Post-primary pulmonary tuberculosis, also known as reactivation TB or secondary TB occurs years later, frequently in the setting of a decreased immune status. In the majority of cases, post-primary TB within the lungs develops in either:

1. posterior segments of the upper lobes
2. superior segments of the lower lobes

Typical appearance of post primary TB is that of patchy consolidation or poorly defined linear and nodular opacities.

Post-primary infections are far more likely to cavitate than primary infections, and are seen in 20-45% of cases. In the vast majority of cases they develop in the posterior segments of the upper lobes (85%). The development of an air-fluid level implies communication with the airway, and thus the possibility of contagion. Endobronchial spread along nearby airways is a relatively common finding, resulting in a relatively well-defined 2-4 mm nodules or branching lesions tree-in-bud appearance on CT.