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[Various aspects of acinar lesions--the key finding of pulmonary tuberculosis on HRCT].

Acinar lesions, a pathologist's naming for granulomatous lesions formed in the peripheral air space, that is, in the bronchiole or its adjacent alveolar space, is very characteristic and pathognomonic for tuberculosis on HRCT imaging. As a radiological term, it is equal to centrilobular nodule or branching shadow, or tree-in-bud appearance in the recent trend. It is universally seen in most of tuberculosis cases, irrespective of its stage or extensity. Although thus common, its appearance is not always uniform. Firstly they are not well defined in some cases. Exudative tendency in pathological process may explain for this appearance. Secondarily they are not always arranged in an orderly manner or in other words centrilobular manner on CT, but often in a random fashion. Pathologically this phenomenon can be explained by the randomness of formation site of granulomas or by scarring in spontaneous healing process of the disease. Finally, although rare, an extreme pattern, in which acinar lesions are diffusely disseminated in both lung fields without other type of lesions, is well known as Oka's Classification of Pulmonary Tuberculosis Type IIB. This rare type of tuberculosis could be formed through indolent dissemination of bacilli via the airway or from the hematogenous dissemination. It should also be noted that in tuberculous pneumonia, especially when it develops in emphysematous lung, acinar lesions is not seen, making differential diagnosis difficult.