Internal auditory canal (IAC) MRI protocol
The hearing-loss protocol. You are imaging cranial nerves VII and VIII inside a fluid-filled canal, so a heavily T2-weighted steady-state sequence makes the nerves black against bright CSF: a 'myelogram' of the canal.
By the ScanFlo Editorial Team · Updated
In this section · Protocols
When this study is used
- Sensorineural hearing loss or tinnitus.
- Suspected acoustic neuroma (vestibular schwannoma).
- Facial-nerve symptoms or vertigo.
Patient & coil positioning
- Head-first supine in a high-channel head coil; head straight so the two canals are symmetric for comparison.
- Centre on the glabella; immobilise firmly for the thin high-resolution slices.
- Warn the patient this is a still, quiet-demanding exam.
Localizer
Three-plane localizer; the axial must be straight through both IACs. If the head is tilted, reposition the patient rather than relying on angling alone.
Sequence planning
For each sequence below you get the reference line the slice group follows, how much it covers, and why the sequence is in the protocol at all. Plan against the localizer, not by eye.
Axial high-res T2 (CISS / FIESTA)
AxialPlanning: Thin sub-millimetre steady-state slices angled to the nerve bundle as it exits the brainstem; both canals.
Why: The VII/VIII complex as black filling-defects inside bright CSF. This is the core sequence.
Axial T1 + contrast fat-sat
AxialPlanning: Matched coverage.
Why: Schwannomas enhance vividly against suppressed fat.
Coronal T1 + contrast
CoronalPlanning: Through the canals.
Why: Second plane confirming an enhancing lesion and its extent.
Representative parameters
Treat these as ranges. Field strength, coil and vendor all move the numbers. What must not move is understanding why each sits where it does.
| Sequence | Slice | Notes |
|---|---|---|
| CISS / FIESTA | ≤1 mm | Steady-state; nerves black in bright CSF |
| T1 +C fat-sat | ≤3 mm | Schwannoma enhancement |
| Coronal T1 +C | ≤3 mm | Second plane |
Artifacts & how to fix them
| Artifact | Typical cause | Mitigation |
|---|---|---|
| Pulsation over brainstem | Basilar-artery / CSF pulsation | Phase A–P; flow compensation; presaturation over the neck |
| Asymmetric canals | Tilted head | Reposition straight before scanning |
| Susceptibility (petrous) | Air in the mastoid | Steady-state tolerates it; keep TE short on T1 |
What am I looking at?
You are not reporting the study, but recognising pathology helps you keep it in the field of view and know when to add a sequence or contrast. How the common findings read on the console:
| Finding | Sequence to check | How it reads |
|---|---|---|
| Acoustic neuroma | Axial CISS / T1+C | Dark 'ice-cream-cone' filling defect in the bright canal; enhances on T1 |
| Nerve asymmetry | Axial CISS | One nerve bundle thinner or displaced versus the other side |
Review checklist
- Axial straight through both IACs, symmetric.
- Sub-millimetre steady-state acquired for the nerve complex.
- Post-contrast in two planes if a lesion is suspected.
- Pulsation artefact steered away from the brainstem.
Mistakes that cost repeats
- A tilted head making the canals asymmetric and hard to compare.
- Slices too thick to resolve the individual nerves.
- Pulsation ghosts smearing the brainstem and canals.
- Skipping contrast when a small schwannoma is possible.
Common questions
- Why use a CISS/FIESTA sequence for the IACs?
- It is a heavily T2-weighted steady-state sequence in which fluid is bright and nerves are black. That creates a 'myelogram' effect inside the fluid-filled internal auditory canal, so the tiny VII and VIII nerves stand out and a small tumour shows as a filling defect.
- Why does head symmetry matter for IAC imaging?
- The two sides are compared directly. A tilted head makes one canal look different from the other, which can mask or mimic a lesion. That is why the head is repositioned straight rather than corrected by angling slices.
- How does an acoustic neuroma appear?
- As a dark filling defect within the bright CSF of the canal, classically 'ice-cream-cone' shaped where it extends into the cerebellopontine angle, and it enhances vividly on post-contrast T1.
Try it in ScanFlo: Acquire a thin CISS through the IACs and look for the black nerves against bright CSF, comparing left with right. That symmetry check is how a small filling defect becomes obvious. See ScanFlo MRI →