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

Abstract

Motivation: Currently, geometric distortions in EPI are generally corrected using a method that employs the reversed phase-encoding direction. This approach is usually implemented by applying an extra run of the same protocol, only with the phase-encoding direction changed, leading to a substantial increase in redundant acquisition time and specific absorption ratio. Furthermore, while the distortion correction method has been widely employed in numerous fMRI studies, its impact on submillimetre fMRI analysis has remained largely unexplored. Methods: This work presents an EPI scheme that acquires both the original and reversed phase-encoding data in a single fMRI session. The feasibility of using the method for submillimetre EPI (0.73 x 0.73 mm2) was verified with visual fMRI at 7T. EPI distortions were corrected using the ANTs software, and its performance was evaluated using various criteria, including spatial resolution, functional mapping accuracy, and histogram distribution. Results: The presented scheme effectively reduced redundant acquisition time and therefore total radio-frequency energy. The distortion-corrected fMRI data demonstrated significant improvements in co-registration with anatomical scans and functional mapping accuracy. As a result, the ratio of grey-matter-activated voxels was substantially enhanced (on average, 88.58%). These improvements were achieved without significant degradation of spatial resolution or alternation of the functional activation distribution; a high degree of similarity between the original and distortion-corrected cases (ρ > 0.99) was observed in the t-value histograms. Conclusions: This work presents a simultaneous acquisition scheme for reversed phase-encoding EPI, demonstrating its effectiveness and the benefits achieved through distortion correction in submillimetre fMRI at 7T.