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

Terminology

Radiologists often informally refer to indeterminate small pulmonary nodules as ditzels.

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, allergic bronchopulmonary aspergillosis (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.