Insight into the fundamental trade-offs of diffusion MRI from polarization-sensitive optical coherence tomography in ex vivo human brain

Abstract

In the first study comparing high angular resolution diffusion MRI (dMRI) in the human brain to axonal orientation measurements from polarization-sensitive optical coherence tomography (PSOCT), we compare the accuracy of orientation estimates from various dMRI sampling schemes and reconstruction methods. We find that, if the reconstruction approach is chosen carefully, single-shell dMRI data can yield the same accuracy as multi-shell data, and only moderately lower accuracy than a full Cartesian-grid sampling scheme. Our results suggest that current dMRI reconstruction approaches do not benefit substantially from ultra-high b-values or from very large numbers of diffusion-encoding directions. We also show that accuracy remains stable across dMRI voxel sizes of 1 mm or smaller but degrades at 2 mm, particularly in areas of complex white-matter architecture. We also show that, as the spatial resolution is reduced, axonal configurations in a dMRI voxel can no longer be modeled as a small set of distinct axon populations, violating an assumption that is sometimes made by dMRI reconstruction techniques. Our findings have implications for in vivo studies and illustrate the value of PSOCT as a source of ground-truth measurements of white-matter organization that does not suffer from the distortions typical of histological techniques.

High-fidelity approximation of grid- and shell-based sampling schemes from undersampled DSI using compressed sensing: Post mortem validation

“…This acceleration factor allows the equivalent of 514-direction DSI to be reconstructed from a 171-direction acquisition, thus bringing DSI acquisition times in line with those of state-of-the-art high angular resolution (e.g., multi-shell) acquisitions. As has been shown in previous validation studies, the DSI acquisition scheme achieves high accuracy (Daducci et al, 2013, Maffei et al, 2020, and DSI data can be readily resampled onto q-shells to facilitate any analysis that requires a shell-based acquisition (Jones et al, 2020).…”

“…Following dMRI acquisition of sample 1A, a piece of the tissue block was extracted for imaging with PSOCT ( Figure 1A, right). Details on the setup, acquisition, processing and analysis of the PSOCT dataset used in this paper were previously described in (Jones et al, 2020). Briefly, the sample was imaged with a polarization maintaining fiber (PMF) based, spectral domain PSOCT system developed in-house (Wang et al, 2016).…”

A 48-Channel Receive Array Coil for Mesoscopic Diffusion-Weighted MRI of Humanex vivoBrain Imaging on the 3T Connectome Scanner

“…Therefore, it the case of an MB = 8 acceleration scheme, the combination of the enhanced SMS encoding power and the increased baseline SNR of the 48ch coil, provides an up to 4.5-fold SNR improvement when compared to the 64ch head coil. Previous work comparing ex vivo dMRI to optical imaging suggests that high spatial resolution (1 mm or higher) improves the accuracy of dMRI-derived axonal orientation estimates, and may have a greater impact than high angular resolution or ultra-high b-values [15]. That work utilized small human samples that were scanned in a small-bore MRI scanner.…”

“…Previous work comparing ex vivo dMRI to optical imaging suggests that high spatial resolution (1 mm or higher) improves the accuracy of dMRI-derived axonal orientation estimates, and may have a greater impact than high angular resolution or ultra-high b -values [15]. That work utilized small human samples that were scanned in a small-bore MRI scanner.…”

“…The impressive level of anatomical detail that can be resolved by ex vivo dMRI has already been demonstrated on a variety of human tissue samples [9][10][11][12][13]. Ex vivo dMRI, in combination with optical imaging, is an excellent tool for validating dMRI acquisition and analysis methods in human brain tissue [14,15]. Higher spatial resolution comes at the cost of lower signal-to-noise-ratio (SNR).…”