Do Hand-Held Magnets = Ferromagnetic Detection?

In short, no. Hand-held magnets do not do the same job that ferromagnetic detectors do.

In many MRI facilities, foreign materials brought by people to the MR suite are tested for magnetic field hazards with high strength hand-held magnets. Ones designed specifically for MRI screening are far stronger than the ones holding up my daughter’s artwork on my refrigerator. Some of these ‘test’ magnets can be 1 Tesla at the surface (10,000 gauss)!

Example of hand-held MRI test magnet

These extremely powerful hand-held magnets can help users differentiate between superficial materials that are, and are not, ferromagnetic, but the extraordinary strength of these magnets introduces a number of additional cautions which limit their use.

First, the key word in the paragraph above is ‘superficial.’ The magnetic field of all permanent magnets drops off precipitously (field strength drops with the cube of distance… double the distance and the magnetic field is cut to 1/8th the original value), so permanent magnets will be useful for distinguishing ferromagnetic materials only at or near the surface of an object. Ferromagnetic components below the surface may go undetected by a hand-held magnet, but rest assured that the MRI will find them if those objects make it into the scanner room!

Second, the potential forces exerted on a ferromagnetic body with magnetic field strengths of near 1 Tesla mean that shallow ferromagnetic material within the body of the patient could be moved, perhaps dangerously, by these very strong magnetic forces. But if the purpose of screening is to prevent accidents instead of preemptively causing them, hand-held magnets are poorly suited for patient screening.

Third, if screening medical equipment instead of patients, even some pieces of equipment designed for use in MRI scanner rooms have maximum allowable static and dynamic magnetic field values. Sticking a 1 Tesla magnet all over an anesthesia machine may wind up having some unintended consequences with regard to operation.

Lastly, 1-Tesla magnets stick hard to things. While the hand held magnets aren’t weighty, their magnetic force can require a bit of elbow-grease to get them separated from the cart or medical gas cylinder to which they got stuck. No, it’s not like it becomes epoxied on, but wielding one of these high strength permanent magnets is not a trivial affair.

Each ferromagnetic detection product has its own limitations, so I’m not attempting to state that FMD systems are the perfect solution to the hand-held magnet problem. Hand-held magnets can be useful, in a limited range of uses.

When it comes to the recommendations of the ACR Guidance Document for Safe MR Practices, or the Joint Commission Sentinel Event Alert (#38) on MRI Accidents and Injuries, or the U.S. Veterans Administration’s new MRI Design Guide, the experts all seem to have recognized the benefits of ferromagnetic detection and made a clear distinction between the new technology and the old custom of using permanent magnets to test for safety.

Tobias Gilk, President & MRI Safety Director
Mednovus, Inc.

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