Plain Radiography

Chest radiography, which is the most commonly ordered radiologic examination, evaluates the lung parenchyma, cardiac and mediastinal size, and the bony structures of the chest wall. The examination is preferably done in the radiology department with the patient in the standing position. This is the standard posterior-anterior (PA) view (the beam of the x-ray passes posterior to anterior). The patient must be able to take a deep breath and hold while the picture is taken. A "good inspiration" is defined as visualizing the ninth rib above the diaphragm. Adding the lateral film visualizes the posterior lung bases and the retrosternal area, and also helps to localize infiltrates and masses anatomically.

For unstable patients, portable chest radiography is performed at the bedside, with patients sitting upright. Portable chest radiography is limited by the low power of the equipment and variations in radiographic technique. The bedside study is obtained as an anterior-posterior (AP) view (the x-ray beam passes anterior to posterior) and thus magnifies the mediastinal structures. For a trauma victim on a backboard, the portable film may be more difficult to interpret. Skinfolds and clothing under a patient may mimic a pneumothorax. Supine chest radiographs will detect only 40 percent of pneumothoraces,1 because air rises anteriorly, making the diagnosis of a small pneumothorax very difficult. Likewise, fluid collections layer posteriorly in supine patients, causing a diffuse haziness in the lung fields, which can be confused with infiltrate or contusion. Therefore, chest radiographs are limited by a patient's overall condition, the patient's ability to cooperate with directions, and the technique chosen. The PA and lateral chest radiographs remain the best views.

The chest radiograph should be systematically analyzed so as not to miss key information. To ensure that the radiograph is of the correct patient, always identify the study by patient name. Assess the technique and the quality of the study. In a good-quality study, the trachea is visible in the midline, the medial borders of the clavicle are centrally located over the superior mediastinum, and the thoracic spine is visible through the mediastinal structures. Note the positioning of all lines, the endotracheal tube, and the nasogastric tube. The trachea should be followed to the left and right main-stem bronchi and to the mediastinal structures. Look at the mediastinum for evidence of free air, for density behind the heart, and for cardiac size (which should be less than 50 percent of the thoracic size). Assess position of the diaphragms, looking at position, contour, and subdiaphragmatic free air: the right hemidiaphragm should be slightly elevated compared with the left (0.5 to 2.5 cm is normal). Assess the lung fields and pleura, looking for masses, infiltrates, free air, or effusions. Finally, assess the soft tissue and bones, looking for subcutaneous air or fractures.

Other views may be obtained. The most useful is the expiratory film, which may accentuate a free air-lung interface, enabling a small pneumothorax to be diagnosed. Up to 10 percent of small pneumothoraces may be missed in inspiratory film alone.2 In foreign-body aspiration, the expiratory film shows hyperinflation on the affected side as air is trapped behind the foreign body. The lateral decubitus film may be useful to assess a pleural effusion, because free fluid will layer along the dependent portion of the chest.

Specific abnormalities to note on the chest radiograph are pulmonary edema, infiltrates, lung nodules and masses, hilar size and contour, cardiac size and configuration, pleural effusions, and pneumothorax.

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