Time-efficient slab-selective water excitation for 3D MRI.

Spectral-Spatial (SPSP) radiofrequency pulses are simultaneously selective in both the spectral and spatial domains. To selectively excite water spins and exclude fat, the individual subpulses that make up a SPSP pulse must be short (<1 ms at 4 T). A short subpulse duration limits the sharpness of the spatial slabs that can be excited when using a traditional SPSP pulse design approach. In this manuscript, the authors present an algorithm for designing SPSP pulses with substantially reduced maximum B(1) amplitudes and specific absorption rates. The proposed algorithm alternates between iterative design of the radiofrequency waveform for a given gradient shape and minimum-time variable-rate selective excitation reshaping of the gradient waveform. This approach is shown to reduce peak B(1) amplitudes in iteratively designed SPSP pulses by an order of magnitude. Unlike the use of regularization to control peak B(1) or specific absorption rate, the proposed method does not comprise the quality of the excitation profile. To achieve high-quality profiles, it was necessary to design the radiofrequency pulses for a measured rather than ideal gradient waveform. Slab-selective water excitation pulses with durations of 4.1 and 9.2 ms (fractional transition widths of 0.14 and 0.073, respectively) are demonstrated at 4 T.