What Is Mosaic Attenuation and What Does It Mean?

Mosaic attenuation is a pattern seen on a CT scan of the lungs in which patches of lung tissue appear lighter or darker than their neighbors, creating an uneven, patchwork-like appearance across the lung fields.¹ If your radiology report mentions it, the term itself is not a diagnosis but a descriptive finding that tells your doctor something uneven is happening in how air, blood, or both are distributed in your lungs. Figuring out what is behind that unevenness is where the real clinical work begins, because the list of possible causes ranges from mild asthma to serious vascular disease.

What You Are Actually Seeing on the Scan

A normal CT of healthy lungs shows tissue that is roughly uniform in density. When mosaic attenuation is present, you get a geographic patchwork: some areas look darker than usual (meaning they are more air-filled or less blood-perfused), and adjacent areas look relatively brighter. The borders between these zones tend to be sharp and follow anatomic boundaries like the edges of lung lobules, which gives the pattern its characteristic mosaic or tile-like look. Radiologists sometimes describe the darker patches as “hyperlucent” and the brighter patches as “ground glass” depending on which side of the contrast they consider abnormal.

This distinction matters more than it might seem. In some patients, the darker areas are the problem: air is getting trapped there because small airways are obstructed, so those patches stay inflated and dark. In other patients, the brighter areas are the problem: they represent lung tissue that is filled with fluid, inflammation, or cellular debris, while the darker patches are simply normal lung for comparison. And in a third group, the issue is blood flow rather than air: certain regions receive less blood, making them appear darker, while the well-perfused zones look relatively bright. These three mechanisms, airway obstruction, infiltrative disease, and vascular compromise, account for nearly all cases of mosaic attenuation.

Airway Disease and Air Trapping

The single most common reason for mosaic attenuation is disease of the small airways. When tiny bronchioles become narrowed or blocked, air flows in during a breath but cannot escape efficiently on exhalation. The affected patches of lung stay hyperinflated and look abnormally dark on the scan. Meanwhile, the surrounding healthy lung deflates normally, creating a contrast between trapped and non-trapped regions.

Two conditions dominate this category. Asthma, especially when poorly controlled, can cause patchy bronchoconstriction severe enough to produce the pattern. Bronchiolitis obliterans, a condition in which chronic inflammation scars and narrows the small airways, is the other major culprit. A study correlating expiratory CT findings with breathing tests found that air trapping on scans in patients whose inspiratory images looked normal was most often linked to bronchiolitis obliterans and asthma.² Other small-airway conditions that can produce the same appearance include hypersensitivity pneumonitis (an allergic reaction deep in the lungs to inhaled particles), chronic obstructive pulmonary disease, and certain infections.

The mechanism connecting obstructed airways to the patchwork pattern on CT is not just about trapped air. Research on asthma patients with induced bronchoconstriction showed that the dark patches on CT are largely driven by a reflex called hypoxic pulmonary vasoconstriction: when a region of lung is poorly ventilated, the blood vessels feeding that region constrict to redirect blood toward better-ventilated areas.³ So the dark patches are dark for two reasons at once: they hold more trapped air and they receive less blood. This makes the contrast with neighboring healthy tissue even more striking.

Vascular Causes

When the primary problem is blood flow rather than airflow, the mosaic pattern is often called “mosaic perfusion.” The logic is straightforward: if a blood vessel supplying part of the lung is blocked or narrowed, that region receives less blood and appears darker on CT, while neighboring areas with normal perfusion look relatively bright.

Chronic thromboembolic pulmonary hypertension (CTEPH) is the condition most strongly associated with this type of mosaic attenuation. In CTEPH, old blood clots that were never fully dissolved remain lodged in the pulmonary arteries and organize into scar tissue, permanently reducing flow to parts of the lung. A study of patients across different types of pulmonary hypertension found that half of those with CTEPH showed mosaic attenuation on CT, a rate significantly higher than in other pulmonary hypertension groups, where it appeared in roughly 15 to 28 percent of patients.⁴ The pattern was present in nearly all CTEPH patients in another imaging study comparing two different CT techniques.⁵

What makes this clinically important is that CTEPH is one of the few forms of pulmonary hypertension that can be treated surgically. If old clots are the cause, a procedure called pulmonary thromboendarterectomy can remove the organized clot material and dramatically improve blood flow. Recognizing mosaic perfusion on CT is sometimes the first clue that sends a patient down this diagnostic path. Researchers have even developed methods to quantify how much of the lung appears underperfused on CT, finding that the extent of underperfusion correlates with the pressure in the pulmonary arteries and with vascular resistance, both key measures of how severely the disease is affecting the heart.⁶

Infiltrative and Parenchymal Disease

The third major category involves diseases that fill the air spaces or thicken the tissue of the lung itself, but do so unevenly. When inflammation, fluid, or abnormal cells occupy some regions and spare others, the affected zones appear brighter on CT (ground glass opacity) while the normal zones look darker by comparison. The result is a mosaic-like contrast, but here the brighter patches are the abnormal ones.

Conditions that produce this kind of patchy ground glass include certain pneumonias, pulmonary hemorrhage, some forms of pulmonary edema, and early or patchy interstitial lung diseases. Any acute or chronic process that causes ground glass opacity can create a mosaic appearance if it does not affect the lungs uniformly. This is the category where the differential diagnosis is broadest, because many different diseases can produce patchy infiltrates.

How Doctors Tell the Three Categories Apart

Given that three very different mechanisms can produce the same visual pattern, radiologists use several tricks to narrow down the cause. The most useful is the expiratory CT scan. In a standard CT, you take a deep breath in and hold it. An expiratory scan asks you to breathe out and hold, and the scan is taken at that low-lung-volume state.

If the mosaic pattern is caused by air trapping, the dark patches will stay dark, or become even more obvious, on the expiratory scan. Healthy lung tissue deflates and gets brighter, but trapped-air regions cannot deflate, so the contrast between the two increases. If the pattern is caused by vascular disease, the mosaic typically looks the same on both inspiratory and expiratory images because blood flow, not airflow, is the issue. And if infiltrative disease is the cause, the brighter patches may become even brighter on exhalation as the normal surrounding lung gets denser, but there should not be sharply defined regions that refuse to deflate.

Vessel caliber provides another clue. In mosaic perfusion from vascular causes, the blood vessels running through the dark patches tend to look smaller than the vessels in the bright patches, because less blood is flowing through them. In airway-mediated mosaic attenuation, the vessels in the dark (air-trapped) zones may also appear small due to the vasoconstriction reflex described earlier, but the expiratory behavior of the lung tissue helps sort the two apart. In infiltrative disease, the vessels in both the bright and dark areas tend to be roughly the same caliber, because the problem is in the tissue, not the circulation.

Clinical context and breathing-test results round out the picture. A study from the late 1990s classified patients with uneven lung density on CT into airway, vascular, infiltrative, and mixed groups using pulmonary function tests alongside the imaging findings.⁷ That combination of breathing tests plus paired inspiratory and expiratory scans remains the core diagnostic approach today, though newer tools are making the process more precise.

When It Is a False Alarm

Not every patchwork pattern on a lung CT represents real disease. One of the most common pitfalls is inadequate inspiration: if you did not take a deep enough breath before the scan, parts of the lung may remain partially collapsed, creating darker and lighter zones that mimic true mosaic attenuation. Radiologists can often spot this by looking at the trachea. If the back wall of the trachea appears bowed inward (partially collapsed), it is a sign the patient was not fully inflated, and the apparent mosaic pattern may be artifact rather than pathology.

Gravity-dependent density differences can also create an uneven appearance. When you lie on your back in a CT scanner, the lung tissue at the bottom (posterior) naturally compresses slightly under its own weight, appearing a bit brighter than the tissue at the top (anterior). This gradient is normal and should not be confused with pathologic mosaic attenuation. Experienced radiologists distinguish the two based on the pattern’s geography: gravity effects follow a smooth top-to-bottom gradient, while true mosaic attenuation produces a patchy, sharply demarcated pattern that does not respect gravitational planes.

If your report mentions mosaic attenuation and your doctor seems unconcerned, one of these technical explanations may be the reason. A follow-up scan with proper breath-hold coaching, or an expiratory series, can settle the question.

Occupational and Environmental Exposures

Some of the more unusual causes of mosaic attenuation involve workplace or environmental inhalation injuries. One that drew considerable attention is so-called “popcorn lung,” formally known as bronchiolitis obliterans caused by inhaling diacetyl, a buttery flavoring chemical used in microwave popcorn factories and other food-processing settings. Workers exposed to diacetyl developed scarring of their small airways, and their CT scans characteristically showed mosaic attenuation with air trapping on expiratory images. The same airway-scarring pattern has been reported with other industrial inhalants.

Hypersensitivity pneumonitis, triggered by chronic exposure to mold spores, bird proteins, or certain chemicals, can also produce mosaic attenuation. In this condition, the mechanism is a combination of small-airway inflammation and patchy infiltrative disease, so the CT appearance can show features of both airway and parenchymal involvement simultaneously. Occupational and environmental history is therefore an important piece of information when a radiologist or pulmonologist is trying to interpret the pattern.

Swyer-James Syndrome

One cause of mosaic attenuation that tends to surprise people is Swyer-James syndrome, sometimes called Swyer-James-MacLeod syndrome. This is an acquired condition that develops after a severe respiratory infection in childhood, such as a viral pneumonia or whooping cough. The infection damages the small airways and blood vessels of part of one lung during a critical growth period, leaving that region permanently underdeveloped.⁸

The affected portion of lung ends up with fewer and smaller blood vessels, and the small airways are scarred (a form of post-infectious bronchiolitis obliterans). On a chest X-ray, the hallmark is a lung that looks abnormally dark on one side. On CT, the affected area shows both air trapping and reduced perfusion, which can produce a mosaic pattern, particularly if the disease is patchy rather than involving the entire lung.⁹ Many people with Swyer-James syndrome are not diagnosed until adulthood, when a CT ordered for an unrelated reason reveals the characteristic findings. The condition is usually managed conservatively, though the presence of bronchiectasis (permanently dilated, damaged airways) in the affected region can predispose to recurrent infections.

Newer Tools for Measuring Mosaic Attenuation

Traditionally, radiologists evaluated mosaic attenuation by eye: looking at the scan, noting the pattern, and making a subjective call about how extensive it was. This approach works well in experienced hands, but it is inherently variable between readers and makes it hard to track changes over time. Several newer approaches aim to make the assessment more objective.

Automated quantification tools use software to measure the density of each small region of the lung and flag zones that fall outside the expected range. One research group developed an automated thresholding method to quantify mosaic attenuation on inspiratory CT and air trapping on paired expiratory CT, then compared these computer-derived measurements to pulmonary function test results and symptom scores.¹⁰ The goal is to move beyond “present or absent” toward precise measurements of how much lung is affected and whether it is getting better or worse.

Deep learning, the same type of artificial intelligence used in image recognition, has also been applied to detecting air trapping on expiratory CT. One study using a deep-learning algorithm found that quantitative air trapping scores increased over a two-year follow-up period in some patients, demonstrating the tool’s ability to track disease progression in a way that the human eye might miss.¹¹

Dual-energy CT is another advancing technology. By scanning at two different X-ray energy levels simultaneously, this technique can generate maps of blood perfusion throughout the lung, essentially showing where blood is flowing and where it is not. In patients with CTEPH, dual-energy CT perfusion maps showed a strong correlation with the visually assessed mosaic attenuation pattern, suggesting that this technology could help quantify vascular-type mosaic attenuation more reliably than conventional CT alone.¹²

Can Mosaic Attenuation Go Away

Whether mosaic attenuation resolves depends entirely on what is causing it. In conditions where the underlying disease can be treated, the pattern can improve or disappear. A study of patients with Behçet’s disease, a systemic inflammatory condition that can involve the pulmonary arteries, found that mosaic attenuation areas resolved in most patients after treatment.¹³ Similarly, asthma-related mosaic attenuation may improve substantially when bronchoconstriction is brought under control with appropriate therapy, because the air trapping and reflex vasoconstriction are reversible.

On the other hand, conditions that cause permanent structural damage to the airways or blood vessels, like bronchiolitis obliterans or long-standing CTEPH, tend to produce mosaic attenuation that persists. Even in CTEPH, surgical removal of organized clot material can restore blood flow and reduce the perfusion mismatch, but fibrotic changes in the small airways or vessels are generally irreversible. Swyer-James syndrome is another example of permanent mosaic change, since the affected lung tissue never recovers from the childhood insult that caused it.

For patients whose mosaic attenuation was discovered incidentally and who have no symptoms, the finding sometimes leads to watchful monitoring rather than immediate intervention. Your doctor may order breathing tests to see if airflow obstruction is present, check for signs of pulmonary hypertension, or recommend a follow-up scan with expiratory images if those were not obtained the first time. The goal is always to determine whether the pattern reflects active, treatable disease or a stable finding that simply needs to be documented.

Post-COVID Air Trapping

An area of growing interest is the appearance of mosaic attenuation and air trapping in patients recovering from COVID-19. Some studies have found air trapping on expiratory CT scans in patients who were still experiencing breathlessness months after their initial infection, even after the acute pneumonia had resolved. The mechanism is thought to involve small-airway inflammation or damage from the viral infection, similar in concept to other post-infectious bronchiolitis patterns. Researchers investigating this phenomenon have recommended using low-dose scanning techniques with paired inspiratory and expiratory images to minimize radiation exposure while still capturing air trapping in these follow-up scans. Whether COVID-related small-airway changes are permanent or slowly reversible remains an open question, and longitudinal studies using the kind of quantitative CT tools described earlier are underway to track outcomes over time.