High Yield Overview
MRI Neurography: Peripheral Nerve Imaging
—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
Examiner's 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
Exam 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.
MRI Neurography 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.
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.
Pathological Nerve Findings
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
Memory 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.
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
EXAMINER
"A 45-year-old presents with hand numbness and thenar weakness. Nerve conduction studies confirm carpal tunnel syndrome. The hand surgeon requests MRI."
EXCEPTIONAL ANSWER
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 POINTS TO SCORE
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 TRAPS
✗Measuring nerve at wrong level
✗Missing thenar denervation
✗Not recognising proximal enlargement
VIVA SCENARIOStandard
EXAMINER
"A patient presents after a motorbike accident with a flail arm. Clinical examination suggests brachial plexus injury. What would you expect on MRI?"
EXCEPTIONAL ANSWER
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 POINTS TO SCORE
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 TRAPS
✗Missing pseudomeningocele
✗Not assessing paraspinal muscles
✗Incomplete plexus coverage
VIVA SCENARIOStandard
EXAMINER
"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."
EXCEPTIONAL ANSWER
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), fusiform shape following nerve, slow growth. Concerning features for MPNST (malignant): Size greater than 5cm, irregular margins, heterogeneous signal (necrosis, haemorrhage), rapid growth, invasion of surrounding structures, loss of target sign, and importantly, association with NF1 (higher risk of malignant transformation). For this patient, I would assess these features and recommend specialist referral. Biopsy planning should be discussed with the tumour team as the track requires excision.
KEY POINTS TO SCORE
Size: Greater than 5cm concerning for malignancy
Heterogeneous signal suggests malignancy
Target sign seen in benign tumours
NF1 increases MPNST risk
Biopsy track must be excisable
COMMON TRAPS
✗Assuming target sign excludes malignancy
✗Not asking about NF1 history
✗Inappropriate biopsy without tumour team input
MRI Neurography Quick Reference
High-Yield Exam 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