RF power, B1+rms, and SAR variation with RF coils, conductive metallic implants, and ionic solutions at 1.5T and 3T

Background and PurposeThe magnetic resonance imaging (MRI) access for patients with active and passive implants is limited by radiofrequency (RF) safety. The time-averaged root-mean-square RF field (B1+rms) and specific absorption rate (SAR) are being evaluated to monitor and control RF-induced heating near conductive metallic implants, such as deep brain stimulation (DBS) leads, during MRI. However, experimental methods to assess the relationship between RF power, B1+rms, and SAR are lacking for RF coils, metallic implants, and ionic solutions. Materials and MethodsA method is developed to evaluate the variation of RF power, B1+rms, and SAR with RF coils, metallic implants, and ionic solutions using phantoms consisting of water (H2O) and sodium chloride (NaCl) with four ionic concentrations (0, 1, 2, 3 %), four metallic wavelengths (0,{lambda} /2,{lambda} , 2{lambda}), two RF coils (body, head) transmit/receive (Tx/Rx) combinations, and five RF pulse flip angles (30{degrees}, 45{degrees}, 60{degrees}, 75{degrees}, 90{degrees}) in two B0 fields (1.5T and 3T). ResultsThe scanner-reported RF power and SAR varied with RF pulse sequences, RF coils, Tx/Rx, metallic implants, and ionic solutions, whereas B1+rms varied only with RF pulse sequences. The RF power, B1+rms, and SAR relationship depends on RF pulse sequences, RF coils, Tx/Rx, implant wavelengths, and ionic concentrations. SAR (whole-body, head) scaled with RF power by absorption ratios () variable with experimental conditions. ConclusionsB1+rms is insensitive to the presence and absence of conductive metallic implants and ionic solutions, implant wavelengths, ionic concentrations, RF coils, and Tx/Rx combinations. RF power must be monitored because scanner-reported SAR may vary unpredictably with experiments.