Sonography of the Fetal Heart: Normal Variants and Pitfalls

1251 Pictorial Essay Sonography of the Fetal Heart: Normal Variants and Pitfalls Douglas L. Brown,1 Donald N. DiSalvo,1 Mary C. Frates,1 and Ira A. Parness2 3 Sonographic evaluation of the fetal heart is an important part of obstetric sonography. The sonographer and sonologist should be familiar with the sonographic appearance of the normal fetal heart and with common structural abnormalities. Occasionally, normal structures in or adjacent to the fetal heart may simulate an abnormality. Although one should seek consultative sonography in instances of uncertain or questionable findings, unnecessary referral and concern may be avoided in some cases if the sonologist is familiar with normal variants and pitfalls. In this pictorial essay, we present several such pitfalls that we and others have observed in the four-chamber view (Fig. 1) and in views of the ventricular outflow tracts (Figs. 2 and 3). Four-Chamber View Echogenic Focus in Ventricle A small, discrete, echogenic focus in the lumen of the left on right ventricle is sometimes seen in the region of the papillary muscles (Fig. 4). This finding is thought to be of no dinical significance and most likely represents normal papillary muscle [1, 2], although prominence of the chordae tendineae also has been proposed [3]. In one study, such an echogenic focus was identified in the left ventricle in 20% of normal fetuses and in the night ventricle in 2% of normal fetuses [2]. Echogenic Focus at Edge of Myocardium Peter M. Doubilet,1 Carol B. Benson,1 Faye C. Laing,1 small tumor on myocandial calcification (Fig. 5A). We have observed this when the imaging plane includes just the antenor end of a nib (Fig. SB) on part of the sternum. Prominent Moderator Band One of the sonognaphic characteristics of the morphologidaily right ventricle is a focal area of echogenicity at its apex, causing a blunted appearance. This is due to the moderaton band (also called the septomanginal tnabecula), which may vary greatly in its development. Occasionally, this musculan tissue in the apex of the right ventricle may be pnominent enough to mimic a cardiac tumor (Fig. 6). Pseudo-Ventricular Septal Defect Occasionally, when imaging the normal fetal heart, echoes may be absent in the thinner, membnanous portion of the ventricular septum, suggesting a ventricular septal defect [1]. This simulated lesion typically occurs when the ventnicu- Ian septum is aligned parallel to the ultrasound beam (Fig. 7). It is probably due to the relative thinness of the membranous septum and to the fact that when parallel to the ultrasound beam, the septum provides little to no interface to reflect echoes back to the transducer. Linear Structures Within Right Atrium Occasionally, a brightly echogenic focus located in on at the periphery of the ventricular myocardium suggests a Linear structures may be observed in the night atrium of normal fetuses. These could be mistaken for an atypically Received December 1 1, 1 992; accepted after revision February 2, 1993. 1 Department of Radiology, Brigham and Women s Hospital, Harvard Medical School, 75 Francis St., Boston, MA 02115. 2Department of Cardiology, Children s Hospital, Harvard Medical School, 300 Longwood Ave., Boston, MA 02115. 3Present address: Department of Pediatrics, Division Pediatric Cardiology, Mt. Sinai Medical Center, 1 Gustave Levy Place, New York, NY 10029. AJR 1993;160:1251-1255 0361-803X/93/1606-1251 American Roentgen Ray Society

1252 BROWN El AL. AJR:160, June 1993. - Fig. 1.-Four-chamber view of normal fetal heart. Sonogram shows that right and left sides of heart are relatively symmetric in size. Apex of right ventricle (RV) is blunted (curved arrow) partly because of moderator band. Ventricular septum is intact and tricuspid valve (straight arrow) inserts slightly more apically on septum than does mitral valve. Septum primum (arrowhead) is seen in left atrium (LA). LV = left ventricle, RA = right atrium..;c-; Fig. 4.-Sonogram shows normal echogenic focus (arrow) in left ventricle. in most cases, it is easily distinguished from a cardiac tumor by its typical brightly echogenic appearance, extreme smallness, and location. moving septum pnimum (also known as the flap of the fonamen ovale), which is normally seen in the left atrium. These linear structures in the night atrium, the eustachian valve and Chiani s network, represent different remnants of the embryonic valves of the sinus venosus [4]. The eustachian valve, also called the valve of the inferior vena cava (IVC), is usually present, having developed from the right valve of the sinus venosus. It appears as a relatively fixed linear echo that extends from the anterior junction of the IVC with the night atrium toward the foramen ovale (Fig. 8A). Infrequently, one observes several thin, filamentous, mobile echoes within the night atrium (Fig. 8B), which most likely represent Chiani s network. Chiani s network results Fig. 2.-Long-axis view of normal left ventricular outflow tract. This view is obtained by angling transducer toward fetal head, from fourchamber view, and rotating transducer. Sonogram shows that ventricular septum (arrowhead) is continuous with wall of aorta (curved arrow). Straight arrow is in lumen of aorta and points to aortic valve. LV = left ventricle. Fig. 3.-Short-axis view of normal great arteries shows right ventricular outflow tract. Sonogram indirectly shows relatively perpendicular orientation of great arteries at valve level: aorta (straight solid arrow) is seen in cross section whereas a longitudinal section of pulmonary artery (curved arrow) is seen. Arrowhead = right pulmonary artery; open arrow = ductus arterlosus, RV = right ventricle. Fig. 5.-Sonograms of echogenic focus at edge of myocardium. A, Four-chamber view shows small echogenic focus (arrow) at apex of left ventricle. B, If transducer is rotated into a plane that shows a longer section of rib (arrow), pitfallcan be recognized. when the nesonptive process of the embryonic night valve of the sinus venosus leaves a fenestrated on reticulan network of fibers [4]. Chiani s network is believed to have no functional significance [4], although it has been reported in a fetus with premature atnial and ventricular contractions [5]. Pseudothickening of Tricuspid Valve A thickened tricuspid valve may indicate tricuspid valve dysplasia. However, the panietal band (also called supnaventniculan crest), an arch-shaped band of muscle located between the tricuspid and pulmonary valves, can be mistaken for a thickened tricuspid valve (Fig. 9).

Fig. 6.-Sonogram shows prominent moderator band. Myocardium in apex of right ventricle (open arrow) on this slightly oblique four-chamber view is more prominent than usually seen. Features that favor a normal prominent moderator band include its singularity and apical location (tumors may be multiple and occur in any chamber), its isoechogenicity with respect to myocardium (tumors are usually hyperechoic), and its concave edge (curved arrow) that tends to hug endocardial surface (tumors tend to have convex margins and are spherical or ovoid). LV = left ventricle. Fig. 8.-Sonograms of eustachian valve and Chiari s network in right atrium. Septum primum (curved arrow) is seen in left atrium. A, In this slightly oblique four-chamber view, eustachian valve (straight arrow) appears as a linear structure within right atrium (RA). B, In another fetus, thin curvilinear structures (straight arrows) are present in right atrium and were observed to move freely at real-time sonography. We believe that these thin structures represent Chiarl s network. Fig. 7.-Sonograms of pseudo-ventricular septal defect. A, On this apical four-chamber view (where septum is parallel to ultrasound beam), defect in ventricular septum (arrow) appears to be present. B, This artifact can be confirmed by scanning same portion of septum from a different orientation, Ideally with ultrasound beam more perpendicular to septum (as in this image), showing continuity (arrow) of septum. Fig. 9.-Sonograms of pseudothickening of tricuspid valve. A, By angling imaging plane superioriy from usual four-chamber view, one can image panetal band (arrows) and simulate thickening of tricuspid valve. Tricuspid valve leaflets, however, are actually out of image plane. B, An Image slightly more inferior shows normal, thin, tricuspid valve leaflets (arrows).

1254 BROWN ET AL. AJR:160, June 1993 Entrance of Coronary Sinus into Right Atrium The coronary sinus returns venous blood from the myocandium to the right atrium. It is located in the posterior left atnioventniculan groove on the external surface of the heart and opens into the night atrium between the tricuspid valve and the entrance of the IVC. In some imaging planes, the entrance of the coronary sinus into the right atrium may simulate a defect in the lower portion of the atnial septum (Fig. 10). Although it can enlarge in certain forms of anomalous systemic on pulmonary venous return, the coronary sinus can be identified in many normal fetal hearts. Pericardial Fluid A small amount of penicandial fluid can be seen in normal second- and third-trimester fetuses [1] (Fig. ha) and does not imply abnormality. Distinguishing a normal amount of Fig. 10.-Normal coronary sinus (straight arrows) is seen on sonogram angled slightly inferoposteriorly from usual four-chamber view of heart. Coronary sinus may simulate a small defect in atrial septum where it enters (curved arrow) right atrium (RA). Its linear orientation and location help identity it as coronary sinus. fluid from a small pathologic effusion can occasionally be difficult, as the upper limit of normal for the thickness of the rim of penicandial fluid has not been precisely defined, but appears to be at least 2 mm [6-8]. A related pitfall is the peripheral hypoechoic part of the myocandium, which can mimic penicandial fluid [9] (Fig. 11B). Ventricular Outflow Tracts Pseudooverriding of Aorta An important normal finding on the long-axis view of the left ventricular outflow tract is visualization of continuity of the ventricular septum with the anterior wall of the aorta. In tetralogy of Fallot, one typically sees interruption of the ventricular septum and displacement of the aorta oven the septum, producing overriding of the aorta. Occasionally, overriding of the aorta can be simulated in a normal heart [1] (Fig. 12). Although the origin of this artifact is speculative, it Fig. 11.-Sonograms of pericardial fluid. A, A trace of penicardial fluid (curved arrow) is seen in this four-chamber view of a normal fetus. Fluid is peripheral to myocardium, which itself can be relatively hypoechoic peripherally (straight arrow). Note that fluid does not surround entire heart but is seen adjacent to a small segment of heart. B, On short-axis view through ventricles of another fetus, normal peripheral hypoechoic part of myocardium (curved arrows) can be distinguished from true fluid by observing its continuation into ventricular septum (straight arrow). Fig. 12.-Sonograms of pseudooverriding of aorta. A, Long-axis view of left ventricular outflow tract suggests discontinuity and overriding (arrowhead) between wail of aorta (Ao) and ventricular septum (curved arrow). in this case, apparent interruption is immediately distal to aortic valve (straight arrow), suggesting that artifact may sometimes be related to a sinus of Valsalva. B, Artifactual nature of this finding is confirmed with a slightly different imaging plane with which overriding is not seen, that is, the septum and aorta appear continuous (arrowhead). LV = left ventricle.

AJR:160, June 1993 SONOGRAPHY OF THE FETAL HEART 1255 Fig. 13.-Sonogram shows parallel great arteries distally. Farther cephalad than level of semilunar valves, aorta (Ao) and pulmonary artery-ductus arteriosus continuum do course parallel. Such parallel orientation at this level Is normal. DA = ductus arteriosus; PA = pulmonary artery; S = spine. Arrow indicates superior vena cava. is probably due to a combination of effects, including thinness of the membranous ventricular septum, the normal path of the left ventricular outflow tract, which courses slightly over the plane of the ventricular septum, and penhaps partial volume artifact with the pulmonary outflow tract, which is immediately superior to this area, on with a sinus of Valsalva. To prevent mistaking this artifact for an abnormality, a long-axis view in a slightly different plane should be obtained. True overriding should be seen in several views whereas pseudooverniding is usually evident in only one plane. Allan [1] reports that if the long-axis view is obtained so that the posterior wall of the aorta is continuous with the anterior cusp of the mitral valve, the ventricular septum should be continuous with the anterior wall of the aorta in the normal heart (Fig. 2). Parallel GreatArteries Distal to Valves Parallel orientation of the aorta and pulmonary artery at the level of the semilunar valves is abnormal and should suggest a transposition abnormality. Morphologic identification of the great arteries and their ventricular chambers of origin then becomes important in determining the exact type of transposition abnormality. However, distal to the semilunar valves, at approximately the level of the aortic arch, the aorta and pulmonary artery-ductus arteniosus continuum do course parallel for a short segment. Parallel orientation at this level should not be mistaken for abnormality (Fig. 13). Fig. 14.-Sonograms of normal slight dilatation of pulmonary artery. A, Diameter of pulmonary artery increases slightly (curved arrow) just distal to puimonic valve (straight arrow). This slight increase in caliber is typically seen when pulmonary artery is imaged in such a way that right pulmonary artery is just out of imaging plane. Ao = aorta. B, Slight alteration of imaging plane shows right pulmonary artery (solid arrow) arises at this location, where pulmonary artery appears to bifurcate into right pulmonary artery and ductus arteniosus (open arrow). Left pulmonary artery Is out of imaging plane. Ao = aorta. Normal Slight Dilatation of PulmonaiyArteiy Normally, the caliber of the pulmonary artery increases slightly just distal to the pulmonic valve, where the main pulmonary artery tnifuncates into the left and night pulmonary arteries and the ductus arteniosus (Fig. 14). This normal finding should not be mistaken for poststenotic dilatation associated with pulmonic stenosis. If needed, Doppler examination and imaging of valve leaflet motion may be useful in determining whether pulmonic stenosis is present [1]. REFERENCES 1. Allan LD. Manual of fetal echocardiography. Lancaster, England: MTP Press Limited, 1986:11-47, 97-115, 126-139, 142 2. Levy DW, Mintz MC. The left ventricular echogenic focus: a normal finding. AJR 1988:150:85-86 3. Schechten AG, Fakhry J, Shapiro LR, Gewitz MH. In utero thickening of the chordae tendineae: a cause of intracardiac echogenic foci. J Ultrasound Med 1987:6:691-695 4. Edwards JE. Congenital malformations of the heart and great vessels: malformations of the atrial septal complex. In: Gould SE, ed. Pathology of the heart. Springfield, IL: Thomas, 1960:260-289 5. Clements J, Sobotka-Plojhar M, Exalto N, van Geijn HP. A connective tissue membrane in the right atrium (Chiari s network) as a cause of fetal cardiac arrhythmia. Am J Obstet Gynecoll982:142:709-712 6. Jeanty P, Romero R, Hobbins JC. Fetal pericardial fluid: a normal finding of the second half of gestation. Am J Obstet Gynecol 1984:149:529-532 7. Shenken L, Reed KL, Anderson CF, Kem N. Fetal pericardial effusion. Am J Obstet Gynecoll989:160:1505-1 508 8. Yagel 5, Hurwitz A. Fetal pericardial fluid (letter). Am J Obstet Gynecol 1985;152:721-722 9. Brown DL, Cartier MS, Emerson DS, Shanklin DR. Smith WC, Felker RE. The peripheral hypoechoic rim of the fetal heart. J Ultrasound Med 1989: 8:603-608