Improving diffusion-weighted imaging of post-mortem human brains: SSFP at 7 T

Post-mortem diffusion imaging of whole, human brains has potential to provide data for validation or high-resolution anatomical investigations. Previous work has demonstrated improvements in data acquired with diffusion-weighted steady-state free precession (DW-SSFP) compared with conventional diffusion-weighted spin echo at 3 T. This is due to the ability of DW-SSFP to overcome signal-to-noise and diffusion contrast losses brought about by tissue fixation related decreases in T(2) and ADC. In this work, data of four post-mortem human brains were acquired at 3 T and 7 T, using DW-SSFP with similar effective b-values (b(eff) ~ 5150 s/mm(2)) for inter-field strength comparisons; in addition, DW-SSFP data were acquired at 7 T with higher b(eff) (~ 8550 s/mm(2)) for intra-field strength comparisons. Results demonstrate that both datasets acquired at 7 T had higher SNR and diffusion contrast than data acquired at 3 T, and data acquired at higher b(eff) had improved diffusion contrast than at lower b(eff) at 7 T. These results translate to improved estimates of secondary fiber orientations leading to higher fidelity tractography results compared with data acquired at 3 T. Specifically, tractography streamlines of cortical projections originating from the corpus callosum, corticospinal tract, and superior longitudinal fasciculus were more successful at crossing the centrum semiovale and projected closer to the cortex. Results suggest that DW-SSFP at 7 T is a preferential method for acquiring diffusion-weighted data of post-mortem human brain, specifically where the primary region of interest involves crossing white matter tracts.