In this article a practical approach is given for the interpretation of HRCT examinations

We will discuss the following subjects:
  • Anatomy of the secondary lobule
  • Basic HRCT patterns
  • Distribution of abnormalities
  • Differential diagnosis of interstitial lung diseases
 Secondary lobule

 Secondary lobule
Knowledge of the lung anatomy is essential for understanding HRCT.

  • The secondary lobule is the basic anatomic unit of pulmonary structure and function.
  • Interpretation of interstitial lung diseases is based on the type of involvement of the secondary lobule.
  • It is the smallest lung unit that is surrounded by connective tissue septa.
  • It measures about 1-2 cm and is made up of 5-15 pulmonary acini, that contain the alveoli for gas exchange.
  • The secondary lobule is supplied by a small bronchiole (terminal bronchiole) in the center, that is parallelled by the centrilobular artery.
  • Pulmonary veins and lymphatics run in the periphery of the lobule within the interlobular septa.
  • Under normal conditions only a few of these very thin septa will be seen.
  • There are two lymphatic systems: a central network, that runs along the bronchovascular bundle towards the centre of the lobule and a peripheral network, that is located within the interlobular septa and along the pleural linings.

Centrilobular area is the central part of the secondary lobule.
It is usually the site of diseases, that enter the lung through the airways ( i.e. hypersensitivity pneumonitis, respiratory bronchiolitis, centrilobular emphysema ).
Perilymphatic area is the peripheral part of the secondary lobule.
It is usually the site of diseases, that are located in the lymphatics of in the interlobular septa ( i.e. sarcoid, lymphangitic carcinomatosis, pulmonary edema).
These diseases are usually also located in the central network of lymphatics that surround the bronchovascular bundle.

Basic Interpretation
A structured approach to interpretation of HRCT involves the following questions:
  • What is the dominant HR-pattern:
    • reticular
    • nodular
    • high attenuation (ground-glass, consolidation)
    • low attenuation (emphysema, cystic)
  • Where is it located within the secondary lobule (centrilobular, perilymphatic or random)
  • Is there an upper versus lower zone or a central versus peripheral predominance
  • Are there additional findings (pleural fluid, lymphadenopathy, traction bronchiectasis).
 These morphologic findings have to be combined with the history of the patient and important
clinical findings.

  • When we study patients with HRCT, we have to realize that we are looking at a selected group of patients.
  • Common diseases like pneumonias, pulmonary emboli, cardiogenic edema and lung carcinoma are already ruled out.
  • So uncommon diseases like Sarcoidosis, Hypersensitivity pneumonitis, Langerhans cell histiocytosis, Lymphangitic carcinomatosis, Usual Interstitial Pneumonitis (UIP) and many others become regular HRCT diagnoses.
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