CT Anatomy of the heart

CT Anatomy of the heart Lawrence M. Boxt Division of Cardiology, North Shore University Hospital, Manhasset, New York, USA Received 14 July 2004; accepted in revised form 26 October 2004 Key words: heart, anatomy, computed topography Abstract Contrast-enhanced ECG-gated multidetector CT provides high contrast and spatial resolution for imaging the thoracic organs. High photon flux, very rapid gantry rotation, and very sensitive photon detectors optimize the scanner for cardiac imaging. ECG-gating (and pharmacological intervention, i.e., beta- blockade), have increased temporal resolution. The structure of the heart is revealed in a manner conve- nient for all radiologists. The atria and ventricles display their characteristic morphology. Antero-posterior and left-to-right relationships are apparent. Viewing through space, or reconstructing in orthogonal planes enforces the supero-inferior relationships as well. Appreciation of normal structure is the foundation for detecting the abnormal. Introduction An important advantage of computed tomography is that it displays the organs of the chest in axial section. Observers can rapidly and intuitively seg- ment the heart and great arteries and veins into their anterior–posterior, and left-to-right rela- tionships. In axial section, the relationships of the four cardiac chambers is apparent, and the rela- tionships of the atrioventricular and semilunar valves can be evaluated as well. Non-contrast- enhanced CT examination is limited to detection and, using electron-beam or ECG-gated multide- tector acquisition, quantitation of cardiac calcium. The exquisite sensitivity for calcium may be used to advantage in quantitation of coronary arterial atherosclerotic load, evaluation of chronic myo- cardial infarction, and characterizing pericardial disease. Quantitative analysis of aortic valve cal- cium by EBCT and MDCT has been found to quantitatively assess severity of aortic stenosis, correlating with Doppler echo-derived aortic flow velocity measurements. The clinical value, and thus, role of CT examination for screening asymptomatic patients is an area of active re- search. Use of contrast-enhanced acquisition by electron-beam or spiral CT technique may pro- vide high resolution and high contrast image data sets which can be reconstructed, post-processed and displayed in arbitrary anatomic section. Use of such computer-based reconstruction and ren- dering software allows construction and inspec- tion of the heart in the intrinsic cardiac axes, allowing broad understanding by both radiology and cardiology imagers, and of significant importance, comparison with imagery obtained by other modalities. Examination of the heart in terms of its intrinsic axes encourages better understanding of the relationships among the cardiac chambers, as well as visualization of pathologic sequelae of cardiac disease. Recon- struction of axial acquisition imagery into 3- dimensional surface renderings allows evaluation of the surface anatomy of the heart as well. In this article, I will discuss the normal anatomy of the heart, as displayed in a recently obtained contrast-enhanced ECG-gated 16-detector com- puted tomographic acquisition. The acquisition The International Journal of Cardiovascular Imaging (2005) 21: 13–27 � Springer 2005 DOI 10.1007/s10554-004-5340-z protocol is spelled out in the Appendix A. The discussion of anatomy as depicted in axial section will be supplemented by additional views recon- structed from the original axial data sets. It is important not to lose sight of this important benefit of computed tomographic examination. The evolution of very fast CT scanning has not only produced sharp visualization of cardiac structure in axial section, but also has allowed Figure 1. Arrow 1, pre-hilar right pulmonary artery. Figure 2. Figure 3. Arrows 1, pulmonary artery sinuses of Valsalva. Arrow 2, LAA. Arrow 3, left upper lobe pulmonary artery. Arrows 4a, 4b; left upper lobe pulmonary veins. Figure 4. Short arrows, right ventricular outflow tract myo- cardium. Arrow 1, LAD. Arrow 2, RUL. Arrow 3, right upper lobe pulmonary artery. 14 application of 3D image reconstruction software to these data sets, allowing direct visualization the relationship among intracardiac structures, and visualization in the intrinsic cardiac axes, and thus reproducible characterization and quantitative functional analysis. Figure 5. Arrow 1, LAD. Arrows 2a, 2b; LAD D. Arrow 3, pulmonary valve leaflet. Arrow 4, RUL. Figure 6. Arrow 1, pulmonary valve leaflet. Arrows 2a, 2b; calcified plaques, LAD. Arrow 3, low attenuation, fatty plaque, LAD. Arrow 4, proximal LCx. Arrow 5, RUL. Figure 7. Arrow 1, LMCA. Arrow 2, LCx. Arrow 3, LAD. Arrow 4, GCV. Arrow 5, pulmonary valve leaflet. Arrow 6, RUL. Figure 8. Arrow 1, LCx. Arrows 2a, 2b; pectinate muscles of the RAA. Arrow 3, RUL. 15 CT Anatomy of the Heart The heart is contained within the middle medias- tinum by the pericardium. The visceral pericar- dium is adherent to the ventricular myocardium, and cannot be visually separated from the epicar- dial fat. The parietal pericardium may be identified Figure 9. Arrow 1, LAD. Arrow 2, LCx. Arrow 3, CS. Arrow 4, LUL. Arrow 5, RUL. Arrow 6, SA. Figure 10. Arrow 1, LAD. Arrow 2a, LCx. Arrow 2b, CS. Arrow 3, sinus venosus interatrial septum. Arrow 4, origin of the RCA. Arrow 5, RUL. Figure 11. Arrow 1, LAD. Arrow 2, P1. Arrow 3, CS. Arrow 4, RCA. Arrow 5, RUL. Arrow 6, anterior mitral leaflet. Arrow 7, chorda tendinea. Figure 12. Arrow 1, LAD. Arrow 2, CS. Arrow 3, right lower lobe pulmonary vein. Arrow 4, membranous interventricular septum. Arrow 5, anterior mitral leaflet. Arrow 6, posterior mitral leaflet. 16 as a paper-thin high signal intensity surface surrounding the heart and great arteries (Figures 5–14, 23, 30 and 35). Direct visualization of the parietal pericardium depends upon the presence and extent of low-density fatty deposition in the pericardial fat pad and middle mediastinum. Figure 13. Arrow 1, LAD. Arrow 2, CS. Arrow 3, RCA. Figure 14. Arrow 1, LAD. Arrow 2, RCA. Arrow 3, P2. Figure 15. Arrow 1, LAD. Arrow 2, posterior descending coronary artery. Arrow 3, CS. Arrow 4, RA. Arrow 5, RCA. Figure 16. Arrow 1, LAD. Arrow2, CS. Arrow 3, RCA. 17 The superior vena cava drains into the right atrium just posterior to the orifice of the right at- rial appendage (Figures 1–11, 26, 42 and 43). The opacified superior vena cava is the brightest object in every image. EB and MDCT scanners acquire image data more rapidly than the contrast bolus circulates to the heart and lungs and back out to the systemic circulation. It continues to fill (and thus appears highly attenuated), as compared to the arterial circulation. The posterior wall of the Figure 17. Coronal reconstruction. Arrows point to calcifica- tion of the costal cartilage. Figure 18. Coronal reconstruction. Figure 19. Coronal reconstruction. Figure 20. Coronal reconstruction. 18 SVC is separated from the cavity of the left atrium as the the superior cava enters the right atrium (Figure 10). This wall is the sinus venosus portion of the interatrial septum. The right atrium is gen- erally round in shape, and forms the right lower border of the heart (Figures 11–14, 24–27 and 38– 42). The lateral RA wall is very thin; the distance between the cavity of the RA and the outer lateral border of the heart should be no greater than 3 mm. Increased thickening is pathological, Figure 21. Coronal reconstruction. Figure 22. Coronal reconstruction. Figure 23. Coronal reconstruction. Figure 24. Coronal reconstruction. Arrow 1, anterior aortic sinus of Valsalva. 19 usually indicating pericardial effusion or pericardial thickening, or infiltration of the right atrial wall by malignancy. The coronary sinus extends from the confluence of the great cardiac vein, between the LA and left ventricle in the posterior atrioventricular ring, and then passes beneath the left atrium to the diaphragmatic surface of the heart to drain into the right atrium medial and slightly superior to the entry of the IVC (Figures 9–15, 27–29 and 40–43). Figure 25. Coronal reconstruction. Arrow 1, calcification of the preoximal LAD. Arrow 2, pectinate muscle of the RAA. Figure 26. Coronal reconstruction. Figure 27. Coronal reconstruction. Arrows 1a,1b: mitral leaflet tissue. Figure 28. Coronal reconstruction. 20 The interatrial septum extends from the pos- terior aspect of the inferior-most portion of the superior vena cava, (the sinus venosus septum, vida supra). The mid-portion of the interatrial septum is called the secundum septum (Figures 10–13 and 42). It comprises the bulk of the interatrial septum, and usually bows toward the right atrium. The thickness of the interatrial septum depends heavily on the amount of fat deposited in it. That is, in the absence of fat, Figure 29. Coronal reconstruction. Figure 30. LAO sagitttal reconstruction. Figure 31. LAO sagitttal reconstruction. Figure 32. LAO sagitttal reconstruction. Short arrows, tra- becular right ventricular myocardium. 21 scanners cannot resolve the interface between the high attenutation contrast-filled atria. The primum interatrial septum is the inferior and medial-most portion of the septum. The right atrial appendage is a broad-based, triangular extension of the right atrial cavity. It is contained within the pericardium, which extends from just anterior to the entry of the superior vena Figure 33. LAO sagitttal reconstruction. Figure 34. LAO sagitttal reconstruction. Figure 35. LAO sagitttal reconstruction. Figure 36. LAO sagitttal reconstruction. Arrow 1, crista su- praventricularis. Arrow 2, pulmonary valve leaflet. 22 cava into the right atrium, obliquely cephalad to- ward the ascending aorta (Figures 4–10, 21–25 and 35–41). The appendage is usually collapsed when right atrial pressure and volume are normal. The lumen of the appendage is filled with small parallel muscle bundles, the pectinate muscles. On contrast-enhanced CT examination, these bundles appear as intracavitary filling defects (Figures 8 Figure 37. LAO sagitttal reconstruction. Arrow 1, crista su- praventricularis. Arrow 2, marginal branch, RCA. Arrow 3, SA. Figure 38. LAO sagitttal reconstruction. Small arrows, anterior mitral leaflet. Figure 39. LAO sagitttal reconstruction. Arrow 1, calcification of the LAD. Figure 40. LAO sagitttal reconstruction. 23 and 9) analogous to trabecular myocardial bundles in the right ventricle. The tricuspid valve is contained within the anterior atrioventricular ring. The ring itself is characterized by fatty attenuation, and may be visualized in axial CT sections. The septal and anterior tricuspid leaflets appear as long filling defects attached to the AV ring. They are con- nected to the RV free wall and septum by very fine chordae, and papillary muscles of varying size. The chordae are not reliably visualized; the pap- illary muscles are difficult to visualize due to their small size, and variable distribution. The right ventricle resides immediately posterior to the sternum, more or less in the midline (Fig- ures 3–16, 19–25 and 32–36). Unless hypertro- phied, the right ventricular free wall myocardium is only about 2–3 mm in thickness, and difficult to visualize. The shape of the RV can be surmised by visualizing the ventricle as the sum of the axial sections obtained during CT examination. From the level of the pulmonary valve, moving caudad, the shape of the ventricle changes. The right ven- tricular outflow tract is round in shape, sur- rounded by the ventricular infundibulum, and lies to the patient’s left (Figures 3–8). Moving in a caudad direction, the chamber increases in size, assuming a triangular shape; the base formed by the AV ring, and the apex at the intersection of the free wall and interventricular septum. The right ventricular inflow is defined by the plane of the Figure 41. LAO sagitttal reconstruction. Short arrows, pos- terior mitral leaflet. Figure 42. LAO sagitttal reconstruction. Figure 43. LAO sagitttal reconstruction. 24 tricuspid valve. It lies to the right of the inflow to the left ventricle (the plane of the mitral valve) (Figure 12). Atrioventricular concordance (con- nection between right atrium and ventricle and left atrium and ventricle) is the result of D-ventricular looping. The relationship of the inflows allows us to presume D-looping. The tricuspid valve is sep- arated from the pulmonary valve by the infun- dibulum. The right ventricular surface of the interventricular septum is irregular. The crista supraventricularis (Figures 36 and 37) forms the superior-most extension of the septomarginal trabeculation. Although the septomarginal trabe- culation may not always be identified, papillary muscles extending from it to the tricuspid valve leaflets, and myocardial bundles running from the interventricular septum to the RV free wall are commonplace (Figures 9–11 and 32). The inferior- most of these is the moderator band, which carries the conducting bundle. The interventricular sep- tum normally bows toward the right ventricle. The pulmonary valve lies slightly out of the axial plane, so may appear elongated in conventional axial acquisition (Figures 4–8 and 36). The caliber of the main pulmonary artery should be about (no greater than) the caliber of the ascending aorta at this anatomic level (Figure 2). The left pulmonary artery is the extension of the main PA over the top of the left atrium (Figure 1–3). When the PA crosses the left bronchus, it becomes the left PA. The right PA originates from the underside of the main PA, passes along the roof of the left atrium, posterior to the ascending aorta and superior vena cava, to enter the right hilum (Figures 2–9, 27, 28 and 40–43). The pericardium is reflected over the top of the main PA. The upper lobe pulmonary veins lie anterior to their respective pulmonary arteries. As the left upper lobe vein courses inferiorly, it passes in front of the left PA, and enters the left atrium immedi- ately posterior to the orifice of the left atrial appendage. The right upper lobe vein lies anterior to the right pulmonary artery. It passes from anterior to posterior and inferiorly to enter the left atrium immediately posterior to the entrance of the superior vena cava into the right atrium (Fig- ures 3–9,28,29,42 and 43). The left lower lobe pulmonary vein always courses in a caudad direction directly anterior to the descending thoracic aorta before entering the posterior left aspect of the left atrium. The right lower lobe vein drains to the right posterior inferior aspect of the left atrium. The left atrium lies posterior, superior, and to- ward the left with respect to the right atrium (Figures 10–12 and 43). The left atrium is just about the same size as the right atrium. The inner surface of the LA is bald smooth (Figures 9–13, 28, 29, 42 and 43). The confluence of the left upper lobe pulmonary vein and orifice of the left atrial appendage is a redundant endothelium, which may appear to be thickened in its medial-most aspect. The left atrial appendage is long and finger-like (Figures 1–8, 26– 28 and 39–41). Analogous to the right atrial appendage, it contains pectinate musculature. However, these myocardial trabeculations are al- ways smaller in caliber than those of the RAA, and almost never cross from one face of the appendage to the other. The LAA runs from caudad to cephalad, around the left aspect of the heart, below the level of the pulmonary valve. The mitral valve lies within the posterior atrio- ventricular ring, immediately subjacent to the cir- cumflex coronary artery (Figures 11, 12, 27, 28 and 38). Fibrous continuity between the anterior mitral leaflet and the aortic annulus is demonstrated on axial acquisition (Figures 11 and 12). Ordinarily, the chordae tendineae of the anterior and posterior mitral leaflets are not visualized on CT examination. However, introduction of ECG-gated 4-, 8-, and 16-detector systems have improved the spatial and temporal resolution to a point where these struc- tures are now commonly identified (Figure 11). The left ventricular papillary muscles are always seen as filling defects in the LV cavity (Figure 11, 14, 25 and 35–37). Analogous to visualization of the chordeae, attachment of the papillary muscles to the chordeae is frequently visualized on the newer scanners. The posterior AV ring also contains the coronary sinus. This vein lies anterior to the circumflex artery, and passes around the ring between the LA and LV, to run beneath the LA prior to its drainage into the RA (Figure 9–13). Before entering the RA, is receives other venous tributaries, which run along the epi- cardial surface of the heart (Figures 7, 8, 16, 24, 27, 28, 37, 39 and 40). 25 The left ventricle lies posterior and to the left with respect to the RV (Figures 9–15, 21–25 and 32–37). The left ventricular myocardium is nearly uniform in thickness (1 cm at end diastole). However, in axial acquisition, the poster LV wall may appear thicker than the septal or apical myocardium, because it has been cut obliquely with respect to its internal axis. Although some trabecular myocardial filling defects may be iden- tified within the ventricular cavity, the LV is characterized by its smooth walls and two large papillary muscles. These always originate from the posterior wall of the ventricle. The plane of the interventricular septum is directed anterior to the coronal plane, and inferiorly toward the left hip. It normally bows toward the RV (Figures 12–14, 21– 24 and 31–37). The aortic valve shares the fibrous trigone of the heart and is, as previously described, in continuity with the anterior mitral leaflet. That is, there is no left ventricular infundibulum that separates the aortic from the mitral valve. The aortic valve has three sinuses of Valsalva, the anterior (Figures 10, 11, 24 and 37), posterior left (Figure 8, 26, 40 and 41) and posterior right (Figures 11, 12, 25, 40 and 41). The right coronary artery originates from the anterior sinus. The left main coronary artery arises from the posterior left sinus. The posterior right sinus is the most inferior sinus, and provides no coronary artery. This so-called non-coronary sinus abuts the right and left atria. The left main coronary artery (Figures 7, 26 and 40) leaves the posterior left sinus, and continues beneath the left atrial appendage to become the circumflex artery. The circumflex coronary artery is the extension of the left main in the posterior atrioventricular groove (Figures 6–10, 27, 28 and 41). The anterior descending coronary artery arises from the left main. It passes behind the right ventricular outflow, and runs within the fat, along the superior interventricular sulcus to the cardiac apex (Figures 4–7, 9, 10, 12–16, 20–25 and 31–39). The right coronary artery originates from the anterior aortic sinus of Valsalva, and travels embedded in the fat of the anterior atrioventricu- lar ring, between the right atrium and ventricle (Figures 10, 13–16, 22–24 and 36–40). In gated acquisitions, the sinoatrial (Figures 9, 23, 36 and 37), conus and right ventricular marginal branches are frequently visualized. Visualization of the right coronary artery depends almost entirely on accu- rately acquired ECG-gated imagery. Appendix A: Image acquisition protocol Siemens Sensation 16 (Siemens Medical Solutions, Iselin, NJ) CT scanner: • 120 kV, 550 mAs, • tube rotation time ¼ 500 ms, • 2.8 mm/ 500 ms table velocity, • 1.0 mm slice thickness, • 0.75 mm slice overlap. 18 ga venocath in an antecubital vein: • Ultravist 150 (Berlex Laboratories, Montville, NJ) in- jected at 4 cc/s for 30 sec (120 cc total dose), • 20 sec delay between commencement of contrast injection and image acquisition. Images reconstructed on a Vitrea workstation (Vital Images, Inc, Plymouth, MN) to 1 mm thickness in a