Sa2RAGE - A new sequence for rapid 3D B1-mapping with a wide sensitivity range

Introduction: At high magnetic field strengths (≥3T) the RF wavelength becomes smaller than the sample size, making transmit magnetic field (B1) inhomogeneity more prominent. RF-shimming [1] and Transmit SENSE [2] have been proposed as methods to minimize this inhomogeneity. To take advantage of these methods it is important to accurately map the B1 field. Saturation prepared with 2 RApid Gradient Echoes (Sa2RAGE) is a new technique which allows the execution of 3D B1-mapping in under a minute with high precision over a 10 fold range of B1. Methods: Sa2RAGE (Fig. 1) can be seen as a fast saturation recovery sequence with two different delay times. To reduce T1-sensitivity, the first delay time, TD1, is kept as short as possible while the second, TD2, is kept long making the ratio between the two acquired signals Sa2RAGE1/Sa2RAGE2 mostly dependent on the saturation pulse performance. The predicted Sa2RAGE ratio for several B1 values was numerically calculated after solving the Bloch Equations taking into account the steady state condition. The observed signal was considered to arise from the point where the center of k-space is acquired [3]. From these numerical calculations, a lookup table was established between Sa2RAGE and B1 (Fig. 2). Contrast to noise ratio by unit of time (CNR) of a B1,i estimate was defined as: RAGE Sa B B i i TR B B i i 2 2 2 , 1 1 , 1 / / ) ( , 1 1 , 1 σ σ + − + + . The noise of the B1,i, σB1,i, was estimated