Imaging of Cardiac Anatomy

Anatomy

Respective to the midline of the sternum, the heart lies with approximately two thirds of its mass on the left and one third on the right (see Fig. 2-2A ). The RA and the RV are anterior relative to the LA and LV (see Fig. 2-2B, C ). The RV lies immediately behind the sternum, whereas the LV is rather posterior than leftward. The apex of the LV is leftward, forming the cardiac apex. Similarly, the RA is anterior to the LA, which is actually the most posteriorly sited chamber (see Fig. 2-2D ). Due to the obliquity of the interatrial septum (IAS) and the different level of insertion of the atrioventricular valves, the LA also lies superiorly to the RA.

Imaging Technique

Computed tomography is certainly the best imaging technique for understanding the positions of the cardiac chambers and their spatial relationship in relation to the thorax and other organs. A dedicated algorithm, by increasing the opacity of the intracavitary contrast and, at the same time, reducing the opacity of the myocardium, creates a perfect electronic cast of the cavities. Maintaining the spine and the sternum as anatomic references, this imaging modality allows a precise perception of the position of the cardiac chambers in the thorax ( Fig. 2-2A-D ).

Anatomy

The esophagus lies posteriorly in close proximity with the posterior wall of the LA. The course of the esophagus is variable ( Figs. 2-3 and 2-4 ). Usually, it runs parallel to the PVs, although an oblique direction (i.e., from the left superior to the right inferior veins) is not infrequent. Between the esophagus and the LA wall there is a thin layer (2-4 mm) of fibro-fatty tissue, which contains the periesophageal plexus of the vagus nerve, small esophageal arteries, and lymph nodes.

Imaging Techniques

Both contrast-enhanced CT (see Fig. 2-3 ) and CMR (see Fig. 2-4 ) may provide excellent images of the esophagus and its anatomic relationships with the roof of the LA and the PVs.

However, it must be kept in mind that the esophagus is a dynamic organ that may change shape and position from the time of CT or CMR acquisition to the time of the procedure. Thus although useful, the CT or CMR image of the anatomic relationship between esophagus and atria may not correspond to the real position of the esophagus at the time of ablation procedures. Only real-time techniques such as an esophagram with barium swallow or intracardiac echocardiography may precisely define the position of the esophagus during the procedure and alert the electrophysiologist when regions that are too close are being ablated.

Anatomy

The external aspect of the RA consists of two main components: the venous component or sinus venosus and the RAA. The first, located posteriorly, receives the systemic venous blood flow from the SVC and IVC and presents a smooth internal surface. The second is a large triangular pouch that extends laterally and anterosuperiorly, partially covering the anterior aspect of the ascending aorta. Characteristically its internal surface contains numerous pectinate muscles (PMs). These two components are divided externally by a groove filled with fat named the sulcus terminalis. The internal aspect of the RA consists of several components particularly relevant for electrophysiologists such as the TC, the cavotricuspid isthmus (CVTI), and surrounding structures (i.e., the eustachian valve and the ostium of the CS), and the right side of the IAS.

Imaging Techniques

General Concepts

The best imaging modality to illustrate the external aspect of the RA is certainly the volume-rendering modality of the CT ( Fig. 2-5 ). There are several reasons for its supremacy over other imaging techniques. First, the voxel (volume elements) at the base for 3D reconstruction approximates to a small cube of 0.6 mm side with the x -, y -, and z -axis of equal dimensions (isotropic). Thus the technique enables the defining of boundaries of two adjacent structures that are close to each other, no less than 0.6 mm in all three spatial directions. Second, the ability of the system to “fly around” or “fly through” (in other words, to observe a given organ from different perspectives or to go inside) allows an intuitive perception of three-dimensionality. Finally, the algorithm used to transform serial axial CT slices into a 3D image classifies each voxel based on the probability that it contains a specific type of tissue (i.e., soft tissue, vessels, bones). By assigning different colors to different tissues, the volume-rendering algorithm produces CT images nearly identical to anatomic specimens (see Fig. 2-5 ).

Unfortunately, the same technique is inadequate to image the internal aspect of the RA. Indeed, CT is usually dedicated to image the left side of the heart (in particular coronary arteries and aorta), and acquisition of the images follows precise temporal protocols (i.e., the acquisition occurs when the first pass of contrast is within the vessels of interest); consequently, in routine CT images the RA and RV are usually poorly contrasted. Even when, mistakenly or for research purpose, the acquisition occurs earlier (i.e., with the contrast in the right sided cavities), the CT delineates the contour of atrial structures in 2D format. Moreover, in this case blooming artifacts (due to the contrast flowing from the SVC to the RA) may partially obscure RA structures. Finally, usually CT acquisition occurs in a short period of the diastole (prospective acquisition). Obtaining images in motion with CT needs the acquisition of multiple phases (retrospective acquisition) with a high radiation dose exposure (up to 25 to 30 mSv). Even when the retrospective acquisition of multiple phases is performed with the new generation of CT machines (which allows obtaining motion images with an exposure of less than 10 mSv) the frame rate is as low as 10 frames/sec, making the motion less fluid than the other imaging techniques.

Contrary to CT, CMR does not use radiation and provides dynamic images of right chambers of the same quality as those of left chambers. However, as for CT, CMR provides a 3D volume-rendering imaging of the external aspect of the heart. Because acquisition of CMR images are intrinsically three-dimensional, it is likely that dedicated CMR software may enable the reconstruction in three dimensions of the internal structures of the RA; however, currently in most CMR laboratories the internal structures of the RA are usually visualized in 2D format.

Three-dimensional TEE provides 3D images of the endocardial surface of the RA of excellent quality. Indeed, the large difference in acoustic impedance between blood and the internal surface of the wall makes right atrial structures fully visible. Moreover, the capability of displaying these structures from different perspectives permits defining anatomic details with a fidelity that approximates the anatomic specimens.

Anatomy

The TC is a muscular ridge that delineates the smooth wall of the sinus venosus from the trabeculated surface of the RAA. This muscular ridge originates from the anteromedial wall of the RA near the interatrial sulcus and runs laterally adjacent to the anterior margin of the SVC; then it descends toward the IVC, where it splits into fine trabeculations. An extensive array of PMs originates perpendicularly from the TC lining the internal surface of the RAA. Both PMs and TC may have variable sizes and shapes. In particular, the TC may appear as a thin, almost invisible muscular protuberance or a thick, broad-based muscular bump mimicking an atrial mass.

Imaging Techniques

Although CT and CMR may delineate in 2D format the TC protruding into the right atrial cavity, to our mind, the best imaging technique for visualizing this elusive structure remains the 3D TEE. When CT protrudes into the RA, 3D TEE is capable of defining in three dimensions its entire course, providing images that resemble the anatomic specimens ( Fig. 2-6 ).

Anatomy

The RAA derives embryonically from the primitive RA and, typically, has an irregular surface because of the PMs. In between the PMs, the atrial wall shows variable thickness, in some parts having a very thin, pouch-like configuration. One of the PMs, known as the sagittal bundle (SB) or tenia sagittalis (which means sagittal worm), is usually prominent and crosses the RAA transversally. The SB may form an incomplete ring around the RAA apex, delimiting an anterolateral pocket-shaped area of thin muscular myocardium. During lead implantation or ablation for accessory pathway, the tip of the catheter could be stuck in a thin area delimited by the SB and TC, with the risk of RAA perforation.

Although volume-rendering CT is the ideal technique for imaging the external surface of the RAA (see Fig. 2-5 ), 3D TEE is currently the only technique that provides a panoramic view of the internal surface of the RAA in three dimensions, showing the course of PMs and of the SB ( Fig. 2-7 ). Moreover, by rotating and angulating the 3D image we can obtain perspectives that clearly show the spatial relationship between PM, SB, and TC ( Fig. 2-8 ).

Anatomy

The CVTI is not a well-defined anatomic structure, but rather an irregular quadrilateral, that comprises the inferior-posterior surface of the RA. The electrically inert tricuspid hinge line anteriorly, and the eustachian valve and the eustachian ridge posteriorly, clearly delimit this area. The medial and lateral borders are, on the contrary, rather indistinct and roughly correspond to the inferior border of the CS ostium and to the final ramification of the TC.

Electrophysiologists divide the CVTI in three sectors: paraseptal, inferior, and inferolateral isthmus. Usually, the preferred site of linear ablation is the inferior isthmus because the distance between the orifice of the IVC and tricuspid valve in this sector is short and makes linear ablation relatively simple. Moreover, the wall thickness of the inferior isthmus is thin and presents areas of fatty-fibrous tissue. Both these conditions make the tissue more susceptible to complete transmural ablation. Finally, linear ablation in the inferior isthmus avoids delicate areas such as that of the right coronary artery or the atrioventricular (AV) node.

However, this region may present with highly variable anatomy. In particular, four anatomic variants of the inferior isthmus may make linear ablation difficult or ineffective, forcing the electrophysiologist to move to the inferolateral or paraseptal isthmus. These variants are:

• 1.

  Subeustachian pouch, which may cause lack of catheter-tissue contact, making the ablation line incomplete.

• 2.

  Prominent PM invading the inferior isthmus. They represent a true anatomic barrier, which makes the line of ablation problematic, with the tip of the catheter trapped in the crevices between the PM.

• 3.

  Prominent and rigid eustachian valve, which, acting like a fulcrum, may make the catheter manipulation difficult, requiring tricky catheter angling for obtaining a good tip-tissue contact.

• 4.

  Long CVTI and/or an angulation close to 90 degrees between the IVC and CVTI.

When unexpected, these variants may prolong the procedural time, increase the number of radiofrequency applications, or make the ablation inadequate with recurrence of the arrhythmias. Of course, identifying these variants before the procedure may result in precious time saving for electrophysiologists who may be able to minimize difficulties by adapting their strategy and equipment.

Imaging Techniques

Right atrial angiography before the onset of the procedure may define the isthmus profile. Angiographic studies have shown that CVTI length and the presence of pouch-like recesses significantly increase the time of procedure, x-ray exposure, and the number of radiofrequency applications.

Both CT and CMR have become a valid alternative to angiography to evaluate anatomic characteristics that may cause difficulties in CVTI ablation. Both techniques, in fact, have provided data confirming that long and irregular CVTI, prominent eustachian ridge, subeustachian pouch, and angle between IVC and the CVTI floor around 90 degrees may cause longer and more difficult procedures.

Technically, CT has a higher spatial resolution than CMR and therefore may better define size and thickness of the CVTI. Moreover, contrary to other imaging techniques, CT provides fine images of the right coronary artery. A preprocedural knowledge of the course of the right coronary artery and its branches (including the nodal arteries) may be particularly useful in some circumstances to avoid damage during ablation. Indeed, especially at the level of the lateral isthmus, the distance between the atrial endocardium may be as short as 5 mm (more or less the distance to obtain a transmural lesion) with a potential risk for right coronary artery injury. Finally, CT accurately shows, in volume-rendering format, the external region of CVTI delimiting, when present, the out-pouching (corresponding internally to the subeustachian pouch) and its spatial relationship with the right coronary artery ( Fig. 2-9 ).

However, as emphasized in a previous paragraph, CT has at least two main limitations. First, to obtain interpretable images of RA anatomy, we need a dedicated protocol because the usual CT protocols do not provide contrasted RA images. Second, to obtain dynamic images, it is necessary to perform a retrospective ECG-gated scan acquisition that exposes patients to high x-ray dosage.

Unlike CT, CMR does not require dedicated protocols (excluding proper planes to visualize the CVTI), x-ray exposure, or potential nephrotoxic contrast. Moreover, cine images provided by CMR clearly show the dynamic nature of CVTI, which shortens and lengthens during the cardiac cycle (see Fig. 2-9 ). However, CMR has its own limitations. First, definition of the eustachian valve or the insertion of the tricuspid valve are not always optimal. Second, as for CT, CMR visualizes the internal aspect of CVTI in a two-dimensional plane thus making it difficult to make a distinction in paraseptal, inferior, and inferolateral isthmus.

3D Transesophageal Echocardiographic Images of the Cavotricuspid Isthmus

In our opinion, 3D TEE may represent an ideal technique for imaging CVTI and its anatomic variants for the following reasons. First, the technique provides imaging of the entire surface of the CVTI with its elusive structures (eustachian valve, CS ostium, tricuspid hinge line, and final ramifications of the TC), in en face view allowing a precise definition of the paraseptal, inferior, and inferolateral sectors ( Fig. 2-10 ).

Second, appropriate rotation of the 3D dataset allows obtaining 3D perspectives of CVTI with the same orientation as that provided by the fluoroscopic LAO and RAO projections ( Fig. 2-11 ).

Third, appropriate perspectives and cuts may illustrate anatomic details practically undetectable with the other imaging techniques, such as the eustachian ridge and thebesian valve ( Fig. 2-12 ).

As for CT and CMR, the technique may demonstrate those anatomic variants that may cause prolonged and difficult ablation. Characteristically, a large eustachian valve is associated with a deep subeustachian pouch ( Fig. 2-13 ).

Finally, quantitative measurements are possible in either multiplanar imaging format (i.e., using 2D planes derived by a 3D volumetric dataset) or more recently, directly on 3D images. As described with CMR, the length of the CVTI (from the eustachian valve to tricuspid valve) and the depth of the subeustachian pouch vary from diastole to systole ( Fig. 2-14 ).

Because the echo machine is easily transportable in the electrophysiologic laboratory, and the technique provides 3D images in real time (with therefore the capability of tracking catheter movements), 3D TEE is theoretically an ideal guide during the ablation procedure. Recent data have shown that when used as a companion to fluoroscopy, 3D TEE significantly reduces the procedural time and radiation exposure. However, monitoring the procedure with a 3D TEE probe has its own limitations: it requires general anesthesia (with an additional time due to induction, intubation, extubation, and recovery). Moreover, the presence in the electrophysiologic laboratory of both anesthesiologist and expert echocardiographers with a fully equipped echo machine certainly impose additional costs and logistical burdens. For these reasons, the use of 3D TEE for guiding a relatively simple and straightforward procedure such as the ablation of typical atrial flutter does not appear applicable. However, in selected patients in whom, for example, reducing fluoroscopic time is an important issue, guiding the procedure using 3D TEE may be justified.

Anatomy

The right side of the atrial septum (AS) embraces the fossa ovalis (FO), also referred to as the septum primum, and its surrounding muscular rim. The FO represents the primitive septum and essentially consists of a flap of connective tissue, which, on its left side, overlaps the muscular rim. Seen from a right atrial perspective, the FO appears as an oval-shaped depression resembling a crater (especially in a case of an abnormal thickness of the muscular rim). The muscular rim around the FO, known as the septum secundum, is not really a septum as the term might suggest, but rather an extensive and deep infolding of the posterior-superior atrial wall between the vena cavae and the right PVs. This infolding contains epicardial fat and small vessels.

Computed Tomography, Cardiac Magnetic Resonance, and 3D Transesophageal Echocardiography Images of the Right Side of the Atrial Septum

Given its high spatial resolution, CT provides detailed 2D, cross sectional images of the AS that clearly differentiate the thin FO from the thicker septum secundum. Moreover, different cross sections may display the AS at different levels. However, because of its 2D format, the AS always appears as a dark line separating the two atrial cavities. Furthermore, the imperceptible difference in attenuation between fat and muscular tissue makes the septum secundum appear as a nearly homogeneous structure. CMR imaging of the septum maintains the same 2D format. However, compared with CT, this technique allows the distinction between muscular and fatty tissue. Indeed, CMR typically uses a specific pulse sequence for visualizing the AS, called steady-state free procession (SSFP). This sequence provides a high signal-to-noise ratio and an optimal blood/myocardial contrast, which allows a precise definition of the endocardial borders. Furthermore, the strength of the signal originating from different tissues in this sequence depends on T1/T2 ratio. Because the water (i.e., blood) and the fat have the same high T1/T2 ratio, both tissues produce a very high signal (represented in the final image in light gray). Conversely, the muscular tissue has a low T1/T2 ratio and the signal originating from this tissue is weak (represented in the final image in dark gray). The sequence therefore not only shows very well-defined images of the AS, but also is capable of distinguishing the fat sited in the fold of the septum secundum ( Fig. 2-15 ).

Unlike CT and CMR, 3D TEE is capable of showing the entire surface of the septum in en face perspective. A color scale allows the perception of depth. Thus the oval-shaped depression of the FO appears well defined (beige) compared with the muscular septum around (brown). Moreover, the same pyramidal dataset may visualize surrounding structures such as the eustachian valve, CS ostium, or the aortic sinus abutting against the AS (aortic mound). The anatomic relationship between the FO, muscular rim, and the aortic mound is relevant for interventionists and electrophysiologists in transseptal crossing. However, in contrast to CMR, the negligible difference in acoustic impedance does not allow distinguishing between fat and the muscular tissue of the SS ( Fig. 2-16 ).

Crossing the Atrial Septum

Transseptal catheterization is used in a wide spectrum of interventional and electrophysiologic procedures such as occlusion of the LAA, percutaneous edge-to-edge repair of the mitral valve, closure of paravalvular leaks, and ablation of PVs. The awareness that the true AS (i.e., the membrane that separates the two atrial cavities) is practically restricted to the flap of the FO has relevant practical implications. Indeed, the safe transseptal puncture must occur only through the FO. Puncturing the SS may lead to the risk of exiting the heart. Moreover, even though the needle crossing the SS eventually enters the LA, maneuvering catheters through the thick septum is particularly troublesome. The anterosuperior part of the atrial wall lies just behind the aorta. Puncturing this region may cause aortic perforation. Nevertheless, experienced interventionists and electrophysiologists cross the FO using only fluoroscopy with the help of anatomic references provided by positioning a catheter in the CS or in the ascending aorta. The use of a pigtail advanced in the ascending aorta has the advantage of defining with certainty the lowest border of the right aortic sinus and at the same time recording aortic blood pressure. The risk of complications during the septal puncture, although rare (<1%), are likely to occur in patients with abnormal anatomy such as enlarged aorta, redundancy of the tissue of FO, enlargement and rotation of the atrial cavities, and lipomatous hypertrophy of the AS (which is nothing else than an exaggerated increase of the fat within the septum secundum). In these cases, even experienced electrophysiologists or interventionists may have difficulty in finding the FO with only fluoroscopy and may ask for an additional imaging technique. Intracardiac or 2D TEE are the most used techniques for guiding transseptal puncture. Pushing the needle against the FO causes, in fact, a protuberance toward the LA (tenting) that exactly indicates the site of the puncture.

We certainly prefer 3D TEE because the pyramidal dataset provides a panoramic view, visualizing the tenting, catheter, and surrounding structures ( Fig. 2-17 ).

It may happen that a very elastic tissue of the FO may form a marked tenting protruding deeply in the LA. However, the catheter is still in the RA. In this scenario, the use of fluoroscopy only may be misleading, showing an advanced tip of catheter as if it was in the LA ( Fig. 2-18 ).

Finally, electrophysiologists often cross the septum with two catheters. In that case 3D TEE results, in our opinion, in an ideal imaging technique showing both catheters and their relationships with surrounding structures ( Fig. 2-19 ).

Imaging Techniques

Retrograde venous angiography can evaluate the anatomy of CS and its branches at the time of CRT implantation. However, given the variability of coronary venous anatomy it would be preferable to have a preprocedural anatomic assessment of the venous system, instead of discovering anatomic anomalies “on the table” that may result in an endocardial approach of the CS difficult or impossible.

CT, CMR, and 3D TEE can visualize the CS ostium and the thebesian valve. The first two techniques can visualize these structures in 2D format; 3D TEE may show the CS ostium in en face view with its thebesian valve appearing as a semilunar flap partially covering the OS (see Fig. 2-12 ). Proper 3D TEE perspectives may show the lumen of the CS in short and long axis view ( Fig. 2-20 ).

CT is definitely the only technique that may provide a panoramic view of the epicardial anatomy of the coronary venous system in 3D format. The technique allows imaging the entire course of the CS and its side branches with a quality and finesse that for some aspects surpasses the real anatomy derived by anatomic specimens. CT also shows the spatial relationship between CS and circumflex artery and its side branches ( Fig. 2-21 ).

Variations of Coronary Sinus Anatomy

CS anatomy may present a number of congenital anatomic variations, most of them without hemodynamic consequences.

However, from an electrophysiologic standpoint they may complicate or make impossible the cannulation of the CS. Specific examples include the presence of stenosis and diverticula, variation in shape (filiform, varicoid, or bifid CS) ( Fig. 2-22 ) and an unusually large size because of a persistent left superior vena cava (LSVC), which is the embryologic connection of the left common cardinal vein and the left horn of the sinus venosus ( Fig. 2-23 ). In this last circumstance, the CS dilates because it receives an increased volume of systemic venous blood from the LSVC. The LSVC originates at the confluence of left internal jugular and subclavian veins, passes lateral to the aortic arc, and ends into the CS (see Fig. 2-23 ). The LSVC occurs in nearly 0.3% of the general population, rising to 3% to 10% in patients with congenital heart disease. In a majority of patients with LSVC, a right SVC is also present. Finally, in a minority of patients, the IVC is interrupted because of failure of the hepatic and prerenal veins of the developing IVC to fuse into a common channel. CT remains the best way for visualizing variations of CS anatomy.

Anatomy

The LA receives the pulmonary venous drainage through the four PVs. With regard to its location, the LA is the most posterior of the heart chambers, sited posteriorly, slightly superior and to the left of the RA. The LA is roughly an ovoid-shaped chamber with a lateral small finger-like appendage. This chamber is bordered inferiorly by the vestibule and mitral valve orifice and medially by the AS. The anterior wall of the LA lies just posterior to the transverse sinus and the aortic root. This wall is probably the thinnest area measuring in some areas no more than 2 mm. The superior wall is the roof of the LA and is probably the thickest area (due to the presence of the Bachmann bundle) measuring up to 6 mm; externally, this wall is close to the bifurcation of the pulmonary trunk and to the right pulmonary artery. The posterior wall is adjacent to the esophagus and descending thoracic aorta. Finally, the superior wall receives the entrance of the PVs.

The major part of the internal surface of the LA is smooth, because PMs are exclusively visible in the LAA. The smooth part embraces the venous component with the orifices of the four PVs and the vestibular component, an annular muscular band that surrounds the mitral valve orifice. Between these two regions, there are no specific anatomic landmarks.

Imaging Techniques

The best imaging modality to visualize the external aspect of the entire LA and the spatial relationship with cardiac and external vascular structures is certainly the electronic cast derived from volume-rendering CT ( Fig. 2-24 ).

Several anatomic structures of the LA are relevant from an electrophysiologic point of view. In particular, we focus our attention on the following structures:

• 1.

  The left atrial isthmus

• 2.

  The left atrial appendage

• 3.

  The lateral ridge

• 4.

  The pulmonary veins

• 5.

  The left side of the atrial septum

Anatomy

From a strict anatomic point of view these regions are not boundaries, presenting generally a smooth surface without specific landmarks, although pathologists describe small crevices along the line of ablation. The average transmural thickness ranges from 2 to 4 mm, being thinnest near the mitral annulus. The length of the LAI is also highly variable, measuring from 2 to 5 cm. One of the complications of ablation in this area is injury to the adjacent vessels. Indeed both the left circumflex artery and CS run in proximity to the external aspect of the LAI.

Imaging Techniques

Two techniques may best visualize the LAI providing different data: the CT 2D cross sectional imaging and 3D TEE. The first reveals the proximity of the left circumflex artery and CS, whereas the second offers a panoramic view of the entire internal surface of the LAI revealing, when present, crevices protruding into the cavity or other irregularity on the endocardial surface ( Fig. 2-25 ).