Introduction to Cardiac Imaging

Heart Size and Cardiac Position

Change in the size or gross appearance of the heart on plain film examination of patients examined in the early surgical era (1950-1970) was found to be a helpful marker of cardiac disease, as well as an index of its severity. The normal heart is usually found in the midline of the chest, with a conspicuous cardiac apex pointing toward the left ( Figure 1-4 ). An increase in the cardiothoracic ratio of greater than 50% is a useful indicator of cardiac disease, usually reflecting left ventricular enlargement. Right ventricular enlargement, commonly found in right heart failure or left-to-right intracardiac shunts, is associated with a (looking from below) clockwise rotation and leftward displacement of the heart, which changes the contour of the left heart border. However, it doesn’t necessarily increase the cardiothoracic ratio ( Figure 1-5 ).

When an abnormality in embryonic cardiac looping exists, the cardiothoracic ratio may be normal, but the heart often assumes an abnormal position within the chest ( Figure 1-6 ). When the bulk of the cardiac mass (and if visualized, the cardiac apex) lie to the right of the midline, dextrocardia is said to exist. When the heart resides in the midline (and no apparent cardiac apex can be identified), then mesocardia is said to exist ( Figure 1-7 ). Pericardial disease is often asymmetric or focal in distribution, which may have the effect of increasing the cardiothoracic ratio ( Figure 1-8 ).

Chest films are two-dimensional maps of the three-dimensional structure of the chest. The high contrast between the air-filled lungs and the periphery of the soft tissue cardiac silhouette allows us to define normal and differentiate between normal and abnormal cardiac contours. In other words, we judge a structure to be abnormal in a plain film exam because the contour of a portion of the cardiomediastinal silhouette differs from our empirically-derived expectations. The 4-view “cardiac series” (often with the administration of oral barium to opacify the thoracic esophagus) is hardly ever obtained anymore; however, posteroanterior, and less commonly, posteroanterior and lateral views continue to be obtained for cardiac examination.

An interpretive process that associates the appearance of the peripheral contours of the cardiothoracic silhouette with pathologic chamber changes lends itself to the limited contrast resolution of conventional (film–screen) plain film imagery. An abnormality of contour is a general description of what catches our eye in an organized search. Identification of an “abnormal contour” can be very subjective, and is tied to image quality, the viewing environment, and the experience of the observer. An organized review of a series of segmental evaluations of particular portions of the cardiothoracic silhouette will improve observational accuracy. Utilizing common descriptors of each segmental abnormality (e.g., increased in caliber, decreased in caliber) one constructs a running differential diagnosis based on the accumulated findings. Upon “completion” of the observation, the signs of physiologic sequelae are put together in a physiologically logical manner, and a differential diagnosis is constructed.

The organized search begins with assessment of radiographic technique, patient position, and the projection in which the heart and great vessels are viewed. We expect that a chest film examination is obtained upright with the patient in deep inspiration. In the upright position, the bulk of pulmonary blood flow is to the lower lobes causing them to appear greater in caliber than the upper lobe branches. Furthermore, we traditionally utilize posteroanterior projection, keeping the heart and great arteries close to the detector, thus minimizing magnification. Generally, intensive care radiography is obtained in a manner convenient for the patient (i.e., with the patient supine or only partially upright) and the x-ray source in front of the patient, and the film/screen combination or digital plate behind the patient (anteroposterior, or AP projection). In the supine position ( Figure 1-9 ), there is a redistribution of blood flow to the now gravity-dependent upper lobe vessels; the heart may appear enlarged because of magnification caused by having the detector behind the patient.

Patient rotation away from a position normal to the x-ray beam (anteroposterior or posteroanterior projection) changes the contours of the cardiomediastinal silhouette by rotating some expected heart contour forming structure off the contour, replacing it with another, normally not heart border forming structure. In this way, a normal structure may be exaggerated and appear abnormal, an abnormal structure not normally viewed in posteroanterior projection may become apparent, or a normal structure may no longer be evident. Estimating the degree of rotation is based upon an analysis of the distance between the clavicular heads and the midline and the relations between the thoracic spine and posterior ribs, as well as the relative size of each lung.

Careful evaluation of the bony thorax may reveal pertinent abnormalities, including scoliosis (which may be associated with congenital heart or lung disease) or pectus excavatum, evidence of prior thoracotomy (surgical palliation or repair of congenital or acquired heart disease), or the presence of indwelling intravenous and intraarterial lines, valvular, vascular, or intracardiac prostheses, pacers, defibrillators, or other devices ( Figure 1-10 ).

The heart is evaluated in terms of its borders. The left heart border may be divided into (1) the aortic arch segment, (2) the main pulmonary artery (MPA) segment, (3) the left atrial appendage (LAA) segment, and (4) the left ventricular contour. The right heart border is formed by (1) the superior vena cava segment, (2) the ascending aortic segment, and (3) the right atrial segment ( Figure 1-11 ). The internal structure of the heart is beyond the contrast resolution of chest film examination. However, plain films are especially sensitive to calcium deposits. Calcification is an indicator of disease, and the geographic distribution and character of the calcific deposits may be diagnostic.

Aortic Arch Segment

On the plain chest film, the aortic arch (Ao) segment ( Box 1-1 ) is formed from the projection of the aorta as it passes from anterior-to-posterior and right-to-left to form the descending thoracic aorta. We expect the aortic arch to be left sided. By definition, a left aortic arch displaces the trachea toward the right. The arch (measured from the lateral-most aspect of the arch to the lucency of the left side of the tracheal air shadow) should be no greater than 25 mm, and not calcified. If the aortic caliber is greater than 25 mm, then we might call the arch segment dilated or enlarged. A dilated arch segment may indicate the presence of an atherosclerotic aortic aneurysm, an aortic dissection, an aortic pseudoaneurysm, or an adjacent mass silhouetting the arch, giving the impression of increased caliber. Separation of (intimal) arch calcification from the lateral aspect of the aortic shadow by greater than 10 mm indicates pathologic thickening of the aortic wall, and is strongly suggestive of an aortic hematoma or dissection.

If the aortic arch segment appears smaller than 25 mm in caliber, or if the arch segment is not visualized (i.e., does not displace the tracheal air shadow to the right), then we can classify this aortic arch as small or inconspicuous ( Figure 1-12 ). Coarctation of the aorta is a maldevelopment of the aortic arch associated with hypoplasia of the distal aortic arch and focal narrowing at or near the insertion of the ductus arteriosus onto the aorta ( Figure 1-13 ). It is commonly associated with a bicuspid aortic valve. The hypoplastic segment corresponds to the portion of the aorta forming the aortic arch segment; thus, the aortic arch segment is small, or not well visualized. If the arch is severely hypoplastic, then tracheal displacement toward the right will certainly not be apparent. Similarly, interruption of the aortic arch may present with an inapparent aortic arch segment and midline trachea (the segment that ordinarily displaces the trachea toward the right is interrupted, or absent). In a duplicated aortic arch, the left-sided arch component is hypoplastic and lies inferior to its usual place, rendering the aortic arch segment small or inapparent ( Figure 1-14 ). The right-sided arch component of the duplicated arch is generally larger than the left and lies higher in the superior mediastinum, but it does not displace the trachea toward the left. Commonly, when the left-sided aortic arch contour is absent, it is associated with a round mass to the right of the trachea, which displaces the trachea toward the left; this is a right-sided aortic arch ( Figure 1-15 ). Right aortic arches come in two varieties; associated with a retroesophageal aberrant left subclavian artery (right aortic arch with aberrant left subclavian artery or “posterior right aortic arch”), or with so-called mirror-image branching. The former may be associated with a complete esophageal ring if a patent ductus arteriosus or duct remnant connects the aberrant left subclavian artery with the proximal left pulmonary artery. These patients present with stridor or dysphagia. Mirror-image branching may be an isolated aortic anomaly or may be associated with congenital heart disease (the most common form of right arch in individuals with Tetralogy of Fallot [TOF]).

Pulmonary Artery Segment

The pulmonary artery ( Box 1-2 ) is usually supported by the right ventricular infundibulum and therefore lies to the left of the ascending aorta. The MPA segment lies just inferior and to the left of the aortic arch segment. It is the round radiodensity found just superior to the air attenuation of the left bronchus (see Figures 1-4, 1-5, 1-11 , and 1-15 ). We expect that right-sided cardiac output will equal left-sided cardiac output; therefore, allowing for differences in arterial pressure and vascular compliance (the aorta is thicker than the pulmonary artery), the caliber of the MPA segment should be about the caliber of the Ao segment. Thus, the MPA segment may appear normal in caliber, dilated (greater in caliber than the Ao segment), or decreased in caliber (smaller in caliber than the Ao segment).

Increased caliber of the MPA segment ( Figure 1-16 ) may be associated with increased pulmonary artery pressure, increased pulmonary blood flow, both increased pulmonary blood flow and pressure, and in an exception to the rule, valvular pulmonic stenosis (PS). Increased pulmonary artery pressure (i.e., pulmonary hypertension), reflects the increased work of the right ventricle to maintain forward blood flow in the face of increased pulmonary resistance. That is, the increased right ventricular pressure generated in these individuals is reflected in the increased caliber of the MPA segment. In an analogous way, increased pulmonary blood flow will increase the caliber of the MPA segment as well. Although all cases of pulmonary hypertension and left-to-right shunts are associated with increased MPA caliber, many shunts are not associated with increased MPA pressure, and most individuals with pulmonary hypertension do not have left-to-right shunts. An exception to the rule of increased MPA caliber is the individual with valvular PS. In this situation, right ventricular emptying is restricted by the (usually congenitally) narrowed valve orifice, resulting in (1) right ventricular hypertrophy, the sequel of generating increased RV pressure to eject across the obstruction, and (2) turbulent flow just distal to the malformed pulmonary valve orifice. The latter is associated with pathologic change in the pulmonary arterial wall, which results in wall weakening and increased caliber in the face of normal MPA pressure (the gradient is across the pulmonary valve; thus, RV pressure is elevated and MPA pressure is normal). An individual with valvular PS will have a dilated MPA segment, but pressure and pulmonary blood flow in the MPA are both normal. Furthermore, in valvular PS the turbulent insult often extends into the left pulmonary artery, resulting in main and left pulmonary artery dilatation. Blood flow returns to Newtonian character before reaching the hilar right pulmonary artery, leaving this structure normal in caliber.

When obstruction to right ventricular emptying is proximal to the pulmonary valve, flow through the MPA segment is decreased, and the segment appears flat or, in severe cases of RV obstruction (i.e., pulmonary atresia), concave toward the right ( Figure 1-17 ). Common causes of decreased MPA caliber include TOF and variants of TOF including pulmonary atresia with ventricular septal defect, and TOF with pulmonary atresia. Furthermore, other forms of subvalvular pulmonary obstruction, including anomalous right ventricular muscle bundles (the so-called double chambered right ventricle), result in decreased RV output and a smaller MPA segment. Individuals with a double-outlet right ventricle, in whom pulmonary venous return is directed toward the ascending aorta (the so-called ventricular septal defect variety), present with smaller MPA segments.

Obstruction to flow into the right ventricle results in decreased RV filling of the MPA and decreased MPA caliber. This may be seen in individuals with tricuspid atresia or Ebstein malformation of the tricuspid valve. In tricuspid atresia, there is no antegrade filling of the RV from the right atrium. Right ventricular chamber size and pulmonary blood flow are significantly related to the amount of right-to-left flow across the atrial septum, as well as left-to-right flow across a ventricular septal defect. Nearly everyone with tricuspid atresia has a small right ventricle and decreased MPA blood flow, resulting in a small MPA segment. In the Ebstein malformation, the tricuspid valve is severely regurgitant, resulting in right atrial and right ventricular enlargement. One would expect that the MPA in such a circumstance would be normal, or even dilated. However, the nature of the valvular pathology in the Ebstein malformation is that the tricuspid valve leaflets fail to separate from the septal and RV free-wall myocardium. Thus, the proximal portion of the right ventricle is “atrialized”; although the RV is volume loaded, there is a decreased complement of right ventricular myocardium to drive the blood into the MPA. As a result, there is progressive tricuspid regurgitation but poor RV emptying and a flat or concave MPA segment ( Figure 1-18 ).

The expected appearance of the MPA segment presumes the presence of a main pulmonary artery. Thus, in individuals in whom there is no anatomic MPA segment, that portion of the left heart border will be flat or concave. In individuals with persistent truncus arteriosus, for example, the MPA segment is concave because there is no MPA segment. Rather, in these individuals, the pulmonary blood flow is derived from the common truncus, which itself branches to give a left and right pulmonary artery. In an analogous manner, individuals with D -transposition of the great arteries present with a small MPA segment. In these individuals, a MPA exists, but it is supported by the left ventricle, which lies posteriorly. Therefore, there is no pulmonary artery to form that portion of the left heart border, and it appears concave.

As we can see, the differential diagnosis of an abnormal MPA segment includes a rather broad range of congenital and acquired abnormalities. How might we more finely classify such a differential diagnosis? This can be accomplished by a more detailed examination of the pulmonary parenchymal vascular anatomy. Using this approach, we can define four basic pulmonary vascular patterns: normal, pulmonary venous hypertension, pulmonary arterial hypertension, and shunt vascularity.

Normal pulmonary blood flow is associated with a normal MPA segment (see Figures 1-2 B , 1-4 , and 1-11 ). In addition, both the left and right hilar pulmonary artery segments are normal in caliber (no greater than a bronchus in diameter), and the lower and upper lobe vessels have a normal appearance. That is, because most of our pulmonary blood flow goes to the lower pulmonary segments, the lower lobe pulmonary artery segments are greater in caliber than those in the upper lobes. All pulmonary segments contain a blood vessel, and the vessels are sharp in appearance. Furthermore, although we know that the pulmonary artery segmental and subsegmental arteries extend out to the pleura, we only see blood vessels in normal individuals extending out about 2/3 of the way from the hilum (limited by the spatial resolution of the imaging system).

The pulmonary veins have no valves, so increased left atrial volume, or pressure, will result in increased caliber of the body of the LA, as well as changes in the parenchymal pulmonary vessels. The earliest vascular changes in left atrial hypertension involve unsharpening of the lower lobe pulmonary venous edges. Left atrial pressure transmitted back to the pulmonary venules stretches the cells of the venules, creating microscopic gaps that water molecules can pass through. Moving from the intravascular space to the sheaths of the pulmonary venules acts to silhouette the lower lobe veins, making them appear a bit unsharp, as compared to the upper lobe veins ( Figure 1-19 A ). Continued LA hypertension or an increase in LA pressure will further stretch the pulmonary venules, allowing more fluid to leave the intravascular space. Soon, fluid passes into the pulmonary parenchyma, and this fluid further unsharpens the vessel edge. At the same time, the increasing pulmonary interstitial edema accumulation increases resistance to lower lobe pulmonary blood flow. The lung is like a sponge. When dry, it is quite springy and resilient. However, when edematous, it becomes less compliant, increasing resistance to local blood flow resulting in an intrapulmonary shunt. Blood flow that normally would have gone to the lower lobes is redirected, or “redistributed” to the collapsed, low resistance upper lobe veins for return to the heart (see Figure 1-19 B ). Thus, in an individual with left atrial hypertension, we see a spectrum of change, associated with progressive increase in left atrial pressure. Changes commence with vague lower lobe vascular unsharpness, lower lobe unsharpness associated with upper lobe pulmonary vein dilatation, and subsequent upper lobe vein unsharpness. Continued LA hypertension greater than 22 mm of Hg (the colloid oncotic pressure) is associated with the rapid movement of intracellular fluid into the alveolar space, producing alveolar edema (see Figure 1-19 C ). Unless the pathologic insult is severe and the changes chronic, removal of the primary cause of left atrial hypertension results in a return to normal appearance with decrease in LA pressure.

Increased pulmonary artery pressure is associated with dilatation of the main and both left and right hilar pulmonary arteries ( Figure 1-20 ). The hilar PAs are extraparenchymal and therefore will increase in caliber with increasing pulmonary artery pressure. However, the appearance of the parenchymal segments reflects the intraluminal pressure, the state of the segmental pulmonary interstitium, and pathologic change in the arterial wall. The pulmonary arteriolar bed is vasoconstricted in individuals with pulmonary hypertension. Thus, the lungs appear blacker, fewer parenchymal vessels are visualized, and those seen do not appear to extend as far to the pleura as in hypertension is the disparity in calibers between the main and hilar pulmonary arterial segments and the parenchymal pulmonary arterial segments. Pulmonary hypertension reflects the adaptation of the right ventricle to increased pulmonary arterial resistance. Resistance is elevated because of pulmonary parenchymal destruction, distal or proximal pulmonary arterial bed obstruction, or cardiac disease which affects pulmonary venous return to the heart (i.e., the differential diagnosis of pulmonary venous hypertension). Primary pulmonary hypertension (always the last choice in the differential diagnosis of pulmonary hypertension) is a genetically-linked group of diseases affecting the intima and media of the pulmonary arterioles and venules, resulting in small vessel obliteration and increased pulmonary resistance. Thus, individuals with pulmonary hypertension share a common radiographic appearance: dilated central and hilar PAs with small or sparse parenchymal pulmonary artery branches ( Figure 1-21 ). Characteristic pulmonary changes are found in examinations of individuals with pulmonary hypertension of different etiologies.

When right-sided cardiac output is greater than that of the left, we presume that a left-to-right shunt is present ( Figure 1-22 ). The increased right ventricular cardiac output increases the caliber of the MPA segment but is also carried through the entire pulmonary bed, causing increased caliber of the hilar as well as lower and upper lobe vessels. The parenchymal vessels are greater in caliber, are sharp, and extend farther out to the periphery of the lung than normally expected. We define “shunt vascularity” as the association of a dilated MPA segment with dilated hilar and parenchymal arterial segments. In newborn and very young individuals, the pulmonary parenchymal bed hasn’t fully matured, and therefore, increased pulmonary blood flow may be associated with interstitial pulmonary changes as well. Until about the age of 12 years, shunts may look like failure, and vice versa. However, after 12 years of age, “adult” findings may be expected, and we do not often see interstitial change (indicating left atrial hypertension) associated with shunt vascularity. The differential diagnosis of a shunt (atrial septal defect, ventricular septal defect, patent ductus arteriosus) is based upon the presence of shunt vascularity and is characterized by the association of cardiac chamber changes seen with the shunt vessels. That is, a shunt with right heart dilatation and a normal left heart is an atrial septal defect until proved otherwise. A shunt with biventricular and left atrial enlargement is a ventricular septal defect until proved otherwise. A shunt with left heart dilatation and a normal right heart is a patent ductus arteriosus until proved otherwise.

Left Atrial Appendage Segment

The LAA ( Box 1-3 ) segment contains the extension of the left atrial cavity which forms the left heart border segment just inferior to the MPA segment. With normal left atrial volume and pressure, the LAA is collapsed and flat or concave toward the right (see Figures 1-4, 1-10 , and 1-11 ). Straightening, or leftward curvature of this segment is a sign of left atrial volume or pressure loading. Although change in the contour of the LAA segment is an early sign of LA abnormality, it is commonly associated with other, chronic signs of LA enlargement, including a well-defined “double density” in the midline posterior aspect of the heart, and posterior displacement and elevation of the left bronchus ( Figure 1-23 ). Left atrial enlargement can be caused by volume loading (mitral regurgitation, left-to-right shunt, left heart failure) or pressure loading (mitral stenosis, LV myocardial ischemia) the left atrium. In acute mitral regurgitation, there may be a change in the left atrial size ( Figure 1-24 ).