Introduction

Technologic advances continue to propel cardiac visualization forward. Advances in miniaturization, computer processing, and algorithms continue to have significant momentum. New acquisition technologies, coupled with the ability to process, display, and quantify enormous amounts of data, have pushed echocardiography forward into becoming a modality that will change cardiac intervention as well.^1–13 Today, three-dimensional transesophageal echocardiography (3DTEE) has generated near-optical cardiac images. An understanding of cardiac mechanical motion in all of its spatial and temporal dimensions is providing valuable general physiologic insight for echocardiography as well as precise diagnoses for patients.

It is expedient to address the technologic advances in terms of the operating sequence in an ultrasound system for echocardiography. This allows a framework consisting of transduction, beamforming, display, and quantification to be used. Echocardiographic instrumentation is unparalleled with respect to its portability, cost, lack of ionizing radiation, and ubiquitous presence.

Transducer Technology

Among imaging modalities, the defining aspect of echocardiography is the transducer. Transduction refers to the ability to convert one form of energy to another. Thus, an ultrasound transducer converts electrical energy into mechanical energy on “transmit” and acts as a microphone on “receive.” All ultrasound transducers perform this operation, but what sets ultrasound advancements apart is the ability to steer in two or three dimensions. The earliest M-mode transducers created an image composed of one spatial dimension and one temporal dimension. The element is composed of specialized material that traditionally uses lead-zirconate-titanate. Newer single-crystal materials that contain homogeneous solid-state domains are more efficient in the transduction process and have higher bandwidth (more upper and lower frequency content). This creates a concomitant increase of echo penetration and resolution. Although the M mode was an advance over the stethoscope, it was limited by its lack of field of view. M mode used a “transmit-listen-wait” duty cycle to determine the distance of targets along an unsteered scanline, and the operator needed to point the transducer to examine different cardiac structures. The development of the phased-array paradigm allowed scan lines to be steered.

Conventional two-dimensional (2D) imaging, commonly used in echocardiography, uses transducers with several elements (Figure 3-1). Specifically, elements are oriented in a single row containing 48 to 128 elements, with each element electrically isolated from the others. Individual wavefronts are generated by firing elements in a certain sequence. Each element, constructively and destructively, adds and subtracts pulses, respectively, to generate a focused wave that has direction. This creates the radially propagated scan line. For example, if the farthest element on the right fires first and a timed sequence propagates along the element to the left, the beam will be steered to the left (Figures 3-2 and 3-3). Each element fires with a delay in phase with respect to a transmit initiation time. To further clarify this point, this one-dimensional array of elements can fire in two dimensions: radially and azimuthally (laterally). This spatiotemporal orientation of elements and their phase-timed firing sequence form the underpinning of any modern phased-array system.

Figure 3-1 Transthoracic imaging transducers for three-dimensional echocardiography in the adult. Three generations of transducers from left to right: ×4, ×3-1 and ×5-1. Miniaturization is the key technological trend in the evolution of matrix array technology. The ×4 has 24 application-specific integrated circuits and the ×5 has one. This reduces power consumption and also allows a more ergonomic handle. Other changes that have evolved include changes to the physical and nose apertures to optimize acoustic coupling in intercostal spaces.

Figure 3-2 Integrated circuit miniaturization in the ×5-1 transducer. The inset shows previous technology using multiple application-specific integrated circuits (left). These perform the micro-beamforming and allow steering of acoustic lines in three-dimensional space both azimuthally and elevationally. Inset at right, These circuits have been condensed to one circuit whose dimensions are designed to match the aperture shown in red on the transducer face.

Figure 3-3 Portion of a three-dimensional beamforming application-specific integrated circuit (ASIC). Shown are electrical contact points that couple the transducer piezoelectric elements to the beamforming component circuitry. The ASIC is fully capable of performing M-mode, two-dimensional, and 3D imaging as well as spectral and color Doppler.

Transducer material is cut, or “diced,” by a diamond-tipped saw to create the checkerboard pattern. Elements are then electrically connected to a system. Early-generation systems were composed of sparsely sampled arrays, that is, arrays whose elements were not all electrically active. These sparse arrays created the first instantaneous 3D images; early clinical research was conducted using this type of transducer. It is advantageous to have every element independently active to control the ultrasound beam with more precision. The spacing, or pitch, of these elements depends on the desired frequency of operation (typically λ/4). Otherwise, undesirable diffraction effects such as grating lobes appear. This means that higher frequency transducers have finer pitches. Increased technologic challenges have emerged in the creation of these element connections. The major advance that allowed a fully sampled matrix array to be fabricated was the ability to develop electrical interconnections to every element.

The key aspects pertinent to 3D echocardiography (3DE) imaging involve imaging moving structures. If a static structure that is not moving in space needs to be visualized, a 2D imaging transducer could be swept if the third spatial dimension could be additionally registered within the coordinate space. By using electromagnetic trackers, early gated 3D methodologies exploited this paradigm. Naturally, the 2D images needed to be gated to the electrocardiogram (ECG). This could lead to error caused by movement or arrhythmias, and these lengthy scans could take tens of minutes, requiring a few hundred heart cycles. Ultimately, high-resolution cardiac imaging requires instantaneous imaging to overcome these limitations. The key difference between a 2D imaging transducer and an instantaneous 3D imaging transducer is the arrangement of elements (Figures 3-4 to 3-7). Although a one-dimensional row is used for 2D imaging, a 2D matrix or checkerboard is used to steer an ultrasound scan line in the azimuthal as well as the elevational direction. A conventional 2D imaging transducer of one row steers energy in the azimuthal plane but unintentionally propagates elevational energy above and below the scanning plane. This checkerboard pattern allows phasic firing of elements to generate a radially propagated scan line that can be steered laterally and in elevation. Thus, the true 3D imaging transducer is born.

Figure 3-4 Internal component of a transthoracic three-dimensional imaging probe. Shown at the bottom is the cable coupling to the housing. The electrical cable connections to the main beamformer in the ultrasound system pass through the rectangular connector (shown in light green) in the middle of the handle. At the top is the beamforming application-specific integrated circuit and matching layer.

Figure 3-5 Three-dimensional transesophageal imaging transducer.

Figure 3-6 Three-dimensional transesophageal echocardiography imaging probe (left) compared with a multiplane probe (right).

Figure 3-7 Three-dimensional transesophageal echocardiography (TEE) imaging probe tip with external housing removed. Shown is the matrix array in both azimuthal and elevational dimensions. Unlike a conventional multiplane TEE probe, there are no moving parts within the housing. Moreover, special shielding is built in to prevent electrical interference from external sources such as electrocautery.

Micro-beamforming is the process of using coarse and fine steering. This is implemented by putting fine-delay circuitry into special, application-specific integrated circuits (ASICs). The first commercial, fully sampled matrix array transducer used this methodology by placing 24 to 26 ASICs into the transducer handle. Approximately 3000 elements were electrically connected to these ASICs. Fine steering was performed using subsections of the element matrix known as patches. Coarse steering is performed within the system and through a conventional cable. Specially engineered ASICs allow an individual element to be electrically active but simultaneously keep the size of the transducer cable small, since a significant portion of beamforming has already taken place in the handle. Early transducers were specialized “3D-only probes,” but it is now possible to perform all the transducer functions such as imaging, color flow, and spectral Doppler within the same transducer. Moreover, the transducer aperture should be large enough to allow sufficient lateral and elevational resolution but small enough to “fit” into the intercostals space. One of the most difficult aspects of transducer engineering is what is known as thermal management. The electronics generate heat, potentially more so at high mechanical indexes (e.g., higher waveform amplitudes). These issues need to be resolved if 3DE was to move to the operating room.

3DE depends on micro-beamforming miniaturization. By reducing the electronic substrate required to beamform onto a single chip, the transducer chip is miniaturized sufficiently to pass into the human esophagus. It also significantly reduces the power requirement and hence the amount of heat generated by live circuitry (Figures 3-8 to 3-10).

Figure 3-8 Three-dimensional transesophageal echocardiography transducer with probe handle components displayed. Shown are steering cables for the bending neck. The bending neck, which is proximal to the probe tip, allows anteroflexion and retroflexion within the esophagus. Four electrical connectors allow post–micro-beamformed acoustic information to pass within the cable.

Figure 3-9 Three-dimensional (3D) steering acoustic lines in 3D space. By steering transmit lines, a fully functional matrix array can interrogate anatomy in 3D.

Figure 3-10 Three different volume scanning paradigms. At left is a volume used for live scanning. The center shows live scanning but in a smaller zoom fashion. At right is a larger volume with multiple gated slabs put together.

Beamforming

3DE beamforming consists of steering and focusing of ultrasound energy both as transmitted and received scanlines. This creates useful signals that can be displayed or quantified. It is both advanced science and art. Significant advancements that maximize frame rate, scanning volume size, and resolution continue to occur. Resolution is defined as the ability to distinguish two point targets as distinct. The limiting item in current 3DE systems is the speed of sound, not computing power. Ultrasound image quality improves by firing more transmit lines with more closely associated line spacing. This slows the frame rate since there are many more duty cycles for the system to deal with.