Forensic dental radiography 197of a thin slice of tissue selected from within the object being imaged. CBCT
provides numerous new and traditional image projections related to various
tasks related to dental diagnosis, such as panoramic, cephalometric, and TMJ
images. The device also allows the creation of multiplanar, real-time recon-
structions (MPRs) of the maxillofacial structures from three planes: axial,
sagittal, and coronal, all from one scan. The three planes are also linked to
each other so that any entity within the object may be observed from three
different views in an interactive manner.
This CT system is termed cone beam because the beam is not thin or fan
shaped such as that used in typical medical CT units. The beam is instead three-
dimensional and produces three hundred or more full-size images of the selected
tissue field from 360° of rotation about the patient. Each individual projection
is similar to a lateral or posterior-anterior (PA) cephalometric radiograph. The
transmitted energy from the tube is received by either an image-intensifying
tube married to a charge coupled device (CCD) or a flat-panel receiver com-
posed of amorphous silicon. The CBCT image is comprised of three-dimensional
image units known as voxels. A voxel is similar to a pixel except that it includes
the third dimension. Where a pixel is a lateral and vertical image unit, the voxel
is in the shape of a cube (e.g., 0.2 × 0.2 × 0.2 mm). The equal-sided voxels are
termed isotropic, unlike medical CT systems in which voxels are rectangular
with an axial measurement of 2 mm and cannot reach the resolution pro-
vided by CBCT. The resolution of the images is determined by the voxel size
and can range from 0.4 mm to as small as 0.125 mm in some units. Cone
beam CT also produces fewer streak or star artifacts than medical CT when
the beam strikes metallic objects such as dental restorations within the scan
zone. The scatter artifact is also primarily confined to the horizontal plane.^16
After the captured images are acquired, they are reconstructed together
in all planes by the system’s computer algorithm and displayed on a com-
puter monitor. Other than that fundamental principle, CBCT units vary in
size, shape, image receptor technology, and the volume size of the tissue field
that may be captured. At least two machines currently available scan the
subject in the supine position, which makes them suitable as morgue and
mass disaster equipment.
The imaging possibilities with CBCT technology and its possible forensic
applications are almost limitless. Any intraoral or extraoral film image view
can be reconstructed from one scan, with the operator having the ability to
select any desired slice location and orientation. Thus, it is possible to plot a
plane within the confines of the dental arches and produce a panoramic image
without the overlying ghost images and artifacts of conventional panoramic
radiography. The thickness of the panoramic image layer may also be deter-
mined in an unlimited manner. Also, computer algorithms correct for the
geometric distortions that are present within all plane film images. Thus,