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MRI
HeadDemonstration data

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.