X-rays are radiant energy, like light, except that these waves are shorter and can pass through opaque objects. They are produced by bombarding a tungsten target with an electron beam and are channeled so that a narrow but diverging beam is emitted from the tube. When an x-ray exposure is taken, the tube is usually aimed so that the rays pass through the subject to the x-ray film in either a posterior to anterior (posteroanterior) or anterior to posterior (anteroposterior) direction. Because the x-rays are diverging and subject to reflection (scatter), structures more distant from the film are magnified and less distinctly outlined. In general, chest radiographs are taken in the posteroanterior direction because this places the heart, an anterior structure, closer to the film, resulting in less magnification and allowing the cardiac outline to be seen clearly.

When using conventional radiology methods, the chest x-ray image is recorded on a film that is chemically processed. Computerized digital chest radiology utilizes a special phosphor plate instead of traditional film. The digital x-ray image is produced by scanning the phosphor plate with a laser beam that causes light to be released from the phosphor plate. This image is then digitized and converted to an image by computer.² The computer image is then viewed from monitors and can also be converted to radiographic film providing a hard copy. Digital images afford many advantages over traditional chest x-rays. Digital images are easily stored and transferred making them more accessible from a variety of remote viewing stations. Digital images can be easily manipulated by changing magnification or relative density, which may add substantial information to the examination without exposing the patient to repeated imaging.³

Anteroposterior chest radiographs are often taken in cardiac care units (CCU) because it is difficult to put the x-ray tube behind the patient. The x-ray film is therefore placed behind the patient. Because the heart is relatively far away from the x-ray film, its outline is somewhat less distinct and the heart size is magnified. Moreover, the distance between the tube and the patient in CCU is shorter than usual to cut-down x-ray scatter, which also results in greater magnification.

The degree of darkness of the x-ray film depends on how much x-ray energy traverses the patient and exposes the film. This depends on the density of the material through which the x-ray beam passes. The chest has four major types of tissue densities through which rays must pass: bone, water, fat, and air. Because bone is the densest of these tissues, fewer and less energetic x-rays pass through bone. Thus, the shadow on the x-ray film cast by bone is light. (An x-ray image is like a photographic negative, with white color indicating lack of exposure and black color indicating intense exposure.) The lung, which is largely air, is least dense; therefore, it appears black on a chest radiograph. Soft tissues and blood are largely water, with similar densities, between those of bone and air. Fat is usually visibly less dense than other soft tissues. Thus, a chest radiograph is actually a shadowgraph.

The reason a structure can be outlined is that the shadow of one density contrasts with that of an adjacent density. If two structures are of equal density and adjacent to each other, then a single combined shadow results. If two structures of similar density are in different planes or are separated by a structure of a different density, then the two structures are seen on x-ray film separately. This property of the x-ray shadowgraph is helpful in determining where a certain density lies. For example, if a density on a posteroanterior chest radiograph is inseparable from and therefore adjacent to the descending thoracic aorta, then the observer knows that this abnormal density is in the posterior chest; if the density is inseparable from the right heart border, then the density is in an anterior position, because the heart is an anterior structure.

The chest radiograph is read as though the reader were looking at the patient. Traditionally, the x-ray film is placed on a view box or light box that allows the radiograph to be backlit so it can be
viewed and interpreted. More recently, digital imaging technology is used increasingly in radiology allowing for rapid viewing of films on monitors rather than on light boxes. Computerized radiographs can be viewed immediately on monitors on the CCU and stored images allow the provider to readily compare current films with previous images.⁴ To ensure that all anatomic structures are seen, radiographs are read according to a certain pattern. This method is called the directed search method. It is common practice to look at soft tissues, bones, and diaphragms first, then at the lungs from apex to base, and finally at the outline of the heart and the aorta. Except for the heart, most structures in the chest are bilateral. Thus, if an abnormality is found on one side of the chest, the other side should be observed to ensure that this “abnormality” is not present there. Even if an obvious abnormality is present, a directed search should be completed so that additional disease is not missed. Figure 12-1A is a normal posteroanterior chest radiograph; Figure 12-1B is a normal lateral chest radiograph. Figure 12-2A shows the location of the lung lobes on the frontal chest radiograph. Because some lobes are anterior and some are posterior, an abnormality in a certain area on a frontal chest radiograph can be in one of two lobes. Obtaining a lateral film or noticing whether an anterior or posterior structure is obliterated by an abnormal density can help with localization. Figure 12-2B shows the location of the lung lobes on a lateral radiograph. Abnormalities of the right middle lobe and lingula would go undetected with posterior chest auscultation.

Bedside radiographs are used not only to assess for cardiopulmonary abnormalities, but also to evaluate placement of lines, tubes, and devices used in acute care. In addition to providing valuable information regarding the patient’s cardiopulmonary status, the chest radiograph allows for early recognition of complications related to line placement as well as to evaluate therapeutic result after interventions such as drainage of a pleural effusion by chest tube placement. Table 12-1 lists invasive lines, tubes, and devices commonly used in acute cardiovascular care and describes radiologic findings. Figures 12-3, 12-4, 12-5, 12-6, 12-7, 12-8, 12-9 and 12-10 demonstrate radiologic appearance of a variety of invasive lines and devices.