4 Chest X-ray
Despite the availability of modern imaging methods, the chest X-ray remains an essential component of diagnostic measures in pediatric cardiology. An X-ray image provides information on the size and shape of the heart, enlargement of any parts of the heart, the position of the heart in the thorax, lung perfusion, lung parenchyma, abdominal situs, and any bone anomalies that may be associated with heart defects.
Routine projections in childhood are anterior–posterior (AP) or posterior–anterior (PA) and lateral images. In neonates and infants, the AP projection is preferred; a PA projection is preferred for older children as it is for adults.
4.2 Heart Size
As a rule of thumb, the transverse diameter of the heart is half that of the thorax. The size of the heart can be more correctly described using the cardiothoracic ratio, which is calculated by dividing the greatest transverse diameter of the heart by the greatest inner diameter of the thorax (Fig. 4.1). A cardiothoracic ratio of over 0.5 is a sign of cardiomegaly except in neonates.
4.3 Heart Shape
A diagram of the structures normally forming the borders of the heart is shown in Fig. 4.2. In AP projection, the right border of the heart is formed by the superior vena cava and right atrium. On the left side, the cardiac silhouette is bordered from top to bottom by the aortic knob, the pulmonary artery, and the left ventricle in AP projection. In a neonate, the left border of the heart may still be formed by the right ventricle, but at a later age this is pathological.
In the lateral projection, the right ventricle is located directly behind the sternum. The posterior border is formed by the left atrium at the top and the left ventricle at the bottom. The left ventricle and inferior vena cava overlap just above the diaphragm (Fig. 4.2).
In neonates the cardiac silhouette is frequently concealed by the presence of a still large thymus. The thymus is located in the anterior and superior mediastinum and often appears as an unusually wide base of the heart. In addition, the heart is wide at the diaphragm in neonates. The thorax of the neonate appears broader and deeper. The ribs are horizontal. Only as the heart continues to grow does it assume the typical, almost vertical position in adults. The contours of the cardiac silhouette are then better defined. In neonates, the cardiac silhouette appears more rounded. Since the right ventricle in neonates still forms the left border of the heart, the apex appears to be higher. A cardiothoracic ratio of over 0.5 can still be normal in neonates and young infants.
4.4 Assessment of Atria, Ventricles, and Great Vessels
Enlarged atria and ventricles have different effects on the shape of the heart. Changes in heart shape therefore provide information on enlargement of certain parts of the cardiac. However, most cardiac defects involve not only one atrium or one ventricle, but generally a combination.
Enlargement of the left atrium
The left atrium is directly adjacent to the tracheal bifurcation. An enlarged left atrium therefore increases the angle of the bifurcation (Fig. 4.3) and typically displaces the main bronchus upward. In addition, in AP projection, an enlarged left atrium can appear as a dense shadow in the cardiac silhouette. The left atrial appendage can also form the left border of the heart. The bulge in the posterior superior cardiac silhouette is prominent in the lateral projection.
Enlargement of the left ventricle
Since the left ventricle normally forms the left border of the heart, the left border is more prominent if it is enlarged. In addition, the cardiac apex is displaced downward. The cardiac apex appears to “dip” into the diaphragm.
In the lateral projection, the posterior inferior border of the cardiac silhouette is displaced further in a posterior and inferior direction. The lower border of the heart (formed by the left ventricle) and the inferior vena cava do not intersect until below the diaphragm. (Fig. 4.4)
Enlargement of the right atrium
When the right atrium is dilated, the lower right edge of the cardiac silhouette becomes more prominent (Fig. 4.5). In the lateral projection, the inferior retrocardiac space is not constricted unless the right atrium is massively enlarged.
Enlargement of the right ventricle
The cardiac apex is elevated when the right ventricle is enlarged. An enlarged right ventricle can also displace the left ventricle to the left and the right atrium to the right, widening the cardiac silhouette. The right ventricle does not normally form the right border, but if it is greatly enlarged, the right ventricle may form the right border.
Right ventricular enlargement is often best detected in the lateral projection, in which the retrosternal space is constricted (Fig. 4.6).
Dilatation of the pulmonary artery
Dilatation of the pulmonary artery results in a prominent pulmonary artery segment in the X-ray (Fig. 4.7). It may be a sign of poststenotic dilatation of a pulmonary artery stenosis, increased blood flow associated with a shunt defect, or elevated pulmonary artery pressure.
“Absent” pulmonary artery segment
Hypoplasia or even atresia of the pulmonary artery leads to a concavity of the pulmonary artery. This is termed an “absent” pulmonary artery segment (Fig. 4.8). The cardiac waist is then prominent. This configuration of the cardiac silhouette is also described as “boot shaped.”
Dilatation of the ascending aorta and aortic arch
The ascending aorta does not usually form the right border. It does not cross the superior vena cava. If the ascending aorta is dilated it can form the right border, or the entire aortic arch may be widened (Fig. 4.9). The cause may be aortic insufficiency (volume overload from regurgitant blood flow), poststenotic dilatation of the ascending aorta associated with an aortic stenosis, or a shunt defect that results in increased perfusion of the aorta.
Left and right aortic arch
Normally, a left aortic arch crosses the left main bronchus and proceeds downward at the left of the spinal column. The course of the aorta along the left edge of the spinal column can generally be followed on the X-ray image. In addition, with a left aortic arch, the esophagus and trachea are displaced slightly to the right of the midline.
If a right aortic arch descends on the right, the aortic silhouette is detected at the right of the spinal column. A right aortic arch also forms the right border. The trachea and esophagus are typically displaced to the left.
A right aortic arch is frequently found with tetralogy of Fallot, pulmonary artery atresia with ventricular septal defect (VSD), or with a truncus arteriosus communis. In particular if none of these congenital heart defects is present, a pulmonary annulus must always be considered if there is a right aortic arch (Chapter 15.26).
Congenital heart defects
Characteristic radiological findings associated with some congenital heart defects are summarized in Table 4.1.
Characteristic radiological finding
Dextro-transposition of the great arteries (d-TGA)
“Egg on side” appearance. Elevated cardiac apex, narrow annulus
Tetralogy of Fallot, pulmonary atresia with ventricular septal defect
“Boot-shaped” heart. Elevated cardiac apex, prominent cardiac waist
Total anomalous pulmonary venous connection, supracardiac type
“Snowman” or “figure 8.” The upper segment of the 8 or snowman is formed by a dilated vertical vein on the left, through which the common pulmonary venous sinus drains into the brachiocephalic vein. The lower part of the figure is caused by an enlarged cardiac silhouette.
Coarctation of the aorta in older children
Rib notching. Defects at the lower edge of the ribs from collateral blood flow through the intercostal arteries
“Globe-shaped” silhouette with a narrow waist. The enlarged right atrium usually extends the cardiac silhouette to the right.