Normal Anatomy and Flow Patterns on Transthoracic Echocardiography

2


Normal Anatomy and Flow Patterns on Transthoracic Echocardiography





Basic Imaging Principles



Tomographic Imaging


Echocardiography provides tomographic images of cardiac structures and blood flow, analogous to a thin “slice” through the heart. Two-dimensional (2D) echocardiographic images provide detailed anatomic data in a given image plane, but complete evaluation of the cardiac chambers and valves requires integration of information from multiple image planes. Small structures that transverse numerous tomographic planes (such as the coronary arteries) are difficult to evaluate fully. In addition, structures may move in and out of the imaging plane as a result of motion caused by cardiac contraction or respiratory movement of the heart within the chest. Respiratory variation in cardiac location is recognized easily by its timing, but movement of the heart during the cardiac cycle is more problematic because it may not be obvious on the 2D image. Cardiac motion relative to surrounding structures is described in three dimensions as:



Even if the 2D image plane is fixed in position, the location of underlying structures may vary between systole and diastole. For example, in the apical four-chamber view, adjacent segments of the LV (which may be supplied by different coronary arteries) may be seen in systole versus diastole. Compared to tomographic imaging, three-dimensional (3D) imaging provides a wider field of view, but it currently has poorer resolution and a slower frame rate, and it still is affected by respiratory and cardiac motion (see Chapter 4). Thus, both modalities are used as appropriate during the echocardiographic study.



Nomenclature of Standard Views


Each tomographic image is defined by its acoustic window (the position of the transducer) and view (the image plane) (Table 2-1). The standard three orthogonal echocardiographic image planes are determined by the axis of the heart itself (with the LV as the major point of reference) rather than by skeletal or external body landmarks (Fig. 2-1). The primary reference points on the heart are the apex, defined as the tip of the LV, and the valve planes at the cardiac base. The four standard image planes are:





image Long-axis plane: Parallel to the long axis of the LV, with the image plane intersecting the LV apex and center of the aortic valve, aligned with the anterior-posterior diameter of the mitral annulus


image Short-axis planes: A series of image planes perpendicular to the long axis of the ventricle, resulting in circular cross-sectional views of the LV, mitral valve, and aortic valve


image Four-chamber plane: An image plane from apex to base, perpendicular to the short-axis view, that includes both ventricles and atria, aligned with the mediolateral diameter of both the mitral and tricuspid annulus


image Two-chamber plane: An image plane from apex to base that includes the LV and left atrium (LA), perpendicular to the short-axis view, and rotated to be midway between the long-axis and four-chamber views


In addition to apical versus basal, other standard directional terms are medial versus lateral (the horizontal axis in a short-axis or four-chamber view) and anterior versus posterior (the vertical axis in a short-axis or long-axis view). This standard terminology also applies to visualization of cardiac anatomy with 3D echocardiography.


Acoustic windows are transducer positions that allow ultrasound access to the heart. The bony thoracic cage and adjacent air-filled lung limit acoustic access, making patient positioning and sonographer experience critical factors in obtaining diagnostic images. Transthoracic images typically are obtained from parasternal, apical, subcostal, and suprasternal notch acoustic windows. The transducer motions used to obtain the desired view are described as follows (Fig. 2-2):





Image Orientation


Most laboratories follow the American Society of Echocardiography (ASE) guidelines for image orientation in adults, although some pediatric cardiologists use alternate formats. The recommended orientation is with the transducer position (narrowest portion of the sector scan) at the top of the screen so that structures closer to the transducer are at the top and structures farther from the transducer are at the bottom of the image. Thus, a transthoracic four-chamber view is displayed with the apex at the top of the image (because it is closest to the transducer), whereas a TEE (transesophageal echocardiograph) four-chamber view is displayed with the apex at the bottom of the image (because it is most distant from the transducer). This orientation aids in prompt recognition of ultrasound artifacts, shadowing, and reverberations because the display of the origin of the ultrasound signal is the same for all acoustic windows and image planes.


The lateral (in short-axis views) and basal (in long-axis views) cardiac structures are displayed on the right side of the screen, which is similar to the format used for other tomographic imaging techniques. Short-axis views can be thought of as the observer looking from the apex toward the cardiac base; long-axis views, as the observer looking from the left toward the right side of the heart. The four-chamber plane is displayed with lateral structures on the right side of the screen and medial structures on the left side (as for the short-axis view).



Examination Technique


The echocardiographic examination is performed by a physician or by a trained cardiac sonographer under the supervision of a qualified physician. Guidelines and recommendations for education and training in diagnostic echocardiography for both sonographers and physicians have been published, as referenced in Chapter 5.


At the time of a transthoracic echocardiographic examination, relevant clinical data, prior imaging studies, and the indication for the study are reviewed. Blood pressure is recorded along with age, height, and weight. The patient is positioned comfortably for each view in either a left lateral decubitus or supine position. Electrocardiographic (ECG) electrodes are attached for display of a single lead (usually lead II) to aid in timing cardiac events. Specially designed echocardiographic examination stretchers provide apical cutouts for optimal transducer positioning at the apex. The transducer is applied to the chest and upper abdomen using a water-soluble gel to obtain good contact without intervening air. The time needed to perform an echocardiographic examination depends on the specific clinical situation—from a few minutes in a critically ill patient to document cardiac tamponade to more than 1 hour to quantitate multiple lesions in a patient with complex valvular or congenital heart disease.



Technical Quality


Image quality depends on the degree of ultrasound tissue penetration, transducer frequency, instrument settings, and the sonographer’s skill. Ultrasound tissue penetration or “acoustic access” to the cardiac structures is largely determined by body habitus, specifically how the heart is positioned relative to the lungs and chest wall. Conditions that increase transducer distance from the heart (e.g., adipose tissue), decrease ultrasound penetration (e.g., scar tissue), or interpose air-containing tissues between the transducer and the heart (e.g., chronic lung disease, recent cardiac surgery) all lead to poor image quality. TEE images tend to show better definition of cardiac structures given the shorter distance between the transducer and heart, the absence of interposed lung, and the use of a higher transducer frequency. On transthoracic studies, optimal patient positioning for each acoustic window brings the cardiac structures against the chest wall. In addition, respiratory variation can be used to the sonographer’s advantage by having the patient suspend breathing briefly in whichever phase of the respiratory cycle yields the best image quality. Unfortunately, even with careful attention to examination technique, echocardiographic images remain suboptimal in some patients.



Echocardiographic Image Interpretation


The physician uses the tomographic 2D echocardiographic images to build a mental 3D reconstruction of the cardiac chambers and valves or uses a 3D echocardiographic data set to examine anatomy in specific image planes (see Chapter 4). To do this, an understanding of image planes and orientation and the technical aspects of image acquisition (e.g., in recognizing artifacts) is needed, along with a detailed knowledge of cardiac anatomy (Table 2-2). Recording images as the tomographic plane is moved between standard image planes is important for this analysis and ensures that abnormalities lying outside or between our arbitrary “standard” views are not missed. Three-dimensional imaging may be helpful for elucidating anatomic relationships in complex cases and may aid in identifying the optimal image planes for display of abnormal findings. Information obtained from anatomic imaging then is integrated with physiologic Doppler data and clinical information in the final echocardiographic interpretation.




Transthoracic Tomographic Views


Normal echocardiographic anatomy is described below for each tomographic view. The best views for specific cardiac structures are indicated in Table 2-3.




Parasternal Window



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 may be visualized by moving the transducer cephalad one or two interspaces. The upper limit of normal for aortic end-diastolic dimension in adults is 1.6 cm/m2 at the annulus and 2.1 cm/m2 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, though 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 and the more basal minor axis bisected in the long-axis view.



The left atrium is seen posterior to the aorta and has a similar anteroposterior dimension as the aortic sinuses in normal adults. The right pulmonary artery lies between the proximal ascending aorta and superior aspect of the LA but may not be well seen on transthoracic images. 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, while a pleural effusion will be 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, 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 right ventricle (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, 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 (RA), tricuspid valve, and RV (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, while 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, may be 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, there are two principal papillary muscles (anterior and posterior) 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 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 to 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 extending 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 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 of 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, since 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, which 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 short-axis, the pulmonic valve is seen in 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 there is considerable individual variability. 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 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.


Jul 10, 2018 | Posted by in CARDIOLOGY | Comments Off on Normal Anatomy and Flow Patterns on Transthoracic Echocardiography
Premium Wordpress Themes by UFO Themes