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The popularity of 3T scanners is growing in Europe. Between 50 and 60 of the powerful MR scanners have already been installed, about a dozen in the U.K. alone. And demand is rising, particularly in teaching and research institutions, according to industry pundits. The proliferation of these scanners is raising concerns about their use.
"MR compatibility with implants has received much attention in the past, and this attention is now extended to 3T applications," said Hans Engels, MR safety manager for Philips Medical Systems in the Netherlands. "Patients with implants are being routinely scanned with 3T."
The increasing use of 3T scanners, combined with the higher number of patients with implants and the growth of MR as the imaging modality of choice in high-risk patients such as those with cancer and heart diseases, is prompting concerns among U.K. regulators about the compatibility of new-generation MR scanners and medical implants.
"We have quite a bit of information about certain implants at 0.5T and 1.5T but very much less as the strength goes up," said Dr. Susanne Ludgate, devices clinical director at the London-based Medicines and Healthcare Products Regulatory Agency (MHRA) of the U.K.
Who should address these concerns-the makers of implants or the manufacturers of MR scanners-is undecided. Adding to the uncertainty is debate over the severity of the problem.
Several potential problems have been identified: heating, implant displacement, interruption of function of certain devices such as pacemakers, and the production of artifacts. Of particular concern to the U.K. regulatory body is local heating. The MHRA has received several reports of burns in patients with hip implants. Another concern is implant displacement, which could be especially dangerous when it involves vascular devices such as stents.
Although just two serious incidents have occurred involving patients with implants who have undergone MR scans in the U.K., hospitals are erring on the side of caution because they do not have access to up-to-date information on MR compatibility with implants, said Janet De Wilde, manager of MagNET, an independent MR evaluation program supported by the MHRA.
The MHRA is looking for solutions to address the information gap. The regulatory agency appears to favor making the implant manufacturers responsible for generating safety and compatibility data for their devices, rather than the scanner manufacturers.
"A new product comes onto the device market every day. Testing machines on all new devices is impractical," Ludgate said.
One option put forward by Ludgate is to request information on equivalent ferromagnetic mass in relation to the mass of the device. This will require the cooperation of MR vendors as well as the manufacturers of implants.
The rationale is that different magnets affect implants differently. Low-field vertical open magnets may actually exert more force than 3T and 4T magnets, according to a spokesperson for GE Healthcare, which has installed three 3T scanners in the U.K. Similarly, small-bore magnets may exert more force than large-bore magnets, even if they operate at the same field strength.
Ideally, implant manufacturers would provide information on the force exerted on the implant for a given "force-product." Scanner manufacturers would provide the maximum force-product for their scanners. Putting this information together would allow estimation of the maximum force for any magnet, according to GE.
Implant heating may be less difficult to address. Radio-frequency heating limits are independent of magnetic field strength. Consequently, there is no reason to expect that general heating or implant heating will be higher at 3T than at 1.5T, provided the scanner complies with regulatory limits, according to International Electrotechnical Commission standards.
Underscoring this argument, the product monitoring group of Siemens' MR business is not aware of any incident involving implant heating resulting from the use of any Siemens 3T units, said Ioannis Panagiotelis, ultrahigh-field segment manager at Siemens Medical Solutions in Germany. That does not mean, however, that such a problem cannot occur.
"There is a contraindication for MR exams for patients with conductive or ferromagnetic implants, unless properly tested and released for the specific system condition," he said.
He advised that caution be exercised whenever MR is involved, regardless of field strength.