X-rays are high-energy electromagnetic waves that have been used in diagnostic imaging since their discovery in 1896 by Wilhelm Conrad Roentgen. They consist of electromagnetic components oscillating in orthogonal planes. For routine clinical use, a finely controlled x-ray beam of known energy and quantity is targeted toward the site of interest.1 An x-ray tube in its most elementary form consists of a tube filament and target (Figure 1–1). The tube filament (cathode) emits electrons when heated. A target (anode) emits x-rays when struck by these high-energy electrons. In diagnostic radiology, these x-rays pass through a patient and are attenuated to various degrees based on the density of the tissue. They then strike an image detector, leading to formation of an image. On the processed image the “lighter” areas are the attenuated x-rays, with “darker” areas of the film corresponding to areas of less attenuation. Bone, the densest tissue, appears bright white on plain film, while soft tissue is of intermediate attenuation, and air appears black (Table 1–1).
Table 1–1Appearance of Structures on Plain Film |Favorite Table|Download (.pdf) Table 1–1 Appearance of Structures on Plain Film
|Density ||Substance ||Appearance on X-Ray |
|Least ||Air/gas ||Black |
| ||Fat ||Dark grey |
|Soft tissue ||Grey |
|Most ||Bone/calcium/metal ||White |
Schematic illustration of an x-ray tube.
The aim of the sections that follow are twofold:
To provide a brief overview as to the interpretive approach to chest and abdominal x-rays
To describe salient findings in common clinical conditions encountered in the emergent surgical setting.
The chest radiograph or chest x-ray (CXR) is one of the most commonly ordered examinations in the emergency department. It provides a quick preview of the thorax with minimal cost, minimal patient discomfort, and minimal radiation risk. A single chest x-ray provides a radiation dose of approximately 0.01 to 0.02 mSv.2 This dose is equivalent to a background radiation dose for an entire day, and the clinical benefits conferred by a chest film far outweigh this negligible radiation exposure.
A CXR view depends on the direction in which the rays pass from the tube through the patient.3
Postero-anterior (PA) CXR: This is obtained with the patient standing and facing the film cassette with a distance of 180 cm from tube to cassette. The x-rays pass from the posterior to anterior direction. This is the standard preferred projection of a CXR and should be obtained whenever feasible. It allows accurate and valid comparison between separate previous radiographs (Figure 1–2).
Antero-posterior (AP) CXR: An AP CXR is obtained when the patient is unable to sit or stand. With the advent ...