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Experimental Evaluation of RF Shield at 1.5 and 3.0 Tesla MRI

Christopher Favazza

C Favazza*, K Gorny, J Felmlee, Mayo Clinic, Rochester, MN

PO-BPC-Exhibit Hall-7 Saturday  Room: Exhibit Hall

Purpose: To evaluate radiofrequency (RF) shield (Accusorb, Axonics, Inc. USA) for prospective use during 1.5T and 3.0T MRI examinations of patients with implanted metallic devices.

Methods:To assess attenuation of RF field, a standard 60x132 cm blanket (Accusorb, Axonics, Inc., USA) was wrapped around an acrylic tube and positioned at the center of a clinical 1.5T and 3.0 T MRI scanners (GE, Signa Excite). Single RF pulses were executed in 1-second intervals at a constant power output and an RF probe consisting of two orthogonal single-loop coils and a shorted coaxial cable (to account for offset voltage) was moved along the z-axis at 2cm increments. Induced voltages were measured using an oscilloscope. Measurements were repeated without the RF shield for comparison. RF shield effects on RF-induced heating was evaluated using fluoroptic thermometers attached to ends of three parallel 40cm straight copper wires submerged 20cm apart inside the ASTM head-and-torso phantom, see Figure 1b. Temperature history during execution of high SAR RF pulse sequence was recorded with and without RF shield around the phantom. The heating of the RF shield itself was measured by 5 fluoroptic thermometers placed at various positions on the surface of the shield.

Results: At 1.5T, the RF attenuation inside the RF shield increased from 5dB near the edges to more than 10dB at 9cm from the edges, exceeding 30dB at the center of the shield. At 3.0T, the RF attenuation reached 30dB at the center of the shield. This attenuation was sufficient to reduce wire heating below levels detectable by fluoroptic thermometry. A measurable amount of heating of the shield was observed, which varied depending on position, but was less than 1.3 degrees C.

Conclusion:The RF attenuation from the shield and the resulting reduced RF heating of metallic wires is significant and merit further evaluation.

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