Growing demand for 3T MR raises concerns about safety

The wider use of 3T MR scanners, combined with the increasing number of patients with implants and the growth of MR as the preferred imaging modality in numerous applications, is prompting concern among regulators. They have raised questions about the compatibility of new-generation 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 U.K. Medicines and Healthcare products Regulatory Agency (MHRA).

Potential problems include heating, displacement, interruption of function of certain devices such as pacemakers, and production of artifacts. Local heating is of particular concern. The MHRA has received several reports of burns in patients with hip implants. Displacement of implants, such as stents, is the other main concern.

Hospitals are erring on the side of caution because they do not have access to up-to-date information on MR compatibility with implants, according to Dr. Janet De Wilde, manager of MagNET, an independent MR evaluation program supported by the MHRA. Two serious incidents have been reported involving patients with implants who underwent MR scans in the U.K., she said. Twelve 3T machines are currently installed in U.K. hospitals, and they are becoming more popular in teaching and research institutions. Industry estimates of the number of 3T systems operating throughout Europe vary from 50 to 60.

The MHRA is seeking 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 placing the onus on the MRI manufacturers.

"A new product comes onto the device market every day. Testing machines on all new devices is impractical," Ludgate said.

One option is to request information on equivalent ferromagnetic mass in relation to the mass of the device. According to a spokesperson for GE Healthcare, which has three 3T scanners installed in the U.K., low-field vertical open magnets may exert more force than 3T and 4T magnets because magnet designs are not the same.

"For the same field strength, small-bore magnets generally exert more force than whole-body magnets. So tests by implant manufacturers are essential. Ideally, if implant manufacturers provided information on the force exerted on the implant for a given 'force-product,' and if manufacturers provided the maximum 'force-product' for their scanners, the maximum force could be estimated for any magnet," the spokesperson said.

With reference to implant heating, radio-frequency heating limits are independent of magnetic field strength. Therefore, there is no reason for general heating or implant heating to be higher at 3T than at 1.5T, provided the scanner complies with regulatory limits, according to International Electrical Commission standards.

"MR compatibility of 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. "Patients with implants are, however, routinely scanned with 3T."

Siemens Medical Solutions' MR division has not heard of any incident involving heating of implants from its installed base of ultrahigh-field systems, according to Ioannis Panagiotelis, ultrahigh-field segment manager at Siemens.

"Regardless of field strength, 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.