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3-D Imaging in Dentistry
Allan G. Farman, BDS, PhD, MBA, DSc
It is now almost a decade since the first CBCT system, the NewTom (QR, Inc, Verona, Italy; now a Cefla Company) received FDA approval for use in the United States. That system required the patient to be placed supine, and in appearance, this CBCT mimicked fan-beam CT scanners used in medicine. The NewTom provided a low-dose, reasonably affordable 3-D radiograph imaging system for use in the dental office. To that point, 3-D imaging had usually meant referral to a medical radiology office, where the CT system was focused at the whole body rather than the maxillofacial region, and the operators were not always cognizant of the diagnostic needs of dentists or the relatively high radiation exposure to the patient.
CBCT systems now abound, both in numbers and variety, and are already quite common in dental practices and dental imaging centers. There are supine systems (eg, Cefla/QR NewTom 3G; Cefla/Myray SkyView), sit-down systems (eg, ISI/i-CAT; Gendex CB500; Soredex Scanora 3D; J. Morita Accuitomo; Prexion 3D; 3M Iluma Elite; Genesys) and stand-up units (eg, Cefla/NewTom VG; J. Morita Veraviewepocs 3D; Kodak 9000 and 9500; E-Woo/Vatech Picasso Trio; Suni 3D; Sirona Galileos). There are large field-of-view (FOV) systems that can be used in evaluating craniofacial anomalies and planning orthognathic surgery, where multislice CT would have been used previously, at much higher radiation dosages to the patient. Generally, such full FOV systems are employed at relatively low isotropic voxel resolution (ie, 0.2 mm to 0.4 mm) to reduce the dose needed while reducing image noise, and also to permit reasonable reconstruction times.
At the other end of the spectrum, there are small FOV systems that usually provide higher resolution (ie, often 0.1-mm isotropic voxel resolution or better) that are ideal for such situations as endodontic assessments. These small FOV systems may be hybrid, providing 2-D digital panoramic and/or cephalograms. Hybrid systems are available at less than US $90,000, a price comparable to an upper-level 2-D pan/ceph not so many years ago, and perhaps less in constant dollar value. Small FOV systems can provide limited (ie, “focused field”) volume images of several teeth for approximately the same dose as two traditional intraoral radiographs. Given that multiple traditional images at different angles could be needed to evaluate an endodontic problem, small FOV CBCT might actually result in a dose savings to the patient.
CBCT is an adjunct to 2-D imaging in dentistry. 3-D imaging provides a clear relationship between structures that could be obscure on 2-D images. CBCT is useful for assessing impacted teeth, particularly the relationship between mandibular third molars and mandibular canals. It is also valuable in assessing implant positioning and pre-implant bone augmentation to provide the best possible prosthodontics reconstructive outcome.
Small FOV CBCT is valuable in assessing failed endodontics and perhaps also in primary evaluation of certain teeth prior to endodontics. However, CBCT does not replace 2-D imaging of dental caries. Beam hardening artefact from restorations and tooth enamel would result in a very large number of false positives for dental caries should current CBCT systems be used for caries detection.
Most controversial is the use of CBCT in orthodontic analysis, given the absence of evidence for improved treatment outcomes in a young population with a relatively high susceptibility to untoward complications of radiation. The effects of radiation in a teenager can be three times as high as in a young adult, and approximately an order of magnitude greater than in the older population most likely to receive dental implants. For this reason, in my role as president of the American Academy of Oral and Maxillofacial Radiology (AAOMR), I appointed Dr. Mansur Ahmad as position paper editor and initiated agreements for several position paper task groups to develop CBCT use guidelines. It is expected that these guidelines should be available before the end of this year. The AAOMR is joining with the American Association of Orthodontists (AAO) to develop imaging use guidelines for orthodontics. The AAOMR is also collaborating with the American Association of Endodontists (AAE) to develop CBCT use guidelines for endodontics. Additional guideline updates are being developed for temporomandibular joint evaluation and for imaging in dental implantology. There is also an agreement with the Academy of General Dentistry (AGD) to develop guidelines for dental caries diagnosis.
While the recent graduates from dental school may have some grounding in 3-D imaging and direct experience with CBCT during their studies, this varies between institutions. Most dentists already in practice have limited or no training in using 3-D images for dental practice, and there are few existing pointers for optimizing CBCT patient image selection. In some countries (eg, Greece, Denmark, and Germany) training to use CBCT is mandatory for all users. The AAOMR, under the leadership of its chairman of continuing education, Dr. Maria Mora, intends to introduce basic CBCT training and certification that is vendor-neutral. Such courses will cover radiation safety, image and image parameter selection based upon task, and basic 3-D anatomy, among other topics. Such courses might best be taken before purchasing a system to ensure that CBCT specifications match the tasks to be performed in the purchaser’s practice. The vendors of CBCT systems are eager for these courses to start, since there are many clinical questions after each sale.
While the average dentist should be able to fully understand the anatomic and disease findings from a small “focused field” image volume, there is still the need to train the dentist in image segmentation methods in order to get the most out of the available information, even with these systems. With large FOV systems, careful review of the full information contained in the image volume takes more time and expertise. In such cases, it is probably most cost-effective for practitioners to refer the image volume out for a careful review by a specialist in oral and maxillofacial radiology. There can be many findings within the CBCT volume significant to the health of the patient, and such findings are particularly common in older individuals attending dentists for dental implant treatment. The individuals ordering and making the CBCT volumes are certainly responsible to make a full interpretation, just as they are with panoramic and other 2-D images. Practitioners are no less responsible for failure to diagnose with CBCT than with any other radiographic image. To indicate otherwise would be a disservice.
3-D imaging provides accurate anatomic relationships between structures and is much easier to explain—often with simulations—to the patient. One might not always be able to preserve the integrity of the mandibular canal when extracting a third molar, even given a 3-D image, but one is more aware of potential complications and best approaches. The patient also is better informed before consenting to the procedure.
There might be practitioners who have relied upon panoramic images to place dental implants for three decades or more, and these “gurus” often do not see any need to move from what they perceive to be success, until they experience 3-D images and see where they were actually placing the implants previously. After that enlightenment, 3-D imaging becomes the rule. One can teach an “old dog” new tricks, at least when it comes to dental implantology. This is a benefit for both the dentist and the patient.
CBCT images can be used to provide virtual and actual models, even replacing initial alginate impressions in some applications. They can be used to produce surgical stents and devices to speed surgical procedures. This is possible using secure web-based services often located in another state or overseas. Similar image transmission is possible for second opinions that have the potential to improve access to care.
It is no longer technologically necessary for the patient to take a plane, train, or automobile to obtain one or more opinions from different geographic regions. These opinions are more frequently sought by a practitioner on behalf of their patient. Unfortunately, however, some state dental boards have either not kept up with the technology, or have created interesting legal issues surrounding out-of-state electronic consultations. For example, rather than the electronic data being seen as the equivalent of the patient visiting a dentist in another state, under the rules of the board in the state where that dentist works, some state boards believe the receiving dentist is “electronically transferred” back to the state where the patient resides. It is conceivable that the “remote” dentist could be accused of working without a license or malpractice coverage in the state where the patient resides. The simple answer might be a directive that the surrogate patient transferred electronically is equivalent to that patient being actually transported themselves. Hopefully this glitch will be resolved and the electronic era can reach its full potential.
Technology has arrived, and the profession now needs to deliver the necessary guidelines and education opportunities.