The Coronary Venous Anatomy a Segmental Approach to Aid Cardiac Resynchronization Therapy Jagmeet P

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provided by Elsevier - Publisher Connector Journal of the American College of Cardiology Vol. 46, No. 1, 2005 © 2005 by the American College of Cardiology Foundation ISSN 0735-1097/05/$30.00 Published by Elsevier Inc. doi:10.1016/j.jacc.2005.04.017 Viewpoint The Coronary Venous Anatomy A Segmental Approach to Aid Cardiac Resynchronization Therapy Jagmeet P. Singh, MD, PHD,* Stuart Houser, MD,† E. Kevin Heist, MD, PHD,* Jeremy N. Ruskin, MD* Boston, Massachusetts

The coronary sinus is the gateway for left ventricular (LV) epicardial lead placement for cardiac resynchronization therapy. The implanting electrophysiologist is usually challenged by a high degree of variability in the coronary venous anatomy, making it important to have a more consistent and uniform segmental approach to describe the coronary venous tree and its branches. Classifying the coronary sinus branches and tributaries by the segment of their location rather than by conventional anatomic names (i.e., middle cardiac vein, great cardiac vein, and so on), would provide more relevant anatomic and functional information at the time of LV lead placement. This would enable the implanting physician to proactively correlate the venous anatomy with the segmental wall motion abnormalities or dyssynchrony, as deﬁned by echocardiography and other imaging modalities. The current viewpoint calls for a more systematic segmental approach for describing the coronary venous anatomy. (J Am Coll Cardiol 2005;46:68–74) © 2005 by the American College of Cardiology Foundation

The cardiac venous system, which has always been simplistic and has been mostly directed at placement of overshadowed by the proximate presence of the coronary the lead along the lateral wall of the LV (10). Recent data arterial tree, has recently begun to attract more attention. have suggested that mechanical asynchrony is variable Its role in invasive cardiology has been directed at either and that a simplistic approach may not always provide the targeted drug delivery (1) or retrograde cardioplegia maximal hemodynamic benefit. The optimal site for LV administration (2). Of late, there has been increasing lead implantation may vary depending on the region interest in the role of the cardiac venous system, toward and/or extent of dyssynchrony (11,12). Transvenous LV providing a potential conduit to bypass coronary artery lead placement is dependent on the availability of a vein, stenosis (3) and to delivery of stem cells to infarcted and because of the variable coronary venous anatomy, myocardium (4). there may not always be a suitable major vein in the In the arena of electrophysiology, the cardiac venous region of interest that can accommodate a pacing lead system has always been of strategic interest. Coronary with acceptable pacing parameters. sinus cannulation has allowed access to the left atrial and Conventional coronary venous anatomy has primarily left ventricular (LV) epicardium, enabling a spectrum of focused on the site of origin of various venous branches diagnostic and mapping maneuvers to aid in the deter- from the main body of the coronary sinus rather than the mination of the type of arrhythmia as well as permit the delivery of ablative energy (5,6). More recently, the particular segment of myocardium that a particular vein coronary sinus has become the gateway to LV epicardial or venous tributary overlies. Although this conventional lead placement to achieve biventricular pacing. The anatomic classification remains important, the great vari- recent surge in the number of implanted biventricular ability in the course of coronary veins and tributaries pacemakers and defibrillators stems from numerous stud- makes it difficult to directly correlate conventional coro- ies that have clearly shown the significant benefit pro- nary venous anatomy with specific regions of the under- vided by cardiac resynchronization therapy in patients lying LV. with congestive heart failure (7,8). To achieve this In the present review of the coronary venous anatomy and therapeutic goal, it is critical that the LV epicardial lead its variability, this report highlights the need for a segmental be positioned appropriately in the region with delayed classification to map the coronary veins and tributaries in electrical activation and mechanical dyssynchrony (9). So relation to the LV wall in a manner comparable to that of far, the approach for lead positioning has been rather echocardiography and LV angiography. This stresses the need for an organized practical approach to coronary sinus From the *Cardiac Arrhythmia Service and the †Cardiac Pathology Service, angiography (similar to that of coronary arteriography), Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts. with careful attention being paid to the tributaries of main This work was funded in part by the Clinical Investigator Training Program Award (supported by Harvard University and Massachusetts Institute of Technology). venous branches and emphasis put on their course and Potential conflicts of interest: Dr. Ruskin (Medtronics, Inc., consultant/advisor); Dr. dimensions to facilitate LV lead placement. This classifica- Singh (Guidant Corporation, consultant/advisor). Manuscript received December 14, 2004; revised manuscript received March 23, tion system attempts not to supplant but rather to add 2005, accepted April 5, 2005. information to the conventional coronary venous anatomic JACC Vol. 46, No. 1, 2005 Singh et al. 69 July 5, 2005:68–74 Coronary Venous Anatomy

course in relation to the lateral wall of the heart and drain Abbreviations and Acronyms either into the great cardiac vein (81%) or into the coronary CT ϭ computed tomography sinus (19%), this may not be definitive (20). Implantation of LV ϭ left ventricle/ventricular the coronary sinus lead usually involves the lateral and posterior branches, which are quite variable in their number, tortuosity, dimensions, and angulation with respect to the nomenclature, which is of particular relevance to the inva- main trunk of the atrioventricular venous ring (16). sive cardiologist and electrophysiologist. The coronary sinus is the most constant feature of the CONVENTIONAL CORONARY VENOUS ANATOMY cardiac venous system, although several congenital anomalies have been described (21). There is also a high degree of Based on the region being drained, cardiac veins can be variability in the number of branches between the middle grouped into the following: 1) the coronary sinus and its cardiac vein and the anterior interventricular vein. Other tributaries, which return blood from almost the whole heart; variable features of the coronary venous anatomy include the 2) the anterior cardiac veins, which primarily drain the presence of ostial valves of the cardiac veins (Vieussens anterior regions of the right ventricle and the right cardiac valves), inter-branch collateralization, and intramural versus border; and 3) the thebesian veins (venae cordis minimae), epicardial course, all of which may significantly impact which open directly into any of the four chambers. Al- selection and cannulation of a cardiac vein, as well as the though the coronary sinus invariably lies in the atrioventric- stability and pacing threshold of an implanted lead. ular groove, its branches and their locations are far more This conventional anatomic classification of the coronary variable than the those of the coronary arterial system venous tree is limited by the large variability in its branches (13,14). The coronary sinus opens into the right atrium and their course across the epicardial surface of the heart. posteromedially, with its opening being guarded by the Lately, because of the increased need to identify LV veins highly variable thebesian valve, which can hinder cannula- for lead implantation, there has been a resurgence of interest tion of the coronary sinus os (15). The coronary venous in the anatomic variations of the cardiac venous circulation. system can be considered as first-order tributaries originating from the main coronary sinus (i.e., the small, great, posterior, and middle cardiac veins), which then branch into second- and third-order tributaries (Fig. 1). The anterior interventricular vein ascends in the anterior interventricular sulcus (parallel to the left anterior descending coronary artery) from the apex toward the base of the heart and ends in the great cardiac vein. It then turns laterally at the base of the heart along the left atrioventricular groove (parallel to the left circumflex coronary artery) and wraps around the left side of the heart, going posterior to merge with the coronary sinus. In addition to several smaller tributaries from the left atrium and ventricles, the great cardiac vein receives two main branches, namely the large left marginal vein, along the lateral border of the heart, and the posterior LV branch (also known as the posterolateral branch). The great cardiac vein terminates in the coronary sinus, a junction defined by the presence of the left atrial oblique vein. This transition point is usually marked by the presence of intravenous valves, which can obstruct catheter and lead placement. Another important branch is the middle cardiac vein, which runs in the posterior interventricular grove, parallel to the posterior descending coronary artery. Of all of the branches of the coronary venous system, the Figure 1. Coronary sinus angiogram of a cadaveric human heart. This heart is splayed out in an anterior view to show the entire length of the great cardiac and middle cardiac vein are the two most atrioventricular venous ring on the left side of the heart. The dotted lines consistently present branches (16). Unlike the middle car- depict the near-equal segmental divisions along the horizontal (short) axis. diac vein, the great cardiac vein varies considerably in its The horizontal line extends from the coronary sinus ostium (origin of MV) to the point where the AV terminates into the great cardiac vein. course (17,18). Lateral and posterior venous branches to- Each of the segments are defined as follows: A ϭ anterior segment; B ϭ gether are seen in Ͻ50% of human hearts, unlike the lateral segment; C ϭ posterior segment. The segment refers to the region anterior interventricular and middle cardiac veins, which are under the respective letters A, B, and C. Conventional branches are labeled as follows: AV ϭ anterior interventricular vein; LV ϭ lateral marginal vein; seen in more than 90% (19). Although Ortale et al. (13) PV ϭ posterior cardiac vein; MV ϭ middle cardiac vein. This figure have indicated that the left marginal vein may vary in its highlights the second- and third-order tributaries and collateral circulation. 70 Singh et al. JACC Vol. 46, No. 1, 2005 Coronary Venous Anatomy July 5, 2005:68–74

system reflect the entire zone that may be approached by LV leads implanted via the coronary sinus, but excludes the ventricular septum. It is bordered superiorly by the main body of the coronary sinus, anteriorly by the anterior interventricular vein, posteriorly by the middle cardiac vein, and inferiorly by the apex of the LV. According to this classification system, the anterior interventricular vein will always lie in an anterior position and the middle cardiac vein will always lie in a posterior position, with both major veins coursing from base to apex. Although the middle cardiac vein typically originates from the coronary sinus, occasionally it has a separate ostium. Various branches originating from the main body of the coronary sinus can be classified as anterior, lateral, or posterior according to their site of origin. The course of the tributaries can be further described as coursing medially or laterally, away from main branches in the anterior and posterior segments or anteriorly or posteriorly from main branches in the lateral wall segment (Figs. 1 and 3). Importantly, this approach identifies the course of these veins and their tributaries across the LV epicardial surface. Figure 2. Coronary sinus angiogram of a cadaveric human heart. This is a In this way, a particular branch can be described both by vertical-axis (longitudinal) look at the heart. The dotted lines depict the conventional nomenclature (i.e., a branch from the middle segmentation of the heart into three equal parts, i.e., the basal, mid, and cardiac vein . . .) and by segmental classification (i.e., . . . a apical regions. This is a lateral view showing the area between the lateral marginal vein (LV) and the anterior interventricular vein (AV). second-order tributary of the posterior segmental branch that courses across the lateral region of the LV at the Previous attempts to categorize different patterns have ascribed more importance to variations in the direction of drainage rather than the presence or absence of the above- mentioned tributaries and their branches. What is most important for the invasive electrophysiologist is a classification system that is simple and is directed primarily at identifying the LV site underlying a particular vein or tributary. SEGMENTAL CLASSIFICATION Classifying the regions of drainage of the coronary venous system in a segmental manner will appropriately help to identify and match the location of the left-sided cardiac venous tree to the underlying LV myocardial segment. This correlation is important from the electrophysiological aspect because it will allow a more precise positioning of the LV lead for enhancing cardiac synchrony. This method of classification separates the left-sided cardiac veins according to a horizontal (short) axis, a vertical (longitudinal) axis, and the course of the tributaries. Along the horizontal (short) axis, the atrioventricular venous ring from the coronary sinus ostium to the anterior interventricular groove can be divided into three equal segments: anterior, lateral, and posterior (Fig. 1). Subclas- sification along the longitudinal axis (from base to apex) Figure 3. Coronary sinus angiogram in a patient with ischemic cardiomy- refers to the origin of second-order tributaries and also opathy (right anterior oblique 30° view). This angiogram highlights lateral incorporates three zones: basal, mid, and apical (Fig. 2). segmental branch and the multiple second-order tributaries originating in This classification creates nine segments of the LV in a 3 ϫ the basal (B), mid (M), and apical (A) regions, coursing in the anterior (apical branch) and posterior direction (i.e., branches from the basal and 3 grid, which is comparable to that described by other LV mid-region). LV ϭ lateral marginal vein; GV ϭ great cardiac vein; AV ϭ imaging modalities. The regions of the LV described by this anterior interventricular vein. JACC Vol. 46, No. 1, 2005 Singh et al. 71 July 5, 2005:68–74 Coronary Venous Anatomy mid-ventricular level). These two pieces of information are The transverse diameter of the epicardial veins remains complementary; they both describe how to access a region constant during systole and diastole, with flow occurring (i.e., via the middle cardiac vein) and describe the region in as a bolus during each cardiac cycle. Therefore, uniform relation to the LV epicardium (i.e., the mid-lateral LV). opacification of the entire cardiac venous system includ- Classification in this way will provide maximal information ing tributaries is difficult with a single bolus of contrast to the invasive cardiologist and electrophysiologist attempt- delivered at the proximal end (22). To address this ing to use the coronary venous system to access specific problem, better catheterization and more systematic an- regions of the LV. This is also advantageous especially in giographic techniques are currently being developed. circumstances in which there is no conventionally named Newer investigational angiographic techniques with a major venous branch in the region of interest. The segmen- double balloon occlusive approach can specifically isolate tal approach is also of value when trying to cross-reference the branch and tributaries of interest in a particular the venous anatomy to the LV segment with maximal segment (Fig. 4). These branches play a key role in dyssynchrony. This approach also emphasizes the impor- positioning the guidewires for support and LV lead for tance of recognizing the presence of second- and third- pacing during the implantation procedure. Furthermore, order branches, which may course over the lateral wall and because there may be no first-order lateral segment serve as a good site for lead placement, especially in the branches, it is important to recognize second-order trib- absence of a first-order lateral branch. utaries from the anterior and posterior segment branches, which may course over the desired segment of interest. CORONARY VENOUS IMAGING As seen in Figure 3, the second-order tributaries originating in the basal, mid, and apical third of the lateral Retrograde venography via conventional balloon occlusion segmental branch course both anteriorly and posteriorly, angiography has enabled the delineation of the number and providing several sites for lodging the lead. Notably, the caliber of the large venous branches (16). This technique dimensions of the coronary sinus and its tributaries are requires central venous access and cannulation of the coro- directly affected by the LV end-diastolic pressure and nary sinus. Unlike anterograde coronary arteriography, cor- may differ considerably between patients with cardiomy- onary venography is often limited by the vigorous backwash opathies that differ in severity and duration. of the injected contrast, which impairs the ability to define the detailed anatomy of the tributaries and collateral circulation. Cardiac venous angiography, like coronary arteriography, must be systematic with appropriate fluoroscopic angulations to clearly define the second- and third-order tributaries for coronary sinus lead implantation. Although the posterior and lateral branches are usually first choices for lead placement, their presence, position, and size is variable. Therefore, it is also important to define second-order tributaries from the anterior and posterior segmental veins (inclusive of the anterior interventricular vein and middle cardiac vein), which cross laterally over the myocardial segment of interest. This segmental approach also calls for a more systematic approach to cardiac venous angiography. Coronary venous angiography performed in at least two different views with the necessary caudal or cranial angulations to separate the branches and display the course of the main branches is key to the segmental approach. Patients receiving cardiac resynchronization therapy devices have an underlying cardiomyopathy and may have had extensive remodeling and rotation of the heart. Importantly, the interventricular septum can be consistently profiled by delineating the middle cardiac vein (posteriorly) and the anterior interventricular vein (anteriorly) branches. Using these as reference veins, the lateral wall venous branches can be profiled by individualizing the different radiologic views (with craniocaudal angulations). Sub-selective angiography Figure 4. Volume computed tomography image of the coronary veins in a of the main branches is often important to clearly define the cadaveric heart. Double balloon occlusive venography was performed to isolate the lateral segment of the coronary venous tree. The lateral vein (LV) and its pattern of distribution of the second- and third-order tributaries can be seen in great detail. 1 ϭ the LV, which is the first-order tributaries. tributary; 2 ϭ second-order tributary; 3 ϭ third-order tributary. 72 Singh et al. JACC Vol. 46, No. 1, 2005 Coronary Venous Anatomy July 5, 2005:68–74

Recent developments in rotational contrast venography sinus, its main branches, and the second-order tributar- (Allura FD 10) offer a dynamic multi-angle visualization ies. This information should prove useful both to deﬁne of the coronary venous tree. The coronary sinus angio- an individual patient’s coronary venous anatomy and to grams can be obtained using a 4-s isocentric rotation of tailor an LV lead implant strategy that targets a myocar- the imaging camera over a 110° arc. The rotational dial segment of interest. images can then be reviewed over a full range of angles, Electron beam computed tomography (CT) and multi- providing the operator with considerably more informa- detector row CT enable three-dimensional reconstruction tion about the coronary venous tree and its branches than of tomographic images of the beating heart and provide a standard images. Also, three-dimensional models of the detailed, minimally invasive deﬁnition of the coronary venous tree can be reconstructed (Figs. 5A and 5B) to venous anatomy (19,23). Unlike retrograde venography, CT give more information regarding the size of the coronary angiography allows for simultaneous imaging of the coro-

Figure 5. Rotational venous angiogram, with still frames in anteroposterior (A) and left anterior oblique 30° (B). PV ϭ posterior vein; MV ϭ middle cardiac vein; GV ϭ great cardiac vein; AV, anterior cardiac vein. Alongside are three-dimensional reconstructed images of the coronary venous tree, facilitating segmental visualization of the coronary venous anatomy. The coronary venous ring is divisible into three segments: posterior (C), lateral (B), and anterior (A), in both views. (B) The mid-ventricular position of a lateral segmental branch (1), closely corresponding to the second-order lateral tributary from an anterior segmental branch, extending over the mid-ventricular region (2). JACC Vol. 46, No. 1, 2005 Singh et al. 73 July 5, 2005:68–74 Coronary Venous Anatomy nary arteries and allows for assessment of arteriovenous CONCLUSIONS relationships (Fig. 6). A comprehensive CT evaluation of This paper calls for a more systematic approach to detailing the coronary venous anatomy of a patient before implanta- the LV regions supplied by branches of the coronary venous tion of a cardiac resynchronization device could help to system. The high degree of variability in coronary venous determine the suitability of the patient for transvenous LV anatomy makes it important to have a uniform segmental pacing from a known venous tributary over the desired classification system for approaching these vessels. It will be location before an invasive procedure is undertaken. Such useful to the electrophysiologist to classify coronary sinus minimally invasive imaging may thus facilitate the predic- branches and tributaries by their epicardial location in tion of technical difficulty, selection of appropriate equip- addition to their anatomic classification. Correlating the ment, and identification of the venous tributary with an venous anatomy with the segmental wall motion abnormal- appropriate diameter in the region of interest. As of yet this ities or dyssynchrony, as defined by echocardiography and approach has not been tested prospectively, and may be other imaging modalities, may provide more relevant ana- limited in some cases by difficulty in visualizing second- and tomic and functional information at the time of LV lead third-order tributaries in sufficient detail. As a disadvantage, placement. current CT technology requires at least 60 ml of contrast, Future approaches to epicardial lead placement may which would be an additional dye load to the 10 to 40 ml involve a noninvasive preprocedural definition of the coro- required for the intraprocedural retrograde venogram. In nary venous anatomy, followed by more selective angio- addition, the effective radiation dose received from a CT graphic techniques to enable better definition of the venous coronary venogram is approximately 9 to 11 mSv (24). anatomy for LV lead placement. Further recognition of the Advances in magnetic resonance imaging may allow this venous tributaries in terms of their distribution, angulation, imaging modality to be used in a similar manner in the tortuosity, and dimensions will enable the development of future. Real-time integration of coronary venous (segmen- lead positioning tailored to each individual patient. This will tal) branch location with echocardiographic or magnetic allow for translation of current advances in our understand- resonance imaging-guided measurements of LV segmental ing of regional mechanical dyssynchrony into the clinical dyssynchrony, along with improvement in LV lead technol- practice of LV lead implantation. ogy, will significantly influence our approach to treating patients with heart failure and ventricular dyssynchrony. Acknowledgments The authors thank Volker Rasche (Philips Medical Sys- tems) for his assistance with the three-dimensional reconstruction of the coronary venous tree from the rotational venogram.

Reprint requests and correspondence: Dr. Jagmeet P. Singh, Massachusetts General Hospital, Cardiac Arrhythmia Service, GRB 109, 55 Fruit Street, Boston, Massachusetts 02114. E-mail: [email protected].

REFERENCES

1. Miyazaki A, Tadokoro H, Drury JK, et al. Retrograde coronary venous administration of recombinant tissue-type plasminogen activator: a unique and effective approach to coronary artery thrombolysis. J Am Coll Cardiol 1991;18:613–20. 2. Gundry SR, Kirsh MM. A comparison of retrograde cardioplegia versus antegrade cardioplegia in the presence of coronary artery obstruction. Ann Thorac Surg 1984;38:124–7. 3. Oesterle SN, Reifart N, Hauptmann E, et al. Percutaneous in situ coronary venous arterialization: report of the ﬁrst human catheter- based coronary artery bypass. Circulation 2001;103:2539–43. 4. Thompson CA, Nasseri BA, Makower J, et al. Percutaneous transvenous cellular cardiomyoplasty. A novel nonsurgical approach for myocardial cell transplantation. J Am Coll Cardiol 2003;41:1964–71. 5. Cappato R, Schluter M, Weiss C, et al. Mapping of the coronary sinus and great cardiac vein using a 2-French electrode catheter and a right Figure 6. Standard multidetector computed tomography angiogram (Bril- femoral approach. J Cardiovasc Electrophysiol 1997;8:371–6. liance 16P scanner) delineating the coronary venous branches in close 6. Sanders P, Jais P, Hocini M, et al. Electrical disconnection of the proximity to the coronary arteries. No appreciable posterior or posterolat- coronary sinus by radiofrequency catheter ablation to isolate a trigger eral vein. The lateral marginal vein seems to be diminutive. Conventional of atrial ﬁbrillation. J Cardiovasc Electrophysiol 2004;15:364–8. branches are labeled as follows: LV ϭ lateral marginal vein; MV ϭ middle 7. Abraham WT. Cardiac resynchronization therapy: a review of clinical cardiac vein; CS ϭ coronary sinus; PDA ϭ posterior descending coronary trials and criteria for identifying the appropriate patient. Rev Cardio- artery. vasc Med 2003;4 Suppl 2:S30–7. 74 Singh et al. JACC Vol. 46, No. 1, 2005 Coronary Venous Anatomy July 5, 2005:68–74

8. Bradley DJ, Bradley EA, Baughman KL, et al. Cardiac resynchroni- 16. Gilard M, Mansourati J, Etienne Y, et al. Angiographic anatomy of zation and death from progressive heart failure: a meta-analysis of the coronary sinus and its tributaries. Pacing Clin Electrophysiol randomized controlled trials. JAMA 2003;289:730–40. 1998;21:2280–4. 9. Ansalone G, Giannantoni P, Ricci R, et al. Doppler myocardial 17. Kawashima T, Sato K, Sato F, et al. An anatomic study of the human imaging to evaluate the effectiveness of pacing sites in patients cardiac veins with special reference to the drainage of the great cardiac receiving biventricular pacing. J Am Coll Cardiol 2002:39:489–99. vein. Ann Anat 2003;185:535–42. 10. Gasparini M, Mantica M, Galimberti P, et al. Is the left ventricular 18. Bales GS. Great cardiac vein variations. Clin Anat 2004:17:436–43. lateral wall the best lead implantation site for cardiac resynchronization 19. Gerber TC, Sheedy PF, Bell MR, et al. Evaluation of the coronary therapy? Pacing Clin Electrophysiol 2003;26:162–8. venous system using electron beam computed tomography. Int J Car- 11. Fung JW, Yu CM, Yip G, et al. Variable left ventricular activation diovasc Imaging 2001;17:65–75. pattern in patients with heart failure and left bundle branch block. 20. Baroldi G, Scomazzoni G. Coronary Circulation in the Normal and Heart 2004;90:17–9. the Pathologic Heart. Washington, DC: Ofﬁce of the Surgeon 12. Lambiase PD, Rinaldi A, Hauck J, et al. Non-contact left ventricular General, Department of the Army, 1967:59–96. endocardial mapping in cardiac resynchronisation therapy. Heart 21. Mantini E, Grondin CM. Congenital anomalies involving the coro- 2004;90:44–51. nary sinus. Circulation 1966;33:317–27. 13. Ortale JR, Gabriel EA, Iost C, et al. The anatomy of the coronary 22. Hodgson EJ, Armour JA, Klassen GA. Functional anatomy of the sinus and its tributaries. Surg Radiol Anat 2001;23:15–21. epicardial coronary veins. Can J Cardiol 1993;9:821–8. 14. von Ludinghausen M. The venous drainage of the human myocar- 23. Schafﬂer GJ, Groell R, Peichel KH, et al. Imaging the coronary venous dium. Adv Anat Embryol Cell Biol 2003;168:I-104. drainage system using electron-beam CT. Surg Radiol Anat 2000;22: 15. Shinbane JS, Girsky MJ, Mao S, Budoff MJ. Thebesian valve imaging 35–9. with electron beam CT angiography: implications for resynchroniza- 24. Morin RL, Gerber TC, McCullough CH. Radiation dose in computed tion therapy. Pacing Clin Electrophysiol 2004:27:1566–7. tomographic imaging of the heart. Circulation 2003;107:917–22.