Background

Echocardiography has evolved into the most predominant diagnostic imaging technique in cardiology. Over the last five decades the diagnostic capability of echocardiography has increased dramatically from M-mode to twodimensional (2-D) imaging. Recent advances in ultrasound instrumentation and computer technology have led to three-dimensional (3-D) echocardiography, introducing a new era in cardiovascular imaging.1

Every imaging technique in cardiology aims at a complete visualisation and comprehensive assessment of cardiac morphology and pathology, as the heart is a complex geometric structure. Analysis of the heart in motion in all three or four (including time) dimensions can therefore further facilitate and enhance the diagnostic capabilities of echocardiography. Three-dimensional echocardiography is still in its evolution and at the phase of early adaptation with respect to its clinical application. It should complement current echocardiographic techniques by providing better understanding of the topographical aspects of pathology and refined definition of the spatial relationships of (intra)cardiac structures. Furthermore, it provides new measures not described by 2-D echocardiography and makes the existing measures more accurate.2

The assessment of patients with mitral valve disease is one of the most challenging and promising clinical applications of 3-D echocardiography. The 3-D anatomy of the mitral valve, as well as the feasibility, accuracy and incremental value of 3-D echocardiography in the evaluation of mitral valve disease, will be discussed.

3-D Reconstruction

There are two main approaches of 3-D reconstruction. The first is random or freehand scanning, which is based on free motion of the ultrasound transducer. Its position in space is located by an acoustic, electromagnetic or mechanical arm location device. A transthoracic approach is used in this mode of acquisition. The limitation of this method is that accurate endocardial border identification is not possible because of big spaces between imaging planes. The second approach is sequential scanning, where the ultrasound motion is predetermined in linear, fan-like or rotational ways. A transthoracic or trans-oesophageal approach is used for this mode of data acquisition. Both the sequentially or randomly collected 2-D images are processed off-line by the computer using interpolation algorithms so that the gaps between individual images are filled and, finally, a volumetric 3-D data set is generated. There are several ways to present the data in the 3-D volumetric data set. The following are the most used.

Anyplane Echocardiography

This mode of presentation allows the examiner to generate 2-D tomographic images in any desired orientation that can be physically unobtainable by conventional 2-D echocardiography.

Volumetric Rendering Technique

This 3-D reconstruction creates images resembling the true anatomy of the heart. By choosing a cutting plane and reconstructing the image beyond this plane the heart can be opened as if by a surgeon. Different structures can be examined en face with increased perception of the anatomic relationships.

The ideal for 3-D reconstruction is realtime 3-D echocardiography (RT3D). The system uses a novel matrix phased-array transducer with parallel processing to scan a pyramidal volume. In a pyramidal volume, the images are displayed as anyplane or volume rendered images immediately. Second-generation matrix transducers recently became available for clinical studies.

Further advances in computer technology have enabled encoding of colour flow Doppler data together with greyscale imaging and 3-D presentation of Doppler flow events with surrounding cardiac anatomy.³

Functional Anatomy of the Mitral Valve

3-D Topography of the Mitral Valve

Viewing the mitral valve from the left atrium with 3-D echocardiography shows that the anterior and posterior mitral leaflets have several indentations dividing them into segments or scallops (see Figure 1). The anterior leaflet has three segments – A1 (anterolateral), A2 (middle) and A3 (posteromedial). Similarly, the posterior leaflet has three segments – P1 (anterolateral), P2 (middle) and P3 (posteromedial). The anterior and posterior leaflets are fused for 3mm to 8mm medially and laterally at the trigones and usually form distinct commissures (anterolateral and posteromedial). The anterior leaflet comprises roughly two-thirds of the valve area, is approximately twofold longer than the posterior leaflet and is somewhat triangular. The posterior leaflet is more elongated and rectangular. The anterior leaflet is attached to the septum and fibrous annulus of the heart and it is relatively non-distensible. Although the anterior leaflet accounts for two-thirds of the mitral valve area, its attachment to the mitral annulus accounts for only approximately one-third of the mitral annular circumference. The anterior mitral leaflet spans the distance between the two fibrous trigones and is in direct continuity with the non-coronary aortic valve leaflet. The posterior mitral valve leaflet is attached to the posterior two-thirds of the mitral annulus, which runs along the free wall of the left ventricle (LV) and is primarily muscular with little fibrous tissue (explaining its tendency to distend and elongate).