The junction between the right atrial appendage (RAA) and the smooth posterior right atrial wall is marked externally by a fat-filled groove, the sulcus terminalis, which corresponds internally to a curved muscular band protruding into the right atrial cavity, which is named the crista terminalis (CT). The CT originates from the anteromedial wall of the right atrium, merges with the Bachmann bundle, continues along the anterior margin of the orifice of the superior vena cava (SVC), arches laterally, and descends towards the IVC, where it divides into thinner branches. An array of pectinate muscles arises from the CT and spreads throughout the RAA wall. The nonuniform architecture of the myofibers at the junction of the CT with the pectinate muscles may cause an anisotropy, which may be the substrate for reentry. Indeed, the CT is one of the most frequent sources of focal atrial arrhythmias; in patients without structural heart disease, two thirds of atrial arrhythmias originate along the long axis of the CT, with a gradient of frequency from the high to the low part of the crista.

Because the CT originates along the anterior border of the orifice of the SVC and runs towards the IVC, the volumetric data set derived from a two-dimensional (2D) TEE bicaval view should include this structure. It is difficult to obtain clear 3D TEE images of the CT, however, and it can be distinguished as a discrete muscular crest only when it is thick and prominent. Using 3D TEE perspectives that enhance shadowing between the margins of the CT and the surrounding atrial walls may help in differentiating the CT from the surrounding wall (Fig. 9.1).

Anatomically, the CVTI is an endocardial region that is roughly quadrilateral; it is delineated anteriorly by the annular insertion of the septal and lateral tricuspid leaflets and posteriorly by the IVC border and the Eustachian valve and ridge. The lateral and medial borders are rather indistinctly defined by the terminal bundles of the CT and the lower border of the CS orifice, respectively. From an electrophysiologic standpoint, the CVTI may be divided into three regions: the septal isthmus between the CS ostium and the tricuspid annulus; the inferior isthmus between the orifice of the IVC and the posterior annulus of the tricuspid valve; and the inferolateral isthmus, which is the most lateral region bordered by the CT. The septal isthmus is shorter than the central isthmus, and the central isthmus is shorter than the inferolateral isthmus. Most commonly, the ablation line is traced along the inferior isthmus. This region presents the thinnest wall and includes the musculature and fibro-fatty tissue. Both these characteristics make the tissue less resistant to a complete transmural ablation.

The 3D TEE image of the CVTI can be obtained by focusing the volumetric pyramid on the aorta and enlarging the angle (or moving laterally) to include the entire right atrium. Once the entire pyramidal data set has been acquired, progressive cropping along the z-axis may disclose a panoramic view of the CVTI, including the inferolateral, the inferior, and the septal isthmus. In this plane, the short-axis view of the aorta and the sinuses can also be imaged (Fig. 9.2a). Because structures are opaque in 3D format, a prominent aortic root may partially cover the septal isthmus. In such a case, two maneuvers may be useful: further cropping along the same axis, which displays the septal isthmus by excluding the aorta (Fig. 9.2b), or a right-to-left rotation around the y-axis, which progressively reveals the CS ostium and the surface of the septal isthmus in an en face view (Fig. 9.3).

The posterior border of the CVTI is defined by the Eustachian valve (EV). This flap of tissue guards the junction of the IVC and the right atrium. In fetal life, the EV directs the flow of oxygen-rich blood from the IVC into the left atrium through an open foramen ovale, preventing blood from flowing into the right ventricle. Following birth, the EV persists as a remnant of the embryologic valve, without a specific function. This flap of fibrous or fibromuscular tissue continues medially into the Eustachian ridge (ER), a prominence of tissue between the fossa ovalis and CS orifice. The EV may vary from a thin, almost invisible flap to a large, crescent-shaped fibromuscular tissue. In about 2 % of patients, the EV has a reticulum of lacelike strands of varying sizes, called the Chiari network.

Three-dimensional TEE is an ideal tool for imaging the EV. The valve can be visualized from several perspectives. Notably, when it is seen from the right atrial perspective, the EV is easily recognizable; its triangular shape is well delineated from the surrounding atrial wall. Conversely, when it is observed from a “caval” perspective, the valve forms a continuous surface with that of the IVC (Fig. 9.4).

When the EV is thin, fine-tuning of the gain may be necessary to display the entire surface of the valve without dropout artifacts. Nevertheless, these artifacts sometimes cannot be avoided (Fig. 9.5).

Anatomic details of the EV and ER may be discovered with magnified images and unusual perspectives. Figure 9.6a shows the EV from a caval perspective. Right-to-left rotation of the image around the y-axis progressively reveals the EV and the ER from a lateral perspective and eventually an atrial perspective. In particular, the ER can be distinguished from the EV as a prominence between the fossa ovalis and the CS.

The orifice of the CS is guarded by a thin, crescent-shaped flap of tissue called the Thebesian valve. The shape of this valve varies from a clear, crescentic flap (with or without fenestrations) to a thin, strand-like, nearly invisible structure. An extensive, imperforate flap covering the orifice to some extent can be seen by 3D TEE, in both an en face and longitudinal view (Fig. 9.7).

Several anatomic variants may create difficulties in an ablation procedure. This unusual anatomy can be discovered by 3D TEE.