Ferromagnetic Detection In The Palm Of Your Hand.

By Tobias Gilk

It is no secret that I am a supporter of the development of ferromagnetic detection as a supplement to regular patient and device screening for MRI suites. Unlike conventional ‘airport-style’ metal detectors which alert on aluminum, brass and non-ferrous stainless steels, the new ferrous-only detectors don’t alarm on materials that aren’t missile threats in the MRI environment. In short, they’re specifically designed to identify those materials that threaten patient safety and magnet operation.

I’ve been fortunate enough to personally test two current products at radiology trade shows, a ferrous detection walk-through portal offered by Kopp Development, marketed as Ferralert, and a hand-held detector by Mednovus, marketed as the SafeScan Target Scanner. I like each company’s product and recently Mednovus provided me with a demonstration unit to test which has reaffirmed my belief in ferrous detection, in general, and impressed me with this product’s performance, in particular.

The Target Scanner is about 15 inches long, weighs slightly less than 2 pounds and looks like a thickened paddle. It is powered by a wall-plug and a generously long power cord, which doubles as a tether to keep the instrument out of the magnet room, and it is quite simple to use.

Turning the instrument on, it runs through a few seconds of diagnostics, LED’s lighting up and the speaker buzzing, sounding almost like a fax machine’s ‘handshake’ tones. After its diagnostics, it emits a low ‘hum’, indicating that it’s on and ready.

On the surface of the paddle is a raised area, roughly 2 inches by 4 inches, which is the detector area. Inside the case is a DC magnet, which induces a magnetic field in ferromagnetic materials in proximity to the raised detector area. By rubbing the paddle flat over a person, it is quite easy to identify ferrous materials in a subjects’ pockets, ferromagnetic grommets, clasps, zippers or jewelry that at person may be wearing, or hairpins or barrettes which may not be readily visible.

When the detector passes over a ferromagnetic object, it lights up an array of LED’s on all sides and emits a ‘squawk.’ The number of LED’s illuminated and the pitch of the audible alert indicate the strength of the signal. The device is sensitive enough that, if you rotate it quickly in your hand, it will slightly register the ambient magnetic field of the Earth.

The detection sensitivity depends on factors such as an object’s size, ferrous content and distance from the detector surface. To ascertain just how sensitive it was, we ran a short battery of tests. I extracted the ball from a ballpoint pen, a sphere of slightly less than 1 mm in diameter, and cut a single standard office staple into pieces, using one leg of a single staple (roughly .25 mm x .5 mm x 5 mm), and half of a leg (roughly .25 mm x .5 mm x 2.5 mm).

While all of my sample materials were ferromagnetic, this test was intended to provide anecdotal results and not highly scientific data suitable for a technical paper. I can’t speak to the specific metallurgical properties of any of them, or the degree of material similarities between the sphere and the staples. Though less than a finely-controlled experiment, it does provide a rough basis for evaluation.

I tested each sample at different distances from the scanner, approximately 1 mm, 13 mm, and 25 mm, and recorded whether the Target Scanner alerted fully, partially, or nominally – just above the background levels.

A standard 1/4 inch binder clip, tested at 25 mm, caused the instrument to alert fully, as did other larger ferromagnetic objects at that distance. Though larger objects caused the scanner to alarm at distances beyond 25 mm, Mednovus states that the scanner should be within an inch (25 mm) of the area to be scanned.

All of the distance tests were performed using a 25 mm (1 inch) stack of paper as a spacer, demonstrating that the scanner can easily penetrate clothing and potentially identify discreet ferromagnetic body piercings on a subject's body in addition to pocketed threats. Facilities wishing to test the sensitive depth of their own Target Scanner may want to use a resource that should be handy, the 2006 edition of the Reference Manual for Magnetic Resonance Safety, Implants and Devices by Frank Shellock, Ph.D., which is almost exactly 25 mm thick.

The sensitive distance also demonstrates that it is capable of alerting on shallow ferromagnetic objects under the skin. Despite the fact that the product is not a medical device and not FDA approved to find subcutaneous materials, Dr. Emanuel Kanal of the University of Pittsburgh Medical Center, has used it successfully to identify ferromagnetic properties of shallow shrapnel and metal slivers in patients. This is in addition to its standard use of scanning the pocket areas of all patients that are to be scanned in their street clothes, as well as any guests that accompany them in the scan room.

While we recommend gowning all MRI patients, for those facilities who scan subjects in their clothing we strongly recommend a final verification through ferromagnetic detection that the patient or visitor isn't carrying any inappropriate or potentially dangerous materials into the magnet room. Additionally, ferromagnetic detection is encouraged for screening all non-patient visitors and those who may not be able to assist in their own screening for any reason (including pediatric, sedated or unconscious patients, or from perceptual or language barriers).

In addition to patient screening, the device is also useful for determining the ferromagnetic properties of tools or devices that may be brought to the MRI suite. Though limited to roughly an inch of sensitivity depth, the device will readily identify ferromagnetic gas cylinders, hand-tools, walkers, and other devices with exposed surfaces. For bulkier objects, such as furniture, anesthesia machines, medication pumps, the detector will not be able to register the ferromagnetic properties of deeper components and should not be used to verify the missile-threat potential of these objects.

For all of its sensitivity, there are potential concerns about the magnetic field it uses to be capable of detecting tiny threats. The Target Scanner emits a field which is about 100 gauss at one inch from the detector surface (diminishing to about 1 gauss at 12 inches). As a point of reference, test magnets provided by pacemaker manufacturers to put pacemakers into test mode can reach field strengths of up to 500 gauss.

Magnetic fields of the strength of the Mednovus Target Scanner can disrupt the operations of some biostimulation implants, though both Medtronic and St. Jude have informed Mednovus that short exposures to fields of this strength will not disrupt the operation of their contemporary devices. Older devices, or devices from other manufacturers, may not live up to this standard, so caution with all implants is prudent. However, any contraindicated devices should have been identified long before the patient is scanned with any ferromagnetic detector. Ferrous-only detectors should be used as a quality check after all other patient screening has been completed. Should the Target Scanner (or any ferrous-only detector) reveal a previously unknown biostimulation device, thank your lucky stars you found it before placing the patient in the bore of a 15,000 gauss scanner!

The other potential risk posed by the Target Scanner’s magnetic field is that it could potentially move shallow ferromagnetic objects within a patient’s body. The only area where this is believed to represent a significant potential hazard is for intra-ocular bodies not bound by scar tissue. To mitigate this risk, Mednovus advises that the scanner should not be rubbed immediately around the eye. To screen eyewear, patients should remove their glasses to have them screened off of their body.

In general, I found the Mednovus SafeScan Target Scanner very easy to use and surprisingly effective at identifying even very small threats. It does take a few minutes to get the technique. It’s not like the wands you see at the airport, which are held a couple of inches off of a subject. This instrument needs to be right up against the patient to be effective. And while I wouldn’t want to carry it around all day, it isn’t unduly heavy. The background hum the device makes when on is a little distracting, so we switched it off between tests, but starting it back up only takes about 5 seconds. The newest versions of the product come with a two-position volume switch, however, allowing a ‘quiet’ setting.

Many have debated the efficacy and reliability of ferromagnetic detection for MRI facilities. Based on our tests, I feel that the technology – and certainly this product – is ‘ready for prime time.’ The risks of patient injury, shim disturbances, equipment damage and lost throughput are just too great to not seriously consider ferromagnetic detection devices as an adjunct to conventional clinical and physical screening protocols.

If you'd like any further information on ferromagnetic detection and how it can be employed to enhance the safety of your facility, contact Robert Junk, AIA or Tobias Gilk. We will be pleased to help you.