Mastoid air cells

What it means

The bump of bone you can feel just behind each ear is hollowed out into a network of small air pockets, a bit like the inside of a sponge. These pockets connect to the middle ear and share its lining, and they help equalise pressure and dampen sound. On a scan, healthy pockets appear black because they are full of air. When they fill with fluid or thickened tissue, they look grey or white instead, and the radiologist describes this.

Why it appears on a CT or MRI report

Because these structures sit at the side of the head, just inside the skull, they show up on almost every brain scan. Reports may say they are clear and well-aerated, or note that one or both sides show fluid, thickening, or partial opacification. Reports may also describe the bony walls — whether they look intact or eroded — and may mention if the chambers look smaller than expected, which can be a sign of long-standing ear problems from childhood.

What it usually means

Clear, air-filled pockets are the most common and reassuring finding. Some haziness or a little fluid here is also very common, especially after a recent cold, ear infection, or flight, and often clears on its own. Persistent fluid or thickening on one side can point to ongoing middle ear inflammation, blocked drainage of the ear, allergies, or a problem with the Eustachian tube that connects the ear to the throat. In children, fluid in these spaces is often part of a glue ear picture. Less commonly, reports describe bony erosion or unusual soft tissue, which can suggest a deeper infection (mastoiditis) or a benign growth such as a cholesteatoma — both deserve ear-nose-throat review. The vast majority of findings in this region are minor and incidental.

When to follow up

If these pockets are reported as clear, no action is needed. If mild fluid or thickening is described and you have no ear symptoms, watchful waiting is usually enough. Discuss with your doctor or an ear specialist if you have ongoing ear pain, hearing loss, drainage, or repeated infections, especially on the same side as the imaging finding. Seek urgent care for severe pain or swelling behind the ear with fever, redness, the ear sticking out compared with the other side, or any new facial weakness — these can signal infection spreading from this region.

A plain-language way to picture it

Imagine a small, dry sponge tucked inside the bone right behind your ear. The thousands of tiny holes in the sponge are usually full of air, which makes the bone lighter and helps with hearing pressure. When you catch a cold or your ear gets blocked, a little water can soak into the sponge — usually it dries out again on its own. The radiologist is essentially checking whether each sponge is dry, slightly damp, or wet.