A CT scan of the sinuses produces detailed cross-sectional images that reveal the bone, soft tissue, and air-filled spaces of all four pairs of paranasal sinuses. It can identify inflammation, infection, structural abnormalities, polyps, cysts, and masses with far more precision than a standard X-ray. It’s considered the gold standard for evaluating the nasal cavity and surrounding sinuses, and it’s the primary imaging tool doctors use to confirm chronic sinusitis, plan surgery, or investigate unexplained facial pain.
The Anatomy It Maps Out
Your skull contains four pairs of air-filled sinuses, and a CT scan images all of them in three planes (front-to-back, side-to-side, and top-to-bottom). The maxillary sinuses sit beneath your eyes and are the largest. The frontal sinuses are above your eyes within the forehead bone. The ethmoid sinuses are a cluster of small air cells between your nose and eyes. The sphenoid sinuses sit deep in the skull behind the ethmoid sinuses.
Beyond the sinuses themselves, the scan reveals the bony shelves inside your nose called turbinates, the thin wall dividing your nasal cavity (the septum), and the narrow drainage pathways that connect each sinus to the nose. It also shows structures that border the sinuses, including the thin bone separating the sinuses from the eye socket (the lamina papyracea) and the floor of the skull base above. These landmarks matter because disease in the sinuses can sometimes extend toward the eyes or brain.
Signs of Sinusitis
The most common reason for a sinus CT is to evaluate sinusitis, and the scan picks up several telltale signs. Mucosal thickening, where the lining of a sinus appears swollen, is the most frequently detected abnormality. In acute sinusitis, the scan may show air-fluid levels, visible as a sharp horizontal line where fluid pools inside a sinus, or foamy-appearing secretions. A sinus that is completely filled with fluid or swollen tissue, called complete opacification, signals more advanced disease.
Chronic sinusitis looks different. Instead of fluid levels, the scan typically shows persistent mucosal thickening along with thickening of the bony sinus walls themselves, a sign that inflammation has been present long enough for the bone to remodel. Clinical guidelines recommend CT imaging to objectively confirm chronic rhinosinusitis when symptoms have lasted 12 weeks or more and a physical exam alone isn’t conclusive.
One important caveat: mild mucosal thickening in the maxillary and ethmoid sinuses is extremely common even in people with no symptoms at all. A systematic review of nearly 17,000 asymptomatic subjects found that about 18% had maxillary mucosal thickening of 2 mm or more, and 13% had mucous retention cysts. Thickening up to 6 mm has been documented in people with no sinus complaints. So a small amount of thickening on your scan doesn’t automatically mean you have sinusitis. Doctors interpret the images alongside your symptoms, not in isolation.
Structural Abnormalities
CT is the best tool for spotting structural issues that may contribute to sinus problems or breathing difficulty. A deviated septum, defined on imaging as a shift of more than 4 mm from the midline, is one of the most common findings. The scan shows exactly where the deviation occurs, how severe it is, and whether it’s blocking a sinus drainage pathway.
Another frequent finding is concha bullosa, an air pocket inside one of the turbinates that causes it to balloon outward. This can narrow the nasal passage on that side and potentially obstruct sinus drainage. CT scans also detect bone spurs along the septum, enlarged turbinates, and variations in the anatomy of the narrow channels where sinuses drain. These details help explain why some people develop recurrent sinus infections while others don’t.
Polyps, Cysts, and Masses
Nasal polyps appear on CT as soft-tissue growths, often in clusters, that can fill the nasal cavity or sinuses. They’re distinguishable from simple mucosal thickening by their shape and location. Retention cysts, which are fluid-filled sacs in the sinus lining, are also clearly visible and are one of the most common incidental findings on any head CT. Most retention cysts are harmless and don’t need treatment.
Mucoceles are a more significant finding. These are expanding, mucus-filled lesions that form when a sinus drainage pathway becomes completely blocked. On CT, they appear as soft-tissue masses that push outward, sometimes eroding into the surrounding bone and extending toward the eye socket or the space around the brain. CT is particularly good at showing this bone erosion, which helps doctors determine whether surgical drainage is needed.
For suspected tumors, a CT scan can reveal a mass and show whether bone has been destroyed, which raises concern for malignancy. However, CT alone can’t always distinguish a benign growth from a malignant one. When a mass looks suspicious, doctors often follow up with an MRI, which is better at characterizing soft tissue and differentiating tumor from trapped mucus or fluid.
Preoperative Mapping for Surgery
If sinus surgery is being planned, the CT scan serves as a three-dimensional roadmap. Surgeons study the scan to identify the precise location of critical structures they need to avoid, including the thin bone between the sinuses and the eye socket, the floor of the skull base at the level of the smell nerves, the bony canal protecting the tear duct, and the openings where each sinus drains.
Every person’s sinus anatomy is slightly different. Some people have a very thin or partially absent wall between their sinuses and eye socket. Others have a skull base that dips lower than usual near the ethmoid sinuses. The CT scan flags these variations before the surgeon ever picks up an instrument, reducing the risk of complications during the procedure.
What the Scan Itself Is Like
A sinus CT is fast and painless. The actual scanning takes less than a minute, and the entire visit, including positioning, is usually done within 10 minutes. You’ll lie flat on your back on the scanner table, though sometimes you may be positioned face-down with your chin elevated. Straps or pillows help you stay still.
You’ll need to remove jewelry, eyeglasses, hearing aids, hairpins, and any piercings near your head. Bras with metal underwire also need to come off, since metal creates artifacts that degrade the images. There’s no preparation needed beforehand, and you can eat and drink normally.
Most sinus CTs are done without contrast dye. A non-contrast scan is sufficient for evaluating sinusitis, structural problems, and routine surgical planning. Contrast is typically reserved for cases where a tumor, abscess, or complicated infection is suspected and the doctor needs to see how blood flows through the tissue.
Radiation Exposure
A standard sinus CT delivers roughly 0.7 mSv of radiation for a standard-dose scan, which is below the 1 mSv you absorb annually from natural background sources like the sun and the ground beneath you. Low-dose protocols, increasingly used for sinus imaging, bring that number down to about 0.05 mSv, a fraction of a single chest X-ray. The dose is low enough that repeat imaging for surgical follow-up or chronic disease monitoring is generally considered safe.