Optimizing signal-to-noise ratio of high-resolution parallel single-shot diffusion-weighted echo-planar imaging at ultrahigh field strengths.

The potential signal-to-noise ratio (SNR) gain at ultrahigh field strengths offers the promise of higher image resolution in single-shot diffusion-weighted echo-planar imaging the challenge being reduced T(2) and T(2) * relaxation times and increased B(0) inhomogeneity which lead to geometric distortions and image blurring. These can be addressed using parallel imaging (PI) methods for which a greater range of feasible reduction factors has been predicted at ultrahigh field strengths-the tradeoff being an associated SNR loss. Using comprehensive simulations, the SNR of high-resolution diffusion-weighted echo-planar imaging in combination with spin-echo and stimulated-echo acquisition is explored at 7 T and compared to 3 T. To this end, PI performance is simulated for coil arrays with a variable number of circular coil elements. Beyond that, simulations of the point spread function are performed to investigate the actual image resolution. When higher PI reduction factors are applied at 7 T to address increased image distortions, high-resolution imaging benefits SNR-wise only at relatively low PI reduction factors. On the contrary, it features generally higher image resolutions than at 3 T due to smaller point spread functions. The SNR simulations are confirmed by phantom experiments. Finally, high-resolution in vivo images of a healthy volunteer are presented which demonstrate the feasibility of higher PI reduction factors at 7 T in practice.