Introduction

Three-dimensional (3D) echocardiography is a recently introduced technique, which offers new possibilities for evaluation of heart size, function and valve integrity. This presentation reviews the potential utility of 3D echocardiography in small animals.

Three-dimensional Echocardiographic Modalities

Echocardiography has over the last four decades evolved from single-beam and two-dimensional (2D) imaging to 3D techniques. The 3D technique was initially based on dataset reconstructed from series of 2D images, which limited the utility of this technique due to long acquisition and post-processing time as well as image quality and alignment concerns. Technical improvements in 3D hardware and software have enabled a more widespread clinical application of 3D echocardiography. The first real-time three-dimensional (RT3D) echocardiographic system was introduced in the early 1990s, and further developments in design and engineering have led to the recent commercialization of RT3D echocardiographic systems. Modern RT3D echocardiographic systems utilize high-frequency matrix array transducers that consist of more than 3000 individual crystals, which simultaneously acquire data from the heart in a 3D pyramidal fashion in real time, thereby allowing simultaneous visualization of the beating heart. Some currently available RT3D echocardiographic modalities of imaging acquisition include live 3D, 3D zoom, 3D full volume, and 3D color Doppler flow analysis. Live 3D is a real-time live imaging mode, which provides a wedge-shaped 3D sector with a sector volume expanding in height and width at greater imaging depth. 3D zoom mode provides a smaller magnified real-time pre-cropped image. 3D full-volume acquisition technique is based upon rapid reconstruction of wide-angled pyramidal live 3D volumes, which are stitched together sequentially beat-by-beat over 4 to 7 ECG gated cardiac cycles. The 3D color Doppler flow modality is, in similar fashion to full-volume 3D acquisition, rapidly reconstructed over several heart beats; resulting in a sample volume approximately half the size of the standard 3D full­volume acquisition but with color Doppler flow imaging superimposed on the anatomic structures. All 3D modes can be further manipulated, cropped and quantified in three orthogonal planes according to the particular region of interest either on the ultrasound platform or off-line using commercial digital software.

Assessment of Left Ventricular Size and Shape

Evaluation of left ventricular (LV) morphology is one of the most common reasons for performing echocardiography in cardiac patients. RT3D echocardiography allows for superior anatomical delineation of the LV in real time. Due to the ability to manipulate the plane to align the true short and long axes of the LV, the problems with chamber foreshortening and oblique imaging planes can be reduced; thus providing more anatomically correct views than conventional 2D echocardiography. A physics-based modeling algorithm, which makes no assumptions regarding LV geometry, is used in modern RT3D software when assessing global LV volumes. The RT3D dataset can further be divided into regional segments by sectioning the LV from base to apex, perpendicular to the LV long axis, thereby allowing regional volume assessment. Global and regional function curves, reflecting the ventricular function over time, can also be derived from the LV RT3D dataset. The potential of LV volume and shape assessments by transthoracic RT3D echocardiography has been demonstrated in dogs with myxomatous mitral valve disease (MMVD), which is the most common heart disease affecting dogs.

Assessment of Left Atrial Size

The left atrium (LA) is a 3-dimensional structure, and LA dilatation may occur in all directions (i.e., craniocaudal, media-lateral and ventrodorsal direction) although perhaps not uniformly. Estimation of LA volume by RT3D echocardiography has been reported to be a major predictor of clinical outcome in human patients with severe LV dysfunction and the clinical value of RT3D techniques was reported superior to 2D techniques. Assessment of LA size provides valuable prognostic information also in dogs with MMVD, and the potential for optimized LA size assessment using RT3D techniques has been demonstrated in dogs.

Assessment of Valvular Anatomy and Function

Detailed assessment of valvular anatomy and function can be achieved by advanced RT3D echocardiography. RT3D echocardiography provides unique en face views of valve structures because of its flexibility in plane alignment. A comparative study in human patients with mitral valve prolapse showed that segmental analysis of the prolapsing valve using transthoracic (TTE) RT3D echocardiography was as accurate as with transesophageal (TEE) 2D echocardiography. Transthoracic RT3D echocardiography also has a potential to become a valuable tool for assessment of mitral valve structures in canine patients, and the potential has recently been demonstrated in canine studies. Modern RT3D digital software provides quantification programs for performing shape analysis of mitral valve morphology (leaflets and annulus) from images collected by RT3D echocardiography.

Assessment of Congenital Deformations

RT3D echocardiography displays the relationship of cardiac structures in space and has the potential to optimize visualization of complex cardiac structures; which makes it appealing for use in patients with congenital heart disease. RT3D echocardiography can provide novel views of septal defects and estimation of ventricular septal defects by RT3D echocardiography has been shown to better correlate with surgical findings compared to 2D echocardiography in human cardiac patients. RT3D echocardiography can also be a valuable tool for assessing aortic and pulmonic stenoses due to improved visualization of the valve and root.

Intracardiac masses, such as thrombi or tumours, are often of irregular shapes, which challenge size determination when using 2D echocardiography. By including the entire volume of the mass, morphology, spatial location and extent of cardiac masses can be further characterized in multiple planes, allowing more accurate assessments of structures by RT3D echocardiographic techniques.

Summary

In summary, 3D echocardiography has evolved from a time-consuming research tool to a clinically applicable diagnostic technique with the potential of providing supplementary information to conventional 2D echocardiography. With increasing clinical experience in animals, and with further advances in computer technology, RT3D echocardiography will likely continue to expand the range of applications in routine clinical assessment of cardiac patients.

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