[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Radiology](https://durgical.com/en/radiology/) › [Chest](https://durgical.com/en/radiology/areas/chest/) › Cavities and air-fluid levels in the lung # Cavities and air-fluid levels in the lung - Finding A cavity is a hole with a wall inside the lung, where tissue has died and drained; it may hold air and a flat fluid line. Updated Nov 13, 2026 ## Key points - Air-fluid level needs a horizontal beam: upright or decubitus film. - Thick wall + air-fluid level after aspiration = abscess; upper-lobe cavity = tuberculosis until proven otherwise. - Abscess: round, same-length level on both views; empyema: lens-shaped, different-length levels. - Thick irregular wall in a smoker: think squamous cell carcinoma. ## What it is A cavity is a gas-filled space inside an area of consolidation, a mass or a nodule, formed when the centre dies (necrosis) and drains through a bronchus. On the chest X-ray it looks like a dark, rounded lucency surrounded by a wall of soft-tissue density; if it holds pus or fluid as well, an upright or decubitus film shows a straight horizontal air-fluid level. A cyst differs by its thin, smooth wall. The commonest causes are infection (lung abscess, tuberculosis, necrotising pneumonia, fungi, septic emboli) and tumour (especially squamous cell carcinoma), with autoimmune disease and infarcts less often. The wall, the number of cavities, their location and the clinical story narrow the list: a thick-walled cavity with an air-fluid level in a dependent segment after aspiration suggests an abscess; upper-lobe cavities with weight loss and cough suggest tuberculosis, which in Colombia must always be considered and isolated for. ## What it looks like - Cavity: a dark lucency with a definite wall inside an opacity; a cyst has a thin, smooth wall. - Air-fluid level: a straight horizontal line, seen only with a horizontal beam (upright or decubitus), not on a supine film. - Lung abscess: thick-walled, often with an air-fluid level; in aspiration, the posterior upper-lobe or superior lower-lobe segments, more often right. - Tuberculosis reactivation: apical and posterior segments of the upper lobes and superior segments of the lower lobes. - Thick, irregular, nodular wall favours cancer; a thin, smooth wall favours a benign cause. - Septic emboli: several peripheral nodules that cavitate, often of different ages. ## Why it looks that way - Infection or tumour outgrows its blood supply; the dead centre liquefies and empties into a bronchus, leaving air and some fluid. - Fluid lies at the bottom and air rises, so a horizontal beam shows a flat line between them. ## Differential and pitfalls - Abscess vs empyema: an abscess is round, with an air-fluid level of the same length on PA and lateral views and an acute angle with the chest wall. - An empyema with air is lens-shaped along the chest wall, its air-fluid level differs in length between views and it meets the wall at an obtuse angle. - Other causes: granulomatosis with polyangiitis, rheumatoid nodules, pulmonary infarct, fungi (an aspergilloma is a fungus ball inside an old cavity). - Mimics: bullae and cysts (thin walls, no fluid), a hiatus hernia behind the heart with its own air-fluid level, overlapping bowel. - In children and after trauma: pneumatoceles (thin-walled, often transient) and congenital lung lesions. - A cavity in a smoker who does not respond to antibiotics is a cancer until proven otherwise. ## Next step - Suspected tuberculosis: airborne isolation, sputum for acid-fast bacilli, a molecular test and culture, and an HIV test. - Suspected abscess: blood cultures and sputum culture; CT of the chest to confirm a cavity and separate it from an empyema. - Multiple cavitating nodules with fever: blood cultures and echocardiography to look for right-sided endocarditis. - Empyema on CT: refer for pleural drainage (chest tube or surgery); an abscess rarely needs drainage. - No response to treatment, a thick nodular wall or a smoker: CT and bronchoscopy, or referral to pulmonology for tissue diagnosis. ## Memory aid CAVITY: Cancer, Autoimmune (granulomatosis with polyangiitis, rheumatoid), Vascular (septic emboli, infarct), Infection (abscess, tuberculosis, fungi), Trauma (pneumatocele), Young (congenital lesions). ## Test yourself Think of the answer, then tap the question to see it. ## Why is an air-fluid level not seen on a supine chest X-ray? The line between fluid and air is horizontal; only a horizontal beam (upright or decubitus film) runs along it and shows it as a straight edge. ## How do you tell a lung abscess from an empyema with air on PA and lateral films? An abscess is round with an air-fluid level of the same length on both views; an empyema is lens-shaped and its level differs in length between views. ## Where do tuberculosis reactivation cavities usually lie? In the apical and posterior segments of the upper lobes and the superior segments of the lower lobes. ## Which lung cancer type most often cavitates? Squamous cell carcinoma, typically with a thick, irregular wall. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/radiology/cavities-and-air-fluid-levels-in-the-lung/) ## Elsewhere in DurgiPedia - [Cavitation syndrome](https://durgical.com/en/clinical-exam/cavitation-syndrome/) ## Connections ## Related 5 - [Consolidation and air bronchograms](https://durgical.com/en/radiology/consolidation-and-air-bronchograms/) - [A nodule or a mass on the chest X-ray](https://durgical.com/en/radiology/a-nodule-or-a-mass-on-the-chest-x-ray/) - [High-resolution CT patterns: ground glass, crazy paving, tree-in-bud](https://durgical.com/en/radiology/high-resolution-ct-patterns-ground-glass-crazy-paving-tree-in-bud/) - [Pleural effusion on the chest X-ray](https://durgical.com/en/radiology/pleural-effusion-on-the-chest-x-ray/) - [Reading a chest CT step by step](https://durgical.com/en/radiology/reading-a-chest-ct-step-by-step/) ## Related conditions 5 - [Pulmonary tuberculosis](https://durgical.com/en/conditions/pulmonary-tuberculosis/) - [Lung abscess](https://durgical.com/en/conditions/lung-abscess/) - [Aspiration pneumonia and pneumonitis](https://durgical.com/en/conditions/aspiration-pneumonia-and-pneumonitis/) - [Lung cancer](https://durgical.com/en/conditions/lung-cancer/) - [Aspergillosis](https://durgical.com/en/conditions/aspergillosis/) [← All of radiology](https://durgical.com/en/radiology/) Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols. Sources: [Lung Abscess](https://www.ncbi.nlm.nih.gov/books/NBK555920/) (StatPearls (NCBI Bookshelf), 2024); [Chest and Mediastinal Imaging](https://www.ncbi.nlm.nih.gov/books/NBK559145/) (StatPearls (NCBI Bookshelf), 2026). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/radiology/cavities-and-air-fluid-levels-in-the-lung/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/radiology/cavities-and-air-fluid-levels-in-the-lung/