Normal Anatomy and Flow During the Complete Examination

Three-Dimensional Views: Replicating the Surgeon’s View

Noninvasive cardiac imaging centers on the ability to visualize and quantitate cardiac structure and function. To perform these tasks, there are currently multiple modalities at the clinician’s disposal (e.g., magnetic resonance imaging [MRI], computed tomography [CT], and nuclear medicine). Although CT and MRI technology provide the imager with excellent image resolution and three-dimensional (3D) volumetric quantification capabilities, these imaging modalities entail space and infrastructure requirements that render them impractical for the operating room environment. Conversely, echocardiography is highly mobile and easily integrated into the operating room environment. Recently, technological advances enabling real-time 3D imaging have made echocardiography competitive with these more expensive and operationally intensive modalities. The introduction of real-time 3D transesophageal echocardiography (TEE) into clinical practice within the last several years offers the perioperative team new qualitative and quantitative imaging capabilities.

For two-dimensional (2D) TEE, the landmark paper published by Shanewise et al. ¹ is considered the definitive standard for a complete perioperative TEE exam. This complete exam consists of 20 views (see Chapter 1). The orientation of these views is determined by the anatomic relationship of the esophagus to the heart (transesophageal views) or the stomach to the heart (transgastric views). Unfortunately, because of this orientation and the 2D nature of the display on screen, interpretation is rarely intuitive, and months of dedicated training are required to master the skill of 2D echocardiography. An experienced echocardiographer creates a “mental” reconstruction of a 3D image of cardiac structures of interest by assimilating information from multiple 2D views, but less experienced imagers or non-echocardiographers find interpreting 2D views challenging, and this impairs transfer of information in the perioperative setting.

A major advantage of 3D echocardiography is the ability to spatially orient and crop the acquired data block at the discretion of the echocardiographer. Consequently, image orientation and 3D structure should facilitate communication between surgeon and echocardiographer, especially if the image is displayed in an orientation that replicates the surgeon’s view intraoperatively. This chapter will provide an overview of 3D echocardiography and demonstrate image orientations that best simulate the views of cardiac structures seen by surgeons in the most common surgical approaches.

Transducer Design

The foundation of echocardiography revolves around the transduction of electrical energy into mechanical energy (i.e., vibrational waves) and vice versa. The parameters that constrain echocardiography (the “acoustic triangle of constraint”) are: (1) frame rate, (2) sector/volume size, and (3) imaging resolution. In general, increasing the requirement of one of these causes a drop in either or both of the other two, assuming other parameters are maintained constant, such as the number of transmit events ( Fig. 9-1). While clinicians who are inexperienced in echocardiography often believe 3D TEE will provide images of superior quality when compared to 2D TEE, it is important to note that 3D echocardiography is subject to the same laws of acoustic physics as 2D echocardiography. Artifacts such as ringing, reverberations, shadowing, and attenuation occur in 3D as well as 2D and M-mode.

Figure 9-1 Triangle of constraint.

Conventional 2D imaging transducers transmit and receive acoustic beams in a flat scanning plane. The to-and-fro sweeping action of a scanline enables a sector to be imaged. Typically, a modern-day conventional 2D imaging transducer consists of 128 linear (1D) “elements” that are interconnected electronically to sum radiofrequency (RF) scanlines. By varying the spatiotemporal phase of initiating each element’s transmit event, the ultrasound beam can be steered. These principles represent the basis for any phased array system ( Fig. 9-2; also see Chapter 6).

Figure 9-2 Phased array transducer.

In contrast to 2D imaging transducers, 3D imaging matrix arrays have both columns and rows of elements. Instead of a single dimension (column) of 128 elements, a matrix array comprises more than 50 rows and 50 columns of elements. As described in Chapter 6, the 2D block of approximately 2500 elements is created by “dicing” a block of transducer material by a diamond-tipped saw to create each element ( Fig. 9-3). This 2D block allows for 3D scanning and forms the foundation of a matrix array transducer ( Fig. 9-4). Additional innovations, such as incorporation of materials that allow more acoustic bandwidth (simultaneous high and low frequencies), enable matrix array transducers to enhance both tissue penetration and image resolution.