Dressings Containing Silver and MRI - Clarification: Antimicrobial Dressings Containing Silver Aren't Necessarily Hazardous during MR Scans*

*ECRI Institute. Antimicrobial dressings containing silver may cause pain and burns during MR scans - Hazard Report and Clarification. Health Devices 2007 Jul;36(7):232-3 and 2008 Feb; 37(2):60-2. Copyright 2007, 2008 ECRI Institute. Reprinted with permission.

In our July 2007 issue, we published a Hazard Report titled "Antimicrobial Dressings Containing Silver May Cause Pain and Burns during MR Scans" in which we reported on a patient who suffered pain during a magnetic resonance (MR) scan. The patient was wearing dressings containing silver, which acts as an antimicrobial agent. We discussed the possibility that the silver in the dressings might have been heated by the radio-frequency (RF) energy that is present during any MR scan.

Our report on this incident has given some readers the impression that patients wearing dressings containing silver should never undergo MR scans. That is not the case. We are publishing this update to further explain the issue and clarify our recommendations. In particular, we want to make the following points clear:

First, the fact that a patient is wearing silver-containing dressings should not keep the patient from having an MR examination.

Second, the reason that some silver-containing dressings are contraindicated by the dressing manufacturer for use during MR scans is because they could produce artifacts and distortions in the MR image, not because of any risk they might pose to the patient (although such risk cannot be conclusively ruled out).

Third, the exact cause of the patient's pain in the reported incident was never determined; the silver-containing dressings are only one of the possible causes.

DISCUSSION
Our position on silver-containing dressings in MR scans.
We believe that the presence of antimicrobial dressings containing silver should not disqualify a patient from undergoing an MR scan. We do advise, whenever feasible, that such dressings be removed before a scan and that the wound be washed with water to remove as many traces of silver as possible. The reason for doing so is to minimize any detrimental effects the silver might have on the quality of the image. If it isn't feasible to remove the dressings, we believe the scan should still be carried out.

However, clinicians should be aware that there may be a risk, albeit remote, that the silver in the dressings - some of which will have been absorbed into the wound and its exudate - could become heated by the RF energy present during the scan (see Possible Causes of Pain, below). As in any MR procedure, the patient should be monitored closely, and any report of unusual pain or heating should be promptly addressed, as described in our Recommendations.

Note that the dressings are only likely to be a concern if the dressed area will be included in the scanned area. If, for example, a head scan is being performed and there are silver-containing dressings on the patient's foot, there is probably no reason the dressings need to be disturbed.

Possible causes of pain.
In the reported incident, the patient was an amputee whose stump was being scanned. The stump had been covered with silver-containing dressings. During the MR acquisition, the patient experienced significant pain, and the scan was aborted.

A week later, the study was continued, after the dressing had been removed and the wound lightly washed with water at the advice of the dressing manufacturer. However, the patient still experienced significant pain, and the reporting hospital noted that the amputation stump retained a silvery sheen despite the washing. It was suspected that the MR energy had interacted with the silver. But no further testing was undertaken to verify that suspicion.

Silver is not ferromagnetic, so the patient's pain could not have been caused by any magnetic effects of the MR field on the silver. As discussed above, though, MR scans create RF energy, which can induce electrical currents within electrical conductors. These currents can cause heating and lead to patient burns. This has been previously reported by Wagle and Smith (2000) and Franiel et al. (2006). However, empirical testing with implants and devices shows that excessive heating only occurs for those that have an elongated shape forming a closed loop of a certain diameter (Dempsey and Condon 2001). In the case that we reported, it is conceivable that the silver formed a large conducting loop around the wound; but the large amount of silver that would be needed makes this unlikely. Also, although the patient felt a burning sensation, there were no visible signs of a burn. So the evidence is far from conclusive.

RECOMMENDATIONS
Until further empirical testing is done to establish the required conditions, if any, necessary to produce a heating effect, the following revised recommendations should be followed:

(1) Alert MR healthcare workers that some silver-containing antimicrobial dressings are contraindicated for MR exams in order to avoid artifacts. Therefore, if contraindicated dressings are close to the region being scanned or within the transmit RF coil, they should be removed if possible. However, a patient wearing such a dressing should not be prevented from having an MR procedure.

In some cases, it may be possible to reapply the same dressing to the patient after the scan. Whether this can be done depends in part on the amount of exudates on the dressing. The decision to reapply a dressing must be made by qualified staff.

(2) As with any MR procedure, the patient should be carefully monitored and instructed to immediately report any unusual pain or heating. All such cases should be investigated immediately. If it is not possible to determine the cause of the pain and alleviate the problem (e.g., by applying a cold compress), then the procedure should be abandoned.

(3) Report any similar occurrences of patient pain or burns to the dressing manufacturer and to ECRI Institute. Full details of the procedure should be recorded, including the field strength, coil used, and scan parameters.

REFERENCES
Dempsey MF, Condon B. Thermal injuries associated with MRI. Clin Radiol. 2001 Jun;56:457-65

Franiel T, Schmidt S, Klingebiel R. First-degree burns on MRI due to nonferrous tattoos. AJR Am J Roentgenol. 2006;187(5):W556

Tope WD, Shellock FG. Magnetic resonance imaging and permanent cosmetics (tattoos): survey of complications and adverse events. J Magn Reson Imaging. 2002;15:180-4.

Wagle WA, Smith M. Tattoo-induced skin burn during MR imaging. AJR Am J Roentgenol. 2000;174:1795.

Wagner M, Lanfermann H, Zanella F. MR-induced burn-reaction in a female patient with "permanent make-up". Rofo. 2006;178:728-30.

Yuh WT, Fisher DJ, Shields RK, Ehrhardt JC, Shellock FG. Phantom limb pain induced in amputee by strong magnetic fields. J Magn Reson Imaging. 1992;2:221-3.