Evaluation of Feasibility of 1.5 Tesla Prostate MRI Using Body Coil RF Transmit in a Patient with an Implanted Vagus Nerve Stimulator.

PurposeTo assess risks of RF-heating of a vagus nerve stimulator (VNS) during 1.5T prostate MRI using body coil transmit and to compare these risks with those associated with MRI head exams using a transmit/receive head coil. MethodsSpatial 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 1.5T MRI scanner bore. Measurements were obtained along the center axis of the scanner and laterally offset by 15cm (body coil) and 7cm (head coil). RF-field measurements were supplemented with direct measurements of RF-heating of 15cm 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. ResultsB1 field measurements demonstrated greater than 10dB reduction in RF power at distances beyond 28cm and 24cm from isocenter for body and head coil, respectively. Moreover, RF power from body coil transmit at distances greater than 32cm 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 degrees C at 0cm to no appreciable heating at locations beyond 40cm. ConclusionsFor the particular scanner model evaluated in this study, positioning an implanted VNS farther than 32cm 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.