Pleural effusion: fluid accumulation in the pleural spaces. This occurs when the rate of production exceeds its rates of re-absorption, causes:

1- Increased microvascular pressure in lungs (HF).
2- Decreased plasma oncotic pressure (e.g. hypoprotienaemia).
3- Increased microvascular permeability (e.g. pleurisy).
4- Reduced lymphatic drainage from pleural space (e.g. lymphangitis).
5- Passage of peritoneal fluid across defects in diaphragms.

Pleural effusions may be transudate, exudates, pus, blood or chyle. Radio logically these produce similar shadow and therefore, indistinguishable. However, there may be clinical data to point to the etiology, or chest film may show other abnormalities, such as evidence of HF or trauma.
1- Transudate: contain less than 3 gm/dl of protein, or the plasma protein/ pleural protein < 0.5 or less than 70% 0f of serum protein level. Usually clear or faintly yellow, watery fluid. Pleural transudate may be called a hydrothorax. They are often bilateral. Commonest cases:
- Heart failure, effusions usually accumulate first on right side before becoming bilateral. Other causes: hypoprotienaemia (e.g. nephrotic syndrome, liver cirrhosis and anaemia), constrictive pericarditis, Meigs syndrome and myxoedema.
2- Exudates: contain more than 3 gm/dl of protein, or the plasma protein/ pleural protein > 0.5 or more than 70% 0f of serum protein level. It vary in color from amber, slightly cloudy, (which often clot on standing), to frank pus. A purulent pleural effusion is termed empyaema. The commonest causes of pleural exudates are bacterial pneumonia, pulmonary TB, carcinoma of lung, metastatic malignancy and pulmonary infarction. Less common causes are: Subphrenic infection, connective tissue diseases, (especially SLE and Rh.arthritis), and non bacterial pneumonias:
Unusual causes include: post myocardial infarction, acute pancreatitis and primary pleural neoplasm.

Radio logical findings:
1- Free fluid: pleural fluid casts a shadow of density of water or soft tissue on chest radiography. In the obscene of pleural adhesions, the position and shape of this shadow will depend upon the amount of fluid, state of underlying lung and patient position. The most dependent recess of the pleura is the posterior costophrenic angle.

CXR:
• > 200 ml fluid required to be visible on PA CXR.
• > 75 ml fluid required to be visible on lat. CXR.
• As little as 25 ml fluid detected on lateral decubitus x-ray.
• US and/or CT scan can detect small amount of pleural effusions.
• The fluid, as increased in volume, extend upward around posterior > lateral > anterior thoracic wall as the mediastinal portion fixed by pulmonary Lig. + Hilum), forming meniscus – shaped semicircular (concave upper edge), higher laterally than medially obscuring the diaphragmatic shadow.
• Associated collapse of ipsilateral lung (relaxation type).
• Massive pleural effusion:
- Radiopacity of entire hemithorax.
- Displacent of mediastinum to contralateral side.
- Severe depression / flattening / inversion of ipsilateral hemidiaphragm.
The majority of massive unilateral pleural effusions are malignant (e.g. lymphoma, metastatic disease, primary lung CA).

2- subpulmonic/ subdiaphragmatic/ infrapulmonary pleural effusion:
- lateral Displacent of the peak of hemidiaphragm.
- Increased distance between the stomach bubble and lung.
- Blunted posterior costophrenic angle (on lat. CXR).
- CT Scan:
- Fluid outside the diaphragm.
- Fluid elevating the crus of diaphragm.

3-Lamellar effusion: are shallow collections between the lung surface and the visceral pleura, some types sparing the costophrenic angle: lamellar effusions represent interstitial pulmonary edema.
4-Loculated fluid: the pleural space may be partially obliterated by pleural disease, causing adhesions of the visceral and parietal pleural layers. Encapsulated fluid can be distinguished from free fluid by gravitational methods, but it may be difficult to differentiated from extrapleural opacity, parenchymal lung disease, or mediastinal mass.

Radiological findings:
- Loculated fluid has little depth (height), but considerable width rather like a biconvex lens. (Best assessed by fluoroscope).
- Fluid may become loculated in one or more interlobar fissure, most often seen in heart failure, the opacity may disappear rapidly following treatment of the cause, and hence known as pseudo- or vanishing tumor, they may recur in the subsequent episode of heart failure.