Repeatable tract-based diffusion metrics on a portable 64 mT MRI: a foundation for global population neuroscience

Diffusion MRI underpins much of modern population neuroscience, yet scanner access remains concentrated in high-income settings, excluding the genetic, developmental, and disease diversity needed for generalizable discovery. Portable low-field MRI systems (field strengths below 0.1 T) offer a cheaper, infrastructure-light alternative that could extend diffusion imaging to under-represented populations, but only if the measurements are sufficiently repeatable. Here we provide the first systematic test-retest evaluation of tract-based diffusion tensor MRI (DT-MRI) metrics at 64 mT. Ten healthy participants were scanned at two time-points (median interval 13 days) with an 18-direction (b = 900s/mm²) diffusion protocol and a T ₂-weighted structural sequence on a portable 64 mT scanner. After correction for distortions and gradient imperfections, diffusion tensors were estimated and constrained spherical deconvolution was performed to enable bundle-specific tractography. Tract averaged fractional anisotropy, mean diffusivity, and radial diffusivity were extracted from a selection of white-matter tracts spanning projection, association, and commissural fiber categories. Bland-Altman analysis and within-subject coefficients of variation indicated strong scan-rescan agreement, while intraclass correlation coefficients were variable across tracts and metrics. Compared with high-field data, coefficients of variation were 3-10 times larger and standard deviations in the means were approximately an order of magnitude higher. Despite this, statistical power calculations yielded feasible sample-size estimates for detecting group differences in a two-tailed t-test: for a 4% difference in means, approximately 25 participants per group were sufficient for the majority of tracts in MD and RD, and around 65 for FA. Although absolute DT-MRI metric values diverge from high-field references due to partial volume effects and noise-floor bias, continued advances in acquisition, reconstruction, and processing are expected to further narrow this gap. The results demonstrate that portable 64 mT MRI can deliver repeatable DT-MRI metrics, establishing a foundation for democratizing advanced neuroimaging and enabling population-scale studies in regions that have historically been excluded from brain research.