By Ronald A. Bulard DDS*
and Scott Keating, VP Engineering, IMTEC (a 3M Company)
What is Cone Beam Computed Tomography (CBCT)?
More and more dentists are using X-ray Cone Beam Computed
Tomography (CBCT) scanners for patient imaging and
diagnostics as a new and vital part of their practices. The result
is advanced, state-of-the-art volumetric images that increase
the quality and accuracy of radiographic dental care. When
using CBCT imaging, clinicians have the most accurate
anatomic information to plan the placement of dental implants
in optimal sites by using technologically advanced digital
imaging devices.
CT software and hardware options
have been improved considerably,
while the cost to own this technology
has come down in recent years. This
has resulted in many more doctors
making the decision to implement
CBCT into their practices.
Computerized tomography is advancing rapidly. The imaging
source-detector and the method of data acquisition distinguish
cone beam tomography from traditional CT imaging.
Traditional CT uses a high-output rotating anode X-ray tube,
while cone beam tomography utilizes a low-power, medical
fluoroscopy tube that provides continuous imaging throughout
the scan. Traditional CT records data with a fan-shaped, helical
X-ray beam onto image detectors arranged in an arc around the
patient, producing a single slice image per scan. Each slice
must overlap slightly in order to properly reconstruct the
images. The advanced Cone Beam technology uses a coneshaped
X-ray beam that transmits onto a solid-state area sensor
for image capture, producing the complete volume image in a
single rotation. The sensor contains an image intensifier and a
CCD camera or an amorphous silicon flat panel detector.
Bone Density
Maximum Intensity Projection
Cephalometric Image
Dental Reformat Axial
The single-turn motion image-capture used in cone beam
tomography is quicker than traditional spiral motion, and can
be accomplished at a lower radiation dose as a result of no
overlap of slices. This type of imaging exposes a patient to less
radiation than traditional CT scanners. Manufacturers are designing
Cone Beam Scanners with the physical space
available in clinics and the patient's comfort in mind. For
example, upright seating is used in CBCT scanners with the
X-ray tube and panel detector rotating around the
patient's head.
Current generation CBCT software capabilities
CBCT imaging provides important information about the 3-D
structure of nerve paths, soft tissue and bone.
3-D software can shade images to differentiate varying
densities of facial structures. Grayscale shading provides the
ability to view the relationships of common internal anatomy.
Traditional CT imaging renders an 8-bit grayscale (256 shades)
or 12-bit grayscale (4,096 shades). Present-day scanners render
images in 14-bit grayscale, providing 16,384 shades. Color
coding the image by density further distinguishes anatomical
structures, enabling the clinician to view pertinent anatomy
while planning implant cases, such as nerves and nasal cavities,
and mandibular and maxillary dimensions. Segmentation
literally cuts the volume rendering, conceding top views, side
views, and CT slices that produce unlimited axial, coronal and
sagittal views. CBCT slices are as thin as 0.1 mm, compared to
1 mm for a conventional fan CT scan.
Image data may be obtained for a complete dental/maxillofacial
volume or a limited region of interest. With the current
generation of Cone Beam scanners, scan times for these types of
images vary from about 20 to 40 seconds for the complete
volume and as few as 8 seconds for the regional scan.
Several software programs on the market allow dentists to
segment and transform the digital images into tessellated
polygonal models, e.g., STL, which are used in the creation of
surgical guides.
Multi-Planar Reconstruction
Panoramic With Nerve Mapping
Pan, Ceph and 3-D in one scan
CBCT devices may be used for traditional forms of
radiography, in addition to advanced 3-D volumetric
renderings. Conventional cephalometric measurements may be
obtained through 3-D volumetric images by rendering the
image as a 2-D projection resembling a radiograph or a
panoramic image. It is also possible to digitize cephalometric
points in 3-D, resulting in the introduction of multiple
analyses.
Non-conventional renderings are also realized using CT data.
In fact, X-ray projections that are non-physical are possible.
For example, a cephalogram can be created where the patient's
neck vertebrae have been omitted. The advantage of this type
of rendering is that in a conventional ceph, the vertebrae may
obfuscate the dentition, whereas in a non-conventional
rendering, they do not.
