3D Echocardiography: A Review of the Current Status and Future Directions

Abstract

Ultrasound technology has improved markedly in the past 10 to 15 years, prompting echocardiographers to extend its use in studying cardiac structure and function. New ultrasound equipment and techniques offer superior image quality, greater accuracy, and expanding capabilities. As a result, more and improved imaging modalities are available for evaluating cardiac anatomy, ventricular function, blood flow velocity, and valvular diseases. Three-dimensional (3D) echocardiography offers the ability to improve and expand the diagnostic capabilities of cardiac ultrasound. However, as with any emerging technology, the enthusiasm to embrace a new technique must be tempered by a critical appraisal of the evidence supporting its use. It is essential to assess the limitations as well as the unique capabilities it provides. Cardiac imaging should be safe, accurate, versatile, comprehensive, and cost-effective, while providing important clinical information. Criteria for appropriate utilization should be based on current evidence and updated as new information becomes available. To justify the use of a new 3D modality, its unique contribution to clinical practice must be critically analyzed. In this article we review the status of 3D echocardiography, examine the evidence for its use in various clinical situations, and propose guidelines for appropriate application of this technique based on available evidence. Attempts to record and display ultrasound images in 3D format were first reported in the 1960s. One of the earliest studies described the acquisition of a series of parallel scans of a human orbit to reconstruct 3D anatomy.1Baum G. Greenwood I. Orbital lesion localization by three-dimensional ultrasonography.N Y State J Med. 1961; 61: 4149-4157PubMed Google Scholar Despite the limited technology of the day, these initial studies demonstrated that complex anatomic structures were ideally displayed using 3D techniques. Concerns about image quality and the computational power needed for storage and reconstruction greatly limited the early application of this methodology. More than a decade later, investigators began to obtain 3D ultrasound images of the heart.2Dekker D.L. Piziali R.L. Dong Jr., E. A system for ultrasonically imaging the human heart in three dimensions.Comput Biomed Res. 1974; 7: 544-553Abstract Full Text PDF PubMed Scopus (84) Google Scholar Through the careful tracking of a transducer, a sequence of 2-dimensional (2D) echocardiograms could be recorded, aligned, and reconstructed into a 3D data set. This methodology was limited by the need for offline data processing to create and display the 3D images. In the early 1990s, von Ramm and colleagues3Sheikh K. Smith S.W. von Ramm O. Kisslo J. Real-time, three-dimensional echocardiography: feasibility and initial use.Echocardiography. 1991; 8: 119-125Crossref PubMed Google Scholar developed the first real-time 3D (RT3D) echocardiographic scanner, capable of acquiring volumetric data at frame rates sufficient to depict cardiac motion. More recently, further improvements in design and engineering have led to the commercialization of RT3D echocardiography. This methodology has evolved quickly, and different versions of RT3D imaging are currently available on several platforms. Early approaches to 3D echocardiography were based on the principle that a 3D data set could be reconstructed from a series of 2D images. In this method, serial 2D images are obtained using either freehand scanning or a mechanically driven transducer that sequentially recorded images at predefined intervals.4King D.L. Harrison M.R. King Jr, D.L. Gopal A.S. Martin R.P. DeMaria A.N. Improved reproducibility of left atrial and left ventricular measurements by guided three-dimensional echocardiography.J Am Coll Cardiol. 1992; 20: 1238-1245Abstract Full Text PDF PubMed Google Scholar, 5Siu S.C. Rivera J.M. Guerrero J.L. Handschumacher M.D. Lethor J.P. Weyman A.E. et al.Three-dimensional echocardiography: in vivo validation for left ventricular volume and function.Circulation. 1993; 88: 1715-1723Crossref PubMed Google Scholar, 6Jiang L. Vazquez de Prada J.A. Handschumacher M.D. Guererro J.L. Vlahakes G.J. King M.E. et al.Three-dimensional echocardiography: in vivo validation for right ventricular free wall mass as an index of hypertrophy.J Am Coll Cardiol. 1994; 23: 1715-1722Abstract Full Text PDF PubMed Google Scholar, 7Gopal A.S. Schnellbaecher M.J. Shen Z. Boxt L.M. Katz J. King D.L. Freehand three-dimensional echocardiography for determination of left ventricular volume and mass in patients with abnormal ventricles: comparison with magnetic resonance imaging.J Am Soc Echocardiogr. 1997; 10: 853-861Abstract Full Text Full Text PDF PubMed Google Scholar, 8Handschumacher M.D. Lethor J.P. Siu S.C. Mele D. Rivera J.M. Picard M.H. et al.A new integrated system for three-dimensional echocardiographic reconstruction: development and validation for ventricular volume with application in human subjects.J Am Coll Cardiol. 1993; 21: 743-753Abstract Full Text PDF PubMed Google Scholar With freehand scanning, a series of images is obtained by manually tilting the transducer along a fixed plane, and a spatial locator attached to the transducer translates the 3D spatial location onto a Cartesian coordinate system. This approach has several practical limitations, including the relative bulk of the acoustic spatial locators, which makes transducer manipulation difficult, and the need for a clear and direct path between the acoustic locators and the transmitter. For electromagnetic spatial locator systems, an additional problem is the potential for interference of the electromagnetic field by ferromagnetic material in close proximity to the transducer (eg, material in hospital beds and medical equipment).9King D.L. Errors as a result of metal in the near environment when using an electromagnetic locator with freehand three-dimensional echocardiography.J Am Soc Echocardiogr. 2002; 15: 731-735Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar An alternative to freehand scanning is the use of a mechanized transducer to obtain serial images at set intervals in a parallel fashion or by pivoting around a fixed axis in a rotational, fanlike manner. Because the intervals and angles between the 2D images are defined, a 3D coordinate system can be derived from the 2D images in which the volume is more uniformly sampled than with the freehand scanning approach. More recently, the use of a transesophageal or transthoracic multiplane probe has emerged as a readily available method to obtain rotational images at defined interval angles around a fixed axis.10Pai R.G. Jintapakorn W. Tanimoto M. Cao Q.L. Pandian N. Shah P.M. Three-dimensional echocardiographic reconstruction of the left ventricle by a transesophageal tomographic technique: in vitro and in vivo validation of its volume measurement.Echocardiography. 1996; 13: 613-622Crossref PubMed Google Scholar, 11Franke A. Flachskampf F.A. Kuhl H.P. Klues H.G. Job F.P. Merx M. et al.Three-dimensional reconstruction of multiplanar transesophageal echocardiography images: a methodologic report with case examples.Z Kardiol. 1995; 84 ([in German]): 633-642PubMed Google Scholar, 12Roelandt J. Salustri A. Mumm B. Vletter W. Precordial three-dimensional echocardiography with a rotational imaging probe: methods and initial clinical experience.Echocardiography. 1995; 12: 243-252Crossref PubMed Google Scholar, 13Otsuji Y. Handschumacher M.D. Schwammenthal E. Jiang L. Song J.K. Guerrero J.L. et al.Insights from three-dimensional echocardiography into the mechanism of functional mitral regurgitation: direct in vivo demonstration of altered leaflet tethering geometry.Circulation. 1997; 96: 1999-2008Crossref PubMed Google Scholar, 14Pandian N.G. Roelandt J. Nanda N.C. Sugeng L. Cao Q.L. Azevedo J. et al.Dynamic three-dimensional echocardiography: methods and clinical potential.Echocardiography. 1994; 11: 237-259Crossref PubMed Google Scholar, 15Hozumi T. Yoshikawa J. Yoshida K. Akasaka T. Takagi T. Yamamuro A. Three-dimensional echocardiographic measurement of left ventricular volumes and ejection fraction using a multiplane transesophageal probe in patients.Am J Cardiol. 1996; 78: 1077-1080Abstract Full Text PDF PubMed Google Scholar Typically, images are collected over a 180-degree rotation at set intervals. To minimize reconstruction artifacts, sequential images are gated to both electrocardiography (ECG) and respiration. Acquisition of a complete data set typically takes 1 to 5 minutes, depending on respiratory and heart rates and the predefined spatial intervals. During cardiac surgery, respiration can be suspended during acquisition to minimize the effects of respiratory motion. The quality of 3D reconstructions from 2D images depends on a number of factors, including the intrinsic quality of the ultrasound images, the number (or density) of the 2D images used to reconstruct the 3D image, the ability to limit motion artifact, and adequate ECG and respiratory gating. In general, the greater the number of images obtained (ie, the smaller the space intervals between images), the better the 3D reconstruction. However, increasing the number of images also lengthens the acquisition time, which can potentially introduce motion artifact. Consequently, the optimal number of images necessary for 3D reconstruction depends on the cardiac structure being examined and the resolution required. For example, 4 to 6 serial images are usually adequate for volume reconstructions of the left ventricle (LV), whereas more images are often needed to visualize more complex, rapidly moving structures, such as mitral and aortic valves. Once the 2D images have been obtained, they are processed offline with customized or commercially available software. The cardiac structures are manually or semiautomatically traced to the 3D spatial coordinates to reconstruct a 3D image (Figure 1). Several studies have demonstrated that 3D reconstruction from serial 2D images provides accurate anatomic information suitable for quantitative analysis.4King D.L. Harrison M.R. King Jr, D.L. Gopal A.S. Martin R.P. DeMaria A.N. Improved reproducibility of left atrial and left ventricular measurements by guided three-dimensional echocardiography.J Am Coll Cardiol. 1992; 20: 1238-1245Abstract Full Text PDF PubMed Google Scholar, 5Siu S.C. Rivera J.M. Guerrero J.L. Handschumacher M.D. Lethor J.P. Weyman A.E. et al.Three-dimensional echocardiography: in vivo validation for left ventricular volume and function.Circulation. 1993; 88: 1715-1723Crossref PubMed Google Scholar, 6Jiang L. Vazquez de Prada J.A. Handschumacher M.D. Guererro J.L. Vlahakes G.J. King M.E. et al.Three-dimensional echocardiography: in vivo validation for right ventricular free wall mass as an index of hypertrophy.J Am Coll Cardiol. 1994; 23: 1715-1722Abstract Full Text PDF PubMed Google Scholar, 7Gopal A.S. Schnellbaecher M.J. Shen Z. Boxt L.M. Katz J. King D.L. Freehand three-dimensional echocardiography for determination of left ventricular volume and mass in patients with abnormal ventricles: comparison with magnetic resonance imaging.J Am Soc Echocardiogr. 1997; 10: 853-861Abstract Full Text Full Text PDF PubMed Google Scholar, 8Handschumacher M.D. Lethor J.P. Siu S.C. Mele D. Rivera J.M. Picard M.H. et al.A new integrated system for three-dimensional echocardiographic reconstruction: development and validation for ventricular volume with application in human subjects.J Am Coll Cardiol. 1993; 21: 743-753Abstract Full Text PDF PubMed Google Scholar, 10Pai R.G. Jintapakorn W. Tanimoto M. Cao Q.L. Pandian N. Shah P.M. Three-dimensional echocardiographic reconstruction of the left ventricle by a transesophageal tomographic technique: in vitro and in vivo validation of its volume measurement.Echocardiography. 1996; 13: 613-622Crossref PubMed Google Scholar, 11Franke A. Flachskampf F.A. Kuhl H.P. Klues H.G. Job F.P. Merx M. et al.Three-dimensional reconstruction of multiplanar transesophageal echocardiography images: a methodologic report with case examples.Z Kardiol. 1995; 84 ([in German]): 633-642PubMed Google Scholar, 12Roelandt J. Salustri A. Mumm B. Vletter W. Precordial three-dimensional echocardiography with a rotational imaging probe: methods and initial clinical experience.Echocardiography. 1995; 12: 243-252Crossref PubMed Google Scholar, 13Otsuji Y. Handschumacher M.D. Schwammenthal E. Jiang L. Song J.K. Guerrero J.L. et al.Insights from three-dimensional echocardiography into the mechanism of functional mitral regurgitation: direct in vivo demonstration of altered leaflet tethering geometry.Circulation. 1997; 96: 1999-2008Crossref PubMed Google Scholar, 14Pandian N.G. Roelandt J. Nanda N.C. Sugeng L. Cao Q.L. Azevedo J. et al.Dynamic three-dimensional echocardiography: methods and clinical potential.Echocardiography. 1994; 11: 237-259Crossref PubMed Google Scholar, 15Hozumi T. Yoshikawa J. Yoshida K. Akasaka T. Takagi T. Yamamuro A. Three-dimensional echocardiographic measurement of left ventricular volumes and ejection fraction using a multiplane transesophageal probe in patients.Am J Cardiol. 1996; 78: 1077-1080Abstract Full Text PDF PubMed Google Scholar However, this methodology is subject to technical limitations during image acquisition and requires significant offline data processing. The development of RT3D echocardiographic systems circumvents many of the disadvantages of reconstructive methods. RT3D echocardiography uses a transducer with ultrasound elements arranged in a grid fashion (Figure 2). The earliest devices, developed by von Ramm and colleagues,3Sheikh K. Smith S.W. von Ramm O. Kisslo J. Real-time, three-dimensional echocardiography: feasibility and initial use.Echocardiography. 1991; 8: 119-125Crossref PubMed Google Scholar, 16von Ramm O.T. Smith S.W. Real-time volumetric ultrasound imaging system.J Dig Imaging. 1990; 3: 261-266Crossref PubMed Scopus (139) Google Scholar, 17Snyder J.E. Kisslo J. von Ramm O. Real-time orthogonal mode scanning of the heart, I: system design.J Am Coll Cardiol. 1986; 7: 1279-1285Abstract Full Text PDF PubMed Google Scholar used a sparse-array matrix transducer transmitting at a frequency of 2.5 or 3.5 MHz. These transducers consisted of 256 nonsimultaneous firing elements and acquired a pyramidal volume data set measuring 60° × 60° within a single heartbeat. However, the resolution and image quality of this first-generation sparse-array transducer were relatively poor and often inferior to standard 2D images; frame rates were low; and the pyramidal volume had a narrow sector angle of 60°, resulting in an inability to accommodate larger ventricles. Moreover, the images obtained with this system were not volume-rendered online; instead, they consisted of computer-generated 2D cut planes derived from the 3D volume data set. These features limited clinical application of this pioneering technology. Current RT3D systems use matrix-array transducer technology with a greater number of imaging elements, typically containing more than 3000 imaging elements, compared with the 256 in the sparse-array transducer. These current matrix-array transducers offer improved resolution and are rapidly becoming the primary technique for 3D data acquisition in clinical and research practice. However, recent improvements in transducer technology have resulted in (1) a smaller transducer footprint, (2) improved side-lobe suppression, (3) greater sensitivity and penetration, and (4) harmonic capabilities that may be used for both gray-scale and contrast imaging. In addition, these matrix-array transducers display either online 3D volume-rendered images or 2 to 3 simultaneous orthogonal 2D imaging planes (ie, biplane or triplane imaging). RT3D systems generally have 3 acquisition modes: real time (narrow), zoom (magnified), and wide angle. The real-time mode displays a pyramidal data set of approximately 50° × 30° (Figure 3A,video clip 1).18Sugeng L. Weinert L. Thiele K. Lang R.M. Real-time three-dimensional echocardiography using a novel matrix array transducer.Echocardiography. 2003; 20: 623-635Crossref PubMed Scopus (70) Google Scholar The zoom mode displays a smaller, magnified pyramidal data set of 30° × 30° at a higher resolution (Figure 3B). The wide-angle mode provides a pyramidal data set of approximately 90° × 90°, which allows inclusion of a larger cardiac volume (Figure 3C, video clip 2). This wide-angle mode requires ECG gating, because the wide-angle data set is compiled by merging 4 narrower pyramidal scans obtained over 4 consecutive heartbeats. To minimize reconstruction artifacts, data should be acquired during suspended respiration if possible. Although wide-angle data sets provide a larger pyramidal scan, this is at the cost of lower resolution, which is decreased compared with the narrow-angle 3D mode. Once a 3D data set is acquired, it must be sliced or “cropped” to visualize the cardiac structures within the pyramid (Figure 4). Multiple cropping methods are available, but a common method displays 2 or 3 imaging planes simultaneously (video clips 3a and 3b). Each of these imaging planes can be manipulated separately to appropriately align the cardiac structures. Another cropping method involves a single-slice plane that can be manually adjusted to expose and display the cardiac structures of interest (video clip 4). The technical aspects of acquiring a high-quality, diagnostic 3D echocardiogram are similar to those for 2D echocardiography. As with any new imaging technique, a learning curve exists, and recognizing and avoiding potential artifacts is critical. Many of the artifacts are related to respiratory or ECG gating and/or incorrect gain settings. The use of optimal gain settings before acquisition is essential for accurate diagnosis. Low gain settings can artificially eliminate certain structures that then will not be viewable during postprocessing. Alternatively, using high gain settings can mask structures and lead to significant misdiagnoses. Therefore, overcompensating for the brightness of the image using the time-gain compensations is recommended, to allow the overall gain to be set at midrange values. This maneuver will allow maximum flexibility with postprocessing settings. Most 3D echocardiographic systems use some form of gating to obtain volumetric data. Gated data sets are most challenging in patients with arrhythmias or respiratory difficulties. The confounding effects of the gating artifacts can be minimized in different ways. For example, if the gated system acquires sector slices in a sweeping motion parallel to the reference image, then every image viewed parallel to the reference image will appear normal, whereas the gated artifacts will be most noticeable when viewed from a plane orthogonal to the reference image. Segmentation is the process by which anatomic features are extracted from the raw ultrasound data. Segmentation can be accomplished using low-level techniques, such as edge detection (in 2D) and surface detection (in 3D) based on local features, such as the spatial gradient in ultrasound intensity. More sophisticated techniques attempt to extract entire boundaries or surfaces at once based on local features and the anticipated shape of the overall structure. Compression is a mathematical technique that can be applied to the original digital image file to reduce the amount of data, thereby decreasing storage requirements and improving retrieval rates. A single-volume data set from a typical RT3D echocardiographic system consists of 64 × 64 × 512 bytes (approximately 2 MB), or more than 50 MB for a 1-second loop, a load that can overwhelm storage systems. Compression of the digital data files can reduce this load by about 3:1. The motion-JPEG algorithm currently used by DICOM (Digital Imaging and Communications in Medicine) and applied to individual 2D slices could be expected to achieve an approximate 20:1 compression. More advanced algorithms, such as wavelets (JPEG-2000), potentially can yield better results. The use of compression algorithms can decrease the size of data files, optimizing storage efficiency without sacrificing image quality.19Hang X. Greenberg N.L. Shiota T. Firstenberg M.S. Thomas J.D. Compression of real time volumetric echocardiographic data using modified SPIHT based on the three-dimensional wavelet packet transform.Comput Cardiol. 2000; 27: 123-126PubMed Google Scholar A complete 3D echocardiographic study includes an assessment of ventricular function, valvular morphology, and hemodynamic status. Unlike 2D echocardiography, in which standard views are described based on the plane through which they pass, 3D echocardiography is inherently volumetric. As such, it permits both an external view of the heart and multiple internal perspectives (through cropping). Table 1 lists the components of a complete 3D echocardiographic study. A general approach is to describe cardiac structures using both the ultrasound plane and the viewing perspective. Three orthogonal planes are recommended: (1) the sagittal plane, which corresponds to a vertical, long-axis view of the heart; (2) the coronal plane, which corresponds to a 4-chamber view; and (3) the transverse plane, which corresponds to a short-axis view (Figure 5). Each plane can be viewed from 2 sides, which represent opposite perspectives; for example, the transverse plane, which represents the short-axis view, can be visualized from the perspective of the apex or base; the coronal plane can be viewed from above or below; and the sagittal plane can be viewed from the left or right. The choice of narrow-angle or wide-angle imaging acquisition modes depends on the cardiac structure to be examined. For imaging of the ventricles, it is best to use a wide-angle acquisition in the apical window (4-chamber) so as to include the entire ventricle. For smaller structures, such as the aortic valve, a narrow-angle acquisition may be adequate.Table 1A complete 3D echocardiographic protocol•Wide-angle acquisition, parasternal long-axis window: 3D color interrogation of the aortic and mitral valves; 3D color interrogation of the tricuspid and pulmonic valves•Wide-angle acquisition, apical 4-chamber window: 3D color interrogation of the mitral, aortic, and tricuspid valves•Wide-angle acquisition, subcostal window: 3D color interrogation of the atrial and ventricular septa•Wide-angle acquisition, suprasternal notch: 3D color interrogation of the descending aorta Open table in a new tab As an alternative to a complete 3D study, 3D echocardiography can be performed selectively as a complement to a 2D study. Instead of a complete 3D echocardiogram, a more focused 3D imaging study may be appropriate in some cases. For example, in a patient with mitral stenosis, the 3D portion of the study may be limited to visualization and quantification of the mitral orifice. Focused 3D imaging for LV volume calculation, typically performed with an apical 4-chamber wide-angle acquisition, also can be used to complement standard 2D imaging. The ability to extract hemodynamic information derived from 3D color Doppler ultrasonography is currently being investigated. To capture and analyze color flow imaging in 3 dimensions, the area of interest should be obtained within the 3D data set, with the angle of the ultrasound beam aligned as parallel as possible to the direction of blood flow. Depth and sector settings should be optimized for color Doppler resolution. Extraneous flows can be cropped so that only the area of interest is displayed. The color Doppler flow patterns can be analyzed in multiple views to provide a complete assessment of the color Doppler data (Figure 6). 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