Secondary Pulmonary Lobule

High resolution computed tomography (HRCT) scans provide an extraordinary amount of information to practitioners. In fact, many findings on HRCT have terms which are incorrectly used for ordinary CT scans (honeycombing, tree-in-bud appearance, ground glass opacities, etc.)

I start reviewing a HRCT by looking at functional units of the respiratory zone – the secondary pulmonary lobules. These polyhedral units consist of ~50 primary pulmonary lobules and include alveolar sacs and ducts. They are supplied by terminal bronchioles and pulmonary artery branches to comprise the gas exchange interface. A connective tissue meshwork (“interlobular septa”) separates adjacent lobules which also harbors the pulmonary veins and peripheral lymphatics.

Knowing how these structures are arranged within the lobule will help diagnose pathology when certain findings are seen on HRCT. If the interlobular septa (dark arrows) are prominent, think of issues with venous and/or lymphatic drainage. If the centrolobular regions (white arrows) are involved, think pulmonary artery and/or terminal bronchiole pathology.

HRCT (Image: Sage Journals)

Tables 1, 2, and 3 in this Radiology review article provide excellent differential diagnoses based on HRCT findings involving the pulmonary lobule: link to article

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