Mosaic attenuation is a pattern seen on a CT scan of the lungs where some areas appear lighter and others appear darker, creating a patchwork look. It’s not a diagnosis on its own. Instead, it’s a visual clue that something is causing uneven airflow, blood flow, or tissue density across different regions of your lungs.
If you’re reading this, you probably saw the term on a radiology report and want to know what it means for you. The short answer: it points your doctor toward a handful of possible causes, some minor and some worth investigating further.
How It Looks on a CT Scan
On a CT image, healthy lungs have a fairly uniform shade of gray. With mosaic attenuation, you see a patchwork of lighter and darker zones with well-defined borders. These borders follow the natural divisions of lung tissue called secondary pulmonary lobules, which are small, self-contained units of lung architecture. The result looks a bit like a mosaic tile floor, which is where the name comes from.
The key question for radiologists is figuring out which zones are abnormal: the lighter ones, the darker ones, or both. That distinction matters because it points toward completely different causes.
The Three Main Causes
Mosaic attenuation falls into three broad categories based on what’s creating the uneven pattern.
Small Airway Disease
When tiny airways in some parts of the lung become narrowed or blocked, air gets trapped in those regions. Trapped air makes those zones look darker on the scan, while the surrounding healthy lung looks brighter by comparison. The blood vessels in the darker areas also appear smaller because the body redirects blood flow away from poorly ventilated zones. One hallmark of this cause is that the pattern gets more obvious on an expiratory CT scan, where you breathe out and hold. Healthy lung deflates normally, but areas with trapped air stay inflated, making the contrast between dark and bright zones even sharper.
Constrictive bronchiolitis (also called obliterative bronchiolitis) is a classic example. In one study of 115 patients with this condition, every single patient showed mosaic attenuation on CT, and 90% had additional signs like bronchial wall thickening or scarring. Asthma, chronic bronchitis, and infections can also cause this pattern through the same air-trapping mechanism.
Pulmonary Vascular Disease
When blood clots or other vascular problems block flow to parts of the lung, those areas receive less blood and appear darker on the scan. The well-supplied areas look brighter. Again, the blood vessels in the darker zones appear noticeably smaller than in the brighter zones. One important condition in this category is chronic thromboembolic pulmonary hypertension (CTEPH), where old blood clots permanently obstruct pulmonary arteries. In CTEPH imaging studies, mosaic attenuation was detectable in about 75% of lung segments examined.
Radiologists look for specific clues to distinguish vascular causes from airway causes. If the main pulmonary artery appears enlarged compared to the aorta, that suggests pulmonary hypertension. If a pulmonary artery branch looks much larger than the airway running next to it, that’s another vascular red flag. The airways themselves typically look normal in purely vascular disease.
Infiltrative Lung Disease
This is where the pattern works in reverse. In conditions like pneumonia, certain types of lung inflammation, or early fibrosis, patches of lung tissue become denser and appear brighter on the scan. The darker zones are actually the normal lung. The giveaway here is that blood vessels look roughly the same size throughout both the light and dark areas, because blood flow isn’t being redirected. The problem is in the tissue itself, not in the airways or blood vessels.
How Doctors Tell the Causes Apart
When your radiologist sees mosaic attenuation, they use a few specific techniques to narrow down the cause. The most important is the expiratory CT scan. You’ll be asked to blow out and hold your breath while an additional set of images is taken. If the darker areas become even more prominent during expiration, that confirms air trapping and points toward small airway disease. If the pattern stays the same whether you’re breathing in or out, the cause is more likely vascular or infiltrative.
The size of blood vessels in the lighter versus darker zones is another critical clue. Smaller vessels in the dark areas suggest either airway or vascular disease. Uniform vessel size throughout points toward infiltrative disease. Radiologists also check for signs of enlarged pulmonary arteries, bronchial wall thickening, mucus plugging, or scarring, each of which helps refine the diagnosis further.
When It’s Not a Concern
Mild mosaic attenuation is surprisingly common and often clinically insignificant. Current guidance from radiology experts suggests that if the lighter zones affect less than about a quarter of the lung volume on an expiratory scan, it can generally be disregarded, even in younger patients. Small amounts of air trapping are a normal finding, especially in otherwise healthy people.
If the pattern is more extensive but the blood vessels look normal and there are no signs of airway inflammation, bronchial wall thickening, or enlarged pulmonary arteries, it’s still reasonable to consider it insignificant. The pattern becomes more concerning when it’s widespread and accompanied by those additional findings.
What Happens Next
If mosaic attenuation shows up on your CT and your radiologist considers it significant, the next steps depend on what the pattern suggests. For suspected small airway disease, you may be referred for pulmonary function tests to measure how well air moves in and out of your lungs. For suspected vascular disease, further imaging like a CT pulmonary angiogram or a ventilation-perfusion scan can map blood flow more precisely. For infiltrative causes, the underlying condition (infection, inflammation, early fibrosis) usually guides the workup.
In many cases, mosaic attenuation is mentioned in a radiology report as a descriptive finding and doesn’t change your care at all. It’s a pattern, not a disease. Its significance depends entirely on how much of the lung is involved, what other abnormalities accompany it, and whether you have symptoms that need explaining.