Accurate measurement of gradient waveform errors can often improve image quality in sequences with time varying readout and excitation waveforms. Self-encoding or offset-slice method sequences are commonly used to measure gradient waveforms. However, the self-encoding method requires a long scan time, while the offset-slice method is often low precision, requiring the thickness of the excited slice to be small compared to the maximal k-space encoded by the test waveform. This disclosure describes a novel hybrid of those methods, referred to as variable-prephasing (VP). Like the offset-slice method, VP uses the change in signal phase from offset-slices to calculate the gradient waveform. Similar to the self-encoding method, repeated acquisitions with a variable amplitude self-encoding gradient mitigates the signal loss due to phase wrapping, which, in-turn, allows thicker slices and greater SNR.