An HRCT Approach to Diffuse Nodular Lung Disease

Diffuse small pulmonary nodules have a broad differential diagnosis. On high-resolution CT, the most useful first step is to determine their relationship to the secondary pulmonary lobule. Nodule margin, attenuation, size uniformity, craniocaudal predominance, and accompanying parenchymal or extrapulmonary findings then refine the interpretation.

Perilymphatic distribution

Perilymphatic nodules involve the peribronchovascular interstitium, interlobular septa, fissures, and subpleural surfaces. Sarcoidosis, silicosis or coal-worker pneumoconiosis, and lymphangitic carcinomatosis are key considerations. Upper-lung predominance and symmetric hilar or mediastinal lymphadenopathy support sarcoidosis. Irregular septal thickening, pleural effusion, and asymmetric involvement may favor lymphangitic tumor spread, while occupational exposure and conglomerate upper-lobe masses support pneumoconiosis.

Random distribution

Random nodules show no consistent relationship to airways, pleura, or lobular structures and may reach the immediate subpleural lung. This pattern suggests hematogenous dissemination. Important alternatives include metastatic disease, miliary tuberculosis, and disseminated fungal infection. Nodule size uniformity, immune status, known malignancy, and systemic symptoms are essential discriminators; imaging should not assign an infectious cause without clinical and microbiological correlation.

Centrilobular distribution

Centrilobular nodules are centered several millimeters from the pleural surface and fissures. A tree-in-bud pattern suggests bronchiolar impaction or endobronchial spread, commonly from infection or aspiration. Ill-defined ground-glass centrilobular nodules can occur with hypersensitivity pneumonitis, respiratory bronchiolitis, and other inflammatory small-airway disorders. Associated mosaic attenuation and expiratory air trapping strengthen an airway-centered interpretation.

Systematic reporting

• Define the dominant distribution: perilymphatic, random, centrilobular, or mixed.
• Describe nodule size, attenuation, margins, cavitation, calcification, and upper- or lower-lung predominance.
• Record tree-in-bud change, ground-glass opacity, consolidation, fibrosis, emphysema, cysts, and expiratory air trapping.
• Assess lymph nodes, pleura, airways, and signs of pulmonary hypertension.
• Integrate exposure, immune status, malignancy history, symptoms, and prior imaging before giving a focused differential.
• If the pattern is mixed or indeterminate, state the uncertainty and the feature that limits a narrower conclusion.

Example report wording

Numerous small nodules in both lungs have upper-lobe predominance and track along the fissures and peribronchovascular interstitium. The distribution is perilymphatic. Symmetric bilateral hilar and mediastinal lymph nodes are also present. In the appropriate clinical setting, the findings favor sarcoidosis. There is no random hematogenous distribution or airway-centered tree-in-bud pattern.

Use the HRCT protocol and distribution analysis guide for technique and secondary-lobular localization.