Fast B1 mapping based on interleaved-three-flip-angle (ITFA) excitation

Abstract

Fast B1 mapping based on short-TR sequences is prone to T1-induced errors. The purpose of this study is to develop a novel fast B1 mapping method that is less prone to T1-induced errors. The authors acquired three gradient echoes by applying three RF pulses of different flip angles in an interleaved manner. The new method, named interleaved-three-flip-angle (ITFA) method, employs a short TR for fast scan. Since the pixel intensity of the gradient echo images is dependent on both B1 and T1, the authors could compute a B1 map from the three gradient echo images with excluding the T1-effects. The authors simulated the proposed B1 mapping method for various T1 values, and they found optimal flip angles for ITFA experiments for a given repetition time. To evaluate the B1 mapping performance, the authors made a human-brain-mimicking phantom that had six compartments with different T1 and T2. The authors performed B1 mapping experiments at 3T on the phantom and a volunteer using the ITFA method, the actual flip angle imaging (AFI) method, and the double angle method (DAM), and then, the authors compared the B1 mapping results. Using a birdcage coil as a transceiver at 3T, the authors performed ITFA scans of the phantom and a volunteer with TR of 60 ms and the nominal flips angles of (25°, 70°, 80°). The authors also performed AFI scans with TR1/TR2 of 30/150 ms and the nominal flip angle of 60°. In both the phantom and human head imaging performed with the same scan times for ITFA and AFI, ITFA showed smaller average B1 errors than AFI when they were compared to DAM. ITFA excitations made it possible to reduce the T1-effects on B1 mapping of the human-brain-mimicking phantom and the human brain at 3T. The authors expect the ITFA method can be used for B1 shimming once the optimal flip angles have been predetermined for the target imaging region and for the preferred TR.