Secondary Lobule Pathology

Lung pathology on high resolution CT centers around the basic anatomic unit of the lung, the secondary lobule.
- The secondary lobule has a central zone which is in close proximity to the bronchovascular bundle containing the terminal bronchiole and its immediate offshoots, as well as the supplying artery
- Diseases that affect the centrilobular zone typically enter through the airways:

  • Hypersensitivity pneumonitis: inflammation of the alveoli caused by immune reaction to inhaled organic dusts
  • Respiratory bronchiolitis: inflammation of the respiratory bronchioles typically seen in heavy smokers
  • Centrilobular emphysema: expansion of the central and proximal acinus and alveoli, also seen mainly in smokers

- The secondary lobule has a peripheral zone, containing the interlobular lymphatic channels
- The peripheral lymphatics are also a common site for disease:

Pattern of involvement:
- Reticular pattern: increased thickness of the lines representing interlobular septa, due to fluid, cellular infiltration (as in cancer), or fibrous tissue
  • Smooth thickening in pulmonary edema, lymphangitic carcinomatosis, alveolar proteinosis
  • Nodular thickening in lymphangitic carcinomatosis, sarcoid, silicosis
  • Lymphangitic carcinomatosis more likely to be unilateral or focal; also associated with hilar adenopathy
  • Pulmonary edema associated with gravity-dependant distribution of septal thickening and ground glass opacities, peribronchial cuffing, fissural thickening, and pleural effusion
  • Alveolar proteinosis classically associated with "crazy paving" describing reticular and ground glass opacities in a patchy distribution, but "crazy paving" is not specific for this condition
  • "Crazy paving" also seen in bronchoalveolar carcinoma, sarcoidosis, ARDS, pulmonary hemorrhage, and non-specific interstitial pneumonia (NSIP)
  • "Honeycombing" is a reticular pattern of septal thickening bounding cystic spaces in the lungs, associated with usual interstitial pneumonia (UIP)
- Nodular pattern: