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Computed Tomographic Evaluation of the Normal Cardiac Anatomy
C o m p u t e d Tom o g r a p h i c Evaluation of the Norma l Card iac Anatomy Jill E. Jacobs, MD KEYWORDS
Accurate interpretation of cardiac computed tomography (CT) requires fundamental knowledge of the normal cardiac anatomy and its common variations. Multidetector CT technology with submillimeter collimation and gantry rotation times of less than 0.5 seconds allows motion-free isotropic imaging of the cardiac structures to a degree that was previously not possible. The ability to perform multiplanar postprocessing enables evaluation of the cardiac structures in various imaging planes, necessitating a thorough understanding of the cardiac anatomy and the structural and functional relationships of the normal cardiac structures. This article reviews the normal anatomy of the coronary arteries, cardiac chambers, and cardiac valves.
CORONARY ARTERIES The right and left coronary arteries arise from the right and left sinuses of Valsalva, respectively, near the aortic sinotubular ridge. A short-axis view of the trileaflet aortic valve shows that the right aortic cusp is the most anterior, the noncoronary cusp is the closest to the interatrial groove, and the left coronary cusp is the most cephalad (Fig. 1). The size of the coronary arteries correlates strongly with the ventricular mass and body size.1,2 Several studies have also shown that even after correcting for left ventricular (LV) mass, gender remains an independent predictor of coronary artery size.3–5 Kucher and colleagues4 demonstrated that after normalization for LV mass,
the coronary artery cross-sectional areas in women were significantly smaller than those in men (left coronary artery [LCA]: 7 3 vs 9 3 mm2/100 g LV mass, P<.0001; right coronary artery [RCA]: 3 1 vs 4 1 mm2/100 g LV mass, P<.002). The pattern of coronary artery distribution and coronary size also vary with coronary artery dominance. As cited by Chuadhry,6 coronary artery dominance was first reported by Bianchi in 1904. In 1940, the criteria for arterial dominance were first described by Schlesinger,7 who stated that the dominant artery is that which gives rise to the posterior descending artery (PDA) or supplies the posterior interventricular septum and the crux cordis (the crux of the heart). The crux of the heart is located at the point of transection of the atrioventricular (AV) groove and the posterior interventricular septum. In right dominant coronary artery systems, the RCA gives rise to the PDA, which extends down the posterior interventricular groove. In addition, at least one other branch of the RCA extends laterally to the PDA in the AV groove, giving off one or more posterolateral (PL) branches to supply the inferior surface of the LV (Fig. 2A). In left dominant coronary artery systems, the PDA and PL branches arise from the left circumflex (CX) artery (see Fig. 2B). With this dominance pattern, the RCA is typically small in caliber, usually tapering and terminating near the acute margin of the heart (see Fig. 2C). In right and left dominant systems, the AV nodal artery also arises from the RCA and
Department of Radiology, NYU Langone Medical Center, 560 First Avenue, TCH HW205, New York, NY 10016-6497, USA E-mail address: [email protected] Radiol Clin N Am 48 (2010) 701–710 doi:10.1016/j.rcl.2010.05.001 0033-8389/10/$ – see front matter ª 2010 Elsevier Inc. All rights reserved.
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Normal anatomy Cardiac Computed tomography
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Fig. 1. Short-axis view through the aortic valve showing the 3 aortic valve cusps. The noncoronary cusp lies closest to the interatrial groove, the right cusp is the most anteriorly positioned, and the left coronary cusp is the most cephalad. L, left coronary cusp; LA, left atrium; NC, noncoronary cusp; PA, pulmonary artery; R, right coronary cusp; RA, right atrium.
LCA, respectively (see Fig. 2D). In codominant coronary artery systems, portions of the LV diaphragmatic wall are supplied by the RCA and the CX arteries (see Fig. 2E). The reported dominance rates in the literature are variable, with RCA dominance occurring in approximately 70% to 80% of the population, left dominance in approximately 8% to 10%, and codominance in the rest. The coronary arteries normally course in an epicardial position along the surface of the heart, surrounded by fat.
LCA The left main (LM) coronary artery courses for a variable distance from the left coronary sinus before giving rise to the left anterior descending (LAD) and CX arteries (Fig. 3A). The length of the LM artery has been reported to vary from 5 to 20 mm.8 In approximately 15% of cases, a third vessel, the ramus intermedius (RI) artery, arises from the LM artery between the LAD and CX arteries and courses laterally to supply the LV free wall (see Fig. 3B). The LAD artery courses anterolaterally in the interventricular groove toward the apex of the heart, giving rise to lateral diagonal branches,
which supply the anterolateral LV free wall; medial septal branches, which supply the anterior twothirds of the interventricular septum; the AV bundle; and the proximal bundle branch (Fig. 4). The diagonal branches and septal perforator branches are numbered sequentially from the most proximal in origin to the most distal. The CX artery courses in the left AV groove, giving rise to obtuse marginal (OM) branches (Fig. 5). The OM branches are also numbered sequentially from the most proximal in origin to the most distal. The CX and OM branches supply the LV free wall and a variable portion of the anterolateral LV papillary muscle. In the majority (80%– 85%) of the population, the CX branch terminates at the level of the obtuse margin of the heart, distal to the origin of the first OM branch. In left dominant coronary systems, as previously described, the CX artery continues distally to the obtuse margin of the heart to give off the PDA and PL branches that supply the inferior surface of the heart (see Fig. 2B).
RCA The RCA courses from the right coronary sinus along the right AV groove and curves posteriorly at the acute margin of the right ventricle (RV) to reach the crux of the heart. In the majority of the population who are right dominant, the RCA then gives rise to the PDA and PL branches (see Fig. 2A). In approximately half of the population, the most proximal branch of the RCA is the conus artery, which supplies the RV outflow tract and also forms an anastomosis with the LCA via the circle of Vieussens (Fig. 6A). In the rest of the population, the conus artery originates directly from the aortic root (see Fig. 6B). The sinoatrial (SA) nodal branch arises from the RCA in approximately 60% of patients, coursing posteriorly along the anterior interatrial groove toward the superior cavoatrial junction (Fig. 7A). In approximately 40% of the population, the SA nodal branch originates from the proximal CX artery (see Fig. 7B).9,10 Multiple ventricular branches also arise from the RCA, supplying the RV free wall. The largest of these branches is the acute marginal branch (Fig. 8).
Coronary Artery Segmentation Identifying and reporting the coronary segments in a standardized fashion facilitates universal understanding of the location of coronary pathologic changes. To date, a 15-segment coronary model has most commonly been used.11 This model has recently been adapted for coronary CT angiography (CCTA), facilitating identification of the
CT Evaluation of the Normal Cardiac Anatomy
Fig. 2. (A) Volume-rendered posterior view of a right dominant heart shows the distal RCA (notched arrowhead) giving rise to the PDA (solid black arrow), which courses down the posterior interventricular septum, and a PL artery (open arrow), which courses over the LV, supplying the diaphragmatic surface of the ventricle. (B) Volume-rendered posterior view of a left dominant heart shows the distal circumflex artery (notched arrowhead) giving rise to the PDA (solid black arrow) and a PL artery (open arrow). (C) Volume-rendered view of the RCA (arrow) in the left dominant patient shown in panel B shows the RCA to be small in caliber, tapering and terminating just beyond the acute margin of the heart. (D) Maximum intensity projection reformat of the RCA in a right dominant patient shows the AV nodal artery arising from the distal RCA (arrow). (E) Volume-rendered posterior view of a codominant heart shows the distal RCA (arrowhead) and distal circumflex artery (notched arrowhead), both giving rise to PL arteries (arrows), which supply the diaphragmatic surface of the ventricle. The structure in the posterior interventricular septum (open arrow) is the middle cardiac vein. There is no PDA. LA, left atrium.
Fig. 3. (A) Axial maximum intensity projection image of the LM artery (arrow) shows the artery to be short before bifurcating into the LAD (notched arrowhead) and CX (arrowhead) arteries. (B) Volume-rendered image of the anterior heart in a different patient shows the LM artery (arrow) trifurcating into the LAD (notched arrowhead), CX (arrowhead), and ramus intermedius (RI) (notched arrow) arteries. The RI is located in the crotch between the LAD and CX arteries and bifurcates at its distal aspect.
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Fig. 4. (A) Maximum intensity projection image of the LAD artery shows the diagonal branches (arrows) extending laterally toward the LV free wall and the septal perforator branches (open arrows) extending medially into the anterior interventricular septum. (B) Volume-rendered image of the anterior heart shows the diagonal branches (arrows) and a septal perforator branch (open arrows).
coronary segments on standard CCTA multiplanar reconstructions.12 Using this model, the LM segment of the LCA extends from the left coronary ostium to the bifurcation into the LAD and CX arteries, the proximal LAD artery extends from the end of the LM artery to the first large septal or diagonal branch (whichever is most proximal), the mid-LAD extends from the end of the proximal LAD to half the distance to the apex, and the distal LAD extends from the mid-LAD to the termination of the LAD. The proximal CX branch extends from the end of the LM artery to the origin of the first OM branch, and the mid and distal CX arteries extend from distal to the origin of the first OM branch to the end of the vessel or to the origin of the PDA (if it is a left dominant coronary artery system). The proximal RCA extends from the right coronary ostium to half the distance to the acute margin of the heart, the mid-RCA extends from the end of the proximal RCA to the acute margin of the heart, and the distal RCA extends from the end of the
mid-RCA to the origin of the PDA (assuming a right dominant coronary artery system) (Fig. 9).
RIGHT CARDIAC CHAMBERS AND VALVES The right-sided cardiac chambers are seen to a variable degree on CCTA, depending on the injection protocol used. Triple-phase injection protocols, using an initial injection of intravenous contrast material, a second-phase injection of a mixture of contrast and saline, and a final-phase injection of a saline chaser, facilitate adequate opacification of the right-sided chambers to enable identification of pathology without precluding RCA assessment because of streak artifact from residual high-density contrast material in the superior vena cava and right atrium (RA) (Fig. 10).
RA, Atrial Appendage, and Tricuspid Valve The RA forms the right lower heart border and receives inflowing blood from the superior vena Fig. 5. (A) Oblique volume-rendered image of the left side of the heart shows the CX artery in the left AV groove (arrow) and a bifurcating OM artery (open arrow) with its branches extending over the lateral LV (arrowheads). (B) Multiplanar reconstruction of the CX artery shows an OM branch (arrowhead) extending laterally over the LV free wall.
CT Evaluation of the Normal Cardiac Anatomy Fig. 6. (A) Axial maximum intensity projection (MIP) image shows the conus artery (arrow) arising as the first branch of the RCA, heading anteriorly toward the RV outflow tract. (B) MIP image of the RCA in a different patient shows the conus artery (arrow) arising directly from the aorta.
cava, inferior vena cava, and coronary sinus. The sinus venosus, the smooth-walled portion of the RA, is located between the superior and inferior vena caval orifices, mainly involving the PL wall of the atrium. The terminal groove, a lipomatous groove on the epicardial side of the atrium, corresponds internally to the crista terminalis and contains the sinus node and the terminal segment of the SA nodal artery.13 The crista terminalis, a variably sized fibromuscular ridge formed by the junction of the sinus venosus and the primitive RA, separates the sinus venosus portion of the RA from the atrial appendage and gives rise to the pectinate muscles, the largest of which anteriorly is the septum spurium (Fig. 11). Superiorly, the crista terminalis extends to the anterior interatrial groove and merges with the Bachmann bundle, the largest anatomic interatrial electric connection structure (see Fig. 11A).14 By facilitating rapid interatrial conduction, the Bachmann bundle helps
to maintain synchronous contraction of the right and left atria. The right atrial appendage typically is pyramidal, has a wider base than the left atrial appendage (LAA), and has slightly larger pectinate muscles than the LAA (Fig. 12). The eustachian valve, located between the RA and the inferior vena cava, serves to direct inflowing blood toward the foramen ovale in utero.15 The valve typically inserts medially into the eustachian ridge, which represents the border between the coronary sinus and oval fossa. The thebesian valve is located at the entrance of the coronary sinus into the RA and prevents reflux of blood into the coronary sinus (Fig. 13). The RA vestibule is a smooth muscular rim surrounding the orifice of the tricuspid valve (TV). The TV separates the RA from the RV and is composed of anterior, posterior, and septal leaflets, which are connected to the RV papillary muscles by chordae tendineae. Unlike the mitral
Fig. 7. (A) Axial maximum intensity projection (MIP) image of the RCA shows the SA nodal branch (arrow) coursing posteriorly toward the superior cavoatrial junction. (B) Axial MIP image of the LCA in a different patient shows the SA nodal branch (arrows) arising from the proximal CX artery (open arrow). Also note the RI branch in this patient (arrowhead) between the CX and the LAD arteries (notched arrowhead).
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Jacobs valve (MV), which is in direct continuity with the aortic valve, the TV is separated from the pulmonary valve (PV) by a muscular ridge, the crista supraventricularis. Also, unlike the MV, the TV has a direct connection to the interventricular septum. These characteristics help to differentiate the TV from the MV in complex congenital heart disease cases.
RV and PV
Fig. 8. Oblique volume-rendered image of the RCA shows the acute marginal branch (arrow) and a smaller right ventricular branch (arrowhead) coursing over the RV.
The RV is the most anterior cardiac chamber and is characterized by a heavily trabeculated apex and the presence of septomarginal bands. The moderator band, also known as the septomarginal trabeculation or trabecula septomarginalis, extends from the interventricular septum to the base of the anterior papillary muscle, contributing to its perfusion (Fig. 14). The moderator band contains the right bundle of the AV bundle, also known as His bundle, along with one or more arteries supplied by the LCA system. Reig and colleagues16 demonstrated that the moderator band artery most commonly originates from the second anterior septal artery from the left coronary system. In addition, these investigators consistently found anastomoses between the moderator band artery and the right marginal artery or right ventricular branches, both of which originate from the RCA.16 Therefore,
Fig. 9. Society of Cardiovascular Computed Tomography coronary segmentation diagram. From Raff GL, Abidov A, Achenbach S, et al. SCCT guidelines for the interpretation and reporting of coronary computed tomographic angiography. J Cardiovasc Comput Tomogr 2009;3(2):127, copyright 2009, Elsevier; with permission.
CT Evaluation of the Normal Cardiac Anatomy LEFT CARDIAC CHAMBERS AND VALVES Left Atrium, Atrial Appendage, and MV
Fig. 10. Four-chamber view of the heart from a CCTA performed with a 3-phase injection protocol shows optimal opacification of the left and right sides of the heart. Opacifying the right and left atria and ventricles improves conspicuity and aids accurate identification of the interatrial and interventricular septum, respectively.
the moderator band serves as a potential anastomotic connection between the right and left coronary arteries, theoretically protecting the right ventricular myocardium in case of a proximal RCA occlusion. The smooth, muscular infundibulum (conus) of the RV is located immediately inferior to the PV and is the outflow tract for blood from the RV through the PV and into the pulmonary artery (Fig. 15). The PV divides the RV outflow tract from the main pulmonary artery, but, as previously stated, it is separated from the TV by the crista supraventricularis. The PV, like the TV, is trileaflet, composed of right, left, and pulmonary leaflets.
Similar to the RA, the left atrium (LA) has a venous portion, a vestibule, and an appendage. The superior and inferior right and left pulmonary veins typically drain into its posteriorly located venous component (Fig. 16), but many common variants of pulmonary venous anatomy exist. Most of the LA is smooth walled, although the LAA, which arises from the superolateral LA, is tubularly shaped and trabeculated (Fig. 17). The pectinate muscles in the LAA are typically smaller than those in the right atrial appendage.15 Assessment of the left atrial size can be easily performed by calculating the left atrial area, exclusive of the LAA and pulmonary veins. An area of less than 20 cm2 is normal, 20 to 29 cm2 is mildly enlarged, 30 to 40 cm2 is moderately enlarged, and greater than 40 cm2 is severely enlarged.17 The vestibular component of the LA surrounds the orifice of the MV. The MV separates the LA and LV, and its apparatus is composed of 5 parts: an annulus, 2 leaflets, 2 commissures, 2 papillary muscles, and chordae tendineae. The annulus, a saddle-shaped fibrous ring embedded in the myocardium, functions to anchor the MV leaflets and is structurally continuous with the aortic annulus via 3 fibrous trigones. The MV is the only bileaflet cardiac valve, containing an anterior and a posterior leaflet (Fig. 18). Because of its position, the anterior leaflet functions to separate the inflow and outflow tracts of the LV. Chordae tendineae are fibrous connections between the mitral leaflets and the anterolateral and posteromedial LV papillary muscles (Fig. 19).
Fig. 11. (A) Axial CCTA image of the heart shows the crista terminalis (arrow) in the RA and the Bachmann bundle (arrowhead). (B) The septum spurium (arrow) is the largest of the pectinate muscles originating from the crista terminalis.
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Fig. 14. Axial CCTA image of the heart shows the RV moderator band (arrow) extending from the right side of the interventricular septum toward the anterior papillary muscle.
Fig. 12. Volume-rendered image of the anterior heart demonstrates the pyramidal shape of the right atrial appendage (RAA) and the narrower, more fingerlike appearance of the LAA (arrow).
LV and Aortic Valve The normal LV is thick walled and lies posterior to the RV. In contradistinction to the heavily trabeculated RV, the LV contains fine trabeculations. The LV has 2 papillary muscles, anterior and posterior, which connect directly to the ventricular myocardium and function as part of the MV annulus to
Fig. 13. Axial CCTA image of the heart shows the thebesian valve (arrow) at the entrance of the coronary sinus into the RA.
Fig. 15. Multiplanar reconstruction of the right side of the heart shows the outflow tract of the RV. The RV conus (C) is located inferior to the PV (arrow).
Fig. 16. Segmented volume-rendered image of the LA shows the superior and inferior right and left pulmonary veins (arrows) each entering the atrium via a separate ostium.
CT Evaluation of the Normal Cardiac Anatomy
Fig. 17. Axial CCTA image through the LA and LAA shows the curvilinear, low-density pectinate muscles within the LAA (arrows). Ao, ascending aorta.
Fig. 19. Three-chamber view through left heart and the left ventricular outflow tract shows the thin fibrous chordae tendineae (arrows) attaching the MV leaflets to the papillary muscles. Also note the fibrous continuity between the mitral and aortic valves (arrowhead). Ao, ascending aorta.
ensure proper function of the MV leaflets (see Fig. 19). The outflow of the LV is into the aorta via the aortic valve. The aortic valve is tricuspid and composed of the right, left, and noncoronary cusps (see Fig. 1). The 3 aortic cusps are half moon shaped; hence, the aortic valve is commonly referred to as a semilunar valve. In the closed position, each cusp forms a pocket that opens into the ascending aorta. Behind each cusp is a dilation of the aorta known as the sinus of Valsalva. The right and left coronary arteries arise from the sinuses of
the right and left cusps, respectively. The posterior cusp lacks a corresponding coronary artery and is therefore called the noncoronary cusp. Three aortic commissures are roughly equally spaced around the valve annulus and separate the 3 aortic cusps (Fig. 20). Unlike the MV, the aortic valve lacks chordae tendineae and papillary muscles. As previously described, the MV and aortic valve have fibrous continuity (see Fig. 19). This feature helps to distinguish the cardiac chambers and valves of the left side from those of the right side in complex congenital heart disease cases.
Fig. 18. Short-axis view through the open MV in diastole shows the anterior (arrow) and posterior (arrowhead) leaflets.
Fig. 20. Short-axis view through the aortic valve shows the 3 commissures (arrowheads) adjacent to the aortic cusps. L, left coronary cusp; R, right coronary cusp; NC, noncoronary cusp.
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Jacobs SUMMARY Knowledge of the normal cardiac anatomy and its common variations is fundamental for accurate assessment of the heart on CCTA. Using standardized terminology to describe and localize the coronary segments and cardiac structures facilitates accurate communication of cardiac CT examination results.
REFERENCES 1. Gardin JM, Savage DD, Ware JH, et al. Effect of age, sex, and body surface area on echocardiographic left ventricular wall mass in normal subjects. Hypertension 1978;9(Suppl II):1136–9. 2. Gutsell HP, Rembold CM. Growth of the human heart relative to body surface area. Am J Cardiol 1990;65: 662–8. 3. Kim SG, Apple A, Mintz GS, et al. The importance of gender on coronary artery size: in-vivo assessment by intravascular ultrasound. Clin Cardiol 2004;27: 291–4. 4. Kucher N, Lipp E, Schwerzmann M, et al. Gender differences in coronary artery size per 100 g of left ventricular mass in a population without cardiac disease. Swiss Med Wkly 2001;131:610–5. 5. Yang F, Minutello RM, Bhagan S, et al. The impact of gender on vessel size in patients with angiographically normal coronary arteries. J Interv Cardiol 2006;19:340–4. 6. Chuadhry MS. Some observation on the coronary artery pattern and inter-coronary anastomoses in human hearts. Medicus 1965;30:160–72. 7. Schlesinger MI. Relation of anatomic pattern to pathologic condition of the coronary arteries. Arch Pathol 1940;30:403–15. 8. Miller SW. Cardiac angiography. Boston: Little, Brown; 1984, xii. p. 430.
9. Berdajs D, Patonay L, Turina MI. The clinical anatomy of the sinus node artery. Ann Thorac Surg 2003;76:732–5. 10. Sow ML, Ndoye JM, Lo EA. The artery of the sinuatrial node: anatomic considerations based on 45 injection-dissections of the heart. Surg Radiol Anat 1996;18:103–9. 11. Austen WG, Edwards JE, Frye RL, et al. A reporting system on patients evaluated for coronary artery disease. Circulation 1975;51(Suppl 4):3–40. 12. Raff GL, Abidov A, Achenbach S, et al. SCCT guidelines for the interpretation and reporting of coronary computed tomographic angiography. J Cardiovasc Comput Tomogr 2009;3(2):122–36. 13. Saremi F, Krishnan S. Cardiac conduction system: anatomic landmarks relevant to interventional electrophysiologic techniques demonstrated with 64detector CT. Radiographics 2007;27:1539–67. 14. Saremi F, Channual S, Krishnan S, et al. Bahman bundle and its arterial supply: imaging with multidetector CT—implications for interatrial conduction abnormalities and arrhythmias. Radiology 2008; 248(2):447–57. 15. Malouf JF, Edwards WD, Tajik AJ, et al. Functional anatomy of the heart. In: Fuster V, Alexander RW, O’Rourke RA, et al, editors. Hurst’s the heart. 11th edition. New York: McGraw-Hill; 2005. p. 45–82. 16. Reig J, Alberti N, Petit M. Arterial vascularization of the human moderator band: an analysis of this structure’s role as a collateral circulation route. Clin Anat 2000;13:244–50. 17. Lang RM, Bierig M, Devereux RB, et al. Recommendations for chamber quantification: a report from the American Society of Echocardiography’s Guidelines and Standards Committee and the Chamber Quantification Writing Group, developed in conjunction with the European Association of Echocardiography, a branch of the European Society of Cardiology. J Am Soc Echocardiogr 2005;18(12):1440–63.
Accurate interpretation of cardiac computed tomography (CT) requires fundamental knowledge of the normal cardiac anatomy and its common variations. Multidetector CT technology with submillimeter collimation and gantry rotation times of less than 0.5 seconds allows motion-free isotropic imaging of the cardiac structures to a degree that was previously not possible. The ability to perform multiplanar postprocessing enables evaluation of the cardiac structures in various imaging planes, necessitating a thorough understanding of the cardiac anatomy and the structural and functional relationships of the normal cardiac structures. This article reviews the normal anatomy of the coronary arteries, cardiac chambers, and cardiac valves.
CORONARY ARTERIES The right and left coronary arteries arise from the right and left sinuses of Valsalva, respectively, near the aortic sinotubular ridge. A short-axis view of the trileaflet aortic valve shows that the right aortic cusp is the most anterior, the noncoronary cusp is the closest to the interatrial groove, and the left coronary cusp is the most cephalad (Fig. 1). The size of the coronary arteries correlates strongly with the ventricular mass and body size.1,2 Several studies have also shown that even after correcting for left ventricular (LV) mass, gender remains an independent predictor of coronary artery size.3–5 Kucher and colleagues4 demonstrated that after normalization for LV mass,
the coronary artery cross-sectional areas in women were significantly smaller than those in men (left coronary artery [LCA]: 7 3 vs 9 3 mm2/100 g LV mass, P<.0001; right coronary artery [RCA]: 3 1 vs 4 1 mm2/100 g LV mass, P<.002). The pattern of coronary artery distribution and coronary size also vary with coronary artery dominance. As cited by Chuadhry,6 coronary artery dominance was first reported by Bianchi in 1904. In 1940, the criteria for arterial dominance were first described by Schlesinger,7 who stated that the dominant artery is that which gives rise to the posterior descending artery (PDA) or supplies the posterior interventricular septum and the crux cordis (the crux of the heart). The crux of the heart is located at the point of transection of the atrioventricular (AV) groove and the posterior interventricular septum. In right dominant coronary artery systems, the RCA gives rise to the PDA, which extends down the posterior interventricular groove. In addition, at least one other branch of the RCA extends laterally to the PDA in the AV groove, giving off one or more posterolateral (PL) branches to supply the inferior surface of the LV (Fig. 2A). In left dominant coronary artery systems, the PDA and PL branches arise from the left circumflex (CX) artery (see Fig. 2B). With this dominance pattern, the RCA is typically small in caliber, usually tapering and terminating near the acute margin of the heart (see Fig. 2C). In right and left dominant systems, the AV nodal artery also arises from the RCA and
Department of Radiology, NYU Langone Medical Center, 560 First Avenue, TCH HW205, New York, NY 10016-6497, USA E-mail address: [email protected] Radiol Clin N Am 48 (2010) 701–710 doi:10.1016/j.rcl.2010.05.001 0033-8389/10/$ – see front matter ª 2010 Elsevier Inc. All rights reserved.
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Fig. 1. Short-axis view through the aortic valve showing the 3 aortic valve cusps. The noncoronary cusp lies closest to the interatrial groove, the right cusp is the most anteriorly positioned, and the left coronary cusp is the most cephalad. L, left coronary cusp; LA, left atrium; NC, noncoronary cusp; PA, pulmonary artery; R, right coronary cusp; RA, right atrium.
LCA, respectively (see Fig. 2D). In codominant coronary artery systems, portions of the LV diaphragmatic wall are supplied by the RCA and the CX arteries (see Fig. 2E). The reported dominance rates in the literature are variable, with RCA dominance occurring in approximately 70% to 80% of the population, left dominance in approximately 8% to 10%, and codominance in the rest. The coronary arteries normally course in an epicardial position along the surface of the heart, surrounded by fat.
LCA The left main (LM) coronary artery courses for a variable distance from the left coronary sinus before giving rise to the left anterior descending (LAD) and CX arteries (Fig. 3A). The length of the LM artery has been reported to vary from 5 to 20 mm.8 In approximately 15% of cases, a third vessel, the ramus intermedius (RI) artery, arises from the LM artery between the LAD and CX arteries and courses laterally to supply the LV free wall (see Fig. 3B). The LAD artery courses anterolaterally in the interventricular groove toward the apex of the heart, giving rise to lateral diagonal branches,
which supply the anterolateral LV free wall; medial septal branches, which supply the anterior twothirds of the interventricular septum; the AV bundle; and the proximal bundle branch (Fig. 4). The diagonal branches and septal perforator branches are numbered sequentially from the most proximal in origin to the most distal. The CX artery courses in the left AV groove, giving rise to obtuse marginal (OM) branches (Fig. 5). The OM branches are also numbered sequentially from the most proximal in origin to the most distal. The CX and OM branches supply the LV free wall and a variable portion of the anterolateral LV papillary muscle. In the majority (80%– 85%) of the population, the CX branch terminates at the level of the obtuse margin of the heart, distal to the origin of the first OM branch. In left dominant coronary systems, as previously described, the CX artery continues distally to the obtuse margin of the heart to give off the PDA and PL branches that supply the inferior surface of the heart (see Fig. 2B).
RCA The RCA courses from the right coronary sinus along the right AV groove and curves posteriorly at the acute margin of the right ventricle (RV) to reach the crux of the heart. In the majority of the population who are right dominant, the RCA then gives rise to the PDA and PL branches (see Fig. 2A). In approximately half of the population, the most proximal branch of the RCA is the conus artery, which supplies the RV outflow tract and also forms an anastomosis with the LCA via the circle of Vieussens (Fig. 6A). In the rest of the population, the conus artery originates directly from the aortic root (see Fig. 6B). The sinoatrial (SA) nodal branch arises from the RCA in approximately 60% of patients, coursing posteriorly along the anterior interatrial groove toward the superior cavoatrial junction (Fig. 7A). In approximately 40% of the population, the SA nodal branch originates from the proximal CX artery (see Fig. 7B).9,10 Multiple ventricular branches also arise from the RCA, supplying the RV free wall. The largest of these branches is the acute marginal branch (Fig. 8).
Coronary Artery Segmentation Identifying and reporting the coronary segments in a standardized fashion facilitates universal understanding of the location of coronary pathologic changes. To date, a 15-segment coronary model has most commonly been used.11 This model has recently been adapted for coronary CT angiography (CCTA), facilitating identification of the
CT Evaluation of the Normal Cardiac Anatomy
Fig. 2. (A) Volume-rendered posterior view of a right dominant heart shows the distal RCA (notched arrowhead) giving rise to the PDA (solid black arrow), which courses down the posterior interventricular septum, and a PL artery (open arrow), which courses over the LV, supplying the diaphragmatic surface of the ventricle. (B) Volume-rendered posterior view of a left dominant heart shows the distal circumflex artery (notched arrowhead) giving rise to the PDA (solid black arrow) and a PL artery (open arrow). (C) Volume-rendered view of the RCA (arrow) in the left dominant patient shown in panel B shows the RCA to be small in caliber, tapering and terminating just beyond the acute margin of the heart. (D) Maximum intensity projection reformat of the RCA in a right dominant patient shows the AV nodal artery arising from the distal RCA (arrow). (E) Volume-rendered posterior view of a codominant heart shows the distal RCA (arrowhead) and distal circumflex artery (notched arrowhead), both giving rise to PL arteries (arrows), which supply the diaphragmatic surface of the ventricle. The structure in the posterior interventricular septum (open arrow) is the middle cardiac vein. There is no PDA. LA, left atrium.
Fig. 3. (A) Axial maximum intensity projection image of the LM artery (arrow) shows the artery to be short before bifurcating into the LAD (notched arrowhead) and CX (arrowhead) arteries. (B) Volume-rendered image of the anterior heart in a different patient shows the LM artery (arrow) trifurcating into the LAD (notched arrowhead), CX (arrowhead), and ramus intermedius (RI) (notched arrow) arteries. The RI is located in the crotch between the LAD and CX arteries and bifurcates at its distal aspect.
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Fig. 4. (A) Maximum intensity projection image of the LAD artery shows the diagonal branches (arrows) extending laterally toward the LV free wall and the septal perforator branches (open arrows) extending medially into the anterior interventricular septum. (B) Volume-rendered image of the anterior heart shows the diagonal branches (arrows) and a septal perforator branch (open arrows).
coronary segments on standard CCTA multiplanar reconstructions.12 Using this model, the LM segment of the LCA extends from the left coronary ostium to the bifurcation into the LAD and CX arteries, the proximal LAD artery extends from the end of the LM artery to the first large septal or diagonal branch (whichever is most proximal), the mid-LAD extends from the end of the proximal LAD to half the distance to the apex, and the distal LAD extends from the mid-LAD to the termination of the LAD. The proximal CX branch extends from the end of the LM artery to the origin of the first OM branch, and the mid and distal CX arteries extend from distal to the origin of the first OM branch to the end of the vessel or to the origin of the PDA (if it is a left dominant coronary artery system). The proximal RCA extends from the right coronary ostium to half the distance to the acute margin of the heart, the mid-RCA extends from the end of the proximal RCA to the acute margin of the heart, and the distal RCA extends from the end of the
mid-RCA to the origin of the PDA (assuming a right dominant coronary artery system) (Fig. 9).
RIGHT CARDIAC CHAMBERS AND VALVES The right-sided cardiac chambers are seen to a variable degree on CCTA, depending on the injection protocol used. Triple-phase injection protocols, using an initial injection of intravenous contrast material, a second-phase injection of a mixture of contrast and saline, and a final-phase injection of a saline chaser, facilitate adequate opacification of the right-sided chambers to enable identification of pathology without precluding RCA assessment because of streak artifact from residual high-density contrast material in the superior vena cava and right atrium (RA) (Fig. 10).
RA, Atrial Appendage, and Tricuspid Valve The RA forms the right lower heart border and receives inflowing blood from the superior vena Fig. 5. (A) Oblique volume-rendered image of the left side of the heart shows the CX artery in the left AV groove (arrow) and a bifurcating OM artery (open arrow) with its branches extending over the lateral LV (arrowheads). (B) Multiplanar reconstruction of the CX artery shows an OM branch (arrowhead) extending laterally over the LV free wall.
CT Evaluation of the Normal Cardiac Anatomy Fig. 6. (A) Axial maximum intensity projection (MIP) image shows the conus artery (arrow) arising as the first branch of the RCA, heading anteriorly toward the RV outflow tract. (B) MIP image of the RCA in a different patient shows the conus artery (arrow) arising directly from the aorta.
cava, inferior vena cava, and coronary sinus. The sinus venosus, the smooth-walled portion of the RA, is located between the superior and inferior vena caval orifices, mainly involving the PL wall of the atrium. The terminal groove, a lipomatous groove on the epicardial side of the atrium, corresponds internally to the crista terminalis and contains the sinus node and the terminal segment of the SA nodal artery.13 The crista terminalis, a variably sized fibromuscular ridge formed by the junction of the sinus venosus and the primitive RA, separates the sinus venosus portion of the RA from the atrial appendage and gives rise to the pectinate muscles, the largest of which anteriorly is the septum spurium (Fig. 11). Superiorly, the crista terminalis extends to the anterior interatrial groove and merges with the Bachmann bundle, the largest anatomic interatrial electric connection structure (see Fig. 11A).14 By facilitating rapid interatrial conduction, the Bachmann bundle helps
to maintain synchronous contraction of the right and left atria. The right atrial appendage typically is pyramidal, has a wider base than the left atrial appendage (LAA), and has slightly larger pectinate muscles than the LAA (Fig. 12). The eustachian valve, located between the RA and the inferior vena cava, serves to direct inflowing blood toward the foramen ovale in utero.15 The valve typically inserts medially into the eustachian ridge, which represents the border between the coronary sinus and oval fossa. The thebesian valve is located at the entrance of the coronary sinus into the RA and prevents reflux of blood into the coronary sinus (Fig. 13). The RA vestibule is a smooth muscular rim surrounding the orifice of the tricuspid valve (TV). The TV separates the RA from the RV and is composed of anterior, posterior, and septal leaflets, which are connected to the RV papillary muscles by chordae tendineae. Unlike the mitral
Fig. 7. (A) Axial maximum intensity projection (MIP) image of the RCA shows the SA nodal branch (arrow) coursing posteriorly toward the superior cavoatrial junction. (B) Axial MIP image of the LCA in a different patient shows the SA nodal branch (arrows) arising from the proximal CX artery (open arrow). Also note the RI branch in this patient (arrowhead) between the CX and the LAD arteries (notched arrowhead).
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Jacobs valve (MV), which is in direct continuity with the aortic valve, the TV is separated from the pulmonary valve (PV) by a muscular ridge, the crista supraventricularis. Also, unlike the MV, the TV has a direct connection to the interventricular septum. These characteristics help to differentiate the TV from the MV in complex congenital heart disease cases.
RV and PV
Fig. 8. Oblique volume-rendered image of the RCA shows the acute marginal branch (arrow) and a smaller right ventricular branch (arrowhead) coursing over the RV.
The RV is the most anterior cardiac chamber and is characterized by a heavily trabeculated apex and the presence of septomarginal bands. The moderator band, also known as the septomarginal trabeculation or trabecula septomarginalis, extends from the interventricular septum to the base of the anterior papillary muscle, contributing to its perfusion (Fig. 14). The moderator band contains the right bundle of the AV bundle, also known as His bundle, along with one or more arteries supplied by the LCA system. Reig and colleagues16 demonstrated that the moderator band artery most commonly originates from the second anterior septal artery from the left coronary system. In addition, these investigators consistently found anastomoses between the moderator band artery and the right marginal artery or right ventricular branches, both of which originate from the RCA.16 Therefore,
Fig. 9. Society of Cardiovascular Computed Tomography coronary segmentation diagram. From Raff GL, Abidov A, Achenbach S, et al. SCCT guidelines for the interpretation and reporting of coronary computed tomographic angiography. J Cardiovasc Comput Tomogr 2009;3(2):127, copyright 2009, Elsevier; with permission.
CT Evaluation of the Normal Cardiac Anatomy LEFT CARDIAC CHAMBERS AND VALVES Left Atrium, Atrial Appendage, and MV
Fig. 10. Four-chamber view of the heart from a CCTA performed with a 3-phase injection protocol shows optimal opacification of the left and right sides of the heart. Opacifying the right and left atria and ventricles improves conspicuity and aids accurate identification of the interatrial and interventricular septum, respectively.
the moderator band serves as a potential anastomotic connection between the right and left coronary arteries, theoretically protecting the right ventricular myocardium in case of a proximal RCA occlusion. The smooth, muscular infundibulum (conus) of the RV is located immediately inferior to the PV and is the outflow tract for blood from the RV through the PV and into the pulmonary artery (Fig. 15). The PV divides the RV outflow tract from the main pulmonary artery, but, as previously stated, it is separated from the TV by the crista supraventricularis. The PV, like the TV, is trileaflet, composed of right, left, and pulmonary leaflets.
Similar to the RA, the left atrium (LA) has a venous portion, a vestibule, and an appendage. The superior and inferior right and left pulmonary veins typically drain into its posteriorly located venous component (Fig. 16), but many common variants of pulmonary venous anatomy exist. Most of the LA is smooth walled, although the LAA, which arises from the superolateral LA, is tubularly shaped and trabeculated (Fig. 17). The pectinate muscles in the LAA are typically smaller than those in the right atrial appendage.15 Assessment of the left atrial size can be easily performed by calculating the left atrial area, exclusive of the LAA and pulmonary veins. An area of less than 20 cm2 is normal, 20 to 29 cm2 is mildly enlarged, 30 to 40 cm2 is moderately enlarged, and greater than 40 cm2 is severely enlarged.17 The vestibular component of the LA surrounds the orifice of the MV. The MV separates the LA and LV, and its apparatus is composed of 5 parts: an annulus, 2 leaflets, 2 commissures, 2 papillary muscles, and chordae tendineae. The annulus, a saddle-shaped fibrous ring embedded in the myocardium, functions to anchor the MV leaflets and is structurally continuous with the aortic annulus via 3 fibrous trigones. The MV is the only bileaflet cardiac valve, containing an anterior and a posterior leaflet (Fig. 18). Because of its position, the anterior leaflet functions to separate the inflow and outflow tracts of the LV. Chordae tendineae are fibrous connections between the mitral leaflets and the anterolateral and posteromedial LV papillary muscles (Fig. 19).
Fig. 11. (A) Axial CCTA image of the heart shows the crista terminalis (arrow) in the RA and the Bachmann bundle (arrowhead). (B) The septum spurium (arrow) is the largest of the pectinate muscles originating from the crista terminalis.
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Fig. 14. Axial CCTA image of the heart shows the RV moderator band (arrow) extending from the right side of the interventricular septum toward the anterior papillary muscle.
Fig. 12. Volume-rendered image of the anterior heart demonstrates the pyramidal shape of the right atrial appendage (RAA) and the narrower, more fingerlike appearance of the LAA (arrow).
LV and Aortic Valve The normal LV is thick walled and lies posterior to the RV. In contradistinction to the heavily trabeculated RV, the LV contains fine trabeculations. The LV has 2 papillary muscles, anterior and posterior, which connect directly to the ventricular myocardium and function as part of the MV annulus to
Fig. 13. Axial CCTA image of the heart shows the thebesian valve (arrow) at the entrance of the coronary sinus into the RA.
Fig. 15. Multiplanar reconstruction of the right side of the heart shows the outflow tract of the RV. The RV conus (C) is located inferior to the PV (arrow).
Fig. 16. Segmented volume-rendered image of the LA shows the superior and inferior right and left pulmonary veins (arrows) each entering the atrium via a separate ostium.
CT Evaluation of the Normal Cardiac Anatomy
Fig. 17. Axial CCTA image through the LA and LAA shows the curvilinear, low-density pectinate muscles within the LAA (arrows). Ao, ascending aorta.
Fig. 19. Three-chamber view through left heart and the left ventricular outflow tract shows the thin fibrous chordae tendineae (arrows) attaching the MV leaflets to the papillary muscles. Also note the fibrous continuity between the mitral and aortic valves (arrowhead). Ao, ascending aorta.
ensure proper function of the MV leaflets (see Fig. 19). The outflow of the LV is into the aorta via the aortic valve. The aortic valve is tricuspid and composed of the right, left, and noncoronary cusps (see Fig. 1). The 3 aortic cusps are half moon shaped; hence, the aortic valve is commonly referred to as a semilunar valve. In the closed position, each cusp forms a pocket that opens into the ascending aorta. Behind each cusp is a dilation of the aorta known as the sinus of Valsalva. The right and left coronary arteries arise from the sinuses of
the right and left cusps, respectively. The posterior cusp lacks a corresponding coronary artery and is therefore called the noncoronary cusp. Three aortic commissures are roughly equally spaced around the valve annulus and separate the 3 aortic cusps (Fig. 20). Unlike the MV, the aortic valve lacks chordae tendineae and papillary muscles. As previously described, the MV and aortic valve have fibrous continuity (see Fig. 19). This feature helps to distinguish the cardiac chambers and valves of the left side from those of the right side in complex congenital heart disease cases.
Fig. 18. Short-axis view through the open MV in diastole shows the anterior (arrow) and posterior (arrowhead) leaflets.
Fig. 20. Short-axis view through the aortic valve shows the 3 commissures (arrowheads) adjacent to the aortic cusps. L, left coronary cusp; R, right coronary cusp; NC, noncoronary cusp.
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Jacobs SUMMARY Knowledge of the normal cardiac anatomy and its common variations is fundamental for accurate assessment of the heart on CCTA. Using standardized terminology to describe and localize the coronary segments and cardiac structures facilitates accurate communication of cardiac CT examination results.
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9. Berdajs D, Patonay L, Turina MI. The clinical anatomy of the sinus node artery. Ann Thorac Surg 2003;76:732–5. 10. Sow ML, Ndoye JM, Lo EA. The artery of the sinuatrial node: anatomic considerations based on 45 injection-dissections of the heart. Surg Radiol Anat 1996;18:103–9. 11. Austen WG, Edwards JE, Frye RL, et al. A reporting system on patients evaluated for coronary artery disease. Circulation 1975;51(Suppl 4):3–40. 12. Raff GL, Abidov A, Achenbach S, et al. SCCT guidelines for the interpretation and reporting of coronary computed tomographic angiography. J Cardiovasc Comput Tomogr 2009;3(2):122–36. 13. Saremi F, Krishnan S. Cardiac conduction system: anatomic landmarks relevant to interventional electrophysiologic techniques demonstrated with 64detector CT. Radiographics 2007;27:1539–67. 14. Saremi F, Channual S, Krishnan S, et al. Bahman bundle and its arterial supply: imaging with multidetector CT—implications for interatrial conduction abnormalities and arrhythmias. Radiology 2008; 248(2):447–57. 15. Malouf JF, Edwards WD, Tajik AJ, et al. Functional anatomy of the heart. In: Fuster V, Alexander RW, O’Rourke RA, et al, editors. Hurst’s the heart. 11th edition. New York: McGraw-Hill; 2005. p. 45–82. 16. Reig J, Alberti N, Petit M. Arterial vascularization of the human moderator band: an analysis of this structure’s role as a collateral circulation route. Clin Anat 2000;13:244–50. 17. Lang RM, Bierig M, Devereux RB, et al. Recommendations for chamber quantification: a report from the American Society of Echocardiography’s Guidelines and Standards Committee and the Chamber Quantification Writing Group, developed in conjunction with the European Association of Echocardiography, a branch of the European Society of Cardiology. J Am Soc Echocardiogr 2005;18(12):1440–63.