The radiological opacities in a chest x-ray are a result of the pathological processes taking place in the lungs. TB causes alveolar consolidation, necrosis, cavitation and fibrosis, features, which it shares with a variety of lung diseases. That is why TB also shares its radiological features with many other pulmonary diseases.
Hence, there is no radiological feature that is absolutely typical of pulmonary TB and other diseases may mimic TB radiologically. The following features, however, when seen on a chest skiagram, suggest a diagnosis of TB:
• Unilateral or bilateral upper zone opacities with or without associated fibrosis.
• Cavity lesions.
• Opacities with or without cavitation in apical segments of either lower lobe.
• Mediastinal and unilateral hilar lymphadenopathy.
• Bilateral miliary mottling.
• Pleural effusion.
The unreliability of radiological features in previously treated patients
TB, even after a complete cure, usually leaves behind residual opacities in the patient’s chest x-ray for life. That is why, when a previously treated patient presents with recurrence of respiratory symptoms, ATT should not be started on a radiological basis alone, without sputum smear or culture positivity for AFB.
His symptoms may be due to intercurrent bacterial infection in areas of post-tubercular bronchiectasis or due to some other lung disease. A significant radiological deterioration, from the chest x-ray taken on completion of previous treatment, may be made the basis of starting ATT only in the rarest case for pressing clinical reasons, i.e. the patient is seriously ill and needs urgent treatment.
There is no radiological feature that is diagnostic of pulmonary TB. A definite diagnosis can never be made on the basis of x-ray alone. It must be complemented with vigorous attempts to isolate AFB in sputum (by smear or culture) or directly from the lesion itself.
Most patients with a cavity on an x-ray chest are smear positive for AFB. Thereafter, when the sputum of a patient with cavitary lesions on the x-ray is persistently negative, consider other diagnosis.
Although computed tomography is currently an important investigation in the management of pulmonary diseases, it is practically of no value in the work up of a patient suspected to have TB. It should not be routinely ordered when investigating such a patient.
In rare situations it can be used to define mediastinal and pulmonary lesions more clearly. Wherever computed tomography is resorted to, it must be complemented, wherever possible, with a guided fine needle aspiration biopsy to arrive at a definite pathological or microbiological diagnosis.