Nodules on HRCT

In general, nodules can be separated into three distinct categories. For this discussion, "tree-in-bud" nodules (a common sign of endobronchial spread of infection such as TB) will not be included, though it could be considered a subset of the centrilobular category). It seems to be fairly easy for everyone to see, and it's rarely confused with the other patterns.

Perilymphatic

Perilymphatic nodules are those which are peripheral with respect to the secondary pulmonary lobule. They are easily identified when they are subpleural or along major vessels. They can give an "interface" sign (which means making the adjacent structures border appear shaggy). The major differential considerations are:

• Sarcoid -- Mid to upper lung field distribution, nodules have irregular margins, may have adenopathy
• Silicosis/CWP -- Mid to upper lung field distribution, posterior, nodules have sharp margins, may have adenopathy
• Lymphangitis Carcinomatosis -- Usually a lot of septal thickening as well

Centrilobular

Centrilobular nodules are those which are central with respect to the secondary pulmonary lobule. They are regullarly- spaced and do not touch the pleura or fissures. The major differential considerations are:

• Infection -- Endobronchial spread. May or may not be tree-in-bud
• Hypersensitivity Pneumonitis/EAA
• Langerhans Histiocytosis

Random

Random nodules are those which are neither perilymphatic nor centrilobular. In general, this is refers to a hematogenous process, and thus the differential is that of a miliary pattern:

• Miliary TB
• Miliary Fungal Infection -- Any granulomatous-type infection
• Metastases -- Thyroid, melanoma, others