An ideal modality used for noninvasive imaging of congenital heart defects should be able to delineate all aspects of the cardiac anatomy, including extracardiac vessels; evaluate physiological parameters such as measurement of blood flow, pressure gradients across cardiac valves or blood vessels, and ventricular function; be cost effective, portable, and noninvasive with least risk and discomfort; and include no exposure to ionizing radiation. No single modality satisfies all of these requirements. Chest radiographic films, the original imaging tool, provide only indirect evidence of intracardiac defects that manifest primarily with volume overload. They do not provide images of the defect itself. They are less informative on pressure overload lesions that do not result in chamber enlargement. Echocardiography has become the main noninvasive imaging modality since the 1980s, providing direct images of intracardiac and some extracardiac anatomy. In recent years, noninvasive radiologic techniques such as magnetic resonance imaging (MRI) and computed tomography (CT) have emerged as supplementary modalities in areas for which echocardiography studies are deficient.
In this chapter, two-dimensional (2D) and M-mode echocardiography are presented in some detail. This will be followed by a brief discussion of the two radiologic techniques with emphasis on how to choose an optimal radiologic technique for a given patient.
Echocardiography
Echocardiography (echo) uses ultrasound beams reflected by cardiovascular structures to produce characteristic lines or shapes caused by normal or abnormal cardiac anatomy in one, two, or three dimensions, which are called M-mode, 2D, and three-dimensional (3D) echocardiography, respectively.
An echocardiographic study currently begins with real-time 2D echo, which produces high-resolution tomographic images of cardiac structure, their movement, and vascular structures leaving and entering the heart. The Doppler and color mapping study has added the ability to easily detect valve regurgitation and cardiac shunts during the echo examination. These tests combined provide reliable quantitative information such as ventricular function, pressure gradients across cardiac valves and blood vessels, and estimation of pressures in the great arteries and ventricles. Echo examination can be applied in calculation of cardiac output and the magnitude of cardiac shunts, although this is rarely used. Reliable noninvasive hemodynamic evaluation and confident delineation of cardiovascular structures by echo have dramatically reduced the necessity for cardiac catheterization. Increasingly, patients undergo valvular or congenital heart surgery on the basis of an echo diagnosis. Transesophageal echocardiography (TEE) has markedly improved resolution of echo images. Real-time 3D echocardiography provides improved accuracy of imaging the global perspective visualization of various cardiac anomalies, but this is not presented here because it is not widely used.
Discussion of instruments and techniques is beyond the scope of this book. Normal 2D echo images and M-mode measurements and their role in the diagnosis of common cardiac problems in pediatric patients are the focus of this presentation.
Two-Dimensional Echocardiography
Two-dimensional echo examinations are performed by directing the plane of the transducer beam along a number of cross-sectional planes through the heart and great vessels. A routine 2D echocardiogram is obtained from four transducer locations: parasternal, apical, subcostal, and suprasternal notch (SSN) positions. From each transducer position, images of the long- and short-axis views are obtained by manually rotating and angulating the transducer. The parasternal and apical views usually are obtained with the patient in the left lateral decubitus position and the subcostal and suprasternal notch views with the patient in the supine position. Figures 5-1 through 5-9 illustrate some standard images of the heart and great vessels. Modified transducer positions and different angulations make many other views possible. Measurement of important cardiac structures can be made on the freeze frame of 2D echo studies. Normal values of the dimension of the cardiac chambers, great arteries, and various valve annulus are shown in several tables in Appendix D (see Table D-1 , Table D-2 , Table D-3 , Table D-4 , Table D-5 ).
The Parasternal Views
For parasternal views, the transducer is applied to the left parasternal border in the second, third, or fourth space with the patient in the left lateral decubitus position.
Parasternal Long-Axis Views
The plane of sound is oriented along the major axis of the heart, usually from the patient’s left hip to the right shoulder. Three major views are recorded; standard long axis, long axis of the right ventricular (RV) inflow, and long axis of the RV outflow (see Fig. 5-1 ).
1
The Standard Long-Axis View
is the most basic view that shows the left atrium (LA), mitral valve, and left ventricular (LV) inflow and outflow tracts (see Fig. 5-1 , A ). This view is important in evaluating abnormalities in or near the mitral valve, LA, LV, left ventricular outflow tract (LVOT), aortic valve, aortic root, ascending aorta, and ventricular septum. In the normal heart, the anterior leaflet of the mitral valve is continuous with the posterior wall of the aorta (i.e., aortic–mitral continuity). The trabecular septum (apical ward) and infracristal outlet septum (near the aortic valve) constitute the interventricular septum in this view. Therefore, ventricular septal defects (VSDs) of tetralogy of Fallot (TOF) and persistent truncus arteriosus are best shown in this view. Detailed discussion of localizing other types of VSDs is presented in Chapter 12 . Pericardial effusion is readily imaged in this view. This is the view to evaluate mitral valve prolapse (MVP). Frequently, the coronary sinus can be seen as a small circle in the atrioventricular (AV) groove (see Fig. 5-1 , A ). An enlarged coronary sinus may be seen with left superior vena cava (LSVC), TAPVR to coronary sinus, coronary AV fistula, and rarely with elevated right atrial (RA) pressure.
2
In the RV Inflow View
a long-axis view of the RV and RA is obtained. In this view, abnormalities in the tricuspid valve (tricuspid regurgitation [TR], prolapse) and inflow portion of the RV are evaluated ( Fig. 5-1 , B ). The ventricular septum in this view consists of the inlet muscular septum (near the AV valve) and trabecular septum (apical ward). The right atrial appendage (RAA) can also be seen in this view. This view is good for recording the velocity of TR (to estimated RV systolic pressure).
3
In the RV Outflow View
the RV outflow tract (RVOT), pulmonary valve, and proximal main pulmonary artery (PA) are visualized ( Fig. 5-1 , C ). The supracristal infundibular (outlet) septum is seen in this view.
Parasternal Short-Axis Views
By rotating the transducer used for the long-axis views clockwise, one obtains a family of important short-axis views (see Fig. 5-2 ). This projection provides cross-sectional images of the heart and the great arteries at different levels. The parasternal short-axis views are important in the evaluation of the aortic valve (i.e., bicuspid or tricuspid), pulmonary valve, PA and its branches, RVOT, coronary arteries (e.g., absence, aneurysm, stenosis), LA, LV, ventricular septum, AV valves, LV, and right side of the heart.
1
The Aortic Valve
The aortic valve is seen in the center of the image with the RVOT anterior to the aortic valve and the main PA to the right of the aorta (“circle and sausage” view) (see Fig. 5-2 , A ). The right, left, and noncoronary cusps of the aortic valve are best seen from this view, having the appearance of the letter “Y” during diastole. Stenosis and regurgitation of the pulmonary valve is best examined in this plane. Stenosis of the PA branches can be evaluated by Doppler interrogation and color-flow mapping and Doppler interrogation of the ductal shunt is obtained in this plane. Color-flow studies show the membranous VSDs just distal to the tricuspid valve (at the 10 o’clock direction) and both the infracristal and supracristal outlet VSDs (at the 12 to 2 o’clock direction) anterior to the aortic valve near the pulmonary valve.
2
Coronary Arteries
With a slight manipulation of the transducer from the above plane, the ostia and the proximal portions of the coronary arteries are visualized. The right coronary artery (RCA) arises from the anterior coronary cusp near the tricuspid valve, which should be confirmed to connect to the aorta; there are some venous structures that run in front of the aorta (cardiac vein) but do not connect to the aorta. The left main coronary artery arises in the left coronary cusp near the main PA. Its bifurcation into the left anterior descending and circumflex coronary artery can usually be seen clearly. The proximal coronary arteries can also be seen in other long-axis views.
3
Mitral Valve
The mitral valve is seen as a “fish mouth” during diastole. This view is good for measuring the mitral valve area in patients with mitral stenosis and it is the best view to identify a cleft mitral valve (see Fig. 5-2 , C ).
4
Papillary Muscles
Two papillary muscles are normally seen at the 4 o’clock (anterolateral) and 8 o’clock (posteromedial) directions. The trabecular septum is seen at this level. This view is good in the assessment of apical portion of the LV such as hypertrophic cardiomyopathy, noncompaction of the apex and apical mass (see Fig. 5-2 , D ).
The Apical Views
For apical views, the transducer is positioned over the cardiac apex with the patient in the left lateral decubitus position.
Apical Four-Chamber View
The plane of sound is oriented in a nearly coronal body plane and is tilted from posterior to anterior to obtain a family of apical four-chamber views (see Fig. 5-3 ). This view displays all four chambers of the heart, the ventricular and atrial septa, and the crux of the heart. This is the best view to image the LV apex, where an apical VSD is commonly seen.
1
Coronary Sinus
In the most posterior plane, the coronary sinus is seen to drain into the right atrium (see Fig. 5-3 , A ). The ventricular septum seen in this view is the posterior trabecular septum.
2
The Apical Four-Chamber View
(see Fig. 5-3 , B ) evaluates the atrial and ventricular septa and size and contractility of atrial and ventricular chambers, AV valves, and some pulmonary veins and identifies the anatomic RV and LV and detecting pericardial effusion. Normally, the tricuspid valve insertion to the septum is more apicalward than the mitral valve (5–10 mm in older children and adults), with a small portion of the septum (called the AV septum) separating the two AV valves. A defect in this portion of the septum may result in an LV–RA shunt. In Ebstein’s anomaly, the insertion of the septal tricuspid valve is displaced more apically. The inlet ventricular septum (where an endocardial cushion defect occurs) is well imaged in this view just under the AV valves. VSDs in the entire length of the trabecular septum are well imaged, including apical VSD. The membranous septum is not imaged in this view. The anatomic characteristics of each ventricle are also noted, with the heavily trabeculated RV showing the moderator band. Abnormal chordal attachment of the AV valve (straddling) and overriding of the valve are also noted in this view. The relative size of the ventricles is examined in this view.
3
Apical Five-Chamber View
Further anterior angulation of the transducer demonstrates the so-called apical five-chamber view. This view shows the LVOT, aortic valve, subaortic area, and proximal ascending aorta. In this view, color-flow imaging allows qualitative assessment of aortic regurgitation. The membranous VSD is visualized just under the aortic valve, and the infracristal outlet VSD is also imaged in this plane.
Apical Long-Axis Views
The apical long-axis view (or apical three-chamber view) shows structures similar to those shown in the parasternal long-axis view (see Fig. 5-4 , A ). In the apical two-chamber view, the LA, mitral valve, and LV are imaged. The left atrial appendage can also be imaged (see Fig. 5-4 , B ). The view of the LV apex provides diagnostic clues for cardiomyopathy, apical thrombus, and aneurysm.
The Subcostal Views
Subcostal long axis (coronal) and short-axis (sagittal) views are obtained from the subxyphoid transducer position, with the patient in the supine position.
Subcostal Long-Axis (Coronal) Views
are obtained by tilting the coronal plane of sound from posterior to anterior (see Fig. 5-5 ). The coronary sinus is seen posteriorly draining into the RA , similar to that shown for the apical view (see Fig. 5-5 , A ). Anterior angulation of the transducer shows the atrial and ventricular septa. This is the best view for evaluating the atrial septum, including atrial septal defect (see Fig. 5-5 , B ). Further anterior angulation of the transducer shows the LVOT, aortic valve, and ascending aorta (see Fig. 5-5 , C ). The parts of the ventricular septum visualized in this view (apical ward) are membranous, subaortic outlet, and trabecular septa. The junction of the superior vena cava (SVC) and the RA is seen to the right of the ascending aorta (see Fig. 5-5 , C ). Further anterior angulation shows the entire RV including the inlet, trabecular and infundibular portions, pulmonary valve, and main PA (see Fig. 5-5 , D ). The ventricular septum seen in this view includes the (apical ward) supracristal outlet, infracristal outlet, and anterior trabecular and posterior trabecular septa.
Subcostal Short-Axis (or Sagittal) Views
(see Fig. 5-6 ) are obtained by rotating the long-axis transducer 90 degrees to the sagittal plane.
To the right of the patient, both the superior and inferior venae cavae are seen to enter the right atrium (see Fig. 5-6 , A ). A small azygous vein can be seen to enter the SVC, and the right PA can also be seen on end beneath this vein (see Fig. 5-6 , A ).
A leftward angulation of the transducer shows the RVOT, pulmonary valve, PA, and the tricuspid valve on end (see Fig. 5-6 , B ). This view is orthogonal to the standard subcostal four-chamber view, and both views combined are good for evaluation of the size of a VSD.
Additional leftward angulation of the transducer will show the mitral valve (not shown) and papillary muscle (see Fig. 5-6 , C ), similar to those seen in the parasternal short-axis views.
Subcostal Views of the Abdomen
Abdominal short- and long-axis views ( Fig. 5-7 ) are obtained from the subxyphoid transducer position, with the patients in supine position.
1
Abdominal Short-Axis View
is obtained by placing the transducer in a transverse body plane (see Fig. 5-7 , left ). It demonstrates the descending aorta on the left and the inferior vena cava (IVC) on the right of the spine as two round structures. The aorta should pulsate. Both hemidiaphragms, which move symmetrically with respiration, are imaged. Asymmetric or paradoxical movement of the diaphragm is seen with paralysis of the hemidiaphrgam.
2
Abdominal Long-Axis View
is obtained by placing the transducer in a sagittal body plane. The IVC is imaged to the right (see Fig. 5-7 , right, A ), and the descending aorta is imaged to the left of the spine ( Fig. 5-7 , right, B ). The IVC collects the hepatic vein before draining into the right atrium. The eustachian valve may be seen at the junction of the IVC and the RA. The failure of the IVC to join the RA indicates interruption of the IVC (with azygous continuation, which is frequently seen with polysplenia syndrome). Major branches of the descending aorta, celiac artery, and superior mesenteric artery are easily visualized. A pulsed-wave Doppler examination of the abdominal aorta in this view is important to identify coarctation by demonstrating persistent diastolic flow and delayed upstroke of systolic flow.
The Suprasternal Views
The transducer is positioned in the suprasternal notch to obtain suprasternal long-axis (see Fig. 5-8 , A ) and short-axis (see Fig. 5-8 , B ) views, which are important in the evaluation of anomalies in the ascending and descending aortas (e.g., coarctation of the aorta), aortic arch (e.g., interruption), size of the PAs, and anomalies of systemic veins and pulmonary veins. In infants, the transducer can be sometimes placed in a high right subclavicular position.
The Suprasternal Long-Axis View
(see Fig. 5-8 , A ) is obtained by 45-degree clockwise rotation from the sagittal plane in the suprasternal notch to visualize the entire (left) aortic arch. Failure to visualize the aortic arch in this manner may suggest the presence of a right aortic arch. Three arteries arising from the aortic arch (the innominate, left carotid, and left subclavian arteries in that order) are seen. The innominate vein is seen in cross-section in front of the ascending aorta and the right PA behind the ascending aorta. Manipulation of the transducer farther posteriorly and leftward will image the isthmus and upper descending aorta, a very important segment to study for the coarctation of the aorta.
The Suprasternal Short-Axis View
( Fig. 5-8 , B ) is obtained by rotating the ultrasound plane parallel to the sternum. Superior to the circular transverse aorta, the innominate vein is seen to connect to the (right) superior vena cava, which runs vertically to the right of the aorta. The right PA is visualized in its length under the circular aorta. Beneath the RPA is the LA. With a slight posterior angulation of the transducer, four pulmonary veins are seen to enter the LA.
The Subclavicular Views
The Right Subclavicular View
(see Fig. 5-9 , A ) is obtained in the right second intercostal space in a sagittal projection. This view is useful in the assessment of the SVC and right atrial junction as well as the ascending aorta. The right upper pulmonary vein and the azygous vein can also be examined in this view.
The Left Subclavicular View
(see Fig. 5-9 , B ) is useful for examination of the branch PAs. The transducer is positioned in a transverse plane in the second left intercostal space and a little tilted inferiorly. The main PA is seen left of the ascending aorta (circle), and it bifurcates into the right and left PA branches.
Quantitative Values Derived from Two-Dimensional Echocardiography
1
Dimensions of Cardiovascular Structures
Several tables of normal values of cardiovascular structures that were measured from still frames of 2D echocardiography are shown in Appendix D . These tables are frequently useful in the practice of pediatric cardiology. They include M-mode measurements of the LV ( Table D-1 ); stand-alone M-mode measurements of the RV, aorta, and LA ( Table D-2 ); aortic root and aorta ( Table D-3 ); pulmonary valve and PAs ( Table D-4 ); and AV valves ( Table D-5 ). The normal dimension of the coronary arteries is shown in Table D-6 .
2
Left Ventricular Mass
LV mass is an indication of LVH. Although the thickness of LV walls (interventricular septum and LV posterior wall) identifies people with increased LV mass, LV mass has become a valuable marker of end-organ damage in patients with systemic hypertension. LV mass can be estimated from M-mode echocardiography or 2D echocardiography measurements.
- a.
M-mode echo method. LV mass is usually derived from 2D-guided M-mode echo measurement, with assumptions that the LV is spherical in shape and the thickness of the wall measured at the basal area of the LV is representative of the entire LV. It is assumed that a smaller sphere formed by the endocardium is inside a larger sphere formed by the pericardium, and therefore, the difference in the volume of the two spheres would be the volume of LV muscle. Although less accurate than the 2D echo method, the M-mode method is simpler to obtain and thus more popular than the 2D method. There is controversy as to indexing the LV mass for body size, having been variably indexed by body weight, height, body surface area, or 2.7 power of height. M-mode–derived LV mass indexed to 2.7 power of height is popular and normal data by that method are presented in Appendix D ( Table D-7 ).
- b.
2D echo method. In the 2D-measured echo technique for calculating LV mass, the LV is assumed to be a bullet shape rather than a sphere, and this technique has been shown to be more accurate than the M-mode method. The LV volume can be estimated from the short- and long-axis views in systole or diastole. The LV area in the short-axis view is calculated by the biplane Simpson’s method. The formula for such a volume would be 5/6 the LV area of the short axis multiplied by the length of the ventricle (obtained from the long axis):
Volume = 5 / 6 Area × Length
The LV volume so calculated is then converted to mass by multiplying it by the specific gravity of muscle (usually taken as 1.05). This cumbersome method is less popular and not routinely performed in most laboratories. Normal values of 2D echo-derived LV mass indexed to the body surface area are presented in Appendix D ( Table D-8 ).
M-Mode Echocardiography
M-mode echocardiography, which graphically displays a one-dimensional slice of cardiac structure varying over time, was one of the earliest tools of the echocardiography. Currently, an M-mode echo is obtained as part of 2D tomographic images. M-mode echo is used primarily for the measurement of the dimension (wall thickness and chamber size) and LV function (fractional shortening, wall thickening). It is also useful for assessing the motion of cardiac valves (mitral valve prolapse, mitral stenosis, pulmonary hypertension) and movement of cardiac wall and septa (in RV volume overload).
Figure 5-10 shows examples of M-mode measurements of the dimension of the RV, LV, LA, and aorta and LV wall thickness during systole and diastole. Line 1 passes through the aorta and LA, where the dimensions of these structures are measured. Line 2 traverses the mitral valve. Line 3 goes through the main body of the RV and LV. Along line 3, the dimensions of the RV and LV and the thickness of the interventricular septum and posterior LV wall are measured during systole and diastole. Pericardial effusion is best detected at this level.
Normal M-Mode Echo Values
Two frequent applications of M-mode echo in clinical practice are measurements of dimension of cardiac structures and LV systolic function.
Cardiac Chamber Dimensions
Most dimensions are measured during diastole, coincident with the onset of the QRS complex; the LA dimension and LV systolic dimension are exceptions (see Fig. 5-10 ). The dimensions of the cardiac chambers and the aorta increase with increasing age and thus normal values are expressed as function of growth (see Appendix D ).
Left Ventricular Systolic Function
LV systolic function is evaluated by the fractional shortening (or shortening fraction) or ejection fraction. The ejection fraction is a derivative of the fractional shortening and offers no advantages over the fractional shortening. Serial determinations of these measurements are important in the management of conditions in which LV function may change (e.g., in patients with chronic or acute myocardial disease).
Fractional Shortening
Fractional shortening (or shortening fraction) is derived by the following:
FS ( % ) = Dd − Ds / Dd × 100
Mean normal value of FS is 36%, with 95% prediction limits of 28% to 44%. FS is decreased in a poorly compensated LV regardless of cause (e.g., pressure overload, volume overload, primary myocardial disorders, doxorubicin cardiotoxicity). It is increased in volume-overloaded ventricle (e.g., VSD, patent ductus arteriosus, aortic regurgitation, mitral regurgitation [MR]) and pressure overload lesions (e.g., moderately severe aortic valve stenosis, hypertrophic obstructive cardiomyopathy).
Ejection Fraction
Ejection fraction relates to the change in volume of the LV with cardiac contraction. It is obtained by the following formula:
EF ( % ) = ( Dd ) 3 − ( Ds ) 3 / ( Dd ) 3 × 100
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