Most complex objects are not of uniform density; they contain a variety of substances of varying densities, which produce shadows on a radiograph. There are four major categories of radiographic densities that appear on a radiograph; in order of decreasing density these are metal or bone, water, fat, and gas (or air).³ Metal is extremely dense, and it is the most radiopaque of materials. Pure metals block or absorb the entire x-ray beam and produce a bright white shadow on the radiograph. Because bones contain a large amount of calcium, they are nearly as dense and radiopaque as metal and also produce white images on the radiograph. Water and other fluids are fairly radiodense; they block a significant amount of the x-ray beam and have a gray appearance. Body tissues or cavities containing water or fluid (such as the heart) will produce a very light or white image on the radiograph. However, because water and fluids are not as dense as bones, bones and metals will still create whiter (more opaque) images than water. Because fat (including muscle) is not as dense as water, it does not block as much of the x-ray beam and consequently is less radiopaque or more radiolucent than bone or fluid. As a result, the radiograph shadow that fat produces is less dense than that produced by bone or water. Fat is contained in subcutaneous tissue and in some muscle; the x-ray image produced by these fat-containing tissues will have a dark gray or charcoal appearance. Gas or air is the least dense of substances visible on the chest radiograph. Because gas does not absorb much of the x-ray beam, it is radiolucent and produces a black image. Gas or air density is normally seen in the lung fields and in the air-filled stomach. The ability to discern muscle, blood, internal organs, fat, and tissues requires the understanding of normal anatomy.^5,^7,^16,¹⁷

All parts of the body contain one or more of the four densities—metal or bone, fluid, fat, or gas. The juxtaposition of body parts or chambers of differing densities will create contrasts on the radiograph. When objects of varying densities are in contact, their borders will be apparent because of the contrasts in their radiographic images, and the difference in their images creates an edge or silhouette.³ When structures are visible on a radiograph, it is easy to evaluate their size, shape, and position. When two objects with similar densities are in contact or in the same plane as the x-ray beam, no spectrum of density is seen; therefore, there is no visible border between the objects on the radiograph.¹⁷ This finding may be normal based on normal anatomy. The silhouette sign describes the loss of a normal interface or silhouette, which is suggestive of abnormal tissues densities, often indicating a pathologic condition.^6,¹⁶ This observation of abnormal density or silhouette sign can often confirm a diagnosis of inappropriate tissue, fluid, or air accumulation. The silhouette sign is commonly applied to the heart, mediastinum, chest wall, and diaphragm.

An x-ray film creates a two-dimensional image of three-dimensional objects as it compresses the image into one plane. As a result, the depth of structures often cannot be appreciated by evaluation of only one radiograph view. Many times, studies must be taken from two or more views so that the images can be compared to better evaluate the relative position of objects.¹⁴ Healthcare providers are aware that it is impossible to confirm correct placement of a tracheal tube from an anteroposterior (AP) or a posteroanterior (PA) x-ray alone, because such a film compresses the anterior-posterior dimension. The AP film can be used to evaluate depth of insertion and placement in or displacement from the midline. Only on the lateral view of the chest will it be possible to correctly evaluate the position of the tracheal tube relative to the trachea and esophagus.

The radiograph views are typically labeled by patient position (e.g., upright, lateral) and by the body part nearest to the x-ray projection tube. The optimal position for chest radiography is the upright or erect position. The standard chest film is an upright PA film. This film is typically obtained in the radiology department. The patient stands facing the radiographic film cassette, and the x-ray beam is directed from the back of the patient, through the patient, to the film. Thus, the patient’s back (posterior aspect) is closest to the x-ray tube.

An AP chest film is the view most frequently obtained in the critical care unit because it can be taken with a portable x-ray machine. When an AP view is obtained, the film cassette is placed under or behind the child and the child faces the x-ray tube. The child optimally should be in a sitting position; however, if there is difficulty in positioning the child because of acute illness or general immobility, the child is positioned supine. The x-ray beam is then directed from in front of the child through the child and to the film. As a result, the front of the child (anterior aspect) is nearest the x-ray tube when the film is taken. The AP film tends to magnify anterior chest structures, including the heart, because of the divergence of the x-ray beam; this makes it difficult to compare heart size in an AP versus a PA film. Heart size is most magnified in an AP supine film because of horizontal elongation of the ribs, higher placement of the diaphragm, and the impression of decreased lung volume.¹⁷

Whenever heart size is evaluated on chest radiographs, it is important to know whether the film was taken with a PA or an AP approach, although the difference is not as significant in infants and small children as it is in older children and adults.¹⁹ Changes in heart size, or in the size of any organ, can best be appreciated by comparing two images obtained using the same approach. Characteristically, chest films obtained in the radiology department under controlled conditions are clearer and of better quality than those obtained with a portable machine. Therefore, the films should be obtained in the radiology department whenever practical.

When PA or AP chest films are obtained, evaluation of lateral relationships of structures is possible, but assessment of AP relationships is not possible because of the compression of the image onto a single plane. If a determination of depth is necessary or if localization of a density is required, a lateral film is taken. Lateral chest films are usually obtained as part of a complete radiographic study of the heart and lungs. To obtain a lateral film, the patient is upright, the film cassette is placed on one side of the patient—typically the patient’s left side—and the beam is directed from the other side of the patient. The lateral film is labeled according to the side of the patient that is nearest the x-ray tube. If the patient’s left side is against the film cassette and the right side is nearest the X-ray tube, the resultant film is a right lateral film. Conversely, if the patient’s right side is nearest the film cassette and left side is nearest the x-ray tube, the film is a left lateral film.²²

Lateral views allow evaluation of the AP relationships of body structures, but they do not allow determination of their lateral relationships. Therefore, the comparison and evaluation of both a PA and a lateral film provide much more information than either film does separately. An additional advantage of obtaining radiographs in two views is that thin structures (such as fissures) may be visible on only one film if the x-ray beam strikes the structure parallel to its long axis.^9,²⁴ When films of the same object are obtained using two different views, there is a greater chance that small structures will be apparent on one of the views.

A lateral decubitus view is obtained with the patient lying on one side or the other. The film cassette is placed at the patient’s back, and the x-ray beam is then aimed horizontally or parallel to the floor.⁹ A lateral decubitus view can be obtained in the critical care unit, and it is often helpful in determining the presence of air-fluid levels in the chest, such as those be seen when a lung abscess or pleural effusion is present. Positioning has a significant influence on the appearance of air, fluid, and blood vessels within the chest.

Interpretation of film technique

Patient Position and View

The evaluation of a chest radiograph requires knowledge of the exposure conditions of the film, the angle of the x-ray beam, and the alignment and position of the patient. If the x-ray tube is positioned close to the film and the patient, the x-ray image of the patient will be magnified. Conversely, if the x-ray tube is farther away from the film and the patient, the image of the patient will be smaller but sharper. The x-ray tube should be positioned so that the x-ray beam is exactly perpendicular to the plane of the film. If the patient is positioned properly, the x-ray beam will be perpendicular to the horizontal or vertical axis of the patient. If the x-ray beam or patient is slanted, and not perpendicular, a lordotic (oblique) view will be obtained and will show the body or chest at an angle. An oblique view is undesirable if it is obtained unintentionally, because structures farthest from the beam will be shortened while those closest to the beam will be enlarged.²² The child’s nurse should assist the radiology technologist in ensuring that the child is not rotated during exposure of the image. This can reduce the need for additional radiographs, with the associated increase in radiation to the child.

In a lordotic film, the clavicles are projected higher than normal and may be seen to lie above the lung apices, and the ribs appear more horizontal. In addition, it may cause hazy opacification at the lung bases as a result of overlapping soft tissues with magnification of the heart and mediastinum. If the apices of the lungs are not visible above the clavicles on an AP or a PA chest film, a lordotic view has been obtained, and this must be considered when evaluation of lung size and chest expansion is made.¹⁷

The alignment and the position of the patient at the time the film is taken also must be considered (Fig. 10-1). To determine whether a patient was positioned appropriately during the radiograph exposure, several structures should be evaluated. This evaluation includes the position of the trachea, curvature of the spine, position of the clavicles, length and symmetry of the ribs, and cardiac landmarks. If the patient is positioned straight and not rotated on the image, the trachea should project in the midline within the upper third of the chest. The spine should be straight with the intervertebral bodies outlined as separate from the disc spaces. With good alignment, the clavicles are of equal size and length. The medial ends of the clavicles should be equidistant from the thoracic spinous processes; they are normally projected over the lung apices and overlap the anterior first ribs. If the clavicles both appear horizontal and of equal size and length, a true AP or PA view was obtained.¹⁴ If, on the other hand, one clavicle appears to be smaller than the other (because it was farther away from the x-ray tube at the time the film was taken), or if one clavicle is at a different angle than the other, the patient was probably rotated, and an oblique view of chest structures appears on the radiograph, creating the appearance that one side of the chest image (and one clavicle) is larger than the other. As noted previously, if the apices of the lungs are not visible above the clavicles on an AP or a PA chest film, a lordotic view has been obtained.¹⁷

Fig. 10-1 In this chest radiograph the child is rotated to the left. Compared with Fig. 10-2, A, which demonstrates evidence of proper positioning, in this film the proximal ends of the clavicles are not symmetrical in appearance. The patient’s left clavicle appears larger. The ribs on the patient’s right side are more curved posteriorly, and ribs in the patient’s left chest are more elongated with greater visualization of the anterior ribs. The cardiac silhouette is more prominent in the left chest with a normal cardiothoracic ratio, but the heart shape is altered by the rotated view.

It is important to consider patient alignment when making any observations about heart size. If the patient is rotated, the oblique image changes the heart shape and distorts the image, causing a spurious increase in cardiac size.

When the chest film is obtained, the radiology technologist and the nurse must be sure to note the position of the patient, whether upright (sitting or standing) or supine. The patient’s position is important because gravity will influence the appearance, position, and location of any free air or fluid in the chest. Free pleural fluid (such as that accumulating as a result of a hemothorax, chylothorax, or pleural effusion) will assume a dependent position. If the child is upright, free fluid tends to accumulate along the bases of the lungs and the diaphragm, and it will appear as a dense opacification obscuring adjacent structures. The fluid usually reaches a higher point along the lateral chest wall than along the mediastinum, which is referred to as the meniscus sign. If the film is taken with the child supine, free pleural fluid will accumulate in the back of the child’s pleural cavity, posterior to the lungs, and produce a more diffuse opacification throughout the affected hemithorax. Vascular markings are usually visible within the lung situated anterior to the fluid, which on an AP film can make this fluid difficult to distinguish from intrapulmonary congestion.

To determine the presence, quantity, and location of free intrapleural fluid, the child should be placed upright before and while the radiograph is taken. If this position is impossible or an inconclusive reading is obtained, a lateral decubitus film often will be obtained. In the lateral decubitus position, the patient is positioned with the affected side down, and any free fluid will accumulate in the dependent portion of the lateral pleural space.

Air tends to rise to the highest point within the chest cavity. As a result, in an upright view, free pleural air (such as a pneumothorax) will rise toward the apex of the lung. On a supine film, the highest point in the chest lies adjacent to the heart and mediastinum. When the child is supine for an AP film, free pleural air usually is seen along the diaphragm or the sides of the lung fields. Free air will typically cause increased lucency at these points, often with no lung edges or lung vascular markings visible.^9,¹⁴ If free air is suggested on a supine view, it can be confirmed by a decubitus view. When the child is positioned on the dependent side opposite to the free air, any air in the pleural cavity will rise to lie along the lateral chest wall.¹⁷

Ideally, chest radiographs should be obtained with the child in the upright position. In this position, air and fluid levels are more readily identified. In addition, when the child is upright, diaphragm excursion is usually better, enabling the child to inspire more deeply so that a better inspiratory film is obtained.

Film Technique

The exposure of the chest film will affect the intensity of the images on the chest radiograph. With the correct exposure, the spinous process of the lower thoracic vertebral bodies should be just visible through the cardiac shadow. If the radiograph is penetrated appropriately, the vertebral bodies will be clear and well delineated behind the heart, and disc spaces will be visible between the vertebral bodies. In addition, some pulmonary vascular markings will be projected over the heart shadow, because they are present behind the heart.

When the exposure of the film is not correct, the change in exposure will affect the entire film appearance. If the chest radiograph is underpenetrated (underexposed), all the structures on the radiograph will appear lighter in appearance or diffusely opaque (i.e., too white). The resulting images are difficult to interpret, preventing a detailed view of the mediastinal, retrocardiac, and spinal anatomy. Pathologic findings in the left lower lobe can be missed easily.²⁷ In addition, pulmonary vascular markings will appear to be more prominent, with less distinct borders; thus, they may be mistakenly interpreted to be increased or hazy resulting from interstitial edema. If a film is overpenetrated (overexposed), all the images on the radiograph will be darker or diffusely lucent. The lungs will appear blacker than usual. Overpenetration can obliterate shadows and can make pulmonary vascular markings appear to be reduced.¹⁶

Unless other orders are specifically given, chest radiographs should be obtained during inspiration. This practice maximizes the size of the lung fields, allows detection of intrapulmonary abnormalities, and makes the cardiac image sharp. To determine whether the radiograph was obtained during inspiration or exhalation, the nurse should count ribs visible above the diaphragm and evaluate rib interspaces. With good inspiration and chest expansion, the domes of the hemidiaphragm are forced down to the ninth or tenth ribs posteriorly. In addition, the normal trachea should appear to be straight. If fewer than nine ribs are visible above the diaphragm, the child probably was exhaling while the film was taken.

If the film is obtained during expiration, the heart will appear larger and less well defined. The lung fields will appear to be hazier and the pulmonary vascular markings more prominent. Hypoinflation of the lung fields can lead to misinterpretation of the images and misdiagnosis of a basilar pneumonia or cardiomegaly. Furthermore, if maximal inspiration is not present, the lungs can appear more congested and the trachea will appear to buckle to the right.^21,²⁴ Figure 10-2 illustrates these differences with two views obtained from a normal child during inspiration and expiration.

Fig. 10-2 Inspiratory and expiratory chest radiograph. Normal chest films obtained from the same child during inspiration and expiration. A, Inspiratory phase. Nine ribs can be counted above the diaphragm, indicating good inspiration. Alignment is good (note similarity of clavicles). Penetration of film is good (all vertebral bodies are visible; some pulmonary vascular markings can be seen). Intercostal spaces are equal; both sides of the diaphragm are visible. The mediastinum and trachea are straight (arrows). Heart borders are sharply defined, and heart size is normal. Pulmonary vascular markings are visible in the proximal two thirds of lung fields (normal). B, Expiratory phase. Only eight ribs are visible above the diaphragm (see numbers on ribs), indicating inadequate lung expansion. Alignment is good. Penetration of film is good. Intercostal spaces are narrow because expiration is occurring. Both sides of the diaphragm are hazy, and the left hemidiaphragm is not readily identifiable. The mediastinum appears widened, and the trachea seems to buckle to the right (arrows). Heart appears much larger than in A, and the heart borders are obliterated, but this is caused by expiration and reduction in apparent lung volume. Silhouette sign appears to be present, cardiothoracic size calculated from this view would be large, and pulmonary vascular markings appear prominent, but these are all artifacts caused by expiration.

(Courtesy H. Rex Gardner, Rush Presbyterian Saint Luke’s Hospital, Chicago, IL.)

The child must not move while the film is taken because motion can blur cardiothoracic structures. Blurring of the diaphragm can mimic the appearance of pulmonary infiltrates.¹⁹ If the nurse or radiology technologist suspects that the child moved when the film was taken, this should be noted for consideration when the film is interpreted. If excessive motion artifact is present, another film should be obtained.

Not all opacities on chest films originate in the lungs. Common artifacts that appear on chest radiographs include electrocardiogram leads, jewelry, snaps, ventilator or oxygen tubing, intravenous catheter lines, and wire sutures. When possible, before the chest film is taken, remove any external metal objects or extraneous items that can cause artifacts on the radiograph. Occasionally, long hair, wrinkles in clothing, or skin folds produce artifacts that resemble pulmonary infiltrates, air-fluid interface, or pneumothorax.

Interpretation of chest radiograph

It is good practice to develop a routine for reviewing chest radiographs. The first step is to verify that the chest radiograph is the correct film from the correct patient. Verify the patient’s name, date of birth, hospital number, and sex. Next, ensure that the correct film is being reviewed by verifying the date and time that it was taken. Last, confirm that the image is actually the type and view of image to be reviewed: AP, lateral decubitus, or other image type. Examine the film for technical qualities, as described previously. Include verification of the side marker, indicating left or right side of the patient, in addition to supine or lateral view.

Systematic Approach

When an organized systematic approach is used to examine the radiographic image, the nurse will be less likely to overlook significant abnormalities. It is often advisable to initially ignore the most striking or obvious features of a radiograph to avoid overlooking other equally important features. Table 10-1 provides one method of organizing the review of a chest film. Because there are many correct sequences that can be used to interpret the film, the nurse must develop an individual, comfortable style. The most important thing about any approach is that it must include all aspects of the film review. Once a healthcare provider develops a systematic and comprehensive approach to reviewing a film, the provider should use this method every time to ensure that nothing is missed during the review.

Table 10-1 An Organized Approach to Chest Radiograph Examination

Focus of Examination	Aspects of Examination
Technique	1. Check alignment and position (check clavicles)
	2. Check degree of inspiration (9 to10 ribs should be visible with full inspiration)
	3. Check penetration (vertebral bodies should be easily visualized)
Soft tissues of chest and neck	1. Check for subcutaneous emphysema
	2. Examine extrathoracic structures
Bony thorax and intercostal spaces	1. Examine clavicles, scapula, ribs, humeri, and cervical and thoracic vertebrae
	2. Check for fractures
	3. Evaluate width of intercostal spaces (should be equal)
Diaphragm and area below diaphragm	1. Note clarity of diaphragm and location (check for elevation)
Diaphragm and area below diaphragm	2. Check for location of gastric bubble (normal: patient’s left)
Pleura and Costophrenic angles	1. Check for presence of fluid or air between pleural layers (look between bony thorax and lung)
Pleura and Costophrenic angles	2. Costophrenic angle should be sharp
Mediastinum	1. Borders should be sharp
Mediastinum	2. Check for lateral shift
Trachea	1. Should be straight (may buckle to left on expiratory film if right aortic arch is present)
Trachea	2. Check for narrowing or deviation
Heart and great vessels	1. Borders of heart and aorta should be distinct
	2. Obliteration of heart borders is called silhouette sign (see Table 10-2)
	3. Measure cardiothoracic ratio—normally approximately 0.5 for children and adults (half the width of the chest)
Lung fields	1. Compare right with left
	2. Note presence and location of any opacification; describe it
	3. Look for air-fluid levels
Hili and pulmonary vascularity	1. Pulmonary vascularity is most prominent at hili
	2. Peripheral pulmonary vascular markings usually are visible in proximal two thirds of lung fields (note if markings are increased or decreased)
	3. Prominent but hazy pulmonary vascular markings may result from pulmonary edema or pulmonary venous congestion
Check location and continuity of all tubes, catheters and wires	1. Note position of head when evaluating endotracheal tube position
	2. Tip of endotracheal tube should be at level of third rib (1-2 cm above carina)
Compare with previous films

Current films should not be reviewed in isolation. They are most valuable when compared with the patient’s previous films to better appreciate changes. With the advent of the PACS, previous images are readily available for review and comparison with recent images. PACS allows for the sequential review of older images and can also allow for side-by-side comparison with the most recent image.

After review of the technical aspects of the chest film, the nurse can begin examination of the structures outlined by the radiograph. In this chapter, emphasis is placed on examination of the AP chest film because this is the projection most frequently obtained in the critical care setting.

Soft Tissues and Bony Thorax

Examine the soft tissues of the chest wall for evidence of subcutaneous emphysema (air density between the skin and bony thorax), which can result from an air leak around the chest tube or from a penetrating chest wound. In addition, tissue swelling can indicate the presence of an injury. Check the soft tissues of the neck for subcutaneous emphysema. In addition, if the child is intubated, note the position of the child’s head, because flexion or extension of the head can change the position of the tip of the endotracheal (ET) tube.¹⁵

Examine the bony thorax and the shape of the chest. Infants and young children normally have round chests with a horizontal orientation of their ribs (see Fig. 10-2). However, older children and adults have chests that are wider than they are deep, and their ribs angle downward from back to front. A round chest in an older child or adult is abnormal and may be the result of chronic respiratory disease with air trapping. As discussed, nine or ten ribs should appear above the dome of the diaphragm if a good inspiratory film has been obtained.

Check the continuity of vertebrae, ribs, scapulas, and clavicles for fractures and bony destruction. The ribs and intercostal spaces should be symmetrical. A fracture often will create a dark line in the bone because of separation of the bone fragments (Fig. 10-3). The vertebral bodies, particularly the cervical vertebrae, should be checked closely for fractures if the child has been admitted following trauma. The presence of multiple rib fractures, especially both posterior and anterior, and in various stages of healing is pathognomonic (positive predictive value of 95% or higher in children 3 years of age and younger) for child abuse.⁴ Healing fractures often have an enlarged area consisting of cartilage and woven bone creating a callus. Rib notching, or erosion of the underside of ribs caused by enlargement of the intercostal arteries, can be seen in older children with coarctation of the aorta, because the intercostal arteries become enlarged to carry collateral circulation around the area of coarctation into the descending aorta. Abnormalities of the ribs may be noted if the child has had surgery to alter the size or shape of the rib cage. Splinting can produce narrowing of the intercostal spaces on one side of the chest.