MRI Neurography: Peripheral Nerve Imaging

Compression. The signs of entrapment are read at the tunnel and just above it.

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Carpal tunnel. Median nerve cross-sectional area at the pisiform is the most discriminating measurement, but there is no validated MRI cut-off. In the study cited on this page the mean was 18.8 mm² in carpal tunnel syndrome against 12.1 mm² in controls, so the familiar 10 mm² figure, which comes from ultrasound, would call the average normal wrist abnormal.

Reading the wrist. Compare with the other side, and read the area alongside the qualitative signs:

• Palmar bowing of the flexor retinaculum
• Nerve flattening within the tunnel
• T2 hyperintensity
• Proximal nerve enlargement
• Sometimes, thenar denervation

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Cubital tunnel. The ulnar nerve enlarges proximal to the cubital tunnel, behind the medial epicondyle, and may sublux over the epicondyle with flexion. T2 hyperintensity indicates inflammation, and there may be denervation of FCU or of the intrinsic muscles.

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Other tunnels. The same reading applies wherever a nerve passes a known fibro-osseous tunnel: signal increase and calibre change at the tunnel, supported by denervation in the muscles the nerve supplies.

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The normal nerve. Six features describe the normal appearance, each with its significance.

The fascicular pattern. On high-resolution axial images it appears as multiple round or oval low-signal dots (the perineurium) surrounding intermediate-signal fascicles. This honeycomb pattern is lost in nerve injury, tumour and inflammation.

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Why the grade matters. The grade of a nerve injury drives prognosis and the decision to wait or operate. MRI shows continuity but cannot reliably separate axonotmesis from neurotmesis. The full classification and the biology of degeneration and regeneration are developed in the nerve-injury-regeneration topic; the focus here is the imaging correlate.

Why imaging cannot replace time. MRI reliably shows nerve continuity, calibre, signal and secondary muscle denervation. Early on, though, it cannot distinguish a recovering axonotmesis from a neurotmesis, because both show a swollen, T2-bright, denervating nerve.

The neuroma-in-continuity. Sunderland grades II to IV, and the mixed grade VI of Mackinnon, all keep the nerve in continuity. A neuroma-in-continuity may need excision or grafting, but its presence on MRI does not by itself indicate surgery.

What decides. Serial clinical examination, electrodiagnostic evidence of re-innervation and the passage of time (no recovery by the expected re-innervation distance, advancing Tinel sign) remain essential. Intra-operative nerve action potentials across a neuroma-in-continuity are the definitive test of whether to resect and graft.

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The muscle points to the nerve. If you see muscle oedema in a specific nerve distribution, look for the nerve pathology upstream.

Oedema, then fat. Oedema-like high T2 signal comes first, fatty replacement with atrophy later, and time determines reversibility. The early changes are potentially reversible; chronic denervation with fatty infiltration indicates poor recovery potential.

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Nerve sheath tumours. The target sign is central low T2 signal (fibrous tissue) with peripheral high T2 signal (myxoid tissue). It is seen in both schwannoma and neurofibroma, and is classic but not specific for benign nature. Perineurioma, which is rare and benign, is fusiform, runs along the nerve and is T2 hyperintense.

The split-fat sign. A rim of fat partly surrounds a fusiform intraneural mass, reflecting the neurovascular bundle's fat being displaced rather than destroyed. Its presence favours a benign nerve sheath tumour; its absence is one of the most useful pointers to MPNST.

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Why it matters. Mild nerve T2 signal can be normal, and the magic-angle effect 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; this is the nerve-specific pitfall.

The physics. 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 interactions are minimised, T2 lengthens, and the structure becomes artefactually bright on short-echo-time sequences (T1, proton density, PD fat-sat, short-TE gradient echo).

Where it catches you. A nerve curving around a tunnel or angling through the brachial plexus can pass through this orientation and light up, mimicking oedema.

How not to be fooled. Suspect magic angle rather than disease when a nerve is bright only on short-echo-time sequences, only where it bends towards 55 degrees to the main field, and is of normal calibre with preserved fascicles and no muscle denervation. Confirm on a long-echo-time sequence, where the artefact fades and true oedema persists, and against the contralateral side.

Related pages: Nerve Root vs Peripheral Nerve for localising the lesion before requesting the scan, and Nerve Repairs for what the imaging findings commit you to. For the entrapments imaged most often see Carpal Tunnel, Cubital Tunnel Syndrome, Tarsal Tunnel Syndrome, Meralgia Paraesthetica and Morton's Neuroma, where MR neurography and high-resolution ultrasound are complementary. For the masses, Schwannoma, Neurofibroma and Malignant Peripheral Nerve Sheath Tumour — remembering that no imaging sign reliably separates benign from malignant. Plexus anatomy is in Brachial Plexus Anatomy and Lumbosacral Plexus Anatomy, with Adult Brachial Plexus Injury for the traumatic case.

Ultrasound, MRI or nerve conduction studies? There is no consensus first-line imaging test for entrapment. High-resolution ultrasound is cheaper, dynamic and operator-available; MR neurography offers deep and proximal coverage (plexus, sciatic) and muscle denervation mapping. Reported diagnostic accuracy largely reflects local expertise and scanner availability rather than an inherent superiority of one modality, and electrodiagnostics remain the functional reference standard.

Is routine MRI needed in carpal tunnel syndrome? Most guidelines reserve MRI for atypical presentations, revision surgery, or a suspected secondary or structural cause such as a mass, an anomalous muscle, a persistent median artery or a bifid nerve. Cross-sectional-area thresholds vary by the level measured and overlap with controls, so MRI is not recommended to confirm routine idiopathic carpal tunnel syndrome.

Quantitative neurography. Fractional anisotropy and apparent diffusion coefficient from diffusion tensor imaging, and quantitative T2 mapping, are promising biomarkers of nerve injury and recovery but remain largely research tools. Lack of standardised acquisition, normative values and reproducibility across vendors limits routine clinical use.

What nerve T2 signal means. Mild fascicular T2 hyperintensity occurs in asymptomatic volunteers and from the magic-angle effect, so signal change alone is non-specific. Interpret it alongside calibre change, fascicular architecture, muscle denervation and the clinical and electrodiagnostic picture.