Median and ulnar nerve fascicle imaging using MR microscopy and high-resolution ultrasound

Understanding nerve microanatomy is important as different neuropathies and some nerve neoplasms present with fascicle enlargement. The aim of our study was to gain clinically oriented knowledge on nerve fascicular anatomy using imaging modalities.

On a cadaveric upper extremity, high-resolution ultrasound (HRUS) scan with 22 MHz probe was performed. Sections of the median and ulnar nerves were excised at the level of the distal arm and after magnetic resonance microscopy (MRM), histological cross-sections (HCS) were prepared. Cross-referencing of the MRM and HRUS images with HCS was performed. Fascicle and nerve contouring was performed with morphometric software in order to assess nerve and fascicular cross-sectional area (CSA), fascicle count, and interfascicular distances. Based on fascicle differentiation, factual fascicle (FF) group and fascicular cluster (FC) group were defined.

On the cross-referenced imaging material, fascicles were differentiated in 92.7% on MRM and in 57.3% on HRUS. High to very high positive correlation among imaging material was observed for the fascicle CSA. FF depiction was 30.1% on HRUS. In comparison to the FF group, the FC group had significantly larger fascicle CSA and shorter interfascicular distances.

The findings of our study contribute to understanding of fascicle depiction on imaging modalities. HRUS offers good visualization of fascicles. The capability of differentiating fascicles is modality specific and depends on the fascicle CSA and the amount of interfascicular epineurium.