Internal Auditory Canal / CPA MRI

High-resolution MRI of the internal auditory canals and cerebellopontine angle is the definitive investigation for asymmetric sensorineural hearing loss and suspected vestibular schwannoma. Submillimetre heavily T2-weighted sequences outline the individual cranial nerves within the CSF-filled canal, and targeted post-contrast imaging characterises enhancing lesions.

Indications

Asymmetric or unilateral sensorineural hearing loss and tinnitus
Suspected vestibular (acoustic) schwannoma or other CPA mass
Vertigo and imbalance with suspected retrocochlear pathology
Pre-operative and pre-cochlear implant assessment of labyrinth and cochlear nerve patency
Facial nerve dysfunction and suspected facial schwannoma or perineural spread
Follow-up of known schwannoma or post-treatment surveillance

Contraindications & Cautions

Non-conditional pacemakers, ICDs, neurostimulators and ferromagnetic aneurysm clips
Cochlear implants (non-conditional) and unstable metallic intra-ocular foreign bodies
Severe claustrophobia without sedation
Gadolinium caution with eGFR <30 mL/min/1.73 m2 if contrast is required
Pregnancy: high-resolution T2 screening is unenhanced; reserve gadolinium for essential characterisation

Patient Preparation

MRI safety screening and removal of ferromagnetic objects and hearing aids
Provide ear protection given acoustic sequence noise
Explain the need for absolute stillness for submillimetre resolution
Intravenous cannula only if a lesion is anticipated or a screen is positive
Correlate with audiometry to determine the symptomatic side

Technique & Parameters

Heavily T2-weighted 3D steady-state sequences (CISS/FIESTA-C/DRIVE) at submillimetre isotropic resolution through the IAC and CPA
Thin axial source images with coronal and oblique reformats along the nerve course
Axial T2 and DWI of the posterior fossa to assess brain parenchyma and exclude epidermoid
Post-gadolinium thin-section T1 (with fat saturation for the petrous apex) when an enhancing lesion is suspected
Whole-brain FLAIR/T2 to screen for demyelination and other causes of retrocochlear symptoms
1.5T or 3T with a head coil; 3T improves nerve conspicuity

Systematic Review

Identify the four nerves in the IAC on axial CISS: facial (antero-superior), cochlear (antero-inferior), superior and inferior vestibular (posterior)
Assess cochlea, vestibule and semicircular canals for fluid signal and patency
Evaluate the CPA cistern and IAC fundus for a filling defect or mass
Characterise any lesion: T2 signal, enhancement, extension into the canal (ice-cream cone), and effect on adjacent structures
Review the brainstem, cerebellum and fourth ventricle for compression or oedema
Compare both sides for symmetry of nerves and labyrinthine signal

Key Findings & Significance

Vestibular schwannoma: avidly enhancing intracanalicular and/or CPA mass with an ice-cream-cone shape and IAC widening
Filling defect within the bright T2 CSF of the IAC on CISS indicating a small intracanalicular schwannoma
Meningioma: broad dural base, dural tail, homogeneous enhancement, eccentric to the porus
Epidermoid: CPA mass following CSF on T1/T2 but restricting on DWI (distinguishes from arachnoid cyst)
Labyrinthitis: enhancement of the membranous labyrinth; labyrinthitis ossificans shows loss of fluid signal
Facial nerve schwannoma extending along the labyrinthine/geniculate segment

Differential Considerations

CPA mass: vestibular schwannoma, meningioma, epidermoid, arachnoid cyst, aneurysm
Enhancing IAC lesion: schwannoma, meningioma, metastasis, neuritis, sarcoid/inflammatory
Loss of labyrinthine fluid signal: labyrinthitis ossificans, schwannoma, tumour
Restricted diffusion CPA lesion: epidermoid versus (non-restricting) arachnoid cyst
Bilateral vestibular schwannomas: neurofibromatosis type 2

Pearls & Pitfalls

CISS/FIESTA outlines individual nerves; a normal cochlear nerve is essential to confirm before cochlear implantation
DWI is the key sequence separating a restricting epidermoid from a non-restricting arachnoid cyst
A dural tail and acute angle with the petrous bone favour meningioma over schwannoma
Bilateral vestibular schwannomas are pathognomonic of NF2 and warrant genetic assessment
Fat-saturated post-contrast imaging avoids masking petrous apex enhancement by marrow fat

Structured Report

State field strength, high-resolution T2 technique and whether contrast was given
Report symptomatic side, lesion presence, size, location (intracanalicular, CPA or both) and morphology
Describe enhancement, labyrinthine signal and cochlear nerve status
Note mass effect on brainstem/cerebellum and any hydrocephalus
Provide an impression, differential and follow-up or surgical recommendation

References

Lin EP, Crane BT. Radiographics: The management and imaging of vestibular schwannomas
ACR-ASNR-SNIS-SPR Practice Parameter for the Performance of MRI of the Brain
Fortnum H, et al. Imaging protocols for asymmetric sensorineural hearing loss
Bonneville F, et al. Unusual lesions of the cerebellopontine angle. Radiographics

Educational clinical decision support only. Protocols vary by institution and equipment; always confirm with a qualified radiologist and local guidelines before clinical use.

Internal Auditory Canal / CPA MRI

📋 Indications

⛔ Contraindications & Cautions

🧑‍⚕️ Patient Preparation

⚙️ Technique & Parameters

🔎 Systematic Review

🩻 Key Findings & Significance

🧠 Differential Considerations

💡 Pearls & Pitfalls

📝 Structured Report

📚 References