Chest radiography was an essential part of cardiac evaluation before the echocardiographic (echo) studies became widely available to cardiologists. This simple test remains very useful to physicians who do not have access to the echocardiograph. In addition, cardiovascular abnormalities may be incidentally suspected by chest radiographic films.
Information to be gained from chest radiographs includes (1) heart size and silhouette, (2) enlargement of specific cardiac chambers, (3) pulmonary blood flow (PBF) or pulmonary vascular markings (PVM), and (4) other information regarding lung parenchyma, spine, bony thorax, abdominal situs, and so on.
I. Heart Size and Silhouette
Heart size: The cardiothoracic (CT) ratio is obtained by dividing the largest transverse diameter of the heart by the widest internal diameter of the chest ( Fig. 3.1 ). The CT ratio is calculated by the following formula.
CT ratio = (A + B)/C
A CT ratio of more than 0.5 beyond infancy is considered to indicate cardiomegaly. However, the CT ratio cannot be used with any accuracy in neonates and small infants, in whom a good inspiratory chest film is rarely obtained.
Normal cardiac silhouette: The structures that form the cardiac borders in the posteroanterior and lateral projections of a chest radiograph are shown in Fig. 3.2 . In the neonate, however, a typical normal cardiac silhouette as shown in Fig. 3.2 is rarely seen because of the presence of a large thymus.
Abnormal cardiac silhouette: The overall shape of the heart sometimes provides important clues to the type of cardiac defect ( Fig. 3.3 ).
Boot-shaped heart with decreased PVM is seen in infants with cyanotic TOF and in some infants with tricuspid atresia ( Fig. 3.3A ).
Narrow waist and egg-shaped heart with increased PVM in a cyanotic infant strongly suggest TGA ( Fig. 3.3B ).
Snowman sign with increased PVM is seen in infants with the supracardiac type of TAPVR ( Fig. 3.3C ).
II. Cardiac Chambers and Great Arteries
Individual chamber enlargement
Left atrial enlargement (LAE): Mild LAE is best recognized in the lateral projection by posterior protrusion of the LA border ( Fig. 3.4 ). An enlargement of the LA may produce a double density on the posteroanterior view of a heavily exposed film. With further enlargement, the left atrial appendage becomes prominent on the left cardiac border, and the left main-stem bronchus is elevated.