Small pulmonary nodules (HRCT chest approach)

Small pulmonary lung nodules refer to an HRCT chest imaging descriptor for 5-10 mm lung nodules and are divided into three main categories based on their distribution pattern:

• centrilobular
• perilymphatic
• random

Approach

Distribution

Firstly, determine whether the nodules are perilymphatic in distribution which relates to their location to the pleural surface and fissures:

• >10% of nodules are subpleural, they are more likely to be perilymphatic
• <10% are fissural or subpleural more likely to be random
• subpleural and fissural sparing, the nodules are more likely to be centrilobular

If there are centrilobular nodules, the next step is to look for any tree-in-bud pattern to help narrow the differential even further:

• if present it can indicate bronchiolar diseases such as endobronchial spread of infection, aspiration or mucus retention in small or large airway diseases such as asthma, ABPA or bronchiectasis
• if absent and with evenly distributed nodules ~1-2 cm apart with immediate subpleural sparing are indicative of either bronchiolar diseases or vasculitides; the two main diseases to consider are hypersensitivity pneumonitis (absent smoking history) or respiratory bronchiolitis (occurs in smoker)

Morphology

The nodules can be either well-defined in cases of vascular and lymphatic origin or ill-defined ground glass if they are air space (alveolar) in origin.