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2. Perilymphatic

Reticular refers to a pattern of too many “crisscrossing” lines creating a net-like pattern, either as a result of thickening of the interlobular septa or as a result of fibrosis as in honeycombing. Random, centrilobular and perilymphatic refer small nodular patterns on CT scan. The distribution of nodules is the most important factor, short of biopsy, for making an accurate diagnosis when small nodules are encountered at thoracic CT (1). In most cases small nodules can be placed into one of these three categories.

Random refers to no preference for a specific location in the secondary lobule (Figure 2)- nodules are seen in relation to the center of the lobule as well as the periphery of the lobule (in contact with visceral pleura and interlobular septae).

Figure 2. Panel A: Schematic showing random distribution of lung nodules. Panel B: thoracic CT scan from a patient with miliary tuberculosis showing a random pattern.

Small random nodules are seen in:

Unlike perilymphatic and random nodules, centrilobular nodules spare the pleural surfaces. The most peripheral nodules are centered 5-10 mm from fissures or the pleural surface (Figure 3).

Figure 3. Panel A: schematic of centrilobular nodule distribution. Centrilobular distribution in a patient with hypersensitivity pneumonitis.

Centrilobular nodules are seen in:

In patients with a perilymphatic nodular distribution, nodules are seen in relation to pleural surfaces, interlobular septa and the peribronchovascular interstitium.
Nodules are almost always visible in a subpleural location, particularly in relation to the fissures (Figure 4).

Figure 4. Panel A: schematic of perilymphatic distribution of nodules. Panel B: On the left a typical case of perilymphatic distribution of nodules in a patient with sarcoidosis.
Notice the nodules along the fissures indicating a perilymphatic distribution (red arrows).
In this case we see the majority of nodules located along the bronchovascular bundle (yellow arrow).

Which of the following diseases is the most common cause of a perilymphatic distribution? (Click on the correct answer to proceed to the fourth of six panels)

  1. Coal-worker's pneumoconiosis
  2. Lymphangitic spread of carcinoma
  3. Sarcoidosis
  4. Silicosis

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