This step-by-step guide is for MRI students, radiographers, and technologists who wish to improve their planning skills and master the internal auditory canal (IAC/IAM) MRI protocol.
What you will learn:
Key factors in IAC/IAM MRIs, including trade-offs.
Patient and scanner setup tips.
Best pulse sequences and planning techniques.
Ways to avoid common artifacts.
What great IAC/IAM images should look like.
Key Takeaways
Because the IAC contains millimeter-scale structures, resolution comes first. The IAC and inner ear hold the cochlea, the semicircular canals, and four cranial nerve branches. We therefore 1) prioritize resolution, 2) maintain enough SNR to keep bright fluid clearly separated from dark nerves, and 3) optimize scan time last. If the patient cannot hold still, time trumps everything.
The two 3D heavily T2-weighted sequences do most of the diagnostic work. 3D axial bSSFP (CISS, FIESTA-C, TrueFISP) and 3D axial T2 TSE (SPACE, CUBE, VISTA) give sub-millimeter isotropic voxels and strong contrast between bright CSF and dark nerves. Tiny filling defects are found here first. Then, we use 2D thin-slice and post-contrast T1 FS sequences to characterize them.
Avoid these 5 common IAC/IAM artifacts.
ArtifactSolution – How to Avoid ItMotion artifactsImmobilize the patient’s head firmly with cushions. Shorten the long 3D sequences using parallel imaging. Clearly tell the patient not to swallow during the important 3D scans.Banding artifactsDo careful local shimming over the temporal bones. If dark zebra-like bands still appear over the nerves, use dual phase-cycled sequences (FIESTA-C or CISS) to cancel them out. Switch to 3D T2 TSE if banding continues.Susceptibility artifactsSwitch from gradient-echo sequences (like bSSFP) to fast spin-echo sequences such as 3D T2 TSE. Spin-echo uses 180-degree refocusing pulses that recover signal lost to magnetic field distortions near air and bone.Wrap-around (aliasing) artifactsTurn on phase oversampling (fold-over suppression). IAC scans use a very small field of view for high resolution, so this stops outside parts of the head (nose or back of skull) from folding into the image.Failed fat suppressionIncrease receiver bandwidth and make sure fat saturation is strong and even on all post-contrast T1 images. If standard fat-sat still fails near the complex skull base, switch to Dixon technique.
Intro to IAC/IAM MRIs
The internal auditory canal (IAC), also called the internal auditory meatus (IAM), is a short bony tunnel inside the petrous part of the temporal bone. It connects the posterior cranial fossa to the inner ear. The canal averages around 8.5 mm in length and roughly 4 mm in diameter, is lined by dura, and is filled with cerebrospinal fluid.
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