Navigator-free multi-shot EPI with shift-invariant kernel extraction in subspace

Motivation: In functional and diffusion MRI, multi-shot EPI enhances spatial resolution and minimizes distortion compared to single-shot scans. However, its vulnerability to shot-to-shot phase variations presents a significant challenge, with various proposed methods having drawbacks. Goal(s): We propose a robust, navigator-free, computational efficient multi-shot method without SNR penalty. Approach: In readout-segmented multi-shot EPI, we exploit the k-space overlapped regions between adjacent segments to extract relative phase fluctuations. This method, inspired by ESPIRiT and nonlinear gradient calibration, efficiently extracts shot-dependent phase variations in subspace. Results: Our ex-vivo and in-vivo scans, including diffusion-weighted imaging, successfully achieves a high in-plane resolution of about 0.6mm without ghost artifacts. Impact: Our proposed multi-shot technique eliminates the needs for time-consuming navigators, provides robust high-resolution diffusion and potentially functional imaging, and could be easily adapted for interleaved Cartesian and spiral EPI allowing robust phase error estimation from merely small k-space regions.