Invisible Metallic Microfiber in Clothing Presents Unrecognized MRI Risk for Cutaneous Burn

Discussion

Thermal burns are among the most commonly reported safety hazards for MR imaging patients. A recent Joint Commission Sentinel Event Alert, citing the Manufacture and User Facility Device Experience data base of the FDA determined that 70% of MR imaging complications were related to thermal burns.⁶ To the best of the authors' knowledge, this is the first reported case of MR imaging−related burns associated with clothing containing invisible SMF.

MR imaging−related burns may be due to direct radio-frequency energy deposition or, more commonly, to gradient magnetic field−induced eddy currents in electrical conductors.^7,8 Such currents have the potential to generate heat sufficient to cause cutaneous burns. This risk is thought to be greatest when the conductive material forms a closed loop of large diameter.⁹ MR imaging safety recommendations include ensuring that potentially conductive materials (eg, wires, leads, patient limbs) do not form loops and that no low-resistance electrical conductors contact the patient at >1 location, to minimize the possibility of forming a conductive loop involving the patient.⁹

We suspect that our patient sustained cutaneous burns secondary to conductive SMF embedded in her undershirt. We retrospectively imaged the undershirt by high-resolution digital radiography (Fig 3) and demonstrated an interleaving weblike pattern of radiopaque SMF. We further demonstrated the enhanced electrical conductivity of the SMF fabric relative to control polyester and cotton fabrics by measuring the resistance by using a volt/ohm multimeter. The undershirt demonstrated low resistance (on the order of 10 ohms), which increased monotonically as a function of the distance separating the probes. In contrast, the resistance for both the cotton and polyester fabrics was infinite.

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Fig 3.

A digital radiograph demonstrating the metallic content of the fibers of the undershirt. Left image shows the radiopaque silver microweave fabric from the Boston Silver T. Comparison right image shows cotton fabric from a thermal undershirt.

We hypothesize that during the MR imaging examination, electromagnetic eddy currents generated within the shirt fiber concentrated at the fabric seam and led to second degree skin burns along the sites of skin contact. Because the undershirt was longer than the overlying shirt, her right wrist was in direct contact with the seam and was also affected.

Attention to the prevention of thermal burns related to clothing must be emphasized as MR imaging systems with increasing field and gradient strengths become more common. The “ACR White Paper on MR Safety” advised that all patients undergoing MR imaging must remove all clothing that contains metallic fasteners, hooks, zippers, loose metallic components, or metallic threads.¹⁰ Furthermore, it stated that it is advisable to require that patients wear a site-supplied gown with no metal fasteners during the MR imaging procedure when feasible.¹⁰ We believe our previous policy conformed to these guidelines because we advised patients to wear a hospital-supplied gown, though we did permit patients to wear loose-fitting nonmetal-containing clothing of their own by request. Our limited survey of New England outpatient and hospital-based MR imaging facilities determined that this practice was common. We have since revised our policy and now require all patients to change into site-supplied MR imaging−compatible outer clothing and to wear nonmetallic cotton or other safe non-trade name undergarments for their examinations. Only in exceptional circumstances, because of severe physical constraints, will outpatients wearing clearly labeled 100% nonmetallic non-trade name fabrics be scanned without changing their clothing. Although fabric-content labels, regulated by the Federal Trade Commission, allow ≤5% impurity,¹¹ we believe that our current policy represents a reasonable compromise between patient care and safety.

Our patient's Boston Silver T shirt showed no visible or labeling evidence to indicate that it contained SMF and was purchased to be used under external appliances (eg, a back brace) because of its perspiration wicking and antimicrobial characteristics. The manufacturer has since voluntarily modified its label to identify the silver content with the trade label X-Static (Nobel Biomaterials, Scranton, Pennsylvania) and to specifically warn against use during MR imaging examinations. The shirt still contains no attached content label.

Alarmingly, silver-impregnated textiles, marketed for antibacterial and odor-fighting properties, have seen recent expanded use^12⇓–14 and X-Static, made with 99.9% pure silver bonded to textile fibers,¹⁵ is now incorporated in products ranging from athletic apparel (eg, golf wear, running apparel, and sports bras) to socks, orthotics, and apparel to be worn under orthoses.^16,17 These products are sold under various brands including Reebok (Canton, Massachusetts), Adidas (Beaverton, Oregon), and New Balance (Boston, Massachusetts).¹⁷ Similarly, other metals with antimicrobial properties, such as copper, are being incorporated into fabrics for socks, pants, shirts, briefs, and bras¹⁷ under the trade name of Cupron and distributed by companies such as Aetrex (Teaneck, New Jersey) and Pacific Brands (Melbourne, Australia)¹⁸ The common practice of not identifying the metallic components of multipurpose fabrics results in significant potential risk in the MR imaging environment.

Risk of burn is enhanced in the sedated patient who is unable to identify or vocalize early signs of heating. Therefore, even greater scrutiny must be used with these patients.

Use of metal detectors for screening patients before entering the scanner room is controversial. The ACR recommends using ferromagnetic detectors to primarily mitigate “missile effect” risk. This device would have been unlikely to detect SMF. We routinely use metal-detector wands to supplement patient screening. This process did not prospectively identify the metallic content of the undershirt. Retrospectively, only 1 in 4 of our wands (White's Matrix 100; New Concept Metal Detectors, Lafayette, Indiana) detected the silver content. We, therefore, cannot recommend the routine use of metal detectors to ensure clothing safety.

In summary, we demonstrate how, despite careful prescreening, invisible metallic microfiber in garments can fail to be detected before entering the MR imaging environment and potentially lead to patient thermal burns. To avoid this hazard, all patients should change into site-supplied MR imaging−compatible outer clothing and wear nonmetallic cotton or other safe non-trade name undergarments for their examinations. When exception is necessary, only patients wearing clearly labeled nonmetallic, non-trade name fabrics should be scanned without changing. We believe that those patients who are sedated or otherwise have diminished responsiveness are at greatest risk for thermal burns and, therefore, require the greatest scrutiny.