Purpose To develop an efficient distortion‐ and blurring‐free multi‐shot EPI technique for time‐resolved multiple‐contrast and/or quantitative imaging. Methods EPI is a commonly used sequence but suffers from geometric distortions and blurring. Here, we introduce a new multi‐shot EPI technique termed echo planar time‐resolved imaging (EPTI), which has the ability to rapidly acquire distortion‐ and blurring‐free multi‐contrast data set. The EPTI approach performs encoding in ky‐t space and uses a new highly accelerated spatio–temporal CAIPI sampling trajectory to take advantage of signal correlation along these dimensions. Through this acquisition and a B0‐informed parallel imaging reconstruction, hundreds of “time‐resolved” distortion‐ and blurring‐free images at different TEs across the EPI readout window can be created at sub‐millisecond temporal increments using a small number of EPTI shots. Moreover, a method for self‐estimation and correction of shot‐to‐shot B0 variations was developed. Simultaneous multi‐slice acquisition was also incorporated to further improve the acquisition efficiency. Results We evaluated EPTI under varying simulated acceleration factors, B0‐inhomogeneity, and shot‐to‐shot B0 variations to demonstrate its ability to provide distortion‐ and blurring‐free images at multiple TEs. Two variants of EPTI were demonstrated in vivo at 3T: (1) a combined gradient‐ and spin‐echo EPTI for quantitative mapping of T2, T2*, proton density, and susceptibility at 1.1 × 1.1 × 3 mm3 whole‐brain in 28 s (0.8 s/slice), and (2) a gradient‐echo EPTI, for multi‐echo and quantitative T2* fMRI at 2 × 2 × 3 mm3 whole‐brain at a 3.3 s temporal resolution. Conclusion EPTI is a new approach for multi‐contrast and/or quantitative imaging that can provide fast acquisition of distortion‐ and blurring‐free images at multiple TEs.

Purpose: To develop new encoding and reconstruction techniques for fast multi-contrast/quantitative imaging. Methods:The recently proposed Echo Planar Time-resolved Imaging (EPTI) technique can achieve fast distortion-and blurring-free multi-contrast/quantitative imaging. In this work, a subspace reconstruction framework is developed to improve the reconstruction accuracy of EPTI at high encoding accelerations. The number of unknowns in the reconstruction is significantly reduced by modeling the temporal signal evolutions using low-rank subspace. As part of the proposed reconstruction approach, a B0-update algorithm and a shot-to-shot B0 variation correction method are developed to enable the reconstruction of high-resolution tissue phase images and to mitigate artifacts from shot-to-shot phase variations. Moreover, the EPTI concept is extended to 3D k-space for 3D GE-EPTI, where a new 'temporal-variant' of CAIPI encoding is proposed to further improve performance. Results:The effectiveness of the proposed subspace reconstruction was demonstrated first in 2D GESE EPTI, where the reconstruction achieved higher accuracy when compared to conventional B0-informed GRAPPA. For 3D GE-EPTI, a retrospective undersampling experiment demonstrates that the new temporal-variant CAIPI encoding can achieve up to 72× acceleration with close to 2× reduction in reconstruction error when compared to conventional spatiotemporal-CAIPI encoding. In a prospective undersampling experiment, high-quality whole-brain T2 * and QSM maps at 1 mm isotropic resolution was acquired in 52 seconds at 3T using 3D GE-EPTI with temporal-variant CAIPI encoding. Conclusion:The proposed subspace reconstruction and optimized temporal-variant CAIPI encoding can further improve the performance of EPTI for fast quantitative mapping.