DOI: 10.1148/rg.276075003
RadioGraphics 2007;27:1539-1565
© RSNA, 2007
Cardiac Conduction System: Anatomic Landmarks Relevant to Interventional Electrophysiologic Techniques Demonstrated with 64-Detector CT1
Farhood Saremi, MD and
Subramaniam Krishnan, MD
1 From the Department of Radiological Sciences and Cardiology, Division of Cardiothoracic Imaging (F.S.) and the Department of Medicine, Division of Cardiology (S.K.), UCI Medical Center, 101 City Dr South, Rte 140, Orange, CA 92868. Recipient of Magna Cum Laude and Excellence in Design awards for an education exhibit at the 2006 RSNA Annual Meeting. Received January 5, 2007; revision requested March 29 and received April 20; accepted May 17. S.K. is with the speakers’ bureau of Medtronic; F.S. has no financial relationships to disclose.
Address correspondence to F.S. (e-mail: fsaremi{at}uci.edu).
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Abstract
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The rapid development of clinical cardiac electrophysiology has triggered a renewed interest in the anatomy of the heart. A thorough knowledge of cardiac anatomy is a prerequisite for successful electrophysiologic procedures. Accurate description of the cardiac anatomy requires the use of a common language in describing this anatomy, as well as close interaction between radiologists, cardiologists, and surgeons. Given its capacity to provide relevant anatomic information in exquisite detail, multidetector computed tomography (CT) has the potential to allow faster and more accurate placement of intracardiac ablation catheters and pacemaker leads relative to the anatomy of interest. High-resolution reformatted images from 64-detector CT data provide accurate anatomic information for locating important landmarks relative to the cardiac conduction system or to current electrophysiologic interventions and cardiac resynchronization therapy.
© RSNA, 2007
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LEARNING OBJECTIVES FOR TEST 1
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After reading this article and taking the test, the reader will be able to:
- Identify the cardiac anatomic landmarks relevant to tachyarrhythmia.
- Describe reformation techniques for localizing the anatomy of interest.
- Discuss imaging findings that are pertinent to the electrophysiologist.
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Introduction
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The past 2 decades have witnessed a revolution in the treatment of patients with cardiac arrhythmias. Advances in this field have included the development of (a) techniques of catheter ablation that often require the precise destruction of minute amounts of arrhythmogenic tissues, and (b) techniques of resynchronization therapy that require the pacing of different parts of the atria as well as of the ventricular branches of the coronary venous system (1–3). These requirements underlie the increasing use of cardiac imaging procedures such as multidetector computed tomography (CT). With recent technologic advances in multidetector CT scanners, especially the introduction of high-resolution 64-detector scanners, it has become possible to perform a "virtual dissection" of the heart and cardiovascular system (4–7), which brings the radiologist’s role in the interpretation of cardiac images to a higher level. By providing the electrophysiologist with an anatomic "road map," the radiologist will make the ablation and pacing procedures much easier, and results as well as complications will be recognized immediately.
In this article, we describe the normal cardiac anatomy as well as anatomic landmarks of interest to electrophysiologists, discussing and illustrating these landmarks in terms of their anatomic localization in different planes, their relationships with other structures, and their anatomic variants. We also discuss the common arrhythmias and electrophysiologic procedures so that the radiologist can better understand these procedures.
Most of the images shown herein were obtained with a 64-detector CT scanner and a standard electrocardiographically gated coronary CT angiography protocol. Orally or intravenously administered metoprolol was used to achieve a target heart rate of less than 65 bpm as needed. A sublingual nitroglycerin tablet (0.4–0.8 mg) was given 1 minute before image acquisition unless contraindicated. Beta blockers are the drug of choice for treating the majority of supraventricular arrhythmias and are not contraindicated in patients with a history of these arrhythmias. Nitrates can also be used safely in this group.
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Diagnostic Electrophysiologic Study
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A diagnostic electrophysiologic study involves the insertion of multiple catheters via the venous or arterial system to study and record electrical activity from the atria, ventricles, and other parts of the conduction system, as well as to induce various arrhythmias (Fig 1). These studies are generally performed to obtain information on the specific type of rhythmic disturbance that may have occurred clinically and to provide the treating physician with more details concerning the underlying mechanism. Under fluoroscopic guidance, catheters are passed into the right atrium or right ventricle. Arteriovenous conduction is studied by positioning a separate catheter across the tricuspid annulus and obtaining a His bundle electrogram. To record activity from the left atrium and sometimes from the left ventricle, a catheter is guided into the coronary sinus. Left-sided procedures involving the left atrium and left ventricle are performed with a transseptal approach (Fig 2) or with a retrograde approach from the femoral artery. The general region of interest is usually determined first, after which more precise mapping is used to localize the arrhythmia focus or the reentrant circuit. Fluoroscopy is routinely used to localize anatomic landmarks for ablation. Multidetector CT can depict various cardiac structures that are difficult to visualize at fluoroscopy (eg, oval fossa, crista terminalis, eustachian ridge, coronary sinus, pulmonary vein ostia) and provides anatomic data for easier placement of intracardiac catheters.
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Figure 1a. Interventional electrophysiologic approaches. Short-axis (a, d), four-chamber (b), and right ventricular inflow-outflow tract two-chamber (c) CT scans show how the eustachian valve directs inferior vena caval (IVC) blood toward the foramen ovale (FO) and superior vena caval (SVC) blood toward the tricuspid valve. Thus, transseptal cardiac catheterization is easier via the IVC and right ventricular instrumentation via the SVC. Because of the angle of the entrance into the coronary sinus (CS) (yellow arrow in d), cannulation is easier via the SVC. Left-sided ablations are performed with a transseptal approach or, less commonly, a retrograde aortic approach. A coronary sinus approach is used for biventricular pacing and specific left ventricular ablations. LA = left atrium, MPa = main pulmonary artery, PV = pulmonary vein, RA = right atrium, RAA = right atrial appendage, RV = right ventricle.
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Figure 1b. Interventional electrophysiologic approaches. Short-axis (a, d), four-chamber (b), and right ventricular inflow-outflow tract two-chamber (c) CT scans show how the eustachian valve directs inferior vena caval (IVC) blood toward the foramen ovale (FO) and superior vena caval (SVC) blood toward the tricuspid valve. Thus, transseptal cardiac catheterization is easier via the IVC and right ventricular instrumentation via the SVC. Because of the angle of the entrance into the coronary sinus (CS) (yellow arrow in d), cannulation is easier via the SVC. Left-sided ablations are performed with a transseptal approach or, less commonly, a retrograde aortic approach. A coronary sinus approach is used for biventricular pacing and specific left ventricular ablations. LA = left atrium, MPa = main pulmonary artery, PV = pulmonary vein, RA = right atrium, RAA = right atrial appendage, RV = right ventricle.
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Figure 1c. Interventional electrophysiologic approaches. Short-axis (a, d), four-chamber (b), and right ventricular inflow-outflow tract two-chamber (c) CT scans show how the eustachian valve directs inferior vena caval (IVC) blood toward the foramen ovale (FO) and superior vena caval (SVC) blood toward the tricuspid valve. Thus, transseptal cardiac catheterization is easier via the IVC and right ventricular instrumentation via the SVC. Because of the angle of the entrance into the coronary sinus (CS) (yellow arrow in d), cannulation is easier via the SVC. Left-sided ablations are performed with a transseptal approach or, less commonly, a retrograde aortic approach. A coronary sinus approach is used for biventricular pacing and specific left ventricular ablations. LA = left atrium, MPa = main pulmonary artery, PV = pulmonary vein, RA = right atrium, RAA = right atrial appendage, RV = right ventricle.
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Figure 1d. Interventional electrophysiologic approaches. Short-axis (a, d), four-chamber (b), and right ventricular inflow-outflow tract two-chamber (c) CT scans show how the eustachian valve directs inferior vena caval (IVC) blood toward the foramen ovale (FO) and superior vena caval (SVC) blood toward the tricuspid valve. Thus, transseptal cardiac catheterization is easier via the IVC and right ventricular instrumentation via the SVC. Because of the angle of the entrance into the coronary sinus (CS) (yellow arrow in d), cannulation is easier via the SVC. Left-sided ablations are performed with a transseptal approach or, less commonly, a retrograde aortic approach. A coronary sinus approach is used for biventricular pacing and specific left ventricular ablations. LA = left atrium, MPa = main pulmonary artery, PV = pulmonary vein, RA = right atrium, RAA = right atrial appendage, RV = right ventricle.
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Figure 2a. Transseptal intervention. Under fluoroscopic guidance, the foramen ovale is probed, or, if the foramen ovale is not patent, a transseptal puncture is made. (a) Short-axis CT scan obtained at the level of the IVC demonstrates a catheter with its tip at the orifice of the left inferior pulmonary vein (IPV). Transseptal puncture is easier via the IVC. LA = left atrium, RA = right atrium. (b) Right atrial view of a dissected human heart shows transillumination of the thin, membranous oval fossa. (c) Right anterior oblique radiograph shows two catheters positioned for transseptal puncture. The catheters have been introduced from the IVC and positioned over the oval fossa (OF) and along the Koch triangle to obtain a His bundle recording. Note the proximity of the His bundle catheter to the pigtail catheter positioned in the noncoronary sinus. A catheter has been introduced via the SVC into the coronary sinus (CS).
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Figure 2b. Transseptal intervention. Under fluoroscopic guidance, the foramen ovale is probed, or, if the foramen ovale is not patent, a transseptal puncture is made. (a) Short-axis CT scan obtained at the level of the IVC demonstrates a catheter with its tip at the orifice of the left inferior pulmonary vein (IPV). Transseptal puncture is easier via the IVC. LA = left atrium, RA = right atrium. (b) Right atrial view of a dissected human heart shows transillumination of the thin, membranous oval fossa. (c) Right anterior oblique radiograph shows two catheters positioned for transseptal puncture. The catheters have been introduced from the IVC and positioned over the oval fossa (OF) and along the Koch triangle to obtain a His bundle recording. Note the proximity of the His bundle catheter to the pigtail catheter positioned in the noncoronary sinus. A catheter has been introduced via the SVC into the coronary sinus (CS).
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Figure 2c. Transseptal intervention. Under fluoroscopic guidance, the foramen ovale is probed, or, if the foramen ovale is not patent, a transseptal puncture is made. (a) Short-axis CT scan obtained at the level of the IVC demonstrates a catheter with its tip at the orifice of the left inferior pulmonary vein (IPV). Transseptal puncture is easier via the IVC. LA = left atrium, RA = right atrium. (b) Right atrial view of a dissected human heart shows transillumination of the thin, membranous oval fossa. (c) Right anterior oblique radiograph shows two catheters positioned for transseptal puncture. The catheters have been introduced from the IVC and positioned over the oval fossa (OF) and along the Koch triangle to obtain a His bundle recording. Note the proximity of the His bundle catheter to the pigtail catheter positioned in the noncoronary sinus. A catheter has been introduced via the SVC into the coronary sinus (CS).
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Catheter-based Ablation
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Catheter-based ablation has become the standard therapy for many tachyarrhythmias, with its use surpassing that of pharmacologic and surgical methods (1–3,8). Ablation is the intentional destruction of arrhythmogenic myocardial tissue to control or cure arrhythmias. Radiofrequency (RF) current is the most commonly used ablative energy source. The principal method of tissue destruction with RF current is thermal in nature. Typical RF lesions are small, usually 3–6 mm in diameter and up to 3 mm deep when a standard catheter tip is used (1,8). Old RF ablation sites may be smaller than acute lesions.
Other energy sources that can be used include lasers and microwaves.
The success of catheter ablation varies depending on the type of arrhythmia and the clinical situation. High success rates (>90%) and low complication rates (<3%) are seen in ablation procedures for arrhythmias such as atrioventricular (AV) nodal reentry, accessory pathways, atrial flutter, idiopathic ventricular tachycardia, and AV node or junction ablation. Lower success rates (<90%) and higher complication rates (>3%) are seen in ablation procedures for atrial fibrillation (AF) and postinfarction ventricular tachycardia (1,9). Ablation in patients with underlying structural heart disease is usually less successful (10).
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Tachycardias and Anatomic Considerations for Treatment
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The cardiac conduction system consists of the sinoatrial node, the AV node, the His bundle, and the right and left bundle branches, as well as the fascicles and Purkinje fibers (11–14). Tachyarrhythmias are categorized according to the width of the QRS complex at electrocardiography (Fig 3). Narrow QRS complex tachyarrhythmias are generally supraventricular in origin. Supraventricular tachycardias are defined as those in which the atrium, including the AV node and the AV junctional portion, is critical to the perpetuation of the tachycardia. The three types of supraventricular tachycardias are AV node reentrant, AV reentry, and atrial tachycardia. Generally, atrial flutter and AF are considered to be distinct entities. A variety of tachyarrhythmias can be targeted for percutaneous catheter ablation (Fig 4).
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Figure 4. Diagram illustrates anatomic considerations in the treatment of supraventricular and ventricular arrhythmias. A variety of tachyarrhythmias can be targeted for percutaneous catheter ablation, including both atrial and ventricular arrhythmias that are either focal or make use of reentrant circuits. LV = left ventricular, MI = myocardial infarction, RV = right ventricular, VT = ventricular tachycardia.
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Paroxysmal Supraventricular Tachycardia
The most common type of supraventricular tachycardia is AV node reentrant tachycardia, followed by AV reentry tachycardia. Both types are due to a reentrant mechanism and require intact conduction over the AV node; hence the term AV node dependent (15,16). In AV node reentrant tachycardia, the anatomic substrate or abnormality is the presence of dual AV node pathways. The pathways are described as "slow" or "fast" depending on their relative conduction velocities (16). These pathways vary in anatomic location but are generally located within the Koch triangle. Catheter ablation of AV node reentrant tachycardia is performed by targeting the slow pathway. AV reentry tachycardia is a reentrant tachycardia that makes use of the AV node and an accessory pathway (16). The fundamental anatomic abnormality or substrate for AV reentry tachycardia is the presence of a second connection between the atria and ventricles (normally, the AV node is the only connection). This second connection is called an accessory pathway and is the abnormality seen in Wolff-Parkinson-White syndrome. Catheter ablation of both AV node reentrant tachycardia and AV reentry tachycardia is effective in over 95% of patients (17).
Atrial Flutter
The most common type of atrial flutter is isthmus-dependent atrial flutter, in which the reentrant circuit is confined to the tricuspid annulus with the wavefront progressing in either a counterclockwise or clockwise direction across the cavotricuspid isthmus (CTI) between the IVC and the tricuspid annulus (Fig 5) (18). This is a relatively narrow target but can easily be reached with ablation catheters introduced from the IVC. With new catheter techniques, the success rate for ablation of this form of atrial flutter is over 95% (19). Cardiac CT may be helpful in characterizing the CTI, including its size, its depth, and its anatomic relationship with the IVC, eustachian ridge, and coronary sinus ostium. Cardiac CT may also depict the pouches and recesses that are commonly present along the CTI and that sometimes make it difficult to create a complete line of block in the isthmus.
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Figure 5a. (a) Diagram shows the reentrant circuit (arrows) of the common variety of right atrial flutter (type I). The right atrium is shown in a right anterior oblique projection. The reentrant circuit is confined to the right atrium by the annulus of the tricuspid valve (TV) and barriers to conduction within the right atrium, and it circulates in a counterclockwise direction. Yellow hatched area between the tricuspid valve and the IVC indicates the critical isthmus of tissue that is targeted for ablation of this type of atrial flutter. CS = coronary sinus, CT = crista terminalis, ER = eustachian ridge, OF = oval fossa, RAA = right atrial appendage. (b) Endoscopic view of the right atrium demonstrates the spatial relationships between the coronary sinus (CS), IVC, and oval fossa (OF). NCS = noncoronary sinus, TV = tricuspid valve.
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Figure 5b. (a) Diagram shows the reentrant circuit (arrows) of the common variety of right atrial flutter (type I). The right atrium is shown in a right anterior oblique projection. The reentrant circuit is confined to the right atrium by the annulus of the tricuspid valve (TV) and barriers to conduction within the right atrium, and it circulates in a counterclockwise direction. Yellow hatched area between the tricuspid valve and the IVC indicates the critical isthmus of tissue that is targeted for ablation of this type of atrial flutter. CS = coronary sinus, CT = crista terminalis, ER = eustachian ridge, OF = oval fossa, RAA = right atrial appendage. (b) Endoscopic view of the right atrium demonstrates the spatial relationships between the coronary sinus (CS), IVC, and oval fossa (OF). NCS = noncoronary sinus, TV = tricuspid valve.
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Atrial Fibrillation
AF is a common rhythmic disturbance that is increasingly being recognized in the aging population. AF is now believed to be due to multiple reentrant wavelets or to a rapidly firing focus in the pulmonary vein (20,21). Ectopic activity of pulmonary vein sleeves has been identified as a major cause of paroxysmal AF in patients with structurally normal hearts (20,22). Typically, access to the pulmonary veins and left atrium is gained via a patent foramen ovale or, if the foramen ovale is not patent, by means of transseptal puncture (Fig 2). Pulmonary vein isolation procedures or circumferential left atrial ablation is increasingly being performed to cure AF. Circumferential left atrial ablation involves the creation of circumferential ablation lines around the left and right pulmonary veins 1–2 cm from the ostia of the veins. Ablation lines in the posterior left atrium and mitral isthmus are often created as well (Fig 6). The ablation success rate is about 80% for paroxysmal AF and 70% for chronic AF (21). Although the majority of foci triggering AF have been mapped to the pulmonary veins, AF may also be triggered by arrhythmogenic foci originating from the right atrium, left atrium, coronary sinus, SVC, or vein of Marshall (22–25). Preprocedural magnetic resonance (MR) imaging or CT allows excellent anatomic characterization of the veins that may help plan the procedure. At present, catheter ablation of AF is by far the most common electrophysiologic indication for cardiac multidetector CT. Postprocedural imaging can also be used to monitor the development of complications such as pulmonary vein stenosis (26,27).
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Figure 6a. AF ablation. LIPV = left inferior pulmonary vein, LSPV = left superior pulmonary vein. (a) Endoscopic image shows how two catheters are introduced into the left atrium through a transseptal puncture, including a deflectable circular mapping catheter (blue) and a deflectable ablation catheter (yellow). LAA = left atrial appendage, MV = mitral valve. (b) Posterior three-dimensional (3D) image of the left atrium and pulmonary veins demonstrates circumferential pulmonary vein ablation. Circumferential ablation lines (red) around two pulmonary vein lesions are connected by a roof ablation line (green). A mitral isthmus ablation line (blue) was created between the left inferior pulmonary vein and the lateral mitral annulus (arrows). AAo = ascending aorta, LA = left atrium, LCx = left circumflex artery, RIPV = right inferior pulmonary vein, RSPV = right superior pulmonary vein.
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Figure 6b. AF ablation. LIPV = left inferior pulmonary vein, LSPV = left superior pulmonary vein. (a) Endoscopic image shows how two catheters are introduced into the left atrium through a transseptal puncture, including a deflectable circular mapping catheter (blue) and a deflectable ablation catheter (yellow). LAA = left atrial appendage, MV = mitral valve. (b) Posterior three-dimensional (3D) image of the left atrium and pulmonary veins demonstrates circumferential pulmonary vein ablation. Circumferential ablation lines (red) around two pulmonary vein lesions are connected by a roof ablation line (green). A mitral isthmus ablation line (blue) was created between the left inferior pulmonary vein and the lateral mitral annulus (arrows). AAo = ascending aorta, LA = left atrium, LCx = left circumflex artery, RIPV = right inferior pulmonary vein, RSPV = right superior pulmonary vein.
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Anatomic Landmarks of the Right Atrium
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On 3D images of the heart obtained from above, the right atrium is positioned to the right and anterior, whereas the left atrium is situated to the left and mainly posterior (Fig 7a). The right atrium consists of three components: the appendage, the venous part, and the vestibule (11). A prominent muscular ridge known as the crista terminalis separates the smooth-walled venous part (sinus venosus) from the appendage. The vestibule is a smooth muscular rim that surrounds the tricuspid orifice. The terminal groove, or sulcus terminalis, is a fat-filled groove on the epicardial side and corresponds internally to the crista terminalis (Fig 7c). The sinus node and the terminal segment of the sinoatrial nodal artery are located in this groove, close to the cavoatrial junction.
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Figure 7a. Atrial epicardial views. AAo = ascending aorta, CS = coronary sinus, LA = left atrium, LIPV = left inferior pulmonary vein, LSPV = left superior pulmonary vein, LV = left ventricle, RAA = right atrial appendage, RIPV = right inferior pulmonary vein, RSPV = right superior pulmonary vein, RV = right ventricle. (a) Three-dimensional image (superior view) shows the cavity of the right atrium to the right and anterior, whereas the left atrium is to the left and mainly posterior. (b) Three-dimensional image (posterior view) shows the anatomic boundaries of the sinus venosus of the right atrium (shaded area). (c) On a right lateral 3D image, the dominant feature is the large, triangular right atrial appendage. The terminal groove (TG) (arrows) lies between the sinus venosus (SV) and the right atrial appendage. Note the sinoatrial nodal artery (SANa) coursing in this groove. (d) Left lateral 3D image shows the LAA as a small lobulated structure. Because of its trabeculated margin and narrow neck (arrows), the LAA is a potential site for the deposition of thrombus.
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Figure 7b. Atrial epicardial views. AAo = ascending aorta, CS = coronary sinus, LA = left atrium, LIPV = left inferior pulmonary vein, LSPV = left superior pulmonary vein, LV = left ventricle, RAA = right atrial appendage, RIPV = right inferior pulmonary vein, RSPV = right superior pulmonary vein, RV = right ventricle. (a) Three-dimensional image (superior view) shows the cavity of the right atrium to the right and anterior, whereas the left atrium is to the left and mainly posterior. (b) Three-dimensional image (posterior view) shows the anatomic boundaries of the sinus venosus of the right atrium (shaded area). (c) On a right lateral 3D image, the dominant feature is the large, triangular right atrial appendage. The terminal groove (TG) (arrows) lies between the sinus venosus (SV) and the right atrial appendage. Note the sinoatrial nodal artery (SANa) coursing in this groove. (d) Left lateral 3D image shows the LAA as a small lobulated structure. Because of its trabeculated margin and narrow neck (arrows), the LAA is a potential site for the deposition of thrombus.
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Figure 7c. Atrial epicardial views. AAo = ascending aorta, CS = coronary sinus, LA = left atrium, LIPV = left inferior pulmonary vein, LSPV = left superior pulmonary vein, LV = left ventricle, RAA = right atrial appendage, RIPV = right inferior pulmonary vein, RSPV = right superior pulmonary vein, RV = right ventricle. (a) Three-dimensional image (superior view) shows the cavity of the right atrium to the right and anterior, whereas the left atrium is to the left and mainly posterior. (b) Three-dimensional image (posterior view) shows the anatomic boundaries of the sinus venosus of the right atrium (shaded area). (c) On a right lateral 3D image, the dominant feature is the large, triangular right atrial appendage. The terminal groove (TG) (arrows) lies between the sinus venosus (SV) and the right atrial appendage. Note the sinoatrial nodal artery (SANa) coursing in this groove. (d) Left lateral 3D image shows the LAA as a small lobulated structure. Because of its trabeculated margin and narrow neck (arrows), the LAA is a potential site for the deposition of thrombus.
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Figure 7d. Atrial epicardial views. AAo = ascending aorta, CS = coronary sinus, LA = left atrium, LIPV = left inferior pulmonary vein, LSPV = left superior pulmonary vein, LV = left ventricle, RAA = right atrial appendage, RIPV = right inferior pulmonary vein, RSPV = right superior pulmonary vein, RV = right ventricle. (a) Three-dimensional image (superior view) shows the cavity of the right atrium to the right and anterior, whereas the left atrium is to the left and mainly posterior. (b) Three-dimensional image (posterior view) shows the anatomic boundaries of the sinus venosus of the right atrium (shaded area). (c) On a right lateral 3D image, the dominant feature is the large, triangular right atrial appendage. The terminal groove (TG) (arrows) lies between the sinus venosus (SV) and the right atrial appendage. Note the sinoatrial nodal artery (SANa) coursing in this groove. (d) Left lateral 3D image shows the LAA as a small lobulated structure. Because of its trabeculated margin and narrow neck (arrows), the LAA is a potential site for the deposition of thrombus.
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Sinus Venosus
The sinus venosus is located mainly in the posterolateral wall of the right atrium between the orifices of the SVC and IVC (Fig 7b). From the 3rd week after the development of the primitive heart tube, the primitive atria are separated from the sinus venosus by a structure known as the sinoatrial ring (28,29). The sinus venosus has two horns. The right horn gives rise to all the intercaval regions of the right atrium, including the crista terminalis, eustachian ridge, and thebesian valve. The left horn gives rise to the coronary sinus.
Crista Terminalis
The crista terminalis is a fibromuscular ridge formed by the junction of the sinus venosus and the primitive right atrium (10,11). Superiorly, it arches anterior to the orifice of the SVC, extends to the area of the anterior interatrial groove, and merges with the interatrial bundle, commonly known as the Bachmann bundle (Fig 8a). The inferior border of the crista terminalis near the IVC orifice is indistinct and merges with small trabeculations of the inferior portion of the CTI (30). The crista terminalis gives rise to a series of relatively thick bundles, the anterior pectinate muscles, which fan out anteriorly. The "septum spurium" is the most prominent anterior pectinate muscle arising from the crista terminalis (Fig 8c). The septum spurium is prominent in 80% of hearts, has a mean thickness of 4.5 mm, and should not be mistaken for intraatrial disease (30). The crista terminalis varies in size and extent among individuals (Fig 9a–9d). A large crista terminalis due to fatty infiltration has been reported in lipomatous hypertrophy of the atrial septum and may mimic a mass (31). Awareness of the variability in the size and extent of the crista terminalis and familiarity with the normal appearance of a prominent crista terminalis will minimize misdiagnosis of this structure.
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Figure 8a. Crista terminalis. Axial (a) and short-axis (b, c) CT scans show how the crista terminalis (CT) (red arrows) extends from the SVC to the IVC. The origin of the crest at the interatrial groove is confluent with the Bachmann bundle (BB) (yellow arrows in a). The septum spurium (SS) (blue arrows in c) is the most prominent anterior pectinate muscle arising from the crista terminalis. AAo = ascending aorta, LA = left atrium, RA = right atrium.
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Figure 8b. Crista terminalis. Axial (a) and short-axis (b, c) CT scans show how the crista terminalis (CT) (red arrows) extends from the SVC to the IVC. The origin of the crest at the interatrial groove is confluent with the Bachmann bundle (BB) (yellow arrows in a). The septum spurium (SS) (blue arrows in c) is the most prominent anterior pectinate muscle arising from the crista terminalis. AAo = ascending aorta, LA = left atrium, RA = right atrium.
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Figure 8c. Crista terminalis. Axial (a) and short-axis (b, c) CT scans show how the crista terminalis (CT) (red arrows) extends from the SVC to the IVC. The origin of the crest at the interatrial groove is confluent with the Bachmann bundle (BB) (yellow arrows in a). The septum spurium (SS) (blue arrows in c) is the most prominent anterior pectinate muscle arising from the crista terminalis. AAo = ascending aorta, LA = left atrium, RA = right atrium.
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Figure 9a. (a–d) CT scans show how the crista terminalis (arrowhead) varies in size and thickness in different individuals, appearing as a small (a), thin (b), valvelike (c), or broad-based (d) structure. (e–h) CT scans show differing amounts of fat infiltration of the crista terminalis (arrowhead) and varying degrees of lipomatous hypertrophy of the septum (double-headed arrow in f–h). A large crista terminalis can mimic a mass at echo studies.
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Figure 9b. (a–d) CT scans show how the crista terminalis (arrowhead) varies in size and thickness in different individuals, appearing as a small (a), thin (b), valvelike (c), or broad-based (d) structure. (e–h) CT scans show differing amounts of fat infiltration of the crista terminalis (arrowhead) and varying degrees of lipomatous hypertrophy of the septum (double-headed arrow in f–h). A large crista terminalis can mimic a mass at echo studies.
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Figure 9c. (a–d) CT scans show how the crista terminalis (arrowhead) varies in size and thickness in different individuals, appearing as a small (a), thin (b), valvelike (c), or broad-based (d) structure. (e–h) CT scans show differing amounts of fat infiltration of the crista terminalis (arrowhead) and varying degrees of lipomatous hypertrophy of the septum (double-headed arrow in f–h). A large crista terminalis can mimic a mass at echo studies.
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Figure 9d. (a–d) CT scans show how the crista terminalis (arrowhead) varies in size and thickness in different individuals, appearing as a small (a), thin (b), valvelike (c), or broad-based (d) structure. (e–h) CT scans show differing amounts of fat infiltration of the crista terminalis (arrowhead) and varying degrees of lipomatous hypertrophy of the septum (double-headed arrow in f–h). A large crista terminalis can mimic a mass at echo studies.
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Figure 9e. (a–d) CT scans show how the crista terminalis (arrowhead) varies in size and thickness in different individuals, appearing as a small (a), thin (b), valvelike (c), or broad-based (d) structure. (e–h) CT scans show differing amounts of fat infiltration of the crista terminalis (arrowhead) and varying degrees of lipomatous hypertrophy of the septum (double-headed arrow in f–h). A large crista terminalis can mimic a mass at echo studies.
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Figure 9f. (a–d) CT scans show how the crista terminalis (arrowhead) varies in size and thickness in different individuals, appearing as a small (a), thin (b), valvelike (c), or broad-based (d) structure. (e–h) CT scans show differing amounts of fat infiltration of the crista terminalis (arrowhead) and varying degrees of lipomatous hypertrophy of the septum (double-headed arrow in f–h). A large crista terminalis can mimic a mass at echo studies.
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Figure 9g. (a–d) CT scans show how the crista terminalis (arrowhead) varies in size and thickness in different individuals, appearing as a small (a), thin (b), valvelike (c), or broad-based (d) structure. (e–h) CT scans show differing amounts of fat infiltration of the crista terminalis (arrowhead) and varying degrees of lipomatous hypertrophy of the septum (double-headed arrow in f–h). A large crista terminalis can mimic a mass at echo studies.
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Figure 9h. (a–d) CT scans show how the crista terminalis (arrowhead) varies in size and thickness in different individuals, appearing as a small (a), thin (b), valvelike (c), or broad-based (d) structure. (e–h) CT scans show differing amounts of fat infiltration of the crista terminalis (arrowhead) and varying degrees of lipomatous hypertrophy of the septum (double-headed arrow in f–h). A large crista terminalis can mimic a mass at echo studies.
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The crista terminalis is a significant structure in several forms of atrial tachyarrhythmias and occasionally is the target for catheter RF ablation. A study by Mizumaki et al (32) suggests that thickening of the crista terminalis may lead to the development of typical atrial flutter. Intracardiac echocardiographic studies have shown that two-thirds of focal right atrial tachycardias seen in the absence of structural heart disease arise along the crista terminalis (33).
Sinoatrial Node and Sinoatrial Nodal Artery
The sinoatrial node is the source of the cardiac impulse. It is composed histologically of cells that are slightly smaller than normal working cells (34). The sinoatrial node is a subepicardial spindle-shaped structure at the superior cavoatrial junction that extends from the SVC along the crista terminalis toward the IVC (30,34,35). It penetrates the musculature of the crista terminalis to lie in the subendocardium and is best seen on axial images (Fig 10a). The sinoatrial node varies in position and length (mean, 20 ± 3 mm) (28). Because of its proximity to the epicardial surface, it may be damaged at selected cardiac surgeries or by extensive pericardial diseases (36). The sinoatrial node surrounds the sinoatrial nodal artery, which can course centrally (70% of cases) or eccentrically within the node (30).
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Figure 10a. Sinoatrial nodal artery and its related anatomy. AAo = ascending aorta, LA = left atrium, PA = pulmonary artery, RA = right atrium, RAA = right atrial appendage, RCA = right coronary artery. (a) Axial (top) and short-axis (right) CT scans obtained at the superior cavoatrial junction show the sinoatrial nodal artery (arrows) arising and coursing anterior to the SVC, with its terminal portion coursing in the myocardial tissue of the crista terminalis (CT). The sinoatrial node is arranged around the artery, a finding that is seen in 75% of cases. Colored arrows indicate corresponding areas. (b, c) Axial coronary CT angiographic images show that the sinoatrial nodal artery (arrows) can arise from the right coronary (b) or left circumflex (LCx) (c) artery.
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Figure 10b. Sinoatrial nodal artery and its related anatomy. AAo = ascending aorta, LA = left atrium, PA = pulmonary artery, RA = right atrium, RAA = right atrial appendage, RCA = right coronary artery. (a) Axial (top) and short-axis (right) CT scans obtained at the superior cavoatrial junction show the sinoatrial nodal artery (arrows) arising and coursing anterior to the SVC, with its terminal portion coursing in the myocardial tissue of the crista terminalis (CT). The sinoatrial node is arranged around the artery, a finding that is seen in 75% of cases. Colored arrows indicate corresponding areas. (b, c) Axial coronary CT angiographic images show that the sinoatrial nodal artery (arrows) can arise from the right coronary (b) or left circumflex (LCx) (c) artery.
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Figure 10c. Sinoatrial nodal artery and its related anatomy. AAo = ascending aorta, LA = left atrium, PA = pulmonary artery, RA = right atrium, RAA = right atrial appendage, RCA = right coronary artery. (a) Axial (top) and short-axis (right) CT scans obtained at the superior cavoatrial junction show the sinoatrial nodal artery (arrows) arising and coursing anterior to the SVC, with its terminal portion coursing in the myocardial tissue of the crista terminalis (CT). The sinoatrial node is arranged around the artery, a finding that is seen in 75% of cases. Colored arrows indicate corresponding areas. (b, c) Axial coronary CT angiographic images show that the sinoatrial nodal artery (arrows) can arise from the right coronary (b) or left circumflex (LCx) (c) artery.
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Modification of the sinoatrial node with an RF ablation catheter has been established as a treatment for inappropriate sinus tachycardia (37). Because of the subepicardial location of the sinoatrial node, approaching from the endocardial surface requires more RF energy to ablate the node (30). Multidetector CT can be used to measure the thickness of the crista terminalis and demonstrate the approximate location of the sinoatrial nodal artery within the nodal tissue. A centrally located sinoatrial nodal artery may provide a cooling effect, reducing the extent of RF damage (30,38). Higher RF energy may not only injure the myocardium of the crista terminalis itself, but can also induce SVC stricture (39). RF ablation of the sinoatrial node is more effective with little risk if the ablation catheter is positioned at the site of the crista terminalis halfway between the caval veins, where the tail of the sinoatrial node is subendocardial (30). This distance can be measured with multidetector CT.
The sinoatrial nodal artery is usually a single branch that arises from the proximal right coronary artery (60% of cases) or LCx artery (40%) (Fig 10b, 10c) (40,41). Regardless of its artery of origin, the sinoatrial nodal artery usually courses along the anterior interatrial groove toward the superior cavoatrial junction. At the cavoatrial junction, the course of the sinoatrial nodal artery becomes variable, with the artery circling either anteriorly (precaval) or posteriorly (retrocaval) to enter the node.
Koch Triangle
Another area of significance to the electrophysiologist is the Koch triangle. The Koch triangle lies in the right atrium at the orifice of the coronary sinus. It is bordered posteriorly by a fibrous extension from the eustachian valve called the tendon of Todaro (42). The anterior border is demarcated by the attachment of the septal leaflet of the tricuspid valve. The apex of the Koch triangle corresponds to the central fibrous body of the heart, demarcating the site of penetration of the His bundle. The midportion of the triangle contains the compact AV node (fast pathway), and the base contains the slow pathway. 
The base of the triangle is bordered by the coronary sinus ostium and the "septal isthmus" (the area between the edge of the coronary sinus ostium and the attachment of the septal tricuspid valve) immediately anterior to it (Fig 11). The septal isthmus is often the target for ablation of the slow pathway in AV node reentrant tachycardia (43). The dimensions of the Koch triangle vary from one individual to another, a fact that is clinically relevant in catheter ablation procedures in this area, which are guided largely by anatomic landmarks. Multidetector CT can depict the boundaries of the triangle and its relationship to adjacent structures.
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Figure 11a. Localization of the Koch triangle. RA = right atrium, RV = right ventricle. (a) Endocardial view of the right AV junction shows the Koch triangle and the right atrial isthmus. The Koch triangle is demarcated by the tendon of Todaro–eustachian ridge (ER) posteriorly (white arrows), attachment of the septal tricuspid valve anteriorly (yellow arrows), coronary sinus (CS) inferiorly, and central fibrous body (CFB) at the apex (red arrow). The septal isthmus (small bracket), the area between the coronary sinus and septal tricuspid valve, is the target for ablation of AV node reentrant tachycardia. The CTI (large bracket) lies between the IVC orifice and the tricuspid valve. A = anterior, I = inferior, P = posterior, S = superior. (b–d) Coronal (b), sagittal (c), and axial (d) CT scans show the Koch triangle (yellow lines) from different perspectives. Arrow in d indicates the central fibrous body at the triangle apex.
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Abstract
LEARNING OBJECTIVES FOR TEST...
