Pulmonary nodules are small, rounded opacities within the pulmonary interstitium. Pulmonary nodules are common and, as the spatial resolution of CT scanners has increased, detection of smaller and smaller nodules has occurred, which are more often an incidental finding.
Pulmonary nodules can be classified according to size, morphology and/or distribution.
- miliary nodules: <2 mm
- pulmonary micronodule: 2-7 mm
- pulmonary nodule: 7-30 mm
- pulmonary mass: >30 mm
- solid pulmonary nodules
- partly solid pulmonary nodules
- ground glass pulmonary nodules
- perilymphatic pulmonary nodules
- centrilobular pulmonary nodules
- random pulmonary nodules
They are generally homogeneous (without air bronchograms or alveolograms) and are well-defined since their margins are sharp and they are surrounded by normal aerated lung. They are quite separate from airspace nodules that often have an irregular margin and are usually ~8 mm in diameter. (For further discussion, see the article on nodular opacification.)
A micronodular or miliary pattern is predominately seen in granulomatous processes, haematogenous pulmonary metastases, and pneumoconioses. Nodules and masses are most often seen in metastatic disease to the lung.