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"Dental CBCT has been available for only about 15 years, but has already found many uses in the practice of oral surgery despite a limited evidence base for its diagnostic efficacy," wrote Keith Horner, BChD, MSc, PhD, Odont Dr, a professor at the University of Manchester School of Dentistry.

Because CBCT is often associated with higher radiation doses than conventional radiographic techniques, "careful attention must be given to justification and optimization," he added.

Much of the published literature involving CBCT is dominated by opinion and case reports, Dr. Horner noted, with few studies focused on patient outcome efficacy.

Given that children represent a significant proportion of orthodontic patients and that similar cone-beam CT exposure settings likely result in higher equivalent doses to the head and neck organs in children than in adults, researchers from the State University of New York at Stony Brook wanted to measure the difference in equivalent organ doses from different scanners under similar settings in children compared with adults.

Two phantom heads were used, representing a 33-year-old woman and a 5-year-old boy. Optically stimulated dosimeters were placed at eight key head and neck organs, and equivalent doses to these organs were calculated after scanning. The manufacturers’ predefined exposure settings were used. One scanner had a pediatric preset option; the other did not.

Scanning the child’s phantom head with the adult settings resulted in significantly higher equivalent radiation doses to children compared with adults, ranging from a 117% average ratio of equivalent dose to 341%, the researchers reported. When the pediatric preset was used for the scans, there was a decrease in the ratio of equivalent dose to the child mandible and thyroid.

Collimation should be used when possible to reduce the radiation dose to the patient, and use of cone-beam CT scans should be justified on a specific case-by-case basis, the study authors concluded.

But there has been little analysis of how dental materials in a patient’s mouth may affect the end result, according to a new study in Dentomaxillofacial Radiology (June 2013, Vol. 42:6).

“Magnetic susceptibility information is not readily available for many materials used in dentistry, especially those containing several components,” wrote the study authors, from the University of California, San Francisco; University of Würzburg; and University of Regensburg. “Partly contradictory results have been reported regarding the severity of image artifacts caused by different dental materials.”

Such artifacts are most likely caused by one of two things: eddy currents induced by alternating gradients and radiofrequency magnetic fields, and distortion of the static magnetic field due to the difference in magnetic susceptibilities of materials and body issues, the researchers noted.

Degree of compatibility

Using a 1.5-tesla MRI system (Magnetom Avanto, Siemens) and spin-echo and gradient-echo pulse sequences, the researchers investigated the potential influence of metal, ceramic, polymer, and composite dental materials on MRI. They then applied the geometric method to determine the magnetic susceptibility of the materials, and classified the materials based upon their degree of compatibility with MRI.

“Although the magnetic susceptibility values of many materials are unavailable, they can be estimated from the caused distortions in an MR image,” they wrote.

Here is a list of the materials included in the study:

AH Plus resin (Dentsply)
Amalgam (Degussa)
Cobalt chrome (CoCr; Amann Girrbach)
Composites: Filtek Supreme XT Universal, Filtek Supreme XT Flowable, Filtek Z250, Filtek P60 posterior restorative (all 3M ESPE); Tetric Ceram, Tetric Flow (Ivoclar Vivadent)
Glass ionomer cementer (3M ESPE)
Gold alloy (DeguDent)
Gold-ceramic crown (DeguDent)
Gutta-percha (Demedi-Dent)
Titanium alloy (Friadent)
Zirconium dioxide (Metoxit)
Orthodontic wires: Nickel-titanium (NiTi) alloy wire, stainless steel wire, stainless steel brackets (all Dentaurum)
“The tested materials showed a range of distortion degrees,” the study authors wrote.

Even so, several were classified as fully compatible, meaning they can be present during an MRI exam even in the tooth of interest, and even if a very precise reconstruction of the tooth surface is required: AH Plus resin-based sealer, glass ionomer cement, gutta-percha, zirconium dioxide, and some composites.

Iron oxide pigments

While none of the 3M ESPE composites studied caused any detectable distortions, all of the Ivoclar Vivadent composites in the study caused “significant” artifacts, the researchers noted.

“Whereas composites of some manufacturers had an almost perfect susceptibility match to water and were therefore compatible for dental MRI, other showed markedly paramagnetic properties and caused significant distortions,” they wrote.

This could be because manufacturers of these products often use iron oxide pigments, and “the smallest contamination by ferromagnetic substances can drastically alter the susceptibility of a magnetically compatible material,” they noted.

In addition to the Ivoclar Vivadent composites, amalgam, gold alloy, gold-ceramic crowns, titanium allow, and NiTi orthodontic wires were classified as compatible I, meaning the material produces “noticeable distortions” and that acceptance when undergoing MRI depends on the application.

The stainless steel orthodontic appliances and the CoCr sample showed the strongest distortions and were classified as noncompatible, meaning these materials produce strong image distortions even when located far from the imaging region, according to the study authors.

“Although solutions for artifact correction are being sought, foreign materials in the body remain an issue,” they wrote.

Their proposed compatibility classification system, which is based on quantifiable magnetic susceptibility, could serve as a guideline for future MRI research in dentistry, they concluded.

MRI’s future in dentistry

Totally noninvasive MRI applied to brain, maxillofacial, and even dental imaging shows a high sensitivity and specificity for many diseases, noted Djaudat Idiyatullin, PhD, who is involved in dental MRI research and development at the University of Minnesota Center for Magnetic Resonance Research.

“MRI certainly has a future [in this field], and it is unfair that in the future somebody having some paramagnetic filling in his mouth would be denied the advantages of MRI screening,” he told DrBicuspid.com. “That is why I think this study analyzing this problem is very important to somehow stimulate the manufacturers to develop new materials with matched magnetic susceptibility, and also to help dentists avoid using MRI noncompatible materials in the practice.”

It would be good to have some regulatory standards requiring the magnetic susceptibility value limit for such materials, he added.

Julian Boldt, PhD, of the University of Würzburg and co-author on the Dentomaxillofacial Radiology study, agreed.

“The materials we tested are commercially available, but there are commercially available alternatives that are MRI compatible,” he told DrBicuspid.com. “Other materials will have to go the same way if MRI gets off the ground in dentistry, which I think it will.”

Boldt and his colleagues are close to completing a prototype of a dedicated dental MRI scanner and are looking for commercialization partners. In addition to diagnostic imaging, MRI has also shown potential for therapeutic applications in dentistry, such as digital impressioning, he noted.

People who received frequent dental x-rays have an increased risk of developing meningioma, the most commonly diagnosed primary brain tumor in the U.S., according to a study published today in Cancer (April 10, 2012).

Ionizing radiation is the primary environmental risk factor for developing meningioma, a largely benign brain tumor, and dental x-rays are the most common artificial source of exposure to ionizing radiation for individuals in the U.S., the study authors noted.

They also studied information from a control group of 1,350 individuals who had similar characteristics but who had not been diagnosed with a meningioma.To examine the link between dental x-rays and the risk of developing meningioma, Elizabeth Claus, MD, PhD, of the Yale University School of Medicine and Brigham and Women’s Hospital, together with colleagues from multiple other academic medical centers, studied information from 1,433 patients who were diagnosed with the disease between the ages of ages 20 and 79 and were residents of the states of Connecticut, Massachusetts, North Carolina, the San Francisco Bay Area, and eight counties in Houston, TX, between May 1, 2006 and April 28, 2011.

To avoid attributing the effect of therapeutic ionizing radiation to dental x-rays, individuals who had received therapeutic radiation to the head, neck, chest, or face were removed from all analyses that assessed the risk associated with dental x-rays, the study authors noted.

“All epidemiology studies are imperfect and only suggest a correlation, not a cause and effect,” Dr. Claus told DrBicuspid.com. “But we did collect information on other sources of common ionizing radiation, particularly therapeutic. We tried to control for other things that we knew would be an issue.”

‘Apparent association’

Over a lifetime, patients with meningioma were more than twice as likely as controls to report having ever had a bitewing exam, the researchers found. Individuals who reported receiving bitewing exams on a yearly or more frequent basis were 1.4 to 1.9 times as likely to develop meningioma as controls. (Risks differed depending on the age at which the exams were done).

An increased risk of meningioma was also linked to panorex exams taken at a young age or on a yearly or more frequent basis. Individuals who reported receiving these exams when they were younger than 10 years old had an increased risk (4.9 times) of developing meningioma. Those who reported receiving them on a yearly or more frequent basis were 2.7 to 3 times (depending on age) as likely to develop meningioma as controls.

Previous studies have reported similar findings, the study authors noted (Neuroepidemiology, 1989, Vol. 8:6, pp. 283-295; Oral Oncology, July 1998, Vol. 34:4, pp. 265-269; Cancer, March 2004, Vol. 100:5, pp. 1026-1034). However, no recent large-scale studies of meningioma risk relative to common ionizing radiation exposure exist, when doses for dental and other procedures have decreased but new radiography procedures have been introduced, they added.

“To our knowledge, this is the largest case-control study to date examining the correlation between dental x-rays and the risk of meningioma,” the study authors wrote. “And because it is the most recent study, it provides an improved examination of the effects of reduced dosing exposure levels over time.”

Dental patients today are exposed to lower doses of radiation than in the past, they noted. Allan Farman, BDS, MBA, PhD, DSc, a professor of radiology and imaging science at the University of Louisville in Kentucky and immediate past-president of the American Association for Oral and Maxillofacial Radiology, agreed.

“This is not the first time that the subjective memories of patients with meningioma have resulted in an apparent association between dental x-ray use and the likelihood of meningioma,” Dr. Farman told DrBicuspid.com. “In each case, the conclusions clearly state that associations are related to radiation received many years ago when dental x-ray exposures were substantially higher than is presently the case.

“Perhaps inclusion of information on radiation exposure of non-dental origin, such as medical CT to the head — which is orders of magnitude greater in dose than that from dental radiographs — would have been beneficial to better view the findings in a wider context.”

Time to ‘Image Wisely’

Other study limitations include the possibility of under- or over-reporting of dental x-rays by the participants, Dr. Claus and her co-authors noted.

“This is a difficult problem in epidemiology because, unlike medical care, which … may be confirmed by a review of centralized medical records, dental care generally is obtained … from numerous dentists, all of which are outside of a health maintenance organization or hospital-based setting, providing little opportunity for researchers to validate dental reports in a timely or cost-efficient manner,” they wrote.

Arthur Goren, past director of radiology at SUNY Stony Brook School of Dental Medicine and currently clinical associate professor in the department of cariology and comprehensive care at New York University College of Dentistry, also questioned the study’s methodology.

“The only way bitewing x-rays can cause brain tumors is through scatter radiation,” he told DrBicuspid.com. “If the scatter reaches the brain, why are tumors not found in the thyroid, eye, parotid, and oral epithelium? We have dosimetry studies of cone-beam CT exposures to the head and neck of adult male and female and juvenile phantoms that show that the other organs received more scatter radiation than the brain and cranium.

“I realize CBCT dosimetry is not an adequate comparison with bitewing dosimetry, but to base findings on anecdotal recollections and not on statistical data is a reach,” he added. “I wonder what the conclusions would be if you substituted cell phone use instead of bitewing x-rays.”

Nevertheless, Dr. Farman said, “this study has value in that it does remind clinicians (including dental practitioners) that x-radiation has been defined by the U.S. Food and Drug Administration as a carcinogen and that radiographs should only be made following professional prescription and never as a mere routine.”

The study presents an ideal opportunity in public health to increase awareness regarding the optimal use of dental x-rays, which, unlike many risk factors, is modifiable, Dr. Claus noted. The ADA’s guidelines for healthy persons suggest that children receive 1 x-ray every one to two years; teens receive one x-ray every one-and-a-half to three years, and adults receive one x-ray every two to three years (Journal of the American Dental Association, September 2006, Vol. 137:9, pp. 1304-1312).

The use of ionizing radiation should always be based upon professional judgment after taking a history and clinical inspection, Dr. Farman added.

“We should always ‘Image Wisely’ irrespective of the region being examined, and this includes radiographs needed for dental diagnosis,” he said. “With careful selection made by a professional, there is no doubt that dental-imaging procedures can reduce pain and suffering and also that early intervention can reduce the costs of treatment intervention involved.”

Image sensor developer Awaiba has introduced what is being dubbed the smallest camera in the world, with potential applications in dental imaging and medical endoscopy, according to a story in PC World and other news reports.

Awaiba sees the NanEye 2B being used for a variety of medical purposes, including dental imaging and medical endoscopy, and it could even be used to give surgical robots a boost, PC World noted.

A University of Missouri engineering team has invented a compact source of x-rays — about the size of a stick of gum — that could be used to create inexpensive and portable x-ray scanners (IEEE Transactions on Plasma Science, January 2013, Vol. 41:1, pp. 106-111).

The researchers expect to have a prototype handheld x-ray scanner using the technology in about three years, according to Scott Kovaleski, PhD, an associate professor of electrical and computer engineering. The cellphone-sized device could improve medical services in remote and impoverished regions and reduce healthcare expenses everywhere, he added.

In dentistry, the compact x-ray generators could be used to take images from the inside of the mouth shooting the rays outward, reducing radiation exposure to surrounding tissues, Kovaleski noted in a university press release.

The device uses a crystal to produce more than 100,000 volts of electricity from only 10 volts of electrical input with low power consumption. Having such a low need for power could allow the crystal to be fueled by batteries, the researchers noted. The crystal, made from lithium niobate, uses the piezoelectric effect to amplify the input voltage.

The accelerator could also be used to create other forms of radiation in addition to x-rays, according to Kovaleski.

“Our device is perfectly harmless until energized, and even then it causes relatively low exposures to radiation,” he said. “We have never really had the ability to design devices around a radioisotope with an on-off switch.”