Abstract
Purpose : Radiofrequency (RF)-induced heating in magnetic resonance imaging (MRI) may pose risks to patients with metallic implants. An electromagnetic wave suppression sheet (EWS-sheet) may be useful in suppressing RF-induced heating. However, no data have proven that EWS-sheets prevent temperature increases in metallic implants. This study assessed the effect of EWS-sheets on attenuation of RF-induced heating of conductive implants in MRI, and investigated appropriate methods of covering the implants with EWS-sheets to reduce temperature increases.
Materials and Methods : The method employed to measure implant heating conformed to the American Society for Testing and Materials F2182-02a criteria. An orthopedic Kirschner wire was selected for testing. All experiments were performed using a clinical 1.5T MR system. MRI was performed at a whole-body average specific absorption rate of 1.5 W/kg for 15 min. Efficacy for reduction of RF-induced heating of implants was investigated by covering test phantoms with EWS-sheets (full, half, and upper covering) ; the distance between the sheet and parallel overlapping of the implant and the vertical distance between the implant and sheet were evaluated.
Results and Discussion : The efficacy of the EWS-sheet for reduction of RF-induced heating depended on the covering method and placement. By ensuring adequate overlap in the parallel direction, full and half covering allowed the implant to resist heating within safety limits. It was noted that deficient overlapping in the parallel direction and a large vertical space between the sheet and implant caused higher temperature increases in the implant than in cases without use of the EWS-sheet, as a result of enhancement of RF power and incident RF pulses.
Conclusion : EWS-sheets may be useful for protecting patients with conductive implants from the risks of RF-induced heating. Further investigation is needed to define the safety guidelines more precisely.
