A little clarity on CBCT imaging

March 21, 2012

By now, we’ve all pretty much heard about just how much additional information and clinical data can be gathered by using cone beam 3D imaging. However, many GPs are not sure exactly which cases are best suited for this evolving technology, and there also has been confusion and concern in recent months about radiation doses and possible health risks with children exposed to too many CBCT scans.

By now, we’ve all pretty much heard about just how much additional information and clinical data can be gathered by using cone beam 3D imaging. However, many GPs are not sure exactly which cases are best suited for this evolving technology, and there also has been confusion and concern in recent months about radiation doses and possible health risks with children exposed to too many CBCT scans.

In response, we asked experts Dr. Allan Farman of the University of Louisville and Dr. Sharon Brooks of the University of Michigan to shed some light on the topic, offer advice for GPs and to provide clarity to aid in decision making around this technology.

“Many newer practitioners will already be familiar with CBCT as it is rapidly becoming part of the regular dental curriculum,” said Dr. Farman, Professor of Radiology & Imaging Science at the University of Louisville School of Dentistry. “Even in such circumstances, I recommend first using the services of a CBCT center while learning how to integrate CBCT into practice. In this way, the practitioner will be selective of cases needing the service. They also can better assess whether owning a system is going to be a good business decision rather than outsourcing CBCT imaging altogether. They also can assess the system that will best suit the diagnostic needs of their patients.”

Dr. Farman, who is the current President of the American Academy of Oral and Maxillofacial Radiology, suggests clinicians thinking about adding CBCT to their practice attend non-commercial training such as that offered by AAOMR in advance of trying to make sense of the best system to buy to meet their specific needs.

“An endodontist needs high precision but a large field of view is not necessary. As cost is largely predicated on the size of the detector it would not make good business sense for an endodontist to purchase a system that can image the whole head,” he said.

Another consideration in making a business decision is whether the practitioner should spend the time and effort in reading the whole image volume or if it is best to outsource that task in a similar manner to the medical model in imaging.

“With the relatively low costs of over-reads and reports, it is probably not worth the average busy practitioner’s time to read any 3D image volumes other than the smallest fields of view restricted to the dental arches,” Dr. Farman said. “Over time, after receiving the reports on many patients, the practitioner may become more comfortable to make some reports themselves…while being ready to refer for another opinion when the need arises.”

Dr. Farman and Dr. Brooks, DDS, MS, Professor, Division of Oral Pathology/Medicine/Radiology, Department of Periodontics and Oral Medicine, University of Michigan School of Dentistry, answered a few of our questions in an effort to make sense of some of the confusion surrounding CBCT use in dental practices and patient radiation exposure.

DPR: What do you suggest general dentists new to CBCT do in terms of becoming educated on radiation dosages? Also, on how to determine what cases/patients may not be best suited for using this technology?

DR. FARMAN: The AAOMR has initiated what will be a series of courses in CBCT at various levels. These are totally independent of influence from vendors eager to sell their wares. The first three Level 1 courses were run successfully at the ADA Annual Congress in Las Vegas and will be re-run at the Annual Congress of the AAOMR at the Knickerbocker Hotel in Chicago on Dec. 7. (More information can be found on aaomr.org.)

This Level 1 course can be taken just for CE credit but also provides a post-program exam for verification of competency of this level of knowledge. The program has seven elements and these include lectures on radiation safety/dose and on the current status of selection criteria. The course also provides details of normal CBCT anatomy, and variations in anatomy including both significant and insignificant pathological findings.

DR. BROOKS: There is quite a bit of information available on radiation doses for CBCT and other dental imaging techniques. One source that is readily available via the Internet is from the SedentexCT Project (sedentexct.eu). The SedentexCT Project is an evidence-based series of guidelines on the use of CBCT in dentistry, put together by a coalition of six European countries.

Everything in their recommendations is referenced to the literature and many other organizations around the world are adopting (or considering adopting) their recommendations. On their homepage, a person can click on “CBCT information” and get a chart that lists the radiation doses for a variety of CBCT machines, along with the references for these doses. There also is a comparison with background radiation doses that might be helpful to the clinician.

Almost all radiation doses reported today are in the form of effective dose. This is a dose calculated from the absorbed radiation dose to a number of tissues and organs. The organ doses are measured and then weighted, based on the radiosensitivity of the tissue, and summed to produce one number. This allows ready comparisons between different radiation techniques, such as CBCT, panoramic, intraoral, medical CT, as well as comparisons with days of background radiation equivalent. The weighting factors come from the International Commission on Radiological Protection (ICRP). In 2007 the ICRP adjusted the weighting factors to include more tissues and to better characterize the radiation risks. One of the tissues added was salivary glands. This is important for dentistry because the salivary glands are in the direct path of the x-ray beam in all dental imaging. For this reason, the effective radiation dose calculated with the 2007 weighting factors is approximately double what the dose was with the older (1990) weighting factors.

When comparing doses from various imaging techniques and equipment, it is important to know which weighting factors were used to calculate the effective dose. Everything published in the last few years should use the new weighting factors. If the dentist needs a refresher on radiation doses in general, consulting a current textbook on dental radiography should be helpful.

DPR: There was some publicity this past year about children being exposed to too much radiation in dental chairs. Do you feel this is a legitimate concern? If so, how can the profession best address it?

DR. FARMAN: As children and adolescents are thought to be the most susceptible to ill effects of radiation, due to the more rapid turnover of cells and the greater lifetime ahead during which untoward effects might develop, it is always best to minimize radiation exposure of the young. The “Image Gently” Alliance that represents many organizations in medicine and pediatrics, as well as the AAOMR, recommends that we child size radiation dosages for the young. There should never be routine use of x-radiation for diagnostic imaging where professional judgment of the individual case is not based in history taking and clinical inspection.

There are definite uses of CBCT in orthodontics on adolescents and children, particularly for evaluation of impacted maxillary canines and for surgical orthodontic cases, however, the AAO is on record as stating that CBCT is not indicated in all patients receiving orthodontics. One should always question the concept of when one only has a hammer the treatment will always be a nail.

Those who recommend routine use of any one technology are not doing their patients, the profession or the CBCT industry any favors. The AAOMR is attempting to educate dentists in the proper use of CBCT including the use of professional judgment. Parents should be particularly concerned should imaging be prescribed before their child is inspected by the clinician to determine imaging needs.

DR. BROOKS: In general, the radiation doses from CBCT are not high and in most cases the benefit of the added information acquired far outweighs the small increase in risk. However, children are much more sensitive to radiation than adults, both because many of their cells are still rapidly dividing as the child grows, making them more sensitive to radiation, and also because they have a much longer lifespan than adults, giving them more years to develop adverse effects of radiation.

The primary concern is with the development of cancer. Even though the risk is small, it is not zero. Thus, dentists owe it to their patients, particularly children, to keep the risks as low as possible. That does not mean that CBCT should never be used in children because there are situations where the 3D information obtained may have a large effect on the treatment provided and the success of the outcome.

However, the dentist should evaluate each patient individually to determine whether the 3D information is worth the small extra risk and not use the technology routinely on all patients. If a dentist does decide to take a CBCT scan on a child, there are ways to minimize the dose, and thus the risk. Examples of dose-reduction techniques include limiting the field of view (FOV) to the area of interest, using the lowest imaging parameters (tube current, tube potential, exposure time) that will still provide the needed diagnostic information.

If a dentist is considering the purchase of a CBCT scanner, selecting one that will allow the customization of imaging size and imaging parameters to the specific diagnostic needs of the individual patients is recommended, rather than one that has “one size fits all.”

DPR: Dentists acquiring this new technology may want to use it as much as possible. How can they best determine which cases do or do not require CBCT?

DR. FARMAN: Receiving training from individuals who are not too involved in promoting a purchase is an excellent way to start. There also are some guidelines. The most comprehensive of these come from the SedentexCT project of the European Association of DentoMaxilloFacial Radiology.

The ADA Council on Scientific Affairs is close to releasing some basic guidelines on CBCT use. Further, the AAOMR has, in conjunction with the AAE, developed guidelines for use of CBCT in endodontics. Additional discipline specific guidelines for CBCT use are in the process of development, including a joint position paper with the AAO. These guidelines are available on the aaomr.org website.

DR. BROOKS: All radiographic examinations, whether bitewings, panoramic radiographs or CBCT, should be based on the need for diagnostic information for the individual patient. The earliest guidelines on selecting patients for dental radiographic examinations were published by the FDA in 1987. The American Dental Association (ADA) in conjunction with the FDA updated these guidelines in 2004 and they are working on another update that should be available in 2012. The basic principle behind these guidelines is the use of selection criteria, the idea that there are situations that suggest the need for information that can be provided by an imaging examination.

The treating clinician is the person who should make the final decision on whether to obtain an imaging examination, including CBCT, which is why the guidelines cited above are not “rules” or “regulations.” The clinician must determine what type of diagnostic information is needed and then which is the best imaging technique to provide that information. It is not necessarily going to be CBCT, although there are many situations where the 3D information provided by CBCT is very helpful and maybe even required.

Today probably the most common use for CBCT is for implant diagnosis and treatment planning, although it is not yet clear whether CBCT is needed for all cases or for specific, more complex, situations, such as limited bone availability and location of anatomic structures. Many orthodontists today also use CBCT in place of traditional orthodontic imaging (panoramic plus cephalometric), but concerns have been raised about the extra radiation given to children with CBCT compared with standard imaging and whether it is “worth it” for most relatively uncomplicated cases. The literature supports the use of CBCT for evaluating impacted canines, for example, but there have been no studies to date that show an improved outcome of treatment in routine cases.

Over the next year there will be a number of position papers published on the use of CBCT (and other types of imaging) for various diagnostic tasks. The AAOMR has published one on endodontic imaging and has several others (implant, TMJ, orthodontic imaging) almost ready. When they are completed they will be posted on the AAOMR website. The endodontic paper is posted there now. Other groups also are working on guidelines related to imaging for their specific specialty.

DPR: Do most manufacturers offer (or require) dentists purchasing this technology the education and training needed to track radiation dosages?

DR. FARMAN: In certain parts of the world it is required to monitor CBCT dose. At least one of the manufacturers of CBCT incorporates a dose area product (DAP) in its NewTom VGi system and so dose is automatically recorded with this system. This is an approach that adds a proportionately small additional cost to manufacture when the total cost of a CBCT is considered. It is an approach that I would like to see followed by other manufacturers. Most dose studies in the literature are based on a single manikin and are often based on manufacturer recommended exposure. These dose figures only apply to the test situation and may well be exceeded for some individual or if the exposure parameters are modified by the user. Further, many of the tabloid non-peer review papers make comparisons that are not realistic and generally maximize levels of dose for the compared technology while minimizing those for CBCT.

DR. BROOKS: I am not aware of manufacturers offering education on tracking radiation doses. In their basic start-up training, they most likely provide information on the doses from their specific machine, but there is not a feasible way for users of the equipment to measure the doses they give.
DPR: Do today’s CBCT units vary greatly in the amount of radiation that the patient is exposed to? Are there studies available to dentists on radiation exposures from this technology and is this something GPs should consult on a regular basis?

DR. FARMAN: In general the newer systems are safer than some of those that were made available a decade ago; however, these older higher dose systems are still in use. Collimating the beam to the region of interest can save dose over use of a larger field of view. This is if the other parameters are kept equal and that may not always be the case. For finer detail (smaller voxels) a higher dose per volume might be used to improve the signal to noise ration where high definition is required.

The other two issues are time sequence with longer exposures (often for more basis images) generally resulting in higher exposures, and beam filtration to remove less penetrating x-rays that will not contribute to the image other perhaps than in adding noise. It is important with most CBCT systems that these be regularly re-calibrated to maximize image quality and the value of the exposure to diagnosis.

DR. BROOKS: Yes, the radiation doses vary significantly between the different machines. Many things can affect the dose including whether the radiation beam is on continuously or is pulsed on and off during exposure; the settings used during the scan (current, tube potential, exposure time); field of view; resolution (in general, high resolution scans require more radiation). The effective doses for many machines have been published in the literature. One easily available source for this information is the SedentexCT Project.

DPR: What one thing would you like to say to the industry in terms of how to make wise choices on using this technology to provide optimal care for dental patients?

DR. FARMAN: The manufacturers seem to be working toward lower dose and better image quality. Their role is indispensable in terms of training users on the efficient and effective use of the individual systems. This requires talented instructors who do not cut corners and should involve at least four hours of instruction using a manikin rather than a patient. Further, it would be appreciated should all companies provide a more balanced viewpoint in their marketing events. CBCT is an invaluable adjunct to other imaging modalities in dentistry but is not a universal tool for support of all dental diagnoses and procedures.

Once it has been decided (professionally) that a CBCT image sequence is needed, then the volume should be used to the maximum benefit of the patient. I am a little concerned about the use of ionizing radiation for CBCT as a replacement impression material should this be made the primary reason for the exposure. When ionizing radiation is to be used there should be a good diagnostic or treatment guidance reason so that the benefits to the patient exceed the risk, however small that risk might be.

DR. BROOKS: The one thing that I would say is to scratch the word “routine” from the vocabulary of both industry and users of CBCT equipment. To maintain the best benefit-risk ratio, the clinician should select the imaging examination based on the individual patient’s needs and then should optimize the examination to reduce the risk. Industry can help this latter by offering equipment with a variety of imaging parameters and a variety of fields of view in a way that is easy for the clinician to select the best one for the individual patient. Clinicians should not need to be radiation physicists to be able to select the best CBCT examination for a specific situation.