High-yield overview
Technique, normal anatomy and pathological patterns for the orthopaedic exam
—Median Nerve Normal Size
10Less than mm² at wrist
—Optimal Field Strength
3T preferred
—Resolution Required
1Less than mm in-plane
—Fascicular Pattern
—Visible on high-resolution MRI
Seddon Classification
Neurapraxia: Conduction block, nerve intact
Axonotmesis: Axon disrupted, endoneurium intact
Neurotmesis: Complete nerve disruption
Key: MRI can show continuity but not differentiate axonotmesis from neurotmesis reliably
Critical Must-Knows
- Normal nerve: intermediate T1, slightly high T2, fascicular pattern
- Pathology: nerve enlargement, T2 hyperintensity, loss of fascicular pattern
- 3T MRI preferred for nerve imaging (higher SNR)
- PD or T2 fat-sat sequences best for nerve visualisation
- Denervation oedema in muscle indicates upstream nerve injury
Clinical Pearls
- "Carpal tunnel: median nerve greater than 10mm² at pisiform level
- "Cubital tunnel: ulnar nerve enlargement proximal to tunnel
- "Brachial plexus: roots, trunks, divisions, cords, branches
- "Nerve tumour: fusiform swelling, target or fascicular sign
- "Muscle denervation: T2 high acutely, fatty replacement chronically
Clinical Warning
MRI neurography is a specialised technique. Know the normal nerve signal characteristics, signs of compression (enlargement, T2 hyperintensity), and the muscle denervation pattern that indicates nerve pathology. Carpal tunnel and cubital tunnel are common clinical applications.
Mnemonic
SCALE-TReading the Abnormal Nerve
| S | S = Size (cross-sectional area enlargement) | L | L = Localisation to a known fibro-osseous tunnel |
| C | C = Calibre change at an entrapment point | E | E = Enhancement (suggests tumour or active inflammation) |
| A | A = Architecture (loss of fascicular / honeycomb pattern) | T | T = T2 signal hyperintensity (oedema) plus muscle denervation |
| S | S = Size (cross-sectional area enlargement) | A | A = Architecture (loss of fascicular / honeycomb pattern) | E | E = Enhancement (suggests tumour or active inflammation) |
| C | C = Calibre change at an entrapment point | L | L = Localisation to a known fibro-osseous tunnel | T | T = T2 signal hyperintensity (oedema) plus muscle denervation |
Hook:Signal change alone is non-specific; combine size, calibre, architecture and muscle denervation before calling pathology.
Overview & Imaging Principles
What MR neurography adds
MR neurography is high-resolution, fat-suppressed MRI optimised to display peripheral nerves and detect pathology that electrodiagnostics localise but cannot anatomically map. It excels at deep or proximal nerves and the brachial and lumbosacral plexus, characterises mass lesions, and demonstrates secondary muscle denervation. It complements - rather than replaces - nerve conduction studies and high-resolution ultrasound.
Where Nerve Imaging Fits
| Question | Best initial test | Role of MR neurography |
|---|---|---|
| Typical carpal/cubital tunnel | Clinical + nerve conduction studies | Reserved for atypical or secondary causes |
| Superficial focal neuropathy | High-resolution ultrasound | Problem-solving, deep extension |
| Plexus / proximal / deep nerve | MR neurography | First-line imaging |
| Suspected nerve tumour | MRI with contrast | Characterisation and surgical planning |
| Functional deficit, localising level | MRI muscle denervation map | Identifies level and chronicity |
Clinical Imaging Technique
Optimal MRI Neurography Protocol
| Parameter | Recommendation | Rationale |
|---|---|---|
| Field strength | 3T preferred over 1.5T | Higher SNR for small structures |
| Coil | Dedicated surface coil | Maximises signal-to-noise |
| Slice thickness | Less than 3mm | Resolves fascicular detail |
| In-plane resolution | Less than 1mm | Visualises nerve architecture |
| Sequences | PD fat-sat, T2 fat-sat, T1 | Nerve-fluid contrast |
| Planes | Axial perpendicular to nerve + along nerve | Cross-section and longitudinal |
Nerve Visualisation Principles
Nerves are best visualised on fat-suppressed T2 or PD sequences where they appear slightly hyperintense against suppressed fat. The fascicular pattern (honeycomb appearance) should be visible on high-resolution imaging. Nerve is intermediate on T1, allowing anatomic localisation.
Systematic Approach: Normal Nerve Appearance
Normal Peripheral Nerve MRI Characteristics
| Feature | Appearance | Significance |
|---|---|---|
| T1 signal | Intermediate (isointense to muscle) | Anatomic localisation |
| T2/PD signal | Mildly hyperintense to muscle | Not as bright as fluid |
| Fascicular pattern | Honeycomb appearance on axial | Intact nerve architecture |
| Size | Consistent along course | Enlargement indicates pathology |
| Enhancement | Minimal or none with Gd | Enhancement suggests pathology |
| Course | Smooth, no deviation | Mass effect causes displacement |
Fascicular Pattern
On high-resolution axial images, the fascicular pattern appears as multiple round/oval low signal dots (perineurium) surrounding intermediate signal fascicles. This 'honeycomb' pattern is lost in nerve injury, tumour, and inflammation.
Key Findings: Pathological Nerve Patterns
MRI Signs of Nerve Compression
| Finding | Description | Significance |
|---|---|---|
| Nerve enlargement | Increased cross-sectional area | Proximal to compression site |
| T2 hyperintensity | Increased signal (brighter than normal) | Oedema, inflammation |
| Calibre change | Abrupt narrowing at compression point | Indicates entrapment location |
| Notching | Flattening at compression site | External compression |
| Loss of fascicular pattern | Homogeneous signal | Fibrosis, chronic compression |
Carpal Tunnel Syndrome
Median nerve cross-sectional area greater than 10mm² at pisiform level is diagnostic. Look for: palmar bowing of flexor retinaculum, nerve flattening within tunnel, T2 hyperintensity, proximal nerve enlargement. May see thenar muscle denervation.
Cubital Tunnel Syndrome
Ulnar nerve enlargement proximal to cubital tunnel (behind medial epicondyle). Nerve may sublux over epicondyle with flexion. T2 hyperintensity indicates inflammation. May see FCU or intrinsic muscle denervation.
Muscle Denervation
MRI Appearance of Muscle Denervation
| Stage | Timeframe | T1 Signal | T2/STIR Signal | Reversibility |
|---|---|---|---|---|
| Acute | Less than 1 month | Normal | High (oedema) | Fully reversible |
| Subacute | 1-6 months | Normal to slightly high | High | Largely reversible |
| Chronic | Greater than 6 months | High (fatty) | Variable | Irreversible fatty infiltration |
Mnemonic
OEDEMA Then FATDenervation Pattern
| O | O = Oedema-like signal (high T2) in acute phase |
| E | E = Early changes potentially reversible |
| F | F = Fatty replacement in chronic phase |
| A | A = Atrophy accompanies fatty change |
| T | T = Time determines reversibility |
| O | O = Oedema-like signal (high T2) in acute phase | A | A = Atrophy accompanies fatty change |
| E | E = Early changes potentially reversible | T | T = Time determines reversibility |
| F | F = Fatty replacement in chronic phase |
Hook:If you see muscle oedema in a specific nerve distribution, look for the nerve pathology upstream. Chronic denervation (fatty infiltration) indicates poor recovery potential.
Specific Nerve Imaging
Brachial Plexus MRI Assessment
| Structure | Location | Key Pathologies |
|---|---|---|
| Roots (C5-T1) | Exit neural foramina | Avulsion (pseudomeningocele), stretch |
| Trunks | Supraclavicular | Trauma, tumour, TOS |
| Divisions | Behind clavicle | Less commonly seen |
| Cords | Infraclavicular | Tumour, aneurysm compression |
| Branches | Axilla, arm | Specific nerve injuries |
Root Avulsion Signs
Traumatic root avulsion: pseudomeningocele (CSF collection at root level), absent nerve root on T2 myelography, denervation of paraspinal muscles (specific for preganglionic injury). MRI can assess root avulsion which has poor prognosis without grafting.
Differential Diagnosis on Nerve MRI
Distinguishing Causes of an Abnormal Nerve on MRI
| Entity | Key MRI Pattern | Discriminating Feature | Pitfall |
|---|---|---|---|
| Entrapment neuropathy | Proximal enlargement, T2 hyperintensity, calibre change at tunnel | Abnormality localises to a known fibro-osseous tunnel | Mild T2 signal can be normal at tunnels (magic-angle, partial volume) |
| Traumatic neuropathy | Discontinuity, neuroma-in-continuity, stump neuroma | History of trauma plus focal fusiform mass at injury level | Perineurial fibrosis may mimic enlargement |
| Schwannoma | Fusiform, eccentric to nerve, split-fat and target signs | Separable from parent fascicles; nerve passes at edge | Cystic/ancient change can look aggressive |
| Neurofibroma | Fusiform, central within nerve, target sign | Nerve enters and exits centrally; cannot be separated | Plexiform type in NF1 may harbour MPNST |
| MPNST | Over 5cm, ill-defined, heterogeneous, perilesional oedema, loss of split-fat | Absent split-fat sign, low ADC on DWI, rapid growth, NF1 | Target sign does NOT exclude malignancy |
| Intraneural ganglion | Cystic T2-bright tubular lesion tracking along nerve | Connection to adjacent joint (e.g. superior tibiofibular) | Mistaken for solid tumour |
| Lipomatosis of nerve (fibrolipomatous hamartoma) | Coaxial-cable / spaghetti appearance, fat between thickened fascicles | Macroscopic fat following fat signal on all sequences | May coexist with macrodactyly |
| Inflammatory / hereditary neuropathy | Diffuse symmetric multi-nerve thickening and T2 signal | Bilateral, non-focal, no tunnel localisation | Misread as multifocal entrapment |
Split-Fat Sign
A rim of fat partly surrounding 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.
Controversies & Areas of Uncertainty
MRI vs ultrasound vs 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/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. Electrodiagnostics remain the functional reference standard; imaging is complementary, not a replacement.
Is routine MRI needed in carpal tunnel syndrome?
Most guidelines reserve MRI for atypical presentations, suspected secondary/structural cause (mass, anomalous muscle, persistent median artery, bifid nerve), or revision surgery. Cross-sectional-area thresholds vary by level measured and overlap with controls, so MRI is not recommended to confirm routine idiopathic CTS.
Quantitative neurography (DTI, T2 mapping)
Diffusion tensor imaging-derived fractional anisotropy and apparent diffusion coefficient, plus 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.
Significance of nerve T2 hyperintensity
Mild fascicular T2 hyperintensity occurs in asymptomatic volunteers and from magic-angle effect, so signal change alone is non-specific. Findings should be interpreted alongside calibre change, fascicular architecture, muscle denervation and the clinical/electrodiagnostic picture.
Guidelines, Registries & Global Practice
Imaging Guidance for Peripheral Nerve Disorders (Society Positions)
| Body | Position on MRI / Neurography | Practical Implication |
|---|---|---|
| AAOS (US, CTS CPG) | Routine imaging not required to diagnose idiopathic CTS; electrodiagnostics support diagnosis | Reserve MRI for atypical or secondary causes |
| ACR Appropriateness Criteria (US) | MR neurography / high-resolution US usually appropriate for focal neuropathy and plexopathy | Modality choice guided by site and local expertise |
| NICE / BOA (UK) | Clinical and electrodiagnostic diagnosis first; imaging for atypical features or masses | MRI targeted, not screening |
| EFORT / European MSK radiology consensus | MR neurography and US complementary; 3T and dedicated coils recommended where available | Protocol and expertise dependent |
| ESSR (European Soc MSK Radiology) | Endorses high-resolution US and MR neurography with standardised nerve protocols | Drives reporting consistency |
Global epidemiology context
Carpal tunnel syndrome is the commonest entrapment neuropathy worldwide (general-population prevalence roughly 3 to 6 percent of adults, higher in women and manual workers). Cubital tunnel is the second commonest. Most are diagnosed clinically and electrodiagnostically; imaging volumes vary enormously with access.
High- vs limited-resource practice
In well-resourced centres, 3T MR neurography with metal-artifact-reduction and 3D isotropic sequences allows multiplanar nerve and plexus assessment. Where MRI access or expertise is limited, high-resolution ultrasound is the pragmatic first-line nerve imaging test, and management often proceeds on clinical and electrodiagnostic grounds alone.
Registry / quality note
There is no implant registry analogue for nerve imaging. Quality is governed by acquisition standards (field strength, dedicated coils, fat suppression, in-plane resolution) and reporting expertise rather than device-survival data. Diagnostic performance figures in the literature should be read as expertise- and protocol-dependent.
Evidence
Median nerve cross-sectional area at the pisiform is enlarged in carpal tunnel syndrome
Park, Won, Oh, Kim, Hwang, Yoo • Asian J Surg (2019)
Key Findings:
- Retrospective comparison of 164 wrists: 67 with clinically and electrodiagnostically confirmed CTS versus 97 controls.
- Mean median nerve cross-sectional area at the pisiform level was 18.8 mm² in CTS versus 12.1 mm² in controls (p less than 0.05).
- Cross-sectional area at the hook of hamate did not differ significantly between groups.
Clinical Implication: The pisiform level is the most discriminating site for measuring median nerve cross-sectional area on MRI in suspected CTS.
3T MRI nerve cross-sectional area correlates with electrodiagnostic severity in CTS
Ikeda, Okada, Toyama, Uemura, Takamatsu, Nakamura • Orthopedics (2017)
Key Findings:
- 70 wrists of 35 patients with unilateral idiopathic CTS imaged at 3T with nerve conduction studies.
- Median nerve cross-sectional area was greatest at the scaphoid body level and positively correlated with distal motor latency.
- Cross-sectional area at the distal radioulnar joint and hamate hook did not correlate with severity.
Clinical Implication: Level of measurement matters; 3T MRI nerve area can reflect electrophysiological severity but thresholds are level-dependent.
Absent split-fat sign and ADC best discriminate malignant from benign nerve sheath tumours
Yun, Lee, Lee, Lee, Kim, Lee, Chung, Lee, Shin • Eur Radiol (2021)
Key Findings:
- 87 peripheral nerve sheath tumours; 55 indeterminate lesions analysed (18 malignant, 37 benign) with conventional and diffusion-weighted MRI.
- Size, margin, perilesional oedema and split-fat, fascicular and target signs differed between benign and malignant tumours.
- Combining mean ADC value with absence of the split-fat sign gave a C-index greater than 0.9 for identifying MPNST.
Clinical Implication: Use ADC plus loss of the split-fat sign, not the target sign, to flag possible MPNST and prompt specialist referral.
MRI features of neurogenic tumours and tumour-like lesions
Abreu, Aubert, Wavreille, Gheno, Canella, Cotten • Eur J Radiol (2011)
Key Findings:
- Review of neurogenic tumours and pseudotumours: traumatic and Morton neuroma, lipomatosis of nerve, nerve sheath ganglion, perineurioma and benign/malignant PNST.
- Defining MRI clues include nerve entering or exiting the mass, fusiform shape, split-fat sign, target sign, fascicular appearance and denervation of the supplied muscle.
- No single finding reliably separates benign from malignant lesions; pattern recognition guides diagnosis.
Clinical Implication: A lesion on the course of a major nerve with split-fat, target or fascicular signs should be treated as neurogenic until proven otherwise.
3D MR neurography enables multiplanar peripheral nerve assessment
Khalilzadeh, Fayad, Ahlawat • Semin Musculoskelet Radiol (2021)
Key Findings:
- Review of high-resolution isotropic 3D MR neurography acquisition and clinical applications.
- 3D techniques allow multiplanar reconstruction along tortuous nerves plus anatomical and functional tissue characterisation.
- Applications span entrapment, trauma, inflammatory/infectious neuropathies and neoplasms.
Clinical Implication: 3D isotropic MR neurography is the modern technique of choice for following nerves and plexus across multiple planes.
MR neurography is feasible around orthopaedic hardware with metal-artifact reduction
Sneag, Zochowski, Tan • Radiology (2021)
Key Findings:
- Technical review of MR neurography for peripheral nerve injury in the presence of metallic hardware.
- Conventional 2D proton-density and T2 fat-suppressed sequences plus 3D reversed steady-state and multispectral techniques reduce susceptibility artifact.
- Real-time radiologist monitoring and protocol optimisation improve diagnostic yield near metal.
Clinical Implication: Pre-existing hardware is not an absolute barrier to nerve imaging when metal-artifact-reduction protocols are used.
MR neurography and high-resolution ultrasound are complementary for nerve imaging
Deshmukh, Sun, Komarraju, Singer, Wu • Magn Reson Imaging Clin N Am (2023)
Key Findings:
- Review correlating MR neurography with high-resolution ultrasound for peripheral nerve assessment.
- Both modalities provide detailed nerve anatomy and pathology with optimised technique.
- Reported diagnostic accuracy largely reflects local expertise and access to current technology.
Clinical Implication: Choose between or combine MRI and ultrasound based on nerve location, depth and local expertise rather than assuming one is universally superior.
Foundational description of MR neurography of peripheral nerves
Aagaard, Maravilla, Kliot • Magn Reson Imaging Clin N Am (1998)
Key Findings:
- Early review establishing direct MR visualisation of normal-sized major peripheral nerves using phased-array surface coils.
- Described the imaging appearance of normal nerve plus traumatic, compressive and neoplastic pathology.
- Illustrated brachial and lumbosacral plexus, carpal tunnel and cubital tunnel imaging.
Clinical Implication: Established MR neurography as a tool able to detect peripheral nerve pathology earlier and more precisely than prior methods.
Clinical Decision Scenarios
Use these scenarios to practise clinical reasoning and management decisions
CLINICAL SCENARIOStandard
CLINICAL PROMPT
"A 45-year-old presents with hand numbness and thenar weakness. Nerve conduction studies confirm carpal tunnel syndrome. The hand surgeon requests MRI."
PRACTICAL APPROACH
On MRI of the wrist, carpal tunnel syndrome findings include: (1) Median nerve cross-sectional area greater than 10mm² at the level of the pisiform (hamate hook) - this is the most specific finding. (2) Flattening ratio - nerve width/height increased within the tunnel. (3) T2 hyperintensity of the median nerve (oedema). (4) Palmar bowing of the flexor retinaculum (greater than 4mm displacement). (5) Proximal nerve enlargement proximal to the tunnel entrance. Additionally, I would look for denervation oedema (high T2 signal) in the thenar muscles (APB, opponens pollicis) which indicates motor involvement.
KEY CLINICAL POINTS
Median nerve area greater than 10mm² at pisiform level
T2 hyperintensity indicates oedema
Nerve flattening within tunnel
Palmar bowing of retinaculum
Thenar muscle denervation if motor involvement
COMMON PITFALLS
Measuring nerve at wrong level
Missing thenar denervation
Not recognising proximal enlargement
FURTHER QUESTIONS
"What is the role of MRI in carpal tunnel syndrome management? When would you order imaging?"
CLINICAL SCENARIOChallenging
CLINICAL PROMPT
"A patient presents after a motorbike accident with a flail arm. Clinical examination suggests brachial plexus injury. What would you expect on MRI?"
PRACTICAL APPROACH
For brachial plexus injury, I would request MRI of the brachial plexus (cervical spine to axilla) at 3T if available. Protocol includes: T2 fat-sat coronal (follows plexus), STIR sagittal/coronal, T1 for anatomy, and consider 3D heavily T2-weighted sequences (CISS/FIESTA) for root assessment. Key findings: (1) Root avulsion - pseudomeningocele (CSF collection at nerve root level), absent root on T2 myelography effect, paraspinal muscle denervation (preganglionic). (2) Trunk/cord injury - nerve discontinuity, neuroma formation, T2 hyperintensity. (3) Muscle denervation pattern - helps localise level. Root avulsion has poor prognosis as it's preganglionic; trunk injuries may recover or be amenable to nerve transfer/grafting.
KEY CLINICAL POINTS
Pseudomeningocele indicates root avulsion
Paraspinal denervation = preganglionic injury
Assess nerve continuity along entire plexus
Muscle denervation pattern helps localise
Root avulsion has poor recovery prognosis
COMMON PITFALLS
Missing pseudomeningocele
Not assessing paraspinal muscles
Incomplete plexus coverage
FURTHER QUESTIONS
"What is the difference between preganglionic and postganglionic injury? How does this affect surgical options?"
CLINICAL SCENARIOChallenging
CLINICAL PROMPT
"A 30-year-old presents with a slowly enlarging painless mass in the forearm. Ultrasound shows a fusiform mass along the course of a nerve."
PRACTICAL APPROACH
MRI features of peripheral nerve sheath tumours: Benign features (schwannoma/neurofibroma): Size less than 5cm, well-defined margins, homogeneous signal, target sign (central low T2, peripheral high T2), split-fat sign, fusiform shape following nerve, slow growth. Concerning features for MPNST (malignant): Size greater than 5cm, irregular margins, heterogeneous signal (necrosis, haemorrhage), perilesional oedema, low ADC on diffusion-weighted imaging, loss of the split-fat sign, rapid growth, and importantly association with NF1 (higher risk of malignant transformation). For this patient, I would assess these features and recommend specialist sarcoma-team referral. Biopsy planning should be discussed with the tumour team as the track requires excision.
KEY CLINICAL POINTS
Size: Greater than 5cm concerning for malignancy
Absent split-fat sign and low ADC favour MPNST
Target sign does NOT exclude malignancy
NF1 increases MPNST risk
Biopsy track must be excisable
COMMON PITFALLS
Assuming target sign excludes malignancy
Not asking about NF1 history
Inappropriate biopsy without tumour team input
FURTHER QUESTIONS
"How would you differentiate schwannoma from neurofibroma? What is the surgical approach to peripheral nerve tumours?"
MRI Neurography Quick Reference
Clinical summary
Normal Nerve Appearance
- •T1: Intermediate (like muscle)
- •T2: Mildly hyperintense to muscle
- •Fascicular pattern (honeycomb)
- •Size consistent along course
Compression Signs
- •Nerve enlargement proximal to compression
- •T2 hyperintensity (oedema)
- •Calibre change at compression point
- •Loss of fascicular pattern
Carpal Tunnel Criteria
- •Median nerve greater than 10mm² at pisiform
- •Palmar bowing of retinaculum
- •Nerve flattening within tunnel
- •Thenar denervation (late)
Denervation Pattern
- •Acute: High T2, normal T1 (oedema)
- •Chronic: High T1 (fatty infiltration)
- •Distribution follows nerve supply
- •Identifies level of injury