Download Three-Dimensional Surface and Volumetric Imaging: Emerging

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southern region
Three-Dimensional Surface
and Volumetric Imaging:
Emerging Orthodontic Tools
Presented by Dr. David Hatcher and Dr. James Mah at the PCSO Southern Regional Meeting, January 28, 2011.
Summarized by Dr. Douglas Hom, PCSO Bulletin Southern Region Editor.
C
one beam computed tomography (CBCT)
scanners were introduced into the U.S. dental
market in 2001 and have been gaining increasing acceptance and use in the orthodontic profession.
As with any new technology, the field has witnessed
rapid development and change. The ability to accurately
describe craniofacial morphology and anatomic relationships offers unprecedented opportunities and challenges
for orthodontic research and clinical practice.
CONE BEAM TECHNOLOGY
There are multiple differences between CBCT and medical CT, and the two technologies should not be confused.
During a CBCT scan, the scanner rotates around the head
and obtains multiple images. The newest machines utilize
flat-panel scanners, which are smaller and lighter than
older CCD-type sensors. Following image acquisition,
the scanning software reconstructs the collected images
into a viewable digital volume format. The smallest subunit in a CBCT volume is the voxel. Voxels are stacked in
rows and columns to form a 3-D matrix. The final output
is in DICOM (Digital Imaging and Communications
in Medicine) format and may be viewed with a variety
of commercial or non-commercial third-party software
packages. Clinicians can look at the entire volume from
any angle, or opt to read coronal, axial and sagittal views.
FUNDAMENTALS OF 3-D IMAGING
Multiple variables must be considered in a CBCT protocol. These include patient immobilization, scan time,
voxel size, field of view, and milliampere settings. The
field of view (FOV) parameter includes small (periapical
size), medium (both jaws) and large (head). The large
FOV is most applicable for orthodontic purposes. The
energy source for CBCT is in the 1 to 10 milliampere
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range, with most machines falling into the 2 to 5 milliampere range. This energy output compares to that of
a Panorex, and is much lower than that of medical CT
(100 to 160 milliamperes). The average scan time ranges
between 5 and 40 seconds, depending upon the type of
machine and protocol. To avoid blur in the final image,
the patient must be stable during image acquisition. For
this reason, shorter scan times present advantages. Image
resolution is affected by voxel size, with smaller voxels
offering potentially higher clarity. Currently, voxel size
varies from 0.04 to 0.07 mm.
PRACTICAL APPLICATIONS OF
CONE BEAM COMPUTED TOMOGRAPHY
Dental morphology and development (hidden supernumeraries and missing teeth): Two-dimensional
panoramic images can fail to show supernumeraries or
ectopic teeth which are hidden behind tooth roots. CBCT
scans allow the clinician to view behind tooth roots and
identify such hidden teeth.
Tooth/root positions: Panoramic tooth position can
vary greatly depending on head rotation and tip. For this
reason, evaluations of tooth/root position from panoramic
data alone are unreliable. CBCT imaging shows a completely accurate picture of tooth/root positions.
Bone volume (thin alveolar bone): Orthodontists encounter cases in which very thin alveolar bone covers the
tooth roots. CBCT imaging now allows the orthodontist
to accurately gauge the limits of root movement in these
cases. Another common situation is maxillary anterior
tooth intrusion in deep bite cases. CBCT can accurately
reveal the vertical dimension of bone apical to the maxillary incisors, and thus the limit of intrusion.
PCSO Bulletin • summer
2011
Implant placement: Specialized software has been
developed to allow virtual implant placement using
CBCT imaging. The final result can therefore be visualized before placement is performed. The process also
allows the clinician to recognize and avoid potential
treatment problems before they arise.
Airway assessment: The causal relationship between
airway disorders and malocclusion was described as
early as 1935. CBCT technology has kindled a renewed
interest in the airway and orthodontic manifestations.
Software-based airway analyses have been developed,
but clinical research must be done in this area to allow
for a fuller understanding.
Impacted teeth: Prior to 3-D imaging, the clinician
relied on a series of two-dimensional radiographs to
assess the position of an impacted tooth. CBCT allows
the clinician to view the impacted tooth in all three
planes of space and to accurately assess the tooth’s relationship to surrounding anatomic structures. With this
information, customized mechanics can be designed for
efficient guided tooth eruption.
CONCLUSION
CBCT has been shown to be a valuable tool in dental
and orthodontic applications. As the technology continues to evolve, new orthodontic research and clinical
applications will follow. Although it is still early in the
development cycle, it appears that CBCT imaging has
the potential to significantly alter the manner in which
orthodontic research, diagnosis, and clinical techniques
are performed.
S
summer
2011 • PCSO Bulletin
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