Imaging plays a central role in the assessment of cardiac function and pathology. Traditional modalities such as chest radiography, echocardiography (echo), cardiac catheterization with angiography, and nuclear imaging are fundamental in the diagnosis and management of cardiovascular diseases. These procedures are being increasingly supplemented by newer techniques, including computed tomography (CT) and magnetic resonance imaging (MRI).
This chapter presents an overview of imaging studies as they are used to assess the cardiovascular disorders described later in this book. On first reading, it would be beneficial to familiarize yourself with the information, but not to memorize the details. This chapter is meant as a reference for diagnosis of conditions that will be explained in more detail in subsequent chapters.
The extent of penetration of x-rays through the body is inversely proportional to tissue density. Air-filled tissues, such as the lung, absorb few x-rays and expose the underlying film (or electronic recording sensor), causing it to appear black. In contrast, dense materials, such as bone, absorb more radiation and appear white, or radiopaque. For a boundary to show between two structures, they must differ in density. Myocardium, valves, and other in tracardiac structures have densities similar to that of adjacent blood; consequently, radiography cannot delineate these structures unless they happen to be calcified. Conversely, heart borders adjacent to a lung are depicted clearly because the heart and an air-filled lung have different densities.
Frontal and lateral radiographs are routinely used to assess the heart and lungs ( Fig. 3.1). The frontal view is usually a posterior–anterior image in which the x-rays are transmitted from behind (i.e., posterior to) the patient, pass through the body, and are then captured by the film (or electronic sensor) placed against the anterior chest. This positioning places the heart close to the x-ray recording film plate so that its image is only minimally distorted, allowing for an accurate assessment of size.
In the standard lateral view, the patient’s left side is placed against the film plate and the x-rays pass through the body from right to left. The frontal radiograph is particularly useful for assessing the size of the left ventricle, left atrial appendage, pulmonary artery, aorta, and superior vena cava; the lateral view evaluates right ventricular size, posterior borders of the left atrium and ventricle, and the anteroposterior diameter of the thorax.
Figure 3.1. Posteroanterior (A and B) and lateral (C and D) chest radiographs of a person without cardiopulmonary disease, illustrating cardiac chambers and valves.
AO, aorta; AV, azygos vein; IVC, inferior vena cava; LA, left atrium; LAA, left atrial appendage; LPA, left pulmonary artery; LV, left ventricle; MPA, main pulmonary artery; MV, mitral valve; RA, right atrium; RPA, right pulmonary artery; RV, right ventricle; SVC, superior vena cava; TV, tricuspid valve.
(Reprinted with permission from Come PC, ed. Diagnostic Cardiology: Noninvasive Imaging Techniques. Philadelphia, PA: J.B. Lippincott; 1985.)
Chest radiographs are useful to evaluate the size of heart chambers and the pulmonary consequences of cardiac disease. Alterations in chamber size are reflected by changes in the cardiac silhouette. In the frontal view of adults, an enlarged heart is identified by a cardiothoracic ratio (the maximum width of the heart divided by the maximum internal diameter of the thoracic cage) of greater than 50%.
In certain situations, the cardiac silhouette inaccurately reflects heart size. For example, an elevated diaphragm, or narrow chest anteroposterior diameter, may cause the silhouette to expand transversely such that the heart appears larger than its actual dimensions. Therefore, the chest anteroposterior diameter should be assessed on the lateral view before concluding the heart is truly enlarged. The presence of a pericardial effusion around the heart can also widen the cardiac silhouette because fluid and myocardial tissue affect x-ray penetration similarly.
Radiographs can depict dilatation of individual cardiac chambers. Ventricular hypertrophy alone (i.e., without dilatation) may not result in radiographic abnormalities, because it generally occurs at the expense of the cavity’s internal volume and produces little or no change in overall cardiac size. Major causes of chamber and great vessel dilatation include heart failure, valvular lesions, abnormal in-tracardiac and extracardiac communications (shunts), and certain pulmonary disorders. Because dilatation takes time to develop, recent lesions, such as acute mitral valve insufficiency, may present without apparent cardiac enlargement.
The pattern of chamber enlargement may suggest specific disease entities. For example, dilatation of the left atrium and right ventricle, accompanied by signs of pulmonary hypertension, suggests mitral stenosis ( Fig. 3.2). In contrast, dilatation of the pulmonary artery and right heart chambers, but without enlargement of the left-sided heart dimensions, suggests pulmonary vascular obstruction or increased pulmonary artery blood flow (e.g., due to an atrial septal defect; Fig. 3.3).
Figure 3.2. Posteroanterior chest radiograph of a patient with severe mitral stenosis and secondary pulmonary vascular congestion.
The radiograph shows a prominent left atrial appendage (arrowheads) with consequent straightening of the left-heart border and suggestion of a double-density right cardiac border (arrows) produced by the enlarged left atrium. The aortic silhouette is small, which suggests chronic low cardiac output. Radiographic signs of pulmonary vascular congestion include increased caliber of upper-zone pulmonary vessel markings and decreased caliber of lower-zone vessels.
Figure 3.3. Posteroanterior chest radiograph of a patient with pulmonary hypertension secondary to an atrial septal defect.
Radiographic signs of pulmonary hypertension include pulmonary artery dilatation (black arrows; compare with the appearance of left atrial appendage dilatation in Fig. 3.2) and large central pulmonary arteries (white arrows) associated with small peripheral vessels (a pattern known as peripheral pruning).
Chest radiographs can also detect dilatation of the aorta and pulmonary artery. Causes of aortic enlargement include aneurysm, dissection, and aortic valve disease ( Fig. 3.4). Normal aging and atherosclerosis may also cause the aorta to become dilated and tortuous. The pulmonary artery may be enlarged in patients with left-to-right shunts, which cause increased pulmonary blood flow, and in those with pulmonary hypertension of diverse causes (see Fig. 3.3). Isolated enlargement of the proximal left pulmonary artery is seen in some patients with pulmonic stenosis.
Figure 3.4. Posteroanterior chest radiograph of a patient with aortic stenosis and insufficiency secondary to a bicuspid aortic valve.
In addition to dilatation of the ascending aorta (black arrows), the transverse aorta (white arrow) is prominent.
Pulmonary Manifestations of Heart Disease
The appearance of the pulmonary vasculature reflects abnormalities of pulmonary arterial and venous pressures and pulmonary blood flow. Increased pulmonary venous pressure, as occurs in left-heart failure, causes increased vascular markings, redistribution of blood flow from the bases to the apices of the lungs (termed cephalization of vessels), pulmonary edema, and pleural effusions ( Fig. 3.5). Blood flow redistribution appears as an increase in the number or width of vascular markings at the apex. Interstitial and alveolar forms of pulmonary edema produce opacity radiating from the hilar region bilaterally (known as a “butterfly” pattern) and air bronchograms, respectively ( Fig. 3.5B). Kerley B lines (short horizontal parallel lines at the periphery of the lungs adjacent to the pleura, most often at the lung bases) depict fluid in interlobular spaces that results from interstitial edema. Pleural effusions cause blunting of the costodiaphragmatic angles.
Figure 3.5. Radiographs of patients with congestive heart failure.
These are anteroposterior views (which may exaggerate the size of the heart because it is further from the x-ray film), taken with portable x-ray machines at the bedside. A. Mild congestive heart failure. Pulmonary congestion is indicated by vascular redistribution from the bases to the apices of the lungs. The white spots labeled “L” are electrocardiographic leads on the patient’s chest. B: Severe congestive heart failure. Increased pulmonary vascular markings are present throughout the lung fields, along with peribronchiolar cuffing (black arrow) and pleural effusion, which is indicated by blunting of the costodiaphragmatic angle and tracking up the right lateral hemithorax (black arrowheads). The presence of interstitial and alveolar edema produces perihilar haziness and air bronchograms (open arrows), which occur when the radiolucent bronchial tree is contrasted with opaque edematous lung tissue.
Changes in pulmonary blood flow may also alter the appearance of the pulmonary vessels. For example, focal oligemia (reduction in the size of blood vessels due to decreased blood flow) is occasionally observed distal to a pulmonary embolism (termed the Westermark sign). The finding of enlarged central pulmonary arteries, but small peripheral vessels (termed peripheral pruning), suggests pulmonary hypertension (see Fig. 3.3).
Table 3.1 summarizes the major radiographic findings in common forms of cardiac disease.
Table 3.1. Chest Radiography of Common Cardiac Disorders
|Congestive heart failure|
|Pulmonic valve stenosis|
|Aortic valve stenosis|
Echocardiography plays an essential role in the diagnosis and serial evaluation of many cardiac disorders. It is safe, noninvasive, and relatively inexpensive. High-frequency (ultrasonic) waves generated by a piezoelectric element travel through the body and are reflected at interfaces where there are differences in the acoustic impedance of adjacent tissues. The reflected waves return to the transducer and are recorded. The machine measures the time elapsed between the initiation and reception of the sound waves, allowing it to calculate the distance between the transducer and each anatomic reflecting surface. Images are then constructed from these calculations.
Three types of imaging are routinely performed during an echocardiographic examination: M-mode, two-dimensional (2D), and Doppler. Each type of imaging can be performed from various body locations. Most commonly, transthoracic studies are performed, in which images are obtained by placing the transducer on the surface of the chest. When greater structural detail is required, transesophageal imaging is performed.
M-mode echocardiography, the oldest form of cardiac ultrasonography, provides data from only one ultrasonic beam and is now rarely used by itself. It supplements the other modalities to provide accurate measurements of wall thicknesses and timing of valve movements.
In 2D echocardiography, multiple ultrasonic beams are transmitted from the transducer through a wide arc. The returning signals are integrated to produce 2D images of the heart on a video monitor. As a result, this technique depicts anatomic relationships and defines the movement of cardiac structures relative to one another. Wall and valve motion abnormalities, and many types of intracardiac masses (e.g., vegetations, thrombi, tumors), can be depicted.
Each 2D plane ( Fig. 3.6) delineates only part of a given cardiac structure. Optimal evaluation of the entire heart is achieved by using combinations of views. In transthoracic echocardiography (TTE), in which the transducer is placed against the patient’s skin, these include the parasternal long axis, parasternal short axis, apical four-chamber, apical two-chamber, apical three-chamber (also known as apical long axis), and subcostal views. The parasternal long axis view is recorded with the transducer in the third or fourth intercostal space to the left of the sternum.
This view is particularly useful for evaluation of the left atrium, mitral valve, left ventricle, and left ventricular outflow tract (LVOT), which includes the aortic valve and adjacent interventricular septum. To obtain parasternal short axis views, the transducer is rotated 90° from its position for the long axis view. The short axis images depict transverse planes of the heart. Several different levels are imaged to assess the aortic valve, mitral valve, and left ventricular wall motion.
Figure 3.6. Transthoracic two-dimensional echocardiographic views.
A. Parasternal long axis view. B. Parasternal short axis view. Notice that the left ventricle appears circular in this view, while the right ventricle is crescent shaped. C. Apical four-chamber view. Ao, aorta; LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle.
(Modified from Sahn DJ, Anderson F. Two-Dimensional Anatomy of the Heart. New York, NY: John Wiley & Sons; 1982.)
Apical TTE views are produced when the transducer is placed at the point of maximal apical impulse. The apical four-chamber view evaluates the mitral and tricuspid valves as well as the atrial and ventricular chambers, including the motion of the lateral, septal, and apical left ventricular walls. The apical two-chamber view shows only the left side of the heart, and it depicts movement of the anterior, inferior, and apical walls.
In some patients, such as those with obstructive airways disease, the parasternal and apical views do not adequately show cardiac structures because the excessive underlying air attenuates the acoustic signal. In such patients, the subcostal view, in which the transducer is placed inferior to the rib cage, may provide a better ultrasonic window.
Doppler imaging depicts blood flow direction and velocity, and identifies regions of vascular turbulence. Additionally, it permits estimation of pressure gradients within the heart and great vessels. Doppler studies are based on the physical principle that waves reflected from a moving object undergo a frequency shift according to the moving object’s velocity relative to the source of the waves.
Color flow mapping converts the Doppler signals to a scale of colors that represent direction, velocity, and turbulence of blood flow in a semiquantitative way. The colors are superimposed on 2D images and show the location of stenotic and regurgitant valvular lesions and of abnormal communications within the heart and great vessels. For example, Doppler echocardiography in a patient with mitral regurgitation shows a jet of retrograde flow into the left atrium during systole ( Fig. 3.7).
Figure 3.7. Doppler color flow mapping (reproduced in gray tones) of mitral regurgitation (MR).
The Doppler image, recorded in systole, is superimposed on an apical view of the left ventricle (LV), left atrium (LA), and mitral valve (short arrow). The retrograde flow of MR into the LA is indicated by the long arrow.
Sound frequency shifts are converted by the echo machine into blood flow velocity measurements by the following relationship:
where v equals the blood flow velocity (m/sec); fs, the Doppler frequency shift (kHz); c, the velocity of sound in body tissue (m/sec); fO, the frequency of the sound pulse emitted from the transducer (MHz); and ?, the angle between the transmitted sound pulse and the mean axis of blood flow.
Transesophageal echocardiography (TEE) uses a miniaturized transducer mounted at the end of a modified endoscope to transmit and receive ultrasound waves from within the esophagus, thus producing very clear images of the neighboring cardiac structures ( Fig. 3.8) and much of the thoracic aorta. Modern probes permit multiplanar imaging and Doppler interrogation. TEE is particularly helpful in the assessment of aortic and atrial abnormalities, conditions that are less well visualized by conventional transthoracic echo imaging.
For example, TEE is more sensitive than transthoracic echo for the detection of thrombus within the left atrial appendage ( Fig. 3.9). The proximity of the esophagus to the heart makes TEE imaging particularly advantageous in patients for whom transthoracic echo images are unsatisfactory (e.g., those with chronic obstructive lung disease).
Figure 3.8. Transesophageal echocardiographic views.
LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle; N, noncoronary cusp of aortic valve; L, left coronary cusp of aortic valve; R, right coronary cusp of aortic valve.
Figure 3.9. Echocardiographic imaging of an intracardiac thrombus.
A. Transesophageal echocardiographic image demonstrates thrombus within the left atrial appendage. (Courtesy of Scott Streckenbach, MD, Massachusetts General Hospital, Boston, MA.) B. Schematic drawing of same image. LA, left atrium; LAA, left atrial appendage.
TEE is also advantageous in the evaluation of patients with prosthetic heart valves. During standard transthoracic imaging, artificial mechanical valves reflect a large portion of ultrasound waves, thus interfering with visualization of more posterior structures (termed acoustic shadowing). TEE aids visualization in such patients and is therefore the most sensitive noninvasive technique for evaluating perivalvular leaks.
TEE is commonly used to evaluate patients with cerebral ischemic events (i.e., strokes) of unexplained etiology, because it can identify cardiovascular sources of embolism with high sensitivity. These etiologies include intracardiac thrombi or tumors, atherosclerotic debris within the aorta, and valvular vegetations. TEE is also highly sensitive and specific for the detection of aortic dissection.
In the operating room, TEE permits immediate evaluation after surgical repair of cardiac lesions. In addition, imaging of ventricular wall motion can identify periods of myocardial ischemia during surgery.
The newest ultrasound imaging modality in development, and entering clinical usage, is 3D echocardiography. The spatial reconstructions afforded by this technique are of particular promise in the assessment of valvular defects, intracardiac masses, and congenital malformations.
Contrast echocardiography is sometimes used to supplement standard imaging to evaluate for abnormal intracardiac shunts. In this technique, often called a “bubble study,” an echocardiographic contrast agent (e.g., agitated saline) is rapidly injected into a peripheral vein. Using standard imaging, the contrast can be visualized passing through the cardiac chambers. Normally, there is rapid opacification of the right-sided chambers, but because the contrast is filtered out (harmlessly) in the lungs, it does not reach the left-sided chambers.
Conversely, in the presence of an intracardiac shunt with abnormal right-to-left heart blood flow, or in the presence of an intrapulmonary shunt, bubbles of contrast will appear in the left-sided chambers as well. Newer perfluorocarbon-based contrast agents have been developed with sufficiently small particle size to intentionally pass through the pulmonary circulation. These agents are used to opacify the left ventricular cavity and, via the coronary arteries, the myocardium, enabling superior assessment of LV contraction and myocardial perfusion.
Echocardiographic techniques can identify valvular lesions, complications of coronary artery disease (CAD), septal defects, intracardiac masses, cardiomyopathy, ventricular hypertrophy, pericardial disease, aortic disease, and congenital heart disease. Typical evaluation includes assessment of cardiac chamber sizes, wall thicknesses, wall motion, valvular function, blood flow, and intracardiac hemodynamics. A few of these topics are highlighted here.