Atypical distribution of small nodules on high resolution CT studies: patterns and differentials

Accurate diagnosis of lung disease with high resolution CT is challenging and relies on a pattern-based approach coupled with knowledge of the distribution of the abnormalities in the lung parenchyma. Some findings and distributions of small nodules are specific for certain diseases, but atypical patterns have been described, especially for granulomatous diseases such as sarcoidosis and tuberculosis. Unusual HRCT aspects that involve the coalescence of small nodules have been termed the "sarcoid galaxy sign" and the "sarcoid cluster sign". Other imaging findings such as the "reversed halo sign" and the "fairy ring sign" can also be composed of small nodules. The aim of this review was to describe and illustrate a range of conditions that manifest with atypical distribution of small nodules on HRCT. We discuss the various aspects, associated findings, and differential diagnosis particularly in sarcoidosis and tuberculosis.