Multidimensional RF pulse design using auto-differentiable spin-domain optimization and its application to reduced...

Develop a general radiofrequency (RF) pulse design algorithm for the design of three-dimensional (3D) spatially selective refocusing pulses and explore their potential applications in reduced field-of-view imaging.

Spin-domain representation was used to describe the effect of RF pulses and to formulate the optimization problem. The implementation was based on an auto-differentiable simulator. Spatially selective excitation and refocusing pulses for a small regions of interest were designed with consideration of hardware limits and B₀ and B₁ ⁺ inhomogeneities. The designed RF pulses were validated in simulation as well as in vivo experiments in brain and prostate on a 3T scanner.

Pulses of short duration (4.15 ms) were designed for 3D tailored excitation and refocusing accounting for B₀ and B₁ ⁺ inhomogeneities. The computation time for each design was approximately 7 min. Improved outer-volume signal suppression in phantom and in vivo spin-echo imaging experiments was achieved by combining 3D excitation and refocusing pulses compared with using only one 3D pulse. The potential of using tailored 3D pulses to obtain images with higher spatial resolution (1 $\times$ 1 $\times$ 3 mm³) and less distortion with the same scan time was demonstrated by acquiring reduced field-of-view 3D images of brain and prostate.

The proposed algorithm jointly optimizes the RF and gradient waveforms given an initial pulse and a desired spin-domain target pattern. Our approach is broadly applicable to the design of excitation and refocusing pulses and complements existing methods based on describing individual spin isochromatic behavior. Combining 3D excitation and 3D refocusing pulses produced better spatial selectivity than using a combination of one-dimensional and 3D pulses.