Pleural fluid may be a transudate or an exudate (see transudate pleural and exudative effusion) and these can be distinguished by biochemical analysis of the fluid content.

Radiological signs of a pleural effusion depend on the amount of fluid and its distribution which is affected by patient position and whether the fluid is freely mobile or loculated. Free fluid tends to collect in dependent areas, initially in the subpulmonary region beneath the lower lobes. As the volume increases, it spills into the posterior and then lateral costophrenic angles. Blunting of the lateral costophrenic angle on an erect frontal chest radiograph occurs when 25525 ml is present. However, as little as 10 ml can be detected with a lateral decubitus view. With an increasing volume of pleural fluid the hemidiaphragm outline is obscured and there is homogeneous opacification of the lower hemithorax associated with a meniscus shape of the fluid at its upper, lateral margin (Fig.1). In the supine patient, pleural fluid collects posteriorly, producing reduced transradiancy of the ipsilateral hemithorax but vascular markings are preserved, enabling differentiation from consolidation. Supine chest radiographs are relatively insensitive for pleural fluid detection. Larger volumes of fluid may produce a peripheral opacity between the lung and chest wall, particularly towards the apex of the lung and there may be thickening of the minor fissures and widening of the paraspinal soft tissues. A massive effusion causes dense opacification of the hemithorax with displacement of the mediastinum to the contralateral side and flattening or inversion of the ipsilateral hemidiaphragm. Collapse of the underlying lung such as occurs with an endobronchial malignancy or extensive pleural thickening should be considered if the mediastinum remains central. The differential diagnosis of a completely opaque hemithorax includes a massive pleural effusion, lung collapse or consolidation (V:1), massive tumour, pneumonectomy or lung agenesis. Unusual radiographic appearances may be produced if fluid lies within the pleural fissures, is loculated or is subpulmonary (see interlobar pleural effusion, loculated pleural effusion and subpulmonary pleural effusion). Fluid may collect adjacent to the mediastinum, especially on the left, producing a triangular retrocardiac opacity, simulating left lower lobe collapse which is differentiated from true collapse by the normally positioned lower lobe bronchus. right-sided pleural effusions may displace the azygoesophageal line to the left producing a retrocardiac opacity. Fluid in the azygoesophageal recess may simulate subcarinal lymphadenopathy.

Chest ultrasonography is collections are seen as lentiform opacities of fixed position. Whilst a very small effusion can be detected it may be difficult to distinguish it from pleural thickening without changing the patient's position. The attenuation value cannot reliably distinguish between different types of effusion. An acute haemothorax is often inhomogeneous and has a higher attenuation than water. Chylothorax occasionally has a low attenuation due to its fat content, but the high quantities of protein also contained within the fluid usually mean the effusion is indistinguishable from other causes. The presence of pleural thickening or enhancement of the extrapleural fat following intravenous contrast enhancement usually indicates that the effusion is an exudate.

The differentiation of pleural fluid within the posterior costophrenic recesses from ascites is aided by bare area, displaced crus and interface signs. (see bare area diaphragm sign, and interface sign).

MRI demonstrates fluid within the pleural space as low signal on T1-weighted images and high signal on T2-weighted images. Motion within the fluid creates flow artefacts and results in a heterogeneous signal intensity. Subacute and chronic haemorrhagic effusions can be recognized by their high signal on both T1- and T2-weighted images and it may be possible to differentiate between the composition of different types of pleural effusion using MRI.

Pleural effusion in the supine patient

Small amounts of fluid are not discernible in the supine chest radiograph, and at least several hundreds of ml of fluid need to be present in order to be detectable. In the supine position, the most dependent pleural spaces are the posterior basilar space and the apex of the hemithorax. Fluid in these locations may be difficult to detect on the supine chest radiographs. Technical limitations, including poor centring, rotation, underpenetration, respiratory motion and the fact that parenchymal disease is often present, also contribute to the difficulty.

The presence of fluid in the posterior basilar pleural space is first reflected by a homogenous increase in density of the lower hemithorax without obliteration of the normal bronchovascular markings (Fig.2). Loss of normal silhoutte of the hemidiaphragm, sharp or indistinct blunting of the lateral costophrenic angle (meniscus sign), elevation of the hemidiaphragm and decreased visibility of the lower lobe vascularity below the level of the hemidiaphragm are the helpful findings suggestive of ipsilateral posterior basilar fluid collection. Bilateral pleural effusions are more difficult to identify because the advantage of having a normal hemithorax for comparison is lost.

A careful search for the radiographic features of fluid in other locations, such as fissures, paraspinal region, and the apex of the hemithorax, may provide clues to the detection pleural effusion in the supine patient. subpulmonary pleural effusion is also common in intensive care unit patients. It may extend into the paramediastinal gutter, which results in triangular configuration of the paraspinal interface. Fluid in the apex of the hemithorax is recognized as a pleural cap. The appearance of intrafissural fluid depends on the shape, orientation and completeness of the fissure. Fluid within the fissures renders them more apparent and thickened on the chest radiographs.

Because the supine chest radiograph is only moderately sensitive and specific in detecting a pleural effusion, and it may miss small effusions or underestimate larger ones, a lateral decubitus radiograph with the suspected side down will identify and quantify pleural effusions in suspected cases. Chest ultrasonograph