Chest Radiography

Technical Aspects

Chest radiography was one of the first clinical studies to use x-rays, which were discovered in 1895 by Wilhelm Conrad Roentgen. X-rays are typically generated by passing a current across a diode, which results in the generation of electrons. The electron beam is aimed at a metal anode, and the resultant interaction produces x-ray photons. The x-ray beam diverges as it exits from the x-ray tube and produces a conical-shaped beam. When x-rays are captured by film or a digital system, the divergence of the beam can lead to geometric distortion, which is a function of the distance from the x-ray source to the object and from the object to the detector. The further an object is from the radiation source, the less geometric distortion and clearer image that will be produced, but higher levels of energy and longer exposure times are required for adequate image production. More energy and longer exposure lead to an increase in radiation exposure for the patient. To balance the competing factors of geometric distortion and radiation exposure, 6 feet is considered the standard source-to-image distance for a typical posteroanterior (PA) and lateral chest x-ray. The radiation exposure of a standard PA and lateral chest x-ray at this distance is approximately 3 millirems or approximately 1/100 of the typical annual rate of celestial radiation for an individual. Because ionizing radiation causes a dose-dependent increase in the risk of genetic alteration and malignancy, as low as reasonably allowable principles of radiation safety are followed to minimize patient exposure.

Anatomic structures with different tissue compositions will produce varying degrees of absorption, blocking, and disruption of the x-ray photons, thereby producing shades of gray or contrast in the image. This contrast allows differentiation of fluid-filled structures (heart and great vessels) from air-filled lungs and the much denser bony structures of the thorax. The x-ray image is produced as x-ray photons strike and alter silver iodide crystals in the x-ray film. Alternatively, digital radiography (filmless technique) can produce images with a flat plate that directly converts incident photons to a digital signal.

The quality of the technique used in obtaining a chest radiograph can be appreciated after a quick survey of the film. The key components are a review of identification, inspiration, penetration, and rotation. Patient identifiers and radiographic markers should be noted. PA chest film should demonstrate the diaphragm along the eighth to tenth posterior rib space or the fifth to sixth anterior rib space. The film should have adequate penetration to allow the intervertebral disc space of the thoracic spine to be barely visualized but not overly penetrated, which would obscure bony details of the spine or identification of pathology within the pulmonary fields. Rotation should be evaluated by confirmation that the thoracic spine lies posterior to the sternum, and that the clavicles are at approximately the first anterior rib.