Spin-echo (SE) BOLD fMRI has high microvascular specificity, but its most common acquisition method, SE-EPI, suffers from T2' contrast contamination with undesirable draining vein bias. To address this, in this study, we extended a recently developed multi-shot EPI technique, Echo-Planar Time-resolved Imaging (EPTI), to laminar SE-fMRI at 7T to obtain pure spin-echo BOLD contrast with minimal T2' contamination for improved specificity. We also developed a framework to simultaneously obtain a series of asymmetric SE (ASE) images with varying T2' weightings, and extracted from the same data equivalent conventional SE multi-shot EPI images with different ETLs, to investigate the T2'-induced macrovascular contribution across the spin-echo readout. A low-rank spatiotemporal subspace reconstruction was implemented for the SE-EPTI acquisition, which incorporates corrections for both shot-to-shot phase variations and dynamic B0 drifts. SE-EPTI was used in a visual task fMRI experiment to demonstrate that i) the pure SE image provided by EPTI results in the highest microvascular specificity; ii) the ASE EPTI image series, with a graded introduction of T2' weightings at time points farther away from the pure SE, show a gradual sensitivity increase accompanied by a larger and larger draining vein bias; iii) a longer ETL in the conventional SE EPI acquisition will induce more draining vein bias. Consistent results were observed across multiple subjects, demonstrating the robustness of the proposed technique for SE-BOLD fMRI with high specificity.
Simultaneous pure T2 and varying T2′-weighted BOLD fMRI using Echo Planar Time-resolved Imaging for mapping cortical-depth dependent responses
