Radiologically visible calcification occurs quite early in tuberculosis, both in the peripheral focus in the lung and in the hilum. As the calcified lung focus shrinks, it may seem to be more dense; the end result is the calcified, healed primary complex (Fig. 5.7). In other patients, the peripheral granuloma will disappear altogether, leaving only residual calcification in the lymph nodes. Calcification seen radiologically is evidence that healing is occurring; unfortunately, it does not always mean that the infection has been conquered. It is possible for a partially calcified lymph node to rupture into a bronchus and cause inhalation bronchopneumonia. Because the lymph nodes, although starting to calcify, may still be large and soft, they may rupture into a bronchus (Fig. 5.8 C-F) or cause external pressure resulting in peripheral atelectasis (see "The Destroyed Lung").

The radiological differential diagnosis of primary lobar tuberculosis includes pyogenic or other lobar pneumonia, especially in those patients in whom the enlarged lymph nodes cannot be seen. Where there is consolidation in the lower lobe particularly, amebiasis will have to be excluded; both paragonimiasis and melioidosis can mimic tuberculosis, although usually at the later, cavitary stage of the disease.

Finally, it must be emphasized that because most tuberculous infections are airborne, a pulmonary lesion may subsequently develop wherever the tubercle bacilli happen to lodge in the lung. Thus, distribution is by chance. The majority of such contacts with bacilli leave no evidence or, at the most, microscopic foci which are not visible radiologically. Careful histopathology of many "normal" lungs has shown that most of these invisible lesions are in the lower lobes and seldom develop further. It is the minority of foci that cause the majority of radiological and clinical findings. Nevertheless, it is very important to recognize that no part of the lung is exempt from any pattern of tuberculosis and in the nonimmune patient there is usually a poor correlation between the radiological pattern and the clinical state of the patient.

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Fig. 5.7A-E. In the majority of patients of any age, the end result of a primary tuberculous infection is a calcified granuloma: this is often a chance finding and seldom of clinical significance. A, B A well-defined, moderate sized calcified granuloma in the lingula of a physician, known to have been present for years. The PPD was positive and the patient was healthy. C Calcified hilar and right paratracheal lymph nodes. There is a very small calcified granuloma in the apical segment of the right lower lobe, seen alongside the paratracheal calcification. D Linear fibrosis can be seen around a small calcified peripheral granuloma. There is calcification in the perihilar nodesE Bronchography (performed for another reason) confirms the distortion of the bronchial pattern, localized emphysema, and some fibrosis, which is in a lower subsegmental branch of the right upper lobe. Such distortion is seldom of clinical significance, although occasionally it is the site of repeated infection. (Courtesy of Semin Roentgenol, 1979).