The secondary pulmonary lobule is the smallest functional unit of lung visible on HRCT and forms the basis for describing pulmonary anatomy and pathology. HRCT plays a crucial role in diagnosing interstitial lung disease (ILD) by visualizing specific patterns of involvement within these lobules.

Anatomy of Secondary Pulmonary Lobule on HRCT

• Shape and Size: The secondary pulmonary lobule is polyhedral (often hexagonal) and measures 1–2.5 cm across.
• Boundaries: It is bounded by connective tissue interlobular septa, which contain pulmonary veins and lymphatics. These septa usually appear as thin lines on HRCT but are more prominent when thickened by disease.
• Core: The center contains a terminal bronchiole and accompanying pulmonary artery (centrilobular artery). The centrilobular region is where airway and vascular diseases often manifest.
• Parenchyma: Each lobule contains 5–15 acini, with alveoli for gas exchange. The parenchyma surrounds the lobular core inside the lobular boundaries.
• Lymphatics: There are two lymphatic systems—central (along bronchovascular bundles) and peripheral (within septa)—which play roles in disease distribution.

On HRCT, normal secondary lobules may show faint boundaries (interlobular septa) and sometimes the centrilobular structures. Pathological changes may make these features more conspicuous.

Role of HRCT in Diagnosis of Interstitial Lung Disease

• Pattern Recognition: HRCT allows precise identification of lesion location (perilymphatic, centrilobular, or random), pattern (reticular, nodular, ground-glass), and distribution (upper/lower zone, central/peripheral).
• Accuracy: HRCT is far more sensitive and specific than chest radiography for detecting, characterizing, and monitoring ILD, often obviating lung biopsy.
• Classification: HRCT differentiates between ILD types (usual interstitial pneumonia, nonspecific interstitial pneumonia, sarcoidosis, idiopathic pulmonary fibrosis, etc.) by assessing characteristic patterns such as:
  • Interlobular septal thickening (seen in edema, lymphangitic carcinomatosis, fibrosis)
  • Centrilobular and perilymphatic nodules (seen in hypersensitivity pneumonitis, sarcoidosis)
  • Honeycombing, traction bronchiectasis, or ground-glass opacities
• Monitoring: HRCT is used to assess disease progression, response to treatment, and detect complications or reversibility.

In summary, HRCT provides high-resolution images of secondary pulmonary lobule anatomy, enabling accurate diagnosis and management of interstitial lung diseases through assessment of specific morphological and distributional patterns.