Tuberculosis: Types, X-ray Findings & Radiological Features

Overview - Natural History

1. Primary Tuberculosis

Pathology

• The inhaled bacilli settle in the mid/lower lung zones and form the Ghon focus (area of consolidation).
• Draining lymphatics carry bacilli to the ipsilateral hilar nodes → together these form the Ghon complex (primary complex).
• In most immunocompetent individuals, the infection is controlled; the lesions undergo caseous necrosis, calcification, and scarring.
• If immunity fails to contain it → Progressive Primary TB (see below).

Chest X-ray Findings

• Middle or lower lung zone opacity/consolidation (unlike reactivation TB, which is upper lobe)
• Ipsilateral hilar/paratracheal lymphadenopathy - this is the most characteristic feature
• Possible pleural effusion
• Possible atelectasis from lymph node compression of airways

2. Progressive Primary Tuberculosis

X-ray Findings

• Rapidly enlarging pulmonary consolidation (lobar pattern)
• Cavitation can form in the primary focus
• Prominent hilar/mediastinal adenopathy
• May be indistinguishable from acute bacterial lobar pneumonia

3. Secondary (Reactivation) Tuberculosis

Pathology

• Reactivation driven by falling cell-mediated immunity (HIV, malnutrition, steroids, DM, old age, TNF inhibitors)
• Localizes to the lung apices because of higher oxygen tension there
• Hallmark: caseous necrosis with cavity formation
• Bronchogenic spread can carry infected material to other parts of the lung

Chest X-ray Findings

• Upper lobe involvement - predominantly apical and posterior segments of the right upper lobe (most common site), also apical-posterior segment of the left upper lobe
• Cavitation - thick-walled cavities, often in the upper lobes (highly characteristic)
• Fibronodular or patchy consolidation
• Volume loss and fibrosis with superior hilar retraction (old healed disease)
• Calcifications in healed lesions (Ranke complex)
• Bronchogenic spread pattern: new acinar/nodular opacities ("tree-in-bud") in ipsilateral lower lobe or contralateral lung
• Absence of hilar adenopathy (in contrast to primary TB)

4. Miliary Tuberculosis

At-risk groups

Chest X-ray Findings

• Diffuse, bilateral, uniformly distributed tiny nodules (1-3 mm) throughout both lung fields
• Nodules are randomly distributed - no zone predilection
• "Snow-storm" or "millet seed" appearance
• May be subtle early on; CT is far more sensitive

5. Tuberculosis in HIV/Immunocompromised Patients

X-ray Findings (Advanced HIV)

• Bilateral hilar lymphadenopathy (prominent, unlike immunocompetent reactivation)
• Diffuse nodular or consolidative opacities
• Lower lung zone involvement
• Cavitation uncommon
• Pleural effusion more frequent

Summary Table

Additional Radiological Notes

• Ranke complex: Calcified primary focus (Ghon focus) + calcified hilar lymph node = healed primary TB
• Simon foci: Small calcified apical nodules representing early hematogenous seeding - can reactivate years later
• CT advantages: Sensitivity 96% vs 48% for plain CXR in active TB. CT better detects lymphadenopathy, "tree-in-bud" pattern (bronchogenic spread), and cavity walls.
• "Tree-in-bud" sign on CT: Centrilobular nodules connected by branching linear densities - highly suggestive of endobronchial spread of TB.
• Activity cannot be determined from a single CXR alone - a cavity may be a sterile old lesion, and a fibrotic-appearing area may be active.