MRI Neurography: Peripheral Nerve Imaging

The summary flags that MRI shows continuity but cannot reliably separate axonotmesis from neurotmesis — a point worth grounding in the injury grading, because the grade drives prognosis and the wait-versus-operate decision. (The full classification and the biology of degeneration/regeneration are developed in the nerve-injury-regeneration topic; here the focus is the imaging correlate.)

The differential table and the controversies both warn that mild nerve T2 signal can be normal and cite the "magic-angle effect" — so it deserves to be explained, because it is the single commonest reason a normal nerve is over-called as neuropathic. (The general MRI physics is developed in the mri-imaging-principles topic; here is the nerve-specific pitfall.)

Highly ordered collagen — in nerve fascicles, tendons and ligaments — normally has a very short T2 because of dipolar interactions between water protons, so these structures are dark on most sequences. When the fibres lie at about 55 degrees to the main magnetic field (B0) those dipolar interactions are minimised, T2 lengthens, and the structure becomes artifactually bright on short-echo-time sequences (T1, proton density, PD fat-sat, short-TE gradient echo). A nerve curving around a tunnel or angling through the brachial plexus can pass through this orientation and light up — mimicking oedema.