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

Abstract

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 T2 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 (beff ~ 5150 s/mm2) for inter-field strength comparisons; in addition, DW-SSFP data were acquired at 7 T with higher beff (~ 8550 s/mm2) 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 beff had improved diffusion contrast than at lower beff 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.

Ex vivo diffusion MRI of the human brain: Technical challenges and recent advances

“…For dwSTE and dwSSFP, the move to UHF is more beneficial, providing more SNR/unit time than 3 T. This is because T 2 (which dominates dwSE) decreases, but T 1 increases, which is leveraged in SSFP and stimulated echo acquisition mode (STEAM). In a comparison study, dwSSFP was found to have considerably higher SNR/unit time at 7 T compared with 3 T and to achieve excellent quality diffusion imaging at 7 T in post‐mortem, whole human brains . Using dwSTE at 9.4 T, high‐quality whole ex vivo human brain dMRI was reported at 400 μm isotropic …”

“…In a comparison study, dwSSFP was found to have considerably higher SNR/unit time at 7 T compared with 3 T and to achieve excellent quality diffusion imaging at 7 T in post-mortem, whole human brains. 15 Using dwSTE at 9.4 T, high-quality whole ex vivo human brain dMRI was reported at 400 μm isotropic. 37…”

Use of multi-flip angle measurements to account for transmit inhomogeneity and non-Gaussian diffusion in DW-SSFP

“…Previous work (Foxley et al, 2014a) demonstrated that with a one-flip angle DW-SSFP acquisition, angular uncertainty in PDD estimates was reduced by increasing field strength from 3T to 7T, providing motivation to move to higher field when performing tractography. This reduction in uncertainty would be expected in local regions of tissue due to the higher SNR associated with an increase in field strength, but would be mitigated by the B1 effects considered in this work (Fig.…”

“…Ultra-high field scanners have potential to enable further gains in spatial resolution, with DW-SSFP providing a valuable tool for addressing the even shorter T2 values at 7T and above (Foxley et al, 2014a). However, DW-SSFP data acquired at 7T are compromised by B1 inhomogeneity (Fig.…”

Towards a High-resolution Fiber Model of the Human Brain with 3D Polarized Light Imaging

“…Diffusion imaging contributes to the understanding of the macroscopic connectivity at the millimeter scale in the living brain, while postmortem diffusion tensor imaging studies already report isotropic resolutions of 1 mm in whole human brains down to 43 microns in rat brains [1]. Hence, not surprisingly, diffusion imaging is of great appeal to neuroscientists as a method for the visualization of connectivity patterns in both clinical and basic research.…”