T2 mapping of individual fascicles of the sciatic nerve at 7 T using multi-echo spin-echo

Neuropathies can selectively affect individual fascicles within peripheral nerves, but diagnosis is limited by MRI spatial resolution and signal-to-noise ratio. This feasibility study aimed to leverage 7 T MRI to measure sub-millimetric T₂ maps of sciatic nerve fascicles in healthy subjects.

Accelerated multi-echo spin-echo sequences were acquired in six healthy volunteers at 7 T. Acceleration included generalised autocalibrating partially parallel acquisition (GRAPPA) and GRAPPATINI, which integrates GRAPPA with model-based accelerated relaxometry. Extended phase graph (EPG) modelling of signal decay was used to compute T₂ maps. Quantitative maps were visually assessed and T₂ values were extracted from regions of interest.

High-resolution imaging (0.4×0.4x3.0 mm³) enabled visualisation of individual fascicles. T₂ values were lower in fascicles (57.0 ms) than in the non-fascicular intraneural compartment (76.2 ms) and intermuscular fat (132.7 ms). Compared with GRAPPA alone, GRAPPATINI reduced acquisition time (11:05 to 3:35 min) while maintaining fascicle depiction and T₂ consistency (mean difference: 1.45 ms). Overlaying T₂ on S₀ maps enhanced fascicle delineation, facilitating anatomical interpretation.

This approach demonstrated the feasibility of high-resolution T₂ mapping of peripheral nerve fascicles at 7 T, offering a potential tool for improved assessment of nerve microstructure and early neuropathy detection.