Cartesian MaxGIRF: Model-based EPI reconstruction incorporating gradient nonlinearity and concomitant field effects

Motivation: Next-generation scanners with strong gradient systems and large-bore low field scanners experience increased gradient nonlinearity and concomitant fields, each of which causes distortions in EPI. Current correction approaches based on image-domain interpolation introduce undesirable spatial blurring. Goal(s): Develop a model-based EPI reconstruction framework that simultaneously corrects distortions from gradient nonlinearity, concomitant fields, and off-resonance during image reconstruction to avoid spatial blurring caused by interpolation-based methods. Approach: Concomitant fields are considered as higher-order encoding and gradient nonlinearity as spatial displacements, which is efficiently handled by the Type-I NUFFT. Results: The proposed method provides better image quality, specifically reduced blurring, compared to conventional correction methods. Impact: Standard corrections for gradient nonlinearity and off-resonance have detrimental effect on image quality and on quantitative diffusion measures. The proposed framework performs all corrections during image reconstruction and provides better image quality by avoiding the smoothing effect of interpolation-based corrections.