DOI: 10.1148/rg.276065747
RadioGraphics 2007;27:1569-1582
© RSNA, 2007
Anatomy of the Heart at Multidetector CT: What the Radiologist Needs to Know1
James P. O’Brien, MD, MBA,
Monvadi B. Srichai, MD,
Elizabeth M. Hecht, MD,
Daniel C. Kim, MD, and
Jill E. Jacobs, MD
1 From the Department of Radiology, New York University Medical Center, 560 First Ave, New York, NY 10016. Received November 30, 2006; revision requested March 12, 2007 and received April 20; accepted May 9. J.E.J. received a research grant from Siemens and is with the speakers’ bureau for GE Healthcare and Siemens; all remaining authors have no financial relationships to disclose.
Address correspondence to J.P.O. (e-mail: obriej04{at}popmail.med.nyu.edu).
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Abstract
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Continued improvements in multidetector computed tomographic (CT) scanners have made cardiac CT an important clinical tool that is revolutionizing cardiac imaging. Multidetector CT with submillimeter collimation and gantry rotation times under 0.5 seconds allows the acquisition of studies with high temporal resolution and isotropic voxels. The volumetric data set that is generated can be analyzed with a depth previously not possible, requiring a solid understanding of the cardiac anatomy and its appearance on CT scans and postprocessed images.
© RSNA, 2007
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Introduction
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The advent of multidetector computed tomography (CT), particularly with scanners having 64 or more detectors, has continued to improve temporal resolution and allows the acquisition of isotropic voxels. With these scanners, the heart and coronary arteries are routinely imaged as a motion-free volume of data. A variety of postprocessing techniques, including multiplanar reformation (MPR), maximum intensity projection (MIP), volume rendering (VR), curved reformation, and cine imaging, allow noninvasive assessment of every aspect of the cardiovascular system. This capability requires a thorough understanding of essential coronary arterial and cardiac anatomy.
In this article, we review the anatomy of the coronary arteries, cardiac chambers, and cardiac valves from a three-dimensional (3D) imaging perspective, with an emphasis on imaging planes and postprocessing techniques used to interpret the relevant findings at dual-source 64-detector CT.
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Multidetector CT Postprocessing Techniques
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The interpretation of cardiac CT angiographic studies performed with multidetector scanners requires real-time interaction with the volumetric data set that is generated. Consequently, radiologists must become proficient with workstation applications and postprocessing techniques. At our institution, interpretation of the cardiac CT angiographic data is accomplished using a combination of the postprocessing techniques described in the following sections.
Multiplanar Reformation
MPR is the basic tool used to interpret cardiac CT angiographic studies. With use of retrospective electrocardiographic gating, data from specific phases of the cardiac cycle are retrospectively referenced to the electrocardiogram for reconstruction. Once the reconstruction is complete, the data are transferred directly to the workstation. The radiologist then interfaces with the reconstructed series in real time at the workstation. Because of variations in the orientation of the heart in the thorax, it is often useful to evaluate cardiac structures along the cardiac planes. The multiplanar capabilities of the workstation allow images of the heart and coronary arteries to be manually rotated for optimal evaluation of the cardiac anatomy. Most workstations with cardiac analysis capabilities can automatically orient volumetric image data sets along the cardiac axes and into the traditionally used cardiac planes (ie, short-axis, horizontal long-axis, vertical long-axis) with the click of a button. This feature is especially useful for evaluating the cardiac chambers and left ventricular (LV) function. Selected reformatted images are sent to the picture archiving and communication system (PACS) for review by the referring clinicians and for long-term storage.
Maximum Intensity Projection
MIP is a postprocessing technique that takes the highest-attenuation voxel in a predetermined slab of data and projects it from the user toward the viewing screen, resulting in a two-dimensional image. MIP images are similar to traditional angiograms, which display intraluminal opacity values (1). Only the highest-attenuation objects, typically contrast material and bone, are preferentially displayed and retained in the image. The limitation of MIP images is that they lack depth and spatial information regarding relationships to adjacent structures (2,3). However, they can allow quick assessment for significant coronary artery stenosis. At our institution, MIP images of each coronary artery are created and transferred to the PACS for every study.
Volume Rendering
VR is a 3D technique in which the CT attenuation values for each voxel can be assigned a specific color (3), thereby producing an overall image of the heart. VR is the only true 3D technique and provides the depth and spatial information that is lacking with MIP (2,3). VR techniques facilitate surface evaluation of the heart and coronary arteries. With respect to diagnosis, we have found this technique to be the most useful for evaluating complex anatomy, including coronary artery anomalies, bypass grafts, and fistulas. At our institution, VR images of each coronary artery are transferred to the PACS. Our referring physicians have found that these images allow them to communicate the major findings of a study to the patient in an easily understandable format.
Curved Reformation
Because normal coronary arteries are often tortuous, accurate evaluation requires assessment of the entire vessel along its center line. Curved reformatted images provide this capability by sampling a given volume (ie, artery) along a predefined curved anatomic plane (3). Most cardiac workstations have software capable of automatically determining the center line of each coronary artery and display the entire length of the artery on a single curved reformatted image (4). This type of reformation is especially helpful in patients with bypass grafts and highly tortuous coronary arteries.
Cine Imaging
Cine images are used to examine the motion and physiologic features of cardiac structures such as the LV and cardiac valves. The capabilities of the multidetector scanner are used to full advantage with this technique, since data from the heart and coronary arteries are typically reconstructed at specific points during the cardiac cycle. For example, at our institution we typically reconstruct the volume data at 10% of the R-R interval throughout the cardiac cycle (0%–90%). Thus, data reconstruction typically occurs at 10%, 20%, 30%, and so on, of the R-R interval. Depending on the individual patient, the 40% series could correspond to end-systole, whereas the 90% series might represent end-diastole, with the remaining data points representing other phases of the cardiac cycle. The reconstructed imaging data can then be manipulated into various imaging planes and displayed in a cine loop, allowing assessment of the motion of cardiac structures throughout systole and diastole. 
This approach is particularly useful for examining LV wall motion and wall thickening and for assessing valve motion in multiple planes. In addition to allowing assessment of myocardial contractility, reconstruction of the cardiac data during both systole and diastole allows determination of quantitative LV functional parameters, including end-diastolic and end-systolic ventricular volumes, stroke volume, and ejection fraction.
Combining the aforementioned postprocessing techniques allows comprehensive evaluation of the heart, great vessels, and right ventricular (RV) and LV function. Normal values have been described at magnetic resonance (MR) imaging (5) and can be applied to the evaluation of cardiac CT angiographic studies (Tables 1, 2). There is good correlation between values obtained with cardiac CT angiography and those obtained with MR imaging (6).
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Coronary Arteries
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Left Coronary Artery
The left coronary artery (LCA) normally arises from the left sinus of Valsalva near the sinotubular ridge (Fig 1). It courses for a variable distance before giving rise to the LAD artery and the LCx artery.
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Figure 1a. Axial MPR image displays the origin of the coronary arteries from the aorta. The LCA (black arrow) bifurcates into the left anterior descending (LAD) artery (white arrowhead) and the left circumflex (LCx) artery (black arrowhead). White arrow indicates the right coronary artery (RCA). (b) VR image shows the LCA (black arrow) arising from the aorta and bifurcating into the proximal LCx artery (arrowhead) and the proximal LAD artery (white arrow).
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Figure 1b. Axial MPR image displays the origin of the coronary arteries from the aorta. The LCA (black arrow) bifurcates into the left anterior descending (LAD) artery (white arrowhead) and the left circumflex (LCx) artery (black arrowhead). White arrow indicates the right coronary artery (RCA). (b) VR image shows the LCA (black arrow) arising from the aorta and bifurcating into the proximal LCx artery (arrowhead) and the proximal LAD artery (white arrow).
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The LAD artery courses anterolaterally in the epicardial fat of the anterior interventricular groove and supplies the majority of the LV (Fig 2). The major branches of the LAD artery are the diagonal and septal perforating arteries. The diagonal branches course laterally and predominantly supply the LV free wall. The septal branches course medially and supply the majority of the interventricular septum, as well as the atrioventricular (AV) bundle and proximal bundle branch (Fig 3) (7).
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Figure 2a. Oblique axial (a) and vertical long-axis (b) MPR images show the normal LAD artery (arrows) coursing in the epicardial fat of the interventricular groove toward the LV apex.
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Figure 2b. Oblique axial (a) and vertical long-axis (b) MPR images show the normal LAD artery (arrows) coursing in the epicardial fat of the interventricular groove toward the LV apex.
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Figure 3a. Oblique axial MPR (a) and VR (b) images show the septal branches (black arrowheads) and diagonal branches (white arrowheads) of the LAD artery. The septal branches quickly reach and penetrate the myocardium, whereas the diagonal branches course laterally to the LV free wall.
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Figure 3b. Oblique axial MPR (a) and VR (b) images show the septal branches (black arrowheads) and diagonal branches (white arrowheads) of the LAD artery. The septal branches quickly reach and penetrate the myocardium, whereas the diagonal branches course laterally to the LV free wall.
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The LCx artery is the other major branch of the LCA. It courses in the left AV groove, giving rise to obtuse marginal branches (Fig 4), sometimes referred to as lateral branches. The LCx artery and its branches supply the LV free wall and a variable portion of the anterolateral papillary muscle (7). It variably gives rise to posterolateral and posterior descending artery (PDA) branches supplying the diaphragmatic portion of the LV (see the section entitled "Dominance").
In approximately 15% of patients, a third branch, the ramus intermedius (RI) branch, arises at the division of the LCA, resulting in a trifurcation (Fig 5) (4). When present, the RI branch courses laterally toward the LV free wall. Its course is similar to that of a diagonal branch of the LAD artery.
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Figure 5a. Oblique axial MPR image shows the RI branch (arrow) arising between the LAD artery (black arrowhead) and the LCx artery (white arrowhead), resulting in a trifurcation of the LCA. (b) VR image shows the RI branch (arrow) arising from the trifurcation. Black arrowhead indicates the LAD artery, white arrowhead indicates the LCx artery.
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Figure 5b. Oblique axial MPR image shows the RI branch (arrow) arising between the LAD artery (black arrowhead) and the LCx artery (white arrowhead), resulting in a trifurcation of the LCA. (b) VR image shows the RI branch (arrow) arising from the trifurcation. Black arrowhead indicates the LAD artery, white arrowhead indicates the LCx artery.
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Right Coronary Artery
The RCA normally arises from the right coronary sinus (CS) and courses in the right AV groove toward the crux of the heart (the point on the posterior surface of the heart where the AV groove transects the line of the interventricular septum and interatrial septum, forming a cross). In approximately 50%–60% of patients, the first branch of the RCA is a conus artery (8). The conus artery can also arise directly from the aorta (30%–35% of patients) (Fig 6) (8). The conus artery supplies the RV outflow tract (conus arteriosis) and forms the circle of Vieussens, an anastomosis with the LAD arterial circulation (7). In approximately 58% of patients, the sinoatrial nodal artery arises from the RCA; in the remaining patients (42%), it arises from the LCx artery (9). Multiple ventricular branches arise from the RCA, the largest of which is called the acute marginal branch.
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Figure 6a. MPR images (a, c) and VR image (b) show the RCA (black arrow in a) and its branches. In this case, the conus artery (arrowhead in a) arises from the aorta. White arrow in a and arrow in b indicate the acute marginal branch, arrowhead in c indicates the sinoatrial nodal branch.
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Figure 6b. MPR images (a, c) and VR image (b) show the RCA (black arrow in a) and its branches. In this case, the conus artery (arrowhead in a) arises from the aorta. White arrow in a and arrow in b indicate the acute marginal branch, arrowhead in c indicates the sinoatrial nodal branch.
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Figure 6c. MPR images (a, c) and VR image (b) show the RCA (black arrow in a) and its branches. In this case, the conus artery (arrowhead in a) arises from the aorta. White arrow in a and arrow in b indicate the acute marginal branch, arrowhead in c indicates the sinoatrial nodal branch.
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Dominance
The coronary artery that gives rise to the PDA and posterolateral branch is referred to as the "dominant" artery, with the RCA being dominant in approximately 70% of cases (Fig 7a). The LCA is dominant in approximately 10% of cases, supplying the entire LV, accompanied by the PDA and posterolateral branches from the LCx artery. In the remaining cases, the RCA and LCA are codominant; that is, portions of the LV diaphragmatic wall are supplied by both the RCA and the LCx artery (Fig 7b). The length of the distal RCA is inversely proportional to the length of the LCA along the inferior aspect of the heart (7). The RCA is typically diminutive compared with the LCx artery in patients with left-dominant systems (Fig 7c).
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Figure 7a. VR image shows the inferior surface of the heart. A right-dominant system is depicted, with the PDA (white arrowhead) arising from the RCA (black arrowhead). A posterolateral branch (arrow) is also seen. (b) VR image shows a codominant system, with the inferior myocardial surface supplied equally by the RCA and the LCx artery. (c) Coronal MPR image shows a left-dominant system, with the RCA being smaller than normal (cf a).
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Figure 7b. VR image shows the inferior surface of the heart. A right-dominant system is depicted, with the PDA (white arrowhead) arising from the RCA (black arrowhead). A posterolateral branch (arrow) is also seen. (b) VR image shows a codominant system, with the inferior myocardial surface supplied equally by the RCA and the LCx artery. (c) Coronal MPR image shows a left-dominant system, with the RCA being smaller than normal (cf a).
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Figure 7c. VR image shows the inferior surface of the heart. A right-dominant system is depicted, with the PDA (white arrowhead) arising from the RCA (black arrowhead). A posterolateral branch (arrow) is also seen. (b) VR image shows a codominant system, with the inferior myocardial surface supplied equally by the RCA and the LCx artery. (c) Coronal MPR image shows a left-dominant system, with the RCA being smaller than normal (cf a).
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Segmental Coronary Arterial Anatomy
A classification scheme that divides the coronary arteries into segments based on specific anatomic structures and arterial branches has been used during conventional coronary angiography for many years (10). This system has been adopted by most centers that perform cardiac CT angiography. Use of a common nomenclature allows reproducibility of results and facilitates accurate communication of the location of cardiac disease among physicians (Fig 8).
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Figure 8a. VR images show the left (a) and right (b) segmental coronary arterial anatomy as described by Austen et al (10), whose system can accurately help localize coronary disease. The images in a show the superior (left) and anterolateral (right) aspects of the heart, including left marginal vein 5 (large white arrow), LCx 11 (large black arrowhead), LAD 6 (large white arrowhead), LAD 7 (black arrow), LAD 8 (small black arrowhead), LAD 9 (small white arrow), and LAD 10 (small white arrowhead). The images in b show the right (left) and inferior (middle and right) aspects of the heart, including RCA 1 (black arrow), RCA 2 (large white arrowhead), RCA 3 (white arrow), RCA 4 (black arrowhead), and RCA 16 (small white arrowhead).
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Figure 8b. VR images show the left (a) and right (b) segmental coronary arterial anatomy as described by Austen et al (10), whose system can accurately help localize coronary disease. The images in a show the superior (left) and anterolateral (right) aspects of the heart, including left marginal vein 5 (large white arrow), LCx 11 (large black arrowhead), LAD 6 (large white arrowhead), LAD 7 (black arrow), LAD 8 (small black arrowhead), LAD 9 (small white arrow), and LAD 10 (small white arrowhead). The images in b show the right (left) and inferior (middle and right) aspects of the heart, including RCA 1 (black arrow), RCA 2 (large white arrowhead), RCA 3 (white arrow), RCA 4 (black arrowhead), and RCA 16 (small white arrowhead).
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Left Coronary Artery.—
The LCA extends from the ostium to its bi- or trifurcation.
LAD Artery.—
The LAD artery is divided into proximal, middle, and distal portions. The proximal LAD artery extends from the left main bifurcation to the origin of the first septal branch. The midportion of the LAD artery extends to the point where the artery forms an acute angle, which may coincide with the origin of the second septal perforator. If this is not the case, the middle and distal LAD artery are split at a distance halfway between the first septal perforator and the apex of the heart. The apical segment represents the termination of the artery.
LCx Artery.—
The LCx artery is divided into proximal and distal segments, based on the origin of the (usually large) obtuse marginal branches. As mentioned earlier, the LCx artery variably gives rise to a posterolateral branch. In addition, it may terminate as the PDA (described later).
Right Coronary Artery.—
The proximal RCA extends from the ostium to a point halfway to the acute margin of the heart. The mid-RCA represents the other half of that distance. The distal RCA courses along the posterior AV groove, from the acute angle of the heart to the origin of the PDA.
Normal Coronary Artery Diameter
Normal coronary artery diameters have been established with catheter angiography. The average size varies with gender (approximately 3 mm in females and 4 mm in males) (11). The average diameters of each coronary artery also vary, ranging from 5 mm (LCA in males) to 2 mm (PDA in females) (11). However, this measurement takes into account only the luminal diameter, and multidetector CT is capable of characterizing the arterial wall with excellent resolution. To our knowledge, the normal coronary artery diameter has not been established with multidetector CT criteria.
Focal abnormal dilatation to more than 1.5 times the diameter of an adjacent normal coronary artery is defined as an aneurysm (12). If the process is diffuse, it is known as ectasia (12). Either process is easily identified with cardiac CT angiography (Fig 9). It has been suggested that individuals with coronary artery aneurysms are at higher risk for ischemia (12).
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Other Cardiac Structures
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The cardiac imaging planes routinely used in cardiac nuclear medicine, MR imaging, and echocardiography are easily created at a 3D workstation using the acquired cardiac CT angiographic data. Evaluation of MPR images in combination with use of these routine cardiac imaging planes allows comprehensive evaluation of cardiac anatomy and function.
Cardiac Imaging Planes and the Left Side of the Heart
Vertical Long-Axis View.—
The vertical long-axis view is a parasagittal plane oriented along the long axis of the LV lumen. The relationship between the left atrium (LA) and the LV is assessed on vertical long-axis images (Fig 10). The inferior and anterior walls of the LV myocardium are optimized on this view. The structure and function of the bicuspid MV and LV are well demonstrated on vertical long-axis cine images, and the LA appendage and CS are routinely depicted (Fig 10).
Horizontal Long-Axis View.—
The horizontal long-axis view, or four-chamber view, is a horizontal plane through the heart that essentially bisects all four cardiac chambers (Fig 11). The resultant display readily allows assessment of chamber size and valve position. The septal, apical, and lateral LV walls can be simultaneously assessed. The lateral wall of the LV is normally thin at the apex (typically 1–2 mm), even in abnormally thickened hearts (13). Subjective evaluation of AV valvular and ventricular function is usually also possible in cine mode.
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Figure 11. Horizontal long-axis MPR image shows the LV (large black *), RV (large white *), LA (small black *), right atrium (RA) (small white *), MV (black arrow), tricuspid valve (white arrow), and pericardium (arrowheads). The latter structure is normally very thin.
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Because most workstations can quickly calculate the area of a structure specified by the user, LA size can readily be determined. A line is drawn along the endocardial border of the LA at the level of the MV on the horizontal long-axis view (Fig 12). This line creates an irregular ellipse, exclusive of the pulmonary veins (PVs) and LA appendage. The area of the ellipse is automatically calculated by the workstation. An area of less than 20 cm2 is normal, 20–30 cm2 is mildly abnormal, 30–40 cm2 is moderately abnormal, and greater than 40 cm2 is severely abnormal (14).
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Figure 12. Horizontal long-axis MPR image illustrates calculation of the LA area. The yellow line drawn along the endocardial border of the LA creates an irregular ellipse. Most workstations can quickly calculate the area contained within the ellipse, which can be used to detect LA enlargement.
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Three-Chamber View.—
The three-chamber view is an oblique long-axis view that optimizes visualization of the LV, LA, aortic root, MV, and aortic valve (Fig 13). It is usually obtained manually, with a plane oriented through the aortic root, aortic valve, MV, and LV on a short-axis view obtained at the base of the heart. The three-chamber view allows evaluation of the LV outflow tract, aortic valve, aortic root, and proximal ascending thoracic aorta. The posteromedial papillary muscles are often seen arising from the LV free (lateral) wall on this view. These muscles are connected to the MV by chordae tendineae, which are linear fibrous bands. During systole, the LV myocardium contracts. The papillary muscles likewise contract, tugging on the MV leaflets to ensure complete closure of the MV and prevent regurgitation (Fig 14).
Short-Axis View.—
The short-axis view is obtained in an oblique coronal plane relative to the thorax, down the barrel of the LV lumen (Fig 15). As one progresses from the MV toward the apex in the short axis, the basal, middle, and apical portions of the LV myocardium can be evaluated. This plane allows easy assessment of LV size and myocardial contractility.
Right Side of the Heart
Depending on the injection protocol used, varying levels of enhancement of the right side of the heart are achieved. If this side of the heart is enhanced with contrast material, the RA, RV, and tricuspid valves can be assessed in detail. The RA receives inflow primarily from the superior vena cava and inferior vena cava, as well as from the CS. The crista terminalis is located at the RA–superior vena cava junction and is a muscular ridge that separates the smooth muscle fibers of the posterior RA from trabeculated muscle fibers anteriorly. The eustachian valve is located at the RA–inferior vena cava junction and directs flow toward the foramen ovale (Fig 16) (7). The thebesian valve prevents reflux from the RA into the CS (Fig 17) (15).
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Figure 16. Sagittal MPR image shows the inferior vena cava (arrowhead), superior vena cava (arrow), LA (black *), and RA (white *). The structures of the right side of the heart can routinely be evaluated depending on the injection protocol used.
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The RV is the most anterior of the cardiac chambers and has a heavily trabeculated apex and papillary muscles whose functions are similar to those of the LV papillary muscles. The smooth, muscular infundibulum (or conus) of the RV is the outflow portion of the RV directly inferior to the pulmonary valve (Fig 18). A characteristic feature of the RV is the moderator band, a muscular band extending from the interventricular septum to the base of the anterior papillary muscle. The moderator band is part of the right bundle branch conduction system (Fig 19). Although the moderator band and the heavily trabeculated apex are distinct features of the RV, other features such as a well-developed infundibulum, septal papillary muscles, and lack of fibrous continuity of the AV valve and outflow tract are key to differentiating the RV from the LV (16). In complex cases of congenital heart disease, the ability to distinguish the LV from the RV may be of paramount importance.
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Figure 18a. Oblique MPR images show the RA (black arrow in a), RV outflow tract (black *), and pulmonary valve (arrowhead). The LA (white *) and the aorta (white arrow in a, arrow in b) are also seen. Cine images could be obtained at this level to assess aortic valvular function. Oftentimes, multiple structures can be evaluated with manually derived MPR images.
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Figure 18b. Oblique MPR images show the RA (black arrow in a), RV outflow tract (black *), and pulmonary valve (arrowhead). The LA (white *) and the aorta (white arrow in a, arrow in b) are also seen. Cine images could be obtained at this level to assess aortic valvular function. Oftentimes, multiple structures can be evaluated with manually derived MPR images.
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Evaluation of the RV can be performed with any multidetector CT study of the thorax and provides significant prognostic information relative to the diagnosis of acute pulmonary embolism. Comparing the size of the RV to that of the LV (RV/LV diameter ratio) on axial images has been shown to correlate with the severity of pulmonary embolism and fatal outcome (17). The RV measurement is obtained at the level of the tricuspid valve and represents the maximum distance between the endocardial surface of the free wall and the endocardial surface of the septal wall. A similar measurement of the LV is obtained at the level of the MV, and the RV/LV diameter ratio is calculated. A ratio of 1 or less is normal, whereas a ratio greater than 1.5 indicates severe pulmonary embolism (17).
Cardiac and Pulmonary Veins
Cardiac CT angiography is excellent for imaging the CS and cardiac veins (Fig 20). The components of the cardiac venous system are variable, but the most constant structure is the CS itself, which runs along the inferior aspect of the heart in the AV groove before emptying into the RA (7,18). The first branch of the CS is the posterior interventricular vein, also known as the middle cardiac vein, which courses in the posterior interventricular groove from base to apex (18). The next two branches are the posterior vein of the LV and the left marginal vein. At this point, the CS becomes the great cardiac vein, which courses in the left AV groove with the LCx artery. It then continues as the anterior interventricular vein in the anterior interventricular groove, coursing from the base of the heart toward the apex adjacent to the LAD artery.
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Figure 20a. VR image shows the great cardiac vein (arrowheads) coursing in the left AV groove. (b) VR image shows the CS (arrowheads) coursing along the inferior surface of the heart and emptying into the RA. In this case, the posterior vein of the LV (white arrow) is prominent and the left marginal vein is absent. Black arrow indicates the posterior interventricular vein. LA = left atrium, RV = right ventricle.
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Figure 20b. VR image shows the great cardiac vein (arrowheads) coursing in the left AV groove. (b) VR image shows the CS (arrowheads) coursing along the inferior surface of the heart and emptying into the RA. In this case, the posterior vein of the LV (white arrow) is prominent and the left marginal vein is absent. Black arrow indicates the posterior interventricular vein. LA = left atrium, RV = right ventricle.
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Variability in the cardiac veins is usually due to absence of either the left marginal vein or the posterior vein of the LV (19). Only approximately 55% of patients have the latter vein, with 83% having a left marginal vein (19,20). Knowledge of this variability is important for the outpatient work-up of patients prior to cardiac resynchronization therapy, which is often performed with cardiac CT angiography. Patients treated with cardiac resynchronization therapy typically undergo implantation of an automatic cardioverter-defibrillator for the treatment of heart failure, ideally with a transvenous approach. During this procedure, the LV pacer lead is most commonly inserted into either the posterior vein of the LV or the left marginal vein (18,19). After cardiac CT angiography, if no suitable vein is present in which to place the LV pacer lead with a transvenous approach, surgical placement may be necessary (19).
The PVs have received significant attention recently. LA muscle can extend into the venous ostia, and ectopic electrical foci originating at this site may be the cause of atrial fibrillation in a significant number of patients (21). The veins can be mapped in detail with multidetector CT, and treatment strategies that make use of radiofrequency catheter ablation performed on the basis of CT findings can be tailored to individual patients (21). Typically, two veins (superior and inferior) drain into either side of the LA (Fig 21). If additional PVs are present, it is important that they be described prior to ablation. They are typically single and occur more commonly on the right side (21). In particular, middle PVs arising on the right side have a stronger association with atrial fibrillation (21).
Atrial Appendages
Patients with atrial fibrillation may develop thrombus in the LA appendage, a condition that can be evaluated with multidetector CT prior to PV ablation. In most adults (>97%), the LA appendages have pectinate muscles measuring greater than 1 mm (22). These muscles are continuous fibers running parallel to each other within the LA appendage and should not be mistaken for thrombus; in contrast, clot manifests as a focal filling defect. The RA appendage also has pectinate muscles (7), although they are slightly larger than those of the LA appendage.
The LA appendage arises from the superolateral aspect of the LA and projects anteriorly over the proximal LCx artery. It is more tubular than the normally pyramidal RA appendage and has a narrower base (7). These features readily allow differentiation between the two appendages (Fig 22), which can be useful when situs is questioned.
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Figure 22a. Vertical long-axis MPR image shows the normal LA appendage (arrow). The linear filling defects in the appendage represent normal pectinate muscles. (b, c) Axial MPR (b) and VR (c) images show the normal RA appendage (arrow) and pectinate muscles.
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Figure 22b. Vertical long-axis MPR image shows the normal LA appendage (arrow). The linear filling defects in the appendage represent normal pectinate muscles. (b, c) Axial MPR (b) and VR (c) images show the normal RA appendage (arrow) and pectinate muscles.
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Figure 22c. Vertical long-axis MPR image shows the normal LA appendage (arrow). The linear filling defects in the appendage represent normal pectinate muscles. (b, c) Axial MPR (b) and VR (c) images show the normal RA appendage (arrow) and pectinate muscles.
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Cardiac Valves
The four cardiac valves are routinely imaged during cardiac CT angiography, and their motion and morphologic characteristics should also be assessed at all cardiac CT angiographic examinations with reconstructed and cine images.
The MV separates the LA from the LV. It is normally connected to the morphologic LV (Figs 10, 11). The MV is composed of two leaflets, the anterior and posterior leaflets; the other valves normally have three leaflets. The MV and aortic valve share fibrous continuity. The MV annulus, or valve ring, is part of the cardiac skeleton and is imbedded in the myocardium (7). Normally, the boundaries of the MV annulus are not readily apparent at cardiac CT angiography. However, calcification of the MV annulus is a common abnormality that makes identification of the annulus possible at cardiac CT angiography. The papillary muscles (described earlier) with their chordae tendineae are also a component of the MV apparatus.
The tricuspid valve separates the RA from the RV (Fig 11) and is composed of the same structures as the MV: leaflets, annulus, commissures (sites where two leaflets come together to attach to the aortic wall), papillary muscles, and chordae tendineae. It is normally connected to the morphologic RV. As its name implies, the tricuspid valve is a trileaflet valve (anterior, posterior, and septal leaflets) and is separated from the pulmonary valve by the crista supraventricularis—a muscular ridge—unlike the MV, which is contiguous with the aortic valve (7).
The aortic valve separates the LV outflow tract from the ascending aorta. It is composed of an annulus, cusps, and commissures. No papillary muscles or chordae tendineae are associated with the aortic valve. The three cusps (right, left, and posterior or noncoronary) of the aortic valve form pocketlike outpouchings that are designed to direct blood into the sinuses of Valsalva during diastole (Fig 23) (7).
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Figure 23. Axial MPR image (superoinferior view) demonstrates the aortic valve and its cusps in relation to the LA. These cusps are the right coronary cusp (white *), the left coronary cusp (black *), and the noncoronary cusp (box).
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The pulmonary valve separates the RV outflow tract from the main pulmonary artery but does not connect directly to the tricuspid valve (Fig 18a). It is otherwise essentially identical to the aortic valve, with right, left, and posterior leaflets.
Pericardium
The pericardium is normally paper thin, measuring 2 mm or less (Fig 11). It is composed of two layers, the parietal layer and the serous layer. The tough outer parietal layer envelops the heart and attaches to the sternum and proximal great vessels; in fact, most of the ascending aorta and main pulmonary artery, portions of the venae cavae, and most of the PVs are intrapericardial (7). The inner, more delicate serous layer lines both the fibrous pericardium and the outer surface of the heart and great vessels (7). The pericardium lining the surface of the heart is known as the visceral pericardium, or epicardium. Multidetector CT routinely depicts the fluid-filled junctions of the visceral and parietal pericardia, which form recesses and sinuses (23). The oblique and transverse sinuses are two of the most commonly encountered sinuses at multidetector CT of the heart and thorax (Fig 24) and are continuous with the pericardial cavity (23). It is important to be aware of the more common recesses and sinuses to distinguish them from lymphadenopathy or abnormal soft tissue (23).
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Figure 24. On an axial MPR image obtained inferior to the right pulmonary artery, the transverse sinus (black arrow) is posterior to the ascending aorta (AA) and the main pulmonary artery (PA) and superior to the LA. The oblique sinus (white arrow) is posterior to the LA and is always separated from the transverse sinus by a fat plane.
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Cardiac Anatomy Reporting
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The origin, course, and segmental coronary artery anatomy are described and used as a "road map" for precise localization of abnormalities. Dominance and the size of the LAD artery, LCx artery, and RCA are described. Typically, the number of diagonal and marginal branches is specified and their disease burden quantified. The overall size of the heart and cardiac chambers should be evaluated. LV function, including ejection fraction and wall motion, should also be assessed. The end-diastolic volume, end-systolic volume, and LV myocardial mass and thickness are easily determined. Other anatomic structures, whether cardiac or extracardiac, are not typically mentioned specifically unless they are abnormal.
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Conclusions
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Evaluation of the heart and coronary arteries with cardiac CT angiography is revolutionizing cardiac imaging. The use of multidetector CT scanners with submillimeter collimation and gantry rotation times under 0.5 seconds allows high temporal resolution and the acquisition of isotropic voxels, which in turn allow multiplanar evaluation of a motionless heart. In this context, detailed knowledge of the cardiac anatomy from a CT perspective is essential for thoracic and cardiac imagers.
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Footnotes
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Abbreviations: AV = atrioventricular, CS = coronary sinus, LA = left atrium, LAD = left anterior descending, LCA = left coronary artery, LCx = left circumflex, LV = left ventricle, MIP = maximum intensity projection, MPR = multiplanar reformation, MV = mitral valve, PACS = picture archiving and communication system, PDA = posterior descending artery, PV = pulmonary vein, RA = right atrium, RI = ramus intermedius, RV = right ventricle, VR = volume rendering, 3D = three-dimensional
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References
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Abstract
Introduction
