What is a Cone-beam Xray?
A new technology is changing the way a general dentist or specialist looks at patients - literally.
No longer do they need to rely only on two dimensional x-rays for diagnosis, treatment planning and patient education. X-rays now come in three and even four dimensions, including manipulation of those images with new programs.
Cone-Beam Computed Tomography (CBCT) in dentistry is high resolution, low distortion, digital imaging of the hard tissues of the head. Instead of pixels, the resolution is measured in voxels, and often is sharper than a conventional CT. Cone-beam refers to the cone shape of the x-ray beam, unlike conventional CT, which uses a fan-shaped beam to create multiple thin slices.
CBCT produces panorex and cephalometric projections, which become 3-D when the data is reformatted in a volume. The images that result can be manipulated with the machine’s visualization software from any point of view: in the axial, coronal, sagittal and cross-sectional planes.
Layers can be “peeled away” to show hidden underlying anatomical structures and defects, slice thickness changed, the structures tilted, zoomed and highlighted. These are views that most dentists have never seen. Treatment decisions can change drastically with the advent of advanced views.
The image can be worked up for soft tissue enhancement, and can even be shown in eye-popping color and video, limited only by what the software can do.
And unlike conventional CT, which must make multiple passes and takes up to 45 minutes for usable images, CBCT 360-degree scans are of a single, large area in which the x-ray source and a reciprocating area detector move in tandem around the patient’s head, and takes only seconds to be created.
This makes a big difference in radiation exposure for the patient: under 100 uSv (microsievert), compared to between 1200-3300 uSv for conventional CT. For comparison, daily background radiation is approximately eight.
CBCT also translates into much lower cost for the patient running from $75 to $400, depending on services and scans provided. Insurance coverage is inconsistent.
A number of x-ray labs and centers are opening across the country to provide these services. Hunter Perret, CEO of iMagDent, which opened in August in Lafayette, La., is the only outpatient dental imaging facility in the state.
“This technology and business are in their infancy,” said Perret. “CBCT is a boutique application of medical imaging for dentistry, and is going to be widely accepted by the dental community. It probably will become the standard of care.
“There is a misconception that it’s only for implants. It’s also for oral surgery, TMJ, orthodontic treatment planning, prosthodontics, and endodontics in the future. ENTs are looking at it for sinus and airway studies. Compared to the ‘medical’ CT, our cone-beam i-Cat (Imaging Sciences) is cheaper for the patient, has extremely less radiation, it’s fast (20 seconds), and there’s no waiting at the hospital.”
At last count, 14 systems were on the market, with more coming under the names Galileos 3D by Sirona, CB MercuRay by Hitachi, Iluma by Imtec/Kodak, and 3D Accuitomo by Morita.
As these companies compete, the equipment is coming down in price. Some systems cost just under $200,000.
Because it is about the same size as a regular panorex X-ray machine, it can fit into most dental offices easily. Larger practices and some individuals are buying equipment for their own offices and not relying on outside centers.
Among incidental findings, Dr. William S. Moore, associate professor of the
University of Texas, Dental Diagnostic Science, said there is some question as to whether scans should be read by a dental radiologist for other abnormalities that may be present.
“The question of liability is still up in the air, though many of these scans are limited in volume to just the jaw or the teeth and won’t have much else to see,” he said. “We can’t read every scan -- it’s not necessary -- and there aren’t enough dental radiologists. Selection criteria on whether or not to order scans will have to be established, like we’re working on over here for orthodontics. Also, better training for clinicians on reading scans and what to look for on them needs to established.”
Moving toward “virtual dentistry,” the merging of 3-D digital photos of the patient’s teeth, along with their CBCT study to create a computerized virtual study model, is tied to dental laboratories for impressions and surgical stents. Dentists will work with these models before they perform work on the actual patient.
Panoramic x-ray has been used for years for implant placement and extractions, but it is 2-D, and has 13 percent visual distortion.
Cone-beam improves selection of implant size and type, confirms the correct angle and depth of placement in the alveolar bone, as well as the amount of bone present. Some machines even assess bone density. The DICOM 3 compliant images can be dropped into programs such as Sim/Plant and NobelGuide for implant treatment pre-planning.
Oral surgeons don’t like surprises when they are operating:
Where is the root of that bicuspid; is it up in the sinus? How complicated is that wisdom tooth going to be; what is the involvement with adjoining teeth and tooth roots? A CBCT study may guide surgical access to a tooth without damaging other vital structures, especially avoiding the inferior alveolar nerve canal in the lower mandible to minimize numbness.
For better overall treatment planning and avoiding mistakes, CBCT provides a lot more information for arch size crowding, unerupted teeth, impacted teeth and jaw asymmetry.
James A. Pearce, DDS, TMD specialist, said “though MRI is still the gold standard in imaging the temperomandibular joint for soft tissue changes, CBCT is very good for bony defects and abnormalities, asymmetry, spurs, and condylar changes.”
Other maxillofacial uses for CBCT include orthognathic surgery, fractures, and jaw pathology.