Minimal-SAR RF Pulse Optimization in Parallel Transmission

INTRODUCTION Parallel transmission is an emerging technique that has great potential in many applications such as reducing selective RF pulse duration and improving B1 and B0 field inhomogeneity in high fields [1-3]. A potential issue in parallel transmission is elevated RF power deposition which is characterized by the specific absorption ratio (SAR). In parallel excitation, the effect of electric field/tissue interactions and superimposed fields from all channels can create localized hot spots”. When high acceleration factors are used, SAR will dramatically increase because of the shortened RF pulse durations. At high fields, this problem is even more serious because SAR can increases quadratically with the static field strength. However, its practical implementation must be strictly subject to the FDA SAR limit. Several methods have been proposed to address this problem, either by exploiting the extra freedom in parallel transmission pulse design, or by using variable-density k-space trajectory [2-8]. To facilitate the parallel transmission imaging, we extend a recent methodology and other previous work by using a Lagrange function to optimize the excitation gradient waveforms in the parallel RF pulse design [4]. A significant feature of the new method is that it can model and optimize the SAR directly when the patient specific information is available. The method can achieve a global optimum under the given excitation pattern constraint. METHOD In this algorithm, we derive SAR as a function of gradient, with the constraints of the same excitation profile and pulse duration.