Empyema Imaging

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. Empyema affects up to 65,000 patients annually, with a mortality of approximately 15%.[1] 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.[1, 2]

Empyema may also result from causes other than bacterial pneumonia. Any process that introduces pathogens into the pleural space can lead to an empyema, such as 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), esophageal tear, iatrogenic introduction at the time of thoracic surgery, and an indwelling catheter that is a nidus for infection.[1, 2]

 

Preferred examination

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.[3, 4]  On the chest radiograph, findings that suggest empyema, as opposed to lung abscess, include extension of the air-fluid level to the chest wall, extension of the air-fluid level across fissure lines, and a tapering border of the air-fluid collection.[1, 4, 2, 5, 6]

The costophrenic angle should be closely inspected on the chest radiograph to assess the presence of fluid that suggests effusion or empyema. 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.[1, 4, 2, 5, 6]

In children, if the radiograph is taken when a younger child is supine, a homogeneous increase in opacity over the whole lung field may be seen, without blunting of the costophrenic angle or a classic pleural-based shadow. If there is a white out, it may not be possible to differentiate between solid underlying severe lung collapse/consolidation and a large effusion.[2]

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, 7]  In patients with complex fluid collections, chest CT imaging has emerged as the study of choice. Chest CT imaging can be used to detect and define pleural fluid and image the airways, guide interventional procedures, and discriminate between pleural fluid and chest consolidation.[8, 9]

Contrast-enhanced CT with tissue phase allows detailed observation of parenchymal abnormalities and may identify the cause of the pleural space infection (ie, bronchogenic carcinoma, endobronchial foreign body, esophageal rupture). Smaller effusions on CT are likely to resolve with antibiotics.[2, 10]  The split pleura sign has a high diagnostic value for empyema.[11]

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, but it poorly demonstrates the thickness of the pleura. When ultrasonography is not used to guide needle aspiration, the failure rate is 12-15%. Color Doppler ultrasonography can also assist in differentiating empyema from peripheral air-fluid pulmonary abscess.[12, 13, 14, 15, 16]

AATS Guidelines

The American Association for Thoracic Surgery (AATS) consensus guidelines for the management of empyema includes the following imaging recommendations[1] :

• Pleural ultrasound should be performed routinely in addition to conventional chest x-ray in the evaluation of pleural space infection, both for diagnostic purposes and for image-guidance for pleural interventions.

• CT of the chest should be obtained when pleural space infection is suspected.

• Positron emission tomography (PET) does not appear useful in the diagnosis of pleural-space infections, because it does not accurately distinguish inflammatory from malignant pleural effusions.

• MRI is useful in distinguishing transudates from exudates and allows assessment of extension into soft tissues, such as in chest wall or spinal involvement.

Intervention

A range of therapeutic options are available, ranging from percutaneous aspiration and intercostal drainage to video‐assisted thoracoscopic surgery (VATS) or thoracotomy drainage. Intrapleural fibrinolytics may also be administered following intercostal drain insertion to facilitate pleural drainage.[17, 18]  Drainage of infected material from the pleural space is a fundamental part of empyema treatment and is associated with improved outcome.[19]  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.[20]

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 shown 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.[1, 4, 21, 2, 5, 6]

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 less than 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.

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.

In children, if the radiograph is taken when a younger child is supine, a homogeneous increase in opacity over the whole lung field may be seen, without blunting of the costophrenic angle or a classic pleural-based shadow. If there is a white out, it may not be possible to differentiate between solid underlying severe lung collapse/consolidation and a large effusion.[2]

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.[22] 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. Although CT signs can be diagnostic of empyema, a pleural tap is indicated for culturing and sensitivity analysis.[1, 18, 2, 5]

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. 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.[4, 7, 8, 9, 2, 10]

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 or more on thoracic CT has been shown to be useful and simple for discriminating between complicated parapneumonic effusion/empyema and parapneumonic effusion before diagnostic thoracentesis.[8] Enhanced CT scans also depict parietal pleural thickening in most cases of empyema.

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%.[9]

Empyema necessitatis is a complication of pleural empyema and occurs when the pleural infection extends beyond the thoracic cavity into the chest wall (see the image below).[23]

 

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.[12]  The diagnosis of empyema is not based solely on ultrasonographic results. Loculated effusions suggest empyema in the proper clinical context, but the diagnosis must be confirmed with thoracentesis.[13, 14, 15, 16]

Ultrasonography has been found to be at least as effective as lateral decubitus radiographs to identify small pleural effusions and to be better than chest radiographs at estimating the volume of the effusion without being hindered by adjacent consolidations or intrathoracic structures. The fact that bedside ultrasound can be performed by nonradiologists makes it ideal for guiding thoracenteses.[1, 2]