Introduction
Appreciation of complex intracardiac anatomy and spatial relationships is inherent to the diagnosis of congenitally malformed hearts. Until recently, the ability of the clinician to image the heart by echocardiography was limited to two dimensions (2DE). The very nature of a 2DE slice, which has no thickness, necessitates the use of multiple orthogonal sweeps. This requires the echocardiographer to mentally reconstruct the anatomy and translate this virtual image into the structure of the report to describe the findings. It is not easy for an untrained observer to decipher the images obtained in the course of a sweep. Furthermore, since myocardial motion occurs in three dimensions, 2DE techniques do not lend themselves to accurate quantitation. Recognition of these limitations of 2DE led to research and clinical interest in three-dimensional echocardiography (3DE).
History of Three-Dimensional Echocardiography
Early approaches were based on 2DE images that were acquired, stacked, and aligned based on the phases of the cardiac cycle to produce a 3DE dataset. The need for time and equipment for offline processing imposed fundamental limitations on their clinical applicability. In 1990, von Ramm and Smith described a transducer that provided live 3DE images of the heart. This transducer was unable to be steered in the third dimension, which has since been termed the plane of elevation; the latter capability, and the ability to render the image in real time, have been developed over the past 15 years.
Advances in Technology
Key advances that have facilitated enhancements in 3DE include matrix-array transducers, piezoelectric materials, and innovations to visualize and quantitate 3D data.
Matrix-Array Transducers
These transducers comprise as many elements in the elevation dimension as they do in the azimuthal dimension, with over 60 elements in each, thus totaling more than 3600 elements. To be able to steer in the plane of elevation, each element must be electrically active and independent; this advance became commercially available in 2002. In 2006, advances in the ability to miniaturize these complex connections led to the development and introduction of a high-frequency pediatric transthoracic 3DE transducer as well as a transesophageal 3DE probe for use in adults.
Piezoelectrical Materials
The piezoelectrical material in a transducer fundamentally determines the quality of the image. Piezoelectrical elements are responsible for delivery of ultrasonic energy into the tissue that is scanned, and for converting waves of reflected ultrasound into electric signals. Their efficiency in converting electrical energy to mechanical energy, and vice versa, is a key determinant of the quality of the image, sensitivity to Doppler shifts, and the ability of transmitted ultrasound to penetrate to increasing depths. One example of new piezoelectrical material involves growing crystals from molten ceramic material, resulting in a homogenous crystal that enables a near-perfect alignment of dipoles, resulting in enhanced electromechanical properties including miniaturization and increased bandwidth and sensitivity, resulting in improvements in both penetration and resolution.
Visualization of Three-Dimensional Echocardiograms
While 3DE techniques provide the ability to acquire wide, trapezoid-shaped data, the visualization of the third dimension (thickness of the slice) is fundamentally challenged within the construct of the 2DE display. Early iterations of software featured variations of grayscale, which limited the perception of depth. Enhancements over the past decade include the use of dynamic schemes that automatically code the near and far fields in contrasting colors ( Fig. 20.1 , and ). More recently, tools such as directional lighting have been used to provide shadows of structures such as wires or devices. A separate advance in visualization has come from the ability to manipulate 3DE images after they have been acquired ( Fig. 20.2 , ). Today, the viewer can interact with the image, performing virtual dissections (cropping) and tilting or rotating the image as desired. More recently, interactive digital holograms have been developed using intraprocedural data from 3D transesophageal echocardiograms. Together, these capabilities constitute fundamental changes to the scope and practice of echocardiography.
Quantification of Cardiac Structures
Quantitative 2DE measurements are based on geometric formulas that rely on (frequently incorrect) assumptions regarding the shapes of cardiac structures. In contrast, 3DE acquisitions have the potential to include entire structures, thus obviating assumptions regarding geometry. Similarly, 3DE software has the potential to quantify cardiac structures accurately regardless of their shape. The software represents the blood pool as a mesh of points and lines for every frame of acquisition, thus providing a moving cast of the cavity of the ventricle through the cardiac cycle. This enables the computation of global and regional volumes, parametric displays of endocardial excursion, and timing of contraction.
Practical Considerations in Pediatrics
Until recently, clinical 3DE experienced organic and unstructured growth, with significant variability in adoption and clinical use between programs. In 2016 and 2017, a document reflecting consensus among experts belonging to the European Association of Cardiovascular Imaging and the American Society of Echocardiography was published. While that document provides details on the acquisition, display, and orientation of images and the added value of 3DE in specific defects, we have summarized key points below.
Acquisition of Images
There are several modes of 3DE acquisition; the appropriate mode must be decided based on the clinical question. While the nomenclature of different modes is specific to each vendor, there are many shared features.
Enhanced Two-Dimensional Echocardiography Capabilities
Matrix transducers enable rotation of the 2DE plane incrementally by 360 degrees. They also allow for simultaneous acquisition and display of multiple orthogonal 2DE images that can be electronically steered in the lateral plane or the plane of elevation. This mode can be used to assess the location, size and shape of atrial and ventricular septal defects, the morphology of valves and outflow tracts, and the nature of regurgitation ( Fig. 20.3 , ).
Real-Time or Live Three-Dimensional Echocardiography
Live 3DE provides a narrow, adjustable pyramidal volume of data. It minimizes the potential for motion and artefacts that are seen with other modes of 3DE, as described below. Acceptable temporal resolution with live 3DE comes at the expense of field of view, which can be a disadvantage in complex defects where multiple spatial relationships need to be determined. Cardiac structures that are relatively static, such as atrial or ventricular septal defects, lend themselves well to live 3DE and are particularly useful when guiding interventions, for example. Some vendors allow adjustments to prioritize temporal resolution at the expense of line density.
Electrocardiographically Gated Acquisitions Over Multiple Beats (Full Volumes)
Full volume 3DE provides a wide pyramidal 3DE volume that is acquired over multiple consecutive cardiac cycles. Full volumes are frequently used in pediatric imaging because they allow the acquisition of a sufficiently large field of view with good temporal resolution. In young children, these datasets are prone to artefacts related to breathing and other movement. This is not an issue in children who can hold their breath or are being ventilated under general anesthesia because ventilation can be briefly suspended. While the capability to acquire a full volume of data over a single cardiac cycle is now available, the current temporal resolution of this latter mode is not acceptable in children.
Three-Dimensional Color Flow
A pyramidal acquisition of 3DE color flow is obtained within a larger acquisition of grayscale data. To ensure adequate temporal resolution, these acquisitions have to be performed over several cardiac cycles.
Principles for Acquisition of Images
Image resolution is best in the axial plane, intermediate in the lateral or azimuthal plane, and lowest in the plane of elevation; this information should be used to decide how best to examine individual structures of interest. Insonation should be performed orthogonal to the plane of the structure of interest. Meticulous attention should be paid to the quality of 2DE images. Adjacent structures that are of clinical relevance should be included. The pediatric probe should be used whenever possible. The width of the scanning sector should be narrowed and the depth of the image should be optimized. Gain and compression must be adjusted to minimize false dropout, typically due to an undergained image, or noise within the cardiac chamber, which is typically due to an overgained image. Either overgained or undergained images may impede visualization ( Fig. 20.4 , optimized gain; , optimized; , overgained; and , undergained). To ensure that full volume acquisitions are free of artifact, careful review of multiplanar images in orthogonal views is recommended.
Display and Orientation of Three-Dimensional Echocardiography Images
Three modes for the display and presentation of 3DE are currently available. Volume-rendered images lend themselves to virtual dissections, enabling the operator to “crop” the heart in multiple sections to view the areas of interest from the perspective that is desired. Surface-rendered images present a cast of the internal surfaces of chambers; these serve as the basis for quantifying ventricular function. Multiplanar reformatting provides orthogonal and adjustable views of the 3DE dataset in a so-called quad screen that has four images. These consist of three rendered 2DE views and the 3DE acquisition. This type of display provides unique 2DE planes that are not available using conventional 2DE devices. It enhances the accuracy of measurements such as diameters and areas, and helps with understanding complex anatomy.
Prior published recommendations for the display of 3DE described the display of cropped images using the three orthogonal planes (right/left, superior/inferior, and anterior/posterior) as they pertain to the heart itself, rather than to the entire body; these guidelines specifically excluded congenital heart disease. The joint European-American consensus document on pediatric 3DE recommends an anatomically correct orientation for image display. The heart is projected in the same orientation as in a person standing upright. Structures that are positioned superiorly in the body are displayed uppermost on the screen, and the diaphragmatic surface of the heart is lowermost. This approach is consistent with other tomographic imaging modalities such as magnetic resonance imaging (MRI) and computed tomography. When applied to 3DE, this approach results in en face views of the septums or valves; these can be rotated either clockwise or counterclockwise to achieve the anatomically correct orientation. Examples of the application of this approach to the atrial septum are demonstrated in Fig. 20.5 and .
“Surgical” View of the Heart
This term has been erroneously used to describe en face views of the cardiac structures. Such views are perhaps more aptly described as the pathologist’s view, or more simply as en face views . The term “surgical view” is more accurately defined as what the surgeon sees, standing to the right of the supine patient, looking into the heart through the right atrium. This generally does not provide the en face projections that can be acquired using 3DE. Annotations on the screen can be useful to ensure correct orientation of the images, particularly when nonstandard views are obtained. For many 3DE images, the anatomic view can be transformed to a more “surgical” view by a 90-degree counterclockwise rotation of the image to mimic the subject moving from an upright to supine position. It is helpful for echocardiographers to appreciate this difference in perspective to aid communication during surgery.
Clinical Applications in Patients With Congenitally Malformed Hearts
3DE imaging has three broad areas of clinical application among patients with congenitally malformed hearts—namely the visualization of morphology, the quantification of sizes of chambers and valves, and the guidance of interventional procedures.
Visualization of Morphology
Centers vary widely in the adoption of 3DE. There have been no randomized trials relating to the success of surgical approaches related to the application of 3DE. Rather, the modality has been adopted on the basis of a clinical need for additional diagnostic information. An expert consensus document provides a consensus on the added value of 3DE to assess broad categories of lesions, including abnormalities of the atrioventricular valves, defects in the atrial and ventricular septums, and more complex intracardiac defects such as double outlet right ventricle (RV). We have provided tables that were published in the pediatric document, listing suggested acoustic windows for specific cardiac structures ( Table 20.1 ) and the roles of 3DE in specific congenital heart defects with normal ( Table 20.2 ) and abnormal ( Table 20.3 ) cardiac connections.
| Subcostal | Apical | Parasternal Long Axis | Parasternal Short Axis | Transesophageal Echocardiography | |
|---|---|---|---|---|---|
| ATRIAL SEPTUM | |||||
| Atrial septal defects | +++ | + (modified) | − | + (modified) | +++ |
| Sinus venosus defects | +++ | +++ | |||
| ATRIOVENTRICULAR JUNCTION AND VALVES | |||||
| Atrioventricular septal defect | +++ | +++ | ++ (left atrioventricular valve) | + (left atrioventricular valve) | +++ |
| Ebstein’s anomaly /tricuspid valve dysplasia | +++ | ++ (anterior angulation) | + | ++ | + |
| Chordal structure of the mitral valve | − | + (smaller patients) | +++ | ++ | +++ |
| Double orifice mitral valve | ++ | ++ | + | ++ | ++ |
| Prolapse of the mitral valve | − | ++ | +++ | ++ | +++ |
| Parachute mitral valve | ++ | ++ | ++ | ++ | ++ |
| Supra mitral membrane | − | ++ | +++ | + | +++ |
| VENTRICULAR SEPTUM | |||||
| Muscular defects (except anterior defects) | +++ | ++ | + | + | ++ |
| Membranous defects | +++ | + | + | + | ++ |
| Doubly committed subarterial defects | ++ | ++ (angled to the pulmonary artery) | + | ++ | |
| OUTLETS | |||||
| Aortic valve | + | ++ | ++ | +++ | |
| Pulmonary valve | + | + | ++ | ||
| Double-outlet right ventricle | +++ | + | + | + | |
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