Conventional two-dimensional echocardiography (2DE) calculates volumes relying on geometric assumptions about the sape of cardiac chambers (e.g area-length method to calculate left ventricular or atrial volume), measures areas by assuming an unverified perpendicular orientation of the 2D view to the axis of the orifice (e.g. mitral valve area planimetry), derives areas from measuring diameters and assuming a circular shape of orifices (e.g. calculation of left ventricular outflow area from the measurement of its anterior-posterior diameter). Conversely, three-dimensional echocardiography (3DE) allows: true measurement of chamber volumes (through a reconstruction of their endocardial surface and counting the voxels contained within the "beutel", thus eliminating the need for geometric assumptions about chamber shape), accurate planimetry of orifice areas (using cut planes which are oriented perpendicularly to the orifice axis and controlling the exact position of the cut plane --> en face view of orifices), and actual measurement of cardiac structure size and shape (through the possibility of cropping the 3D data set in any way to obtain an anatomical display of the desired structure).
2DE calculation of left ventricular volume is mainly based on two algorithms: the area-length and the dic-summation algorithms which can be used in a mono- or biplane approach..
The single plane area-length method assumes an ellipsoid shape of the left ventricle and that the area measured in 4-chamber view is representative of the shape of the left ventricle throughout its circumference since the volume is obtained by a simple 180° rotation of that area around the longitudinal axis of the left ventricle (Figure 1). This assumption may work fairly well in normal ventricles, but it is unreliable in ventricles with distorted geometry (e.g. aneurysms, dilated cardiomyopathies) or with extensive wall motion abnormalities. Any foreshortening of the apical 4-chamber view will result in a significant underestimation of the final volume.
Figure 1. Schematic drawing of the area-length algorithm used to calculate left ventricular volume. The ventricular shape is approximated to an ellipsoid, which volume is calculated from the measurement of its major axis, represented by left ventricular length, and the mid-cavity area obtained from an apical 4-chamber view. The volume is calculated as 0.85 x A2/L.
The biplane disc-summation algorithm is based on the principle that the total left ventricular volume is calculated by summing a stack of elliptical discs. Each disc has a height which is calculated as a fraction (usually 1/20) of the longest left ventricular long axis obtained from the 4- and 2-chamber views (Figure 2).
Figure 2. Schematic representation of the disc summation algorithm.
The cross-area of the discs is calculated from the two diameters obtained from the 4- and 2-chamber views assuming that they are orthogonal. This method is less sensitive to errors related to the irregular shape of the left ventricle than the area-length method. However, it requires that the LV long axes in the 4- and 2-chamber views do not differ with more that 10%, otherwise the measurement of the diameters to calculate the disc volume are not performed at the same level, and is based on the assumption that the 4 - and 2-chamber views are orthogonal. The latter is never met since the two views are at approximately 60° from each other and, even more important, the lack of anatomical landmarks for a correct 2-chamber view makes the position of this view unverifiable.
On top of the listed limitations of the two methods, there is the need of manual tracing of the endocardial border that depends heavily on the experience of the operator.
3DE overcomes the geometric limitations of 2DE. Quantitative analysis of left ventricular volumes starts with the extraction of non-foreshortened, anatomically correct apical views from the voxel-based 3D data set obtained from apical approach. These views are used to trace the endocardial border with a semiautomated detection process (Figure 3).
Figure 3. 3D echocardiographic quantitation of left ventricular volumes. After having identified the endocardial border on the 3 apical slices on the left which share the same apex (no foreshortening anymore), the software automatically detect the whole endocardial surface. The accuracy of endocardial border detection on the whole left ventricular circumference can be checked on the transverse slice which can be moved up and down along the left ventricular longa axis. Once confirmed by the operator, a 3D cast of the left ventricle is developed and the volume is measured by counting the voxels within the volume.
Following identification of the endocardium on 2DE views, the software automatically identifies the 3D endocardial surface using a deformable shell model and a cast of the cavity is then created, Left ventricular volumes are measured by simply counting the number of voxels within the 3D cast and therefore without any geometrical assumption about left ventricular shape.
When compared to 2DE, 3DE measurement of left ventricular volumes has been reported to be significantly more accurate with less underestimation of the volumes measured at cardiac magnetic resonance. There are several explanations for the reduced underestimation of volumes: 1. no foreshortening of left ventricular long axis; 2. inclusion of the left ventricular outflow tract within the measurement. In addition, semiautomatic or completely automatic border detection algorithm significantly improve the reproducibility and repeatibility of the measurements, even if several authors have underlined the role of experienced users and the need to check the accuracy of border detection.
2DE quantification of right ventricular (RV) size and function is challenging, due to the anterior position of the RV in the chest, its complex asymmetric geometry, irregularity of the highly trabeculated endocardial border, impossibility to visualize in the same view both inflow and outflow tracts and lack of realistic geometrical models. 3DE allows the reconstruction of the whole right ventricle with no need of making geometric assumptions about its geometry (Figure 4) and has been demonstrated to have a good accuracy in measuring RV volumes compared to cardiac magnetic resonance. A good correlation between cardiac magnetic resonance and 3DE parameters reflecting RV geometry and function has been demonstrated, although, as for the left ventricle, an underestimation of 3DE RV volumes is often reported.
Figure 4. 3D echo reconstruction of the right ventricular volume. The right ventricle is shown at its end-diatolic (white wire-frame volume) and end-systolic (green beutel) volumes in order to appreciate its systolic function. PV= pulmonary valve; TV= tricuspid valve
3DE has been reported as an accurate and reproducible method to measure left atrial volumes and function. By plotting the volume against time, the phasic functions of the filling and ejection of the atrium may be quantified (Figure 5).
Figure 5. 3D reconstruction of the left atrium with transthoracic 3D echocardiography (left panel). From left atrial volume change over time, parameters of left atrial function can be measured (right panel)
As for the left ventricle, 3DE corrects for atrial cavity foreshortening and does not make any geometrical assumptions about its geometry. Therefore, it results in less volume underestimation than 2DE, when both methods are compared against cardiac resonance imaging.
The normal mitral valve is oval and saddle-shaped, with its lowest points located at the commissures, and its highest points near the aortic root and near the posterior wall. Evidently, 2DE is unable to provide data about the size of mitral leaflets and the shape of mitral annulus, since mental reconstruction from separate 2D views cannot provide the same information as the volume-rendered 3D reconstruction. With 3DE, the elliptical shape of mitral valves is best appreciated from the surgical view of the valve, encompassing the whole annular circumference in one view. Performant tools to precisely quantitate the size of mitral leaflets, size, shape and degree of non-planarity of mitral annulus have been developed in order to better understand mitral valve mechanics and to assist the surgeon in evaluating the feasibility of mitral valve repair (Figure 6).
Figure 6. 3D reconstruction of the mitral valve with superimposed measurements of mitral valve geometry and shape.