What Does Mosaic Attenuation on Chest CT Mean?

Mosaic attenuation on chest CT means the lung looks patchy, with darker and lighter areas side by side, often with edges that follow the small units of lung called lobules. It reflects differences in how much air or blood different areas contain. It is a description of a pattern, and the radiologist and your clinician work out the likely cause from the rest of the scan, your symptoms and further tests.

This guide is for adults who have seen this phrase in a CT report. It explains the common questions and what to ask next; it cannot tell you what is causing the pattern in your lungs.

Do not wait for a planned follow-up if you develop severe breathing problems. NHS advice is to call 999 or go to A&E, or to use your local emergency number, for the signs below. Do not drive yourself.

See a doctor soon, rather than at a routine review, if your breathlessness is getting worse during normal activities or when lying down, if you are breathless with swollen ankles, or if you have had a cough for three weeks or more.

• Severe difficulty breathing, such as gasping, choking or being unable to speak in full sentences.
• A tight or heavy chest, or pain spreading to the arms, back, neck or jaw.
• Lips or skin turning pale, blue or grey.
• Sudden confusion.

Expert radiology guidance from the Pulmonary Fibrosis Foundation groups the causes of mosaic attenuation in a few broad ways. Each group makes some areas of lung look darker or lighter for a different reason.

• Small airways disease: narrowed small airways can trap air, so affected areas look darker. Examples include hypersensitivity pneumonitis, asthma and constrictive (obliterative) bronchiolitis.
• Vascular causes: reduced blood flow to parts of the lung, for example in severe pulmonary hypertension or chronic thromboembolic disease, can also make areas look darker.
• Ground-glass change: hazy, lighter areas from inflammation or other lung changes can create a similar patchwork. Our guide on ground glass opacity explains that finding.

The key tool is often an expiratory CT, meaning images taken after you breathe out fully. The Pulmonary Fibrosis Foundation recommendations describe expiratory imaging as very helpful for identifying airway causes of mosaic attenuation. If small airways are narrowed, air stays trapped in those areas when you breathe out, so they remain dark. This is called air trapping.

Mild air trapping may only be visible on expiratory images, while more severe air trapping can show as mosaic attenuation on the usual breath-in scan. When vascular disease is suspected instead, the clinician may look at heart and blood-vessel tests rather than relying on the CT pattern alone. A European Respiratory Society review notes that CT-based approaches to small airways disease have limits, so the pattern is interpreted together with other findings.

The scan is similar to an ordinary chest CT. The radiographer asks you to breathe in and hold, then breathe out fully and hold, while images are taken. Following the breathing instructions closely matters: the recommendations note that a poor breath-out can hide air trapping, and RadiologyInfo explains that movement can blur CT images.

Some chest CT scans use iodine contrast injected into a vein to show the blood vessels. Tell the team about previous contrast reactions, kidney problems or pregnancy. CT uses a small dose of radiation, and teams adjust the dose to the person and the question being asked.

Lung function tests show how your lungs work, while CT shows how they look. MedlinePlus describes spirometry (how much and how fast you can blow out), lung volume testing (how much air the lungs hold, including air left after breathing out) and gas diffusion (how well oxygen passes into the blood). Results can suggest an obstructive pattern, where air is hard to push out, or a restrictive pattern, where the lungs cannot fully expand.

Normal spirometry does not always rule out small airways disease. The European Respiratory Society review explains that small airways contribute little to routine measurements, so early disease can be missed. Ask whether lung volumes or gas diffusion are being checked as well. Ask the testing team before stopping any inhaler; do not stop medicines on your own.

Your history can be as important as the scan. Cleveland Clinic lists triggers for hypersensitivity pneumonitis, including birds and feather bedding, mould on hay, grain or in damp buildings, and water vapour from hot tubs, humidifiers and poorly maintained heating or air conditioning. Avoiding the trigger is central to its management, which is why identifying it matters.

Also mention smoking, dusts or fumes at work, a past severe chest infection, a lung, bone marrow or stem cell transplant, rheumatoid arthritis or other connective tissue disease, inflammatory bowel disease, previous blood clots, and medicines you take. The European Respiratory Society review links several of these to small airways disease.

Bring the full CT report, any earlier scans and a list of your exposures and symptoms.

• What do you think is the most likely reason for the mosaic pattern in my scan: airways, blood vessels or ground-glass change?
• Did my CT include expiratory images, and did they show air trapping?
• Do I need full lung function tests, including lung volumes and gas diffusion?
• Are heart or lung blood-vessel tests needed?
• Could anything at home or work be relevant, and should I avoid it while tests continue?
• Who will follow this up, and which symptoms should make me seek care sooner?

On SameCase, patients and verified doctors share de-identified cases. Seeing how others with a similar CT phrase described their exposures, tests and questions can help you organise your own history and decide what to ask at your next appointment.

Someone else's case cannot explain your scan or predict your results. Use it to prepare, and rely on your own clinician and radiologist for interpretation.