Echocardiography in Pediatric and Adult Congenital Heart Disease, 2nd Ed.

33. Three-Dimensional Echocardiography in Congenital Heart Diseases

INTRODUCTION

Complex intracardiac anatomy and spatial relationships are inherent to congenital heart defects (CHD). Beginning over thirty years ago and until recently, the clinician’s ability to image the heart by echocardiography has been limited to two-dimensional techniques. In the interim, there have been important advances in 2D echocardiography. Improving transducer technology, beam forming, and miniaturization have led to significant improvements in spatial and temporal resolution using 2DE. However, 2DE has fundamental limitations. The very nature of a 2DE slice, which has no thickness, necessitates the use of multiple orthogonal “sweeps.” The echocardiographer then mentally reconstructs the anatomy, and uses the structure of the report to express this mentally reconstructed vision. This means that the only three-dimensional image of the heart is the “virtual image” that exists only in the echocardiographer’s mind, and is then translated into words. It is not easy for an untrained—albeit interested—observer to understand the images obtained in the course of a sweep: expert interpretation is required. Since myocardial motion occurs in three dimensions, 2DE techniques inherently do not lend themselves to accurate quantitation.

Recognition of these limitations of 2DE led to burgeoning research and clinical interest in the modality of three-dimensional echocardiography (3DE). Early reconstructive approaches were based on 2DE image acquisitions that were subsequently stacked and aligned based on phases of the cardiac cycle, in order to recreate a 3DE dataset. While these approaches proved to be accurate, the need for time and offline processing equipment imposed fundamental limitations on their clinical applicability. In 1990, von Ramm and Smith published their early results with a matrix-array transducer that provided real-time images of the heart in three dimensions. While this was an important breakthrough, this transducer was unable to be steered in the third (elevation) dimension. Over the past five years, dramatic technological advances have facilitated the ability to perform live 3DE scanning, including the ability to steer the beam in three dimensions and to render the image in real time.

3DE TECHNOLOGY

Technologic advances that have facilitated the maturation of 3DE techniques include the following:

■Matrix transducers

■Beam forming and steering in three spatial dimensions

■Display of three-dimensional information

■Software for quantification

Matrix Transducers

Two important advances in transducer technology, namely the organization of elements and the use of novel piezoelectric materials, have been the structural basis for improvements in 3DE matrix transducers. The organization of elements within the transducer is best understood by starting with a brief review of 2DE transducers.

Elements: Two-Dimensional Transducers

Current 2DE transducers transmit and receive acoustic beams in a flat 2DE scanning plane. As opposed to M mode, which provides one spatial and one temporal dimension, 2DE scanning systems sweep a scan line to and fro within this 2DE imaging plane. The angular position of the beam is said to vary in the azimuthal dimension. Even though traditional, flat 2DE scanning comprises two spatial dimensions plus one temporal dimension, this is not three-dimensional imaging. The 2DE transducer itself consists of elements that work in concert to create a scan line. Typically, a conventional transducer consists of 64 to 128 elements arranged along a single row (technically referred to as a one-dimensional array of elements). These elements are spaced according to the ultimate frequency (and hence wavelength) of the acoustic vibrations; these propagate radially along the direction of the scan line. The two spatial dimensions in the image come from sweeping the beam by firing along this row at different times. This array of elements steers the outward ultrasound beam or scan line by using interference patterns generated by varying the spatiotemporal phase of each element’s transmit event.

Elements: Primitive Matrix-Array Transducers

An innovation applied in the last decade was to increase the number of rows of elements from one (in the 2DE transducer described above) to five to seven rows of elements. This “sparse-element” model generated a primitive matrix-array transducer with 5 x 64 = 320 elements. While this represented a dramatic increase in the number of elements, not all elements were electrically active, and the individual elements were not electrically independent from each other. As a result, this transducer did not steer in the third (elevation) dimension. Poor image quality, a large footprint and the lack of portability limited the mainstream acceptability of this approach.

Elements: Contemporary Matrix-Array Transducers

Contemporary matrix-array transducers comprise as many elements in the elevational dimension as they do in the azimuthal dimension, with over 60 elements in each of these dimensions. While the elements are arranged in a two-dimensional grid, this array generates 3DE images. In order to be able to steer in the elevational plane, each element must be electrically independent from all other elements, and each element must be electrically active. The technology and electrical circuitry to electrically insulate and connect each element became commercially available in 2002. As a result, the contemporary matrix-array transducer consists of thousands of electrically active elements that independently steer a scan line left and right, as well as up and down.

Transducers: Piezoelectric Materials

The piezoelectric material in an ultrasound transducer is a fundamental determinant of system image quality. Piezoelectric transducer elements are responsible for delivery of ultrasound energy into the scanned tissue and for converting returning ultrasound echoes into electric signals. Their coupling efficiency in converting electrical energy to mechanical energy or vice versa is a key determinant of image quality, Doppler sensitivity, and penetration. To create an overall piezoelectric effect, these elements must be subject to the application of an external electric field to align dipoles within polycrystalline materials. For almost forty years, a ceramic polycrystalline material, PZT (lead-zirconate-titanate) or PZT composites, have been the standard piezoelectric material used in medical imaging. This material is a uniform powder that is mixed with an organic binder; the resulting compound is baked into a dense polycrystalline structure. At its best, it achieves ~70% alignment of dipoles due to imperfect alignment of the individual dipoles. This leads to a corresponding constraint in the electromechanical coupling efficiency of the material.

One example of new piezoelectric material involves growing crystals from a molten ceramic material, resulting in a homogenous crystal with fewer defects, lower losses and no grain boundaries. When these crystals are poled at the preferred orientation(s), near perfect alignment of dipoles (~100%) is achievable, resulting in dramatically enhanced electromechanical properties (Fig. 33.1). The efficiency of conversion of electrical to mechanical energy improves by as much as 68%–85% when compared to PZT ceramics currently used in ultrasound transducers. The new piezoelectric materials provide increased bandwidth and sensitivity, resulting in both penetration and high resolution. The improved arrangement of atoms in these new piezoelectric materials, and their superior strain energy density, translate into advances in transducer miniaturization. The recent implementation of these advances has led to the availability of high frequency matrix 3DE transducers that have dramatically enhanced the applicability of 3DE to pediatric populations as well as to transesophageal echocardiography.

Figure 33.1. The top panel shows imperfect alignment of dipoles in traditional piezoelectric (PZT) crystals after poling (application of an external electrical field). The bottom panel shows almost perfect alignment of dipoles in new piezoelectric material.

Three-Dimensional Beam Forming and Steering

Beam forming consists of steering and focusing of transmitted and received scan lines. For 3DE, this means that the beam former must be steered in both the azimuthal and elevational planes. This is achieved both in the ultrasound system and within the transducer itself, using highly specialized integrated circuits to create a 3D trapezoid of acoustic information that is processed. These 3DE data are summed, processed, and finally placed into rectangular space using a 3D scan converter.

Display of 3DE

Two-dimensional computer displays consist of rows and blocks of picture elements, termed pixels, that comprise a 2D image. In contrast, a 3DE data set consists of bricks of pixels, termed volume elements or voxels. However, even for a 3DE data set, the two-dimensional nature of the display imposes restrictions on the ability to appreciate depth. As shown initially by the ancient Greeks and subsequently rediscovered during the Renaissance, perspective is used to simulate the appearance of 3D depth, providing objects the appearance of being close to or deeper/further away from the screen. The process of adding perspective is done by casting a light beam through the collection of voxels. The light beam either hits enough tissue so as to render it opaque, or it keeps shining through transparent voxels so as to render it transparent. More recent algorithms apply different hues to the front of the data set (nearest to the screen) as opposed to voxels that are far from the screen (Fig. 33.2, Video 33.1). The user has the ability to rotate and tilt the data set on the computer screen. Tools are available to “cut away” interfering structures, thus performing “virtual dissection.”

Software for Quantification

Quantification requires segmentation of structures of interest from the acquired data. Since myocardial motion occurs in three spatial dimensions, 2DE planes are inherently incapable of capturing the entire motion. Two-dimensional techniques for quantitation are based on geometric formulas that rely on assumptions regarding the shapes of cardiac structures. However, these assumptions are frequently incorrect. In contrast, 3DE acquisitions include the entire extent of the structure, thus minimizing the possibility of foreshortening of the apex or any geometric assumptions regarding shape. Three-dimensional quantitative software tools have the potential to quantify cardiac structures accurately regardless of their shape. Advances in the software tools for processing 3DE datasets have mirrored the rapid advances in transducer technology that have occurred over the past five years.

3DE volumetric techniques traditionally relied on definition of chamber cavities, that is, the blood-endocardium interface. The software constructs this interface by using a process known as surface rendering, and represents it as a mesh of points and lines. This software-generated mesh is calculated for every frame of acquisition, thus providing a moving cast of the cavity of the ventricle during the cardiac cycle (Video 33.2). Since this is digital data, it provides for ease of computation of global and regional volumes, synchrony as well as parametric displays of endocardial excursion and timing of contraction (Fig. 33.3).

3DE quantification tools for the left ventricle are more technologically advanced than for other cardiac structures. Until recently, 3DE LV quantification tools employed the method of disk summation. With improvements in computing speeds and programming, newer tools have been developed to provide instantaneous tracking of the blood pool–endocardium interface at each frame of acquisition. This provides a surface-rendered model that is displayed as a mesh of lines and points. However, these approaches are still based on some basic 3D geometric assumptions regarding left ventricular shape, and therefore their application cannot be extended to the right ventricle or to univentricular hearts. More recently, 3D speckle-tracking echocardiography has been used to quantify left ventricular volumes, ejection fraction, and strain.

Figure 33.2. This is a parasternal short-axis image of a cleft in the anterior mitral leaflet. Asterisks mark the edges of the cleft. A tissue colorization map has been applied to the image. This has the effect of coloring tissues in the near field (near to either the transducer or the front of the 3D image) orange. Tissues in the far field are colored blue. This is a dynamic after effect, which means that as the operator rotates, tilts, or otherwise manipulates the image, the color effect correspondingly changes in real time. L, left; LVO, left ventricular outflow tract; P, posterior; RV, right ventricle; S, septum.

Given the complex shape and architecture of the right ventricle, it is not surprising that tools for quantifying RV volume have been slower to mature. Until very recently, these tools utilized the method of disks for volumetrics. Novel software now provides instantaneous tracking of the blood pool–endocardium interface at each frame of acquisition (Fig. 33.4). This yields a surface-rendered model that is displayed as a mesh of lines and points.

Quantitative software for the mitral valve provides the ability to perform sophisticated analyses of the nonplanar shape of the mitral annulus and to measure 3D structures, including annular diameters, commissural lengths, and leaflet surface areas. Quantitative techniques have also been developed to provide volumetric measurements of 3D color flow using nonaliased color flow data.

Figure 33.3. Software for processing 3DE enables quantitation of left ventricular volumes throughout the cardiac cycle, providing end-diastolic and end-systolic volume as well as ejection fraction. The mesh represents left ventricular volume at end-diastole. The cast of the left ventricular cavity consists of segments of varying colors, each of which represents a subvolume of the ventricular cavity based on the American Society of Echocardiography 16-segment model. The change in volume of each subvolume is represented graphically, with time on the X-axis and volume on the Y-axis.

MODES OF 3DE

Electronically-steered 3DE systems have two major modes of scanning: live and EKG-gated. The live mode is the only one where the system scans in 3D real-time. A defining characteristic of this mode is: if the transducer comes off the chest, the image disappears. The live 3D mode can also be operated within a three-dimensionally shaped zoom box. Live 3DE modes provide narrow (20 to 30 degrees in the elevation plane) datasets that have high voxel density. Live 3DE can be obtained on patients with arrhythmias or with an active precordium; this mode eliminates the potential for motion or stitch artifacts.

Currently, EKG-gated modes are required to provide wider volumes while maintaining adequate frame rates (Fig. 33.5). Gating allows for anywhere from two to eight smaller volumes to be stitched together to generate volumes that are greater than 90 degrees wide in the elevation plane, at frame rates exceeding 30 Hz. Gated modes have comparatively lower voxel density, and are subject to both motion and stitch artifacts. Recent enhancements have improved the ability to acquire gated full-volume data among patients with arrhythmias. Gated modes are available using grayscale or with color flow Doppler.

Figure 33.4. Novel software provides the ability to measure right ventricular volume throughout the cardiac cycle. This yields a surface-rendered model that is displayed as a mesh of lines and points. The change in volume of each subvolume is represented graphically, with time on the X-axis and volume on the Y-axis.

Figure 33.5. This figure depicts the differences between 2D, live 3D, and full-volume (ECG-triggered) 3DE. Conventional 2DE is shown in the left panel. Live 3DE imaging (center panel) adds the elevation plane. The shape of the image is therefore trapezoidal rather than pie-shaped. The right panel depicts ECG-triggered (full-volume) 3DE imaging, which provides a wider trapezoid.

Contemporary 3DE systems provide the user with tools to vary frame rate, 3D volume size, and image resolution. Increasing the requirement in one of these causes a drop in the other two, all things being equal.

Given the potential for motion and stitch artifacts with gated modes and the need for high spatial and temporal resolution, it has been our practice to use live 3DE to delineate anatomy in children. We also use live 3DE for the purpose of image-guidance of interventions, because instant visual feedback is critical in this application. We reserve the use of gated modes for the following:

■Targets that do not fit within a live 3DE window

■Quantitation of chamber volumes 3DE color flow demonstrations of regurgitant jets or shunt flows

Modes

Live 3DE imaging has been commercially available for transthoracic and fetal applications since 2002. Live 3DE transesophageal echocardiographic imaging became commercially available in 2007. This has yielded images never before seen of the beating heart (Fig. 33.6). We anticipate that continuing improvements in transducer technology and the wider applicability of advances in piezoelectrics will enable the application of 3DE technology to an ever-increasing range of patient sizes, windows, and applications.

Figure 33.6. Live 3D transesophageal echocardiography demonstrates a catheter passing through a large atrial septal defect. The viewing perspective is unique: the observer is virtually located within the left atrium, looking rightwards. A, anterior; I, inferior.

CLINICAL APPLICATIONS IN CONGENITAL HEART DISEASE

3DE imaging has three broad areas of clinical application among patients with congenital heart disease: visualization of morphology, volumetric quantitation of chamber sizes and flows, and image-guided interventions.

Visualization of Morphology

Dating from an early stage in the development of 3DE technology, the structural complexity that is inherent to congenital heart disease has been identified as fertile substrate for exploration using 3DE. The recent publication of a consensus document for 3DE image orientation and display has provided the foundation for developing standardization in image display: a key factor because the addition of the third dimension has led to great flexibility and variability in image orientation. We anticipate the development of a similar consensus document for 3DE in congenital heart defects, and expect that such a document would prove of great value.

The Atrioventricular Valves

3DE is valuable in delineating the morphology of the atrioventricular valves. Espinola-Zavaleta et al. described the role of 3DE in delineating congenital abnormalities of the mitral valve. Rawlins et al. demonstrated the additive value of 3DE and improved image quality using intraoperative epicardial 3DE to delineate the anatomy of atrioventricular valves. Seliem et al. studied 41 patients with AV valve abnormalities and found that 3DE imaging was helpful in delineating the morphology of the valve leaflets and their chordal attachments, the subchordal apparatus, the mechanism and origin of regurgitation, and the geometry of the regurgitant volume. Vettukatil et al. examined the role of 3DE in patients with Ebstein anomaly of the tricuspid valve. They demonstrated that 3DE provided clear visualization of the morphology of the valve leaflets, including the extent of their formation, the level of their attachment, and their degree of coaptation. They were also able to visualize the mechanism of regurgitation or stenosis. 3DE provides unparalleled views of cor triatriatum (Video 33.3) as well as en face views of the tricuspid valve (Video 33.4).

In children with hypoplastic left heart syndrome, tricuspid regurgitation is a serious problem that has been associated with decreased capability to successfully complete the Fontan operation. In a series of seminal papers, Nii et al. demonstrated the importance of mitral-tricuspid annular interaction and the role of tricuspid annular function and papillary muscle displacement as a cause of tricuspid regurgitation in hypoplastic left heart syndrome. These investigators subsequently hypothesized that abnormalities of tricuspid annular function and papillary muscle location are directly related to the severity of tricuspid regurgitation in hypoplastic left heart syndrome through disturbances of leaflet function. They found that moderate TR is associated with tethering and prolapse of the TV leaflets, that patients with a tethered TV have lateral displacement of their anterior PM and a planar annulus, in a manner similar to that seen in adults with functional mitral regurgitation. In contrast, a smaller TV septal leaflet area, annular enlargement, increased annular height, and increasing patient age were associated with prolapse. Studies such as these promise to have important practical applications for the surgical management of tricuspid regurgitation in hypoplastic left heart syndrome.

Atrioventricular Septal Defect

Hlavacek et al. studied 52 datasets on 51 patients with atrioventricular septal defects (AVSD) and showed that gated 3DE views could be cropped to obtain en face views of the atrial and ventricular septa. These views provide a clear understanding of the relationships of the bridging leaflets to the septal structures (Fig. 33.7, Video 33.5). These views have been useful to determine the precise location of the interventricular communication relative to the bridging leaflets, and to demonstrate how these relationships determine the level of shunting (atrial, ventricular, or both). They found that 3DE on unrepaired balanced AVSD and repaired AVSD with residual lesions was more often additive/useful (33/36; 92%) than on repaired AVSD without residual lesions or unbalanced AVSD (9/16 (56%), P=0.009). 3DE was additive or useful in all three patients with unbalanced AVSD being considered for biventricular repair. Useful information obtained by 3DE included: precise characterization of mitral regurgitation and leaflet anatomy, unique viewing perspectives such as the surgeon’s view of a cleft mitral valve (Video 33.6), substrate for subaortic stenosis, valve anatomy, and presence and location of additional septal defects. Kutty and Smallhorn recently published a comprehensive review of the role of 3DE in AVSD. This work highlights the role of 3DE in understanding the preoperative anatomy of AVSD and, in particular, postoperative issues such as left AV valve regurgitation and left ventricular outflow tract obstruction. Practical tips on 3D image acquisition are also provided in this review. In a quantitative study of left AV valve regurgitation following repair of AVSD, Takahashi et al. showed that the severity of postoperative left AV valve regurgitation is related to dilatation of the mitral valve annulus, leaflet prolapse, a more acute angle between the anterolateral papillary muscle and the mitral valve annulus, and lateral displacement of anterolateral papillary muscle position.

Figure 33.7. This is an apical four-chamber 3D echocardiogram in a patient with atrioventricular septal defect. Asterisks mark the hinges of the common atrioventricular junction. The bridging leaflets divide the defect into an interatrial (ASD) and interventricular (VSD) component. Note the posterior location of the inferior vena caval orifice (IVC) relative to the septal structures.

The Atrial and Ventricular Septa

Tantengco et al. showed that 3DE reconstructions provided unique en face views of atrial and ventricular septal defects. Cheng et al. studied 38 patients with atrial and/or ventricular septal defects using 3DE, and compared their results to 2DE and surgical findings. They demonstrated novel 3DE views of both atrial and ventricular septal defects and improved accuracy of quantification of the size of the defect by 3DE compared to 2DE (r = 0.92 vs. r = 0.69). This approach has also been used to demonstrate the morphology of muscular ventricular septal defects. We have found live 3DE to be of great value in evaluating the ventricular septum en face (Video 33.7) and to assess malformations of the outflow tract that involve malalignment of the outlet septum (Video 33.8). These early reports were of value in demonstrating the feasibility of 3DE. More recent papers have emphasized practical approaches to 3DE targeting specific lesions and defects. Sivakumar et al. have recently published an elegant series demonstrating a simplified subxiphoid 3DE acquisition technique to visualize ventricular septal defects. Similarly, Faletra et al. and Saric et al. have published elegant practical guidelines on how to demonstrate atrial septal defects.

The Aortic Arch, Pulmonary Arteries, and Aortopulmonary Shunts

3DE color flow Doppler has been used to provide echocardiographic “angiograms” of flow patterns in the aortic arch (coarctation of the aorta), the branch pulmonary arteries (the Lecompte maneuver), and across Blalock-Taussig shunts. These authors examined echocardiographic “angiograms” in 26 patients (Fig. 33.8, Video 33.9). 3DE provided additional diagnostic information in 10 of 26 patients (38%). In 17 of 26 patients (65%), validation of the 3DE diagnosis was available at surgery, cardiac catheterization, MRI, or CT angiography.

The Aortic Valve and Outflow Tract

Sadagopan et al. examined the role of 3DE in 8 children who subsequently underwent surgery for congenital aortic valvar stenosis. They showed that 3DE was accurate in providing measurements of aortic valve annulus and number of valve leaflets, and in identifying sites of fusion of the leaflets as well as nodules and excrescences that characterized dysplastic valves. Bharucha et al. studied 16 patients with subaortic stenosis. Using a form of 3DE reconstruction known as multiplanar reconstruction, which provides access to an unlimited number of 2DE planes, they demonstrated abnormalities of mitral valve leaflet or chordal apparatus attachments (14 patients), abnormal ventricular muscle band (11 patients), and abnormal increased aortomitral separation (2 patients). Noel et al. compared measurements of aortic root diameter by 2DE to those obtained using 3DE. They demonstrated that three-dimensional echocardiography compares favorably with 2DE in the evaluation of aortic root dilation in patients with known histories of aortic root disease. The largest diameter measured by 3DE was significantly larger than on 2DE, which may well be due to the multiplanar nature of 3DE acquisitions, which lends itself well to measuring the largest dimension regardless of the acquisition window. They found that the interobserver variability of measurement of 3DE diameters is low, particularly in the anteroposterior dimension and in identifying the largest diameter.

Figure 33.8. This is a 3DE color flow, ECG-triggered (full-volume) acquisition demonstrating a right-sided Blalock-Taussig shunt (BTS). The grayscale image has been suppressed, thus providing an echocardiographic angiogram. The shunt is seen in its entirety from its origin from the innominate artery (Inn) to its insertion. The proximal and distal right pulmonary artery (pRPA and dRPA, respectively) are well seen. Note the proximity of the superior vena cava (SVC) to the cranial end of the shunt. This dataset can be rotated, tilted, and examined in an infinite number of planes in order to delineate the location of stenosis. I, inferior; L, left.

Characterization of Left Ventricular Noncompaction

Baker et al. evaluated four patients with left ventricular noncompaction using 3DE. They found that 3DE enabled diagnosis and provided detailed characterization of the affected myocardium, including easy visualization of entire trabecular projections, intertrabecular recesses, endocardial borders, and wall motion abnormalities of the affected myocardium. 3DE enabled easy differentiation between compacted and noncompacted portions of the myocardium.

Quantitation of Chamber Dimensions, Valve Apparatus, Function, and Flows

The recent ASE-EAE consensus paper on 3DE image acquisition and display provides a comprehensive summary of the state of quantitative 3DE, particularly as applied to adult echocardiography. There are significant challenges with echocardiographic quantitation in the pediatric age group relating to transducer frequency, gain, and compression. In an elegant in vitro study, Herberg et al. used pediatric-sized tissue phantoms to examine this question, and found that measurement of distance was accurate both in 2D and in real-time 3D echocardiography. Measurements of small areas and volumes differed significantly from true values; 3DE was more accurate than was 2DE. The accuracy of these measurements was significantly influenced by the level of image gain and compression at the time of image acquisition as well as by postacquisition (workstation-related) image gain.

Left Ventricular Volumetrics

3DE quantitation provides both proven and potential value for pediatric echocardiography. Bu et al. compared 3DE measurements of LV volumetrics to those obtained using MRI. Their study showed that 3DE measurements of LV end-systolic volume, end-diastolic volume, mass, stroke volume, and ejection fraction in children using a rapid full-volume acquisition strategy are feasible, reproducible, and comparable with MRI measurements. They found good correlations between the two methods, but a tendency toward mild underestimation of volumes by 3DE. Interestingly, estimates of ejection fraction were in closer agreement. In a study examining the feasibility of 3DE LV volumetrics, Baker et al. demonstrated acceptable resource utilization, a negotiable learning curve, and excellent inter- and intraobserver reproducibility for 3DE LV volumetrics. In a study of neonates and infants with complex CHD, Friedberg et al. have shown that high-quality, full-volume 3D echo acquisitions to measure 3D mass and volumes could be obtained in the majority (87%) of patients. LV diastolic volumes and mass by matrix-array 3D echo compared well to those measured by MRI.

Ashraf et al. have validated the measurement of left ventricular twist using epicardial 3DE in an open-chest porcine model, compared to sonomicrometry. The clinical utility and mainstream acceptance of such elegant techniques is hampered by the known intervendor and interplatform variability of these measurements.

Right Ventricular Volumetrics

The location, complex anatomy, and irregular shape of the RV pose a challenge to traditional imaging. The RV has complex contraction patterns, with its inflow and sinus exhibiting shortening primarily in a longitudinal direction, whereas the outflow tract contracts primarily in a circumferential manner. The accuracy of 3DE RV volumetrics compared to MRI has been well established across a range of patient sizes (ranging from children to adults), both in normal populations and in disease states ranging from tetralogy of Fallot to univentricular hearts.

Two recent studies indicate ways in which 3DE may provide important insights into the remodeling of the univentricular heart with a systemic right ventricle. The NHLBI-funded Pediatric Heart Diseases Clinical Research Network recently published the results of a multicenter, prospective, randomized controlled trial examining the Blalock-Taussig shunt to the RV to pulmonary artery (RVPAS) shunt for initial palliation (Norwood stage I) surgery for hypoplastic left heart syndrome. This study included systematic evaluation of the feasibility and use of 3DE indices of RV volume and EF as secondary end points. Of 484 patients who were enrolled in the study in centers that indicated they were capable of 3DE imaging, 349 (80%) patients underwent 565 3D echocardiograms that were deemed acceptable for quantitative 3DE analysis. This analysis showed that at age 14 months, there was no difference between the two groups in 3DE measures of RV size and function and magnitude of TR. No significant difference by shunt, or indeed any clinically relevant difference by shunt, was found in any of the 3DE variables examined at any stage of repair during the first 14 months after randomization. Volume unloading was seen after stage II, as expected, but the ejection fraction did not improve. Kutty et al. performed a similar single-center prospective study in 18 patients, all of whom underwent an RVPAS. They demonstrated a lack of acute RV volume unloading and a serial decrease in RV ejection fraction over the short-term following stage II palliation.

Notwithstanding these important studies, the currently-available 3DE algorithms for RV volumetrics are limited by their reliance on the interrelationships of normal landmarks such as the mitral and tricuspid valves, and the RV and LV apices. Such reliance makes it difficult to measure RV volumes accurately in the setting of profound ventricular size mismatch, as is encountered in hypoplastic left heart syndrome. The development of algorithms for RV volumetrics that are independent of other landmarks, and more automation, would be important directions for future developments.

Visualization and Quantitation of 3DE Color Flow

Multiplanar reconstruction tools provide the ability not only to visualize but also to trace and measure the area of valvar regurgitant orifices at the level of the vena contracta. While this technique is new and has not been validated, it has been shown to be feasible, and has already yielded new insights into the shape of regurgitant jets, and holds promise as a tool to enhance the echocardiographer’s ability to serially quantify valve regurgitation. This technique was also used in the PHN SVR study that was discussed in the prior section.

Pemberton et al. developed a technique for quantifying nonaliased 3DE color flow jets in vitro. They validated this technique on open-chested pigs; when compared to measurements obtained from flow probes positioned on the ascending aorta, they obtained excellent correlations between the two methods. They compared 143 individual measurements of cardiac output and found excellent correlation between the two techniques (r2 = 0.93). 3DE quantification of color flow has recently been validated in adults by comparison to cardiac outputs obtained by thermodilution. Application of this technique to pathologic states could lead to potentially more accurate measurements of regurgitant volumes and fractions.

Image-Guided Interventions

The high temporal and spatial resolution of transthoracic, and particularly transesophageal, echocardiography has ignited interest in the potential uses of live 3DE to guide interventions. Scheurer et al. demonstrated the use of live 3DE to guide the performance of endomyocardial biopsy in children. In their experience, the use of live 3DE guidance was associated with no complications, including no new tricuspid valve flail leaflet or pericardial effusion. 3DE proved to be a reliable noninvasive modality to accurately direct the bioptome to the desired site of biopsy within the right ventricle. As familiarity with this technique increased, the need for fluoroscopic guidance of bioptome manipulation in the right ventricle was minimized. Del Nido et al. have extended the concept of image-guided intervention to the very novel approach of epicardial live 3DE–guided, open-chest, closed-heart, off-bypass cardiac surgery. They began with in vitro validation of the ability of live 3DE to guide the performance of common surgical tasks and moved on to closure of small atrial septal defects in the porcine model using live 3DE guidance. More recently, they have shown that patch closure of ventricular septal defects and suture annuloplasty of the mitral valve can be performed (ex vivo) on the beating heart. With the recent introduction of live 3D TEE, we have been able to obtain previously unobtainable images of cardiac chambers, septal structures, and valves. Live 3D TEE is now routinely used to guide catheter manipulations, transseptal procedures, closure of atrial septal defects, and percutaneous interventions on the mitral valve (Figs. 33.9 and 33.10). More recently, fusion imaging involving registration of 3D echocardiograms to live fluoroscopic images has led to the ability to provide combined images that present information in the context of a familiar frame of reference.

LEARNING CURVE

The learning curve with 3DE is steep but negotiable. Our experience would suggest that the success of 3DE in a program requires advocacy and an investment of time by both echocardiographers and sonographers. The acceptance of 3DE is improving on a global level, albeit at an early stage of the technology cycle. We have developed and implemented an interactive teaching course that utilizes simulations using 3DE datasets with rehearsal and direct mentoring; this has been shown to be useful in overcoming the steep part of the learning curve.

Figure 33.9. This is a live 3D transesophageal echocardiogram immediately following device closure of a fenestrated atrial septum with a cribriform Amplatzer septal occluder. The viewer is “virtually” placed inside the left atrium, looking rightwards and posteriorly. The entire left atrial disk of the device is seen, with its central umbilication and the meshwork of metal that constitutes its frame. Part of the inferior portion of the right atrial disk (RA disk) and the tricuspid valve apparatus (TV) are also seen. I, inferior; L, left.

Figure 33.10. This is from a live 3DE transesophageal echocardiogram in a patient with a low secundum atrial septal defect. 2DE imaging demonstrated that the defect was in proximity to the coronary sinus, raising doubts about candidacy for device closure. In this 3DE image, the free walls and appendage of the right atrium have been removed. The viewer is looking posteriorly and leftward. Note the high resolution of anatomic detail. The atrial septal defect is separated from the coronary sinus orifice by a reasonable distance. Based on the shape and size of the defect, this was felt by 3DE to be a reasonable candidate for device closure, which was successfully accomplished. A, anterior; S, superior.

FUTURE DIRECTIONS

Over the next decade, advances in the 3DE arena will involve technical enhancements such as improved image resolution, holographic displays, a wide range of validated software tools for quantification, and enhancements to work flow. New multimodality applications will increasingly bring 3DE into the mainstream. Refinements in transducer technology will make high-resolution 3DE available across the spectrum of patient sizes. We anticipate a 3DE TEE probe miniaturized for pediatric usage. With the growing interest in multimodality imaging, 3DE volumetric data will eventually be integrated with the pressure data that is available during cardiac catheterization, yielding pressure-volume loops that can be obtained as a matter of routine clinical practice.

SUGGESTED READING

Acar P, Abadir S, Paranon S, et al. Live 3D echocardiography with the pediatric matrix probe. Echocardiography. 2007;24(7):750–755.

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Questions

1.Which of the following advances have NOT enabled the maturation of 3-D echocardiography (3-DE)?

A.Matrix transducers

B.Beam forming and steering in three spatial dimensions

C.Display of 3-D information

D.Software for quantification

E.Discovery of the fourth dimension

2.Indications for full-volume (noninstantaneous) 3-D echocardiography currently include all of the following except:

A.targets that do not fit within a live 3-DE window.

B.quantitation of chamber volumes.

C.3-DE color-flow demonstrations of regurgitant jets or shunt flows.

D.Live guidance of intracardiac interventions.

3.Which of the following is NOT an appropriate target for 3-DE in congenital heart defects?

A.Determining atrioventricular valve morphology

B.Identifying substrates for atrioventricular valve regurgitation

C.Identifying coronary artery flow characteristics

D.Identifying substrates for outflow tract obstruction

E.Obtaining en-face views of valves and septal defects

4.For which of the following is 3-DE quantification of function most advanced in its application?

A.The mitral valve

B.The tricuspid valve

C.The right ventricle

D.The left ventricle

E.The pulmonary valve

5.Which of the following does NOT affect echocardiographic quantitation of chamber sizes and distances?

A.Image gain at the time of acquisition

B.Image compression at the time of acquisition

C.Post-acquisition image gain

D.Temporal resolution

E.Azimuthal resolution

6.In pediatrics, 3-DE quantitation has been validated (compared to gold standards) for all of the following except:

A.valvar regurgitant volumes.

B.left ventricular volumetrics.

C.right ventricular volumetrics for subpulmonary right ventricles.

D.right ventricular volumetrics for systemic right ventricles.

E.left ventricular mass.

7.In patients with atrioventricular septal defect (atrioventricular canal defect), 3-DE has utility in all of the following except:

A.Understanding the relationships of bridging leaflets to the inter-chamber communications.

B.Evaluating patients for univentricular versus biventricular repair.

C.Repaired defects without residual lesions.

D.Understanding the substrate for postoperative left atrioventricular valve regurgitation.

8.What is Video 33.1 an example of?

A.Rheumatic mitral valve stenosis

B.Mitral valve endocarditis

C.Flail chords affecting the A2 scallop of the mitral valve

D.Parachute mitral valve

E.Cleft mitral valve as seen in atrioventricular septal defect

9.Contemporary 3-DE systems allow the user to vary all of the following except:

A.Volume (frame) rate

B.Size of the 3-DE volume being acquired

C.Overall image resolution

D.Transducer frequency

E.Image resolution in specific dimensions / axes

10.Which of the following is not an advantage of live 3-DE imaging?

A.No motion artifact

B.No stitch artifact

C.Provides high temporal and spatial resolution

D.Allows quantitation of ventricular volumes

E.High frame rates for 3-DE color flow.

Answers

1.Answer: E. The development of matrix transducers, beam forming and steering, displays for 3-D information, and quantification software have all significantly advanced 3-D technology.

2.Answer: D. Full volume 3-DE is indicated for quantification of chamber volumes as well as regurgitant / shunt flow. In addition, targets outside the live 3-D window necessitate a full-volume acquisition to render 3-D spatial information.

3.Answer: C. 3-DE is ideally suited to image AV valve morphology and function as well as defining anatomy of the LVOT and septal defects.

4.Answer: D. Currently, 3-DE is most suited for quantitation of LV volume and function.

5.Answer: E. Quantitation is impacted by image gain (both at the time of acquisition and in post-processing) and compression as well as the temporal resolution.

6.Answer: A. At present, 3-DE has not been validated in the quantitative assessment of regurgitant volume.

7.Answer: C. In repaired defects without residual lesions, 3-DE does not have incremental value over 2-DE.

8.Answer: E. There is a cleft in the anterior mitral leaflet in this video clip.

9.Answer: E. While the overall image resolution can be influenced by the user, image resolution in specific dimensions / axes cannot. Frame rate, image size, and transducer frequency can also be varied in 3-DE.

10.Answer: D. Significant advantages of live 3-DE imaging include lack of motion and stitch artifacts as well as high temporal and spatial resolution. High frame rates for 3-DE color flow are also possible with live 3-DE.