Empyema is inflammatory fluid and debris in the pleural space. It results from an untreated pleural-space infection that progresses from free-flowing pleural fluid to a complex collection in the pleural space (see the images below).
Empyema most commonly occurs in the setting of bacterial pneumonia. About 20-60% of all cases of pneumonia are associated with parapneumonic effusion. With appropriate antibiotic therapy, parapneumonic effusions most often resolve without complications, and they are of little clinical significance. However, some effusions do not resolve; these are called complicated effusions. The resulting infection and inflammatory response can proceed until adhesive bands form. The infected fluid becomes loculated pus in the pleural space.
Thoracic trauma (in about 1-5% of cases)
Rupture of a lung abscess into the pleural space
Extension of a non–pleural-based infection (eg, mediastinitis, abdominal infection)
Iatrogenic introduction at the time of thoracic surgery
An indwelling catheter that is a nidus for infection
Mortality related to empyema is associated with respiratory failure and systemic sepsis, which occurs when the immune response and antibiotics are inadequate to control the infection.
Drainage is performed to remove the collection and to improve outcomes. The proper intervention depends on the severity of the disease and ranges from minimally invasive catheter drainage to open surgical decortication. Early intervention decreases the mortality rate associated with empyema. Prompt diagnosis, treatment, and proper management of empyema are crucial. 
Standard 2-view chest radiography remains the first study for evaluating effusion or empyema. If an effusion is present, bilateral decubitus imaging is indicated for further characterization. These examinations are informative and cost-effective. [2, 3]
Ultrasonography may show small volumes of pleural fluid, and it may provide information about viscosity. Ultrasonography may also quickly demonstrate septa in the pleural fluid collection.
Chest CT provides the most information. CT imaging depicts fluid, loculation, and thickening of the pleural membranes. CT and ultrasonography are also used in the placement of drainage catheters. [4, 3]
Limitations of techniques
Two-view chest radiographs and decubitus views are not always obtainable in the intensive care unit. Radiographs are often limited to bedside supine or semierect anteroposterior views in very ill patients. A small fluid collection in the subpulmonic recess may be present but not detected on radiographs. Ultrasonography may demonstrate septa in the pleural fluid, but it poorly demonstrates the thickness of the pleura.
Free-flowing pleural fluid collects in the dependent portion of the pleural space. On 2-view chest radiographs, pleural fluid obscures the costophrenic angle (as in the image below). 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 the patient instead undergoes a CT examination.
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.
CT scanning 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 (indicated in the first image below). 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 the second and third images below). This sign can also be seen in chronic pleural effusions. The combination of a split pleura sign and total amount of pleural fluid ≥30 mm on thoracic CT has been shown to be useful and simple for discriminating between complicated parapneumonic effusion/empyema and parapneumonic effusion prior to diagnostic thoracentesis.  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 the image below).
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%. 
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.
An enhancing pleura sign can be present in chronic pleural effusion and metastatic disease.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is rarely used to image pleural effusion and empyema (the degree of confidence in the diagnosis of empyema is moderate). It may be useful for evaluating thickening of the pleural membrane when the administration of contrast material is contraindicated. 
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. 
Loculated effusions suggest empyema in the proper clinical context, but the diagnosis must be confirmed with thoracentesis.
The diagnosis of empyema is not based solely on ultrasonographic results.
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 ventilation and perfusion studies may suggest a layering effusion.
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.