Lung attenuation may increase or decrease depending on various conditions. Enhanced lung attenuation may develop when the amount of air in the alveoli and the lumen of the airways decreases, and decreased lung attenuation may develop in conditions such as empyema, cysts, and bronchiectasis.
What Is Lung Attenuation?
Lung attenuation is a radiologic finding called a mosaic pattern, which is not very specific and can be caused by vascular disease, abnormalities in the airways, or ground-glass interstitial or air-space infiltrates.
What Are the Patterns of High-Resolution Computed Tomography (HRCT)?
The basic patterns of HRCT are:
1. Reticular: Several lines can be seen as a result of interlobular septal thickening or honeycombing pattern. The honeycombing pattern is characterized by small cystic spaces lined by bronchiolar epithelium with walls made of thick fibrous tissue. It is typically seen in usual interstitial pneumonia. Thickening of the interstitium is either due to the accumulation of fluid, fibrous tissue, or infiltration of cells. It is more commonly associated with patients with interstitial lung disease.
There are two types of septal thickening:
-
Smooth Septal Thickening: It is most commonly seen in pulmonary edema, the lymphangitic spread of carcinoma, and pulmonary alveolar proteinosis (PAP).
-
Nodular Septal Thickening: It is most commonly seen in sarcoidosis, silicosis, and lymphangitic spread of carcinoma.
2. Nodular: The nodular pattern can be classified based on the distribution of the nodules:
-
Perilymphatic: The nodules are seen at the pleural spaces, interlobular septa, and peribronchovascular interstitium. They are always found at the subpleural location of the fissures. The nodules are seen in sarcoidosis, silicosis, coal worker's pneumoconiosis, and lymphangitic spread of carcinoma. The overlap in the differential diagnosis of perilymphatic nodules and nodular septal thickening in the reticular pattern is also known as the reticulonodular pattern.
-
Centrilobular: The nodules are distributed primarily within the center of the secondary lobule. It usually spares the pleural spaces, unlike perilymphatic and random nodules. It is seen in hypersensitivity pneumonitis, respiratory bronchiolitis in smokers, and infectious airway diseases and is rarely seen in bronchoalveolar carcinoma, pulmonary edema, and vasculitis. These nodules are ill-defined and are of dense or ground glass opacity. They are also known as acinar nodules.
-
Random: The nodules have no definable distribution and are randomly distributed throughout the lung parenchyma in a bilaterally symmetric distribution. The nodules are typically seen in the pleural spaces and fissures but particularly lack subpleural dominance, which is more commonly seen in the perilymphatic distribution. It is commonly seen in hematogenous metastasis, miliary tuberculosis, miliary fungal infections, sarcoidosis, and langerhans cell histiocytosis.
3. High Attenuation: Increased attenuation is also known as ground-glass opacity if the opacification is without obscuration of the underlying blood vessels and consolidation if the opacification obscures the underlying vessels. The increase in lung density is due to replacing air spaces in the alveoli with fluid, cells or fibrous tissues.
4. Low Attenuation: Decreased attenuation can be seen in emphysema, lung cysts, and bronchiectasis.
What Does Ground Glass Opacity in Increased Lung Attenuation Represent?
Ground glass opacity represents the following:
-
Filling the alveolar spaces with pus, edema, hemorrhage, inflammatory mediators causing inflammation, or tumor cells.
-
Thickening of interstitium or alveolar spaces as sewn in pulmonary fibrosis.
-
Partial collapse of alveoli.
-
Increases capillary blood volume.
-
Normal expiration.
The importance of ground glass opacities is either microscopic interstitial alveolar disease or, it indicates the presence of an ongoing, and active lung disease that is potentially treatable in the absence of fibrosis. Ground glass opacity is diagnosed on scans obtained with thin sections, as volume averaging is more with thicker sections, leading to spurious ground glass opacity, regardless of the nature of the abnormality.
The site of abnormalities of the ground glass pattern can be helpful in the diagnosis:
-
Upper Zone Predominance: Respiratory bronchiolitis, Pneumocystis pneumonia.
-
Lower Zone Predominance: usual interstitial pneumonia (UIP), non-specific interstitial pneumonia (NSIP).
-
Centrilobular Predominance: Hypersensitivity pneumonitis, Respiratory bronchiolitis.
A characteristic "Dark bronchus sign" is seen in ground glass opacity which refers to air-filled bronchi appearing dark compared to the surrounding lung parenchyma, which is filled with inflammatory alveolar infiltrates. It is especially useful in diagnosis.
What Are the Conditions with Decreased Lung Attenuation?
Conditions with decreased lung attenuation:
Emphysema: As a result of parenchymal damage, areas of low attenuation are present.
The various types of emphysema:
Centrilobular Emphysema:
-
Centrilobular emphysema is the most common type of emphysema, typically associated with smoking.
-
There is a haphazard distribution with more upper lobe predominance.
-
Irreversible destruction of the alveolar walls in the centrilobular portion of the lobules.
Panlobular Emphysema:
-
It is commonly seen in alpha-1 antitrypsin deficiency and sometimes in smokers.
-
It has a lower lobe predominance.
-
It is usually a diffuse type and affects the entire secondary lobule.
-
There is uniform damage to the underlying structures of the secondary pulmonary lobule, resulting in widespread areas of abnormally low attenuation.
Paraseptal Emphysema:
-
It is usually present adjacent to the pleura and interlobar fissures.
-
It is often associated with bullae formation. Apical bullae may lead to spontaneous pneumothorax. Occasionally, giant bullae may cause severe compression of the adjacent lung tissue.
Lung Cyst: The cysts are thin-walled (less than 3mm), well-defined, and circumscribed air-containing lesions that are lined by fibrous or epithelial cellular epithelium. The common causes are
-
Lymphangiomyomatosis.
-
Langerhans Histiocytosis.
-
Lymphoid interstitial pneumonia.
It should be differentiated from emphysematous bullae, blebs, and pneumatoceles.
Axial HRCT image shows multiple bilateral bizarre-shaped cysts and small centrilobular nodules in a smoker with Langerhans’ cell histiocytosis. An upper lobe predominance in the size and number of cysts is commonly seen.
Lymphangiomyomatosis is a rare, progressive type of proliferation of spindle cells mimicking the smooth muscles. The proliferation of these cells along the bronchioles causes air trapping and results in the development of thin-walled cysts. If these cysts rupture, they may cause pneumothorax. It has a female predilection, aged between seventeen to fifty years.
Bronchiectasis: It is defined as localized irreversible dilatation of the bronchial tree.
HRCT findings of the bronchiectasis include
-
Bronchial Dilatation: The broncho-arterial ratio, where the internal diameter of the bronchus or pulmonary artery exceeds 1>1.5. In cross-section, it appears as “signet ring appearance.”
-
Lack of bronchial tapering.
-
Visualization of peripheral airways.
Conclusion:
Lung attenuation may increase or decrease in various conditions. When the changes are limited, the increased lung attenuation is termed “ground-glass opacity,” which does not obscure the underlying vessels. When the condition becomes more pronounced, that is, when the vessels are obscured, the term “lung consolidation” or “consolidation” is used.
