Imaging of Cavitary Lesions of the Lung

Definition

Plain Chest Radiograph (CXR)

• Thick-walled radiolucency within an area of consolidation or mass
• Air-fluid level - the hallmark of a communicating cavity; the fluid level is horizontal and may change position with posture
• On frontal view, the air-fluid level appears as a horizontal line; the level should be equal or similar in length when compared with the lateral view (a parenchymal lesion). If the air-fluid level is significantly longer on one view, this favors an empyema (pleural collection) rather than a true abscess
• The surrounding infiltrate or consolidation is common with infectious cavities
• Location: aspiration-related abscesses typically occur in the posterior segment of the upper lobes or superior segment of the lower lobes (dependent zones in a supine patient). Due to bronchial anatomy, the right lung is more commonly affected

Limitations of CXR

• Cannot reliably determine wall thickness, inner wall character, or relationship to pleura
• May miss small cavities or those obscured by consolidation
• Cannot differentiate abscess from empyema reliably

Computed Tomography (CT)

• Wall thickness and regularity
• Inner wall character
• Relationship to pleura and adjacent structures
• Associated lymphadenopathy, pleural effusion, satellite nodules

CT Features by Diagnosis

1. Lung Abscess (Infectious/Bacterial)

• Spherical or polygonal shape - similar dimensions on frontal and lateral views
• Thick, shaggy ("ragged") inner wall - highly characteristic
• Air-fluid level within the cavity
• Surrounding consolidation or ground-glass opacity (perilesional inflammation)
• Destroys rather than displaces adjacent structures and pulmonary vessels
• Does not compress adjacent lung
• Lymphadenopathy and pleural effusion may accompany severe infection

2. Malignant Cavitary Lesion (e.g. Squamous Cell Carcinoma)

• Wall thickness ≥ 16 mm strongly favors malignancy (though variable by tumour type)
• Nodular, irregular inner wall (eccentric nodularity)
• Associated lymphadenopathy and pleural effusion favor malignancy
• Squamous cell carcinoma (both primary and metastatic) cavitates more often than adenocarcinoma or small cell carcinoma
• PET scan helps assess extrathoracic disease

3. Empyema (vs. Abscess - critical distinction for management)

4. Tuberculosis

• Reactivation (post-primary) TB: cavitation is common; upper lobes (apical and posterior segments of RUL, apicoposterior segment of LUL) are the classic sites
• Cavities may be thin or thick-walled; satellite nodules ("tree-in-bud" pattern) represent bronchogenic spread
• HIV-related TB: with low CD4 counts, cavitation is less common; lower zone infiltrates and lymphadenopathy predominate
• A cavity on imaging cannot reliably indicate whether TB is active or a sterile residual of old disease from a single study - serial imaging or microbiological data is needed
• Healing results in fibrotic scarring with calcification

5. Septic Emboli

• Multiple distinct nodules, 1-3 cm in diameter, in different stages of cavitation
• Subpleural wedge-shaped infiltrates (representing infarcts)
• Bilateral and peripheral distribution
• Echocardiography is indicated to rule out valvular vegetations (endocarditis)

6. Fungal Infections

• Invasive Pulmonary Aspergillosis: early CT shows the halo sign (ground-glass opacity surrounding a nodule or mass - representing hemorrhagic infarction). As necrosis progresses, the air crescent sign appears - a crescent of air between the necrotic core and surrounding tissue, highly suggestive of invasive fungal infection. Present in 5-12.5% of cases; cannot be distinguished from mucormycosis on imaging alone
• Cryptococcus: solitary or multiple nodules, sometimes cavitating, more common in immunocompromised patients (especially HIV)
• Cavity wall in fungal disease can be thin or thick; the air-crescent sign specifically suggests liquefying fungal ball

7. Granulomatosis with Polyangiitis (GPA / Wegener's)

• Multiple bilateral cavitary nodules; autoimmune; noninfectious
• Associated with upper airway and renal disease
• No air-fluid level typically; more nodular, may wax and wane

8. Rheumatoid Nodules

• Peripheral, subpleural cavitary nodules
• May be thin-walled; may arise in the context of known RA

Additional Imaging Modalities

PET-CT

• Helpful when malignancy is suspected to assess metabolic activity and detect extrathoracic/nodal metastatic disease
• Active infections (TB, fungal) can also show FDG-avidity, limiting specificity

Ultrasound

• Limited role for parenchymal cavities; useful for guiding percutaneous drainage if abscess abuts the chest wall

Image-guided aspiration / CT-guided biopsy

• Required when non-invasive imaging cannot establish the diagnosis - provides material for culture and histology

Key Imaging Approach Summary

• Goldman-Cecil Medicine International Edition (2-Volume Set), Chapter 78: Cavitary Lung Diseases, p. 919
• Murray & Nadel's Textbook of Respiratory Medicine (2-Volume), Chapter 50: Lung Abscess; Chapter 20: Radiographic Features; Chapter 53: Tuberculosis Radiology
• Fishman's Pulmonary Diseases and Disorders (2-Volume Set): Invasive Pulmonary Aspergillosis

Imaging of Cavitary Lesions of the Lung

Source: Grainger & Allison's Diagnostic Radiology

Definition and Imaging Approach

General Radiological Features of Cavitation

1. Lung Abscess

• Rounded opacity with thick, often poorly defined walls
• Air-fluid level when the cavity communicates with a bronchus (see CXR: cavity with air-fluid level in right upper zone below)
• Surrounding consolidation/infiltrate

• Large cavity with a thick, irregular ("shaggy") inner wall with intracavitary thick septa
• Air-fluid level within the cavity
• Surrounding parenchymal consolidation, ground-glass opacity
• Primarily round/spherical shape - unlike empyema
• Does not compress adjacent lung; instead destroys it

• Multiple bilateral nodules, typically 1-3 cm, in different stages of cavitation
• A characteristic CT sign is the "feeding vessel" sign - a vessel coursing directly into a cavitating nodule
• Associated subpleural wedge infarcts

2. Necrotising / Cavitary Pneumonia

• Staphylococcus aureus is the classic organism: necrotising cavitary pneumonia, bilateral in ~40%, with abscess formation (15-30% of cases), pneumatoceles (thin-walled gas-filled cysts, classic sequelae), and pleural effusions (30-50%)
• Klebsiella pneumoniae: lobar consolidation with expansion of the lobe ("bulging fissure" sign), rapidly progressing to cavitation and necrosis
• Pseudomonas aeruginosa (most common nosocomial): multifocal upper lobe consolidation, large random nodules, tree-in-bud, ground-glass, necrosis
• Pulmonary gangrene: rare complication - initially small lucencies within consolidated lung; development of fragments of necrotic lung (pulmonary sequestrum) within an abscess cavity
• On contrast-enhanced CT: areas of decreased or absent enhancement indicate ischaemia/infarction within the consolidation, signifying necrotising pneumonia

• Pneumatoceles: thin-walled, no wall enhancement, anatomically located within the previous pneumonia zone
• Abscess: thick wall with enhancement

3. Empyema (vs. Lung Abscess - Critical Distinction)

4. Tuberculosis

Primary TB

• Homogeneous lobar consolidation (any lobe) with ipsilateral hilar/mediastinal lymphadenopathy (most common manifestation)
• Ghon focus + ipsilateral calcified node = Ranke complex
• Cavitation is rare in uncomplicated primary TB but may occur (progressive primary TB) in one-third of untreated cases

Post-Primary (Reactivation) TB

• Predilection for upper lobes (apical and posterior segments) - absence of lymphadenopathy + upper lobe cavitation distinguishes this from primary TB
• Cavitation indicates active disease: present in 40%-80% of cases of post-primary TB
• Air-fluid levels are unusual but seen in up to 20% of cases
• Rasmussen aneurysm: contrast-enhanced CT shows a contrast-filling aneurysm within parenchymal consolidation adjacent to a cavity - rare but life-threatening (granulomatous weakening of a pulmonary arterial wall)
• "Tree-in-bud" pattern: reflects endobronchial spread; centrilobular branching opacities in the same or contralateral lung
• Healing results in scar formation, severe volume loss, pleural thickening; residual thin-walled cavities may persist in both active and inactive disease
• A tuberculoma may be present with satellite nodules in up to 80% of cases

5. Malignant Cavitary Lesions

Primary Bronchogenic Carcinoma

• Cavitation occurs in tumours of any size and is best shown on CT
• Squamous cell carcinoma is the most likely cell type to cavitate
• Wall is typically ≥ 8 mm thick with irregular thickness; may contain tumour nodules projecting into the lumen
• Sometimes walls can have smooth inner and outer margins
• Fluid levels are common

Cavitating Pulmonary Metastases

• Cavitation is seen more commonly in squamous cell than in adenocarcinoma metastases
• Multiple nodules at various stages of cavitation; bilateral distribution

6. Fungal Infections

Invasive Pulmonary Aspergillosis / Mucormycosis (Immunocompromised Host)

• "Halo sign": nodule or mass surrounded by a halo of ground-glass attenuation (represents haemorrhagic infarction around the fungal lesion)
• "Air-crescent sign": crescent of air between the necrotic fungal core and surrounding tissue; highly suggestive of invasive fungal infection; seen in 5%-12.5% of cases; cannot distinguish aspergillosis from mucormycosis on imaging alone
• Mucormycosis: lobar/multilobar consolidation, nodules, cavitation in 26%-40% of cases

Cryptococcosis

• Masses ranging from 5 mm to very large; may show a halo sign and eventually cavitate
• Seen in AIDS patients (CD4 < 100 cells/mm³)

Histoplasmosis

• Chronic pulmonary histoplasmosis radiologically resembles post-primary tuberculosis: upper lobe contraction, calcification, and cavitation

Coccidioidomycosis

• Cavitation with hilar/mediastinal adenopathy in ~20% of primary lesions
• Characteristic CT: central soft-tissue attenuation with surrounding halo of ground-glass around nodules
• Chronic fibronodular cavitary disease resembles post-primary TB

Paracoccidioidomycosis

• HRCT: ground-glass opacities, nodules, "halo" and "reversed halo" signs, consolidation, cavitation, fibrosis

7. Bronchopleural Fistula

• Sinus tract between bronchus and pleural space
• CT shows communication between a parenchymal cavity and pleural space
• Pneumothorax or hydropneumothorax with air in pleural space
• May result from necrotising pneumonia, abscess rupture, surgery, or malignancy

Differential Diagnosis Summary (Causes of Solitary Pulmonary Mass/Cavity)

• Bronchial carcinoma / carcinoid
• Granuloma (TB, fungal)
• Hamartoma (cavitation is almost unknown)
• Metastasis
• Chronic pneumonia or abscess
• Hydatid cyst
• Pulmonary haematoma
• Fungus ball (aspergilloma in pre-existing cavity)
• Bronchogenic cyst / sequestration
• Arteriovenous malformation / pulmonary infarct

Pulmonary Hydatid Disease - Radiology

Source: Grainger & Allison's Diagnostic Radiology

Aetiology & Background

Cyst Structure

• Pericyst (outer host-derived fibrous layer)
• Laminated (ectocyst) membrane with inner germinal (endocyst) layer

Epidemiology

• Usually solitary
• Multiple and/or bilateral in 10% of cases
• At the time of presentation: two-thirds are ruptured, one-third unruptured
• Size varies from very small to very large

Radiological Features - Classified by Stage

Stage 1 - Intact / Unruptured Cyst

• Well-defined, rounded or oval homogeneous opacity - often with completely smooth margins
• The intact cyst appears as a simple mass indistinguishable from a solid tumour or benign mass on CXR
• No calcification (unlike hepatic hydatid which may calcify)
• No satellite lesions
• May cause adjacent compression of lung and diaphragm elevation with large cysts

• Well-defined, round or oval cystic lesion with a thin, smooth wall
• Water-density (0 HU) content - pure fluid inside the cyst
• The pericyst wall may be visible as a thin rim
• Internal daughter cysts may be seen with E. multilocularis (alveolar type - more complex, infiltrative)

Stage 2 - Early Communication (Pericyst Erosion / Impending Rupture)

• "Meniscus sign" (also called the "crescent sign"): a fine radiolucent crescent of air appears at the upper pole of the cyst between the outer pericyst and the inner laminated membrane
• This is considered a sign of impending rupture

Stage 3 - Ruptured Cyst (Endocyst Rupture into Bronchus)

(A) Water-Lily Sign (Camalote Sign / Nymphaea Sign)

• The crumpled, collapsed endocyst floats on residual fluid at the bottom of the cyst cavity
• On CXR/CT it resembles a water lily (lotus leaf) or floating membrane above an air-fluid level
• Best demonstrated on CT where the floating membrane is clearly visible
• Pathognomonic of a collapsed (ruptured) pulmonary hydatid cyst

(B) Air-Fluid Level

• A horizontal air-fluid level within the now-communicating cyst cavity
• Indicates bronchial communication has occurred and cyst contents are being partially expectorated

(C) Double Wall Sign

• Both the pericyst (outer) and the laminated membrane (inner) are visible as two distinct concentric rings on CT
• The space between them filled with air

(D) Rising Sun Sign / Serpent Sign

• When most fluid has been expectorated, the crumpled membranes lie at the bottom of the cavity
• Rising sun sign: the membrane appears along the lower arc of the cyst
• Serpent sign: the floating membrane has a wavy, snake-like configuration on CT

(E) Empty Cyst Sign

• After complete expectoration of all cyst contents, only an empty collapsed cavity remains
• This is the "empty cyst" or "ghost cyst" appearance - the wall may be visible as a thin ring

Stage 4 - Complications

1. Secondary Infection

• Secondary bacterial infection of a ruptured or partially evacuated hydatid cyst produces a lung abscess
• Appears as a thick-walled cavity with surrounding consolidation and air-fluid level, indistinguishable from a primary pyogenic abscess without clinical context

2. Pleural Rupture

• Rupture into the pleural space causes:
  • Pleural effusion (hydatid fluid + daughter cysts in pleural space)
  • If there is also airway communication: hydropneumothorax
  • Risk of anaphylaxis from spillage of hydatid fluid

3. Massive Haemoptysis / Haemorrhage

• Aggressive invasion of bronchial or pulmonary arteries can result in massive haemoptysis

Classic Radiological Signs - Summary Table

Imaging Modality Comparison