Evaluation of feasibility of 1.5 Tesla prostate MRI MRI using body coil RF RF transmit in a patient with an implanted vagus nerve stimulator

摘要

Purpose To assess risks of RF ‐heating of a vagus nerve stimulator ( VNS ) during 1.5?T prostate MRI using body coil transmit and to compare these risks with those associated with MRI head exams using a transmit/receive head coil. Methods Spatial distributions of radio‐frequency ( RF ) B1 fields generated by transmit/receive (T/R) body and head coils were empirically assessed along the long axis of a 1.5?T MRI scanner bore. Measurements were obtained along the center axis of the scanner and laterally offset by 15?cm (body coil) and 7?cm (head coil). RF ‐field measurements were supplemented with direct measurements of RF ‐heating of 15?cm long copper wires affixed to and submerged in the “neck” region of the gelled saline‐filled (sodium chloride and polyacrylic acid) “head‐and‐torso” phantom. Temperature elevations at the lead tips were measured using fiber‐optic thermometers with the phantom positioned at systematically increased distances from the scanner isocenter. Results B1 field measurements demonstrated greater than 10?dB reduction in RF power at distances beyond 28?cm and 24?cm from isocenter for body and head coil, respectively. Moreover, RF power from body coil transmit at distances greater than 32?cm from isocenter was found to be lower than from the RF power from head coil transmit measured at locations adjacent to the coil array at its opening. Correspondingly, maximum temperature elevations at the tips of the copper wires decreased with increasing distance from isocenter — from 7.4°C at 0?cm to no appreciable heating at locations beyond 40?cm. Conclusions For the particular scanner model evaluated in this study, positioning an implanted VNS farther than 32?cm from isocenter (configuration achievable for prostate exams) can reduce risks of RF ‐heating resulting from the body coil transmit to those associated with using a T/R head coil.