Steady-state double-angle method for rapid B1 mapping

Purpose: To introduce an accurate, rapid, and practical method for active B-1 field mapping based on the double-angle method (DAM) in the steady-state (SS) signal regime. Methods: We introduced and evaluated the performance of the SS-DAM approach to map the B-1 field and compared the results to those calculated from the conventional DAM approach. Similar to DAM, SS-DAM uses the signal intensity ratio of 2 magnitude images acquired with different flip angles using the spoiled gradient recalled echo sequence. However, unlike DAM, in SS-DAM, these 2 spoiled gradient recalled echo images are acquired with very short TR, which allows substantially reduced acquisition time. Numerical, phantom, and in vivo brain imaging analyses, representing a wide range of T(1)s and large B-1 variation, were conducted. Methods for further accelerating acquisition were also investigated. Results: Our results demonstrate the potential of the SS-DAM approach to be applied widely in the clinical setting. B-1 maps derived from SS-DAM were demonstrated to be quantitatively comparable to those derived from DAM but were derived much more rapidly. Large-volume B-1 maps were obtained at a field strength of 3 tesla within clinically acceptable acquisition times. Conclusion: SS-DAM permits accurate B-1 mapping in the clinical setting, with whole-brain coverage in less than 1 min.