A fast, analytically-based method to optimize local transmit efficiency for a transmit array

Purpose: to develop an analytically-based algorithm for rapid optimization of the local RF magnetic (B_1^+) field intensity for a given RF power through a transmit array. The analytical nature of the method will yield insight to optimization requirements and provides a valuable reference for numerically-based searches. Materials and Methods: with knowledge of the B_1^+ field distribution generated by each single coil of the array, both the phases and the amplitudes of each coil current are optimized to maximize the magnitude of the B_1^+ field in a specific location of the body per unit of power transmitted through the array and, consequently, minimizing the whole body SAR for a given pulse sequence. Results: simulations considering the human body show that the proposed method can reduce the whole body SAR for a given B1+ magnitude at the location of interest by a factor of about 6.3 compared to the classic birdcage current configuration, and by a factor of 3.2 compared to phase only shimming in a case with significant coupling between the elements of the array. Conclusion: the proposed method can rapidly provide valuable information pertinent to the optimization of field distributions from transmit arrays.