RF Power, B_1+rms, and SAR Variation with RF Coils, Conductive Metallic Implants, and Ionic Solutions at 1.5T and 3T

Currently, 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, RF pulse sequences, metallic implants, and ionic solutions. A method is developed to evaluate the variation in RF power, B_1+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, λ/2, λ, 2λ), two RF coils (body, head), transmit/receive (Tx/Rx) combinations, and five RF pulse flip angles (30, 45, 60, 75, 90 degrees) in two B₀ fields (1.5T and 3T). 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, metallic implant wavelengths, and ionic concentrations. SAR (whole-body, head) scaled with RF power by absorption ratios (α) variable with experimental conditions.