Any kid with a penchant for magnets knows: Keep a nail on a permanent magnet long enough, and the nail becomes magnetized. The same can happen to steel in rooms exposed to the magnetic fields generated by MR scanners. The magnetic field left behind when
Any kid with a penchant for magnets knows: Keep a nail on a permanent magnet long enough, and the nail becomes magnetized. The same can happen to steel in rooms exposed to the magnetic fields generated by MR scanners. The magnetic field left behind when these scanners are pulled out and the rooms put to another purpose can distort the output of x-ray-based and nuclear medicine systems, or even twist the PC screens used in day-to-day office work.
"It can turn a magnet room into a no man's land for other equipment," said Tobias Gilk, an associate architect with Junk Architects in Kansas City, MO.
PET and CT equipment, conventional nuclear medicine systems, and much computerized office equipment do not work well in magnetic fields. When magnetic contamination reaches such a level, at or even below 1 gauss in some cases, the room may be good for little more than storing paper records, Gilk said.
"Magnetic contamination is a large issue," he said. "Whether that contamination will be at levels that cause problems to the equipment that replaces the MR in the former magnet room will depend on a number of factors."
Major factors are the amount of steel used, how close it is to the magnet, length of exposure, and strength of the magnetic field. Buildings are typically framed with steel, and ferrous metal makes up the rebar in concrete floors. Iron or steel pipe may be built into walls. The steel plating installed to passively shield MR scanners will be magnetized over time.
The risk of magnetic contamination will go up with the adoption of 3T scanners, Gilk said. Field strength is a major factor in magnetic contamination.
"We are concerned that the doubled field strength will impart stronger residual magnetic charges to ferrous materials near the magnet," he said. "This has the potential of making contamination problems greater for a second or third use of a magnet room."
Vendors can do nothing to make their MR scanners less likely to cause magnetic contamination. They can, however, improve the quality of information being provided to facility owners, architects, and engineers.
"By letting them know about the potential for residual magnetic contamination, it will push owners and their designers to anticipate and prevent the problems," Gilk said.
One way to do that is to be clear in the siting information that manufacturers provide owners and architects.
"Many owners and architects take a manufacturer's siting information as gospel," he said. "And if there isn't a warning on the drawings, then there must not be a concern, goes the logic."
The only sure way to reduce the risk of contamination is to build rooms with as little steel as possible. Building products are available that can nearly eliminate ferrous content surrounding a magnet, Gilk said. For example, fiberglass can replace rebar in concrete floors. Using such materials, however, takes the insistence of building owners.
"High-strength nonferrous building materials are relatively new and not widely used in the building trades," Gilk said. "Even with manufacturers' admonitions against using steel, because of the shimming problems it can create, architects and structural engineers make many buildings out of steel as a matter of course."
If steel structures are used, columns and beams closest to the magnet should be oriented perpendicular to it, he said. Aligning them with the axis of the magnet causes these components to be magnetized more quickly. Special care should be taken to minimize steel in structures that cannot be easily replaced.
Rooms already contaminated can be "degaussed," a process that uses electromagnetic fields to strip the polarity from magnetized objects. Degaussing, however, is more an art than a science, according to Gilk, who advises prevention rather than gambling on an uncertain cure.