Pleural effusion on CT

Objectives and capabilities of CT diagnostics

Although radiography and ultrasound remain primary modalities, computed tomography is indispensable in complex clinical cases. Intravenous contrast administration significantly increases the diagnostic value of the study.

Key capabilities of CT include:

• Reliable differentiation of free fluid from loculated (loculated) effusion.
• Precise spatial localization of loculated effusion prior to surgical drainage.
• Differentiation between diseases of the pleura itself and the lung parenchyma.
• Detection of hidden thoracic or abdominal pathology causing the fluid accumulation (e.g., subdiaphragmatic abscess or primary tumor).
• Visual navigation for safe percutaneous aspiration or biopsy.

CT semiotics of fluid accumulations

Free pleural effusion is visualized on tomograms as a crescent-shaped opacity located in dependent areas of the thorax due to gravity. The X-ray attenuation (CT number) of fluid is always lower than that of the adjacent thickened pleura or solid mass.

It is important to remember that effusion density can be misleading. For example, in chylothorax (an accumulation of lymph containing fat), CT attenuation is not reduced due to the concomitantly high protein content.

Specific forms of effusion:

1. Loculated effusion: forms a limited accumulation with a lenticular configuration (lenticular configuration). It has smooth, distinct contours and gently displaces the adjacent lung tissue without destroying it.
2. Massive effusion: occupies the majority of the hemithorax. Coronal reconstructions clearly demonstrate how the fluid displaces mediastinal organs to the opposite side, flattens (depresses) the diaphragmatic dome, and leads to extensive compressive atelectasis of the adjacent lung.

Pleural empyema vs Lung abscess

One of the most important indications for CT is the differential diagnosis between parapneumonic empyema (purulent exudate in the pleural cavity) and abscess (purulent destruction of the lung tissue itself).

Empyema is characterized by the pathognomonic “split-pleura” sign (split-pleura sign). It manifests as marked thickening and intense contrast enhancement of both the parietal and visceral pleura. The subpleural adipose tissue is often infiltrated and expanded due to edema, while the adjacent lung undergoes passive compression.

In contrast, lung abscess is characterized by destruction of the pulmonary parenchyma and the presence of a thick, irregular, and uneven wall of the abscess cavity itself.

Assessment of pleural thickening

When effusion is detected, the diagnostician must thoroughly examine the pleura throughout its extent. The morphology of its thickening is the key to determining the nature of the disease.

• Benign thickening: typically appears uniform and smooth. It occurs in infectious processes and as a sequela of prolonged inflammation.
• Malignant thickening: manifests with irregular, nodular contours and focal masses. The “split-pleura” sign in the absence of overt infection should also raise suspicion for malignant effusion. CT allows for the visualization of the primary tumor (e.g., breast or lung cancer) and metastatic lymph node involvement.

Class and exam questions

What else to study on this topic

• Radiological signs of pleural effusion in the supine position
• Ultrasound differentiation of pleural fluid and solid masses
• Stages of pleural empyema development and their imaging appearance
• Etiology and diagnosis of chylothorax
• Specifics of pleural effusions in renal disease and HIV infection

Other topics in the «Chest Radiology» section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.