What are the typical X-ray and CT scan abnormalities consistent with tuberculosis (TB)?

Radiographic Abnormalities in Tuberculosis

Chest X-ray is the first-line imaging modality for TB, with classic findings including upper lobe infiltrates, cavitation, and fibro-cavitary disease in the apical-posterior segments of the upper lobes or superior segments of the lower lobes. 1

Chest X-ray Findings

Primary TB Pattern

• Lobar pneumonia with hilar and/or mediastinal lymphadenopathy is the hallmark of primary TB, most commonly seen in children and severely immunocompromised individuals 1, 2
• Lymphadenopathy is the most characteristic radiological feature in primary tuberculosis 2
• Infiltrates may appear patchy or nodular 1

Postprimary/Reactivation TB Pattern

• Apical-posterior upper lobe or superior segment lower lobe fibro-cavitary disease with endobronchial spread through airways is typical 1
• Cavitation appears in approximately 50% of postprimary TB cases and is the hallmark finding 2
• Patchy, poorly defined consolidation in characteristic locations 2

Atypical Presentations

• HIV-infected patients frequently present with atypical radiographic patterns, including infiltrates in any lung zone, mediastinal or hilar adenopathy, and less common apical cavitary disease compared to HIV-negative patients 1
• Immunocompromised hosts may present with mediastinal lymphadenopathy alone or a deceptively normal chest radiograph 3, 4

CT Scan Abnormalities

When to Use CT

• CT without IV contrast is recommended when chest X-ray is equivocal (ACR appropriateness rating: 7/9) 3
• CT increases diagnostic specificity by better demonstrating cavitation and endobronchial spread 3, 1
• Consider CT for immunocompromised patients, particularly those with low CD4 counts 1

Characteristic CT Findings

Most Common Abnormalities

• Micronodules and tree-in-bud appearance are the most frequent CT findings, present in 100% of active TB cases in some series 5, 6
• Tree-in-bud pattern consists of centrilobular nodules (2-4 mm) connected to branching linear structures, representing dilated and inflamed bronchioles with mucoid impaction 4, 7
• Consolidations (85% of cases), cavities (85%), bronchiectasis (80%), and lobular consolidations (70%) 5

Specific CT Features by Disease Activity

• Active disease indicators: Centrilobular densities, tree-in-bud appearance, consolidation, cavitation 8, 6
• Inactive disease indicators: Fibrotic lesions, distortion of bronchovascular structures, emphysema, bronchiectasis 6
• The disappearance of tree-in-bud appearance and pleural effusion with the presence of fibrotic change indicates effective treatment 8

Enhanced CT Features

• Hilar and mediastinal lymph nodes with central hypodense areas on contrast-enhanced CT strongly suggest TB 2
• Peripheral vascularity on Doppler imaging is valuable for TB lymphadenitis diagnosis 9

Clinical Pitfalls and Caveats

Common Diagnostic Challenges

• HRCT can differentiate old fibrotic lesions from newly active tuberculous lesions, which is critical for treatment decisions 8, 6
• Minimal exudative tuberculosis can be overlooked on standard chest radiographs but detected on HRCT 6
• CT findings can predict acid-fast bacilli smear positivity, even in smear-negative patients 3, 1

Special Populations

• In severely immunocompromised patients with normal or near-normal radiographs, CT may reveal abnormal lymph nodes or subtle parenchymal disease 3
• Extensively drug-resistant TB (XDR-TB) shows more extensive consolidation and tree-in-bud appearance compared to non-XDR multidrug-resistant TB 5

Imaging Algorithm

1. Perform frontal chest radiograph as initial imaging (ACR appropriateness rating: 9/9) 3, 1
2. Proceed to CT without IV contrast if chest X-ray is equivocal, patient is immunocompromised, or classic findings are absent 3, 1
3. If radiographic findings are consistent with TB, proceed with sputum examination for definitive diagnosis 1