The key difference between 2D and 3D echocardiography is the volumetric approach to data acquisition (Figure 1), visualization (Figure 2 and 3), and quantitation (Figure 4). 
Figure 1. Two-dimensional echocardiography is a tomographic technique which provides "flat" views of the heart and great vessels which thickness is fixed and related to the piezoelectric element vertical dimension. Three-dimensional echocardiography is based on real-time volumetric imaging that allows acquisition of pyramidal data sets.
Figure 2. Imaging of the mitral valve with 2D (left panel) and 3D (right panel) echocardiography) from the ventricular perspective. 2D imaging allows visualization mitral valve leaflet margins and left ventricular wall only. 3D volume rendering of the same valve allows appreciation of the whole leaflets, commissures and relationships with surrounding structures (e.g. left ventricular outflow tract) in a realistic anatomic display.
Figure 3. In contrast with 2D technique, 3D echocardiography allows the visualization of the structure of interest from virtually any desired perspective. For example, by 2D echocardiography the mitral valve can be seen from the ventricular perspective only, while by 3D echocardiography the mitral valve can be visualized both from the ventricular and the atrial ("surgical view") perspectives.
Figure 4. Quantitative analysis with 2D echocardiography is based on geometric assumptions about cardiac structure shape, spatial interpolation among available views and calculations using fixed mathematical formulas irrespective of the actual shape of the cardiac structure of interest. Quantitative analysis with 3D echocardiography is based on voxel count in a volume which has been identified by semi-automated or automated border identification without geometric assumptions about structure shape.
Pros and Cons of 3D versus 2D echocardiography
|3DE provides realistic anatomic images in the beating heart which are easily recognizable and interpreted by the surgeon, interventional cardiologist, pediatric cardiologist, anatomist, etc||3DE data set acquisition and display require a specific training|
|The additional dimension (depth) that is inherent to 3D acquisitions allows data set navigation and cropping to obtain "en face" views of cardiac structure of interest that were not available with tomographic imaging modalities (2D echo, cardiac magnetic resonance, computerized tomography)||Patients with arrhythmias or unable/unwilling to cooperate for breath holding are challenging to image with current 3D technology|
|3DE allows visualization of cardiac structures independent on availability of specific acoustic windows (i.e. en face view of the mitral valve from apical approach) and anatomically sound alignment of the imaging plane to provide accurate planimetri of orifice size (i.e. planimetry of mitral stenosis or interatrial septal defect)||Poor acoustic window limits the application of transthoracic 3DE|
|In contrast with 2D echocardiography, re-aligning planes on 3D data sets to identify maximum longitudinal axis of cardiac chambers and avoid foreshortened views and optimize volumetric quantification is always possible||Good image quality is a prerequisite for an accurate semi-automated or fully-automated volume quantitation|
|3DE provides a unique and reliable quantitation of right ventricular volume by echocardiography, both in healthy subjects and in patients||Right ventricle has an unfavorable position within the chest for transthoracic 3D transducers and severely dilated right ventricles are often difficult to encompass in a pyramidal 3D data set|
|3DE measurements of cardiac chamber volumes do not rely on geometric assumptions about their shape, and are more reproducible and closer to values provided by cardiac magnetic resonance than 2D volumes||Limited evidence exists about the reference values of cardiac chambers 3D volumes and the intervendor consistency of 3D quantitative parameters|
|A comprehensive and time-volume analysis of cardiac chamber geometry and function can be obtained throughout the cardiac cycle from a single 3DE data set||High temporal resolution can only be obtained from an ECG gated multibeat acquisition|
|En face (surgical) views of cardiac structures allows a reliable identification of mechanisms and severity of organic as well as functional diseases of cardiac structures||Technical conditions (image quality, temporal and spatial resolution, gain, compression, drop-outs artifacts, etc.) may significant impact on disease severity assessment|