Comprehensive 4D Parallel Transmission Spatial-Spectral Pulse Design for Slab-Selective Uniform Water-Selective Excitation: Demonstration in the Human Brain at 7 Tesla

Purpose: To propose a new parallel transmission (pTx) spatial-spectral (SPSP) pulse design for achieving slab-selective uniform water-selective excitation without unwanted out-of-slab fat excitation when using bipolar slab-selective gradients to maintain a sharp slab profile. Methods: Our new pTx SPSP pulses were designed by formulating the design problem comprehensively in the 4D space (1D spectral and 3D spatial domains) and by incorporating a SPINS-like 2D excitation k-space trajectory for within-slab flip-angle homogenization. Our new design was validated at 7 T using simulation, phantom and human experiments with the commercial Nova eight-channel transmit RF head coil. Its utility was demonstrated by comparing to traditional multi-spoke pTx SPSP pulses. Results: In both simulation and experiments, our design outperformed traditional approaches, producing slab-selective uniform water-selective excitation with no out-of-slab fat excitation. Quantitatively, coefficient of variation measuring excitation non-uniformity reduced by up to ∼23%. Conclusion: Our proposed new design provides an effective solution for slab-selective uniform water-selective excitation, holding a promise to many applications including mesoscale BOLD fMRI and fat-free body imaging at ultrahigh field.