Simple method for RF pulse measurement using gradient reversal

Purpose To develop and evaluate a simple method for measuring the envelope of small‐tip radiofrequency (RF) excitation waveforms in MRI, without extra hardware or synchronization. Theory and Methods Gradient reversal approach to evaluate RF (GRATER) involves RF excitation with a constant gradient and reversal of that gradient during signal reception to acquire the time‐reversed version of an RF envelope. An outer‐volume suppression prepulse is used optionally to preselect a uniform volume. GRATER was evaluated in phantom and in vivo experiments. It was compared with the programmed waveform and the traditional pick‐up coil method. Results In uniform phantom experiments, pick‐up coil, GRATER, and outer‐volume suppression + GRATER matched the programmed waveforms to less than 2.1%, less than 6.1%, and less than 2.4% normalized root mean square error, respectively, for real RF pulses with flip angle less than or equal to 30°, time‐bandwidth product 2 to 8, and two to five excitation bands. For flip angles greater than 30°, GRATER measurement error increased as predicted by Bloch simulation. Fat‐water phantom and in vivo experiments with outer‐volume suppression + GRATER demonstrated less than 6.4% normalized root mean square error. Conclusions The GRATER sequence measures small‐tip RF envelopes without extra hardware or synchronization in just over two times the RF duration. The sequence may be useful in prescan calibration and for measurement and precompensation of RF amplifier nonlinearity. Magn Reson Med 79:2642–2651, 2018. © 2017 International Society for Magnetic Resonance in Medicine.