Actual flip‐angle imaging in the pulsed steady state: A method for rapid three‐dimensional mapping of the transmitted radiofrequency field

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Abstract
A new method has been developed for fast image‐based measurements of the transmitted radiofrequency (RF) field. The method employs an actual flip‐angle imaging (AFI) pulse sequence that consists of two identical RF pulses followed by two delays of different duration (TR1 < TR2). After each pulse, a gradient‐echo (GRE) signal is acquired. It has been shown theoretically and experimentally that if delays TR1 and TR2 are sufficiently short and the transverse magnetization is completely spoiled, the ratio r = S2/S1 of signal intensities S1 and S2, acquired at the beginning of the time intervals TR1 and TR2, depends on the flip angle (FA) of applied pulses as r = (1 + n * cos(FA))/(n + cos(FA)), where n = TR2/TR1. The method allows fast 3D implementation and provides accurate B1 measurements that are highly insensitive to T1. The unique feature of the AFI method is that it uses a pulsed steady‐state signal acquisition. This overcomes the limitation of previous methods that required long relaxation delays between sequence repetitions. The method has been shown to be useful for time‐efficient whole‐body B1 mapping and correction of T1 maps obtained using a variable FA technique in the presence of nonuniform RF excitation. Magn Reson Med 57:192–200, 2007.