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Inside Dentistry

July/August 2011, Volume 7, Issue 7
Published by AEGIS Communications


Cone Beam CT

In the past 6 months, the one greatest change in the CBCT environment is increasing public awareness of this technology and growing concern about radiation dosage. The origin of this change can be traced to an article that appeared in the New York Times on November 22, 2010, under the lead authorship of Mr. Walt Bogdanich.1 More recently, the Sendai tsunami disaster in Japan and associated problems with the nuclear reactors in Fukoshima has made the public ever more attentive of issues related to radiation dose. The pediatric medical community had preempted the response to this concern through the Image Gently Alliance (www.pedrad.org/associations). The American Academy of Oral and Maxillofacial Radiology (AAOMR) represents the dental community in this Alliance. The counterpart Alliance for adult diagnostic imaging is Image Wisely (www.imagewisely.org). The AAOMR teamed with the American Association of Endodontists to publish guidelines for use of CBCT in endodontics.2

Hopefully the dental industry will now be refocused on using professional judgment and seeking evidence for selecting CBCT, as well as other procedures involving ionizing radiation, in dentistry. Some speakers on the circuit have been particularly cavalier in terms of downplaying the issue of ionizing radiation dose, and in suggesting what essentially was routine use of CBCT for most persons undergoing orthodontics, including replacement of traditional impression materials by ionizing radiation as an alternate “impression” media. What was being ignored is the fact that many patients receiving orthodontic treatment are young and that such individuals are thought to be especially susceptible to the potential untoward effects of ionizing radiation.3,4 Due to the longer life ahead of them and the more rapid turnover of cells in young individuals, it has been calculated that children and young teenagers can be up to an order of magnitude more susceptible to effects of ionizing radiation than mature adults.

Hence, dentistry and medicine should be applying professional judgment on procedure selection in order to image with gentle, “child-size” radiation doses that are As Low As Reasonably Achievable (ALARA). In the Lewis Carol classic, the Queen of Hearts demanded: “Sentence first—verdict afterwards.” As professionals, we should more sensibly first be seeking scientific evidence supporting efficacy before proclaiming our verdict. Unfortunately, many on the speaking circuit have had much in common with the upside-down logic expounded by Lewis Carol’s Queen of Hearts.

Without a doubt, CBCT is a valuable imaging modality wherever 3D details are needed, such as relating implant placement to the mandibular canal, or assessing an impacted third-molar root relationship to that same structure. It is certainly a benefit to use CBCT with low dose settings as a substitute for higher-dose, multi-slice CT, where this had been previously employed in planning orthognathic surgery or surgical orthodontics. The take-home point should be that “routine” is always wrong for X-ray procedures, including—but not restricted to—CBCT. A history should be taken from the patient, who should then be examined by the dentist before prescribing imaging procedures. Image selection should be individualized and always based on professional judgment.

Unfortunately, the jury is still out on the impact of CBCT on outcomes of dental treatment. While there are many nicely illustrated case reports and series, and much “expert opinion,” there is a dearth of scientific publications correlating use of CBCT to treatment outcomes in dentistry. Given the number of factors involved in successful treatment outcomes, it will be difficult to separate the effects of imaging procedures used from other matters impacting such outcomes.

Certainly in endodontics, detecting additional root canals in teeth where these are obscured by superimposed anatomic detail by 2D imaging is one area where limited field of view (or well-collimated FOV) using a voxel resolution of 0.2 mm or better has proven valuable.5 This is particularly the case for mesio-buccal roots of maxillary molars.6,7 For dental implant planning, the AAOMR is on record as recommending 3D imaging, and this is now most accurately achieved at a reasonable dose using CBCT. Any procedure in which there is a likelihood of damaging the mandibular canal has the potential to substantially affect a patient’s quality of life. It is my recommendation that where the mandibular canal is superimposed on the roots of the third molar on a panoramic radiograph, a CBCT is a good approach to achieve an accurate appraisal of the actual relationship and determine the best surgical approach to be used. As a minimum, this will provide a higher level of informed consent prior to surgery. It is recognized that using CBCT does not mean that damage to the inferior dental nerve will always be avoided. CBCT collimated to the maxilla and performed at high resolution can also be valuable in assessing impacted canines and their relation to adjacent teeth. This is certainly one case where CBCT is warranted for orthodontic assessment and treatment planning.

It should be recognized that there is not merely one CBCT, but rather multiple systems to choose from depending on the patient base of any given practice and the tasks to be performed. For the endodontist and general dentist, it perhaps makes most sense to purchase a hybrid panoramic/small FOV system that provides sufficient spatial resolution to demonstrate the periodontal ligament space and relatively fine root canals. For the oral surgeon, this can also be the case if the practice is based largely on third-molar extractions; however, should the practice extend to orthognathic surgery, evaluation of trauma patients for possible fracture, etc, a larger FOV system is indicated. Such a system, optimally, should permit collimation to smaller fields of view when these are appropriate, because the vertical field of view is a major factor determining procedure dose.

The choice is not merely between which CBCT to purchase, but also whether to make this investment or outsource to a local or mobile imaging center. Realistically, as more CBCT systems are deployed, most purchasers will need to make a purchasing decision based on the use of the product in their own practice.

I suggest outsourcing as a way to enter the practice of using CBCT and, based on the experience, to then make the purchase decision. Irrespective of whether you decide to buy, there are still opportunities to outsource some of the work, most especially the formatting of images and having consultations for diagnostic reports. Such outsourcing can provide a greater variety of services for which the average clinical dental practitioner might not wish to invest in software contracts or time from their busy practice.

References

1. Bogdanich W, McGinty JC. Radiation worries for children in dentists’ chairs. New York Times. Nov 22, 2010. Available at: http://www.nytimes.com/2010/11/23/us/23scan.html?_r=1&hp. Accessed April 25, 2011.

2. AAE and AAOMR. Use of cone-beam computed tomography in endodontics joint position statement of the American Association of Endodontists and the American Academy of Oral and Maxillofacial Radiology. Oral Surg Oral Med Oral Pathol Oral Radiol Endod.2011;111:234-237.

3. 1990 Recommendations of the International Commission on Radiological Protection. Ann ICRP. 1991;21(1-3):1-201.

4. SEDENTEXCT Project. Radiation protection: cone beam CT for dental and maxillofacial radiology. Provisional guidelines (v1.1 May 2009). Page 17. Available at: www.sedentexct.eu/content/provisional guidelines-cbct-dental-and-maxillofacial-radiology. Accessed Dec. 21, 2010.

6. Scarfe WC, Levin MD, Gane D, Farman AG. Use of cone beam computed tomography in endodontics. Int J Dent. 2009:634567. Epub 2010 Mar 31.

7. Sauer JN, Clark SJ, Bauman R, et al. Tomografia komputerowa związką stożkową w endodoncji: pole widzenia i rozdzielczość przestrzenna (CBCT in endodontics: field of vies and spatial resolution). eDentico 2010;2(26):54-63.

8. Bauman R, Clark SJ, Morelli JM, et al. Ex vivo detection of mesiobuccal canals in maxillary molars using CBCT at four different isotropic voxel dimensions. Int Endod J. 2011;44:752-758.

About the Author

Allan G. Farman, DSc (Odont)
Professor, Radiology and Imaging Science
Department of Surgical and Hospital Dentistry
University of Louisville
Louisville, Kentucky

President
American Academy of Oral and Maxillofacial Radiology
agfarm01@louisville.edu


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