Clinicians have a number of available tests for cardiac imaging including echocardiography, nuclear testing, magnetic resonance imaging (MRI), computed tomography and positron emission tomography. These can yield overlapping, if not identical information, often with similar or comparable accuracy. Decisions concerning which technique to use must then be based on such factors as local expertise in performance and interpretation, test availability, feasibility, cost, and patient preference. Transthoracic echocardiography (TTE) is associated with little (if any) patient discomfort. Moreover, the use of TTE with exercise or vasoactive drugs such as dipyridamole or dobutamine involves minimal risks of arrhythmia, ischaemia, and hypotension.

Two-dimensional echocardiography (2DE) displays anatomic structures in real-time but only in a single slice i.e. two dimensions (Figure 1).  This modality is widely available for left ventricular (LV) assessment and enables assessment of regional wall motion, valvular function and cardiac haemodynamics. Linear measurements such as those obtained via M mode have been used in many laboratories for many decades to measure dimensions and calculate volumes of cardiac cavities.  However these measurements and calculations have now been proven to have low reproducibility with high variability between sequential studies.  This is because the methods of calculating such measurements rely on geometric assumptions and require all images to be obtained in the correct axis.

Consequently, cardiac MRI has been proposed as a more desirable alternative for LV assessment, especially in trials, because of its good image quality and high spatial resolution.  Over the past years cardiac MRI has become the "gold standard" for LV volumes, LVEF and LV mass; however, because of its inherent problems such as high expense, contraindication in patients with cardiac devices, and non-portability, the use of this modality is limited for routine clinical evaluation.

Both qualitative and quantitative limitations of the two-dimensional imaging have led to the emergence of the 3DE technique.  Until recently, uptake of the 3DE technology has been slow due to limited image quality and longer processing and acquisition times compared with 2DE.  The most recent of the 3DE technologies is the "live" or "real-time" technique (RT3D) (Figure 2a and 2b).  Limitations in the reproducibility of 2DE are most likely attributable to inconsistent positioning of standard imaging views resulting in a high percentage of off-axis images with resultant variations in LV measurements.  The underestimation of LV volumes with 2DE is probably attributable to tangential imaging planes and foreshortening.  By contrast, the "bright blood" display of the cardiac MRI images may overestimate the measured LV cavity size by filling the space between the trabeculae. 

Recently RT3D has been used to evaluate congenital heart disease, stress echocardiograms and cardiac resynchronization therapy.  In patients with congenital heart disease (especially atrial septal or ventricular septal defects), RT3D can show the location, size, configuration, type, and motion of the defect (Figure 3).  RT3D also shows the spatial relations of the defect with the neighbouring structures and the image can be rotated to view the defect from either the left or the right side of the septum.  Studies have shown that RT3D is a technology that allows instant visualization of cardiac anatomic details that could not be well delineated by 2D imaging.

RT3D has been shown to be feasible when performing stress echocardiograms as well.  The peak acquisition time for RT3D has significantly decreased using RT3D due to the simultaneous imaging of the whole LV.  The shorter time for acquisition is achieved by using the full volume or biplane imaging (Figure 4) of the entire LV from one window without moving the transducer and permits more rapid image acquisition compared with 2DE. RT3D volumetric data sets can be matched using the same views at baseline and peak stress for a precise comparison of the same segments in different planes simply by cropping

Quantification of global LV function is especially important in patients with heart failure, where there is a potential for LV dyssynchrony.  LV mechanical synchrony has emerged as a therapeutic target using cardiac resynchronization therapy in selected patients with chronic heart failure.  Previous techniques for assessment of intra-ventricular dyssynchrony including tissue Doppler and M-mode analysis are technically difficult and do not assess the whole LV simultaneously. RT3D can quantify global mechanical dyssynchrony (Figure 5) and may help to identify chronic heart failure patients, previously not considered suitable for resynchronization therapy, who might benefit from such therapy.

The four main areas in which the value of RT3D has been investigated include the analysis of cardiac volumes and LV mass, ischaemic heart disease, congenital heart disease and valvular pathology (Figure 6).  Although many of these research studies have shown that RT3D is a superior technique to 2DE for assessing cardiac structures and LV function its use in the mainstream clinical laboratory is still limited. This is probably due to the learning curve required to assess the heart in planes not seen by 2DE and the technical limitations of RT3D such as the and rotating the three-dimensional volumetric data sets.  Studies have shown that RT3D can accurately visualize planes for a precise comparison of the same segments especially in the apical region. This is because RT3D volume datasets can be optimally aligned by post processing manipulation of the images.

Quantification of global LV function is especially important in patients with heart failure, where there is a potential for LV dyssynchrony.  LV mechanical synchrony has emerged as a therapeutic target using cardiac resynchronization therapy in selected patients with chronic heart failure.  Previous techniques for assessment of intra-ventricular dyssynchrony including tissue Doppler and M-mode analysis are technically difficult and do not assess the whole LV simultaneously. RT3D can quantify global mechanical dyssynchrony (Figure 5) and may help to identify chronic heart failure patients, previously not considered suitable for resynchronization therapy, who might benefit from such therapy.

The four main areas in which the value of RT3D has been investigated include the analysis of cardiac volumes and LV mass, ischaemic heart disease, congenital heart disease and valvular pathology (Figure 6).  Although many of these research studies have shown that RT3D is a superior technique to 2DE for assessing cardiac structures and LV function its use in the mainstream clinical laboratory is still limited.  This is probably due to the learning curve required to assess the heart in planes not seen by 2DE and the technical limitations of RT3D such as the transducer size and spatial resolution.  Recent work on 3DE has shown the same avoidance of cut-plane variation and independence from geometric assumptions giving a reduction of measurement variation. The resultant reduction in test-retest variation might be expected to reduce the large patient numbers anticipated to overcome measurement variations and should now be the test of choice in the clinical laboratory for the calculation of LV volumes.