Chapter 1 Historical Perspective on Three-Dimensional Echocardiography

Introduction

Three-dimensional echocardiography (3DE) has been in existence since the 1970s, albeit with images that barely resembled a heart at that time. Throughout the 1970s and 1980s, 3DE was limited by computing power, both at the workstation and at the ultrasound level. In the 1990s, as both two-dimensional echocardiography (2DE) image quality and computing power dramatically improved, 3DE began to make strides as a possible clinical entity. During these early attempts at 3DE, it was never real time and was approached from the perspective of capturing multiple 2D images of the heart and either tracing a wire frame of the heart or melding the 2D images together using interpolation methods to create a 3D image. The first attempts at real-time 3DE were undertaken by von Ramm et al at Duke University in the 1990s. The major transition from research to clinical tool for 3DE happened in 2002 when the first reasonably user-friendly version of real-time 3DE (RT3DE) was introduced. This was also the first version of RT3DE to have diagnostic-quality 3DE images performed by an ultrasound system that was capable of both 2DE and 3DE.

Earliest Approaches

3DE had its beginnings in the 1970s with primitive equipment and equally primitive images. By the mid-1980s, there was early 3DE performed with standard B-mode ultrasound and tracking devices designed to locate the transducer in space. The initial approach to 3D echo was to obtain multiple 2D images and reconstruct them into a 3D image. This was accomplished by registering the 2D images such that it could be discerned how the individual images fit together to form the 3D image. Such registration of the images was achieved by tracking the transducer in space via mechanical, acoustic, electromagnetic, or optical detection apparatus. In the mechanical approach, the transducer had an actual mechanical arm attached to it and therefore dictated movement in space (Figure 1-1). Dekker and colleagues¹ are credited with this first iteration of 3DE using the mechanical arm approach in 1974. Next came an acoustic attempt at location, the “spark gap” technique by Moritz and Shreve² in 1976. In this technique, the transducer was located by sending pulsed acoustic signals from a device holding the transducer, called a spark gap to a Cartesian locator grid (Figure 1-2). This approach, in theory, allowed free-hand scanning—meaning that the transducer did not have to follow a predetermined pathway and, indeed, was the precursor to free-hand scanning using an electromagnetic locator. Both electrocardiography and respiratory gating were often used, or images were acquired with breath holding. Initially, only end-diastolic and end-systolic images of the left ventricle (LV) were obtained. Over time, complete cardiac cycles could be obtained.^3,⁴ Several groups published articles during this period using a variety of primitive transducer tracking systems and the resultant less than pleasing 3D imaging.⁵^–¹⁰

Figure 1-1 A, The mechanical arm used by Dekker and colleagues ¹ for tracking the ultrasound transducer in space. Note the arrow depicting the actual ultrasound transducer. As might be imagined, this apparatus was not well received by sonographers because moving the transducer around was physically taxing, given the weight and awkwardness of the mechanical arm. B, Image of the heart. Note that the image shows only mild resemblance to the actual heart and heart chambers. A, aorta; LA, left atrium; LV, left ventricle; MV, mitral valve.

Figure 1-2 An acoustic “spark gap” apparatus used by Moritz and Shreve ² for locating the transducer for three-dimensional echocardiography. A, Actual device housing the transducer, with the audio signal sent from the trifurcating mechanism. B, The device in practice, with the coordinates shown on the board to the right of the user.

In 1977, Raab and colleagues ¹¹ developed the magnetic locator, which ultimately led to the most advanced type of free-hand scanning, but this method for transducer location never caught on until the mid-1990s. This method is still used today by some labs, predominantly for research purposes (Figure 1-3; see also Figures 1-13, 1-14).

Figure 1-3 A, Electromagnetic locator device. The sensor is shown attached to a holder for the transducer. B, Summary of the electromagnetic sensor system. The magnet is located in the box, which serves as the transmitter. The magnet is connected to the sensor and indirectly to the ultrasound system. C, The location of the transducer is tracked by the electromagnetic sensor.

In between the spark gap and the free-hand magnetic approaches were myriad tactics that used the technique of transducer movement in a preprogrammed, stepwise fashion while attempting to keep the patient and the sonographer as stationary as possible. The mechanical devices then moved the transducer in a linear, fanlike, or rotational direction. An example of a fanlike acquisition is shown in Figure 1-4.

Figure 1-4 A, A fanlike device for three-dimensional acquisition. This device, made by TomTec, Inc., stepped the transducer through a fanlike motion. The transducer was held by the square apparatus (arrows), and the user held the device in a manner similar to a pistol grip. B, The stepwise approach to fanlike scanning. C, Ultrasound transducer being held by the fanlike mechanical device in the direction of acquisition. D, Mechanical fan device and transducer within.

(B, Courtesy TomTec, Inc., Munich, Germany.)

Fanlike Scanning

Fanlike scanning was, in theory, advantageous for evaluating the right ventricle (RV) compared with rotational or linear scanning, since the RV is not easily accessed from the apex and often is best imaged from the RV inflow tract using a more rightward direction of scanning from the standard parasternal view.¹²

Rotational Scanning

The rotational approach used a cylinder-like mechanical apparatus that those in the industry at that time referred to as the “bazooka” (Figure 1-5). This device had a mechanical stepper motor that moved the transducer sequentially through a semicircular or 180-degree scan with stops every two to three degrees to acquire a 2D image. The acquisition of roughly 60 images was followed by computer software processing, initially using a polar coordinate approach to meld the images into a 3D image. The computer software used significant interpolation between images. The result was frequent “stitch” artifact caused by deficiencies in image line-up from patient movement, transducer movement outside the prespecified position, respiratory or cardiac gating artifact, or all of these factors. Although innovative and potentially very powerful for this era, clear limitations remained. Hence, clinical applicability was very limited.¹³

Figure 1-5 A, A device for rotational scanning with a detachable part for holding different-sized transducers of this era. B, The detachable transducer holder is back in place. C, A transducer (an older mechanical transducer) is shown held in place. D, The rotational device with the attached hardware for connecting to the computer that operated the motor on the rotation stepper. E, Another version of the rotation device produced by TomTec, Inc. F, Rotational scanner shown in close-up. The arrow shows the transducer.G, How rotational scanning was performed with one cardiac cycle obtained every 3 degrees and each cardiac cycle consisting of 15 to 20 frames.

(D and G, Courtesy TomTec, Inc., Munich, Germany.)

Linear or Parallel Scanning

In linear scanning, the transducer was sequentially stepped along a line and obtained images every few millimeters, as opposed to degrees with the rotational and fanlike scans (Figures 1-6 to 1-9). Examples of linear scanning using an intravascular ultrasound, predominantly for coronary imaging, as well as a transthoracic echocardiography (TTE) device are shown in the figures. A device, known as “lobster tail,” for linear pullback using transesophageal echocardiography (TEE) was available as well. In similar fashion as the previously described linear devices, the lobster tail operated by stepwise pullback of the TEE probe in the esophagus, with imaging at each stop. This TEE probe, developed by TomTec, Inc. (Munich, Germany), included both the TEE probe and the “stepper,” that is, the device that moved the probe sequentially (see Figures 1-8 and 1-9). There were several iterations of this technique, including one that involved placing the TEE probe in a water bath within the esophagus to serve as a “standoff” to improve acoustic effects and sharpen the images (see Figure 1-8). The rotational method of scanning with the lobster tail is summarized in Figure 1-9. Further summaries regarding the various types of scanning, early fetal ultrasound, and early computer hardware are shown in Figures 1-10 to 1-12.