Radiological Examination of Pleura

Introduction:

The pleura is a serous membrane divided into the visceral (inner layer) and parietal pleura (outer layer) and it plays a major part in the respiratory tract. There are two pleurae, each one for each lung, which fold to form two layers. The visceral pleura is a slippery inner membrane that lines the surface of the lungs, and it dips into the area that divides the lobes called the hilum. The hilum is the entry point for blood vessels, nerves, and bronchus. The parietal pleura is a thin inner membrane that forms the lining of the inner aspect of the chest wall and diaphragm. The parietal and visceral pleura join at the hilum and are separated by a pleural cavity or intrapleural space, which contains viscous fluid of about 4 to 5 cubic centimeters (ccs). The viscus fluid, called pleural fluid secreted by the mesothelial cells of the parietal pleura and is reabsorbed by lymphatics.

What Are the Functions of Pleura and Pleural Fluid?

Pleura plays a vital part in the respiratory system by doing the following functions:

• Pleura prevents the spread of infection from the lungs to other parts and vice versa.

• The pleural fluid contains macromolecules such as hyaluronan, sialomucin, and phospholipids which act as a lubricator. As a result, the pleural fluid reduces the friction between the two opposing pleural layers during ventilation, which improves pulmonary compliance.

What Are the Imaging Techniques Used to Examine the Pleura?

1) Chest X-ray:

• The normal visceral and parietal pleura are not visible in the chest X-ray (CXR).

• Pneumothorax is a lung condition characterized by an abnormal collection of air in the pleural space that occurs with trauma, and a fractured rib impinges the visceral pleura. If the lung edge is more than 2 cm away from the inner chest wall, it refers to pneumothorax.

• Pleural thickening is seen at the edge of the lung and is identified by the pleura running tangentially to the x-ray beam.

• Asbestos plaques are calcified plaques found in the lung due to multiple exposures to the asbestos, which appears irregular, as holly leaves.

• Pleural effusion is the abnormal collection of fluid in the pleural space, and this fluid collects in the lowest part of the chest. For example, if the chest x-ray is taken in the upright position, the fluid surrounds the base of the lung and appears concave line covering the costophrenic angle and hemidiaphragm. In the supine position, the fluid outlines the posterior aspect of the chest cavity and is not visible in the chest x-ray.

2) Ultrasonography:

• As the pleura is superficial, it is visible in the ultrasound; the pleural abnormalities are characterized by their acoustic properties. Ultrasound is performed using an array transducer through the intercoastal space at a frequency of about 3.5 to 5 MHz.

• Pleural thickening due to inflammation or tumor, which is less than one centimeter, is difficult to find on ultrasound, but it can effectively detect the pleural thickening when it is combined with pleural fluid.

• It can detect even a small amount of pleural fluid and helps in guiding the aspiration when the fluid is a free-flowing, high hemidiaphragm or when there is pulmonary collapse. Pleural fluid appears hypoechoic with a hyperechoic line delineating the lung and visceral pleura.

• Tumors such as lymphomas and neurogenic neoplasm appear as hypo or anechoic structures on ultrasound. In such a case, it can not be differentiated from pleural fluid and hence require needle aspiration.

• A pleural biopsy is performed under the guidance of sonography which identifies the depth of the parietal pleura from the skin.

3) Computed Tomography (CT) Scan:

• CT can detect pleural abnormalities at an earlier stage than other imaging techniques. It can detect the presence, exact location, and extent of the pleural lesion.

• Before the CT scan, a standard lateral and posteroanterior radiograph is taken. The use of contrast material is often not required in the pleural lesion, sometimes, contrast materials help in differentiating cysts and abscesses from solid lesions and empyemas.

• Pleural fluid appears as sickle-shaped radioopacity in the posterior aspect of the thorax. CT scan differentiates pleural effusion from ascites with several signs.

• Displaced Crus Sign - When the fluid displaces the crus away from the vertebral body, it indicates pleural effusion; if it displaces towards the vertebral body, it indicates ascites.

• Bare Area Sign - In pleural effusion, the fluid is seen in the bare area also, but in ascites, it is not seen in the bare area between the coronary ligament.

• Interface Sign - As the diaphragm forms the interface the margins of the organs such as the spleen and liver are seen as sharp in ascites and hazy in pleural effusion.

• Malignant mesothelioma is a rare tumor often due to asbestos exposure which appears as areas of pleural effusion with irregular, thickened, nodular pleura.

• Pleural lymphoma (obstruction of lymphatics due to enlarged mediastinal nodes or hilar) appears as broad-based lymphomatous plaques often involving the pleura and chest wall.

4) Magnetic Resonance Imaging (MRI) Scan:

• In MRI, pleural effusion appears as a low signal intensity on T1 weighted images, and high signal intensity on T2 weighted images due to their water content.

• Subacute or chronic hemorrhage appears as high signal intensity on both T1 and T2 weighted images, and a concentric ring sign is also present. The concentric ring sign is the outer dark rim made of hemosiderin with a bright signal in the center in T1 shortening effects.

• Pleural lipomas appear as high signal intensity on T1 weighted images, and moderate signal intensity on T2 weighted images.

• Invasion of lung carcinoma into the chest wall appears as a high signal within the chest wall, or thickening of the wall appears as high signal intensity on T2 weighted spin echo images.

• The sagittal and coronal view is helpful in determining the invasion of tumors into the base of the neck, spinal canal, brachial plexus, and subclavian artery.

• Pleural mesothelioma appears as high signal intensity on T2 weighted images and intermediate signal intensity on T1 weighted images.

Conclusion:

The anatomical extent of the pleura is similar to that of the lungs, except that the inferior border of the lungs extends to the T10 vertebral level, and the inferior surface of the pleura extends to the T12 vertebral level. The radiologist must consider this while imaging the whole pleura, and also it should be considered while performing upper abdomen biopsies. Though ultrasound is easily available and economical, a CT scan is highly sensitive in detecting pleural masses, pleural fluid, and pleural thickening. Ultrasound is a valuable imaging tool for detecting small quantities of fluid and guiding fluid aspiration.