Introduction
High-resolution computed tomography (HRCT) is used to diagnose interstitial lung diseases. HRCT plays a role in the integrated approach of diagnosis and provides sufficient information to produce a non-invasive definitive diagnosis. Multiple patterns of interstitial lung diseases are observed in HRCT. The common distinct patterns of interstitial lung disease include septal, reticular, linear, nodular, cystic, ground-glass patterns, and decreased lung attenuation patterns.
What Is Reticular and Linear Opacification?
Reticular and linear opacifications are patterns observed in HRCT. The opacifications in the lungs occur due to a decrease in the gas-to-soft tissue ratio. This is caused by a pathological process that is centered in or around the pulmonary interstitium. This is caused by the thickening of the interstitial compartments by tumor, blood, water, cells, fibrosis, or a combination of any of these components. The reticular and linear opacifications are evaluated to arrive at a differential diagnosis. Various interstitial lung diseases mimic each other, and the differential diagnosis is based on the predominant pattern.
What Is Reticular Opacification?
The reticular pattern on HRCT refers to multiple interlacing shadows that form a meshwork. It appears as a complex network of curvilinear opacities. The reticular pattern consists of all or some of the constituents, including peribronchovascular interstitial thickening, interlobular septal thickening, intralobular interstitial thickening, honeycomb appearance, and traction bronchiectasis or bronchiolectasis. The opacities are subdivided into fine, medium, or coarse, based on the size of the lucent spaces created by the intersection lines.
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Fine - Fine refers to the ground glass pattern that is 1 to 2 mm (millimeter) in size. It is observed in interstitial pulmonary edema, where the processes thicken the pulmonary interstitium and produce a fine network of lines.
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Medium - Medium refers to the honeycomb pattern of 3 to 10 mm. This pattern is observed in pulmonary fibrosis involving peripheral and parenchymal interstitium.
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Coarse - Coarse refers to a cystic pattern that is greater than 10 mm in size. The cystic spaces result from parenchymal destruction and are seen in pulmonary Langerhans cell histiocytosis, pulmonary sarcoidosis, and interstitial pneumonia.
What Are the Conditions Associated With Reticular Opacification?
Reticular opacification is the prototype entity in idiopathic pulmonary fibrosis. It presents with subpleural and predominant posterior intralobular interstitial thickening, honeycombing, irregular interlobular septal thickening, traction bronchiectasis, or bronchiolectasis. Ground-glass opacities are also seen in bronchiectasis and bronchiolectasis. The other conditions that mimic idiopathic pulmonary fibrosis and reticular pattern are asbestosis, non-specific interstitial pneumonia, chronic hypersensitivity pneumonitis, desquamative interstitial pneumonia, rheumatoid arthritis, and end-stage sarcoidosis.
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Asbestosis - Asbestosis shows the presence of a reticular pattern as the predominant pattern and mimics interstitial pulmonary fibrosis. It presents with subpleural reticulation at the early stage with dot-like opacities. Areas of ground-glass opacities and honeycombing are present. Parenchymal bands, subpleural lines, diffuse pleural thickening, and pleural plaques are observed.
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Non-Specific Interstitial Pneumonia - Non-specific interstitial pneumonia exhibits a reticular pattern at the lower lobes. It is characterized by the presence of predominant mild subpleural reticulation that depends on the subtype of non-specific interstitial pneumonia. HRCT shows the presence of predominant areas of consolidation with minimal honeycombing.
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Chronic Hypersensitivity Pneumonitis - Chronic hypersensitivity pneumonitis shows the presence of a coarse reticular pattern that is patchy, peripheral, or peribronchovascular. It presents with bronchiolectasis, traction bronchiectasis, and minimal honeycombing. Areas of ground glass opacity and hazy centrilobular micronodules are also noted.
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Desquamative Interstitial Pneumonitis - Desquamative interstitial pneumonitis is a condition that rarely resembles interstitial pulmonary fibrosis because it may present with minor subpleural reticulation. Ground glass opacity is observed, but honeycombing is usually absent.
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Rheumatoid Arthritis - Rheumatoid arthritis presents with a reticular pattern that entirely mimics interstitial pulmonary fibrosis or non-specific interstitial pneumonia. HRCT shows the presence of subpleural and basal reticulation with mil honeycombing.
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End-Stage Sarcoidosis - End-stage sarcoidosis shows the presence of a pure reticular pattern that mimics interstitial pulmonary fibrosis. It is characterized by the presence of thickening of interlobular septae, traction bronchiectasis, intralobular linear opacities, and honeycombing.
Some conditions present mixed ground glass-reticular patterns. The reticular pattern coexists with ground glass patterns in the following conditions:
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Scleroderma - HRCT shows the presence of mixed ground glass and reticular pattern at the lower lobes. It is characterized by the appearance of areas of ground-glass opacity and traction bronchiolectasis.
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Idiopathic pulmonary fibrosis and desquamative interstitial pneumonia present with mixed ground glass and reticular pattern. However, a pure reticular pattern in these conditions is rare.
What Is Linear Opacification?
The linear pattern of opacification is observed as axial or peripheral thickening of the interstitium. Linear interstitial patterns are present in processes that thicken the peripheral pulmonary interstitium or the axial bronchovascular interstitium.
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Axial - The axial interstitium shows the presence of diffuse thickening that is present along the bronchovascular tree. The diffuse thickening is observed as parallel opacities that radiate from the hila or peribronchial cuffing.
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Peripheral - The peripheral interstitium shows medial or lateral thickening of the interstitium, which produces Kerley lines.
What Are the Conditions Associated With Linear Opacification?
Axial interstitial thickening is usually present in interstitial pulmonary edema, and it is difficult to distinguish it from other diseases that cause bronchial wall thickening, like asthma and bronchiectasis. Peripheral interstitial thickening is seen in conditions like lymphangitis carcinomatosis, interstitial pulmonary edema, and acute viral or atypical bacterial pneumonia.
What Is the Significance of Reticular and Linear Opacification?
The reticular and linear opacities are a group of morphological changes observed in interstitial lung diseases on a high-resolution computed tomography scan. It indicates both active inflammatory changes that are potentially treatable and irreversible pulmonary fibrosis. The linear opacities without distortion indicate an active process, while cystic changes, honeycombing, and lung distortion suggest irreversible fibrosis. However, the linear and reticular opacities alone are not reliable for differential diagnosis and must be evaluated in association with other HRCT findings.
Conclusion
High-resolution computed tomography scan plays a major role in the diagnosis of interstitial lung diseases. It reveals the changes that are not visible on a chest radiograph and conventional CT (computed tomography) and helps in identifying the changes even in the smallest structural unit of the interstitium. The reticular and linear opacifications are seen in various interstitial lung diseases. It is an effort to diagnose interstitial lung disease based on the predominant pattern, but it should be correlated with the clinical and laboratory findings.
