Chapter 6 Chest Radiography
Skilled interpretation of chest radiographs is central to the diagnosis and treatment of patients in the cardiothoracic intensive care unit (ICU). In this chapter the common abnormalities of chest radiographs that are encountered in cardiac surgery patients are reviewed. Where appropriate, indications for computed tomography (CT) or ultrasound scanning of the chest are discussed. Before attempting to interpret abnormalities, it is essential to appreciate the findings of the normal chest radiograph and to have a systematic approach to reviewing films. These topics are discussed first.
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.
The right paratracheal space should be less than 3 mm in thickness; widening is suggestive of mediastinal lymphadenopathy. The azygos vein is normally less than 10 mm in diameter. This vein may become distended when right atrial pressure is high.
The left mediastinal contour comprises the left subclavian artery (6); the aortic arch (7); the aortopulmonary window (8); the distal main pulmonary artery (9); the left atrial appendage (10); the left ventricle (11); and the descending thoracic aorta (12).
The central left pulmonary artery (13) is 1.5 cm higher than the right (14). The lower-lobe pulmonary arteries (15) should be less than 16 mm in diameter; enlargement suggests pulmonary arterial hypertension. Collapse or consolidation in the lower lobes can obscure the lower-lobe pulmonary arteries. The peripheral pulmonary vessels are normally more dilated in the lower zones than in the upper zones because of the effect of gravity. Vessels should be clearly defined and without fuzzy margins.
The right ventricle (20) forms the inferior mediastinal contour and may extend up to 50% of the way up the posterior surface of the sternum. The pulmonary outflow tract (21) and ascending aorta (4) are visible. The left brachiocephalic vein (22) may be seen in the retromanubrial region.
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 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.
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.
There may be associated signs of valvular heart disease, such as valvular calcification (see subsequent discussion) or, in the case of aortic valve disease, dilatation of the ascending aorta, which causes a convex bulge in the upper right heart border.
The right ventricle does not make up any portion of the contour of the heart on the frontal radiograph, but it forms the anterior contour on the lateral radiograph. Signs of right ventricular enlargement include:
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.
(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 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.
On thoracic CT, the signs of pulmonary arterial hypertension include a transverse diameter of the main pulmonary artery greater than that of the adjacent ascending aorta as well as the presence of pericardial fluid and minor pericardial thickening.1
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.
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.