Feasibility of brain intra-axonal microstructure imaging with ultrahigh B-encoding using MAGNUS ultra-high-performance gradients.

The MAGNUS high-performance MRI gradient platform delivers G = 200-300 mT/m, and SR = 500-750 T/m/s using standard clinical 3.0T system power electronics. This enables the exploration of an expanded diffusion parameter space (b~7-≥30 ms/μm) with reasonable SNR, along with substantially shorter diffusion encoding pulse-widths, echo times, reduced distortion, and blurring from shorter echo spacing. The choice of high b-value diffusion-encoding space can effectively suppress contributions from extra-axonal water, allowing for simplified biophysical models to be explored for non-invasive mapping of intra-axonal content. In this study, the feasibility and reproducibility of mapping whole-brain effective intra-axonal radius ( ), using MAGNUS was assessed. By making use of a test-retest paradigm, reproducibility and sensitivity were evaluated for this new biomarker. Six healthy volunteers were imaged, after obtaining written informed consent, under local IRB-approved protocols with a focus on utilizing the maximum gradient strength of 300 mT/m. Multi-shell dMRI protocols, with a lower bound b = 7 ms/μm were used for feasibility analysis and short (same-day) and long-term (7-days) test-retest repeatability. To aid in increased precision, a framework for rigorous post-processing incorporating real-valued diffusion data handling and gradient non-linearity correction was integrated. At 300 mT/m, simulations highlight a lower bound threshold for robust detectability of >1.41 μm. The simulated distribution function was consistent with measurements, where a mean = 2.75 ± 0.15 μm was observed for whole-brain white matter (WM) across all volunteers. Left-Right brain white matter asymmetry as a function of was noted with segmentations of well-reported parcels, such as the corpus callosum and corticospinal tract, demonstrating good agreement with prior literature. Data highlighted good repeatability in voxel-wise and parcel-based estimates for short- and long-term test-retest analysis. A mean coefficient of variance of 3.2% for WM parcels across all volunteers was noted, with a reproducibility coefficient of 0.16 μm (6.6%) highlighting a lack of systemic bias. This study reports on the feasibility of investigating using MAGNUS. The analysis of repeatability established the floor of changes in the brain that can be observed in studies leveraging as a neuroimaging biomarker for white matter integrity or for investigating neuroplastic processes in the brain.