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A Basic CBCT Review & Interpretation Protocol John W. Graham, DDS, MD

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By John W. Graham, DDS, MD

The very essence of the difference between standard medical CT scans and cone beam CT scans is that CBCTs are acquired with one, low-energy, cone-shaped beam delivered via an isocentric revolution around the patient's head (similar to a panoramic radiograph) – with reconstruction of the acquired data via complex algorithms. Medical CT scans reconstruct multiple slices of patient anatomy acquired with high-energy, multi-rotational fan-shaped beams. The clear advantage of CBCT technology is decreased radiation to the patient. The details of CBCT capture and acquisition are beyond the scope of this brief article, but excellent articles exist on the subject.¹

The advantages of CBCT data on all patients in a private practice cannot be fully appreciated this early in the technology's development. That being said, some of the potential benefits are worth addressing.

Likely, the greatest of all possible benefits of CBCT datasets is the myriad of radiographic studies that are available to the clinician with one simple scan. Nearly any conceivable skeletal view is at the doctor's fingertips from the day of the scan into perpetuity. Think about that value alone. Gone are the days of wishing you'd taken tomograms or forgetting to take a frontal. You have a view of everything! One scan, with about the same gamma radiation as a high resolution panoramic X-ray solves all these problems.

The potential applications don't stop there; think of the clarity of instruction when a parent or patient can easily visualize the severity of impactions, crowding, or asymmetries as the 3D skull is rotated, sliced, highlighted and magnified. Communication with specialists and referral sources becomes clearer, more precise and more powerful.

The evolution of DICOM interpretation software is quickly moving orthodontists to the exciting and profitable reality of single-scan-based appliance fabrication and "impressionless" initial and final study models.

Protocol-based evaluation of radiographs is the de facto method that ensures thorough and quick assessment of patient records. The inherent advantage of utilizing a systematic protocol on each patient is that repeatability fosters reliability. It is up to each individual clinician to develop and modify their own radiographic evaluation protocol (something that hopefully is already in place for the evaluation of 2D X-rays) to suit their practice needs. What is provided below is only one example and certainly is not meant to imply that it is the only protocol, or the best protocol, to evaluate cone beam radiographs.
  1. Start global: Begin with the full-volume 3D rendering of the skull and teeth. Slowly revolve the skull starting by viewing the patient's right side and rotating around the front and completing on the left side. The benefit of starting with the full-skull rendering and observing from "30,000 feet" is that subtle abnormalities such as mandibular asymmetries, maxillary cants (often observed initially as uneven piriform apertures) and the like may become more obvious than if initially viewed as "slices."
  2. Coronal slices: Starting from the anterior and moving distally scroll through coronal slices looking for abnormalities such as impacted or supernumerary teeth; observing bone thickness in edentulous areas; noting generally sinus symmetry; and presence or absence of sinus wall thickening. Sinus tissues that appear unusually thick are an indication of possible chronic sinus disease, possibly affecting breathing and jaw development. A coronal view provides visualization of the medial and lateral poles of the TMJ, as well as general torque of posterior teeth.
  3. Saggital slices: Start from the right side and scroll through to the left side. Sagittal views allow for visualization of anterior tooth torque, as well as a second scan for abnormalities such as impactions, dilacerations and the like.
  4. Axial slices: Starting from the top of the head and scrolling down, observe general symmetry of sinuses, tooth roots, TMJs and other structures. The axial view is excellent for the visualization of cortical bone adjacent to tooth roots both on the lingual and buccal surfaces. The axial views allow for a third scan for abnormalities.
  5. TMJ view: Utilizing the TMJ views in the DICOM software, observe both joints for obvious abnormalities such as beaking or flattening. Compare the sizes of both sides assuring symmetry. Ensure that the condyles are seated in the fossae evenly on both condyles, and that the cortical bone of each condyle is even and uninterrupted.
To reiterate, this is only one protocol and can be easily modified or added to. Many software programs allow for multiple view windows on one screen. Thus a practitioner may have the 3D volumetric representation of the skull, with three additional windows opened on the same screen representing the coronal, saggital and axial views.

However it may be done in an individual's office, remember that the key is repetition, and that repeatability fosters reliability. The entire evaluation of a patient's CBCT records utilizing a protocol such as this, need not take more than just a few minutes, and is generally done prior to entering the consultation room. Any noted abnormalities may be referred to an oral maxillofacial radiologist for further evaluation and formal interpretation, and multiple services exist which provide simple and secure internet-based transfer of DICOM data for this very purpose. This provides the examining clinician with the peace of mind that a formal radiological exam has been provided and documented in the patient's records. While not necessary on all patients, it is a nice adjunctive service to have at the orthodontist's fingertips.² CBCT technology is here to stay, and as a profession, we should be most grateful. Gone are the days of poor and inaccurate radiographic diagnosis. We have the opportunity now as orthodontists to safely provide our patients with the best and most advanced diagnostic techniques available, and in doing so, provide them with the best treatment possible.

* The above Protocol is offered to show what the authors are using in their practices. As with all new technologies, as more experience is obtained, the protocol may change without further notice. Before adopting any approach, each practitioner should carefully evaluate the protocol for examining and interpreting CBCT image datasets. In addition, each clinician is encouraged to obtain additional education and training in all areas relevant to this important area so as to offer ethically and legally competent care for their patients.

References:
  1. German, D.S., German J.: Cone beam volumetric imaging: a two minute drill, J. Clin. Orthod. 44: 253-265, 2010.
  2. www.beamreaders.com
Author’s Bio
John W. Graham, DDS, MD, graduated from Baylor College of Dentistry then received a medical degree from University of Texas Southwestern Medical School. He completed an internship in general surgery at Parkland Memorial Hospital followed by training in oral and maxillofacial surgery. Graham received his orthodontic certificate from University of Rochester/Eastman Dental Center, New York. He is on faculty at the University of the Pacific, Arthur A. Dugoni School of Dentistry. He maintains a private practice and lectures nationally and internationally on modern orthodontic techniques. Graham is one of a handful of professionals who have training in dentistry, medicine, oral surgery and orthodontics.
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