Imaging of pleural effusions in adults

INTRODUCTION — Detection of pleural effusion(s) and the creation of an initial differential diagnosis are highly dependent upon imaging of the pleural space. Conventional chest radiography and computed tomography (CT) scanning are the primary imaging modalities that are used for evaluation of all types of pleural disease, but ultrasound and magnetic resonance imaging (MRI) have a role in selected clinical circumstances.

The imaging of pleural effusions will be presented here. Imaging of pleural plaques, thickening, tumors, and pneumothorax are discussed separately. (See "Imaging of pleural plaques, thickening, and tumors".)

NORMAL PLEURAL ANATOMY — The term pleura is generally meant to encompass the parietal pleura (lining the inner surface of the chest wall, including the diaphragmatic pleura and the cervical pleura also called dome of pleura or pleural cupola that covers the lung apex and extends into the cervical region), the visceral pleura (lining the outer surface of the lung), and the intervening pleural space.

The parietal and visceral pleura merge at the pulmonary hilum and thus separate the thoracic cavity into two separate hemithoraces [1]. Both visceral and parietal pleural surfaces consist of a mesothelial layer and three to seven connective tissue layers, but the visceral pleura is thicker than the parietal pleura. Together, the visceral and parietal pleural layers and the lubricating liquid in the interposed pleural space (10 to 15 mL per hemithorax) have a combined thickness of 0.2 to 0.4 mm, while the width of the pleural space is 10 to 20 micrometers.

The pleura is drained by a visceral and parietal lymphatic network. The parietal lymphatic pleural network is the main pathway of pleural liquid resorption and communicates with the parietal pleura through gaps in the pleural mesothelium forming stomata, named Kampmeyer foci [2]. (See "Mechanisms of pleural liquid turnover in the normal state".)

Normal pleural anatomy can be displayed by computed tomography (CT) scanning [3]. A 1 to 2 mm thick line of soft-tissue attenuation can be seen at the point of contact between the lung and the chest wall, corresponding to the visceral and parietal pleura and the minimal amount of lubricating pleural liquid (image 1).

Extrapleural fat and the endothoracic fascia, each with a thickness of 0.25 mm, are visible between the pleural line and the ribs (or the transverse thoracic muscle anteriorly, subcostal muscle posteriorly, and innermost intercostal muscles laterally), and together form the so-called intercostal stripe and paravertebral line [4].

The apical part of the endothoracic fascia is thickened and is called Sibson's fascia. Outside this fascia is a space filled with areolar tissue, called Semb's space. The anterior and posterior junction lines are well outlined by lung and contain four layers of pleura: two visceral and two parietal components (image 2A-B). The interlobar fissures and most accessory fissures in the lungs are formed by two layers of visceral pleura, with the exception of the azygos vein fissure, which contains four layers of pleura, ie, two visceral and two parietal layers of pleura.

CONVENTIONAL RADIOGRAPHY — Abnormalities of the pleural space can easily be detected by conventional radiographic methods using frontal, lateral, oblique, and decubitus radiographs. Pleural effusions accumulate in the most dependent part of the thoracic cavity because the lung, which is physically less dense than liquid, floats on the effusion. The otherwise normal lung will follow its intrinsic elastic recoil and decrease in volume while maintaining its shape during collapse.

Because of gravity, pleural liquid initially accumulates in a subpulmonic location, ie, between the inferior surface of the lower lobes and the diaphragm [5]. Up to 75 mL of pleural effusion can occupy the subpulmonic space without spillover. As it accumulates, pleural liquid spills over into the costophrenic sulcus posteriorly, anteriorly, and laterally. It surrounds the lung and forms a cloak, or cylinder, which looks like a meniscoid arc in radiographic projections.

The amount of pleural effusion can be estimated based on standard frontal and lateral radiographs. At least 75 mL are needed to obliterate the posterior costophrenic sulcus, and a minimum of 175 mL is necessary to obscure the lateral costophrenic sulcus on an upright chest radiograph [6]. A pleural effusion of 500 mL will obscure the diaphragmatic contour on an upright chest radiograph; if the pleural effusion reaches the level of the fourth anterior rib, close to 1000 mL are present. On decubitus radiographs and computed tomography (CT) scans, less than 10 mL, and possibly as little as 2 mL, can be identified (image 3) [7].

For quantitation on decubitus views, the rind of layering pleural effusion is measured: small effusions are thinner than 1.5 cm, moderate effusions are 1.5 to 4.5 cm thick, and large effusions exceed 4.5 cm. Effusions thicker than 1 cm are usually large enough for sampling by thoracentesis, since at least 200 mL of liquid are already present [7]. (See "Ultrasound-guided thoracentesis".)

On supine radiographs, as little as 175 mL of effusion can be visible [8], sometimes forming apical caps which disappear on upright imaging. Mobile effusions also layer along the posterior aspect of the thorax in the supine position and produce a filter effect or pleural veil that overlies the aerated lung [9]; a gradient of decreasing opacity towards the apex can be identified. The following features suggest that this appearance in the supine patient is due to an effusion, as opposed to parenchymal lung disease (such as pneumonia or pulmonary edema):

●Pulmonary vessels are clearly visible through the added opacity created by the effusion

●Air bronchograms are absent

Subpulmonic effusions — Subpulmonic pleural effusions elevate the lung base, mimicking an elevated hemidiaphragm (image 4A-B). The apex of the curvature at the lung base is shifted laterally, and its slope slants sharply towards the lateral costophrenic sulcus [10]. This configuration has been dubbed "Rock of Gibraltar sign" and is particularly well seen on the lateral chest radiograph in patients with a subpulmonic pleural effusion (image 5 and image 6) [11]. Large pleural effusions, especially on the left side, can produce diaphragmatic inversion, making the normally convex diaphragm appear concave. On the right side, the depressed, inverted diaphragm displaces the liver caudad. This configuration can lead to paradoxical breathing on the affected side with the hemidiaphragm rising on inspiration and descending on expiration.

On the left side, a marked separation (>2 cm) of the lung from the stomach bubble suggests a subpulmonic effusion. This separation of the stomach gas bubble from the lung base, especially when the bubble appears displaced inferomedially, is of particular importance on the frontal and lateral views [12].

Atypical localization of a pleural effusion generally results from an abnormality in the underlying lung. When the lung cannot expand to fill the thoracic cavity, the relative pleural pressure becomes more negative relative to the atmospheric pressure. The increased negative pleural pressure enhances pleural liquid formation, leading to the accumulation of pleural effusions which accumulate in these areas subtended by lung with the greatest elastic recoil [10,13]. (See "Diagnosis and management of pleural causes of nonexpandable lung".)

Loculated pleural effusions — Pleural effusions can also loculate as a result of adhesions. Loculation (or encapsulation) is most common when the underlying effusion is due to hemothorax, pyothorax, chylothorax, or tuberculous pleuritis. A typical configuration of a loculation along the chest wall, often described as a pleural or extrapleural sign, has the following features (image 7 and image 8) [6]:

●The angles of interface between the pleural "mass" and the chest wall are obtuse, and the mass displays tapered borders

●The surface of the "mass" is usually smooth when seen in tangent, poorly marginated when seen "en face," and only partially visualized when displayed in an oblique projection ("incomplete margin sign") also called "one-edged lesions" [14-16]

●The content is homogeneous

●The "mass" droops on upright images owing to its liquid content and the effect of gravity

COMPUTED TOMOGRAPHY — Computed tomography (CT) detects small pleural effusions, ie, less than 10 mL and possibly as little as 2 mL of liquid in the pleural space. Thickening of the visceral and parietal pleura as well as enhancement of the visceral and parietal pleura after injection of intravenous contrast material (the "split pleura sign") suggest the presence of inflammation and thus an exudative, rather than transudative, effusion [17]. The administration of intravenous contrast material in patients with pleural abnormalities is important, because it facilitates the differential diagnosis of pleural effusions.

Other uses of CT scanning in the evaluation of pleural disease include [18-23]:

●Facilitating measurement of pleural thickness

●Distinguishing an empyema from a lung abscess

●Visualizing small pneumothoraces in supine patients

●Visualizing underlying lung parenchymal processes that are obscured on the chest radiograph by a large pleural effusion

●Determining the exact location of pleural masses and characterization of their composition (see "Imaging of pleural plaques, thickening, and tumors")

●Occasionally identifying peripheral bronchopleural fistulae

●Occasionally identifying a diaphragmatic defect in a cirrhotic patient with hepatic hydrothorax

●Identifying lung parenchymal or upper abdominal abnormalities that may provide a clue to the etiology of the pleural effusion (eg, lung mass, apical cavities, aortic dissection, infradiaphragmatic abscess, liver cirrhosis with ascites leading to hepatic hydrothorax)

●Guiding thoracentesis and tube thoracostomy of loculated empyemas (see "Epidemiology, clinical presentation, and diagnostic evaluation of parapneumonic effusion and empyema in adults", section on 'Diagnostic imaging' and "Epidemiology, clinical presentation, and diagnostic evaluation of parapneumonic effusion and empyema in adults")

ULTRASONOGRAPHY — Ultrasonography permits easy identification of free or loculated pleural effusions, and it facilitates differentiation of loculated effusions from solid masses [24]. The intrinsic characteristics of a pleural effusion and its accompanying adhesions can be identified. (See "Bedside pleural ultrasonography: Equipment, technique, and the identification of pleural effusion and pneumothorax".)

Thoracentesis of loculated pleural effusions is facilitated by ultrasound guidance. However, computed tomography (CT) is the method of choice for more complicated interventional procedures, such as empyema drainage or biopsy of pleural masses. (See "Ultrasound-guided thoracentesis" and "Medical thoracoscopy (pleuroscopy): Equipment, procedure, and complications".)

MAGNETIC RESONANCE IMAGING — Magnetic resonance imaging (MRI) can display pleural effusions, pleural tumors, and chest wall invasion [25]. In selected cases, it can characterize the content of pleural effusions [26,27]. The role of MRI in the imaging of hemothorax is discussed below; other aspects of thoracic MRI are presented separately. (See "Magnetic resonance imaging of the thorax" and "Principles of magnetic resonance imaging".)

FDG-PET SCANNING — This modality has shown only modest accuracy in discriminating malignant from benign pleural effusions. While it can differentiate between exudative and transudative pleural effusions, it is not routinely recommended to facilitate the distinction between benign and malignant pleural effusions [28]. Rarely, if the pleural effusion is accompanied by a solid pleural component, some value may be found in performing positron emission tomography (PET) in individual patients. Technical details and indications for thoracic PET are discussed in detail separately.

TRANSUDATIVE PLEURAL EFFUSIONS — The most common cause of a transudative pleural effusion is left ventricular failure. Pulmonary edema liquid permeates the lung interstitium and the visceral pleura, eventually accumulating in the pleural space, in order to be resorbed by the lymphatics of the parietal pleural [29]. Pleural effusions related to left ventricular failure are bilateral in nearly 90 percent of cases [6]. Other causes of transudative pleural effusion include constrictive pericarditis, hepatic cirrhosis (image 9), and renal failure (table 1). In general, transudative pleural effusions are the product of imbalanced hydrostatic forces. (See "Pleural fluid analysis in adults with a pleural effusion".)

Occasionally these pleural effusions loculate and mimic masses, particularly in the interlobar fissures; these have also been called pseudotumors or vanishing tumors (image 7) [10]. Computed tomography (CT) scanning can sometimes determine the true nature of such a mass by showing its liquid content and its relationship to the fissures, thereby excluding an intrapulmonary origin.

In rare instances, patients have bilateral pleural effusions with markedly different characteristics (image 10 and image 11). This situation is called "Contarini's condition," named after the 95^th Doge of Venice, who died of cardiac decompensation with a unilateral transudative pleural effusion and a contralateral empyema caused by necrotizing pneumonia [30]. CT scanning in such situations can identify small collections of gas, loculations, or pleural thickening and pleural enhancement or the so-called split pleura sign in an empyema, characteristics not found in a transudative pleural effusion.

Hepatic hydrothorax — Hepatic hydrothorax is defined as a pleural effusion, usually greater than 500 mL, in patients with cirrhosis, but without primary cardiac, pulmonary, or pleural disease. The majority occur in the right hemithorax (85 percent). The evaluation of suspected hepatic hydrothorax is discussed separately. (See "Hepatic hydrothorax", section on 'Diagnosis'.)

Demons-Meigs syndrome indicates the association of a right-sided pleural effusion with ascites and a large ovarian fibroma. It is a rare occurrence but can mimic some of the features of hepatic hydrothorax due to its association with ascites and preferential accumulation of pleural effusion in the right hemithorax [31].

EXUDATIVE PLEURAL EFFUSIONS — The most common conditions leading to an exudative pleural effusion are pneumonia (resulting in a sterile parapneumonic effusion or an empyema) and malignant tumors (table 2). Large unilateral exudative pleural effusions in young patients are suspicious for tuberculosis (image 12A-B), whereas in older individuals they frequently indicate a malignant process (image 13) [32]. (See "Pleural fluid analysis in adults with a pleural effusion".)

Empyema — The vast majority of empyemas are due to pulmonary infections; surgical procedures and trauma are other common causes. Empyemas are most often due to streptococcus species, anaerobic bacteria (bacteroides and peptostreptococcus), or mixed aerobic-anaerobic flora [33]. Aerobic bacteria like methicillin-resistant staphylococcus and gram-negative bacteria (eg, Enterobacter) are found in hospital-acquired empyemas; tuberculous mycobacteria and fungi are less frequent causative agents (image 14A-B and image 15 and image 16) [34]. (See "Epidemiology, clinical presentation, and diagnostic evaluation of parapneumonic effusion and empyema in adults".)

The radiologic diagnosis of empyema can be facilitated by computed tomography (CT) scanning. Three stages are recognized in the evolution of empyemas:

●Stage 1 consists of an exudative pleural effusion that contains more than 15,000 leukocytes per microliter

●Stage 2 is a fibrinopurulent stage in which adhesions have already formed

●Stage 3 is the organizing stage, with development of a thick pleural peel

The effusion can be easily drained in stage 1; in contrast, decortication may be required in stages 2 and 3. Ultrasound is able to image early adhesions during the fibrinopurulent stage of an empyema (image 17 and image 18). Linear, irregular, honeycomb-like adhesions predict difficulties in drainage.

In the early, exudative stage of an empyema, the pleural effusion appears on radiography to be freely layering. When the effusion becomes loculated, it forms tapered borders with obtuse angles at its interface with the chest wall, often showing gravity dependent changes in shape, such as "drooping" (image 19A-D and image 20) and on occasion the "incomplete margin" sign. (See 'Loculated pleural effusions' above.)

In the fibrinopurulent and organizing stages, an intravenous contrast material enhanced CT scan shows strong enhancement of the visceral and parietal pleurae, producing the "split pleura sign" (image 21A-B) [17,19]. The pleura is also frequently thickened, exceeding 3 to 5 mm [35].

Empyemas tend to compress the adjacent lung rather than destroy it, thereby allowing differentiation from large lung abscesses [19]. In addition, empyemas typically have thinner, smoother walls than lung abscesses, which tend to have thicker walls and irregular luminal and exterior surfaces. Empyemas tend to form an obtuse angle of interface with the chest wall, compared with lung abscesses, which commonly have an acute angle. However, the angle of interface is probably less useful for differentiation of empyema from lung abscess than the thickness and uniformity of the wall and the effect on adjacent vascular structures, particularly with very large juxtapleural abscesses that abut the chest wall and can, on rare occasions, also form obtuse angles of interface with the chest wall.

The finding of a gas-liquid level in an empyema indicates the presence of a bronchopleural fistula (BPF) (image 22). In this setting, the gas-liquid levels in the frontal and lateral projections on an upright chest radiograph characteristically have unequal linear dimensions and typically extend to the chest wall [36]. CT scanning is also capable of identifying a BPF [20-22]. Central BPFs occur most often after surgical procedures or trauma and can be confirmed by bronchoscopy. In contrast, peripheral BPFs are often a complication of necrotizing pneumonia (image 23 and image 24 and image 25) [20]. (See "Management of persistent air leaks in patients on mechanical ventilation".)

Tuberculous empyemas tend to persist for decades and exhibit extensive calcification of the pleura (image 19A). They were seen more frequently in the past after pneumothorax therapy for tuberculosis.

Malignant pleural effusion — The second most common cause of an exudative pleural effusion is related to a malignant tumor [37,38]. Carcinoma of the lung, breast, or ovary, and lymphoma [39], including primary effusion lymphoma in patients infected with HIV [40], account for approximately 80 percent of all malignant effusions (image 13). The mechanisms of formation include:

●Increased pleural membrane and capillary permeability

●Decreased clearance due to lymphatic obstruction

●Bronchial obstruction leading to atelectasis and a marked regional decrease in intrapleural pressure, which favors pleural liquid accumulation

Findings on CT imaging that suggest a malignant pleural effusion include an irregular, nodular, or thickened pleura. Enhancement of the visceral pleura after administration of intravenous contrast material suggests pleural inflammation or malignancy. The size of malignant effusions varies, but metastatic malignancies are the most common cause of a massive pleural effusion obliterating an entire hemithorax. Malignant effusions can become loculated (image 26).

The diagnosis and management of malignant pleural effusions are discussed separately. (See "Management of malignant pleural effusions".)

Hemothorax — A hemothorax is defined as a bloody pleural effusion with a hematocrit exceeding half the value in peripheral blood [6]. It can be seen after trauma, following an iatrogenic puncture or transection of a vessel, after pulmonary embolism (image 27 and image 28), as a result of metastatic disease, after anticoagulant therapy, or as a sequela of a leaking aortic aneurysm. Large posttraumatic hemothoraces exceeding 500 mL are readily seen on chest radiographs and can contain more than one-half the blood volume of an injured patient per involved hemithorax [41]. Smaller volumes of intrapleural blood may be only visible on CT scans, and have been referred to as occult hemothorax [42].

CT scanning of a hemothorax shows an effusion with relatively high attenuation, exceeding 35 Hounsfield units (HU) when the blood is fresh, and reaching 70 HU with clotted blood (image 29A-D) [43]. Of note an increased attenuation of a pleural effusion with elevated HU may, on rare occasions, be due to the retention of contrast material in the pleural space [44]. A hematocrit effect with a liquid-liquid level can become visible in subacute hematomas, due to the higher attenuation of the sedimented red blood cells compared with that of the supernatant, which contains serum of lower attenuation (image 30 and image 31 and image 32).

MRI is able to identify blood and to estimate the age of the hemorrhage [26,27,33,45]:

●Oxyhemoglobin exists in fresh blood, which has a low signal on T1-weighted spin-echo sequences and a high signal on T2-weighted sequences.

●Deoxyhemoglobin exists in subacute bleeding (ie, several hours to days old), which has a low signal on T1- and T2-weighted sequences.

●Methemoglobin can be seen when blood is several days to several weeks old (image 33). If it is intracellular, it displays a high signal on T1-weighted imaging but a low signal on T2-weighted imaging. When it is extracellular, methemoglobin exhibits a high signal on both T1- and T2-weighted images.

●Hemosiderin shows a low signal on both T1- and T2-weighted imaging and usually indicates blood that is several weeks to several months old.

Chylothorax — Chylothorax is most likely the result of mediastinal tumor involvement by lymphoma or bronchogenic carcinoma [46]. These two neoplasms account for 54 percent of all chylothoraces, while trauma, including surgery, accounts for another 25 percent. Rare causes of chylothorax include filariasis, lymphangioleiomyomatosis, congenital anomalies of the thoracic duct, and so-called idiopathic chylothorax. (See "Etiology, clinical presentation, and diagnosis of chylothorax".)

Disruption of the thoracic duct leads to formation of a chylous duct cyst, which can appear as a posterior mediastinal mass. It can perforate, usually after an interval of 10 days, leading to the delayed development of a pleural effusion. Radiologically, a large pleural effusion is characteristic, and loculation can occasionally be seen (image 34A and image 34B and image 35 and image 36) .

On CT scanning, the pleural accumulation of chyle can display lower attenuation values than other effusions. MRI can show high signal on T1-weighted sequences, due to the high fat content. An additional report suggests that morphological changes of the thoracic duct and accessory lymphatic channels can also be identified on T2 weighted sequences [47]. Non-enhanced magnetic resonance (MR) lymphography uses strongly T2-weighted imaging to identify slow-flowing lymph [2]. Based on the location of the thoracic duct, traumatic right-sided chylothoraces suggest injury to the lower third of the thoracic duct, whereas left-sided chylothoraces suggest a lesion in the upper two-thirds of the thoracic duct (image 34A-B and image 35).

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topics (see "Patient education: Pleural effusion (The Basics)")

SUMMARY AND RECOMMENDATIONS

●Conventional chest radiography and computed tomography (CT) scanning are key to the detection and characterization of pleural effusions; pleural ultrasound and magnetic resonance imaging (MRI) play a role in selected clinical circumstances. (See 'Introduction' above.)

●On conventional chest radiographs, frontal, lateral, and decubitus views are used to detect the presence of a pleural effusion and to differentiate pleural liquid from pleural thickening. Pleural effusions and thickening can both cause blunting of the costophrenic sulcus, but only freely mobile effusions will result in layering of liquid on the decubitus view (image 3). (See 'Conventional radiography' above.)

●The amount of pleural effusion can be estimated based on decubitus chest radiographs; small effusions are thinner than 1.5 cm, moderate effusions are 1.5 to 4.5 cm thick, and large effusions exceed 4.5 cm. Effusions forming a rind thicker than 1 cm are usually large enough for sampling by thoracentesis, because at least 200 mL of liquid are already present. (See 'Conventional radiography' above.)

●Subpulmonic pleural effusions elevate the lung base, mimicking an elevated hemidiaphragm (image 4A-B). The apex of the curvature at the lung base is shifted laterally, and its slope slants sharply towards the lateral costophrenic sulcus. On the left side, a marked separation (>2 cm) of the lung from the stomach bubble suggests a subpulmonic pleural effusion. (See 'Subpulmonic effusions' above.)

●Loculated pleural effusions are differentiated from lung masses by certain characteristics: the angles of interface between a pleural loculation and the chest wall are obtuse with tapered borders; the surface of a loculated pleural effusion is usually smooth or displays the "incomplete border sign", when not imaged in tangent; the content is homogeneous; and pleural loculations appear to droop on upright images owing to the liquid content and the effect of gravity. (See 'Loculated pleural effusions' above.)

●On CT scans, the visceral and parietal pleura and the minimal amount of lubricating pleural liquid between them appear as a line of soft-tissue attenuation 1 to 2 mm thick at the point of contact between the lung and the chest wall (image 1). Extrapleural fat and the endothoracic fascia, each with a thickness of 0.25 mm, may be visible between the pleural line and the ribs. (See 'Normal pleural anatomy' above.)

●CT scans can detect very small pleural effusions, ie, less than 10 mL and possibly as little as 2 mL of liquid in the pleural space. CT is helpful in evaluating complex pleural disease, such as empyemas, pleural liquid loculations, pleural masses associated with pleural effusions, and lung parenchymal processes obscured by pleural effusions. Intravenous administration of iodinated contrast material is important for optimal evaluation of pleural disease by CT scanning. In addition, CT guidance improves the accuracy of positioning thoracostomy tubes for drainage of loculated pleural effusions. (See 'Computed tomography' above.)

●The presence of an exudative, rather than transudative, pleural effusion is suggested by thickening of the pleurae on CT scanning (eg, >3 to 5 mm) and by intravenous contrast material enhancement of the visceral and parietal pleurae. (See 'Computed tomography' above and 'Transudative pleural effusions' above and 'Exudative pleural effusions' above.)

●Thoracic ultrasonography is useful to identify free or loculated pleural effusions and to differentiate loculated effusions from solid masses. Thoracic ultrasound guidance for thoracentesis improves the accuracy and safety of the procedure. (See 'Ultrasonography' above and "Bedside pleural ultrasonography: Equipment, technique, and the identification of pleural effusion and pneumothorax".)

●Pleural effusions and tumors can also be delineated by MRI. The main roles for MRI in the evaluation of a pleural effusion are to characterize a hemothorax and determine whether a pleural tumor extends into the surrounding soft tissues of the chest wall, mediastinum, supraclavicular region, or abdomen. (See 'Magnetic resonance imaging' above and 'Hemothorax' above and "Magnetic resonance imaging of the thorax", section on 'Pleura'.)