2.2.1. Computed tomography
The major advantage of computed tomography CT is increasing the specificity of the diagnosis of TB; therefore, CT is often not necessary in the acute setting, particularly when the disease is already suspected, with appropriate precautions implemented and microbiologic testing underway. CT may be able to better show distinct findings, such as cavitation or endobronchial spread with tree-in-bud nodules, and may be helpful in cases in which the chest radiograph does not show “classic” findings of PTB.13 Although CT has twice the sensitivity to detect cavities,14 it is the presence or absence of cavitation on chest radiograph that is entered on Canada’s case report form and that is used in treatment-duration decisions. CT findings can also better correlate with AFB smear microscopy positivity.15–17 Even in AFB smear-negative patients, CT may suggest the risk that a patient will be TB-culture positive when findings consistent with PTB are present.18 CT may be of value in the severely immunocompromised patient with a normal or near-normal radiograph by revealing abnormal lymph nodes or subtle parenchymal disease.
2.2.2. Magnetic resonance imaging
The accuracy of magnetic resonance imaging (MRI) is similar to CT in describing findings related to culture-positive PTB.19 MRI’s greatest utility, however, is in the diagnosis and management of extra-pulmonary TB (see Chapter 7: Extra-pulmonary Tuberculosis). MRI is a reasonable consideration for use in select patients for whom there is a desire to avoid ionizing radiation.
The noninvasive imaging tool 18F-fluoro-2-deoxyglucose-positron emission tomography/computed tomography (FDG-PET/CT) is used primarily for cancer diagnosis and staging. It identifies areas of active inflammation by mapping where cells with high metabolic demand take up the radioactively labeled glucose analogue. The radiotracer accumulates within inflammatory cells, such as macrophages and neutrophils, and can be quantified as a standardized uptake value (SUV). FDG-PET/CT cannot reliably differentiate PTB lesions from malignant lesions or other infections/inflammatory conditions.20 Radiotracer uptake magnitude is also unrelated to TB disease activity, thus limiting the role of FDG-PET/CT in the diagnosis of PTB.21 Conceivably, FDG-PET/CT may be helpful in identifying other sites of disease in patients with an already confirmed tracer-positive site of involvement.
2.2.4. Future developments in imaging for TB
Deep learning artificial intelligence software for chest radiography detection of PTB have achieved sensitivity and specificity similar to human readers,22,23 exceeding thresholds for triage test criteria by the World Health Organization (WHO).24 This may be valuable in closing diagnostic gaps in resource-limited and remote settings.
In the future, electronic medical records may be able to link clinical and computer-detected radiographic features to aid diagnosis.25 The intensity and duration of treatment may eventually be tailored to the clinical and radiographic presentation of PTB.
We strongly recommend that posterior-anterior and lateral chest radiography should be an integral part of TB diagnosis but should be accompanied by confirmatory microbiological tests for TB disease because of its low specificity (good evidence).
Good practice statements
Chest radiography findings suggestive of pulmonary TB should be immediately reported to the ordering physician.
In pregnant women suspected of having TB, a posterior-anterior chest radiograph should be performed, as the risk to the fetus of undiagnosed pulmonary TB far outweighs any risk from radiation exposure.
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