow the dental arch shape and size. Van
Elslande et al compared the CMM mesiodistal tooth angulation measurements and those
obtained from the CBCT pan-like images and found for 16 of the 28 teeth there were
statistically significant differences. However, when a tolerance limit of ±2.5° is applied,
the mesiodistal angulation of only maxillary laterals and mandibular left canine showed
clinically significant differences.
46
CBCT
Computed tomography (CT) was first developed by an engineer named, Sir.
Godfrey Hounsfield, CT technology has rapidly evolved. The first generation of CT
scanners used a single detector element to capture a beam of X-rays. Both the detector
and source rotated one degree, a design known as the "translate-rotate" or "pencil-beam"
scanner. In 1975, a second generation of CT known as "hybrid" CT was introduced.
These machines used more than one detector and a small fan-beam X-ray, as opposed to a
pencil-beam. In 1976, a third generation CT scanner was introduced. These scanners
used a large, arc-shaped detector that acquires an entire projection without the need for
translation. This rotate-only design utilizes the power of the X-ray tube much more
efficiently than the previous generations. A fourth generation scanner shortly followed,
replacing the arc-shaped detector with an entire circle of detectors. In this design the X-
ray tube rotates around the patient, while the detector stays stationary.
47

15
The introduction of cone beam computed tomography (CBCT) specifically
dedicated to imaging the maxillofacial region heralds a true paradigm shift from 2D to a
3D approach to data acquisition and image reconstruction.
48
CBCT was initially
developed for angiography
49
, but other applications have included mammography.
50

CBCT was originally developed as an alternative to conventional CT to provide a more
rapid acquisition of an image by using a larger field of view (FOV), and a comparatively
less expensive x-ray detector. Advantages of the CBCT are a shorter radiation exposure,
faster x-ray acquisition time, reduced image distortion due to patient movements, and
increased x-ray efficiency. The use of a larger FOV increases the amount of x-ray
scatter. Since the CBCT uses a larger FOV, the image quality related to noise and
contrast resolution is limited.

Figure 13. Traditional CT vs. CBCT; Traditional CT uses a “Fan” of x-rays and need
multiple helical passes around the subject; CBCT uses a “cone of x-rays and only needs a
single pass around the subject.
48

16

Figure 14. Conventional CT vs CBCT; reconstructions of the base projections differ,
CBCT scans are subject to much more radiation scatter.
49

How Cone Beam Computed Tomography (CBCT) works
Typically, CBCT machines scan patients in one of three positions: sitting,
standing, or supine. Unlike having the patient upright, a patient supine during a scan can
alter the 3D soft tissue profile, thus it is important to look at additional photographs to
accurately evaluate the soft tissue.
CBCT imaging is accomplished by using a rotating gantry to which an x-ray
source and detector are fixed. A divergent cone-shaped source of ionizing radiation is
directed throughout the middle area of interest onto an x-ray detector on the opposite
side. To acquire an image, a single rotation of the x-ray source and detector rotates
around a fixed fulcrum within the center of interest (Figure 13). During the rotation,
multiple sequential planar projection images of the FOV are acquired in a complete or
17
sometimes partial arc. This procedure varies from a traditional medical CT, which uses a
fan-shaped x-ray beam in a helical progression to acquire individual image slices of the
FOV. A computer stacks the acquired slices to obtain a 3D representation. Each slice
requires a separate scan and separate 2D reconstruction. Because CBCT exposure
incorporates the entire FOV, only one rotational sequence of the gantry is necessary to
acquire enough data for image reconstruction (Figure 14).
48

Figure 15. Isotropic Voxels vs. Anisotropic Voxels.
48

The resolution of CBCT imaging is gauged by the volume elements or voxels
produced from the 3D volumetric data. The voxel dimension depends on the pixel size on
the detector. The detector typically detects pixels that range from 0.09 mm to 0.4 mm.
Therefore, the detector determines the final resolution and clarity of the 3D volumetric
image. Because CBCT raw data is obtained in one rotation of the x-ray source, CBCT
18
voxels are isotropic, which means that all three dimension are of equal length. Traditional
CT scanners rotate multiple times around the object to produce an image. To create CT
voxels, the computer must combine all the obtained slices, and construct a z dimension
depending on slice thickness. These resulting compositional voxels are anistropic, which
is of unequal dimensions. The voxel shape is columnar and has an equal x and y
dimension, but an unequal z dimension (Figure 15). Once the patient has been scanned
with a CBCT scanner, data must be processed to create a 3 dimensional volume. This
process of reconstruction combines a number of individual base projections, each frame
resembling a lateral cephalogram that contains more than one million pixels (Figure 14).
Each pixel is assigned 12 or 16 bits of data. A computer processes the multiple base
projections to reconstruct the desired 3D volume. 3D image processing time varies based
on voxel size, FOV, number of projections, and computer processing speed.
Image Resolution: Pixel and Voxel Size
A pixel represents the smallest sampled 2D element in an image. The image has
dimensions along two axes in millimeters, dictating in-plane spatial resolution. A voxel is
the volume element, defined in 3D space. Its dimensions are given by the pixel, together
with the thickness of the slice (the measurement along the third axis). Resolution which
determines the ability to distinguish structures as separate and distinct from each other is
inherently related to the acquired voxel volume. The FOV, acquisition matrix, and the
slice thickness determine voxel volume. The pixel size (FOV/matrix) determines the in-
plane resolution. Reducing the FOV, increasing the matrix number, or reducing the slice
thickness results in an image with reduced voxel volume. A smaller voxel produces
19
images with a higher resolution, but a mandibular signal-to-noise ratio (SNR), which may
cause the image to look grainy.
CBCT Radiation Dosage
The effective radiation dosage is 3~11uSv for panoramic radiographs and 5~7
uSv for cephalograms. For a CBCT scan, the radiation dosage is 40~135 uSv, close to
that of a chest X-ray, or equivalent to 4~17 days of natural background radiation.
51,52

The use of a thyroid collar and chin position can substantially reduce the dose by up to
40%. In comparison to a traditional CT, CBCT radiation exposure of the maxillofacial
region is 76.25% to 98.5% less.
51-53
CBCT Accuracy
Traditional orthodontic analysis typically requires a 2D lateral cephalogram and
panoramic x-ray of the skull, which can be highly inaccurate due to magnification,
anatomic superimposition, beam projection angle, and patient position.
54
These
distortions were unavoidable, until the advent of CBCT imaging. In a CBCT imaging,
this distortion can be avoided, because the x-ray beams emitted are cone beamed and the
object is very close to the sensor. After the CBCT scan is complete, the computer
software processes the data, and the resulting image has a 1-to-1 measurement ratio to the
original object. To ensure that error is kept to a minimum and that other operational
systems are functioning correctly, a water phantom is used to calibrate the 3D scanner.
55

In 2005, Hilgers studied the accuracy of CBCT in the TMJ. He found that CBCT
accurately and precisely depicts the TMJ complex in a 3D model. The measurements
were reproducible and significantly more accurate than those made with conventional
20
cephalograms in all 3 dimensional planes.
56
In 2004, Lascala studied the accuracy of
linear measurements obtained by cone beam computed tomography. The group studied
13 measurements in 8 dried skulls and recorded measurements with an electronic caliper.
The skulls were then scanned and analyzed with a NewTom QR-DVT 9000. The
radiographic

distance measurements of the same dry skull were then compared. They
concluded that the CBCT image underestimates the real distances between skull

sites.
Differences were only significant for the skull base, and

therefore, it is reliable for linear
evaluation measurements

of other structures more closely associated with maxillofacial

imaging.
57
CBCT scans are highly accurate and provide a 3 dimensional image of the
maxillofacial structures, which is paramount to effective orthodontic treatment planning.
In 2005, Hutchinson showed that linear and angular dimensions are more accurate
using a CBCT derived panoramic radiograph, compared to traditional radiographs.
47
Just
as three-dimensional imaging in the medical field has revolutionized medical diagnosis
and the delivery of treatment since the introduction of computed tomography (CT) in
1967, the Cone-Beam CT has opened up a new horizon for three-dimensional diagnosis
and treatment planning in dentistry, particularly in orthodontics where shape, form,
structure, and position are of critical importance. Utilizing this new technology, we can
now visualize each tooth in all three planes of space.
Limitations of CBCT
CBCT image artifacts arise because of the inherent polychromatic nature of the x-
ray beam. The x-ray source produces a heterogeneous mix of low and high energy
photons, which passes through an object. The low energy photons are absorbed more than
21
the higher energy photons, resulting in a richer high energy photon penetrating beam or
beam hardening. Beam hardening manifests as a cupping artifact, or streaks and dark
bands. Cupping artifacts occur when x-rays passing through the center of an object
become harder than x-rays passing through the edges of an object. Since the x-ray beam
becomes harder in the center, the image is processed incorrectly and produces the artifact.
Patient motion can cause blurriness of the reconstructed 3D image. This type of
error can be minimized by placing a head restraint on the patient before the scan is taken.
Undersampling can also occur if too few basis projections are used in the
reconstruction. A reduced number of projections can lead to blurriness and a noisier 3D
image. Resolution of fine detail will be affected.
Scanner related artifacts can manifest as a circular or ring shaped artifacts. These
artifacts are caused by poor scanner detection or improper scanner calibration. These
problems will consistently result in circular artifacts.
CBCT image noise is mostly caused by scattered radiation. Since CBCT uses a
cone shaped x-ray beam, scattered radiation is unavoidable. This scattered radiation
becomes omnidirectional and is recorded by the CBCT detector. This nonlinear
attenuation contributes to the overall image noise.
A cone beam effect can be a source of artifacts. Since the x-ray beam diverges
and the beam rotates around the patient, the amount of information for peripheral
structures is reduced. The outer rows of the detector record less than the inner rows,
resulting in image distortion, streaking, and greater peripheral noise.

22
DICOM
DICOM is a universal file type that facilitates data exchange and viewing between
hardware, regardless of the manufacturer. This format was created in response to the need
of a universal file format in medical imaging, such as CT scans, MRI’s, and ultrasound.
Before DICOM was introduced, it was almost impossible to view images by any software
other than manufacturer’s software that generated the images. In 1983, the American
College of Radiology (ACR) and national Electrical Manufacturers Association (NEMA)
joined forces and released their first standard in 1985, ACR/NEMA 300. Soon after its
release, many vendors started to adopt and accept the standard. After its third standard
revision, the format changed its name to DICOM. Currently, the DICOM standard has
achieved a near universal level of acceptance among medical imaging equipment vendors
and healthcare IT organizations.
DICOM files stores massive amounts of data that need to be viewed on dedicated
workstations where the images can be appropriately displayed by a DICOM viewer.
A DICOM file made up of a header and the image itself. The header contains: patient
demographic information, acquisition parameters, referrer, practitioner and operator
identifiers and image dimensions. The remaining portion of the DICOM file contains the
image pixel data. Because DICOM files can contain multiple high-resolution images,
DICOM files sizes tend be large.
DICOM images can be compressed by converting the data into smaller image file
types. Compression is a means to efficiently archive and transfer image data. There are
two main types of data compression: lossless and lossy. Lossless compression allows the
23
file size to be reduced without any loss of information. This allows all the original data to
be recovered if necessary. This type of compression uses a substantial amount of
processing power and makes files slower to open and save. By contrast, lossy image
compression permanently eliminates some of the file data, which can result in a
remarkable reduction in file size. (Figure 16) The goal is to eliminate redundant
information from the dataset without adversely affecting image quality, but excessive
compression inevitably results in image degradation.

Figure 16. Image quality; No image compression vs. image compression.
49

24
Chapter Three: Hypothesis
Research Hypothesis, H
a
:
The mesiodistal tooth angulation measurements from CBCT constructed pan-like images
are significantly different from the measurements directly taken from CBCT volumetric
images
Null Hypothesis, H
0
:
The mesiodistal tooth angulation measurements from CBCT constructed pan-like images
are not significantly different from the measurements directly taken from CBCT
volumetric images

25
Chapter Four: Materials and Methods
This study was approved by the University of Southern California Health
Sciences Institutional Review Board (HSIRB). The IRB approval ID is UP-10-00184.
Case Selection Criteria
The patients in this study are a subset from another study designed to obtain the
standard tooth and torque from each tooth for a group of patients whose occlusions are
close to normal. The selection criteria for the near normal group were comparable with
those used by Andrews: no history of past orthodontic treatment; subjects in good health
and exhibiting normal growth; well-related vertical, transverse, and anteroposterior
relationships; pleasing profile; arches well aligned with no supernumerary teeth; normal-
appearing teeth; low decayed, missing, filled tooth index numerical value; no large
restorations or fixed replacements. Furthermore, all subjects had close to Angle Class I
molar relationships, and overjet and overbite close to within normal limits.
Photo Screening
1. Complete dentition (a few cases with the second molars not fully erupted; as long
as the anatomy of the crowns are preserved, we have accepted some cases with
large fillings or crowns)
2. Molar relationship from <1/2 step Class II to 1/4 step Class III
3. OB/OJ between 0-5 mm
4. Spacing < 6 mm
5. Crowding < 4 mm (limit to 3 teeth max)
26
6. Rotation < 15
0
(limit to 3 teeth max)
7. Dental X-bite limited to no more than 1 tooth
8. No apparent arch form asymmetry
X-Ray Screening
1. Panoramic X-rays show generally parallel roots
2. ANB between -1
o
to 6.5
o

3. FMA between 14
o
to 37
o

4. U1/L1 angulation between 110
o
to 146
o

5. No obvious skeletal PA and vertical asymmetry
From April 2004 to October 2009 approximately 4190 scans with CBCT were
done. From this we initially selected 125 near normal patients. After applying the
inclusion/exclusion criteria above, 76 patients were in the “near normal” group. 33
patients were randomly selected from this group for this study.
Each CBCT scan was taken using a NewTom 3G under the following conditions:
the tube voltage – 110kVP, tube current – 15mA, scan time – 17 seconds, grayscale –
12bit, field of view – 200mm (12 inch). Before the scan was acquired, each patient was
placed in a supine position and oriented perpendicular to the rotation axis of the CBCT x-
ray source. The patient was to remain as still as possible during the entire scan time to
minimize distortion and noise in the final image. Based on our specific requirements and
instructions, Dolphin developed a customized 3D analysis program that allowed us to
digitize each tooth in all three planes of space and obtain the tip and torque measurements
from these digitizations.
27
Global Three-Dimensional Coordinate Systems
A global three-dimension coordinate system is first generated for the proper
orientation of the head and the maxillofacial structure. A saggital plane, a coronal plane,
and a transverse plane, each perpendicular to the other two are used. The saggital plane is
the plane that evenly divides left and right, the coronal plane is the one that is
perpendicular to the saggital plane at the buccal grooves of the right and left first molars,
and the transverse plane is the functional occlusal plane, defined as the plane that
intersects the incisal overbite and the molar overbite (defined at the buccal groove of the
maxillary first molars) (Figure 17).

Figure 17. Image of all 3 planes established at the functional occlusal plane (Intersection
of the overbite of the central incisors and first molars at the buccal groove of the
maxillary molars).

28
Tooth-Specific Three-Dimensional System for Locating Crown
and Root Centers

We chose to use the true long axis of each tooth, defined as the line that connects
the center of the crown and the center of the root (Figure 18-20). A three-perpendicular-
plane system specific for each tooth was used for locating each crown and root center: the
anatomical mesio-distal plane, the anatomical bucco-lingual plane, and the transverse
plane at the crown and root center level. Tooth long axis may be less affected by surface
contour and structure (central developmental lobe) and easier to define with the imaging
software program, and therefore it is less variable. We have also removed another
variable by intentionally ignoring the apical third of the root, which has shown
tremendous variation from the true long axis of many teeth.

Figure 18. A point being placed at the center of the crown utilizing all three planes of
space to accurately determine the proper location.
29

Figure 19. A point being placed at the center of the root utilizing all three planes to
accurately determine the proper location.

Figure 20. All crown and root points digitized and a line connecting them is shown
representing the long axis of each tooth.

30
Tooth-Specific Three-Dimensional System for Measuring Each
Tooth

A different tooth-specific three-plane coordinate system is used to study the
mesio-distal angulation and bucco-lingual inclination of each whole tooth in three-
dimensional space. This system is made of a transverse plane, a mesio-distal plane and
bucco-lingual plane per each tooth. The transverse plane is the same functional occlusal
plane as in the global coordinate system. The mesio-distal plane is also called the arch
plane along the curvature of the dental arch and it changes from tooth to tooth. Each tooth
also has its own bucco-lingual plane. It is also worth noting that the occlusal plane we use
is different from Andrews’ occlusal plane, which touches the incisal edges and the second
molar’s cusp tips. Since maxillary and mandibular incisal edges and molar cusp tips are
at different levels, each arch has a different occlusal plane in Andrews’ studies, and the
two are often not perfectly parallel.
Mesio-Distal Angulation Measurement (Tip)
As illustrated in the figure below, the mesio-distal angulation of the tooth
should be measured from the projection of the true tooth long axis on the mesio-
distal plane to the vertical line formed by the intersection of the mesio-distal and
the bucco-lingual plane (Figure 21). If the root center is distal to the crown
center, the measurement is positive, otherwise it is negative. Ideally this
measurement should agree with Andrews’ crown angulation measurement since
the FACC and the long axis of the whole tooth should have the same angulation.

31

Figure 21. Determining the mesio-distal angulation (tip) of the mandibular right
first molar from the mesio-distal view.

Bucco-Lingual Inclination Measurement (Torque)
The bucco-lingual inclination of the tooth should be measured from the
projection of true tooth long axis on the bucco-lingual plane to the same vertical
line formed by the intersection of the mesio-distal plane and the bucco-lingual
plane (Figure 22). If the root center is lingual to the crown center, the
measurement is positive, otherwise it is negative. This measurement may not
agree with the crown bucco-lingual inclination due to the contour of the labial
surface of the tooth measured.
32

Figure 22. Determining the bucco-lingual angulation (Torque) from the bucco-
lingual plane (Coronal View).

Arch Form
Four teeth on the right side of both the maxillary and mandibular arches
are digitized along the arch outline: the mid incisal tip, the right canine tip, the
right second premolar, and the right second molar. The software program will
duplicate the right side half arch to the left side constructing a symmetrical arch
form (Figure 23-24). If this constructed arch form does not fit the patient’s real
arch form, the midpoint can be adjusted to the left or right to get a better
adaptation of the arch form to the teeth.
33

Figure 23. Determining the arch form of the maxillary arch by placing the points
(shown in yellow) buccal to the midincisal, canine, second bicuspid and second
molar. The left side arch is duplicated from the right side arch and it fits the dental
arch perfectly.

Figure 24. Determining the arch form of the mandibular arch by placing the
points (shown in yellow) buccal to the midincisal, canine, second bicuspid and
second molar. The left side arch is duplicated from the right side arch and it fits
the dental arch perfectly.

Once the arch form is constructed, the customized program will add two lines to
each tooth. The green line represents the bucco-lingual plane that is perpendicular to the
arch plane and the shorter blue line represents the mesio-distal plane that is parallel to a
34
plane that is tangent to the arch plane at the point where the bucco-lingual plane
intersects the arch plane. The dark blue dot represents the crown center of each tooth.
These three planes form the tooth-specific three-dimensional coordinate system from
where the mesio-distal angulation and bucco-lingual inclination are measured.
At this point, the software program can measure and print out the tip and torque
measurements for all the teeth automatically. All 33 cases were measured twice by two
different investigators, who set the head orientation independently. Repeated
measurements of ten random cases were used for Inter-Class Coefficient (ICC)
calculation to test for reliability of the 3D measurements.
Two-Dimensional Constructed Panoramic Images
Using Dolphin software, 2D panoramic images were constructed. The head
orientation was set to the same orientation that was used in the 3D global coordinate
system. Two different panoramic images were constructed, one for the maxillary teeth
and one for the mandibular teeth. The secondary reconstruction to create the pan-like
image was performed by placing the transverse plane at the cervical margin of either the
maxillary or mandibular incisors parallel to the occlusal plane (Figure 25). The points for
the path were plotted at the mandibular posterior border, and the mesial and distal of each
tooth. Two panoramic images were constructed because the arch form and positions of
the maxillary and mandibular teeth are not the same. The image thickness was set to the
narrowest possible that still included all maxillary and mandibular crowns and roots as
the transverse plane was moved from above the maxillary teeth apex level to below the
mandibular teeth apex level.
35

Figure 25. Constructing the panoramic image.

Figure 26. The transverse plane is set to the cervical margin of the mandibular incisors,
parallel to the occlusal plane in the frontal view.
36

Figure 27. Points are plotted at the mandibular posterior border and the mesial and distal
of each tooth.

Figure 28. The thickness is set to the narrowest possible that still included all maxillary
and mandibular crowns and roots as the transverse plane was moved from above the
maxillary teeth apex level to below the mandibular teeth apex level.

The software produced a 2D panoramic image based on the path selection and this
image was saved as a jpeg file. The mesiodistal angulations of the teeth were then
measured using the 3-point line angle tool in the Dolphin software (Figure 29). Point 1 is
placed at the root center, point 2 is placed near the red line that represents the transverse
37
plane. The blue line that connects point 1 and point 2 will go through the crown’s center
for each tooth. Point 3 makes the blue line that connects point 2 and 3 parallel to the red
transverse line. The mesiodistal angulation is defined as the line angle between point 1, 2
and 3. If the crown is mesial to the root, the measurement is positive, and if the crown is
distal to the root, the measurement is negative. The measurements were recorded into a
Microsoft excel spreadsheet.

Figure 29. Using the three point angle to measure the mesiodistal inclinations of the teeth
on the 2D panoramic images.

Data Collection
The following information was obtained and calculated using patients’ treatment
records and personal history forms:
1. Age of patient at time of records (Table 1)
38
2. Sex (Table 2)
3. Ethnicity (Table 3)
4. Tip for every tooth excluding third molars using 3D coordinate system
5. Tip for every tooth excluding third molars on constructed panoramic image
Table 1. Average age of the sample
Average Age Std dev
22.15 9.08

Table 2. Sex of the sample

Gender Frequency
M 9
F 24

Table 3. Ethnicity of the sample

Ethnicity Frequency Percentage
African American 8 24.42
Asian 7 21.12
Caucasian 7 21.12
Hispanic 11 33.33

Assumptions
There are many factors that could potentially influence the radiographic measurements.
Therefore, this study was based on the following assumptions:
1. The 3D scan was taken in the same position by the same operator
2. The machines setting was the same for each patient

3. The protocol used for this study is appropriate and reproducible

39
Method Error
Because of the retrospective nature of this study, the error from the radiographic
procedure could not be studied. However, patients for this study were selected based on
the knowledge that the 3D scans were taken on the same x-ray machine by the same
operator. Variables that could not be controlled include: accuracy of the x-ray unit, and
the ability of the x-ray technician to follow the instructions of the x-ray unit
manufacturer. These sources of error are assumed to be insignificant. All 33 cases were
measured twice by two different investigators, who set the orientation independently.
Repeated measurements of ten random cases were used for Inter-Class Coefficient
calculation to test for reliability of the 3D measurements. For testing reliability of the 2D
measurements, 2D panoramic-like images of ten of the 33 cases were randomly selected
and constructed three times by the same investigator, 1 week apart. The second
constructions were done with the same head orientation as the first and the third
constructions were done with the head orientation set independently. ICC tests between
the first and second time measurements were performed to test for digitization errors and
ICC tests between first and third time measurements were performed to test for errors due
to independent head orientations.
Another set of 2D measurements for the maxillary teeth were made on the
panoramic images constructed specifically for the mandibular teeth (by setting the
transverse plane at the cervical margin of the mandibular incisors and by plotting the path
of the panoramic construction trough at the mesial and distal of the mandibular teeth).
These measurements are compared with the measurements made on panoramic images
40
constructed specifically for the maxillary teeth. Both set of measurements are compared
against the 3D measurements and the frequency of differences over 2.5
0
for each tooth
was calculated.
Statistical Analysis
All data were entered into Microsoft Excel worksheet and were analyzed using
Excel and Statistical Package for Social Sciences (SPSS) version 17.0. The data was
tested for normality and paired t-tests or wilcoxon signed rank test were performed to
compare the 3D and 2D measurements. Significance was established at alpha
=(0.05/7=0.0071) based on the Bonferroni adjustment.

41
Chapter Five: Results
Method Error
Table 4. Overall ICC for repeated measurements from ten cases
Test ICC
3D-3D (two independent head orientations) 0.914
2D-2D (same head orientation) 0.980
2D-2D (two independent head orientations) 0.971

The 3D measurements of all 33 cases were measured by two different
investigators at different times, with the head orientation set independently. Repeated
measurements of ten of the 33 cases were selected randomly to calculate the ICC to test
for reliability of the 3D measurements. Average ICC for all the teeth was 0.914. For ten
2D repeated measurements, when the same head orientations were used, the average ICC
for all the teeth was 0.980; when head orientations were set independently, the average
ICC for all the teeth was 0.971.(Table 4). ICC for all the teeth are listed in Table 5.
Table 5. ICC by tooth
Tooth 3D 2D-2D Head Orientation
UR7 0.991 0.987 0.997
UR6 0.902 0.991 0.991
UR5 0.964 0.996 0.993
UR4 0.905 0.978 0.994
UR3 0.79 0.985 0.983
UR2 0.974 0.997 0.989
UR1 0.937 0.974 0.942
UL1 0.901 0.874 0.929
UL2 0.913 0.988 0.976
UL3 0.912 0.981 0.968
UL4 0.953 0.983 0.973
42
Table 5. (Continued)
Tooth 3D 2D-2D Head Orientation
UL5 0.973 0.991 0.995
UL6 0.941 0.983 0.985
UL7 0.983 0.983 0.989
LL7 0.977 0.988 0.996
LL6 0.772 0.989 0.96
LL5 0.917 0.992 0.982
LL4 0.938 0.995 0.96
LL3 0.962 0.965 0.9
LL2 0.808 0.983 0.925
LL1 0.835 0.991 0.978
LR1 0.969 0.987 0.961
LR2 0.946 0.991 0.965
LR3 0.97 0.912 0.92
LR4 0.948 0.991 0.984
LR5 0.956 0.995 0.994
LR6 0.61 0.992 0.973
LR7 0.954 0.983 0.993
Mean 0.914 0.98 0.971

Right and Left Comparison
Multiple paired t-tests were performed to compare the tooth mesiodistal
angulations between right and left side for all 33 cases. The bonferroni adjustment was
made with the  level set at 0.05/7=0.0071. The differences between the right and left
side measurements were not significant (Table 6) and therefore the measurements were
combined in Table 7 and Table 8.

43
Table 6. T-tests for the right vs. left measurements
Tooth 7 6 5 4 3 2 1
Panoramic
Maxillary 0.13 0.04 0.09 0.78 0.29 0.97 0.38
Panoramic
Mandibular 0.27 0.0075 0.17 0.13 0.71 0.03 0.07
3D
Maxillary 0.47 0.35 0.41 0.69 0.06 0.08 0.05
3D
Mandibular 0.22 0.95 0.82 0.50 0.80 0.36 0.50

Comparison Between Mesiodistal Angulation Measurements
from Constructed Panoramic Images and from 3D Volumetric
Images Directly

Table 7. Measurements of mesiodistal angulations for maxillary teeth
Tooth 1 2 3 4 5 6 7
3D (Mean) 5.52 6.27 10.98 7.72 3.24 0.48 -10.17
Panoramic
(Mean) 3.11 5.09 9.56 6.68 3.17 0.43 -10.74
Mean
Difference
2.41 1.17 1.42 1.04 0.07 0.05 0.57
Std
deviation of
the
difference 2.55 2.96 1.67 1.49 1.38 1.85 1.89
P value
5.75452
E-06
0.02915928
5
2.70682
E-05
0.00034
5
0.76900397
3 0.046
0.09241
1

44
There is a statistically significant difference between the 3D and 2D
measurements for the maxillary central incisors, canines, and first premolars. There is no
significant difference between the measurements for the maxillary lateral incisors, second
premolars, and first and second molars. The difference shows that the roots are displayed
more mesially on the 2D panoramic image on all maxillary teeth. The standard deviation
of the difference is the greatest for the maxillary incisors.
Figure 30. 3D vs. 2D measurements for the maxillary teeth

-significant differences
For both pan-like measurements and the 3D measurements, the mesiodistal
inclinations of the teeth increase from the maxillary central to the lateral and canine and
then decrease as the teeth are more posterior in the mouth. All of the maxillary teeth have
positive mesiodistal inclinations (roots are more distal than the crown) except for the
second molar.

-15
-10
-5
0
5
10
15
1 2 3 4 5 6 7
Pano Upper
3D Upper
45
Table 8. Measurements for mesiodistal inclinations for mandibular teeth
Tooth 1 2 3 4 5 6 7
3D 0.81 -0.48 5.32 5.85 7.86 9.69 19.13
Panoramic 0.31 -0.12 4.60 5.55 8.42 11.92 21.12
Difference 0.5 -0.35 0.72 0.29 -0.55 -2.22 -1.99
Std
deviation 2.00 2.66 2.00 1.68 1.34 1.77 2.11
P value 0.160804 0.451081 0.04607 0.320176 0.023079
3.76672E-
08
6.14E-
06

There is a statistically significant difference in the 3D and 2D tooth mesiodistal
angulation measurements for the mandibular first and second molar. There is no
statistically significant difference in the 3D and 2D tooth mesiodistal inclination
measurements for the mandibular incisors, canine, and premolars. The standard deviation
of the difference is the greatest for the mandibular lateral incisor.
Figure 31. 3D vs. 2D for the mandibular teeth

-significant differences
-5
0
5
10
15
20
25
1 2 3 4 5 6 7
PANO LOWER
3D LOWER
46
For both 2D pan-like measurements and 3D measurements, the mesiodistal
angulation of the teeth increase (root more distal than the crown) as the teeth are more
posterior in the mouth (except the mandibular lateral incisors as compared to the
mandibular central incisors). All of the mandibular teeth have positive average
mesiodistal inclinations except for the lateral incisor, which has slight distal angulation.
Clinical Significance of the Differences Between 2D and 3D
Measurements

Table 9. Frequency and percentage of 2D-3D differences above 2.5° for maxillary teeth
Tooth Frequency Percent
1 24 36
2 24 36
3 24 36
4 11 17
5 7 11
6 22 33
7 22 33
All 134 29

It is generally accepted that clinically acceptable error tolerance for angular
measurements is set at 2.5°. Therefore, the frequency and percentage of each tooth that
show differences larger than 2.5° between 2D and 3D measurements are calculated. The
highest amount of measurements that differed by more than 2.5° were for the maxillary
anterior teeth. The least amount of differences above the 2.5° threshold were the second
premolars. Overall, 29% of the measurements differed by more than 2.5°.
Another set of 2D measurements for the maxillary teeth were made on the
panoramic images constructed specifically for the mandibular teeth. These measurements
are compared with the measurements made on panoramic images constructed specifically
47
for the maxillary teeth. Both sets of measurements are compared against the 3D
measurements and the frequency of differences over 2.5
0
for each tooth was calculated.
Table 10. Frequency of clinically signicant differences for maxillary measurements on
two constructed panoramic images

Md image Mx image
Tooth Frequency Percent Frequency Percent
1 24 36 37 56
2 24 36 42 64
3 24 36 27 41
4 11 17 27 41
5 7 11 14 21
6 22 33 28 42
7 22 33 35 53
All 134 29 210 45

After comparing the above two sets of measurements, it was decided that the
measurements for the maxillary teeth made on the pan-like images made specifically for
the mandibular teeth were more susceptible to larger errors. Therefore, one pan-like
construction may not be good enough for both maxillary and mandibular teeth. All of the
other data for the study, maxillary and mandibular teeth measurements, were made on
separate pan-like images.

48
Table 11. Amount of differences above 2.5° for mandibular teeth
Tooth Frequency Percent
1 20 30
2 32 48
3 21 32
4 15 23
5 11 17
6 27 41
7 29 44
All 155 33

The highest amount of measurements that differed by more than 2.5° were for the
mandibular lateral incisor and second molar. The least amount of difference above the
2.5° threshold were for the second premolars. Overall, 33% of the measurements differed
by more than 2.5°. Overall, 31% of all maxillary and mandibular measurements differed
by more than 2.5°.

49
Chapter Six: Discussion
The primary purpose of this study was to determine if there were differences in
tooth mesiodistal angulation measurements from constructed panoramic images as
compared to direct measurements from 3D volumetric images. It was hypothesized that
there would be no differences between the 3D and 2D measurements.
Significance of the Findings
Ten of the 33 cases were selected randomly and 3D measurements were repeated
by two different investigators to test for reliability. The average of the Intraclass
correlation coefficient for the 3D measurements for all teeth was 0.914. This indicates
that the reliability of the 3D measurements is reasonably good. Another ten of the 33
cases were selected randomly (not necessarily the same ten cases for the 3D repeated
measurements) and 2D measurements were repeated three times, by the same
investigator. When the same head orientations were used between the first and second
time measurements, the average ICC for all the teeth was 0.980; when independent head
orientations were used, the average ICC for all the teeth was 0.971. This shows that the
intraexaminer reliability for the construction and measurements of mesiodistal
angulations on the 2D panoramic images was very good. And due to the stringent
guidelines used in setting the head orientation, resetting the head orientation seems to
have little to no effect on the measurements for the 2D panoramic images. As comparing
the reliability of the 3D and 2D measurements, the 2D measurements seem to be more
consistent. However, this does not necessarily mean the 2D measurements are more
accurate. It could mean that the 2D measurements may be consistently off.
50
When constructing the panoramic images, at first the transverse plane was set to
the cervical margin of the mandibular incisors. Both maxillary and mandibular
measurements were recorded; however, when constructing two different panoramic
images, with the transverse plane first set at the mandibular cervical margin for the
mandibular measurements and the maxillary cervical margin for the maxillary
measurements, the results show that the measurements have less clinically significant
differences. This can be explained because when constructing the panoramic images, the
points were plotted at the mesiodistal of each tooth. However, the maxillary and the
mandibular arches are not identical in their arch form and tooth positions; therefore, more
distortion would be introduced if separate troughs were not drawn. As an orthodontist, it
is important to note that if constructed panoramic images are used, to be the most
accurate in determining the mesiodistal angulations of teeth, two separate panoramic
images should be constructed from the CBCT data.
Orthodontists have different options when it comes to determining the mesiodistal
angulations of teeth at the beginning, middle, and end of treatment. Starting from the
least accurate, orthodontists can determine the mesiodistal angulations by merely looking
at the crowns of the teeth. Dr. Andrews used this technique on 120 cases to determine his
ideal tip for teeth; however, a related study showed that simply looking at the crowns of
the teeth did not accurately depict the long axis and that there may be a change in
angulations between the crown and the roots.
The next possible option is to take conventional panoramic radiographs to
determine the mesiodistal angulations of the teeth. Many orthodontists believe that an
51
understanding of anticipated deviations in axial tooth positions represented by the
panoramic radiograph is important clinically. Studies have shown that 2D radiographic
images have been used regularly at the start, middle, and end of treatment to judge root
parallelism and to guide repositioning of brackets to align roots. However, previous
investigators found significant inaccuracies in mesiodistal tooth angulations in panoramic
radiographs. They attributed the inaccuracy of panoramic images to projection geometry,
focal trough depth, variable vertical and horizontal magnification factors, and patient
positioning errors.
58
The study by Bouwens et al showed the mesiodistal projection of the
maxillary anterior teeth differed from the gold standard (CBCT) measurements by an
average of 4.5°, whereas the average difference for the mandibular anterior teeth was
approximately 6°. There were statistically significant differences in mesiodistal tooth
angulations for 75% of maxillary and 67% of mandibular teeth. Variations up to 5° in
mesiodistal tooth angulation relative to an established reference plane were used.
Application of this clinically significant tolerance limit indicates that 34% of maxillary
and 38% of mandibular image angles from panoramic radiographs were clinically
significantly different from angles represented in the CBCT volumes when evaluated on a
tooth-by-tooth basis.
58

The next level of accuracy would be to use a CBCT constructed panoramic image.
In comparison with the data on conventional panoramic radiographs by McKee et al, who
imaged a single skull with 4 conventional pan units, the accuracy of the projected
mesiodistal root angles on the CBCT pan-like image were superior to the conventional
pan unit.
4
Therefore, 2D constructed panoramic images should have less distortion
52
compared to 2D conventional panoramic radiographs. In this study, the teeth with
significant differences were the maxillary central incisors, canines, first premolars, and
the mandibular molars, while the maxillary laterals, maxillary second premolars,
maxillary molars, and mandibular teeth from central incisors to the second premolars did
not show significant differences.
In a related study, by Vans Eslande et al, with a single skull and multiple scans,
statistically significant differences were found between the CMM and the CBCT pan-like
image angles for the following teeth: maxillary right first molar, second premolar, canine,
maxillary right incisors to the premolar and first molar, mandibular left lateral incisor,
canine, first premolar, and first molar, and mandibular right central incisor, first
premolar, and second molar. The greatest differences were found for the maxillary left
lateral incisor (3.675°), mandibular left canine (2.179°), and maxillary right lateral incisor
(2.006°).
44
However, the previous study used one anatomical device used to simulate
ideal teeth positions. Furthermore, the CMM machine they used cannot be applied to the
patients clinically. Although 33 actual patients’ records were used, the results show less
significant differences and show a trend in the differences as compared to the random
variety of teeth that showed significant differences in the results from Van Eslande et al.
Not only were there differences between the measurements but there was also a
trend in the differences in the maxillary measurements. The differences between the 2D
and 3D measurements show that the roots are displayed more mesially on the 2D
panoramic image on all maxillary teeth, more so for the anterior than the posterior teeth.
These results are similar to those of Mckee et al and Peck et al, who found exaggerated
53
mesial inclination of the roots on anterior teeth. On the other hand, Van Eslande et al
found that the maxillary roots with evidence of a statistically significant difference, with
the exception of teeth maxillary molars, were projected with a greater distal angulation.
44
There were significant differences between the measurements of the maxillary central
incisors, canines, first premolars, and the mandibular molars. To determine if those teeth
were also significantly different clinically, a tolerance of 2.5° was used. The maxillary
centrals and canines had 36% of the measurement differences above 2.5°; however, the
maxillary lateral incisor also had 36% of the measurement differences above 2.5° while
the first premolars only had 17%. The mandibular molars also had 41% and 44% of the
measurements differences above 2.5°; however, the mandibular lateral incisor had the
most differences at 48% yet was statistically insignificant. In all maxillary and
mandibular teeth measurements, 31% of the measurements differed by more than 2.5°.
This finding was less than the majority (61%) found in McKee et al study but more than
that found with CBCT pan-like images where all teeth were within this range with the
exception of tooth maxillary left lateral incisor.
44
However, the previous study had only
one nonanatomic model with teeth artificially set very upright. In this study, the sample
consisted of patients of five different ethnic groups, with more patients having relatively
protrusive facial profiles. According to a study by Garcia-Figueroa et al, torque may
affect mesiodistal angulation measurements.
43

Although most studies use the 2.5° tolerance as being clinically significant, if
using the guidline for the ABO from 2002, which states that the apex of the tooth cannot
deviate by more than 1mm, then the clinically significant amount would be different for
54
different lengths of teeth. The shorter the teeth, the more tolerance for angular deviation,
and yet still meeting the 1mm ABO requirement. Using trigonometry, for a tooth with the
length of 24mm between the crown center to the apex, a discrepancy at the apex of 1mm
is the same as 2.5° change in angulation; if the length is 20mm, the change in angulation
is 3.5°; and if the length is 16mm, the change is 4.5°.
A possible explanation for the significant differences in the maxillary teeth is the
curvature of the plotted trough. On the curved part of the arc, teeth are more likely to
become widened or narrowed. The widths of teeth shown on the constructed panoramic
images are not the actual width of the teeth. The widths actually match the widths of the
center trough, the line that connects the dots that are plotted. The parts of the teeth that
are lingual to the center trough are widened, while the parts of the teeth that are buccal to
the center trough actually get narrowed. For example, for the maxillary anterior teeth, the
roots are inside the center trough and get widened, causing more perceived distal root tip.
On the other hand, the crowns are on the buccal side of the center trough and get
narrowed, causing less perceived distal root tip. The end result will depend on how much
the root and crown points move because of widening and narrowing. The crowns of the
maxillary front teeth are more buccal than the roots are lingual as compared to the plotted
trough; therefore, there is less distal root tip in the constructed panoramic images as
compared with the 3D measurements.
The mandibular anterior teeth did not show significant differences because they
are a lot narrower than the maxillary anterior teeth and both widening and narrowing will
not have a large enough effect. The real 3D tip of the teeth may also be an influence. If
55
the teeth have 0° of tip, both widening and narrowing will not cause any change when the
3D data is converted into a 2D constructed image. All of the tips of the mandibular
anterior teeth were close to 0°. This explains why torque does not affect the differences
between 2D and 3D measurements.
The maxillary first premolars showed a statistically significant difference while
the maxillary lateral incisors did not. This may be a gray area because when comparing
the two sets of measurements, the maxillary lateral incisors actually showed more 3D-2D
differences than the maxillary first premolars (1.17 vs 1.04). However, the standard
deviation of the 3D-2D differences for the maxillary lateral incisors is a lot larger than
that for the first premolars (2.96 vs 1.49). Furthermore, there are significant differences
for the maxillary right lateral incisor, but not for maxillary left lateral incisor and for the
maxillary right first premolar, but not for maxillary left first premolar. It might be
beneficial to look at the clinically significant differences, which showed that there were
actually more clinically significant differences (above 2.5°) for the lateral incisors (36%)
as compared to the first premolars (17%).
The mandibular molars show significant differences between the 2D and 3D
measurements. A possible explanation is when doing the 2D measurements, the root
point might have been placed more apically than when measuring the 3D measurements.
Since both mandibular molars have curved roots and choosing a root point closer to the
apex will give these teeth more distal root tip. This error will only affect the two
mandibular molars because they are the only teeth that have roots curving distally.
56
In summary, the 2D angular measurements from the constructed panoramic image
may vary based on the position of the tooth in the dental arc (curve or straight), the 3D
torque, the 3D tip, the width of the tooth, relative buccal or lingual position of the crown
point and the root point, the length of the tooth, where the center trough is plotted,
whether the crown and root points of one tooth are within one single trough block, and
possibly other variables.
The most accurate way that is available to orthodontists clinically would be using
3D data to find the mesiodistal angulations of the teeth. The accuracy of a CBCT volume
is typically limited only by resolution or pixel size. Several studies have verified the
accuracy of measurements from CBCT image volumes, with most focusing on linear
measurements. Additionally, Marmulla et al concluded that the digital volume
tomographies of the NewTom 9000 provide images that are geometrically correct, and
Mischkowski et al concluded that the CBCT device provided acceptable information
about linear distances and volumes.
60-61

With the increasing body of evidence that shows distortion on 2D radiographs
with no clear trend in those differences, more studies are likely to arise to determine if the
distortions can be quantified. Similar to the study by Mckee et al, the 2D constructed
panoramic images have less significant differences than the traditional panoramic
radiographs. 3D scans are the most accurate and as the resolution of the images are
improved by the emerging imaging technology, new data processing software, and proper
technique is used to avoid patient movement during scanning, there will be a push
towards a 3D model.
57
Limitations
There were several limitations in this study:
1. The accuracy of the 3D measurements relies heavily on the clarity or resolution
of the images. The image quality in this study was not ideal. This was reflected in
the relatively low ICC for the repeated 3D measurements as compared to the ICC
for the 2D constructed panoramic image measurements. This limitation may have
to do with the 3D scanner itself and the software used to render the 3D images. If
improvements in the hardware and the software systems to increase the image
qualities are made, the digitizations will be more accurate.
2. The digitization of the center points for the root and the crown is tedious and very
time consuming. An average of 45 minutes is needed for the complete
digitization of one case. This will limit the use of direct 3D measurements to
research. The process of the digitization is also subjective. This will generate
human errors due to the irregularities in the shape of the crowns and roots of most
teeth, and especially for the teeth with multiple curved roots. The
alternative would be to define the centers mathematically, allowing the software
to find the crown and root centers automatically and objectively. This would
require very complicated algorithms and may be a possibility for the future. Once
the digitizations are made, the tooth-specific coordinate system for measuring
individual tooth tip and torque is done mathematically and the errors are kept to a
minimum.
58
3. The radiation to the patients is much higher than traditional radiographs. The
effective radiation dosage is 3~11uSv for panoramic radiographs and 5~7 uSv for
cephalograms. For a CBCT scan, the radiation dosage is 40~135 uSv, close to that
of a chest X-ray, or equivalent to 4~17 days of natural background radiation. The
challenge for researchers and manufacturers is to make 3D imaging systems that
require less radiation, shorter scans, but offer images with better
resolutions. Caution against excessive radiation has prompted the cessation non-
selective use of CBCT on all orthodontic patients as part of the initial orthodontic
records at USC.
4. The patients included in this study have "near normal" occlusion and the outcome
of this study may not predict measurements that are ideal for all patients.

59
Chapter Seven: Conclusion
Although 2D constructed panoramic images are more accurate than traditional 2D
panoramic radiographs, considerable errors can be generated when tooth angulations are
measured because there are a lot of variations in teeth and the measurements are sensitive
to many factors. The results of this study indicate that there are significant differences
between 2D constructed and 3D measurements in the following teeth: maxillary centrals,
maxillary canines, maxillary first premolars and mandibular first and second molars.
These results can be used by orthodontists when analyzing 2D panoramic images when
determining root alignment and quality of treatment. Further research is needed to help
determine if similar results can be obtained when different variables are introduced into
the study such as patients with non normal occlusion, patients that have undergone
extraction treatment, and patients with conventional panoramic radiographs. The results
of this study should be interpreted with the knowledge that they may only be relevant to
the patients selected in this very specific sample group and caution should be used when
applying it to orthodontic patients.

60
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Abstract (if available)

Abstract The purpose of this study was to evaluate if there are differences between mesiodistal angulations measurements for all the teeth in constructed 2D panoramic images as compared to 3D measurements. Pre-treatment 3D DICOM data was obtained from the University of Southern California’s patient database and mesiodistal angulation measurements from the constructed 2D panoramic images were compared to 3D coordinate system measurements (gold standard). Thirty three cases were chosen based on the specified inclusion and exclusion criteria. Results indicated that there was a relative trend in the differences in the tooth mesiodistal angulation measurements between the 3D and 2D measurements (p<.007). The maxillary central incisors, canine, and first premolar showed significant differences in the measurements, while the lateral incisors, second premolars, and molars did not show significant differences. On the other hand, the mandibular incisors, canine, and premolars did not show significant differences in the tooth mesiodistal angulations, while the molars did show a significant difference in the measurements. From this study, it can be concluded that there are significant differences for some teeth between angulation measurements obtained from the 2D constructed panoramic images and those directly obtained from 3D measurements.

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Asset Metadata

Creator Sakai, Nicole (author)

Core Title Comparison between tooth mesiodistal angulation measurements from constructed panoramic images and three-dimensional volumetric images

School School of Dentistry

Degree Master of Science

Degree Program Craniofacial Biology

Publication Date 03/28/2011

Defense Date 03/10/2011

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag dental,mesiodistal angulation,OAI-PMH Harvest,orthodontics,panoramic radiographs,three-dimensional

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Paine, Michael L. (committee chair), Enciso, Reyes (committee member), Moon, Holly (committee member), Sameshima, Glenn T. (committee member)

Creator Email nicolesakaidds@gmail.com,nsakai@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-m3704

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