Worries about 3T effect on implants cause alarm among U.K. regulators, hospitals

March 21, 2005

The growing popularity of 3T scanners is raising concerns in the U.K. about the safety of patients with medical implants. The worries center on the increased power of this new breed of MR, which imposes a substantially higher specific absorption rate (SAR) than its predecessors.

The growing popularity of 3T scanners is raising concerns in the U.K. about the safety of patients with medical implants. The worries center on the increased power of this new breed of MR, which imposes a substantially higher specific absorption rate (SAR) than its predecessors.

Potential problems include heating; displacement, particularly of stents; interrupted function of certain devices, such as pacemakers; and production of artifacts. Local heating is of particular concern.

The Medicines and Healthcare products Regulatory Agency (MHRA) in the U.K. has received several reports of burns in patients with hip implants. Worries are being heightened by the lack of experience with these high-powered scanners.

"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 MHRA.

Experience with 3T scanners is limited, at least for now. Twelve such machines are installed in U.K. hospitals. Between 50 and 60 are operating at sites throughout Europe, according to consolidated industry estimates. These numbers could grow quickly, however, if manufacturers succeed in their efforts to make 3T the new benchmark for high-field MR.

In hopes of closing the information gap about 3T safety, the MHRA appears to be leaning toward making implant manufacturers responsible for generating safety and compatibility data for their devices, rather than placing the onus on the MR manufacturers.

"A new (implant) product comes onto the device market every day. Testing MR machines on all new (implants) is impractical," Ludgate said.

Support for such testing requirements is coming from the MR industry. A spokesperson for GE Healthcare noted that magnet design may be more a factor than field strength when it comes to determining the relative danger a scanner poses to patients with implants. Low-field vertical open magnets, for example, may exert more force than 3T and 4T magnets, according to GE, which has three 3T scanners installed in the U.K.

"For the same field strength, small-bore magnets generally exert more force than whole-body magnets. So tests by implant manufacturers are essential," the spokesperson said. "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."

One way to determine relative risks posed to different implants under varying field strengths, therefore, may be to determine equivalent ferromagnetic mass in relation to the mass of the device.

In regard to risk from heating, 3T systems may pose no greater risk than other systems, because radio-frequency heating limits are independent of magnetic field strength, 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 routinely scanned with 3T."

The concerns regarding 3T systems, therefore, may be a consequence of the newness of 3T and the perception that these systems may pose a greater risk simply because they are more powerful. Until experience verifies or contradicts this possibility, however, U.K. hospitals are proceeding cautiously, according to Dr. Janet De Wilde, manager of MagNET, an independent MR evaluation program supported by the MHRA.

This caution may continue for some time. U.K. hospitals lack access to up-to-date information on MR compatibility with implants, she said.