Approach to Reading a Plain Chest Radiograph

A disciplined approach is required to obtain the maximal value out of any diagnostic imaging study. A plain chest radiograph may best exemplify this statement. An obvious abnormality such as a 6-cm mass is easily spotted, even by the untrained eye. Such an abnormality tends to monopolize the observer’s attention immediately, which causes more subtle abnormalities, often with equal or even greater diagnostic importance, to go unnoticed. To avoid this pitfall, the observer must develop a step-by-step approach that is applied to reading a plain chest x-ray in a disciplined fashion until it becomes second nature. The following suggestions are broad guidelines, and each observer must formulate an approach that he or she finds comfortable.

In broad terms, the steps in reviewing a chest film are as follows:

In subsequent sections, to discuss common abnormalities that the RT should recognize, the following areas are reviewed: (1) evaluation of the technical quality and adequacy of the film, (2) normal anatomic structures on a chest radiograph, (3) more sophisticated imaging techniques, and (4) major anatomic components seen on the radiograph.

Chest Radiograph Technique and Quality

Several technical factors should be routinely assessed when reading a chest film:

As the first step, the RT should check the patient’s identity on the film or image file and all labels visible on the film. This check helps avoid the mistake of interpreting a chest radiograph for a patient different than the patient being considered and establishes which side is which because labels are often placed to indicate the patient’s left or right side; such labeling of the side is important in cases where the patient’s chest or abdominal contents are reversed—known as situs inversus or dextrocardia.

A plain chest radiograph is taken using one of two techniques: the PA view or the AP view. The views are named for the path of the x-ray beam. In the PA view, the patient puts his or her back to the x-ray source and the chest against the film. The x-ray beam leaves the source, passes through the posterior (P) side of the patient, through the patient and then through the patient’s anterior (A) surface, and finally to the film. The PA view is usually performed in the radiology department with equipment that standardizes the distance from the x-ray source to the film and where the x-ray technician can maximize the quality of each film. In addition, as noted, taking the film with the anterior chest closest to the film minimizes magnification of the heart.

The AP film is usually taken with a portable x-ray machine. The AP technique puts the x-ray source in front of the patient with the film behind the patient’s back. The source of the x-ray beam is usually much closer to the patient than with a PA film, although the distance varies from patient to patient. The closer x-ray source and the position of the patient both lead to a slight magnification of the heart shadow. The AP film is usually taken in the intensive care unit because these patients are too ill to go out of the intensive care unit to the radiology department. Overall, AP portable films are usually of lesser quality compared with PA films. During the reading of the chest radiograph, the RT needs to take into account the view (AP or PA) as he or she interprets the heart size and subtle findings that may be influenced by film quality and technique.

When a chest film is taken using the portable AP technique, it is sometimes difficult to align the patient properly, and a portion of the chest may be missed. Although these problems are seen far more often with portable films, they may occur with PA and lateral films as well. The RT should ask the following questions: (1) Is the whole chest visible on the film? (2) Is the patient well positioned?

Patient rotation can make interpretation more difficult by projecting midline structures (e.g., the trachea) to the right or left. The observer can assess for rotation by comparing anterior structures such as the medial (toward the middle) ends of the clavicles with a posterior structure such as the spinous process (midline structure of the spine). In a perfectly positioned or aligned chest film, the spinous process should be seen midway between the medial ends of the clavicles and in the middle of the tracheal air column (Figure 20-1). Patient rotation makes the mediastinum appear unusually wide and obscures or distorts the appearance of the pulmonary arteries as they emerge from the mediastinum into the lung parenchyma.

The RT also must ensure that the film is adequately penetrated. An improperly penetrated film may conceal important details. A chest radiograph with proper exposure should show the intervertebral disc spaces through the shadow of the heart and should allow the blood vessels in the peripheral regions of the lungs to be visualized. A chest radiograph that is underexposed or underpenetrated (i.e., owing to too-low kilovoltage of the x-ray beam) does not allow visualization of the intervertebral discs through the heart shadow and may make identification of pathology in soft tissue areas such as the mediastinum more difficult. Specifically, an underpenetrated film may cause the normal branching of the pulmonary arteries in the lung to appear abnormal and be misinterpreted as evidence of interstitial infiltrates. Similarly, an overpenetrated radiograph overexposes the film, leaving the lung parenchyma black and no ability to visualize the peripheral blood vessels or abnormalities that may be present (e.g., infiltrates secondary to pneumonia, pulmonary nodules). This overpenetration makes evaluation of the lung parenchyma far more difficult. Adjustment of the contrast and brightness of the chest film on the computer display improves the ability to see certain aspects of a chest x-ray with improper penetration. However, adjusting the display cannot completely overcome the loss of important details caused by an improperly penetrated film.

Anatomic Structures Seen on a Chest Radiograph

After the RT has reviewed the technical aspects of the chest radiograph, it is time to review the anatomic findings in the film. The main structures imaged on a routine chest radiograph are listed next and illustrated in Figure 20-2:

The anatomy seen on the film should be reviewed in a thorough, systematic manner. All of the above-listed anatomic structures must be individually assessed. When first beginning to read films, it is helpful for the RT to create a list of the anatomic structures that must be assessed and to check off the structures as they are reviewed. As the reader gains experience, the checklist becomes second nature and automatic.

Assessment of the chest wall should include looking for symmetry, rib fractures, or other bone changes. Lung evaluation begins by assessing the size and density. Any obvious differences in symmetry must be explained. Of the lung parenchyma, 80% to 90% is overlaid with bone in the form of ribs, clavicles, and spine. The overlying bone may conceal some lung abnormalities. A lateral film is helpful in clarifying the presence or absence of suspicious lung abnormalities on frontal projections. The RT must pay specific attention to areas where subtle abnormalities may be hiding; these include the lung tissue behind the clavicles (especially medially), the area of lung that projects behind the heart, and part of the lung that lies deep in the posterior sulcus (the extreme bottom of the lung projecting behind the dome of the diaphragm on the frontal view).

Review of the lung edge on both frontal and lateral films discloses any pleural abnormalities, such as fluid in the pleural space (e.g., hydrothorax, hemothorax [blood in the pleural space]) and air in the pleural space (pneumothorax). Evaluation of the mediastinum should include assessment of heart size. In the PA projection, the diameter of the heart shadow should not exceed one-half the diameter of the chest. An enlarged heart may occur with congestive heart failure or with a large pericardial effusion (accumulation of fluid within the space that surrounds the heart encased within the pericardium). The lateral contours of the mediastinum should correspond to normal anatomic structures as outlined in Figure 20-3.

Computed tomography (CT) scanning is a very helpful chest imaging technique because it can visualize structures in cross section and can visualize great detail and miniscule structures (e.g., approximately 2 mm) within the lungs. To perform a CT scan, a patient lies on an examination table called a gantry. The gantry is passed into a circular opening in the CT scanner. X-ray sources and detectors surround the opening in the scanner. When the scanning begins, the x-ray source and detectors pass quickly around the patient in a circular motion with the x-ray beam passing through the patient to the detector on the opposite side. The information from the detector is sent to a computer, which calculates the two-dimensional image from the data sent to it. The image created by the scan looks like a slice of the patient. Originally after each CT image was made, the gantry and patient were advanced 1 cm for the next image. The scanning and stepwise advancement of the patient were repeated until the entire chest was imaged.

Newer CT scanners use many x-ray sources and detectors, all connected to a highly capable image processing computer. These new CT scanners allow the patient and the examination table to pass through the scanner rapidly without stopping for each image. The term spiral or helical is often applied to these high-powered CT scanners, which can gather complete images in seconds.

Conventional CT scanning provides an excellent view of the chest and allows imaging of portions of the chest that are poorly seen on plain chest radiographs. Areas such as the mediastinum, the apices and costophrenic sulci of the lungs (the normally sharp shadows where the diaphragm contacts the rib cage laterally), and the pleural surfaces all are easily seen with CT scanning. Injection of iodinated contrast material makes blood appear denser (radiopaque or white) and allows blood vessels to be distinguished from soft tissue structures such as lymph nodes, further enhancing the ability to evaluate areas such as the mediastinum. Conventional CT scanning of the chest is commonly performed to evaluate the following: lung nodules and masses, great vessels of the chest, mediastinum, and pleural disease.

Conventional CT scans display images as slices every 3 to 7 mm. Each slice displays everything within the 3- to 7-mm slice of tissue. CT scans can also evaluate the delicate structures of lung parenchyma. To see lung parenchyma optimally, images need to be displayed with extremely thin slices, often 1 mm thick or less. To limit the number of images to be reviewed, thin-slice CT scans often provide images at intervals of 5 mm or 10 mm. When displayed this way, thin-cut CT scan images provide great detail of lung parenchyma but just a sampling of lung tissue rather than displaying the entire chest. The term high-resolution CT scanning (HRCT) is associated with CT scans designed to evaluate the lung parenchyma using thin-slice images. High-quality CT scanners often acquire much more image data than are displayed. Image data can be easily formatted into either conventional or thin-slice (HRCT) format. If the original image data are saved, a radiologist can easily go back and generate thin-slice images even if the CT scan was first displayed as a “conventional” CT scan. Thin-slice displays are especially helpful in imaging small nodules or the details of parenchymal infiltrates (e.g., interstitial lung disease) because such thin slices allow maximal spatial resolution (i.e., the ability to separate objects that are close together).

Computed Tomography Angiography

The rapid scanning that can be performed on helical CT scanners has made CT angiography possible. To perform CT angiography, a large amount of contrast dye is injected into the patient’s vein. The CT technician monitors the movement of contrast material so the scan can be started when the contrast material has entered the area to be studied. CT angiography of the chest has been used for years to identify pulmonary thromboemboli (Figure 20-4).¹ More recently, CT angiography of the coronary artery has been evaluated; this seems to provide an alternative to routine coronary angiography in many patients.

Three-Dimensional Reconstruction

The imaging processing capabilities of modern CT scanners allow reconstruction of the chest in any direction and production of three-dimensional representations of some areas of the body (Figure 20-5). The term virtual bronchoscopy is used when these reconstructions simulate the view of the airways that a physician would have during bronchoscopy.

Ultrasound

Ultrasound imaging is created by passing high-frequency sound waves into the body and detecting the sound waves that bounce back (echo) from the tissues of the body. The pattern of the returning sound waves is used to generate an image of the tissue studied. Ultrasound of the chest is excellent for evaluating the heart or pleural fluid.³ Ultrasound evaluation of the lung itself is rarely useful because of the poor ability of ultrasound to transmit through the air-filled lungs.

Ultrasound imaging using small portable machines has become common in critical care units. Portable ultrasound units allow rapid assessment of heart function and volume status and are used to assist in many critical care procedures.⁴ Ultrasound is also commonly used to guide the placement of central and arterial catheters. Blood vessels can be easily identified using ultrasound. The compressibility of veins is used to differentiate veins from arteries (Figure 20-6). Because the path the needle is taking is clearly seen on the ultrasound screen, using ultrasound guidance for venous and arterial puncture allows the procedure to be more easily accomplished with less time, risk, and patient discomfort.

The remainder of the chapter outlines commonly encountered abnormalities involving the pleura, lung parenchyma, and mediastinum. The reader is encouraged to fine-tune his or her observational powers for assessment of imaging studies because, as noted by Pasteur, “In the field of observation, chance favors the prepared mind.”

 Rule of Thumb

Three general steps to assessing a chest film are as follows:

1. Content assurance: Is the entire chest visible on the film?

2. Quality assurance: Is the chest radiograph properly exposed and centered?

3. Disciplined application of personalized search pattern

Hydrothorax

In healthy individuals, it is estimated that 1 to 8 ml of pleural fluid is normally present.⁵ Hydrothorax (also called pleural effusion) refers to the accumulation of excessive fluid within the pleural space. Normally, the diaphragm forms a dome that curves down to attach to the chest wall on the lower ribs and thoracic vertebra. On a chest radiograph, the arch of the diaphragm and the chest wall meet to form a point called the costophrenic angle. The costophrenic angle is seen on both PA and lateral views (see Figure 20-2). If the point of the costophrenic angle is rounded rather than sharp, it usually indicates a pleural effusion is present (Figure 20-7).⁶ For a pleural effusion to cause blunting of the costophrenic angle on the frontal view, at least 175 to 200 ml of pleural fluid must have accumulated. The lateral film detects smaller pleural effusions than are detected with the frontal view. The posterior costophrenic angle becomes blunted with 75 to 100 ml of fluid. The best film for detecting small amounts of pleural fluid is the lateral decubitus view, which is a frontal view taken as the patient is lying on the side of the suspected effusion; 5 ml of pleural fluid can be detected on a decubitus radiograph.⁷

Sometimes, fluid can accumulate between the lung and the diaphragm and maintain a sharp costophrenic angle, hiding 500 ml of fluid.⁸ Fluid that accumulates between the lung and the diaphragm is said to be in a subpulmonic location. The subpulmonic location is the first place pleural effusions accumulate in an upright patient.⁹ The earliest sign of a left-sided pleural effusion on an upright chest radiograph is an increased distance between the inferior margin of the left lung and the stomach gas bubble. With a subpulmonic effusion, there may be an associated slight lateral shift of the point at which the diaphragm dips downward on the frontal chest radiograph (i.e., similar to a hockey stick with the blade toward the lateral chest wall).

If both air and fluid are contained within the same space, the interface between the air and the fluid forms a soft tissue density with a straight, level border that has air density above it. The interface may have a small meniscus on both sides. These straight, level interfaces between air and fluid are called air-fluid levels. An air-fluid level in the pleural space indicates a hydropneumothorax (Figure 20-8).

Occasionally, fluid occupies an unusual position, such as within an interlobar fissure (which separates lobes of the lung). Fluid is most commonly seen in the minor fissure, which is between the right middle lobe and the right upper lobe. Fluid within a fissure can be diagnosed on a chest radiograph by a characteristic lenslike, elliptic shape on either the PA or the lateral projection (Figure 20-9).

An increased volume of fluid generally is categorized as either a transudate or an exudate (see Chapter 25), but an exudate cannot be distinguished from a transudate on a chest radiograph. This distinction requires analyzing a sample of the pleural fluid. Loculation of pleural fluid (or trapping so that the fluid does not move freely with changing positions) is more commonly seen in exudative effusions, hemothorax (blood in the pleural space), and empyema (infection of the pleural fluid).

Clues as to whether a pleural exudate results from inflammation or from cancer may be present on the chest radiograph. Clues that favor a malignant cause for a pleural effusion include surgical absence of a breast shadow (breast cancer), evidence of prior axillary (armpit) node dissection (breast cancer), a pulmonary parenchymal mass (lung cancer), or multiple lung masses (metastatic disease).

Computed Tomography

Pleural fluid can be identified easily on CT scans of the chest. In a supine patient, free fluid accumulates in the most dependent area of the pleura, which is posterior. Pleural fluid that does not flow to the posterior thorax is loculated.

The pleural lining is enhanced by contrast media with some forms of pleural disease. Pleural thickening and nodularity are well seen with contrast-enhanced CT scan. An elliptic pleural fluid collection with thickening and enhancement of the surrounding pleura suggests an empyema, which is infected pleural fluid.¹¹ The presence of gas bubbles within the fluid without prior surgery or needle insertion (which can introduce air) establishes the diagnosis of empyema (Figure 20-10).

Pneumothorax

The term pneumothorax refers to the collection of air within the pleural space. The visceral pleura surrounding the lung becomes visible when air accumulates in the pleural space. Pneumothorax may occur spontaneously because of rupture of a bleb (a gas-containing space within the visceral pleura of the lung—a form of pulmonary air cyst) or may result from trauma or an invasive procedure that punctures the pleura, such as a transbronchial biopsy or percutaneous aspiration lung biopsy. Pneumothorax may also occur as a complication of positive pressure ventilation (which is called barotrauma). When the patient is upright, the intrapleural air accumulates over the top of the lung (apex) and pushes the lung away from the chest wall. The clinician can easily detect a pneumothorax by seeing the thin pleural line at the lung margin and noting the absence of bronchovascular markings between the lung margin and the inner aspect of the chest wall (Figure 20-11). If a diagnosis of pneumothorax is suspected, an upright chest radiograph should be obtained. Visualizing a small pneumothorax may be assisted by taking the chest radiograph when the patient exhales.

When the patient is supine, the free air in the pleural space moves to the highest point in the chest, which is the anterior cardiophrenic sulcus (see Figure 20-1).¹² Because air in this region does not create a visible edge between the pleura and the x-ray beam, radiographic clues to the presence of pneumothorax are more subtle in a supine patient.¹² A supine patient with a pneumothorax may have a deep sulcus sign (Figure 20-12),¹³ which refers to air accumulating anteriorly and outlining the heart border below the dome of the diaphragm. In addition, the upper abdomen on the same side often shows increased lucency. If the diagnosis remains in doubt, a decubitus radiograph or a cross-table lateral radiograph (in which the patient lies face up while the x-ray is directed across the body) can help make the diagnosis of pneumothorax.

A pneumothorax may be difficult to diagnose if a patient has bullous emphysema. If after carefully examining the chest film, there is uncertainty about the presence of a pneumothorax, a CT scan of the chest can resolve the question. Skin folds can mimic a pneumothorax. To avoid mistaking a skin fold for a pneumothorax, the clinician needs to look carefully at what appears to be the lung margin. The absence of the pleural line at the lung margin and the presence of bronchovascular markings between the lung margin and the chest wall suggest a skin fold rather than a pneumothorax.

Occasionally, air within the pleural space may be under pressure or tension (Figure 20-13); this is called a tension pneumothorax. Tension pneumothorax is an emergency that occurs when the tear in the pleura (which allows air to leave the lung and enter the pleural space) opens on inspiration but closes on expiration. Air continues to accumulate in the pleural space and can compress the heart and adjacent lung. A tension pneumothorax is suggested on chest films when the hemidiaphragm is pushed down inferiorly or when the mediastinum is shifted toward the opposite lung. A tension pneumothorax requires immediate decompression with a chest tube, Heimlich valve, or needle aspiration of the trapped air.

Alveolar Disease

When alveoli are filled with something denser than air, they have a characteristic radiographic appearance regardless of the material that fills them. The type of fluid that fills the alveoli varies depending on the disease process. In the case of pulmonary edema, the alveoli are flooded with a watery fluid that contains few blood cells. With bacterial pneumonia, the alveoli are filled with an exudative fluid containing numerous white blood cells (pus). In the case of pulmonary hemorrhage, the alveoli fill with blood. In the condition known as pulmonary alveolar proteinosis, the alveoli fill with a fat-rich material derived from pulmonary surfactant. Both pneumonia and a bleeding lung can cause identical-appearing patchy, increased density shadows that tend to coalesce over time on the chest radiograph. These shadows are often referred to as infiltrates.

These shadows, or opacities, often have lucent tubular visible structures running through them that represent air bronchograms (Figure 20-14). Normally, patent airways are invisible in the outer two-thirds of the lung on a chest radiograph. There is no contrast between air in the airway and air in the lung. However, the increased contrast produced by filling of the surrounding alveoli with fluid makes the airways more visible and causes the air bronchogram sign. Air bronchograms are the hallmark of infiltrates that fill alveoli (so-called airspace disease) (Figure 20-15 and Box 20-2).

Pulmonary Edema

Pulmonary edema is one of the most common chest film findings in critically ill patients. Pulmonary edema can be caused by vascular congestion, loss of integrity of the pulmonary capillaries, or some combination of both factors. Edema from vascular congestion can be caused by failure of the left heart (cardiogenic pulmonary edema), renal failure, or fluid overload. Breakdown in the integrity of the lung capillaries can also cause pulmonary edema as in acute respiratory distress syndrome (ARDS; see Chapter 27).

The development of cardiogenic pulmonary edema can be described through a series of changes on the chest film. Before pulmonary edema develops, the pressure in the pulmonary veins increases. The increasing pressure in the pulmonary veins can be seen on the chest film as enlarging blood vessels to the apices of the lungs. If the blood vessels to the apices of the lungs are the same size or larger than the blood vessels to the base, the vessels are said to be “cephalized” (Figure 20-16). Cephalization of the pulmonary blood flow is often caused by left-sided heart failure.

As fluid builds up from the high venous pressures, thickening of bronchial walls (peribronchial cuffing) (see Figure 20-16) and edema in the septa that separate the lung lobules become evident. The thickened septa are most clearly seen as thin lines against the pleural edge that run perpendicularly away from the pleural edge. These lines are called Kerley B lines (Figure 20-17).

The development of edema in the lung itself is seen first in the hila of the lungs by blurring the normally distinct walls of the hilar blood vessels; this is followed by blurring and increased haziness caused by the edema progressing outward toward the pleura. The term bat’s wing appearance is applied to the predominance of edema in the hilar regions of both lungs with progressively less edema in the more peripheral areas of the lungs (Figure 20-18).

In addition to the above-mentioned classic signs of pulmonary edema, many patients with long-standing heart failure have enlargement of the heart or a pleural effusion. Pleural effusions from heart failure are usually bilateral. If the effusion is visible only on one side, it is more commonly on the right side than the left.

The appearance of ARDS is sometimes similar to the appearance of other forms of pulmonary edema. However similar they may appear at first, there are some key differences to help distinguish ARDS from pulmonary edema caused by high vascular pressures. The edema of ARDS is patchy and bilateral and does not predominate in the central hilar regions. A chest film of a patient with ARDS also lacks cardiomegaly, cephalization, and Kerley B lines, which are often seen in cardiogenic pulmonary edema.

Interstitial Disease

Diseases that mainly involve the interstitium of the lung have a different radiographic appearance than alveolar diseases (see Box 20-2). The interstitium of the lung represents the part of the lung that frames the airspaces and supports the vessels and bronchi as they travel through the lung. A pulmonary lobule is the smallest functional unit of the lung.¹⁴ A pulmonary lobule contains alveoli and alveolar ducts built around a central pulmonary arteriole and bronchiole, all surrounded by a thin sheet of fibrous connective tissue called the intralobular septa. Intralobular septa are invisible on a normal chest radiograph. Pulmonary edema secondary to poor left-sided heart function causes edema of the intralobular septa. As noted, short thin lines from the edematous intralobluar septa can be seen perpendicular to the pleura (see Figure 20-16); these are Kerley B lines.

Interstitial lung disease (see Chapter 24) refers to a group of diseases that involve the lower respiratory tract. Chest radiographs of patients with interstitial lung disease may have several different appearances, depending on the stage and type of interstitial lung disease (see Box 20-2). A chest radiograph of a patient with interstitial lung disease usually has diffuse, bilateral infiltrates. The infiltrates may resemble scattered, ill-defined nodules (nodular); a collection of scattered lines (reticular); a combination of both nodules and lines (reticular-nodular); or honeycombing, which is the development of cystic spaces with well-defined walls seen in the periphery of the lung and resembling a bee’s honeycomb. Honeycombing is thought to represent irreversible scarring and indicates end-stage lung disease (Figure 20-19).

There are many types of interstitial lung disease. Causes include infectious (e.g., viral pneumonia) or occupational exposures (e.g., to asbestos [asbestosis] or to silica [silicosis]). The two most common interstitial lung diseases, sarcoidosis and idiopathic pulmonary fibrosis, have no known cause.¹⁵ Because many different types of interstitial lung diseases have the same appearance on a chest radiograph, the chest film rarely helps establish the specific cause of interstitial disease. Clues to specific causes of interstitial lung disease on a plain chest film are reviewed in Table 20-1. HRCT has become an important tool in establishing the specific form of interstitial lung disease that a patient may have. HRCT is particularly helpful in diagnosing idiopathic pulmonary fibrosis.¹⁶

Assessing Lung Volume

Volume loss, or atelectasis, is a common abnormality on chest radiographs, and the location and extent of volume loss produce characteristic chest radiograph patterns. Atelectasis may be localized to a subsegmental portion of the lung, where it has a classic radiographic appearance called plate (or platelike) atelectasis (Figure 20-20).¹⁷ Plate atelectasis is seen occasionally on chest films of normal individuals but is often associated with ventilatory disturbance, including restricted diaphragmatic motion, sometimes with resultant alveolar hypoventilation; retained secretions, producing small airway obstruction; and diminished surfactant production.¹⁸ Atelectasis commonly occurs after abdominal or thoracic surgery, with pleurisy, or after pleural irritation from rib fracture or pulmonary infarction.

Volume loss involving a whole lobe is usually caused by central airway obstruction.¹⁹ The collapsed lobe assumes the shape of a wedge with the apex of the wedge at the hilum and the base of the wedge on the pleural surface. This wedge is visible on a PA or lateral x-ray film, depending on which lobe is collapsed (Figure 20-21). The central bronchial obstruction may be caused by cancer, a foreign body, or a mucous plug (Figure 20-22). As shown in Figure 20-23, a bulging convexity to the apex of the wedge indicates a central tumor.

Atelectasis of a segment or lobe of the lung causes changes to surrounding structures. As lung volumes decrease, surrounding tissues collapse in to fill the space the collapsed segment or lobe usually fills. The diaphragm becomes elevated on the side of the atelectasis, the mediastinum shifts toward the atelectasis, and poor expansion of the chest causes narrowing of the space between the ribs. If the collapsed segment of the lung is in the upper lobe, the hilum is displaced upward, and the minor fissure on the right is displaced upward.

Assessment of lung volumes on a chest radiograph requires several observations. Rib counting is a popular method to assess lung volume. With a good inspiration, the sixth and sometimes the seventh anterior rib should project above the diaphragm. If more than seven anterior ribs are visible above the diaphragm, hyperinflation is present. Obstructive pulmonary disease is classically associated with increased lung volumes (hyperinflation). In patients with chronic obstructive pulmonary disease, there may also be an increase in the AP diameter of the chest, with associated enlargement of the retrosternal and retrocardiac airspaces and flattening of the hemidiaphragms. These all are secondary signs of pulmonary emphysema. The only primary signs of emphysema are loss or shifting of pulmonary vessel markings and the appearance of the walls of bullous airspaces (Figure 20-24).

Because radiographic signs of emphysema are apparent only with more advanced disease, the chest radiograph is generally considered insensitive for detecting obstructive lung disease. However, HRCT is far more sensitive and may show evidence of emphysema even when pulmonary function test results are normal.²⁰ Figure 20-25 shows a case of upper lobe paraseptal emphysema, characterized by cysts on the pleural surface. A chest CT scan may prove useful to help define which patients may benefit from treatments such as lung volume reduction surgery.

Solitary Pulmonary Nodule

A solitary pulmonary nodule (SPN) is a parenchymal opacity smaller than 3 cm in diameter that is totally surrounded by aerated lung. One or two SPNs are encountered in every 1000 chest radiographs. SPNs are important to identify because they may be caused by lung cancer. The reported prevalence of malignancy in SPN ranges from 3% to 6% in large surveys of the general population. In patients with SPN who have surgical resection, 30% to 60% of the nodules are malignant.²¹

When first encountered, SPN should be assessed for features listed in Table 20-2 that may help establish a nonmalignant cause. The goal of imaging SPNs is to avoid resecting benign nodules, while encouraging surgical removal of all potentially curable cancers. The axial anatomic display of CT coupled with better density-discriminating powers make CT a favored tool for evaluating SPN. CT provides a detailed evaluation of the shape and edges of pulmonary nodules, in addition to helping to identify whether calcification is present and, if so, the pattern of its calcification (Figure 20-26).

Central, or lamellar (swirls of concentric rings), calcification strongly suggests a benign cause of SPN or a granuloma. Eccentric (off-center), speckled, or amorphous calcification may be seen in cancers. A smooth-edged, round nodule more often is benign, whereas a lobulated or spiculated (having a spikelike appearance) edge is more likely to be a malignant nodule (see Figure 20-26). PET is often very helpful in evaluating SPNs. Nodules with greater than 1 cm diameter that are avid for the isotope used in PET (fluorodeoxyglucose) and “light up” on the scan generally are more likely to be malignant than nodules without uptake.

Pneumomediastinum

Pneumomediastinum, a form of barotrauma, may result from movement of air into the mediastinum, as may also be seen in cases of esophageal rupture (Figure 20-31). This condition usually occurs in the distal portion of the esophagus in patients who undergo procedures to stretch or dilate the esophagus. Chest trauma may cause rupture of a main bronchus, also allowing movement of air into the mediastinum. Rarely, air dissects down from the soft tissues of the neck after thyroid, parathyroid, or tonsillar surgery. Gas associated with a retrotonsillar abscess may also move down to the mediastinum through the fascial planes of the neck. Air that accumulates in the retroperitoneum may enter the mediastinum via openings in the diaphragm for the aorta or esophagus.

Catheters, Lines, and Tubes

A common use of a chest radiograph is to check on the position of catheters, lines, and tubes after they have been inserted. RTs must be skilled at examining the chest radiograph to determine the position of the endotracheal tube, chest tubes, and hemodynamic monitoring lines.

Endotracheal Tube

Endotracheal tubes are radiopaque or have an opaque marker indicating the end of the tube. Radiographs are routinely obtained at the bedside after intubation to assess correct tube position. The radiograph shows the distal tip of the endotracheal tube and the carina.²² The position of the patient’s neck is important. The neck position usually is neutral, but the position of the tip of the endotracheal tube can vary with neck position. Specifically, the endotracheal tube position can move 4 cm toward the main carina as the neck moves from full extension (high position) to full neck flexion (low position), which is one-third the length of the average adult trachea. Goodman and Putman²³ suggested that when the head and neck are in the neutral position, the endotracheal tube should be positioned in the midtrachea (5 to 7 cm from the carina). Placement below the thoracic inlet ensures that the tube is beyond the vocal cords (usually at C5-6). Figure 20-32 shows a malpositioned endotracheal tube in the right main stem bronchus.

 Rule of Thumb

The distal tip of the endotracheal tube should be positioned approximately 5 to 7 cm above the level of the carina in an adult patient.

Pulmonary Artery (Swan-Ganz) Catheter

A Swan-Ganz catheter is used to measure hemodynamic and central pressure variables such as pulmonary artery occlusion pressure. Pulmonary artery catheters are placed at the bedside and ideally should reside in the proximal right or left main pulmonary arteries. They are floated into position using an inflatable balloon on the catheter tip. Because of this floating, they are placed in the right pulmonary artery more than 90% of the time. When measuring the so-called wedge or pulmonary artery occlusion pressure, the balloon is inflated, and the catheter moves out into a more peripheral vessel. As soon as the reading is accomplished, the balloon should be deflated, and the catheter should be pulled back to a central location. Persistent peripheral placement (i.e., when the catheter tip is far out in the lung parenchyma) can cause infarction of lung distal to the wedged catheter (Figure 20-33).