[HRCT of the lung: nodular pattern: anatomy and differential diagnosis]

Since the spectrum of differential diagnoses is wide, the interpretation of a nodular pattern in lung lesions detected on CT is a frequent problem. Number, size, localization, and morphology (shape, density, margins) contribute to evaluating the most probable differential diagnosis. "Classical" high resolution CT or high resolution image reconstructions from multiple row detector CT helical acquisitions achieve a detail resolution that makes it possible to distinguish findings by their typical predominance in certain anatomical compartments of the lung. The position of bronchial, vascular and lymphatic structures can be determined down to the secondary pulmonary lobule, the smallest subunit of the lung to be separated by septa of connective tissue. Based on this, a centrilobular predominance of nodules, i.e. with a tree-in-bud pattern, is a frequent sign of bronchiolitis. Perilymphatic predominance in the periphery of the lobules is associated with sarcoidosis or lymphangitic spread of cancer. Random distribution of nodules is interpreted as a sign of hematogenic spread of disease. Hence the subtle interpretation of specific findings on HRCT can contribute substantially to clinical decision making, although these signs may not always replace biopsy and histologic workup.