Introduction
Pulmonary tuberculosis is an infection caused by Mycobacterium tuberculosis, spread by respiratory droplets. In the exam you should classify it into primary and secondary (post-primary, reactivation) forms, describe the typical chest radiograph and CT changes of each, and explain how imaging guides diagnosis, differential diagnosis and follow-up.
Risk factors: HIV, poverty, immigration, silicosis, immune compromise, malignancy and others. Any patient with suspected TB should be isolated until sputum assessment has proved acid-fast bacilli. TB is associated with silicosis and HIV, but not with bronchogenic carcinoma.
Classification
Tuberculosis is classified by the stage of infection and the host response. Radiologically, the two main groups differ in age group, location, nodal involvement and cavitation.
X-ray changes in primary tuberculosis
Primary infection follows inhalation of droplet nuclei by a host without prior immunity; it is the disease that develops after initial exposure. The bacilli are taken up by alveolar macrophages and spread to regional lymph nodes. The parenchymal focus and the nodal involvement together form the primary complex. Very few patients have clinical manifestations, but they usually show radiographic manifestations. Patterns include pneumonia, adenopathy, atelectasis and pleural effusion.
- Ghon focus: a small (about 1 cm), ill-defined, subpleural area of consolidation, typically in the middle or lower zone.
- Lobar pneumonia associated with hilar or mediastinal adenopathy at any age = TB; usually upper lobe.
- Lymphangitis: linear streaks linking the focus to the hilum, sometimes visible.
- Hilar or paratracheal lymphadenopathy: ipsilateral, usually unilateral hilar, more common in children and often marked there. A large node may compress a bronchus and cause segmental or lobar atelectasis or air trapping. On CT, enlarged lymph nodes have focal areas of low attenuation and show peripheral enhancement; this can be used to differentiate from secondary TB.
- Ghon complex: the focus plus the ipsilateral nodes (adenopathy and infiltration). After healing with calcification, Ghon complex plus a calcified lymph node is called a Ranke complex.
- Simon's focus: the healed site of primary infection at the apex.
- Pleural effusion: usually unilateral and on the same side as the focus; in young adults it may be the only finding. It is more common in adults than in children. Complications: empyema, fistula formation and bone erosion (Dams line can be seen).
- Progressive primary TB: expansion of the focus, cavitation, endobronchial spread and dissemination. It is more likely in infants, malnourished patients and those who are immunosuppressed.