Stress echocardiography is a widely used clinical technique that provides invaluable information in patients with known or suspected coronary artery disease (CAD). Common applications of stress echo include the diagnosis of reversible ischaemia, myocardial viability, and establishing the functional significance of known coronary lesions. Stress echo has been shown to have similar sensitivity and higher specificity than single photon emission computed tomography (SPECT) studies; furthermore, it is lower in cost and does not involve the use of ionising radiation. Not surprisingly, the demand for stress echocardiography has been rising and industry has been developing new technologies to increase the efficacy of the technique. Despite its popularity as a diagnostic technique, stress echocardiography is not without its problems.
- Rapid acquisition of different scan planes at each stress stage. If this is not achieved, ischaemia-induced wall motion or thickening abnormalities may resolve quickly and reduce the sensitivity of the test.
- In order to ensure that identical myocardial segments are compared at each stage of the stress echo test, it is important that scan planes are reproducible.
- Sub-optimal image quality can make it difficult to appreciate subtle changes in wall motion and thickening and reduces reader confidence when interpreting the images.
The purpose of this article is to highlight how multidimensional four-dimensional (4-D) imaging can help overcome some of these problems.
Multidimensional 4-D Imaging - The Technology
Four-dimensional imaging uses new transducer technology as illustrated in Figure 1. This incorporates a matrix probe and complex electronics within the transducer handle, which means that ultrasound beams can be directed in almost any direction within a 3-D space. Increased processing power incorporated within the state-of-the-art 4-D scanners enables either a realtime 4-D image of the heart with relatively narrow width to be collected or, alternatively, a full volume data set incorporating the whole heart may be generated over four consecutive cardiac cycles. In addition, the matrix array probe may be utilised to collect three simultaneous 2-D scan planes, which, for example, would allow acquisition of all three standard apical views within one cardiac cycle. In this way the need to acquire scan planes rapidly at each stage of stress echo can be overcome because the tri-plane acquisition allows views in which images of every single myocardial segment can be acquired very rapidly without needing to move the transducer between each scan plane.
Alternatively, a full volume 4-D data set can be acquired over four consecutive cardiac cycles and this data set can subsequently be cropped to display any view desired. Multiplane 4-D imaging also helps to ensure reproducibility of scan planes at each stress stage. This is because with multiplane imaging there is a fixed spatial relationship between scan planes, which helps ensure reproducibility. When a 4-D data set is acquired, the cropping and segmentation of the data set can be performed after the study to ensure that views are comparable and reproducible.
Finally, the issue of poor image quality can be overcome by the utilisation of ultrasound contrast agents. While contrast echo has been widely used during stress 2-D echocardiography, there has been little data on its applicability during 4-D and multidimensional stress echo. However, contrast specific imaging software has been incorporated within the image processing for multidimensional or 4-D echo, allowing excellent high-definition images with superb endocardial enhancement to be obtained in even the most difficult echo subjects.
Multidimensional Imaging and Stress Echo
Sugegn et al. published a study in the Journal of the American Society of Echocardiography in 2003, demonstrating that with bi-plane multidimensional imaging all stress echo images could be obtained in an average of 27 seconds whereas conventional 2-D echocardiography took significantly longer at 38 seconds. Heart rate decreases very rapidly following the end of exercise when even a difference of 10 seconds during acquisition could be significant. For example, average peak heart rate at the end of bi-plane acquisition was 135 beats per minute, whereas with conventional 2-D imaging the peak heart rate was 120 beats per minute. It is self evident that by using multidimensional tri-plane imaging, as opposed to bi-plane imaging, even greater reductions in imaging time, and therefore higher post-stress heart rates and sensitivity, would be achieved. The principle for this was also shown in a paper by Dagianti et al., in the American Journal of Cardiology in 1998, who compared treadmill exercise echo with supine bicycle echo. When utilising the latter modality, imaging is performed while the patient continues to exercise and therefore there is no drop in heart rate. In patients who underwent conventional treadmill exercise, stress echo-significant reductions in the degrees of wall motion and thickening abnormalities were seen in patients with one and two vessel disease highlighting the reduced sensitivity that results. Because multidimensional imaging significantly reduces the acquisition time, sensitivity should be improved.
In the author's department conventional 2-D exercise echo has been compared with tri-plane exercise echo in over 50 patients undergoing treadmill exercise studies. The time taken from the end of the exercise to acquisition of the three apical scan planes using tri-plane imaging was 42 seconds, whereas when conventional 2-D echo was used the mean time was 65 seconds, as shown in Figure 2. All imaging was performed by the same experienced sonographer. This meant that the peak heart rate achieved with multidimensional tri-plane echo was 127 beats per minute, whereas by the time the imaging had finished with conventional 2-D echo it had fallen to 117 beats per minute. This was a highly significant reduction as shown in Figure 3.
Modern stress echo reporting software incorporates the ability to compare the same scan plane at different stages during the stress echo. This can now be achieved with the software incorporated into state-of-the-art scanners. For example, the three scan planes acquired at base line and peak exercise echo will be digitally stored and then automatically sorted by the software so that the base line and peak stress images from the same view can be visualised side by side. In addition, the 4-D data set can be semi-automatically separated into nine short axis slices running from the base to the apex. This facilitates detailed evaluation of every myocardial segment. Comparison of changes of stress-induced changes can be made by looking at the different short axis slices side by side.
A mathematical model or cast of the left ventricle can be created from the multi-plane images and this can be animated to provide a graphical 3-D display of myocardial contraction. Images acquired in this way can be compared at rest and stress and rotated in 3-D space to facilitate appreciation and display of subtle changes in wall motion induced by reversible ischaemia. This is demonstrated in Figure 4.
Several studies have shown an excellent correlation between stress-induced wall motion abnormalities detected by contrast-enhanced 4-D multidimensional stress echo and conventional stress echo. An example of contrast-enhanced tri-plane images acquired at rest and stress is shown in Figure 5. Contrast agent infusion pumps allow slow infusion and provide excellent endocardial enhancement without attenuation. Use of contrast-specific imaging settings also provides superior sensitivity and reduces contrast destruction, especially near the apex.
The number of myocardial segments seen adequately or well is significantly higher when contrast is used compared with non-contrast images. In addition, wall motion score indices between the two techniques have also been shown to correlate well. The obvious advantage of using the new technology is the ease and speed of acquisition together with reproducibility. The ability to use contrast to ensure excellent endocardial visualisation in all patients is an added bonus. In addition, as shown in Figure 6, left ventricular (LV) volumes and ejection fraction can be derived from the 4-D data sets. Failure to increase ejection fraction during dobutamine stress has been shown to be a sensitive marker of reversible ischaemia when compared with conventional stress echocardiography.
The potential of multi-plane and 4-D imaging during stress echo is not yet completely exploited. The ability of 4-D echo to look in detail at regional function and dyssynchrony provides further opportunities to detect and assess reversible ischaemia during stress. It has been recognised for a long time that an early marker of ischaemia is LV dyssynchrony and because semi-automated software for analysis of dyssynchrony is being developed, this will lead to more objective and automated analysis of stress echo studies.
Conclusion
In summary, the recent availability of multidimensional and 4-D echocardiography has the potential to help overcome some of the problems that exist with conventional stress echo performed using 2-D imaging. In particular, when exercise echo is utilised, the ability to acquire images rapidly is readily achievable using this technology and will increase the sensitivity of the technique.