Parallel transmission to reduce absorbed power around deep brain stimulation devices in MRI: Impact of number and arrangement of transmit channels

PURPOSE: To assess the mean and variance performance of parallel transmission (pTx) coils for reduction of the absorbed power around electrodes (APAE) in patients implanted with deep brain stimulation (DBS) devices. METHODS: We simulated 4 pTx coils (8 and 16 channels, head and body coils) and a birdcage body coil. We characterized the RF safety risk using the APAE, which is the integral of the deposited power (in Watts) in a small cylindrical volume of brain tissue surrounding the electrode tips. We assessed the APAE mean and variance by simulation of 5 realistic DBS patient models that include the full DBS implant length, extracranial loops, and implanted pulse generator. RESULTS: PTx coils with 8 (16) channels were able to reduce the APAE by >18x (>169x) compared to the birdcage coil in average for all patient models, at no cost in term of flip angle uniformity or global specific absorption rate (SAR). Moreover, local pTx coils performed significantly better than body arrays. CONCLUSION: PTx is a possible solution to the problem of RF heating of DBS patients when performing MRI, but the large interpatient variability of the APAE indicates that patient-specific safety monitoring may be needed.