Types of Chest Radiographs

Two types of frontal chest radiographs may be obtained:

Image Quality

A frontal image is nonrotated when the spinous processes of the thoracic vertebrae are halfway between the medial ends of the clavicles. Rotation of the patient can change the appearance of the mediastinal contour, causing diagnostic confusion. Lung volume at the time of exposure should be as close as possible to total lung capacity. Ideal exposure and processing of the image allows visualization of the pulmonary vessels in the upper zones and also in the lower zones behind the heart. An image that is too dark impedes visualization of vascular detail in the upper zones, possibly resulting in failure to diagnose a pneumothorax, whereas an image that is too light results in lack of detail behind the heart and possible failure to diagnose left lower lobe pathology or pleural fluid. The radiation dose received from a chest radiograph is small, so inadequate images should be repeated.

Normal Structures on the Erect Posteroanterior Chest Radiograph

Normal structures identified on the PA and left lateral radiographs are shown in Figs. 6-1 and 6-2.

Figure 6.1 Normal PA chest radiograph, patient erect. See text for details.

Figure 6.2 Normal left lateral chest radiograph. See text for details.

Normal Structures on the Erect Left Lateral Chest Radiograph

Systematic Approach to Examining Chest Radiographs

A systematic approach to examination of the chest radiograph is essential so as to reduce the risk for missing important findings. One suggested approach is to examine structures by moving from the center of the film to the peripheries, focusing on:

Cardiac Size

Cardiac size can be estimated by measuring the maximum diameter of the heart and comparing it to the maximum diameter of the thorax; this is the cardiothoracic ratio (CTR). The CTR is normally less than 50%, but that is a crude estimate of cardiac size. A patient’s failure to take a full inspiration leads to a reduced AP diameter, causing the CTR to overestimate cardiac size. Fat pads adjacent to the left and right cardiac borders may make accurate measurement difficult.

Cardiac Chamber Enlargement

The PA and lateral chest radiographs can suggest specific valvular or chamber abnormalities.

Left Atrial Enlargement

Radiographic signs of left atrial enlargement include (Fig. 6-3):

1. Enlargement of the left atrial appendage, initially causing a straightening of the upper left heart border on the PA film and progressing to a focal bulge in the left heart contour.

2. A double contour to the right heart border caused by the bulging of the enlarged left atrium into the right atrium, behind the right atrial border.

3. Posterior displacement of the left main bronchus on the lateral radiograph.

4. Elevation of the left main bronchus on the frontal radiograph, causing splaying of the carina.

Figure 6.3 PA chest radiograph demonstrating mitral valve disease. A, Left atrial enlargement is shown by the double contour on the right heart border; B, Enlargement of the left atrial appendage and splaying of the carina are shown.

The size of the left atrium relative to the overall heart size can provide a clue as to whether the mitral valve is predominantly stenosed or regurgitant. A large left atrium combined with normal or nearly normal overall heart size favors mitral stenosis. Patients with mitral stenosis may also have evidence of valvular calcification.

Cardiac Calcification

Calcification can be present in many cardiac structures, including the pericardium, the cardiac valves, and the walls of the cardiac chambers, as well as in organized thrombus and in the coronary arteries due to atheroma. Aortic valve calcification can be seen on the lateral radiograph as a ring of calcification projected centrally over the heart (Fig. 6-4). The presence of aortic valve calcification implies stenosis. Mitral annular calcification is common in patients more than 70 years of age; it appears as a C-shaped ring near the posterior and inferior aspects of the heart on the lateral radiograph (Fig. 6-5). Mitral annular calcification does not imply functional impairment of the mitral valve. In contrast, calcification of the mitral leaflets is associated with rheumatic heart disease and usually indicates significant mitral valve dysfunction, usually stenosis. Mitral leaflet calcification is seen in the same region as annular calcification but is punctate and lacks the C-shape of the more benign annular calcification. The positions of each of the four heart valves on frontal and lateral radiographs are shown in Figure 6-6.

Figure 6.4 Lateral radiograph showing aortic valve calcification (arrows). See text for details.

Figure 6.5 Lateral radiograph showing mitral annular calcification (arrows). See text for details.

Figure 6.6 A and B, Frontal and lateral radiographs of a patient with four prosthetic heart valves. A mechanical prosthesis has been inserted in each of the four heart valve positions.

(Reproduced from Bijl M, van den Brink RB: Images in clinical medicine: four artificial heart valves. N Engl J Med 353:712, 2005. Copyright © 2005 Massachusetts Medical Society.)

Hiatus Hernia

Hiatus hernia results in a retrocardiac mass that is predominantly left-sided but can bulge slightly to the right of the midline on a frontal radiograph. Such hernias are common and are best appreciated on an erect chest radiograph, on which they can usually be seen to contain an air fluid level. On a supine film, a hiatus hernia may be mistaken for a left basal pneumothorax.

Pulmonary Vascularity

Pulmonary Venous Hypertension and Pulmonary Edema

Three changes in pulmonary vascularity occur in patients with raised left atrial pressure:

1. Pulmonary venous hypertension. In the erect chest film, pulmonary venous hypertension results in a progressive increase in the caliber of the vessels in the upper lobes. If vessels in the first anterior interspace have a caliber greater than 3 mm, it is suggestive of either pulmonary venous hypertension or plethora (e.g., due to left-to-right shunt). With pulmonary venous hypertension, the upper lobe vessels are larger than the lower lobe vessels, whereas the usual pattern of lower lobe dominance is maintained with plethora.

2. Interstitial edema. Increased interstitial fluid within the pulmonary parenchyma results in the thickening of the interlobular septa (Kerley A and B lines), in peribronchial thickening, and in fuzziness around vessels. Kerley B lines are seen as horizontal lines perpendicular to the chest wall peripherally, in the mid to lower zones, and measuring 1 to 3 mm in thickness (Fig. 6-7). Kerley A lines are thickened interlobular septa that are seen within the more central lung, passing obliquely toward the pulmonary hilum. Septal lines that persist beyond resolution of heart failure may occur due to hemosiderin or fibrin deposition within the septa.

3. Alveolar pulmonary edema. Alveolar filling by transudate results in a generalized increase in parenchymal density. Classically, this has a perihilar distribution and causes symmetric perihilar consolidation (the “bat-wing” pattern) (Fig. 6-8). Evidence of pulmonary venous hypertension and interstitial edema is usually obscured, but it becomes apparent as the alveolar edema subsides. Alveolar edema can be asymmetric, usually due to patient positioning or underlying lung pathology (e.g., bullous emphysema). Pulmonary venous obstruction is a rare cause of asymmetric alveolar edema.

Figure 6.7 Chest radiograph detail showing Kerley B lines (arrows). See text for details.

Figure 6.8 AP radiograph, patient erect, showing alveolar pulmonary edema. The perihilar and symmetric nature of the infiltrates favors a diagnosis of alveolar pulmonary edema over ARDS (see Fig. 6-17). Interstitial fluid, as demonstrated by the thickening of the horizontal fissure (arrow), can also be seen.

On a supine chest radiograph, the gravitational changes of pulmonary venous hypertension are not identified, but the signs of interstitial and alveolar edema are unchanged. Pleural fluid often coexists with interstitial or alveolar edema.

Chronic Obstructive Pulmonary Disease

Emphysema is associated with hyperinflation of the chest and peripheral pulmonary vascular attenuation. The signs of hyperinflation include inferior displacement of the hemidiaphragms below the sixth to seventh anterior ribs on an inspiratory film, flattening of the domes of the hemidiaphragms, and an increase in the retrosternal airspace on the lateral radiograph. Pulmonary vessels are thin and less profuse in areas of lung affected by emphysema. In centrilobular emphysema, the distribution of emphysema is predominately within the mid and upper zones. In panlobular emphysema, as seen in α-1 antitrypsin deficiency, the distribution is basal. Bullae can be identified on the chest radiograph in one third of patients with emphysema. When bullae are large they can be confused with pneumothoraces (see later discussion).

Bronchiectasis may be evident on the plain radiograph as dilated, nontapering airways with thickened walls, usually in the lower zones. The chest radiograph is commonly normal in patients with chronic bronchitis and asthma.