Interpretation of Chest X-ray: Basic Principles
Interpretation of chest x-ray requires several elements:
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An understanding of some basic principles and terminology of radiography
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A knowledge of the normal appearance of the chest x-ray
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A familiarity with the appearance of important pathologic findings
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A systematic approach to allow all key findings to be observed
We start with some basic principles. If you are already familiar with basic radiographic concepts and the normal appearance of the chest x-ray, move on to the section on pathology. But a few minutes familiarizing yourself with the ideas in this section can pay big dividends some day when you encounter an unfamiliar chest x-ray that does not fit any of the patterns of common pathology discussed in this chapter. Understanding basic principles may allow you to deduce the nature of the abnormality and avoid common pitfalls. A number of outstanding texts are devoted entirely to chest radiography. We have drawn on numerous sources for the following discussion.
Chest X-ray Techniques and Effects on Resulting Images
Familiarity with the chest x-ray technique is important for the emergency physician because it can affect the appearance of important diagnostic findings and the sensitivity and specificity of a chest x-ray for these diagnoses. In some cases, a given chest x-ray technique may falsely simulate pathology; in other cases, the technique many hide important abnormalities. Here, we describe the most common chest x-ray techniques, along with pitfalls of each. The most common variations of the frontal chest x-ray used in the emergency department are the anterior–posterior (AP) view and the PA view. The AP view is commonly performed as a portable study in the patient’s room, and it may be performed with the patient in a supine or upright position. The PA frontal view is performed in the radiology suite, and often a lateral projection PA chest x-ray is obtained at the same time.
Chest x-rays can be characterized by the following:
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The direction of the x-ray beam as it passes through the patient (posterior to anterior or anterior to posterior)
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The distances between the x-ray source, the patient, and the x-ray detector
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The position of the patient (upright, supine, lateral decubitus, lordotic, or oblique)
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The x-ray exposure
Each of these variables has important effects on the resulting image. In some cases, the technique is manipulated intentionally to achieve a desired diagnostic effect. In other cases, the patient’s clinical condition limits the x-ray technique, and we are forced to accept a suboptimal diagnostic image. Understanding the effects of the x-ray technique on the resulting image is invaluable in helping the emergency physician to avoid misinterpretation of the image and misdiagnosis of the patient.
Direction of the X-ray Beam With Respect to the Detector: Posterior–Anterior Versus Anterior–Posterior Technique
When a frontal projection chest x-ray is obtained, the direction of the x-ray beam as it passes through the patient to the detector can be from patient posterior to anterior or from patient anterior to posterior ( Figure 5-1 ). To restate this another way:
Anterior–posterior and posterior–anterior chest x-rays.
The designation refers to the direction of passage of the x-ray beam through the patient to the receptor.
In the case of the PA x-ray, the patient is oriented with the x-ray film or detector in contact with the anterior surface of the thorax. The x-ray beam passes through the patient’s posterior thorax to the detector. The patient faces toward the detector, away from the x-ray source ( Figure 5-1 ).
In the case of the AP x-ray, the patient is oriented with the x-ray film or detector, in contact with the posterior surface of the thorax. The x-ray beam passes through the patient’s anterior thorax to the detector. The patient faces toward the x-ray source, away from the detector.
Why does the direction of the x-ray beam through the patient to the x-ray detector matter? We return to this question in a moment, but first we describe a characteristic difference in the PA and AP x-ray techniques, which contributes to differences in the resulting image.
The PA chest x-ray is typically acquired in a radiology suite, with the x-ray source positioned 6 feet from the x-ray detector. In comparison, the AP chest x-ray is typically acquired as a portable examination, with only 3 feet separating the x-ray source and the detector.
Distance of the X-ray Beam With Respect to the Detector: Posterior–Anterior Versus Anterior–Posterior Technique
Why does the distance from the x-ray source to the detector matter? The answer becomes obvious if we consider the analogy of the children’s game of creating shadows on a wall with a light source ( Figures 5-2 and 5-3 ). You can try our example at home (use a light, not an x-ray) if it is unclear. Place a lamp 6 feet from a wall. Position your hand close to the lamp, and the shadow on the wall appears larger than the actual size of your hand, though rather indistinct at its edges. Most people have used this trick for their amusem*nt, making a hand appear as large as a head, for example. Without moving the lamp, move your hand closer to the wall; the resulting shadow becomes smaller (closer to its actual size), denser (darker), and sharper at its edges. We have now defined one of the two variables: with a light (or x-ray) source a fixed distance from a detector, positioning the object to be imaged close to the detector results in a sharper, truer image without false magnification. Therefore, when performing medical imaging, we should place the body part to be imaged as close to the detector as possible to achieve a sharp, unmagnified image. Now consider the same scenario, with a twist. Imagine that you cannot put your hand close to the wall because of furniture obstructing your path. How can you sharpen the image and reduce magnification? The answer is simple: move the lamp farther from your hand and the wall. Try this experiment, and you will find that the shadow of your hand becomes sharper and less magnified, truer to its actual size. Why would this scenario occur with medical imaging? Imagine placing your thorax against the wall; your ribs prevent you from moving your internal organs closer to the wall, although by facing toward or away from the wall you can position your heart (anterior in your chest) closer to the wall. However, you can easily control the distance of the light source from the wall to reduce magnification and improve the image sharpness. Therefore, when performing medical imaging, we should position the x-ray source as far as possible (within reason) from the body part to be imaged, to improve sharpness and reduce magnification.
A shadow experiment to illustrate principles of chest x-ray technique.
When the light source is a fixed distance from the wall, moving the hand closer to the wall results in a less magnified and sharper image.
A shadow experiment to illustrate principles of chest x-ray technique.
When the hand is a fixed distance from the wall, moving the light source farther from the hand and wall results in a less magnified and sharper image.
Magnification: Asset or Artifact?
Although magnification may sound advantageous, it can introduce clinically deceptive artifacts if some body parts are magnified to a greater degree than others. The most clinically accurate x-ray has no magnification. Why would we not want magnification? Wouldn’t magnification potentially assist in identifying pathology? The problem again can be resolved by considering the shadow experiment. In medicine, we need an accurate estimate of the actual size of body parts and an accurate measure of their size relative to one another. Consider the example of the hand shadow and the shadow of the human head. Although it may be amusing to have your hand mimic a giant dog biting a head, this scenario of varying magnification is highly undesirable in medical imaging. We need to know whether a mass has increased in size or if the heart and mediastinum are enlarged relative to the surrounding thoracic cavity. Our imaging technique must avoid false magnification, particularly when differential magnification of objects in the same image might occur.
Hopefully, we have convincingly demonstrated how the distances from the body part to the detector (assuming a fixed light source) and from the light source to the body part (assuming a fixed distance from body part to detector) affect the resulting images. Let’s briefly consider why this is the case. Figures 5-4 through 5-6 illustrate the effect of distance on object magnification. The angle created between the x-ray source and an object’s edges is determined by the distance from the x-ray source to the object. The shorter the distance from the x-ray source to the object, the greater the angle. Greater angles lead to greater magnification; therefore, a shorter distance from the x-ray source to the imaged object leads to increased magnification. Although increased magnification may appear to be a benefit, in most clinical scenarios this can lead to a false appearance of cardiomegaly or a widened mediastinum. This is particularly a problem if very short distances are employed, as thoracic structures nearer to the x-ray source will be magnified to a greater degree than structures farther from the source. Positioning the x-ray source at a greater distance from the object to be imaged reduces this relative magnification, leading to a truer representation of the object. The AP portable examination places the x-ray source 3 feet from the patient, rather than the 6 feet usually used for a PA examination. Consequently, the AP portable chest x-ray examination typically has a greater degree of false magnification of the heart and mediastinum compared with the PA technique.
Magnification and chest x-ray.
The degree of magnification is influenced by the distance of the x-ray source from the object being imaged. A more distant x-ray source results in a less magnified image.
Magnification and chest x-ray.
The degree of magnification is influenced by the distance of the x-ray receptor from the object being imaged. A more distant x-ray receptor results in a more magnified image. Moving the x-ray receptor closer to the object being imaged results in a less magnified image.
Chest x-ray technique and magnification.
An anterior–posterior chest x-ray technique results in greater magnification of the heart. A posterior–anterior chest x-ray positions the x-ray source farther from the heart, and the x-ray receptor closer to the heart, resulting in less magnification of the cardiac silhouette relative to the thorax.
We asked earlier why the direction of the beam (AP vs. PA) matters to the resulting image. As we described in our shadow experiment earlier, sometimes we cannot fully control the location of objects relative to the x-ray detector. We cannot change the anterior location of the heart in the chest; however, by positioning the detector on the anterior aspect of the chest, we bring the heart and the detector closer together, reducing magnification of the heart. Consequently, the PA x-ray technique results in less magnification of anterior structures compared with the AP technique. Remember this when viewing AP portable chest x-rays, which are more prone to false appearance of cardiomegaly or mediastinal widening ( Figure 5-7 ).
Differences between anterior–posterior (AP) and posterior–anterior (PA) chest x-rays and upright or supine positioning.
As discussed in Figures 5-2 through 5-6 , a PA chest x-ray results in a less magnified appearance of the heart, whereas an AP x-ray results in false magnification that can simulate or exaggerate cardiomegaly. Upright positioning typically results in better lung expansion. Supine positioning typically results in poorer lung expansion and clumping of pulmonary vessels in poorly expanded lungs, with an appearance that can simulate or exaggerate the appearance of pulmonary edema. Supine positioning also exaggerates the mediastinal width. A, An upright PA chest x-ray. B, A supine AP chest x-ray obtained a short time later in the same patient. The cardiac silhouette is enlarged in both but appears larger in B.
Patient Positioning for Chest X-ray: Supine Versus Upright Technique
Now let’s also consider the differences among a true upright, a supine, and an intermediate lordotic x-ray ( Table 5-4 ). Although occasionally other positions of the patient are desirable in specific clinical scenarios, for most applications, a perfectly upright patient position is desirable during chest x-ray acquisition. When the patient is positioned perfectly upright and perpendicular to the direction of the x-ray source, thoracic structures are positioned equal distances from the x-ray source, ensuring equal magnification on the resulting x-ray image. In comparison, if the patient is positioned in a lordotic position with the upper and lower thorax at different distances from the x-ray source, the magnification of superior thoracic structures will differ from that of the lower thoracic structures.
TABLE 5-4
Artifacts of Supine Chest X-ray Technique That Can Lead to Misdiagnosis
| Supine Chest X-ray Finding | Possible Misdiagnosis |
|---|---|
| Heart appears large | Simulates cardiomegaly |
| Mediastinum appears wide | Simulates aortic or mediastinal abnormality |
| Increased vascular markings in upper lung zones | Simulates pulmonary edema |
| Poor inspiration | Simulates pulmonary edema and may hide small nodules |
| Layering of fluid in plane of x-ray detector | May prevent recognition of pleural fluid |
| Distribution of air to anterior chest and abdomen | May prevent recognition of pneumoperitoneum or pneumothorax |
When the patient is positioned in a fully upright position, fluid within structures of the chest will generally reside due to gravity in a dependent position, forming fluid levels. In comparison, if the patient is positioned supine, the horizontal plane in which fluid will spread is parallel to the x-ray film or detector beneath the patient, and no fluid levels will be seen. (see Chapter 6 , Figure 6-2 ). The resulting appearance may be a diffuse increase in the density of the entire affected thorax, sometimes called a “veiling opacity.” This may be mistaken for an increased parenchymal density, rather than being recognized as a broadly layered pleural effusion.
Pleural fluid and fluid within collections such as lung abscesses typically are not visible on an image obtained with a supine patient but are visible on an image obtained with an upright position. In a patient positioned upright, the upper surface of fluid within the potential pleural space usually forms a curved line, higher along the lateral chest wall than at its intersection with the mediastinum. This appearance is termed the meniscus sign ( Figures 5-8 and 5-9 ). In contrast, fluid within an air-filled cavity usually forms a straight horizontal line without a lateral meniscus. Examples include fluid within an air-filled abscess cavity or fluid within a hemopneumothorax ( Figure 5-10 ; see also Figure 5-8 ).
The meniscus sign.
A, On an upright chest x-ray, fluid in the pleural space typically layers with gravity, forming a dependent collection. The upper surface of that collection usually forms a curve that is higher along the lateral chest wall than at its intersection with the mediastinum. This appearance is called the meniscus sign. B, In contrast, fluid within an air-filled cavity does not usually form a prominent meniscus and has a straight, horizontal upper surface. Examples include fluid within a pneumothorax and fluid within a lung abscess.
Pleural effusions: Posterior–anterior (PA) and lateral upright views.
A, A PA upright view, where a pleural effusion is most evident on this patient’s left side. Both costophrenic angles are blunted. The pleural effusion forms a meniscus against the left lateral chest wall. B, The lateral upright view shows two meniscus densities, suggesting bilateral pleural effusions. The posterior diaphragmatic recess is filled with pleural fluid, which forms a meniscus with the posterior chest wall. Compare with Figure 5-8 and 5-10 .
Pleural effusion with air–fluid level.
A, Posterior-anterior (PA) upright x-ray. B, Lateral upright x-ray. This patient has a large fluid collection in the right pleural space following a pneumonectomy. The appearance is somewhat different from that of even a large typical pleural effusion because of the absence of a lung and the presence of air in the pleural space. Normally, an effusion collecting in the pleural space forms a meniscus with the lung margin, coming to a “beak” or sharp point along the lateral chest wall. No air–fluid level is seen in a typical pleural effusion, because the pleural space is a potential space that contains no air. An exception would be a hemopneumothorax, where blood and air would coexist in the pleural space. An air–fluid level might also be seen in an empyema, if gas-forming organisms are present. In this case, air was introduced into the pleural space during the pneumonectomy, and reactive pleural fluid has accumulated. The x-ray alone cannot rule out infection. A, Look carefully in the apex of the right thorax and note the absence of lung markings. Also note the absence of the right main bronchus, which has been surgically removed. Its silhouette is missing, whereas that of the left main bronchus remains. Another possible cause of an absent bronchus silhouette would be a bronchus filled with fluid or tumor, which are both of water density. Compare with Figure 5-8 and 5-9 .
In an upright patient, air within the peritoneal cavity collects beneath the diaphragm, making it visible on x-ray because of the contrast between the density of air and diaphragmatic soft tissue. ( Figures 5-11 through 5-14 ). In a supine patient, air within the peritoneal cavity may collect in the midline anterior abdomen rather than in a subdiaphragmatic position and in addition will spread in the same horizontal plane as the x-ray detector beneath the patient, preventing the air from being visible. An x-ray obtained with an upright patient position is therefore more sensitive for detection of pneumoperitoneum.
Free air (pneumoperitoneum).
Free air (pneumoperitoneum) can be recognized on upright chest x-ray. This finding relies on the silhouette sign described later in this chapter (see Figure 5-26 ): air density is readily seen when in direct contact with water (soft-tissue) density. A, The normal appearance of the upright chest x-ray in the absence of pneumoperitoneum. B, The appearance of the upright chest x-ray in the presence of pneumoperitoneum. On an upright chest x-ray, normally the inferior surface of the right diaphragm is not seen, as the liver (water or soft-tissue density) is in direct contact with the inferior border of the diaphragm (also water density). When air is present within the peritoneal cavity, it may collect inferior to the right diaphragm, superior to the liver. This air may be recognized as a black line or collection, making the inferior border of the diaphragm visible. On the patient’s left side, the normal gastric air bubble can simulate intraperitoneal air, as it is normal for the stomach to contain air. Fortunately, usually the gastric wall and adjacent diaphragm are thicker than the diaphragm alone would be, allowing the normal gastric bubble to be distinguished from subdiaphragmic pneumoperitoneum. In some cases, pneumoperitoneum may extend beneath the central diaphragm, in which case the inferior border of the heart may be partially visible. Normally, the heart, diaphragm, and liver are in contact, with no visible line separating them. Compare with Figures 5-12 through 5-14 .
Free air.
Pneumoperitoneum is a critical finding requiring recognition on chest x-ray. Occasionally, it may be an unanticipated finding on chest x-ray in a patient who cannot provide an adequate history. Remember that normally the inferior surface of the diaphragm cannot be seen, as it is contiguous with a solid organ sharing the same water density on chest x-ray: the liver on the right, the spleen on the left. On the left, the interior surface of the stomach may be shown in relief by air within it. It may be difficult to distinguish this inner surface of the stomach from the inferior surface of the diaphragm, although the diaphragm alone should be thinner than the combined thickness of diaphragm and stomach. Within the abdomen, the external surface of the bowel wall should not be seen, again because of its contiguity with other soft-tissue structures. Air within the bowel is readily seen and makes the internal surface of the bowel wall quite apparent. When pneumoperitoneum exists, the external surface of the bowel can be seen. A normal finding that may simulate this is the presence of two adjacent loops of bowel with their walls abutting. In this case, the internal surface of both walls may be seen, and it may appear that air is present on both sides of the wall of a single loop. In A (upright PA x-ray) and B (upright lateral x-ray), copious free air is present. A, Both diaphragms are outlined, and several bowel loops can be seen with air on both sides of their walls. B, The lateral x-ray also demonstrates this finding. Compare with the CT from the same patient in Figure 5-13 .
Free air, abdominal CT.
Same patient as in Figure 5-12 . A and B are shown on lung windows to highlight the contrast between air and other soft tissues. In these axial CT images, free air is seen outlining loops of bowel. Note how both sides of the bowel wall are clearly visible when contrasted with air. B, Notice that where a loop of bowel abuts the anterior abdominal wall, only the internal surface of the bowel wall is visible. This is because the bowel wall and the abdominal wall share the same soft-tissue density. On CT, air may accumulate along the anterior abdominal wall, as this is the highest point in the abdomen when the patient lies supine for CT. In contrast, in an upright patient undergoing chest x-ray, air accumulates under the diaphragm. Air is an outstanding contrast agent because of its much lower density than abdominal tissues, so no administered contrast agents are required to detect it.
Free air.
This patient presented with epigastric pain and has dramatic free air (pneumoperitoneum) visible under the right diaphragm on this upright chest x-ray. The inferior surface of the diaphragm is readily visible because of air separating it from the liver. Normally, these two structures are indistinguishable. In a more subtle case, the only clue to free air might be a thin black line separating the diaphragm and liver. On a supine x-ray, air might not collect in a subdiaphragmatic position and might not be visible. This patient was found to have a perforated gastric ulcer at laparotomy.
Another advantage of the upright chest x-ray is that the patient typically is able to accomplish a greater degree of inspiration. With greater inspiration, blood vessels become more widely spaced, allowing other abnormalities to be recognized. Radiologists have sometimes compared this with seeing a bird in a tree, which is more easily accomplished in a tree with widely spaced branches. Small lung masses can be more easily seen as a result. A well-expanded chest on an upright view also gives a truer estimate of the heart size. Rarely, a chest x-ray obtained at end-expiration provides diagnostic benefits, which we will review later.
In comparison, a supine chest x-ray is usually characterized by higher diaphragms, with poorer lung expansion and clumping of pulmonary vessels. In addition, blood flow to the upper lung is relatively increased by gravity, and the heart appears larger than on an upright x-ray. This is due to the portable AP technique but also because of the relatively unexpanded chest, which makes the heart appear comparatively larger. In combination, these findings may mimic CHF (see Figure 5-7 ).
Supine and semilordotic views are often obtained in emergency department patients not by design but because patient condition prohibits the desired technique. Trauma patients, unstable medical patients, and intoxicated or neurologically disabled patients are just a few examples of patients who often undergo imaging in supine or semilordotic positions because of their inability to be positioned upright. The emergency physician must be aware of the limitations of these examinations and should recognize the patient position when interpreting the resulting images.
Lordotic and decubitus positions are sometimes obtained intentionally for specific diagnostic purposes, described later under “Additional Chest X-ray Views.”
Normal Appearance of a Chest X-ray
Recognizing pathology requires a strong familiarity with the normal appearance of the chest x-ray. Classic abnormalities are often recognized by their distinct differences from the normal chest x-ray appearance. More subtle abnormalities may be missed by an inexperienced observer, but an experienced reader may immediately recognize that the chest x-ray differs from the norm even before characterizing and articulating the abnormality. Radiologists understand that recognizing familiar patterns of normal and abnormal requires constant exposure, much like recognizing a familiar relative. This pattern recognition approach has been dubbed the “Aunt Minnie” effect for this similarity. We briefly describe the appearance of a normal chest x-ray here. In the sections that follow, we present numerous examples of important pathology. Even with several examples of each type, you will only begin the exposure required to “recognize Aunt Minnie.” Make a habit of looking at your patients’ chest x-rays, even if the radiologist has already rendered an interpretation.
Frontal (Posterior–Anterior or Anterior–Posterior) Upright Chest X-ray View
The normal frontal upright chest x-ray ( Figure 5-15 ) has the following features:
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The airway is midline.
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The bones show no fractures or lytic lesions.
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The cardiac silhouette occupies less than half of the transverse diameter of the thoracic cavity. The cardiac silhouette is crisp, with no adjacent pleural fluid or parenchymal opacities to disrupt the normal silhouette. The mediastinal width is less than 8 cm, and the aortic knob is well defined. The normal appearance of the aorta is described in detail in Chapter 6 , Chapter 7 .
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The hemidiaphragms are visible as smooth curves bilaterally. No air is seen beneath them; the upper surface is not obscured by pleural effusion or infiltrates. The costophrenic angles are not blunted by pleural effusions. The right diaphragm is slightly higher than the left.
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The lung fields are clear, without opacities to suggest pleural effusion, parenchymal disease such as infectious infiltrate, or mass lesion. Lung vascular markings are visible to the periphery, without evidence of pneumothorax. Lung markings are less prominent in the lung apices because gravity diverts blood flow to the lung bases.
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The minor fissure is invisible or subtle in appearance, without significant thickening to suggest fluid accumulation in this potential space.
The normal frontal chest x-ray.
Normal features of the posterior–anterior upright chest x-ray are demonstrated.
Lateral Upright Chest X-ray View: Retrosternal Space, Retrocardiac Space, and the Spine Sign
The lateral chest x-ray provides important diagnostic information. Unfortunately, this view is usually not obtained when a portable x-ray examination is performed—another good reason to send the patient to the radiology suite for imaging if the clinical condition permits this. The lateral view ( Figure 5-16 ) reveals the retrosternal space, which overlies the heart and mediastinum on a frontal projection. This space is usually quite lucent (black) because of the presence of a low-density epicardial fat pad and sometimes lung segments—but when occupied by a soft-tissue mass, this space may appear radiodense (white) ( Figure 5-17 ). The lateral chest x-ray also reveals the retrocardiac space. This space normally should be quite lucent (black) (see Figure 5-16 ). Lower lobe pneumonias may be evident on the lateral view as an abnormally dense retrocardiac region ( Figure 5-18 ). On the lateral view, the diaphragms usually form smooth curves descending from anterior to posterior. The space above the diaphragms is usually lucent (black), as it contains low-density lung tissue. Pleural effusions may be evident on lateral view as dense (white) layering opacities replacing the normal curve of the diaphragm in this space (see Figure 5-9 ). Sometimes pleural effusions form a meniscus against the posterior wall of the thorax, actually reversing the normal curve of the diaphragm. In addition, air beneath the diaphragm (pneumoperitoneum) may be visible on the lateral view (see Figure 5-14 ).
The normal appearance of the lateral chest x-ray.
The normal lateral chest x-ray shows progressive lucency (darkening) of the thoracic spine as it approaches the diaphragm. This normal finding is called the “spine sign.” Failure of the spine to become progressively more lucent as it approaches the diaphragm suggests an overlying opacity such as an infectious infiltrate or pleural effusion. The retrocardiac space should appear lucent (dark) as well, and the posterior diaphragmatic recess is dark and deep. The retrosternal space should be lucent (dark). A, A schematic representation. B, A patient with a normal spine sign and retrosternal and retrocardiac spaces.
Abnormal retrosternal space, suggesting anterior mediastinal mass.
The abnormal lateral chest x-ray shows loss of the normal lucent retrosternal space. This abnormality is easily missed but indicates soft-tissue density in the anterior mediastinum. The differential diagnosis includes the five terrible Ts: thyroid mass (goiter or malignancy), thymoma, teratoma, “terrible” lymphoma, and thoracic aortic aneurysm. A, A schematic representation. B, A patient with a retrosternal density, suggesting anterior mediastinal mass.
The spine sign.
The abnormal lateral chest x-ray shows loss of the normal progressive lucency of the thoracic spine as it approaches the diaphragm, called the spine sign. In addition, the retrocardiac space may be less lucent than normal, and the posterior diaphragmatic recess may appear shallow or less lucent than usual, indicating pleural effusion or infiltrate. A, A schematic representation. B, A patient with a retrocardiac infiltrate, illustrating a pathologic spine sign.
The thoracic spine also is visible on a lateral chest x-ray. The normal appearance of the spine is a gradually more lucent (blacker) appearance moving from cephalad to caudad (see Figure 5-16 ). This is not a result of decreasing spinal density but rather is a normal artifact of the examination technique. When this progressively more lucent appearance is lost, it implies the presence of an abnormal density in the retrocardiac space. This is called the spine sign and is a pathologic abnormality that can be a clue to disease. Remember that the increasing density has a differential diagnosis, including infectious infiltrate, pulmonary edema, pleural effusion, mass, and atelectasis. Other radiographic findings and the patient’s clinical presentation must be used to sort through this differential diagnosis, and additional imaging may be necessary. Nonetheless, this finding can confirm a pneumonia not seen on the frontal projection x-ray. The lateral x-ray is often neglected but is a key additional view that should be obtained whenever possible and carefully reviewed.
Additional Chest X-ray Views
Early radiologists became extremely skilled at deducing clinically relevant information from additional chest x-ray views. The advent of cross-sectional imaging with CT scan has made some of these views less common, as CT imaging is able to determine the three-dimensional location of objects with high accuracy. However, in some cases, the detailed information provided by CT is unnecessary, and more limited information from x-ray may be sufficient for clinical action such as draining a pleural effusion. Although CT could be used in this scenario, the radiation dose from CT is approximately 500 times that of a single additional chest radiograph, and the cost is approximately 10 times higher. In addition, for chest angiography protocols, chest CT requires IV contrast administration, with risks of allergy or contrast nephropathy. X-ray avoids these issues. On an individual basis, the emergency physician should consider the information needed for clinical decision-making. If it is obvious that CT will be required for specific information, such as evaluation of pulmonary embolism or aortic pathology, additional x-rays should generally not be obtained. However, if the information required can be provided by radiographs, these are a better choice for reasons of cost, radiation, and contrast exposure.
Lateral Decubitus Views
Lateral decubitus views allow assessment of radiographically visible mobile fluid collections and foreign bodies, as well as inferences about the presence of radiographically invisible foreign bodies. In a lateral decubitus view, the patient is positioned with one side of the thorax (right or left) in a dependent position ( Figures 5-19 and 5-20 ). The view is labeled based on the side of the chest that is dependent. Thus an x-ray obtained with the patient positioned with the left side of the thorax in a dependent position is a “left lateral decubitus” view. Usually, an x-ray is then obtained using an AP projection, as described earlier.
Lateral decubitus x-rays and pleural effusions.
A density seen on an upright chest x-ray can be pleural fluid, atelectasis, or a parenchymal opacity. When these conditions cannot be differentiated on the upright view, a lateral decubitus x-ray can sometimes prove the opacity to be mobile pleural fluid, which layers with gravity. A parenchymal opacity, atelectatic lung segment, or loculated pleural fluid maintains its position and does not layer with gravity on a lateral decubitus view. A, Schematic upright chest x-ray. B, Schematic right lateral decubitus x-ray, showing mobile pleural fluid layering in a dependent position.
Lateral decubitus x-rays and pleural effusions.
Same patient as in Figure 5-9 . In this patient with a pleural effusion, a decubitus view reveals the effusion to be mobile. A right lateral decubitus view (right side down) shows fluid to layer with gravity in the right chest, emphasizing the quantity and mobility of the right pleural fluid. If the density seen on the posterior–anterior upright x-ray had been a parenchymal infiltrate, atelectasis, or a loculated pleural effusion, it would not have moved with this change in the patient’s position. Compare with Figure 5-9 and Figure 5-19 , A and B.
Decubitus views can be useful in the following scenarios:
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Differentiation of parenchymal consolidation from pleural effusion. When opacities are visible on an upright or supine frontal projection (PA or AP), a lateral decubitus view can differentiate parenchymal consolidation from pleural effusion. A parenchymal opacity will not change in position relative to the patient when the patient is moved to a lateral decubitus position. A pleural fluid collection will layer with gravity if it is not loculated. (see Figures 5-19 and 5-20 ). Alternative means of differentiating these two entities include ultrasound, which readily identifies fluid as a hypoechoic collection, and CT.
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Identification of loculated versus freely mobile pleural fluid collections. When pleural fluid is seen on standard radiographic views, decubitus views can reveal mobile collections which will maintain their position relative to the patient. Ultrasound and CT can also differentiate these.
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Identification of mobile radiopaque foreign bodies. In some cases, the location of a visible foreign body may be further delineated by repositioning the patient. Airway foreign bodies may shift from the midline, while foreign bodies within the esophagus will remain midline despite dependent positioning.
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Identification of radiolucent (invisible) foreign bodies. Some aspirated foreign bodies such as peanuts or plastic are too low in density to be visible on standard radiographs. Their presence can sometimes be deduced from their effect on pulmonary inflation. A foreign body lodged in an airway can occlude the airway in a ball-valve fashion, trapping air within a lung segment and causing relative hyperinflation. Normally, when a patient is placed in a lateral decubitus position, the dependent lung appears hypoinflated because of the weight of other thoracic structures resting upon it. In the case of an obstructing airway, foreign body, air trapped within a lung segment will make the affected lung remain fully inflated despite lateral decubitus positioning. Of course, a foreign body that does not create a ball-valve effect would not be detected by this method.
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Identification of a pneumothorax in a patient who cannot tolerate an upright position. Remember that a pneumothorax may not be visible on a supine x-ray, because air will collect in the anterior chest rather than near the thoracic inlet or lung apices. The patient should be placed in a decubitus position opposite to the side of the suspected pneumothorax (hemithorax with the suspected pneumothorax should be higher), allowing air within the pleural space to rise to the lateral chest wall. As a consequence, the pleural line and absence of lung markings typical of pneumothorax may then be more visible.
Lordotic Views
In a lordotic view, the patient is positioned in a semiupright position relative to the x-ray source. The resulting image provides more detail of upper thoracic structures by altering the position of upper ribs and clavicles relative to the upper lung. Normally, these bony structures obstruct the view of the upper lung; a lordotic position can allow assessment of upper lung masses, infiltrates, or apical abnormalities such as pneumothoraces. The ribs appear more horizontal on a lordotic view. Although lordotic positioning is sometimes used for intentional diagnostic benefit, often lordotic positioning is an undesired effect in an emergency department patient undergoing an AP portable chest x-ray. Artifacts of this technique include magnification of the heart and mediastinum, simulating cardiomegaly, mediastinal mass, or aortic aneurysm. In addition, lordotic positioning may increase the amount of soft tissue projected over the lower abdomen, resulting in increased apparent opacity at the lung bases. This may be mistaken for basilar infectious infiltrates, atelectasis, or pulmonary edema.
Oblique Views
Oblique views, though rarely used in the emergency department, use parallax to identify the location of objects or structures within the lung. Two structures that overlie each other on lateral or frontal projection views can be distinguished by an oblique view. More often in the emergency department, an oblique view is inadvertently obtained because of rotation of the patient. This commonly occurs when AP x-rays are obtained in patients with kyphosis, contractures, or neurologic deficits. Rotated or oblique views can introduce a number of artifacts. Among them, the mediastinum often appears artifactually widened, and the heart and mediastinum may appear shifted ( Figure 5-21 ). Opacities in the lung bases may be hidden if the cardiac silhouette is projected over them because of rotation.
Rotation.
A good-quality chest x-ray should be obtained in a true frontal projection (A), free of rotation. An oblique film alters the apparent width and position of the mediastinum. Assess for this by checking the position of the clavicular heads, which should be symmetrically arrayed about the midline. Rotation of the patient shifts the position of the clavicular heads. Notice how the three-dimensional figures in the upper part of this diagram appear when rotated and then projected in two dimensions, as happens in a chest x-ray. The “heart” (ellipse) appears wider in the rotated views (B and C).
Phase of Respiration: Inspiratory and Expiratory Views
Normally, a chest x-ray is obtained at full inspiration, although in many cases patients may fail to inspire deeply because of chest pain or may be unable to hold their breath in this position because of dyspnea. The chest x-ray captures a frozen, static moment in time, but thoracic structures are actually in motion with patient respiration and cardiac activity. The apparent density of lung tissue varies with the phase of respiration. At end-expiration, lung volumes are very low, the diaphragms appear high, and lung tissue appears dense (whiter). Blood vessels in the lungs appear crowded together, contributing to the apparent density of lung tissue. If the phase of respiration is not considered, this appearance may be mistaken for pulmonary edema. In contrast, a chest x-ray obtained at end-inspiration shows well-inflated lungs, diaphragms that have descended fully, and widely spaced pulmonary vessels. Lung parenchyma under these conditions appears less dense. A clue to the phase of respiration is the number of ribs visible. A rule of thumb is that the diaphragm should lie at the level of the posterior 8th to 10th rib for an adequate respiratory effort in a good-quality chest x-ray. Abnormally dense lung parenchyma such as seen with pneumonia is more visible against the backdrop of fully inflated and lucent (black) lungs. Patients with COPD or asthma may have hyperexpanded lungs with low-density parenchyma and more than 10 ribs visible.
Occasionally, chest x-rays are intentionally obtained in other phases of respiration. Examples include:
- 1.
Pneumothorax: An end-expiratory chest x-ray is sometimes used to exaggerate the appearance of a pneumothorax. At end-expiration, lung parenchyma is deflated and occupies a smaller percentage of the thorax. In contrast, air in the pleural space will not decrease in volume during expiration. As a consequence, a pneumothorax will appear to occupy a larger percentage of the thoracic cavity.
- 2.
Foreign body. Low-density foreign bodies may be difficult to see on chest x-ray. However, they may exert a ball-valve effect by lodging in small airways. As a consequence, they may trap air within lung segments. During end-expiration, a normal lung will decrease in size. A lung with a partially obstructed bronchus due to a foreign body exerting a ball-valve effect may be unable to deflate during expiration. The abnormal lung or lung segment will therefore appear larger and more lucent than the normal side on an end-expiratory radiograph.
Chest X-ray Exposure (Penetration)
The exposure of chest x-ray is particularly critical, as the chest contains structures across a wide range of densities, from air to bone. A fully exposed x-ray film or detector results in a completely black image. For example, x-ray passes readily through air outside the patient, making the background around the patient appear black. Lungs are normally composed mostly of air, with a small density contribution from pulmonary blood vessels, and appear nearly black with a good exposure. A pneumothorax is even less dense, like air outside of the patient, and appears almost completely black. Metals and bones prevent transmission of much x-ray to the detector with a normal exposure; consequently, the detector is not exposed and the image appears white.
From these principles, we can extrapolate that an overexposed chest x-ray will appear black (the entire detector is fully exposed). An underexposed chest x-ray will appear nearly white, as the detector is not exposed. Although the appearance of tissues can be adjusted (brightness and contrast) on a digital picture archiving and communication system (PACS) display, no amount of image manipulation can overcome a badly over- or underexposed image. For example, in a badly overexposed image, the entire detector is fully exposed, and all pixels are completely black. Adjusting the contrast and brightness simply makes the entire image blacker or whiter, without revealing tissue detail. In a badly underexposed image, the entire detector fails to be exposed, and all pixels are completely white. Adjustment of brightness and contrast is not useful in this case, either. As the exposure level is increased above an “optimal exposure,” lung tissue becomes “burned out” (black), and fine details of lung architecture, such as bulla, fissures, pulmonary vascularity, and pneumothorax, are lost. This loss comes with a gain in the visibility of bony detail, as x-ray can now penetrate through less dense regions of bone, including fracture zones. If exposure is lower than an “optimal level,” detail of bone is lost, as no x-ray can penetrate through dense bone. At the same time, soft tissues become more visible, as the lower exposure prevents x-ray from fully exposing the detector behind them. Depending on the clinical presentation, overexposure- or underexposure may be intentionally performed to highlight either bone or soft-tissue detail.
Beware of some common scenarios in which poor exposure can simulate pathology:
- •
In the obese patient, underexposure is common. The detector is in effect “shielded” from the x-ray beam by overlying soft tissues. Consequently, the entire image, including lung tissues, appears brighter. This appearance can be mistaken for pulmonary edema if the overall image exposure is not considered.
- •
On a frontal projection, underexposure makes retrocardiac structures less visible, including the left lower lobe, which extends behind the heart.
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On a frontal projection, breast tissue can increase attenuation of the x-ray beam in the lower lung zones, resulting in relative underexposure of the lung in these regions. Consequently, the lower lung fields may appear brighter white, simulating bibasilar infiltrates, pulmonary edema, or atelectasis.
- •
In a patient with a unilateral mastectomy, an asymmetrical appearance of the lower lung zones can lead to false diagnosis of unilateral basilar pneumonia. Pay attention to this finding to avoid misdiagnosis. Use other x-ray findings such as the silhouette sign (described later) to assess for pathology in the lower lung fields.
Artifacts Outside of the Patient
Opacities and lucencies on the x-ray image may not originate within the patient ( Figures 5-22 through 5-25 ). Overlying skin folds and clothing can create lines simulating pathology such as the pleural line of pneumothorax. Foreign bodies may appear to lie within the patient but may be discerned to be outside of the patient if a lateral view is obtained. Two orthogonal views are needed to confirm an object’s location. Whenever possible, extraneous external foreign bodies should be removed before obtaining x-ray to prevent them from obscuring internal anatomy or being mistaken for objects within the patient. Sometimes surface anatomy such as nipples can simulate internal pathology such as lung nodules. When this is in doubt, marking the surface anatomy with a radiopaque tag can clarify the source of the chest x-ray finding.
Assessing for artifacts: Ring outside body.
A, Upright posterior-anterior (PA) x-ray. B, Upright lateral x-ray. This 39-year-old presented with shortness of breath. Chest x-ray was obtained and revealed an unexpected finding—a ring overlying the patient’s gastric air bubble. The patient then reported having lost her wedding ring some days prior, after removing her shirt. Ingested foreign bodies are a frequent complaint in the emergency department. Plain x-rays can assist in localizing these, in identifying the ingested item, and in predicting the likelihood of passage and need for intervention. When an ingested foreign body is suspected, two orthogonal views (posterior–anterior or anterior–posterior and lateral) are needed to localize the object. In this case, the object was not seen on the lateral view, suggesting that it was not within the patient. On reexamination of the patient, the ring was found in a skin fold on the patient’s back.
Assessing for artifacts: Button battery ingestion.
This 7-year-old child was “washing” a button battery in her mouth in an effort to fix her electric toy when she reported accidentally swallowing the battery. Parents were uncertain whether any ingestion had occurred. When an ingested foreign body is suspected, two orthogonal views (PA or AP and lateral) are needed to localize the object. The anterior–posterior (AP) supine view shown here reveals two radiopaque objects, labeled 1 and 3 on this image. The lateral view ( Figure 5-24 ) reveals 3 radiopaque objects, including an object not seen on this AP image but whose position is labeled 2. Did the patient ingest multiple batteries? Only object 1 is projected in a consistent location overlying the abdomen on both AP and lateral views and is intraabdominal. The other two objects are external to the patient. Object 3 on the lateral view (see Figure 5-24 ) corresponds to the caudad object on the AP view. Object 2 on the lateral view is so far lateral that it is not visible on the AP view. The patient underwent upper endoscopy, but the battery had moved beyond the pylorus and could not be retrieved. X-rays obtained 24 hours later confirmed passage of the battery.
Assessing for artifacts: Button battery ingestion.
Same patient as Figure 5-23 . The lateral view shown here reveals the three radiopaque objects. (See also Figure 5-23 .)
Assessing for artifacts.
This adult male presented in cardiac arrest and was successfully resuscitated. Chest x-ray was obtained and shows popcorn-like densities in the neck and right axilla. Are they a sign of a disease process? Could these be calcifications in lymph nodes related to metastatic disease? No, they are due to ice in bags placed outside the patient for postresuscitation neuroprotective cooling. Densities on x-ray must be considered in clinical context. Not all opacities are caused by pathology within the patient.
