Radiography
Findings
Free-flowing pleural fluid collects in the dependent portion of the pleural space. On 2-view chest radiographs, pleural fluid obscures the costophrenic angle (see Image 1). Approximately 75 mL of fluid is required to blunt the posterior costophrenic angle on a lateral chest radiograph. Almost 200 mL of fluid is required to blunt the lateral costophrenic angles on frontal radiographs. If loculations have formed, fluid opacity may be seen in a nondependent area. The D configuration of loculated fluid bulging out from the chest wall is a classically described but infrequently observed finding.
Although supine or semierect radiographs do not show pleural effusion as well as upright 2-view chest radiographs do, an ill patient is often unable to stand. A unilateral free-flowing effusion results in increased hazy opacity on the side of the affected hemithorax.
If a pleural effusion is suspected, bilateral decubitus views are recommended. When an effusion is identified, the width of the layering fluid may be measured. If the width of the fluid is <10 mm, the effusion may be managed medically and followed up with serial radiographs. However, if the effusion is wider than 10 mm, thoracentesis or catheter drainage should be performed, if clinically indicated. CT or ultrasonographic guidance is best for placing a pleural catheter for drainage.
Degree of Confidence
When 2-view chest radiographs are used to detect pleural fluid, the sensitivity is 67% and the specificity is 70%. Decubitus views increase the degree of confidence. However, decubitus views are often skipped, and instead, the patient undergoes a CT examination.
False Positives/Negatives
Empyema is not diagnosed strictly on the basis of traditional radiographic findings. Further imaging with CT and confirmation of pleural infection with thoracentesis are usually required to diagnose empyema.
Computed Tomography
Findings
CT is the imaging study of choice for evaluating possible empyema. Depending on the expected clinical management, patients can undergo imaging with or without intravenously administered contrast material. If tapping of a clinically significant pleural effusion is clinically indicated, no intravenous contrast medium is necessary to evaluate for the presence and location of pleural fluid.
The typical empyema is lenticular. Nonenhanced CT scans can demonstrate atypical pleural effusions along the mediastinum, thickened pleurae, loculations in the fissures, septa, or gas bubbles in the pleural space (see Image 2). Gas bubbles in the pleural space strongly suggest an empyema in the proper clinical context (ie, in the absence of recent thoracentesis). Lung windows can demonstrate pneumonia adjacent to the abnormal pleural collection. Soft-tissue windows can demonstrate a cause for the empyema, such as esophageal rupture or mediastinal surgery.
With most empyemas, enhanced chest CT scans demonstrate the split-pleura sign (see Image 3). This sign can also be seen in chronic pleural effusions. Enhanced CT scans also depict parietal pleural thickening in most cases of empyema.
Empyema necessitatis occurs when the pleural infection extends beyond the thoracic cavity into the chest wall (see Image 5).
Although CT signs can be diagnostic of empyema, a pleural tap is indicated for culturing and sensitivity analysis.
Degree of Confidence
If no interventional procedure has been performed, gas bubbles in a pleural fluid collection are virtually diagnostic of an empyema. Enhancing pleura and thickened parietal pleura are also strongly associated with empyema. In a study of empyema, pleural enhancement was seen in every case, and pleural thickening was seen in 92%.3
False Positives/Negatives
An enhancing pleura sign can be present in chronic pleural effusion and metastatic disease.
Without gas bubbles in a pleural fluid collection or an enhancing pleura sign, the diagnosis of infection in pleural fluid depends on a high level of clinical suspicion confirmed with findings from thoracentesis. Although pleural thickening is present in empyemas, it can also be seen in other diseases, such as chronic effusion or asbestos exposure. A pleural exudate without pleural thickening most likely represents malignancy or uncomplicated pleural effusion.Magnetic Resonance Imaging
Findings
MRI is rarely used to image pleural effusion and empyema. It may be useful for evaluating thickening of the pleural membrane when the administration of contrast material is contraindicated.
Degree of Confidence
The degree of confidence in the diagnosis of empyema is moderate.
False Positives/Negatives
MRI is not routinely used to diagnose empyema.
Ultrasonography
Findings
Ultrasonography is an important adjunct in defining the characteristics of a pleural effusion. It may be used to detect small effusions. Ultrasonography also provides information about fluid viscosity, the presence of septa, and the free-flowing or loculated nature of the effusion.
Degree of Confidence
Loculated effusions suggest empyema in the proper clinical context, but the diagnosis must be confirmed with thoracentesis.
False Positives/Negatives
The diagnosis of empyema is not based solely on ultrasonographic results.
Nuclear Imaging
Findings
Nuclear medicine scans are not used in the routine workup for effusion and empyema. Effusions may be seen on ventilation and perfusion scans. Diffusely decreased unilateral intensity on both ventilation and perfusion studies may suggest a layering effusion.
Degree of Confidence
Nuclear medicine tests are not used as diagnostic studies for empyema.
Angiography
Findings
Angiography does not have a role in the management of empyema. The interventional radiologist may need to perform thoracentesis with imaging guidance, and an indwelling catheter may be needed.
More on Empyema |
| Overview: Empyema |
 Imaging: Empyema |
| Follow-up: Empyema |
| Multimedia: Empyema |
| References |
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References
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Ploton C, Freydiere AM, Benito Y, Bendridi N, Mazzocchi C, Bellon G. Streptococcus pneumoniae thoracic empyema in children: rapid diagnosis by using the Binax NOW immunochromatographic membrane test in pleural fluids. Pathol Biol (Paris). Oct-Nov 2006;54(8-9):498-501. [Medline].
Sahn SA. Use of fibrinolytic agents in the management of complicated parapneumonic effusions and empyemas. Thorax. Aug 1998;53 Suppl 2:S65-72. [Medline].
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Further Reading
Keywords
organizing effusion, complicated effusion, loculated effusion, pleural pus, intrapleural pus, pleural-space infection
