In Vivo RF Power and SAR Calibration for Multi-Port RF Transmission

Introduction: While a substantially increased degree of freedom inherent of parallel RF transmit facilitates excitation profile control, it meanwhile raises concerns that poorly guided multi-port RF pulse or shimming calculations, or multi-channel hardware failure may inadvertently elevate SAR. One can address the concerns to some extent by monitoring RF power at the ports, where power sensors are implemented to measure individual port forward and reflected power, calculate in real-time net forward power into the subject and stop scan when estimated overall SAR reaches a threshold. To better manage SAR however one must complement the real-time monitoring with a more proactive scheme. In principle, for multi-port RF pulse design one can explicitly minimize SAR by guiding the design with a predictive model that tracks SAR (1). In this work we developed a practical method that is capable of establishing such a model under in vivo imaging conditions. The model predicts, for any set of RF pulse sequences or shimming coefficients, the overall SAR of the multi-port operation. Since RF field and power deposition are substantially subject dependent and both crucial to imaging performance, especially in high field MR, power model calibration is going to play an equally important role as B1 calibration is. The two are expected to guide pulse or shimming calculations on a subject-specific basis, and enable effective control of SAR as well as excitation profile. The two also serve as key inputs to analysis that gauge RF pulse / coil performance with gt factor and ultimate intrinsic SAR (2,3). Methods and Results: When RF signal gets transmitted (Tx) / detected (Rx), the B1 / B1 fields interact with the spin system, forming the basis of MR signal induction / detection. The concomitant E field meanwhile gives rise to RF loss in the object and dictates SAR / noise. Optimizing source configuration and thereby the RF coil currents’ magnitude / phase, temporal modulation and spatial distribution is critical to MR imaging performance. To exert control during Tx, an MR scanner in practice uses a designed RF pulse sequence to update the magnitude and phase of a Larmor-frequency sinusoidal pulse every Δt (e.g., Δt=2usec). The control is multiplied in multi-port Tx, or, parallel Tx, which includes B1 shimming as a special case. For any Δt interval, the magnitude-phase pairs specified by multiple RF pulse sequences, expressed with complex scalars wp (n)