MRI Cranial Nerves — Generic Standard Protocol

MRIninja Knowledge Base | Master / General Protocol Page Related pages: MRI CPA and Inner Ear · MRI Middle Ear and Petrous Bone · MRI Soft Tissues Neck · MRI Parotid Glands · MRI Brachial Plexus Version 1.0 — May 2026

1. Executive Summary

MRI of the cranial nerves is one of the most complex and heterogeneous imaging domains in neuroradiology. Twelve pairs of cranial nerves (CN I–XII) arise from the brain and brainstem, traverse specific foramina and canals in the skull base, and distribute through the face, head, neck, and thoracic viscera. No single MRI protocol can simultaneously optimise for all twelve nerves — they span from the cribriform plate (CN I) to the thoracic inlet (CN X), cross markedly different tissue environments (dural foramina, bony canals, extracranial soft tissue planes), and vary in diameter from 0.5 mm (chorda tympani) to 10–12 mm (CN II optic nerve). The generic cranial nerve MRI protocol is therefore a framework — a collection of shared principles and adaptable sequence strategies — from which nerve-specific or indication-specific protocols are derived.

The unifying clinical rationale for cranial nerve MRI is the evaluation of nerve dysfunction: palsy, paresis, pain, sensory loss, hypersecretion, or autonomic abnormality attributable to a specific cranial nerve. For each clinical syndrome, MRI provides: (1) anatomical localisation of the lesion along the nerve course; (2) lesion characterisation (compression vs infiltration vs inflammation vs demyelination); (3) assessment of the nerve itself (thickening, T2 signal change, enhancement, loss of neural fat signal); (4) identification of the responsible pathology (mass, vascular, inflammatory, infectious).

MRI is uniquely suited for cranial nerve assessment because it provides direct nerve tissue visualisation in the cisternal and intracanalicular segments where CT provides only bony canal detail. In the extracranial soft tissue segments, MRI provides the contrast resolution for perineural spread, neural sheath tumours, and inflammatory nerve involvement that CT cannot match.

The key technical challenge common to all cranial nerve MRI is scale: the nerves are 1–10 mm in diameter, require sub-millimetre to millimetre resolution, and course through tissues with markedly different magnetic properties (CSF, bone, fat, muscle) that challenge fat suppression and susceptibility management.

1.1 Core Strengths

Direct nerve tissue visualisation: MRI is the only non-invasive modality that directly images the nerve fascicles — including their course within CSF cisterns (where they appear as intermediate-dark structures in T2-bright CSF on 3D CISS), through bony canals (where T1 and post-contrast T1 show the nerve against the background of canal marrow fat or air), and through extracranial soft tissues.

Perineural tumour spread (PNS) detection: this is the single most important contribution of MRI to cranial nerve pathology. Perineural spread of mucosal head and neck carcinomas — most commonly along CN V3 (mandibular nerve) via the foramen ovale, CN V2 (maxillary nerve) via the foramen rotundum, and CN VII along the facial canal — is invisible on CT until late-stage neural foraminal enlargement occurs. MRI detects PNS at an earlier, potentially treatable stage through: nerve enhancement; replacement of normal neural fat within the foramen; nerve enlargement; T2 signal change within the nerve. Detection of PNS fundamentally changes tumour staging and radiation therapy planning.

Neural sheath tumours: schwannoma and neurofibroma of any cranial nerve are characterised by their T2 signal (bright, target sign in neurofibroma), T1 signal, enhancement pattern, and nerve-of-origin. Their location along the nerve, their relationship to adjacent structures, and any intracranial extension are defined.

Inflammatory and infectious neuritis: viral (Ramsay Hunt syndrome — CN VII/VIII), autoimmune (immune-mediated neuritis, neurosarcoidosis), and infectious (Lyme disease, tuberculosis) cranial neuropathies show nerve enhancement, T2 signal change, and in some cases nerve thickening. Enhancement of a specific nerve segment in the appropriate clinical context is pathognomonic.

Demyelinating disease: cranial nerve demyelination in multiple sclerosis produces T2-bright plaques within the nerve or adjacent brainstem (optic neuritis: CN II), detectable on fat-suppressed T2 or STIR sequences, and typically enhances acutely on post-contrast T1.

1.2 Intrinsic Limitations of the Generic Protocol

Scale heterogeneity across twelve nerves: no single protocol optimises for all cranial nerves simultaneously. CN II (optic nerve, 3–5 mm diameter) requires dedicated orbital sequences. CN I (olfactory nerve, < 0.5 mm) is virtually unresolvable individually on clinical MRI. CN X (vagus nerve) runs 35–40 cm from the jugular foramen to the thoracic inlet — whole-course imaging requires full neck and thorax coverage, far beyond a single-station head protocol.

Resolution limit for individual fascicle assessment: clinical MRI at 1.5T–3T resolves nerve trunks ≥ 2–3 mm reliably. Smaller individual branches (e.g., the chorda tympani, the lingual nerve, the nerve to stapedius) are below the clinical resolution limit.

Functional assessment: MRI provides structural information only. Electrophysiological testing (EMG, nerve conduction, auditory brainstem response, visual evoked potentials) remains essential for functional assessment.

When dedicated child protocols are required: each cranial nerve, when it is the specific clinical target, requires its own dedicated protocol. CN II (optic neuritis, optic nerve tumour); CN V (trigeminal neuralgia — neurovascular compression; trigeminal schwannoma; perineural spread); CN VII (facial palsy — Bell’s palsy, Ramsay Hunt, schwannoma; intratemporal and parotid assessment); CN VIII (vestibular schwannoma; hearing loss — see CPA/inner ear page); CN IX–XI (jugular foramen — glomus jugulare, schwannoma); CN XII (hypoglossal canal — schwannoma, metastasis); full perineural spread assessment for any head and neck cancer.

2. Main Clinical Indications

2.1 Standard Indications

Cranial neuropathy — unknown cause is the broadest indication. When a patient presents with unexplained cranial nerve dysfunction (facial palsy, diplopia, facial numbness, hypoglossal palsy), the MRI survey covers the full course of the affected nerve from brainstem nucleus to end-organ. The generic protocol — brain sequences covering the brainstem + targeted posterior fossa and skull base + neck sequences for extracranial nerves — provides the initial survey. If the generic protocol identifies a specific lesion, targeted imaging of that lesion follows.

Perineural spread screening in known head and neck cancer. When a patient with diagnosed mucosal carcinoma (oral cavity, oropharynx, salivary gland, skin) has unexplained pain, sensory loss, or motor deficit along a cranial nerve territory, PNS must be excluded. This indication requires targeted post-contrast T1 sequences through the foraminal pathway of the clinically implicated nerve.

Trigeminal neuralgia — evaluation for neurovascular compression (NVC) at the trigeminal entry zone (REZ). The CISS/FIESTA-C sequence at sub-millimetre isotropic resolution identifies vascular loops in contact with CN V at the REZ. This is the single most specific MRI indication for CN V and requires the 3D CISS targeted on the posterior fossa bilaterally.

Facial nerve palsy — atypical or recurrent Bell’s palsy, suspected neoplastic cause, Ramsay Hunt syndrome, trauma. The facial nerve protocol extends from the IAC (CPA/inner ear page) through the intratemporal segments (middle ear page) to the parotid and extracranial branches (neck/parotid page). The generic cranial nerve protocol provides the intracranial and skull base survey; specialist child pages cover the specific segments.

Lower cranial nerve palsy (CN IX–XII) — evaluation of the jugular foramen (CN IX, X, XI) and hypoglossal canal (CN XII). Glomus jugulare, schwannoma, meningioma, and metastatic disease in this region produce combined lower cranial nerve palsies. MRI characterises these lesions.

Optic neuritis and optic nerve assessment — the optic nerve is best assessed by dedicated orbital MRI with fat-suppressed T2/STIR and post-contrast T1. The generic cranial nerve protocol provides the initial intracranial survey; dedicated orbital sequences are the child protocol.

2.2 Urgent Red Flags Requiring Expedited or Emergency Imaging

3. Preparation Reference

Universal MRI safety screening belongs to the general MRI preparation page and is not repeated here.

3.1 Anatomy-Specific Preparation Items

Coil selection is preparation: the cranial nerve protocol spans a large anatomical range. The coil selection must match the nerve being targeted: - Brain/head coil (16–32 channel): for all intracranial segments of CN I–VIII and the skull base portions of CN IX–XII - Head-and-neck coil: for CN IX–XII extracranial segments, CN X into the neck, CN XII in the hypoglossal canal - Orbital surface coil: for CN II (optic nerve) targeted imaging - Neck coil or flexible surface coil: for extracranial CN VII in the parotid, CN IX–X–XI in the carotid space

Prior imaging review: for PNS assessment, the complete prior imaging record (CT, PET/CT, prior MRI, endoscopy reports, biopsy results) must be reviewed before the examination to know which nerve is at risk and which foraminal pathway to prioritise.

Dental metalwork: as documented in the neck and parotid protocols — dental metalwork affects the skull base and parotid region, degrading the assessment of CN V3 and CN VII. At 3T, the susceptibility artefact from posterior dental metalwork can obscure the foramen ovale and the pterygopalatine fossa regions. Document and consider 1.5T for foramen ovale assessment in patients with extensive dental restoration.

Prior surgery: post-radiation changes and post-surgical anatomy can completely alter the expected nerve pathway. Review the operative and radiation records before the examination to understand the altered anatomy.

3.2 Patient Positioning on the MRI System

Standard position: supine, head-first, head in the head coil. The positioning is identical to standard brain MRI for the intracranial segments. For neck coverage (CN X, lower cranial nerves in the carotid space), the head-and-neck coil combination is used with isocentre at the level of the C2–C3 vertebra to cover the skull base to the upper thoracic inlet.

Isocentre consideration: for skull base and posterior fossa assessment (the most common focus of generic cranial nerve MRI), isocentre at the level of the external auditory canal optimises B0 homogeneity at the skull base and posterior fossa — the most critical region for 3D CISS quality. This is the same positioning principle as for CPA/inner ear MRI.

4. Standard Protocol Design

The generic cranial nerve protocol must cover the full course of the suspected nerve from brainstem to extracranial distribution. Because no single protocol is optimal for all cranial nerves, the generic design provides a modular framework with: - Brain/posterior fossa module: covers the cisternal segments of all cranial nerves and the brainstem nuclei - Skull base module: covers the foraminal segments using 3D CISS and post-contrast T1 - Extracranial module: added for CN VII (parotid), CN V (pterygopalatine fossa), CN IX–XII (neck), as needed

4.1 Mandatory Core Sequences

4.2 Conditional Sequences

4.3 Rationale Summary Per Sequence

3D CISS/DRIVE (sub-millimetre isotropic) — the defining sequence for cisternal and foraminal cranial nerve assessment. As described in the CPA/inner ear protocol, the 3D CISS provides T2/T1 ratio-weighted contrast that maximises CSF brightness vs neural tissue darkness. For cranial nerves in the cisterns: - CN III in the interpeduncular cistern - CN IV in the ambient cistern (the thinnest and most difficult to identify: 0.5 mm diameter) - CN V root entry zone and Meckel’s cave - CN VI at the clivus - CN VII/VIII complex in the CPA cistern and IAC - CN IX/X/XI in the lateral medullary cistern - CN XII in the premedullary cistern and hypoglossal canal

At 0.5–0.7 mm isotropic, these nerves are individually resolved in the CSF. This resolution is essential for neurovascular compression assessment (CN V — trigeminal neuralgia; CN VII/VIII — hemifacial spasm, tinnitus) where the contact between a vascular loop and the nerve root entry zone determines surgical candidacy for microvascular decompression (MVD).

T2-weighted TSE (brain) — standard brain T2 provides: - Brain parenchymal lesions contributing to cranial neuropathy (demyelinating plaques, ischaemia, tumour, encephalitis) - Cisternal segment survey at lower resolution - Posterior fossa parenchyma assessment - Meningeal changes

FLAIR — essential for: - Periventricular and subcortical MS plaques (optic neuritis, CN VI, CN III involvement in MS) - Leptomeningeal disease (FLAIR sulcal bright signal = leptomeningeal carcinomatosis) - Subarachnoid haemorrhage (causing CN III palsy from PComA aneurysm) - White matter disease context

Post-contrast T1 fat-suppressed (axial + coronal) — the most clinically important sequence for most cranial neuropathies, particularly PNS. Fat suppression is essential because: - Perineural spread in the pterygopalatine fossa and cavernous sinus replaces the normally T1-bright fat → this fat loss is the primary sign of PNS; post-contrast T1-FS reveals the enhancing nerve within the fat-filled foramen - Enhancement of the nerve itself (neuritis, schwannoma, perineural carcinoma) is only visible against a fat-suppressed background - Normal neural fat in the foramina (foramen ovale, foramen rotundum, hypoglossal canal) provides natural T1-bright contrast that delineates the dark nerve — fat suppression inverts this, revealing pathological enhancement

Coronal T2 fat-suppressed (STIR or SPAIR) — the panoramic survey of the extracranial cranial nerve territories. A coronal slab from the skull base to the thoracic inlet, with fat suppression, provides bilateral simultaneous assessment of: - The parapharyngeal space and parotid (CN VII, CN IX) - The carotid space (CN IX, X, XI, XII) - The pterygopalatine fossa and masticator space (CN V2, V3) - The neck lymph node chains (for PNS-producing primary tumours)

STIR is preferred at isocentre for its B0-independence across the full coronal coverage from skull base to thoracic inlet — the same rationale as the neck, brachial plexus, and WB-MRI protocols within MRIninja.

4.4 Sequence Matching and Cross-Sequence Consistency

The most critical matching requirement for cranial nerve MRI is between the pre-contrast T1 and post-contrast T1 fat-suppressed sequences: the foraminal fat signal (T1-bright, pre-contrast) must be compared with the post-contrast fat-suppressed image to distinguish physiological fat replacement by perineural tumour (loss of fat + enhancement) from normal anatomy (fat retained). Geometric mismatch between pre and post T1 sequences makes this comparison unreliable.

For serial PNS monitoring or post-treatment surveillance, the acquisition parameters — particularly the plane and thickness of the foramen-targeted T1 — must be reproduced at every follow-up.

4.5 Fat Suppression

For cranial nerve MRI, fat suppression strategy follows the same principles as the neck and parotid protocols:

SPAIR is preferred at isocentre (head coil, skull base) where B0 homogeneity is adequate.

STIR is preferred for coronal coverage extending to the neck (off-isocentre) where B0 inhomogeneity from the cervicothoracic junction would fail SPAIR.

Dixon is preferred for post-contrast 3D T1 at 3T for B0-independent fat-water separation across the full skull base to neck coverage.

Non-fat-suppressed T1 is mandatory pre-contrast: the T1-bright fat within the neural foramina is the diagnostic reference against which fat replacement by PNS is assessed.

Post-contrast STIR is absolutely contraindicated as throughout all MRIninja protocols.

5. Optimisation Strategy

5.1 Artifact Reduction by Source

Vascular pulsation artefacts (CN V–VIII): the basilar artery, AICA, PICA, and superior cerebellar artery (SCA) produce pulsatile artefacts in the phase-encoding direction. For posterior fossa axial acquisitions (A-P phase), these artefacts propagate anteroposteriorly and may cross the pontine area where CN VI and CN VII exit. Flow-compensated (GMN) sequences and cardiac gating reduce this artefact, at the cost of complexity and time. For routine clinical assessment, accepting these artefacts and noting them in the report is standard. For high-resolution neurovascular compression assessment (CN V trigeminal neuralgia), flow-compensated 3D CISS reduces vascular artefact specifically at the REZ.

Susceptibility at the skull base (CN V, CN XII, CN IX): the complex air-bone-soft tissue interfaces at the skull base foramina produce local B0 disturbances. The foramen ovale (CN V3) and the hypoglossal canal (CN XII) are particularly affected. Narrow bandwidth T1 sequences increase chemical shift artefact here; wider bandwidth (300–500 Hz/px at 3T) reduces this. Dixon post-contrast T1 is robust across the skull base B0 inhomogeneity.

Motion during 3D CISS (posterior fossa): as documented for CPA/inner ear and middle ear protocols. Any head motion during the 5–7 minute 3D CISS degrades the sub-millimetre resolution required for nerve and vascular loop identification. This is the most common cause of non-diagnostic cranial nerve MRI for the posterior fossa component.

Dental metalwork (CN V3 foramen ovale region): at 3T, posterior dental metalwork produces susceptibility artefacts extending to the foramen ovale (3–5 cm from the posterior molar teeth). This specifically degrades CN V3 PNS assessment in the region of highest clinical importance. Mitigation: 1.5T for CN V3 skull base assessment in patients with extensive dental metalwork; document limitation in report.

5.2 Protocol Efficiency and Throughput

A complete generic cranial nerve MRI — T2 brain + FLAIR + DWI + 3D CISS + post-contrast T1 (axial + coronal) + STIR coronal — requires approximately 35–45 minutes at 3T. For targeted nerve-specific protocols, the time ranges from 20 minutes (posterior fossa only) to 50–60 minutes (full skull base + neck course).

The 3D CISS is the major time investment: 5–7 minutes for the posterior fossa volume. Compressed sensing can reduce this to 3–4 minutes. For routine CN V, VII, and VIII assessment, 3D CISS is indispensable and the time is justified. For CN I (olfactory) or CN XII (hypoglossal), dedicated thin-slice T2 and post-contrast T1 sequences are more efficient than full posterior fossa 3D CISS.

5.3 Field Strength Considerations

3T is preferred for: - Sub-millimetre 3D CISS for CN V, VII, VIII, IX–XI cisternal assessment - Higher spatial resolution for thin nerve branches (CN IV, CN VI — the thinnest cranial nerves at clinical MRI) - PNS post-contrast T1 with Dixon fat suppression (better fat-water separation at 3T)

1.5T is equivalent or preferred for: - Patients with extensive dental metalwork (susceptibility smaller at 1.5T) - Brain parenchymal assessment (T2, FLAIR — adequate at 1.5T) - When 3D CISS quality at 3T is compromised by banding artefact

6. Contrast Use Principles Specific to Cranial Nerve MRI

6.1 Non-Contrast Standard Protocol — Sufficient For

Non-contrast cranial nerve MRI is diagnostically adequate for: - Neurovascular compression assessment (trigeminal neuralgia: 3D CISS alone identifies the vascular loop in contact with CN V REZ) - Initial screening for cisternal nerve tumours (schwannoma: T2-bright filling defect in CISS; enhancement is confirmatory but not necessary for detection of larger lesions) - Demyelinating disease evaluation (optic neuritis in the acute phase: fat-suppressed T2/STIR optic nerve signal change visible without contrast)

6.2 Gadolinium Indicated — Region-Specific Contexts

Post-contrast T1 fat-suppressed is required for: - Perineural spread assessment: the single most important indication — enhancement of the nerve within the foramen (foramen ovale, foramen rotundum, hypoglossal canal) is the primary PNS sign - Cranial nerve neuritis (Bell’s palsy, Ramsay Hunt, CN III neuritis, optic neuritis post-acute): nerve enhancement confirms active inflammation and may localise the segment involved - Leptomeningeal carcinomatosis: enhancement of multiple cranial nerve sheaths in the cisterns - Neural sheath tumours (schwannoma, neurofibroma): enhancement confirms the diagnosis - Cavernous sinus assessment (CN III, IV, V, VI): cavernous sinus lesions (thrombosis, inflammation, tumour) require contrast for characterisation - All suspected PNS cases regardless of clinical certainty: the consequence of missing PNS is under-staging and inadequate treatment

6.3 Post-Contrast Acquisition Timing

Standard equilibrium phase (3–5 minutes post-injection) for all cranial nerve post-contrast sequences. No specific arterial phase is required.

Exception: for arteriovenous fistula or vascular anomaly contributing to cranial nerve dysfunction, MRA (TOF or contrast-enhanced) may require specific timing.

7. Reporting Essentials

7.1 Interpretation Framework

Cranial nerve MRI reporting requires a nerve-by-nerve systematic approach, with the anatomical course of the clinically implicated nerve as the primary axis:

For each implicated nerve: 1. Brainstem nucleus and root exit zone: any T2 signal change, plaque, mass, enhancement 2. Cisternal segment (in CSF): presence of nerve on 3D CISS; any thickening, T2 change; neurovascular contact (for neurovascular compression assessment) 3. Foraminal/intracanalicular segment: fat signal preservation; enhancement; foraminal widening or destruction (CT for bony detail) 4. Extracranial segment: nerve calibre; T2 signal; enhancement; soft tissue mass adjacent

Diagnostic axes: - Intrinsic (nerve itself: neuritis, schwannoma, lymphoma infiltration) vs extrinsic (compression by adjacent mass or vascular loop) - Single nerve vs multiple nerves (leptomeningeal vs individual nerve pathology) - Enhancing vs non-enhancing - Focal vs segmental vs diffuse involvement

The PNS reporting axiom: when a cranial nerve segment shows fat replacement + enhancement + nerve thickening in a patient with head and neck cancer history → report as perineural spread until proven otherwise; describe the full extent from the known primary tumour to the most proximal nerve segment involved.

7.2 Mandatory Reporting Checklist

Technical quality: - [ ] Field strength; coil; 3D CISS voxel size documented - [ ] 3D CISS: banding artefact location — affects target nerve? Yes/No - [ ] Post-contrast T1-FS: fat suppression quality at skull base; foramen ovale level

Brain parenchyma: - [ ] T2/FLAIR: any brainstem signal change at the level of the implicated nerve nucleus/exit zone - [ ] DWI: any restricted diffusion in brainstem or posterior fossa

Cisternal segments (3D CISS): - [ ] Each implicated nerve: visible / not visible; normal calibre / thickened; neurovascular contact present/absent

Foraminal segments (post-contrast T1-FS): - [ ] Each relevant foramen: fat preserved / replaced; nerve enhancement present/absent - [ ] Cavernous sinus: normal / abnormal

Extracranial segments (T2-FS coronal + post-contrast T1-FS): - [ ] Pterygopalatine fossa: normal / abnormal signal or enhancement (CN V2) - [ ] Masticator space: normal (CN V3) - [ ] Parotid: CN VII — normal / mass / invasion - [ ] Neck: carotid space normal / mass

7.3 Structured Reporting

Reports must include: Indication (specific cranial nerve affected; clinical syndrome; prior cancer history if applicable); Technique (field strength, coil, sequences, 3D CISS voxel size, contrast: agent, dose); Comparison (prior MRI date); Findings (systematic nerve-by-nerve review from brainstem to extracranial terminus); Impression (primary finding; PNS extent if present; neurovascular compression grade; mass characteristics); Limitations (dental metalwork at foramen ovale; 3D CISS banding; coverage limits).

7.4 Incidental Findings — Clinical Decision Framework

Usually benign: asymmetric Meckel’s cave size; prominent subarachnoid space along the optic nerve sheath (normal variant); cisternal loops of vessels adjacent to CN V that do not contact the REZ.

May require follow-up: mild asymmetric enhancement of CN V in the absence of known malignancy (neuritis vs early PNS) — clinical correlation and ENT review; small schwannoma-pattern lesion in the IAC without prior imaging.

Urgent communication: unexpected new PNS in a cancer patient (changes staging and treatment); leptomeningeal carcinomatosis pattern with multiple nerve sheaths enhancing; unexpected cavernous sinus mass; CN III palsy with aneurysm pattern on MRA.

8. MRI Technologist Pearls

8.1 Sequence Order Logic

1. Three-plane localiser
2. 3D CISS ← first; most critical for cisternal nerve assessment; motion-sensitive; best when patient is fresh
3. T2 brain axial
4. FLAIR axial
5. DWI
6. T1 axial ← pre-contrast reference
7. STIR coronal ← skull base to neck; before contrast
8. Contrast injection
9. Post-contrast T1-FS axial
10. Post-contrast T1-FS coronal

8.2 Positioning Tricks

Isocentre at the external auditory canal for skull base-targeted protocols: identical to the CPA/inner ear protocol. For intracranial-only protocols (brain T2 + FLAIR + DWI), standard brain isocentre is acceptable. For 3D CISS quality at 3T, isocentre at the level of the posterior fossa target is essential.

Dedicated orbital positioning for CN II: for optic nerve assessment, the patient should have their eyes closed and still during the orbital sequences. Looking laterally stretches and displaces the optic nerve; primary gaze with eyes gently closed is the standard position.

8.3 Fast Salvage Protocol

Approximately 11–12 minutes — covers the primary cranial nerve diagnostic questions for most clinical indications.

8.4 Common Avoidable Errors

9. Quality Control Checklist

• Isocentre verified (posterior fossa level for 3D CISS protocols)
• 3D CISS: banding artefact location assessed; does not cross target nerve(s)
• 3D CISS: posterior fossa bilateral coverage including all target nerves
• 3D CISS: coronal and nerve-targeted reformats generated
• Brain T2/FLAIR: brainstem covered bilaterally including medulla (CN IX–XII exit levels)
• DWI: ADC map generated
• Pre-contrast T1: without fat suppression; foraminal fat signal documented for comparison
• Post-contrast T1-FS: fat suppression quality at skull base and foraminal levels — uniform / failure documented
• Post-contrast T1-FS: coverage includes all relevant foramina (specify: foramen ovale / foramen rotundum / hypoglossal canal / jugular foramen) based on indication
• STIR coronal: covers from skull base to thoracic inlet (for neck nerve segments)
• Correct laterality labelling

11. Evidence Gaps and Ongoing Debate

3D CISS NVC grading and surgical outcome correlation: multiple classification systems for neurovascular conflict (Sindou grade, Barker classification) have been described for trigeminal neuralgia. The MRI-identified NVC grade (point contact vs deviation vs compression) and its correlation with surgical MVD outcome is moderately supported by prospective series but no large RCT has validated MRI NVC grading as a definitive surgical selection tool. False-negative and false-positive NVC on MRI are well-documented.

PNS detection threshold: the minimum extent of PNS detectable on MRI (before symptoms develop, or at early symptomatic stage) has not been formally established. Early sub-clinical PNS (microscopic invasion of the nerve sheath not visible on MRI) is the major cause of PNS staging under-detection on MRI. Whether ultra-high-field (7T) MRI improves PNS detection sensitivity has not been formally studied in prospective surgical series.

CISS vs 3D T2 TSE (SPACE/CUBE) for cranial nerve assessment: 3D T2 SPACE/CUBE provides isotropic T2-weighted imaging that, like CISS, can resolve cranial nerve cisterns. The relative advantages (CISS: higher fluid-to-tissue contrast; SPACE: banding-free at 3T) for each nerve and each clinical question have not been systematically compared in prospective studies across all cranial nerve indications.

Role of DWI in cranial neuropathy: DWI in cranial nerve cisternal segments is technically challenging (EPI distortion adjacent to skull base; tiny nerve size). Research applications include DWI tractography of the optic nerve and facial nerve. The clinical value of DWI for non-cholesteatoma cranial nerve pathology (beyond DWI’s established role in brain ischaemia and epidermoid/cholesteatoma) is not established.