Extraordinary advances in technology have made possible percutaneous catheter-based treatment of a wide spectrum of specific pediatric and adult “structural” heart diseases. Many of these percutaneous interventional procedures require access to the left heart via transseptal catheterization. Being able to see the anatomy can be a considerable advantage. However, septal anatomy is more complex than perceived at first sight. The true interatrial septum comprises a valvelike flap forming the floor of the fossa ovalis. On the right atrial aspect, the muscular rim surrounding the fossa is an infolding of the atrial wall. Hence, the target area for safe crossing, without exiting the heart, is the fossa floor and its immediate margin of the rim. Real-time (RT) three-dimensional (3D) transesophageal echocardiography is a recently developed technique that provides 3D images of the heart. Because of lack of interference from bone and lung and the closer proximity of the transducer to the posterior structures of the heart, this technique provides 3D RT images of atrial structures of unprecedented quality. In this review, the authors describe two key areas: a step-by-step approach for acquiring and processing RT 3D transesophageal echocardiographic images of the interatrial septum and, second, septal anatomy as it is visualized by RT 3D transesophageal echocardiography. To demonstrate their consistency with actual anatomy, several RT 3D transesophageal echocardiographic images are matched to equivalent anatomic specimens.
Extraordinary advances in technology and human technical skill have made it possible to expand interventional procedures from balloon atrial septostomy and treatment of coronary and peripheral vascular disease (before the turn of the 21st century) to a wide spectrum of specific pediatric and adult “structural” heart diseases. These range from the closure of atrial and ventricular septal defects to the most recent percutaneous valve repair procedures, closure of prosthetic dehiscence, and occlusion of the left atrial appendage. Moreover, over the past two decades, rapid expansion of catheter ablation has allowed electrophysiologists to better deal with, and often cure, several cardiac arrhythmias. This is commonly done by transseptal catheterization because many of these procedures require access to the left heart.
However, septal anatomy is significantly more complex than perceived at first sight. The true interatrial septum (IAS) comprises a valvelike flap forming the floor of the fossa ovalis. On the right atrial aspect, the muscular rim surrounding the fossa is an infolding of the atrial wall. Hence, the target area for safe crossing, without exiting the heart, is the floor of the fossa and its immediate margin of the rim. Moreover, anatomic variability in the position and orientation of the fossa ovalis and its surrounding structures may present specific challenges to even those interventional cardiologists with significant transseptal experience.
Two-dimensional (2D) transesophageal echocardiography (TEE) has been used as the imaging modality to guide transseptal crossing. Although 2D TEE can display the IAS in several planes, because of the “tomographic” nature of this technique, the planes always intersect the septum perpendicularly. Consequently, this structure is imaged as a “linear” echo, which may be thicker around the fossa ovalis (muscular rim) and thinner at the level of the floor. Moreover, the spatial relationship with the surrounding cardiac structures is difficult to appreciate, because this technique lacks the third dimension. In the past few years, advances in computer and crystal technology have led to the introduction of matrix-array transducers that have several thousand electrically active elements that can be used in microbeam forming to generate “real-time” (RT) three-dimensional (3D) images. More recently, the reduction in size of the transducer footprint has led to the development of RT 3D TEE. Because of lack of interference from bone and lung, and the closer proximity of the transducer to the posterior structures of the heart (which permits imaging at higher frequencies), this technique provides 3D RT images of unprecedented quality. A unique peculiarity of 3D echocardiographic representation is the ability to visualize septa from an “en face” perspective. This allows the IAS to be imaged as it actually is: a partition dividing the two atria.
Several studies have been published describing the use of 3D transthoracic echocardiography and 3D TEE for imaging the normal features of the IAS and morphology of different types of atrial septal defects. The relevant role of RT 3D TEE, for monitoring transcutaneous closure of atrial septal defects, as well as for other catheter-based procedures, has been recently described. To the best of our knowledge, detailed anatomic descriptions of the IAS and direct comparisons of RT 3D transesophageal echocardiographic images to anatomic specimens have not yet been reported.
In this review, we therefore aim (1) to present a step-by-step approach for acquiring and processing RT 3D transesophageal echocardiographic images of the IAS, (2) to illustrate the septal anatomy as it is visualized by RT 3D TEE, and (3) to emphasize some practical points that may be beneficial for interventional procedures that involve puncture of the IAS. Finally, to demonstrate their consistency with actual anatomy, RT 3D TEE images are matched side by side to equivalent anatomic specimens.
The RT 3D transesophageal echocardiographic transducer (Philips Medical Systems, Andover, Massachusetts) and the acquisition modalities have been described in greater detail elsewhere. In brief, it is a matrix probe with 2,500 elements capable of four different modalities of data acquisition: the Live 3D modality switches the system from 2D to 3D images, acquiring RT 3D imaging without electrocardiographic gating reconstruction. The 3D-Zoom mode displays a truncated pyramidal data set. The dimension of this sector can be adjusted manually to display the region of interest (up to 90° × 90°, although the frame is substantially reduced to compensate). The Full Volume 3D mode combines a series of subvolumes (up to seven) acquired with electrocardiographic gating to create a final larger reconstructed full-volume image. Finally, the 3D Color Doppler modality combines grayscale full volumetric data with 3D color Doppler. In this latter modality, the final representation still remains a narrow pyramid, because two different data sets (i.e., volume and flow) must be acquired simultaneously for each image portion.
Image Acquisition of the Interatrial Septum
RT 3D images of the IAS in the “en face perspective” and surrounding atrial wall can virtually be obtained by RT 3D TEE from any possible 2D angle. According to Saric et al. , using a few simple imaging rotation maneuvers, images of the IAS can be brought to the desired anatomically correct position. The following description refers only to one of these angles, namely, the 90° bicaval plane. Once the 2D transesophageal echocardiographic bicaval view has been obtained, the “zoom acquisition” modality is used. The dimension of the sector should be as large as possible in the x (lateral) and z (elevation) directions, to include the entire IAS and surrounding structures, while the y (depth) direction should be set to include only the left and the right sides of the septum ( Figures 1 A and 1 B). These specific settings allow the IAS to be acquired in high resolution, excluding right-sided surrounding structures that may cover the right aspect of the IAS. The extensive area scanned causes a frame rate as low as 5 Hz, resulting in an image that appears to move less smoothly than at a higher frame rate. Fortunately, the IAS is relatively immobile, so the low frame rate does not have a significant impact. Once the pyramidal data set has been acquired, a 90° up-down angulation shows the entire left side aspect of the IAS in an en face perspective ( Figures 1 C and 1 D). To obtain an anatomically correct orientation, the image should be rotated in such way that the mitral valve is toward the left lower corner of the image ( Figure 2 A). A 180° counterclockwise rotation shows the right side of the septum with the fossa ovalis and the entrance of superior vena cava ( Figures 2 B– 2 D).
Threshold Processing
Several controls are available and are useful for threshold processing of the IAS. Power output, gain adjustment using time-gain compensation, and focus position should be properly set before the 3D acquisition, on the basis of the 2D image, because these settings cannot be changed after image acquisition. The other settings can be modified “offline” (QLAB; Philips Medical Systems) to optimize visualization of the IAS.
The suggested settings are summarized in Table 1 . Briefly, the gain should be set at a low to medium level (10–40 on a scale ranging to 100). We start with a gentle increase of the gain until noise appears. Then we decrease the gain just to remove as much of the noise as possible, maintaining a level that permits anatomic structures to be visualized; this will avoid at the same time that part of the atrial septum (in particular the thin floor of the fossa ovalis), may disappear, creating a false impression of an atrial septal defect. Compression control adds soft echoes, making objects appearing more opaque and larger. Both effects cause an increase in homogeneity in the image, resulting in merging of boundaries among adjacent structures. Conversely, low values make objects more transparent and thinner and enhance the edges of structures. Thus, a low level of compression should be used (0–3 on a scale ranging to 10) to make the margins of the fossa ovalis distinguishable. Smoothing control removes the subtle roughness of the surface, making structures smoother. In our experience, medium to high values (6–8 on a scale ranging to 10) allow us to maintain precise definition of the IAS while reducing the roughness of its surface. Finally, we use blue/bronze color map vision to enhance depth perception, while the vision control is set to H, and the XRES button is set at its highest level.
| Settings | Scale/Control | Suggested setting for right atrial structures |
|---|---|---|
| Gain | 10–40 (1–100) | Fine tuning of gain setting, increasing the gain until noise appears and then decreasing to the lowest level possible to remove noise while maintaining anatomic structures (particularly the fossa ovalis) visible |
| Compression | 1–3 (1–10) | Use low values that, enhancing edges, make the margins of the fossa ovalis distinguishable |
| Smoothing | 6–7 (1–10) | Medium values maintain precise definition of the IAS while reducing the roughness of its surface |
| Vision control | (A–H) | Vision control F, G, and H have the highest resolution |
| Color map vision | Vision | The blue/bronze modality enhances depth perception |
| XRES button | Off/low/medium level | The medium level enhances resolution |
Real-Time Three-Dimensional Transesophageal Echocardiographic Anatomy
Right Atrial Perspective
The IAS is the medial wall of the right atrial cavity. Once the right atrial wall has been removed, the exposed IAS is formed by the fossa ovalis and its surrounding rim. The rim often appears as a ridge of thicker muscle surrounding the oval-shaped depression (the fossa). The fossa ovalis is completely overlapped by its valve, a flap of thin tissue, which lies on the left side of the IAS and is continuous with left atrial wall. In some individuals, incomplete adhesion of the valve to its muscular rim results in a crevice-like patency at its anterosuperior margin that is described as a patent foramen ovale. The RT 3D transesophageal echocardiographic image of the IAS from this perspective is quite typical. When a blue/bronze modality is applied in color map vision, the fossa ovalis is easily distinguishable because it is depicted as a more pallid shade (beige) in comparison with the surrounding muscular rim (brown) ( Figure 3 ). The different shades of color are due to different depths of the two structures and are particularly evident when the rim is thick and protrudes into the right atrial cavity (e.g., “lipomatous” septum; Figure 4 ). From the right perspective of the IAS, a raised area called the “aortic mound” lying anterior to the fossa ovalis can be appreciated. This area can give an erroneous impression of being part of the septum. Instead, it is the anteromedial part of the right atrial wall. Passing a catheter through this area does not enter the left atrium but exits the heart into the transverse pericardial space that is immediately behind the aortic root ( Figure 5 ). Although the aortic mound is easily recognizable in 3D images ( Figure 5 B), the pericardial space is not. This is because the resolution of the system is inadequate to reveal such a small space and because “in vivo,” the deeper part of this “potential” space is almost virtual.
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