Pseudo multi-shot EPI for geometric distortion improvement.

Conventional EPI uses an RF for excitation and prolonged echo train to sample k-space, while off-resonance and T2 * decay effects cause by magnetic susceptibility variation accumulate within each echo, leading to geometric distortion. Multi-shot EPI methods, which divide k space into segments, can shorten the effective echo spacing and reduce the distortion on EPI images. But multiple shots cost longer scan time and render susceptibility to motion. In this study, we propose a new "multi-shot" EPI method termed pseudo multi-shot EPI (pmsEPI), in which phase-encoding lines are segmented as in multi-shot EPI but collected within a single shot. With the magnetization divided into different pathways via interleaved excitation instead of refocusing in a single long echo train, the total phase error accumulation is reduced in each segmented acquisition, thereby improving distortion of the resultant EPI image. The performance of the pmsEPI method is demonstrated by phantom and in vivo brain experiments on 3T scanner. The experimental results show that the distortion displacements of pmsEPI acquisition compared to conventional EPI decrease by 50% with two pseudo shots and 66% with three pseudo shots, validating the ability of the method to obtain images with reduced distortion in a single shot, despite magnetization splitting may induce more than 40% SNR loss and minor artifacts. Specifically, the ability of pmsEPI in diffusion-weighted imaging with different trajectory options is highlighted, and the flexibility is demonstrated in a single-shot blip up and down acquisition.