4 Cardiac Anatomy Jan Bogaert and Andrew M

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Cardiac Anatomy 61 4 Cardiac Anatomy Jan Bogaert and Andrew M. Taylor

CONTENTS quences are now available for this task. To accurately interpret cardiac anatomy with MRI, a thorough 4.1 Introduction 61 knowledge of cardiac anatomy, with reference to 4.2 Cardiac MRI Techniques 61 4.3 Position of the Heart in the Thorax – both the axes of the body and the heart, and of the Gross Cardiac Anatomy 62 different cardiac MRI techniques is required, for 4.4 Cardiac Structures 63 both normal and pathological conditions. Analysis 4.4.1 Atria 63 of cardiac anatomy can sometimes be very difficult. 4.4.1.1 Morphological Right Atrium 63 This is especially true for congenital heart disease, 4.4.1.2 Morphological Left Atrium 65 4.4.1.3 Atrial Septum 66 where a segmental approach, consisting of a careful 4.4.2 Ventricles 66 analysis of the different components (venous struc- 4.4.2.1 Morphological Right Ventricle 66 tures, atria, atrioventricular valves, ventricles, ven- 4.4.2.2 Morphological Left Ventricle 68 triculo-arterial valves, and great arteries), each with 4.4.2.3 Ventricular Septum 70 its own typical characteristics, enables an accurate 4.4.3 Valves 70 4.4.4 Coronary Arteries 72 description of these complex hearts. 4.4.5 Pericardium 73 Radiologists interested in cardiac imaging are of- 4.5 Great Vessels 73 ten not sufficiently familiar with cardiac anatomy 4.6 Key Points 82 and have difficulties visualizing the heart in relation References 82 to its intrinsic cardiac axes, whilst cardiologists are often not sufficiently familiar with the tomographic approach for visualization of cardiac and extracardiac structures on MR images. 4.1 In this chapter on cardiac anatomy, the specific Introduction features of the different cardiac components and their presentation on MR images will be high- The first widely accepted strength of magnetic reso- lighted. A detailed description of the cardiovascular nance imaging (MRI) was its ability to non-inva- anatomy on MR images in the different body axes sively study cardiac morphology and structure. MRI (transverse, coronal, sagittal), and the intrinsic car- provides anatomical images of the heart with high diac axis (short-axis, horizontal long-axis, vertical spatial and contrast resolution, in a fast and reli- long-axis, left ventricular, LV, outflow tract, and able fashion. These images can be acquired in every right ventricular, RV, outflow tract) is available at imaginable plane, without restrictions in image ori- the end of this chapter (Figs. 4.17–4.24). The strate- entation and without the need for administration of gies for cardiac image planning and slice position- contrast agents (Dinsmore et al. 1984; O’Donovan ing are discussed in Chap. 5. et al. 1984; Burbank et al. 1988). Although spin- echo (SE) imaging techniques were initially used for studying cardiac anatomy, a variety of MRI se- 4.2 J. Bogaert MD, PhD Cardiac MRI Techniques Department of Radiology, Gasthuisberg University Hospital, Catholic University of Leuven, Herestraat 49, 3000 Leuven, Assessment of cardiac anatomy with MRI is for many Belgium A.M. Taylor, MD, MRCP, FRCR people still linked with the SE-MRI technique. This Cardiothoracic Unit, Institute of Child Health and Great has a historical explanation: In the early 1980s, the Ormond Street Hospital for Children, London, UK SE-MRI technique was the only available sequence for

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studying the heart, providing “morphologic images” As mentioned above, the gradient-echo technique (Hawkes et al. 1981; Heneghan et al. 1982; Herfkens introduced in the late 1980s to study cardiac dynam- et al. 1983; Higgins et al. 1985). Functional cardiac ics (e.g., function, perfusion), should be considered imaging, although possible, was very cumbersome a very useful method for depicting cardiac anatomy. with SE-MRI (Lanzer et al. 1985; Tscholakoff and This sequence provides bright-blood images, and Higgins 1985; Longmore et al. 1985). It took sev- offers complementary information to the SE-MRI eral years before the availability of the gradient-echo technique about cardiac morphology. With the ad- (GE) technique, with short repetition times, provided vent of the balanced, steady-state free precession a more useful method for analysis of cardiac function (SSFP) GE MRI sequence, combining high contrast with MRI (Frahm et al. 1986; Higgins et al. 1988). between blood and surrounding tissues with high Contrast in cardiac SE-MRI is not only generated temporal resolution, subtle anatomical structures by differences in tissue relaxation, but also by the such as valve leaflets, tendinous chords, muscular flow phenomenon. This allows images of the heart trabeculations, and pectinate muscles can be well vi- and great vessels to be obtained without the need of sualized, especially when using the cine-loop view- contrast agent administration to visualize the blood ing mode (Carr et al. 2001). A similar sequence de- pool. As explained in detail in Chap. 1, the excited sign is used for coronary artery imaging (Chap. 14), spins within the blood pool in the image slice are re- and contrast-enhanced three-dimensional (3D) MR placed by non-excited spins between the excitation angiography for great-vessel imaging (Chap. 16). and read-out pulses, thus creating a “black-blood” Real-time MRI is no longer a research tool, but it or “dark-blood” appearance. The older SE-MRI se- allows the operator to interactively navigate through quences, whereby only one line of N-space per slice, the heart and rapidly determine the cardiac image per heartbeat was acquired, have now been replaced planes (Castillo and Bluemke 2003). It is quite by fast SE-MRI techniques. Newer techniques (e.g., obvious that this task can only be achieved with an double inversion pulse) are also available for generat- extensive knowledge of the 3D cardiac anatomy. ing better suppression of the blood signal (Stehling et al. 1996; Simonetti et al. 1996; Winterer et al. 1999). These innovations have led to an overall im- provement in image quality. Image quality has be- 4.3 come largely independent of the slice direction, slice Position of the Heart in the Thorax – thickness and other influencing parameters. In the Gross Cardiac Anatomy analysis of the SE-MRI, one should be careful not to mistake other anatomical structures devoid of sig- The heart has a central, ventrobasal location in the nal, such as the air-filled trachea and bronchi, for thorax and is bordered bilaterally by the lungs, anteri- vascular structures. For a similar reason, calcium- orly by the sternum, and inferiorly by the diaphragm containing structures (e.g., calcified valve leaflets) (Fig. 4.1). It has an oblique position in the thoracic may be not detected on SE-MRI. cavity, with the cardiac apex in the left hemithorax.

abc

Fig. 4.1a-c. Position of the heart in the thorax. All images are obtained using a breath-hold dark-blood double inversion- recovery SE-MRI technique. a Transverse image; b coronal image; c sagittal image. The arrow on the transverse and coronal image indicates the longitudinal axis of the heart. LL, left lung; RL, right lung. star, bullous emphysema.

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The long axis of the heart is rotated about 45º to both its long axis is not parallel to the long axis of the body. the sagittal and the coronal planes. In younger or In congenitally malformed hearts, the use of right and slender individuals, the heart tends to be more verti- left might be confusing, since morphological “right” cal, whereas it tends to be more horizontal in obese structures may occupy a left-sided position, and vice patients. For the most part, the heart is surrounded by versa. The difficulty is overcome in congenitally mal- the pericardial sac and has no physical connections formed hearts by adding the description “morpholog- with the surrounding structures except posteriorly ical” to “right” and “left”. The relationship of the right and superiorly where the great arteries originate and and left structures of the normal heart are further the caval and pulmonary veins drain into the atria complicated by the marked twisting of the ventricular (Amplatz and Moller 1993). outflow tracts. The aorta, even though emerging from While the position of the heart in the thorax is rela- the left ventricle (and therefore a left component of the tively constant, the position of the different cardiac heart) has its valve in a right-sided position relative to components, their intrinsic relationship, as well as their the pulmonary valve (Anderson 2000). relationship with the great vessels is much more complex and prone to a large number of congenital variations, isolated or in combination with other extracardiac congenital abnormalities. Knowledge of the gross 4.4 cardiac anatomy and of the specific characteristics of Cardiac Structures the different cardiac structures enables description of most complex congenital heart disease (Table 4.1) 4.4.1 The heart is a double, two-chambered pump, usu- Atria ally described in terms of “right-sided” and “left- sided” chambers. In reality, the right chambers are 4.4.1.1 more anteriorly positioned within the chest, the left Morphological Right Atrium chambers more posteriorly, and the ventricles are more inferiorly located than the atria. This is caused by a Both atria can be divided anatomically into a venous primary important feature of the normal heart, is that component, a vestibule of the atrioventricular valve,

Table 4.1. Essentials of the heart

Left Atrium Right Atrium Receives pulmonary veins Receives IVC / SVC – coronary sinus Posterosuperior location Fibromuscular webs Eustachian valve (IVC) Thebesian valve (coronary sinus) Crista terminalis: divides venous component from vestibule Prominent pectinate muscles > left atrium Left Atrial Appendage Right Atrial Appendage Anterosuperior (over LCx) Broad triangular Long/narrow Wide connection with RA Small junction Left Ventricle Right Ventricle Oval or prolate ellipsoid shape Pyramidal shape Fine trabeculations Coarse trabeculationsa Inflow and outflow in contact No connection between inflow and outflowa No infundibulum Infundibuluma Smooth septal surface Chordal attachment of septal leaflet of septuma Feeds the aorta Feeds the pulmonary artery Moderator banda Tricuspid valve more apically positioned than mitra valvea

aCriteria helpful in differentiating morphological left from right ventricles

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a septal component, and an appendage and are sepa- component receives the inferior and superior caval rated by the atrial septum. The right atrium forms veins on its posterior surface, and the coronary sinus the right heart border (Fig. 4.2). Embryologically, the at the inferior junction with the septal component. right atrium is formed from the sinus venous and the Fibromuscular webs attach to the terminal crest in primitive auricle. These two parts of the atrium are the regions of the openings of the inferior caval vein separated on the inside by a ridge, the terminal crest and coronary sinus. These are the so-called venous or crista terminalis, and on the outside by a groove, valves, the Eustachian valve in relation to the inferior the sulcus terminalis. The sinus venosus, forming the caval vein, and the Thebesian valve at the coronary posterior part of the right atrium, forms the venous sinus. The coronary sinus running through the left component, which has a smooth interior because of its or posterior atrioventricular groove opens into the origin as a vessel; while the primitive auricle, having a right atrium above the postero-inferior interventric- rough trabeculated interior, will form the appendage ular groove. Sometimes, the right coronary vein, also (Anderson 2000). Pectinate muscles branch from the draining into the right atrium, is visible in the ante- crest at right angles to run into the appendage. These rior atrioventricular groove. The junction between muscles encircle completely the parietal margin of the appendage and venous component is particular the vestibule of the atrioventricular valve. The venous wide, and the appendage has a broad, triangular appearance, positioned just ventrally to the entrance of the superior vena cava in the right atrium (Figs. 4.3 and 4.4). The vestibule is smooth walled and supports the attachments of the leaflets of the tricuspid valve. The different components, as well as the relationship to the caval veins and the coronary sinus, can be well appreciated on several imaging planes, including the transverse, coronal, and vertical long-axis or sagittal planes (Mohiaddin et al. 1991; Galjee et al. 1995). The terminal crest is routinely visible on dark-blood and bright-blood MRI as a mural nodular or triangular structure adjacent to the lateral wall that con- nects both caval veins. This muscular structure can be misinterpreted as an abnormal intra-atrial mass

Fig. 4.2a,b. Right atrium at a end diastole and b end sys- Fig. 4.3. Right atrial appendage. Transverse image using a 3D tole. The images are obtained in the transverse axis, using balanced-SSFP technique with submillimeter spatial resolu- a balanced steady-state free precession (SSFP) technique. tion The right atrial appendage (raap) is located ventrally to The terminal crest (crista terminalis, ct) divides the venous the superior vena cava (SVC) and laterally to the ascending component (posteromedially) from the vestibule. Note the aorta (Ao). The high spatial and high contrast resolution en- important changes in right atrial (RA) volume and shape able depiction of the pectinate muscles that branch from the during the cardiac cycle. terminal crest and run at right angles to the raap.

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Fig. 4.4a-d. Right and left atrial appendage. Short-axis images using a 3D balanced-SSFP technique with submillimeter spatial resolution. While the right atrial appendage (raap) has a triangular appearance, and a broad communi- cation (star) with the body of the right atrium (RA), the left atrial ap- abpendage (laap) has a narrow junction (two stars) and a long, tubular appearance. Note the relationship of the appendages to the coronary arteries. The raap is dorsally located to the right coronary artery (rca), while the laap overlies the posterior (or left) atrioventricular groove and the left circumflex coronary artery (lcx). Ao, Aorta; LA, left atrium; lm, left main stem coronary artery; PA, pulmonary artery; c d ps, pericardial sac.

(Menegus et al. 1992; Meier and Hartnell 1994; consists of sinus venosus and primitive auricle, and Mirowitz and Gutierrez 1992). Bright-blood MRI they form the same anatomical components as in the techniques with submillimeter spatial resolution, such right atrium. The venous component, also posteri- as currently used for coronary artery imaging, enable orly located and smooth-walled, receives the four visualization of thin structures such as the pectinate pulmonary veins, one at each corner (Fig. 4.5). The muscles in the right atrial appendage. Enlargement of vestibule supports the leaflets of the mitral valve the right atrium will easily displace the adjacent lung, and is also smooth-walled. The pectinate muscles, while right atrial-appendage enlargement encroaches being confined in the appendage, are much less ob- on the upper retro-sternal air space. vious than in the right atrium and never extend around the atrioventricular junction. The append- 4.4.1.2 age, overlying the left atrioventricular groove and Morphological Left Atrium left circumflex coronary artery (LCx), has a narrow junction with the body of the left atrium and The morphological left atrium forms the upper pos- has a long, tubular-shaped appearance (Anderson terior heart border, with its appendage extending 2000; Fig. 4.4). Imaging planes for studying the left anteromedially. It lies just beneath the carina and atrium are similar to those used to study the right anterior to the esophagus. The left atrium extends atrium. The relationship of the left atrium to the cranially behind the aortic root and the proximal carina and main stem bronchi is best visualized on part of the ascending aorta. The close relationship coronal views. The visualization of the entrance of to the esophagus makes the left atrium useful as the pulmonary veins in the left atrium (e.g., to ex- an acoustic window during transesophageal echo- clude abnormal pulmonary venous return) is best cardiography. Embryologically, the left atrium also done using a combination of transverse (or four-

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Fig. 4.5a,b. Left atrium at end diastole (a) and end systole (b). Horizontal long-axis images using a balanced SSFP technique. The entrance of the pulmonary vein from the right lower lobe in the left atrium a b (LA) can be clearly seen.

chamber) views and coronal (or short-axis) views. very well (Fig. 4.6). Sometimes fatty inﬁltration of Enlargement of the left atrium displaces the esoph- the interatrial septum is observed, which can be agus posteriorly and widens the subcarinal angle. easily differentiated from pathological masses by Massive enlargement, exceeding the space in front the characteristic signal intensity corresponding to of the spine, results in encroachment upon the right the subcutaneous fat. It is a mostly benign, usu- lung such that the left atrium becomes border-form- ally asymptomatic condition, with a low frequency ing on the right and may push the right ventricle on autopsy series. The atrial septum is best shown forward. Enlargement of the left atrial appendage in horizontal and longitudinal planes through the displaces the adjacent left lung and might be visible heart (e.g., transverse or four-chamber views). as an additional left border on frontal chest X-ray film. 4.4.2 4.4.1.3 Ventricles Atrial Septum 4.4.2.1 The atrial or interatrial septum separates the left Morphological Right Ventricle from the right atrium. In the atrial septum is an oval depression, the fossa ovalis. The floor of the fossa In a normal heart, the right ventricle sits above the ovalis is the remains of the septum primum. The liver and forms the inferior and anterior heart bor- septal surface on its right atrial aspect is made up der with the exception of the apex. The morphologi- of the floor of the oval fossa and its postero-inferior cal right ventricle can be identified externally by its rim. The superior rim of the fossa (the so-called septum secundum) is no more than an infolding of the atrial wall between the superior caval vein and right pulmonary veins. The septal surface is roughened on its left atrial aspect, as is the flap of the oval fossa. The flap valve, superiorly, overlaps the infolded atrial walls (the “septum secundum”) so that, even if the two are not fused, there will be no shunting across the septum as long as left atrial pressure exceeds that in the right atrium. In most cases the atrial septum can be seen on SE-MRI as a thin line separating the two atria, except at the level of the foramen ovale, which is often too thin to be seen. This finding should not be misinterpreted as an atrial septal defect. Bright-blood cine MRI, providing a higher contrast between bright atrial blood Fig. 4.6. Atrial (interatrial) septum. Horizontal long-axis im- and atrial walls, usually depicts this thin membrane age using a balanced-SSFP technique (end-systolic image).

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pyramidal shape and by its coronary distribution and essential for differentiation of the morphologi- pattern, which is distinctive and typical. The left cal right from left ventricles. The inlet component anterior descending coronary artery (LAD) demar- (tricuspid valve and atrioventricular septum) sur- cates the right from the left ventricle. The right rounds and supports the leaflets and subvalvular ventricle possesses an inlet component, an apical apparatus of the tricuspid valve. The leaflets can be trabecular component, and an outlet component divided into septal, anterosuperior, and inferior (or (Anderson 2000; Fig. 4.7). The presentation of the mural) locations within the atrioventricular junc- different components is specific for each ventricle tion. The most characteristic feature of the tricuspid valve (and thus also of the right ventricle) is the pres- ence of tendinous chords attaching its septal leaflet to the ventricular septum. Chordal attachments to the septal surface are never seen in the morphological left ventricle. The apical trabecular portion of the right ventricle has characteristically coarse trabeculations. The infundibulum is incorporated into the right ventricle and forms the outflow tract, whereas the right ventricle proper forms the inflow tract. This completely muscular ring supports the three semilunar leaflets of the pulmonary valve. The junction between the infundibulum and the right ventricle is composed of the parietal band, the septomarginal band, and the moderator band. This moderator band, a muscular band that contains the con- a tinuation of the right bundle branch, passes from the interventricular septum to the anterior wall and is an essential characteristic of the morphological right ventricle (Fig. 4.8). The septal and moderator bands are also known as the trabeculae septomar- ginalis or septomarginal trabeculation. Only a small part of the infundibulum is a truly muscular septum (Fig. 4.9), while the rest of the posterior margin of the infundibulum, also called supraventricular crest (crista supraventricularis) is caused by an infolding of the roof of the ventricle (also called ventriculoinfundibular fold), and is separated from the aorta by extracardiac space. The separation of the tricuspid and pulmonary valves by the crista supraventricu- b laris is another characteristic of the morphological right ventricle. Additional trabeculations, the sep- toparietal trabeculations, run around the anterior margin of the infundibulum. The internal appearance of the morphological right ventricle is specific for it, though a muscular infundibulum can be rarely from the left ventricle. The muscular trabeculations are relatively coarse, few, and straight, tending to run parallel the right ventricular (RV)

Fig. 4.7a-c. Components of the right ventricle. Horizontal long-axis image (a), RV vertical long-axis image (b), RV inflow and outflow tract image (c) using a balanced-SSFP technique. The inlet, apical and outlet part of the right ventricle are indicated on the different images. Ao, Aorta; LV, c left ventricle; RA, right atrium; RV, right ventricle.

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Fig. 4.8. Moderator band. Horizontal long-axis view, using Fig. 4.9. Infundibulum. Transverse image using the bal- the balanced-SSFP technique. This muscular structure con- anced-SSFP technique with submillimeter spatial resolution. nects the apical ventricular septum with the apical part of The right ventricular outflow tract (rvot) is characterized by the RV free or lateral wall. RV, right ventricle. a complete, thin muscular ring. Ao, Aorta.

inflow and outflow tracts. The papillary muscles of enlargement can also displace the left ventricle left- the right ventricle are relatively small (making right ward and posteriorly, and the apex of the heart up ventriculotomy readily possible) and numerous, and and back, lifting the apex of the heart off the hemi- they attach both to the septal and to the free wall diaphragm on the frontal chest radiograph. surfaces. Because of its numerous attachments to the RV septal surface (mostly to the posteroinfe- 4.4.2.2 rior margin of the septal band), the tricuspid valve Morphological Left Ventricle may be described as “septophilic”. The lining of the chamber becomes smooth in the infundibulum, a The morphological left ventricle in a normal heart funnel leading to the exit from the chamber, the is a thick-walled chamber that forms the apex and pulmonary trunk. The normal right ventricle is a lower part of the left and posterior heart border. The relatively thin wall chamber with an end-diastolic exterior of the left ventricle is shaped like a cone. wall thickness of 3–4 mm. Towards the RV apex, Internally, the left ventricle is demarcated by its fine there is often a thinning of the free wall which is trabeculations, which are numerous, fine muscular not to be mistaken for a wall thinning such as found projections. Like its morphological right counter- in arrhythmogenic RV dysplasia (ARVD or ARVC; part, the morphological left ventricle also possesses see Chap. 9). Although the RV free wall is consid- an inlet, an apical trabecular, and an outlet portion. erably thicker than that of the right atrium, it is The inlet component contains the mitral valve (or LV thinner than the wall of the left ventricle. These valve) and extends from the atrioventricular junc- relative thicknesses reflect the range of pressures tion to the attachments of the prominent papillary in the chambers. In the right atrium, the pressure is muscles (Fig. 4.10). The most characteristic anatomi- usually close to 0 mmHg; in the right ventricle, the cal feature of the mitral valve is that it has no chordal pressure rises up to about 25 mmHg, while in the attachments to the ventricular septum. There are left ventricle the peak pressure is about 120 mmHg. two papillary muscles; the anterior lateral and the The complex cardiac anatomy of the morphological posterior medial (Fig. 4.11). Notably, the papillary right ventricle is best studied using a combination of muscles do not attach to the septum (Anderson different imaging planes, i.e., transverse or horizon- 2000). Since the LV papillary muscles are large and tal long-axis views in combination with short-axis arise only from the free wall surface, this makes views and/or RV outflow tract views. Other interest- left ventriculotomy difficult, except at the apex or ing planes to study the right ventricle are described at the high paraseptal area. In addition to the ante- in Chap. 5. RV enlargement reduces the retrosternal rior descending branch of the left coronary artery airspace. Because this space is normally limited, RV (LCA), which externally marks the location of the

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Fig. 4.10a,b. Components of the left ventricle. Left ventricular inflow–outflow tract flow view at end diastole (a) and end systole (b), using the balanced-SSFP technique. Ao, Aorta; LA, left a b atrium.

a b

Fig. 4.11a-c. Left ventricular papillary muscles. Left ventricular outflow tract (a), vertical long-axis (b) and midventricular short-axis (c) view, using the balanced-SSFP technique. The papillary muscles are clearly depicted as intracavity structures attached to the posterior(medial) and anterior(lateral) LV wall. Their fibrous extensions, i.e., tendinous chords, towards the mitral valve can be seen on high-quality MR images. Note that the LV septal surface on the short-axis view is free of muscular c attachments.

anterior portion of the ventricular septum, anterior feature of the morphological left ventricle, which and posterior obtuse marginal branches of the left contains the fine characteristic trabeculations. The coronary artery course across the LV free wall. Also smooth septal surface also helps in identification, known as diagonals, these branches supply the large since the morphological left ventricle never pos- papillary muscles and the adjacent LV free wall. The sesses a septomarginal trabeculation or a moderator apical trabecular portion is the most characteristic band (Fig. 4.12). While intracavity muscular bands

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ferences are found in the longitudinal direction, with a gradual wall-thinning towards the LV apex. Compared with the lateral LV wall segments (i.e., end-diastolic wall thickness: 7–8 mm in women, 8– 9 mm in men), the LV apex can be extremely thin (approximately 3 mm; Fig. 4.10). Less pronounced variations in wall thickness are seen around the LV circumference (Bogaert and Rademakers 2001). LV enlargement takes place inferiorly and to the left, displacing the left lung.

4.4.2.3 Ventricular Septum

The ventricular or interventricular septum sepa- Fig. 4.12. Essential characteristics of the morphologically rates the left from the right ventricle. It is mainly a right and left ventricle. Horizontal long-axis view using the thick-walled muscular layer, except in the subaortic balanced-SSFP technique. Five essential differences can be seen on this image: 1, fine apical trabeculations in LV apex; region, where it becomes very thin (“membranous 2, LV septal surface free of muscular insertions; 3, rough septum”). It contains muscular fibers coming from apical trabeculations in RV apex; 4, muscular insertions on the LV as well as from the RV free wall. The position RV septal surface; 5, tricuspid valve more apically positioned and shape of the ventricular septum are determined than mitral valve. LV, left ventricle; RV, right ventricle. by the loading conditions. In the unloaded condition, the ventricular septum has a flat appearance. are always present on the right as the moderator In normal loading conditions, the septum has a con- band, sometimes thin intraventricular strands can vex shape towards the right ventricle, and this shape be found in the LV apex, known as false tendons. is maintained during the cardiac cycle. Enhanced The outlet part, with the aortic valve, is in direct RV filling, as during onset of inspiration, may lead continuity with the inlet part, since normally there to a slight flattening of the ventricular septum dur- is little or no conal musculature beneath the aortic ing early diastolic filling. This phenomenon is called valve, which results in aortic-mitral fibrous conti- ventricular coupling (see Chaps. 6 and 11). In sev- nuity. The outlet portion of the morphological left eral cardiac and extracardiac diseases, pathological ventricle is distinguished by its abbreviated nature. ventricular coupling may occur (e.g., constrictive Part of two of the three leaflets of the aortic valve pericarditis, cor pulmonale, atrial septal defects, se- have muscular attachments to the outlet component. vere pulmonary incompetence, pulmonary hyper- The remainder of the leaflets take origin from the tension). The ventricular septum is best studied in fibrous tissue of the aortic root, part of this being the short- and horizontal long-axis views. As with the extensive area of fibrous continuity with the aortic atrial septum, a diagnosis of septal defect cannot leaflet of the mitral valve. This fibrous continuity is be made on the basis of this morphological (i.e., called mitral-aortic intervalvular fibrosa. It is the SE-MRI) finding alone, and additional bright-blood posterior aspect of the roof of the outlet, therefore, and flow measurements should be obtained to make which is particularly short. There is no muscular a definite diagnosis. segment of the ventriculoinfundibular fold in the left ventricle such as separates the arterial and the right ventricular valves. The morphological left ven- 4.4.3 tricle is usually studied along the three intrinsic car- Valves diac axes, i.e., short-axis, horizontal long-axis, and vertical long-axis view. Other interesting imaging Two atrioventricular (AV) (or ventricular) valves planes are the LV outflow-tract view and the LV in- connect the atria to the ventricles, a mitral and a flow- and outflow-tract view. These imaging planes tricuspid valve. Embryologically, the mitral valve is are particularly interesting in patients with obstruc- always connected to the morphological left ventri- tive hypertrophic cardiomyopathy or to depict aor- cle, whilst the tricuspid valve is connected with the tic regurgitation. Wall thicknesses are not uniform morphological right ventricle. There is a difference throughout the left ventricle. Most pronounced dif- in positioning along the longitudinal cardiac axis

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between both valves. The tricuspid valve is always lationship with the leaflets of the aortic valve and somewhat more apically positioned than the mitral the parietal atrioventricular junction, respectively valve, a feature that is very helpful in differentiating (Fig. 4.13). The zone of apposition between the two the ventricular morphology. leaflets has anterolateral and posteromedial ends, The RV (AV) valve, or tricuspid valve, has three the so-called commissures, each supported by one leaflets, these being the septal, inferior, and antero- of the paired LV papillary muscles which are embed- superior leaflets (Fig. 4.13). The muscular support ded in the anterolateral and posteromedial walls of of the tricuspid valve is made up of the anterior mus- the left ventricle. cle, which is the largest and usually arises from the The semilunar valves of both great arteries attach septomarginal trabeculation. The complex of chords across the anatomic ventriculoarterial junction and supporting the anteroseptal commissure is domi- therefore lack a fibrous supporting annulus such as nated by the medial papillary muscle (of Lancisi), supports the atrioventricular valves. They have no a relatively small muscle which arises either as a chordal attachments. The pulmonary artery normally single band or as a small branch of chords from the arises from the infundibulum of the right ventricle. posterior limb of the septomarginal trabeculation The leaflets of the pulmonary valve and the leaflets of (Anderson 2000). The inferior muscle, smallest of the triscupid valve are widely separated by the infun- the three, is usually single and may be represented dibular musculature. The aortic valve cusps are usu- by several small muscles. ally described according to the origin of the coronary The LV (AV) valve, or mitral valve, has aortic arteries – left, right, and non-coronary, although they and mural leaflets, so named because of their re- may also be called anterior, and right and left poste-

Fig. 4.13a,b. Tricuspid and mitral valve. Short- axis view through atrioventricular valves, using balanced-SSFP technique obtained during diastole. The leaflets, commissures and valve orifices can be best appreciated when the ab valves are opened.

Fig. 4.14a,b. Aortic valve cusps in closed (a) and open (b) condition. The right coronary (rcc), left coronary (lcc), and noncoronary (ncc) cusps in closed condition are similar to a “Mercedes Benz” star. At maximal opening during systole, the cusps are nearly completely apposed to the wall of sinus of ab Valsalva.

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rior (Fig. 4.14). Each valve cusp has a small nodule (Fig. 4.15). The RCA courses into the right atrioven- of connective tissue at its mid-point. When the great tricular groove and provides nutrient branches to arteries are normally related, the non-coronary left the infundibulum (infundibular or conal branch) coronary commissure of the aortic valve sits directly and to the RV free wall (Angelini et al. 1999). The above the middle of the anterior mitral leaflet. The extension, and thus the myocardial perfusion terri- non-coronary right coronary commissure sits directly tory of the RCA is highly variable. It may stop proxi- above the membranous septum, which in turn is lo- mally in the right atrioventricular groove or may cated directly above the left bundle of a branch of the continue into the posterior interventricular groove conduction system. to the apex or to the left atrioventricular groove, end- Because the valve leaflets are thin and fibrous, ing in the posterolateral LV branch. Occasionally, it SE-MRI is not ideal for visualizing the cardiac valves might extend up to the LAD. In nearly 85–90% of and for studying valve pathology. Although they are cases, it is normal for the RCA to provide a posterior often seen on SE-MRI, especially when in closed po- descending branch that follows the posterior atrio- sition, appreciation of small changes in thickness, ventricular groove as far as the apex of the heart but structure and integrity are difficult to appreciate. not beyond (dominant RCA), thus supplying the in- Bright-blood MRI techniques demonstrate valve feroseptal part of the LV myocardium. In only 5%, leaflet morphology, valve leaflet motion, abnormal the LCx continues as posterior descending artery valve opening, and valvular flow patterns much bet- (dominant LCx), while, in 10% of cases, both the ter (de Roos et al. 1995). The cardiac valves are best right coronary artery and left circumflex supply the studied in specific imaging planes perpendicularly inferior wall (balanced pattern). or longitudinally oriented through the valve of in- The LCA originates from the middle portion of terest (see Chap. 5). the left anterior sinus of Valsalva, just below the sinotubular junction (Fig. 4.15). The proximal vessel originating from the left ostium, called the left main 4.4.4 stem (LM) or trunk, is only a short conductive arte- Coronary Arteries rial segment (±1 cm) from which the LCx and LAD arteries normally arise. The LAD courses in the ante- Two of the sinuses of Valsalva give rise to coronary rior interventricular groove, the LCx in the left (pos- arteries. These sinuses are the ones adjacent to the terior) atrioventricular groove. The LAD gives off pulmonary truncus. branches to both the septum (perforator branches) The right coronary artery (RCA) arises from an and the anterolateral wall of the left ventricle (diago- ostium located just below the sinotubular junction, nal branches), and the LCx produces branches to the in the middle of the right (anterior) sinus of Valsalva. posterolateral wall of the left ventricle, including the

Fig. 4.15a,b. Origin and proximal course of coronary arteries. Short-axis view through the aortic root (a), and oblique view through anterior (or right) atrioventricular groove, using a balanced-SSFP technique with submillimeter spatial resolution. The right coronary artery (RCA) and left coronary artery originate from the right and left sinus of Valsalva, respectively. The major arteries as well as several branches can be readily depicted. This subject has a LCx dominant system. Ao, Aorta; LM, left main stem coronary artery; RA, right atrium; RV, right ventricle; star, conal branch.

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posteromedial papillary muscle. The LAD terminates The pericardium is best visualized over the right side at the cardiac apex, or 1–2 cm before or after the apex. of the heart and cardiac apex, while it is often invis- The perforators originate from the LAD at a grossly ible along the LV free wall ,where it is interposed perpendicular angle and these branches immediately between the myocardium and the low-intensity left become intramural, coursing within the septum (see lung (see Fig. 11.1). In normal subjects, the peri- Chap. 14). The MRI techniques, as well as the ideal cardium has a thickness of 1.2±0.5 mm in diastole imaging planes, to study the coronary arteries are de- and 1.7±0.5 mm in systole (Sechtem et al. 1986a). scribed in detail in Chaps. 5 and 14. Similar results (i.e., 1.7 mm, range 1.5–2.0 mm) have been found by Bogaert and Duerinckx, evaluating the normal appearance of the pericardium on 4.4.5 breath-hold MRI used to visualize coronary arteries Pericardium (Bogaert and Duerinckx 1995). These values ex- ceed the thickness of 0.4–1.0 mm reported for ana- The pericardium envelops the heart and the origin tomical measurements of pericardial thickness. The of the great vessels and consists of an outer fibrous layer of normal pericardial fluid present in the peri- layer (the fibrous pericardium) and an inner serous cardial space also has a low intensity, and it likely sac (the serous pericardium). This ﬁbrous part is that this thin layer of fluid contributes to the overall attached to the sternum and diaphragm. The serous pericardial thickness as visualized by MRI. Because pericardium consists of an inner visceral layer (the MRI is sensitive to the small amount of normal peri- epicardium), which is intimately connected to the cardial fluid and depicts its anatomical distribution, heart and the epicardial fat, and an outer parietal it should be valuable in detection and quantification layer, which lines the fibrous pericardium. The vis- of even small pericardial effusions (see Chap. 11). ceral layer is reflected from the heart and the root of In patients with constrictive pericarditis, Sechtem the great vessels onto the inner surface of the fibrous et al. have found a pericardial thickness of more pericardium to become continuous with the parietal than 4 mm (Sechtem et al. 1986a). Newer studies, layer. The pericardial cavity lies between these two however, have shown that constrictive pericarditis layers of the serous pericardium. Two serosal tun- might be present in patients with a normal or near- nels can be identiﬁed: the transverse sinus, posterior normal pericardial thickness at surgery (Talreja et to the great arteries and anterior to the atria and al. 2003). Thus, in the absence of a thickened peri- the superior vena cava, and the oblique sinus, pos- cardium, other diagnostic criteria are needed to terior to the left atrium (Groell et al. 1999). The differentiate constrictive pericarditis patients from transverse sinus is divided into the following four restrictive cardiomyopathy patients. A combination recesses: the superior aortic recess, inferior aortic of transverse or long-axis imaging planes and short- recess, left pulmonic recess, and right pulmonic re- axis views ensures the best approach for studying cess. Pericardial sinuses and recesses are frequently the entirety of the pericardial sac. depicted on cardiac MR images (see Figs. 11.2–3). Knowledge of their locations is helpful in the differentiation of normal pericardium from pericardial effusions and mediastinal processes such as lymph 4.5 nodes. Under physiologic conditions, the pericardial Great Vessels space contains 20–25 ml of serous fluid; however, the amount of fluid may vary considerably among The aorta has its origin from the centre point of the individuals, particularly in children and infants. base of the heart and curves upwards to the aortic These differences may explain, at least in part, why arch, where the brachiocephalic vessels have their in some patients, sinuses or recesses may or may not origin. The junction between the aortic root con- be seen. Moreover, clinically asymptomatic patients taining the sinuses of Valsalva and the ascending can have large pericardial fluid collections, espe- aorta is called the sinotubular junction. The course cially when it accumulates over long periods, and it of the aortic arch, as well as the branching pattern of may be found incidentally. the brachiocephalic vessels, can be subject to a large On MR images, the normal pericardial sac is vis- number of congenital variations (see Chaps. 15 and ible as a very thin curvilinear structure of low signal 16). The most frequent presentation is a left-sided intensity surrounded by the high-intensity mediasti- aortic arch running over the main stem bronchus nal and (sub)epicardial fat (Sechtem et al. 1986a,b). with the following branching pattern: right brachio-

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cephalic trunk, left common carotid artery, and fi- nally left subclavian artery (Fig. 4.16). The leaflets of the aortic valve are supported by the three sinuses of Valsalva. The pulmonary trunk orginates from the muscular pulmonary infundibulum, and bifurcates into the right and left pulmonary arteries. Two of the sinuses of the pulmonary trunk are always next to the aorta (also called facing sinuses), while the third sinus is non-facing. The ligamentum arteriosus, a fibrous remnant of the arterial duct (or “ductus arteriosus”) extends from the pulmonary trunk into the descending aorta. The aortic isthmus is defined as the segment between the site of take-off of the left subclavian artery and the aortic insertion of the a duct. A combination of different imaging planes is recommended to study the thoracic great vessels. Systemic venous return to the heart is through the superior and inferior caval veins (venae cavae), which lie on the right side of the spine. The superior caval vein is formed by the confluence of the right and left innominate veins, which lie in front of the brachiocephalic artery. The inferior caval vein has only a small intrathoracic portion. After receiving the hepatic veins, it crosses the diaphragm to enter the posterior aspect of the right atrium. There are two right and two left pulmonary veins, joining the posterior aspect of the left atrium. The right pulmonary veins enter close to the atrial septum. In patients with an atrial septal defect, the b pulmonary venous blood from the right lung drains preferentially into the right atrium. The two left pulmonary veins frequently join the left atrium as a single trunk. The great vessels are usually studied using a combination of black-blood and bright-blood techniques in different imaging planes. This combination of sequences is the best guarantee to see the vessel wall and para-aortic tissues. Often, velocity-encoded flow, cine MRI studies are performed to calculate the flow patterns in blood vessels. A good imaging plane to start with is the transverse imaging plane. Abnormalities in the course and dimensions of the great vessels are readily depicted in this imaging c plane. However, to obtain accurate dimensions, an Fig. 4.16a-c. Great vessels of the thorax and neck. Contrast- imaging plane perpendicular to the long axis of the enhanced MR angiography using volume rendered recon- vessel should be used. Additional imaging in other struction, posterior-anterior view (a), posterior-anterior planes is often necessary to better depict the vascu- view with slightly oblique inclination (b), anterior-posterior lar abnormality (e.g., aortic coarctation) or to better view (c). The aorta (Ao), the brachiocephalic vessels (right visualize the consequences of valvular pathology on brachiocephalic trunk (tbc), right subclavian artery (rsa), right common carotid artery (rcca), left common carotid vascular structures (e.g., post-stenotic aortic dilata- artery (lcca), left subclavian artery (lsa) and left vertebral ar- tion). tery (lva)), the pulmonary artery (PA) and its major branches The outflow tract of the right ventricle, the pul- (pa), as well as the pulmonary veins (pv) entering the left monary trunk, and its bifurcation are well depicted atrium (LA), are clearly depicted.

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a b c d

e f g h

i j k l

m n o p

q r s t

Fig. 4.17a-t. Transverse images. Abbreviations: aavg, anterior (or right) atrioventricular groove; amvl, anterior mitral valve leaflet; Ao, aorta; apm, anterior papillary muscle; av, aortic valve; azv, azygos vein; c, carina; cs, coronary sinus; ct, crista terminalis; Es, esophagus; ev, Eustachian valve; ft, false tendon; gcv, great cardiac vein; hazv, hemiazygos vein; hv, hepatic vein; ias, interatrial septum; ib, intermediate bronchus; IVC, inferior vena cava; ivs, interventricular septum; LA, left atrium; laap, left atrial appendage; lcca, left common carotid artery; liv, left innominate, (or brachiocephalic) vein; llb, left lower lobebronchus; lmb, left main stem bronchus; lpa, left pulmonary artery; lsa, left subclavian artery; LV, left ventricle; lvot, left ventricular outflow tract; maif, mitral-aortic intervalvular ﬁbrosa; mb, moderator band; PA, pulmonary artery (or trunk); pmvl, posterior mitral valve leaflet; ppm, posterior papillary muscle; ps, pericardial sac; pv, pulmonary vein; RA, right atrium; raap, right atrial appendage; rbca, right brachiocephalic artery; rca, right coronary artery; rcv, right cardiac vein; riv, right innominate (or brachiocephalic) vein; rmb, right main stem bronchus; rpa, right pulmonary artery; RV, right ventricle; rvap, right ventricular apex; rvot, right ventricular outflow tract; SVC, superior vena cava; T, trachea; tv, tricuspid valve.

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abc

de f

ghi

jkl

Fig. 4.18a–l. Coronal images. Abbreviations: lm, left main stem coronary artery; mv, mitral valve; sv, sinus of Valsalva; other abbreviations as in Fig. 4.17.

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de f

ghi

jkl

Fig. 4.19a–l. Sagittal images. Abbreviations: lad, left anterior descending coronary artery; lcx, Left circumflex artery; pavg, posterior (or left) atrioventricular groove; puv, pulmonary valve; sv, sinus of Valsalva; other abbreviations as in Fig. 4.17.

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de f

ghi

jkl

Fig. 4.20a–l. Short-axis images. Abbreviations: aivg; anterior interventricular groove; d, diaphragm; lvap, left ventricular apex; pivg, posterior interventricular groove; other abbreviations as in preceding ﬁgures.

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de f

ghi

Fig. 4.21a–k. Horizontal long-axis images. Abbreviations: fo, fossa ovalis; pda, posterior descending artery. Other abbreviations as in preceding jkﬁgures.

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abc

d e f

ghi

kl Fig. 4.22a–j. Vertical long-axis images. Abbreviations as in preceding ﬁgures.

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c d

Fig. 4.23a-d. Left ventricular outflow tract images. Abbreviations as in preceding figures.

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Fig. 4.24a-c. Right ventricular outflow tract images. Abbreviations as in preceding figures.

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