The Simultaneous Measurement of Reversed Phase-Encoding EPI in a Single fMRI Session: Evaluation of Geometric Distortion Correction in Submillimetre fMRI at 7T

The high temporal resolution of echo-planar imaging (EPI) has driven its widespread use in functional MRI (fMRI), and recent advancements in EPI have enabled the mapping of cortical layer-specific functional activities. Notwithstanding these technical innovations, geometric distortion correction in EPI remains essential to ensure accurate functional mapping onto anatomical references. A common approach for distortion correction involves the acquisition of an additional scan, with the phase-encoding direction reversed. However, this extra scan necessitates redundant acquisitions of calibration scans, significantly increasing acquisition time and energy deposition. To address this concern, this work presents a novel EPI scheme that simultaneously acquires both the original and reversed phase-encoding data within a single fMRI session. Furthermore, despite the widespread use of distortion correction, methods for qualitative and quantitative evaluation of its impact on submillimetre fMRI analysis have remained largely unexplored. This study acquires submillimetre visual fMRI data (0.73 × 0.73 mm ² ) at 7T using the suggested EPI scheme and evaluates the impact of distortion correction through various metrics proposed here, including spatial resolution, co-registration accuracy, functional mapping fidelity, and distribution of functional voxels. Our fMRI acquisition scheme effectively reduced redundant acquisition time and total energy deposition to subjects. Distortion-corrected fMRI data demonstrated significant improvements in co-registration with anatomical scans, thereby enhancing functional mapping accuracy. The improvements were achieved without significant degradation of spatial resolution or alteration of the functional activation distribution. These findings were verified through qualitative and quantitative assessments, highlighting the effectiveness of distortion correction in submillimetre fMRI.