Overlap-Kernel EPI: Estimating MRI Shot-to-Shot Phase Variations by Shifted-Kernel Extraction From Overlap Regions at Arbitrary k-Space Locations.

purposeIn multi-shot EPI, shot-dependent phase fluctuations can introduce ghost artifacts, undermining advantages for enhancing resolution or reducing distortion, particularly in diffusion scans. Here, a novel self-navigation strategy based on shift-invariant kernel extraction is proposed, enabling robust estimation of phase inconsistencies from overlap bands at any k-space locations. It is inspired by RF sensitivity auto-calibration in parallel imaging, and extends traditional self-navigation trajectories beyond oversampled k-space center. THEORY AND

methodsOur shot-to-shot correction method only requires shot-to-shot sampling trajectories to slightly overlap at arbitrary k-space locations. Such k-space overlaps can be defined as shot-dependent auto-calibrating-signal regions, where a GRAPPA/ESPIRiT-type operation can be applied to explicitly extract shot-to-shot signal variation maps in one step. This inherits GRAPPA/ESPIRiT's robustness, and offers much greater flexibility in designing self-navigated multi-shot sequences.

resultsOur kernel extraction techniques in four self-navigated 2D SE-EPI and GE-EPI sequences are successfully demonstrated, as well as minor applications in extracting maps from navigator data and B

conclusionThis paper rigorously analyzes subspace algorithms for phase-stabilized multi-shot MRI, significantly broadens the self-navigation concept, and provides a more complete picture of the connections between signal fluctuations calibration in EPI and RF sensitivity calibration in parallel imaging.