Normal Anatomy and Flow Patterns on Transthoracic Echocardiography

Long-Axis Views

With the patient in a left lateral decubitus position and the transducer in the left third or fourth intercostal space, adjacent to the sternum, a long-axis view of the heart is obtained that bisects the long axis of both aortic and mitral valves ( Figs. 2.3 and 2.4 ). In this standard view, the aortic sinuses, sinotubular junction, and proximal 3 to 4 cm of the ascending aorta are seen; further segments of the ascending aorta are visualized by moving the transducer cephalad one or two interspaces. The term “aortic root” often is used to refer to the entire proximal aorta including the annulus, sinuses, sinotubular junction, and ascending aorta. The upper limit of normal for aortic end-diastolic dimension in adults is 1.6 cm/m ² at the annulus and 2.1 cm/m ² at the sinuses.

In the long-axis view, the right coronary cusp of the aortic valve is anterior and the noncoronary cusp is posterior (the left coronary cusp is lateral to the image plane). In systole, the thin aortic leaflets open widely, assuming a parallel orientation to the aortic walls. In diastole, the leaflets are closed, with a small obtuse closure angle between the two leaflets. The leaflets appear linear from the closure line to the aortic annulus because of the hemicylindrical shape of the closed leaflets (linear along the length of the cylinder, curved along its short axis). In normal young individuals, the leaflets are so thin that only the apposed portions at the leaflets' closure line may be seen. The 3D anatomy of the attachment line of the aortic leaflets to the aortic root is shaped like a crown with the three commissures attached near the tops of the sinuses of Valsalva and the mid-portion of each leaflet attached near the base of each sinus ( Fig. 2.5 ). The fibrous continuity between the aortic root and the anterior mitral leaflet (absence of intervening myocardium) helps identify the anatomic LV in complex congenital disease.

The anterior and posterior mitral valve leaflets appear thin and uniform in echogenicity, with chordal attachments leading toward the medial (or posteromedial) papillary muscle seen in the long-axis view, although the papillary muscle itself is slightly medial to the long-axis plane. The anterior mitral leaflet is longer than the posterior leaflet but has a smaller annular length so that the surface areas of the two leaflets are similar ( Fig. 2.6 ). As the mitral leaflets open in diastole, the tips separate, and the anterior leaflet touches or comes very close to the ventricular septum. In systole, the leaflets coapt, with some overlap between the leaflets (apposition zone) and a slightly obtuse (>180°) angle relative to the mitral annulus plane. The chordae normally remain posterior to the plane of leaflet coaptation in systole. Some normal individuals have systolic anterior motion of the chordae resulting from mild redundancy of chordal tissue that is not associated with hemodynamic abnormalities. This must be distinguished from the pathologic systolic anterior motion of the mitral leaflets seen in hypertrophic cardiomyopathy. The mitral annulus (the attachment between the mitral leaflets, LA, and LV) is an anatomically well-defined fibrous structure shaped like a bent ellipse, with the more apical major axis bisected in the four-chamber view and the more basal minor axis bisected in the long-axis view.

The left atrium is seen posterior to the aorta and has an anteroposterior diameter similar to that of the aortic sinuses in normal adults. The right pulmonary artery lies between the proximal ascending aorta and the superior aspect of the LA but usually is not well seen on transthoracic images in adults. The coronary sinus is seen in the atrioventricular groove posterior to the mitral annulus. Dilation of the coronary sinus resulting from a persistent left superior vena cava (which can be confirmed by echo-contrast injection in a left arm vein if needed) is seen in about 0.4% of studies; it is an isolated incidental finding in about half of these cases and is associated with congenital heart disease in the remainder.

Posterior to the LA, the descending thoracic aorta is seen in cross section. A long-axis view of the descending thoracic aorta can be obtained by rotating the transducer counterclockwise. The oblique sinus of the pericardium lies between the LA and the descending thoracic aorta so that a pericardial effusion can be seen between these two structures, whereas a pleural effusion is seen only posterior to the descending thoracic aorta.

The left ventricle septum and posterior wall are seen at the base and mid-ventricular level in the long-axis view, thus allowing assessment of wall thickness, chamber dimensions, endocardial motion, and wall thickening of these myocardial segments. LV end-diastolic and end-systolic measurements of wall thickness and internal dimensions are made in the long-axis view on 2D images from the septal to posterior wall tissue-blood interface or using a 2D-guided M-mode recording when a perpendicular alignment can be obtained (see Chapter 6 ). From the parasternal window, the LV apex is not seen; the apparent “apex” usually is an oblique image plane through the anterolateral wall.

A portion of the muscular right ventricular outflow tract is seen anteriorly. Unlike the symmetric prolate ellipsoid shape of the LV, the RV does not have an easily defined long or short axis. In effect, the RV is “wrapped around” the LV, with an inflow region, an apical region, and an outflow region forming a somewhat anteroposteriorly flattened U-shaped structure. Most standard image planes result in oblique tomographic sections of the RV, so right ventricular size and systolic function are best evaluated from multiple views or with 3D imaging, as discussed more fully in Chapter 6 .

Right Ventricular Inflow and Outflow Views

In the long-axis plane, the transducer is moved apically and then angulated medially to obtain a view of the right atrium, tricuspid valve, and right ventricle ( Fig. 2.7 ). In this RV inflow view, the septal and anterior leaflets of the tricuspid valve are well seen. The RV apex is heavily trabeculated, whereas the outflow tract (supracristal region) has a smoother endocardial surface. The moderator band, a prominent muscle trabeculation that traverses the RV apex obliquely and contains the right bundle branch, is seen in both parasternal and apical views ( Fig. 2.8 ). The papillary muscles are more difficult to identify in the RV than in the LV. Typically, two principal papillary muscles (anterior and posterior) are seen, with a smaller supracristal (or conus) papillary muscle. The moderator band attaches near the base of the anterior RV papillary muscle.

The coronary sinus is identified as it enters the right atrium (RA) adjacent to the tricuspid annulus. By slowly scanning back to an LV long-axis view, the coronary sinus can be followed along its length.

Another normal anatomic feature of the RA ( Fig. 2.9 ) is the crista terminalis, a muscular ridge that courses anteriorly from the superior vena cava to the inferior vena cava and divides the trabeculated anterior portion of the RA from the posterior, smooth-walled sinus venosus segment. The RA appendage is rarely seen on transthoracic imaging, but it is a trabeculated protrusion of the RA that extends anterior to the RA free wall and base of the aorta.

The inferior vena cava is seen entering the RA inferior to the coronary sinus. In some individuals, a prominent Eustachian valve is seen at the junction of the inferior vena cava and RA both in this view and from the subcostal window. When a more extensive fenestrated valve is present, it forms a Chiari network extending from the inferior to the superior vena cava, attached to the crista terminalis posteriorly and the fossa ovalis medially, with a netlike structure that appears as bright mobile echo densities in the RA. Both these findings are considered normal variants.

The interatrial septum is not well seen in the RV inflow view, being just inferior and parallel to the image plane. However, careful angulation between the long-axis and RV inflow views allows examination of the atrial septum with recognition of the thick primum septum at its junction with the central fibrous body, the thin fossa ovalis in the central portion of the atrial septum, the ridge-like limbus located superior to the fossa, and the ridge adjacent to the junction with the coronary sinus.

Moving the transducer toward the base and then angulating laterally, a long-axis view of the RV outflow tract, pulmonic valve, and pulmonary artery is obtained. This view is particularly useful for recording flow velocities in the RV outflow tract and pulmonary artery.

Short-Axis Views

Short-axis views are obtained from the parasternal window by rotating the transducer clockwise 90° and then moving or angulating the transducer superiorly or inferiorly to obtain specific image planes.

At the aortic valve level ( Fig. 2.10 ), the short-axis view demonstrates all three aortic valve leaflets: right, left, and noncoronary cusps. In systole, the aortic leaflets open to a near-circular orifice. In diastole, the typical Y-shaped arrangement of the coaptation lines of the leaflets is seen with three points of aortic attachment, or commissures. Identification of the number of aortic valve leaflets (or commissures) is made most accurately in systole because a bicuspid valve may appear trileaflet in diastole as a result of a raphe in the larger leaflet but the presence of only two commissures in systole. The normal valve leaflets are thin at the base with an area of thickening on the ventricular aspect in the middle of the free edge of each cusp that serves to fill the space at the center of the closed valve. These nodules normally enlarge with age (nodules of Arantius) and can have small, mobile filaments attached on the ventricular surface (Lambl excrescences). These small but normal structures may be seen when echocardiographic images are of high quality and should not be mistaken for pathologic conditions. The origins of the left main and right coronary arteries often can be identified in this view.

The aortic and pulmonic valve planes normally lie perpendicular to each other. Thus, when the aortic valve is seen in the short axis, the pulmonic valve is seen in the long axis. In adults, evaluation of the leaflets of the pulmonic valve is limited; usually only one or two leaflets are seen well, and a short-axis view often is not obtainable. The close relationship between the aortic valve and other intracardiac structures is apparent in this short-axis view ( Fig. 2.11 ). The pulmonic valve and RV outflow tract are seen anterolaterally, adjacent to the left coronary cusp, and portions of the septal and anterior tricuspid valve leaflets are seen anteriorly and slightly medially, adjacent to the right coronary cusp. Posteriorly, the RA, interatrial septum, and LA lie in proximity to the noncoronary cusp of the aortic valve. The LA appendage can be better imaged from this view by a slight lateral angulation and a superior rotation of the transducer. The central location of the aortic valve illustrates how disease processes can extend from the aortic valve or root into the RV outflow tract, RA, or LA. Extension of disease processes into the ventricular septum or anterior mitral leaflet also is possible, as evident in the long-axis view.

At the mitral valve short-axis level ( Fig. 2.12 ), the thin anterior and posterior mitral leaflets are seen as they open nearly to the full cross-sectional area of the LV in diastole and close in systole. The posterior leaflet consists of three major scallops—lateral, central, and medial (also called P1, P2, and P3)—although considerable individual variability exists. The two mitral commissures (the points on the annulus where the anterior and posterior leaflets meet) are located medially and laterally. Note that this parallels the arrangement of the papillary muscles so that chordae from the medial aspects of both anterior and posterior leaflets attach to the medial (or posteromedial) papillary muscle, and chordae from the lateral aspects of both leaflets attach to the lateral (or anterolateral) papillary muscle. Chordae branch at three levels (primary, secondary, and tertiary) between the papillary muscle tip and mitral leaflet with a progressive decrease in chordal diameter and increase in the number of chordae from approximately 12 at the papillary muscle to 120 at the mitral leaflet. Most chordae attach at the free edge of the leaflets (called marginal chordae ), but some (called basal or strut chordae ) attach to the LV surface of the leaflet. Occasionally, aberrant chordae to the ventricular septum or other structures are seen in an otherwise normal individual.

At the mid-ventricular (or papillary muscle ) level ( Fig. 2.13 ), the normal LV is circular in the short-axis view. An elliptical appearance of the chamber usually is due to a nonperpendicular orientation relative to the long axis of the LV. Moving the transducer superiorly with apical angulation resolves this problem. In some normal individuals, the LV appears flattened along the diaphragm in diastole, but it has a normal circular appearance in systole. A noncircular appearance in systole is consistent with myocardial disease, such as myocardial infarction or aneurysm formation, or with abnormal septal curvature due to right heart disease. Although 2D linear measurements of LV diameter are made in the long-axis view, rotating the transducer between the long- and short-axis views at this level ensures that this measurement is both centered in the chamber and perpendicular to the long axis. Oblique measurements result in overestimation of wall thickness and ventricular dimensions.

This view also allows assessment of segmental endocardial motion and wall thickening at the mid-ventricular level. The nomenclature of LV myocardial segments is based on coronary anatomy as discussed in Chapter 8 . Basically, the ventricle is divided into anterior (septum and free wall), anterolateral, inferolateral (also called posterior), and inferior (free wall and septum) segments for consistent descriptors of the location of abnormalities. The segments are further defined by their location along the length of the ventricle as basal, mid-ventricular, or apical. Ventricular septal motion reflects abnormalities other than coronary disease, including RV volume overload, pressure overload, or both; conduction abnormalities; and the post–cardiac surgery state (see Fig. 6.22 ).

The medial and lateral papillary muscles are seen in this short-axis plane and serve as landmarks identifying the mid-ventricular level. Rarely, one of the papillary muscles is bifid, resulting in an appearance of three separate papillary muscles. Note that the apical segments of the LV myocardium are not seen in standard parasternal views. However, in some patients, a short-axis view of the LV near the apex can be obtained by moving the transducer laterally and angling medially. Alternatively, 3D volumetric imaging can be used to display multiple simultaneous short-axis views of the LV from base apex (see Chapter 4 ).

Four-Chamber View

In the apical four-chamber view, the length of the left ventricle is seen in a plane perpendicular to both the short-axis and long-axis planes ( Fig. 2.15 ). The anterolateral wall, apex, and inferior septum lie in this tomographic plane. The LV appears as a truncated ellipse with a longer length than width and a tapered but rounded apex. If the transducer is not positioned at the true apex, the LV will appear foreshortened, with a more spherical shape and little tapering of the apex. Foreshortening of the long-axis plane must be distinguished from disease processes, such as chronic aortic regurgitation, which result in increased sphericity of the ventricle. Although the RV is more trabeculated than the LV, prominent trabeculation also can be seen at the LV apex and must be distinguished from apical thrombus. An aberrant LV trabecula that traverses the ventricular chamber is an incidental finding, often called an LV “chord.”

Medially, the right ventricle is triangular with a cavity about half the area of the LV. The RV apex is less round and more basal than the LV apex, and the moderator band traverses the RV chamber near the apex. The RV can be further evaluated by moving the transducer medially over the RV apex. Considerable individual variability in the shape and wall motion of the RV, particularly at the apex, is seen in normal individuals, so caution is needed in diagnosing an abnormal RV from any single tomographic plane.

The four-chamber view also shows the mitral annulus in its major dimension and the anterior (located adjacent to the septum) and posterior (adjacent to the lateral wall) mitral valve leaflets, along with chordal attachments to the lateral papillary muscle. The mitral leaflet tips separate widely in diastole. In systole the closure plane of the leaflets may appear “flat” (a 180° closure angle) because of the nonplanar “saddle” shape of the annulus, with the four-chamber view bisecting the annulus at its most apical position compared with the more basal annulus segments seen in a long-axis view.

The tricuspid annulus lies slightly (up to 1.0 cm) closer to the apex than the mitral annulus. The tricuspid leaflets show a wide diastolic opening; thin, uniformly echogenic, leaflets; and normal coaptation in systole. The septal leaflet is imaged adjacent to the septum. The tricuspid leaflet adjacent to the free wall may be either the anterior or posterior leaflet, depending on the exact rotation and angulation of the image plane.

The left and right atria are distal from the apical transducer position. Apical views are useful for measurement of LA volume ( Fig. 2.16 ), but ultrasound resolution at this depth is poor, so detailed evaluation of atrial tumors or exclusion of thrombus typically requires TEE imaging. The interatrial septum lies parallel to the ultrasound beam in this view, so “dropout”—absence of reflected signal—from the region of the fossa ovalis is common and should not be mistaken for an atrial septal defect. The descending thoracic aorta is seen lateral to the LA. The pulmonary veins enter the LA posteriorly but are difficult to image at this depth in adults. If the transducer is angulated posteriorly from the four-chamber view, more posterior portions of the lateral and inferior septal myocardium are seen. In addition, the length of the coronary sinus comes into view in the atrioventricular groove.

By angulating the transducer anteriorly, the aortic valve and root are seen in an oblique long view. This view is sometimes referred to as the apical “five-chamber” view. More anterior portions of the septum and lateral wall are seen, especially at the base, as the transducer is angulated anteriorly. This view of the anterior mitral leaflet, LV outflow tract, and aortic valve is at an angle approximately 60° to 90° from the long-axis view. In some adults, further anterior angulation of the transducer allows visualization of the pulmonary artery arising from the RV. A view of the pulmonic valve from the apical window is more easily obtained in young adults and children.

Two-Chamber View

From the four-chamber view, the transducer is rotated counterclockwise about 60° to obtain the two-chamber view of the LV, mitral valve, and LA ( Fig. 2.17 ). The apical two-chamber view is used for evaluation of the anterior LV wall (seen to the right of the screen) and the inferolateral (posterior) wall (seen on the left). Fine adjustments in transducer position may be needed to visualize the anterior wall endocardium because of interference from adjacent lung tissue. To ensure that the proper rotation has been made for a two-chamber view, the transducer is angled posteriorly to intersect both papillary muscles symmetrically. Then the transducer is angled slightly anteriorly so that neither papillary muscle is seen in its long axis in this view. The anterior mitral leaflet is seen en face, so the apparent closure plane of the leaflet relative to the annulus can be misleading. The LA appendage may be visualized adjacent to the anterior wall. A long-axis view of the descending thoracic aorta can be obtained by angulating posteriorly and rotating counterclockwise from the two-chamber view.

Long-Axis View

Rotating the transducer another 60° from the two-chamber view (120° from the four-chamber view) yields a long-axis view similar to the parasternal long-axis view ( Fig. 2.18 ). The aortic valve, LV outflow tract, and mitral valve are seen in long axis. The LV walls visualized in this view are the anterior septum (on the right side of the screen) and posterior or inferolateral wall (on the left). Compared with the parasternal long-axis view, the LV apex now is seen, but the aortic and mitral valves are at a greater image depth (with consequent poorer image resolution).

Other Apical Views

Nonstandard short-axis views of the LV apex that use a higher-frequency transducer (5 or 7.5 MHz) are helpful if an LV apical thrombus is suspected. One useful view is obtained by sliding the transducer laterally from the LV apex and then angulating medially.