Abstract
Advances in computer and transducer technology have allowed the implementation of three-dimensional echocardiography (3DE) in routine clinical practice, with significant impact on patient diagnosis, management, and outcome. 3DE now offers faster and easier data acquisition, superior image quality, immediate display of cardiac structures, and the possibility of online quantitative analysis of heart valves and cardiac chambers. In comparison with two-dimensional echocardiography used for cardiac chamber quantification, 3DE offers the advantage of enabling the measurement of chamber volumes without any geometric assumptions about their shape or errors due to apical view foreshortening. Moreover, 3DE offers a more realistic anatomic and surgical display of heart valves and congenital defects, allowing a better appreciation of the complex morphologic abnormalities and a superior understanding of the functional interaction among neighboring structures in vivo. As interventional and surgical procedures have become more prevalent for treating structural heart disease, transesophageal 3DE has become a valuable imaging technique for patient selection, intra-procedural guidance and detection of potential complications. Undoubtedly, 3DE will rapidly expand its clinical applications and use across echocardiography laboratories in the next years, and progressively emerge as the primary technique of assessing cardiac and valve function by ultrasound.
Keywords
matrix array transducers, physics, real time, three-dimensional echocardiography (3DE)
Introduction
Since 1974, when the first three-dimensional echocardiography (3DE) images of the heart were obtained by Dekker and colleagues, 3DE technology has greatly evolved. The development of the real-time volumetric acquisition technique, along with significant technological advances in computer and transducer technologies, have significantly improved the image quality and the practical feasibility of 3DE, allowing its implementation in clinical practice. 3DE data sets can be acquired from either transthoracic (TTE) or transesophageal (TEE) approach, allowing real-time visualization of the cardiac structures from any spatial point of view. 3DE has been demonstrated to be superior to two-dimensional echocardiography (2DE) in various clinical scenarios, such as (1) quantification of cardiac chamber volumes and function,(2) assessment of the mechanisms and severity of heart valve diseases, (3) evaluation of cardiac complex anatomy and defects in congenital valve diseases, and (4) patient selection and monitoring during cardiac interventional procedures ( Boxes 5.1 and 5.2 ). However, to take the best use of this technique, a full understanding of its technical principles, as well as of its strengths and limitations, is essential.
- 1.
Patients with distorted left ventricular anatomy (aneurysm, extensive wall motion abnormalities, etc.) in whom accurate measurement of volumes will be clinically relevant
- 2.
Patients with left ventricular dysfunction who may be candidates for device implantation or complex surgical procedures
- 3.
Patients with heart failure, or right heart diseases that may affect right ventricular size and function
- 4.
Mitral valve assessment in patients referred to mitral valve surgery
- 5.
Evaluation of mitral stenosis
- 6.
Patients with tricuspid stenosis and/or more than mild tricuspid regurgitation in whom assessment of tricuspid valve morphology and severity of regurgitation will be clinically relevant
- 7.
Congenital heart diseases
- 8.
Patients with acceptable acoustic window quality with unclear anatomy by two-dimensional imaging
- 1.
Assessment of mitral valve anatomy in patients in whom the data will be clinically relevant for management: mitral stenosis, functional or degenerative mitral regurgitation, congenital abnormality, endocarditis
- 2.
Left ventricular outflow tract sizing in patients referred for transcatheter aortic valve replacement (TAVR) who cannot undergo cardiac tomography
- 3.
Assessment of aortic valve anatomy in patients with aortic regurgitation, candidates to aortic valve repair
- 4.
Left atrial appendage orifice sizing in candidates for device closure
- 5.
Assessment of atrial septal defect anatomy and size in candidates for device closure
- 6.
Suspected or known mitral valve prosthesis structural or nonstructural dysfunction or endocarditis
- 7.
Guiding/monitoring interventional procedures in the cath lab
- 8.
Pre- and postoperative assessment of mitral valve and congenital heart disease cardiac surgery
- 9.
Cases with uncertain 2D anatomy in transthoracic and transesophageal studies
Fully Sampled Matrix Array Transducers
Considerable advancements in hardware and software, involving microelectronic techniques, image formation algorithms, and digital processing, have led to the development of fully sampled matrix array transducers, which enabled the volumetric 3DE acquisition with good imaging quality within a short acquisition time ( Fig. 5.1 ). At present, matrix array transducers are composed of nearly 3000 piezoelectric elements (as opposed to only 128 elements in a conventional 2DE phased-array transducer), with operating frequencies ranging from 2 to 4 MHz for TTE and from 5 to 7 MHz and TEE imaging. The piezoelectric elements are arranged in rows and columns to form of a rectangular grid (i.e., matrix configuration), individually connected and simultaneously active (fully sampled). The electronically controlled phased firing of the piezoelectric elements enable to generate a scan line that propagates radially (y, axial direction) and can be steered both laterally (x, azimuthal direction) and in elevation (z, vertical direction) to acquire a pyramidal volumetric data set (see Fig. 5.1 A and B ).
Currently matrix-array probes are available for both TTE and TEE imaging and, in addition to the conventional 2D-Doppler imaging, they enable three different acquisition modalities: multiplane 2DE imaging, real-time (or live) 3DE imaging, and multibeat ECG-gated 3DE imaging, all three with/without color flow information ( Chapter 10 , “3D Image Acquisition”).
Previous 3DE equipment could only acquire and display in real-time volumetric data sets of a relatively small size (about 30° × 50°). These pyramids were sufficiently large to allow a partial display of the ventricles or of the valvular structures; the larger volumes needed to encompass the whole structure required at least four smaller component volumes acquired over a series of consecutive cardiac cycles to yield a 90° × 90° image (see Fig. 5.1C ). However, the technology evolved and the current 3DE systems have the capability of acquiring and displaying single-beat volumes as 90° × 90° pyramids in real time (see Fig 5.1A ), with improved temporal and spatial resolution (even though significantly lower than by multi-beat acquisition).
A major technological breakthrough that allowed manufacturers to develop fully sampled matrix array transducers was the electronics miniaturization and microbeamforming. Several miniaturized circuit boards have been incorporated into the matrix-array transducer, allowing partial beamforming to be performed in the transducer itself, reducing both power consumption and the size of the connecting cable. In addition, more advanced crystal manufacturing processes (such as the PureWave crystal technology), by increasing the efficiency of transduction and of conversion process between electrical power and ultrasound energy, helped reduce heat production.
Physics of Three-Dimensional Ultrasound
3DE is an ultrasound technique; consequently it is limited by the speed of ultrasound in human body tissues (∼1540 m/s in myocardial tissue and blood). The image depth determines the distance a single pulse has to travel backward and forward, resulting in the maximum number of pulses per second. The acquisition is performed by a pyramidal volume with the desired beam spacing in each dimension (spatial resolution), which is related to the volumes per second that can be imaged (temporal resolution). As a consequence, similarly to 2DE, there is an inverse relationship between temporal resolution, acquisition volume size, and spatial resolution, as represented in the equation:
Volume rate ( temporal resolution ) = 1,540 × Number of parallel received beams ( volume size ) 2 × volume width / lateral resolution 2 × Volume depth ( spatial resolution )
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