Imaging Techniques for Detecting Tuberculosis

Tuberculosis is a serious infectious disease caused by Mycobacterium tuberculosis which can affect any organ system, but the majority of the issues are observed in the chest as the lungs are often the initial site of infection. Imaging techniques play a major role in diagnosis, treatment evaluation and detecting disease complications. Although sputum smear microscopy, culture for AFB serve as the initial stage of investigation but imaging techniques are necessary to confirm disease extent and activity. The article has focused on providing an account of the different imaging techniques.

Chest X-ray (CXR):

A patchy or nodular infiltrate may be observed in the chest radiographs mostly in the upper lobe of the lungs. Apical abnormalities can be observed from the lordotic view. This method is implied for the initial investigations of the TB suspects having an unexplained cough or with sputum-positive cases. Pulmonary tuberculosis can also be detected through CXR. The patterns which are mostly found in CXR includes:

● Cav ity Formation due to high bacterial load

● Noncalcified Round Infiltrates

● Homogeneously Calcified Nodules (usually 5-20 mm) depicting old infection

● Appearance of several small, nodular lesions resembling millet seeds in case of Miliary TB

CT chest:

Multi-detector CT (MDCT) can help in detecting radiographically occult disease through the diagnosis of parenchymal lesions, evaluating mediastinal lymph nodes, disease activity and complications assessment. It gives accurate diagnosis of pulmonary lesions and also helps in differentiating the etiologies of pneumonia. Further, bronchiectasis, cavitation, necrosis in the LNs, pathologies of pleural/airway/diaphragmatic pathologies and others can be evaluated.

● Contrast-enhanced CT (CECT) can detect mediastinal LNs and pleural enhancement in empyema.

● High-resolution CT (HRCT) to detect Miliary and centrilobular nodules, ground-glass opacities and air-trapping.

MRI

MRI combined with diffusion-weighted imaging (DWI) and subtracted contrast-enhanced (CE) imaging can help in optimal evaluation of mediastinal nodes and also to assess disease activity in mediastinal fibrosis. Detection of diffusion restriction in LNs and peripheral enhancement indicates active disease.

Positron emission tomography - CT - Fluorodeoxyglucose:

Being highly sensitive in nature, positron Emission Tomography (FDG-PET) can help in detecting infections, inflammation and malignancy, as a result it can detect pyrexia of unknown origin (PUO) more accurately. Active TB often mimics cancer, PET can help in distinguishing them by assessing complete disease extent, detecting occult distant involvement and so on. High radiation exposure limits its application in certain cases.

The imaging techniques, therefore, help in detecting Miliary TB, consolidations, thick walled cavity, acinar/ centrilobular nodules, clustered nodules, Miliary nodules, rim-enhancing LNs, pleural effusion or empyema and also in detecting lymphadenopathy (homogeneously enhanced without calcification) poses a diagnostic dilemma. Apart from these, the imaging techniques also help in investigating the signs of healing or the signs of TB sequelae which include bronchovesicular distortion, fibro-parenchymal lesions, bronchiectasis, emphysema and fibro-atelectatic bands indicative of prior infection with scarring. In the cases where healed TB is detected or patients have some minor symptoms, further imaging is not required.