About a CT scan of the IAMs
The internal auditory meatus (IAM) is a narrow channel in the temporal bone, on each side of the skull, that carries the nerves for hearing, balance and facial movement from the inner ear to the brain. The temporal bone also contains the middle ear, with its three tiny bones (ossicles), and the inner ear, including the cochlea and balance organs. A CT (computed tomography) scan of this area uses X-rays and thin slices to show these small bony structures in fine detail.
Important: if you have hearing loss on one side, tinnitus in one ear or other symptoms that could be caused by a growth on the hearing nerve (an acoustic neuroma, or vestibular schwannoma), NICE recommends an MRI of the IAMs, not CT. MRI shows the nerves and small growths much more clearly and does not use radiation.
A CT of the IAMs and temporal bones is used to look at the bones of the ear, for example:
- conductive hearing loss, where sound cannot pass through the middle ear properly, including otosclerosis (abnormal bone growth around the ear bones) and problems with the ossicles
- cholesteatoma, an abnormal growth of skin in the middle ear, to see how far it has spread and plan surgery
- long-term ear infections and their effect on the bone
- fractures of the temporal bone after a head injury
- the shape of the inner ear, including before a cochlear implant
- when MRI is not possible, for example if you have a pacemaker or other device that is not safe for MRI
What happens during the scan
You lie on your back on the scanner bed with your head resting in a cradle, and pads may be used to help you keep still. Only your head goes into the scanner, which is a short, open ring. The radiographer controls the scanner from the next room, but can see, hear and talk to you throughout. You need to keep your head very still, as the pictures are very detailed. The scan itself takes less than a minute. Contrast dye is not usually needed; where contrast is needed, it is charged separately.
Is CT safe?
CT uses X-rays, which are a type of ionising radiation. A CT of the temporal bones covers a small part of the head, so the dose is similar to or lower than a CT of the head (about 2 mSv, similar to around 1 year of natural background radiation). Centres use techniques to reduce the dose to the eyes. The risk from a single scan is very small, and every CT scan is checked by a radiologist or radiographer to make sure the benefit of the scan outweighs the risk, as required by the Ionising Radiation (Medical Exposure) Regulations. If you have had other CT scans recently, tell us when you book.
What the scan can show
A CT scan of the IAMs and temporal bones can help to look at:
- otosclerosis
- cholesteatoma and its spread
- the ossicles (ear bones)
- chronic ear infection and mastoiditis
- fractures of the temporal bone
- the shape of the inner ear and cochlea
- planning for ear surgery and cochlear implants
When to get urgent help: see a GP or go to an urgent treatment centre the same day if you have sudden hearing loss in one or both ears, as early treatment may help. Go to A&E if you have ear pain with swelling behind the ear, a high temperature, weakness of your face, or a severe headache.
Preparing for your scan
No special preparation is needed, and you can eat and drink normally. Remove hearing aids, earrings, glasses and hair clips before the scan. Upload any hearing test results and letters from your ENT specialist when you book.
Wear loose, comfortable clothing without metal zips, poppers or buttons, and leave jewellery at home, as metal in the scan area can affect the pictures. You may be asked to change into a gown. Unlike MRI, CT is not affected by pacemakers or most metal implants, and the scanner is a short open ring rather than a tunnel, so it is usually easier for people who feel anxious in enclosed spaces.
Tell the team before your scan if you are, or might be, pregnant or breastfeeding, if you have kidney problems, diabetes (especially if you take metformin), asthma, thyroid problems or allergies, or if you have had a reaction to contrast dye before. If you need contrast, you may be asked for a recent kidney blood test result.
Frequently asked questions
Can a CT scan find an acoustic neuroma?
CT can miss small acoustic neuromas. NICE recommends MRI of the internal auditory meati for people with hearing loss that could be caused by one. CT is mainly used to look at the bones of the ear.
Should I have CT or MRI for hearing loss?
For hearing loss on one side, tinnitus in one ear or balance problems, MRI is usually the right test. For conductive hearing loss, cholesteatoma or planning ear surgery, CT is usually used. Your ENT specialist will say which is needed; if you are unsure, contact us before booking.
Are both ears scanned?
Yes. Both sides are usually scanned together, so the radiologist can compare them.
Do I need a referral?
Because CT uses radiation, each scan has to be justified by a qualified professional before it can go ahead. Upload a referral from your GP or consultant when you book if you have one. If you do not, select “no referral” and the scanning centre will review your request and confirm the next steps with you, which may include arranging a referral.
Sources
- NICE NG98 – Hearing loss in adults: assessment and management
- NHS – Otosclerosis
- NHS – Cholesteatoma
- NHS – Acoustic neuroma
- NHS – CT scan
- UKHSA – Patient dose information: guidance
The information on this page has been gathered from various sources and is provided for general information only. It is not medical advice. TOP MRI is not liable for the accuracy or completeness of this information. Please speak to your doctor or the scanning centre about your own situation.