RF power, B 1+rms , and SAR variation with RF coils, conductive metallic implants, and ionic solutions at 1.5T and 3T

Abstract Background and Purpose The 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 (B 1+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, B 1+rms, and SAR are lacking for RF coils, metallic implants, and ionic solutions. Materials and Methods A method is developed to evaluate the variation of RF power, B 1+rms, and SAR with RF coils, metallic implants, and ionic solutions using phantoms consisting of water (H 2 O) and sodium chloride (NaCl) with four ionic concentrations (0, 1, 2, 3 %), four metallic wavelengths (0, λ /2, λ , 2 λ ), two RF coils (body, head) transmit/receive (Tx/Rx) combinations, and five RF pulse flip angles (30°, 45°, 60°, 75°, 90°) in two B 0 fields (1.5T and 3T). Results The scanner-reported RF power and SAR varied with RF pulse sequences, RF coils, Tx/Rx, metallic implants, and ionic solutions, whereas B 1+rms varied only with RF pulse sequences. The RF power, B 1+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 (α) variable with experimental conditions. Conclusions B 1+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.