Mosaic attenuation is a descriptive term used by radiologists to characterize a specific pattern observed on high-resolution computed tomography (HRCT) scans of the lungs. This finding is not a diagnosis in itself, but indicates that the lung tissue is not uniform. The pattern is defined as a geographic patchwork of regions showing differing X-ray attenuation, meaning areas of varying density or brightness. Understanding this pattern is important for determining underlying lung conditions, as the heterogeneous appearance guides physicians toward the specific physiological process causing the uneven look.
Visualizing Mosaic Attenuation on CT Scans
The term “attenuation” in the context of a CT scan refers to how much the lung tissue absorbs the X-ray beam. Air-filled lung tissue, which is less dense, absorbs less radiation and appears dark (low attenuation or lucent) on the image. Conversely, denser tissue, such as blood, fluid, or inflamed lung, absorbs more radiation and appears lighter or brighter (high attenuation).
The mosaic pattern is visually reminiscent of a tiled floor or a geographical map, with sharply demarcated borders between adjacent regions of different densities. The pattern involves areas that are darker than normal lung tissue interspersed with areas that appear normal or slightly lighter. This alternating appearance of light and dark patches across the lung fields gives the finding its name.
Radiologists typically analyze this pattern on an inspiratory scan, where the patient has taken a full breath in. The appearance of the pattern is heterogeneous, meaning the distribution of the light and dark areas is uneven across the lung. The finding is objective and requires further investigation to determine the specific cause.
Underlying Mechanisms Causing the Pattern
The visual pattern of mosaic attenuation arises from a non-uniform distribution of disease within the lungs, traceable to three primary physiological mechanisms. Determining which areas are abnormal—the darker or the lighter patches—is the initial step in distinguishing these causes. The resulting mosaic appearance is caused by regional differences in ventilation, blood flow, or tissue density.
Air Trapping (Small Airway Disease)
One major cause is air trapping, which occurs due to obstruction in the small airways. When these passages become narrowed or blocked, air can easily enter the distal lung during inhalation but becomes trapped during exhalation. This trapped air causes the affected lung segments to become overinflated, leading to reduced tissue density.
These regions of trapped air appear darker (hypoattenuated) on the CT scan because they contain more air relative to tissue and blood. The adjacent lung tissue, which is normally ventilated, appears relatively lighter or normal in density. In this mechanism, the darker, low-attenuation areas represent the abnormal lung tissue.
Vascular Differences (Reduced Perfusion)
Another mechanism involves differences in blood flow, or perfusion, to different areas of the lung. Conditions that affect the pulmonary arteries, such as chronic thromboembolic disease or pulmonary hypertension, can cause certain lung segments to receive significantly less blood flow. This is often referred to as “mosaic perfusion.”
Lung segments that are hypoperfused appear darker than normal lung tissue because blood contributes significantly to overall lung density. Adjacent, non-diseased segments may receive compensatory increased blood flow, making them appear relatively lighter. The darker areas are abnormal due to a lack of blood, and the pulmonary vessels within these dark patches often appear smaller than in the lighter areas.
Interstitial/Alveolar Disease (Ground-Glass Opacity)
The third mechanism involves changes in the lung tissue itself, specifically the interstitium or the air sacs. Mild inflammation, fluid accumulation, or cellular infiltration within the air spaces can cause the affected lung tissue to become denser. This increased density is often described as ground-glass opacity, a hazy appearance that is lighter than normal lung.
When this ground-glass opacity is distributed unevenly, the resulting areas of increased density contrast against the adjacent, normal, air-filled lung. In this scenario, the lighter, hyperattenuated areas are the abnormal tissue, while the darker patches represent the normal lung. This mechanism results in a pattern where the vessel size remains uniform across both the light and dark regions.
Diagnostic Significance and Differentiating Causes
Determining which of the three mechanisms is responsible for the observed mosaic pattern is the major challenge for physicians. The correct interpretation is achieved by integrating the CT findings with the patient’s clinical history and specific imaging techniques. Distinguishing the cause is paramount because treatment strategies for small airway disease, vascular disease, and parenchymal disease are completely different.
The most valuable tool for differentiation is the use of expiratory CT scans. A standard inspiratory scan is often followed by a scan taken during forced exhalation. If the darker, low-attenuation areas are due to air trapping, they will fail to increase in density or decrease in volume during expiration, confirming small airway obstruction.
If the pattern is due to vascular disease or parenchymal disease, the lung tissue generally shows a uniform increase in density upon exhalation, and the mosaic pattern may disappear. Radiologists also look for associated findings, such as the size of the pulmonary vessels. Smaller vessels in the dark areas, combined with central pulmonary artery enlargement, strongly suggest a vascular cause like pulmonary hypertension.
Secondary signs, such as bronchiectasis (widening of the airways) or centrilobular nodules, may suggest a small airway disease. The mosaic attenuation pattern points to a category of disease, including small airway diseases, pulmonary vascular diseases, and specific interstitial lung diseases. The final diagnosis relies on compiling all these imaging clues with the patient’s symptoms and laboratory results.