Real-time 3D echo correlates well with MR, but with caveats
In a real-world clinical setting, real-time 3D echocardiography (RT3DE) analysis of left ventricular volumes correlates highly with cardiac magnetic resonance (CMR) imaging . However, the tool is more reliable when users understand the drawbacks, according to a multicenter study in the July issue of the Journal of the American College of Cardiology – Cardiovascular Imaging.

The main question researchers sought to answer was whether or to what extent average end-users of RT3DE equipment and volumetric analysis software could expect their LV volume measurements to be interchangeable with those performed with the current standard reference technique, namely CMR imaging.

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Effects of RT3DE Image Quality on Endocardial Visualization: These examples of short-axis cut planes extracted from real-time 3D echo (RT3DE) datasets demonstrate how spatial resolution may affect the perception of endocardial boundaries. In one patient (top), endocardial trabeculae can be well visualized and clearly differentiated from the myocardium and, thus, appropriately included in the left ventricular (LV) cavity. In contrast, in the second patient (bottom), the spatial resolution of the RT3DE image is not sufficient to provide this kind of detail and is likely to result in erroneous exclusion of the trabeculae from the LV cavity. Source: Victor Mor-Avi, PhD, University of Chicago.  
“This question is of particular practical importance because these tools are becoming widely available and are anticipated by some to provide a quick, relatively inexpensive and portable alternative to CMR imaging,” the authors wrote.

Victor Mor-Avi, PhD, and colleagues from the University of Chicago; Brisbane, Australia; Aachen, Germany; and Linz, Austria evaluated 92 patients with a wide range of ejection fractions with CMR and RT3DE imaging.

Echo images were acquired with the iE33 imaging system and an X3-1 matrix array transducer, and volumetric analysis was done with QLAB, 3DQ-Advanced (Philips Medical Systems).

CMR images were obtained using a 1.5T scanner with a phased-array cardiac coil. Equipment manufacturers varied between institutions and included Philips (Intera Achievea), Siemens (MAGNETOM Sonata), and GE Healthcare (Sigma Excite).

Researchers obtained LV end-systolic volume and end-diastolic volume. They assessed reproducibility using repeated analyses. The investigation of potential sources of error included: phantom imaging, intermodality analysis-related differences, and differences in LV boundary identification, such as inclusion of endocardial trabeculae and mitral valve plane in the LV volume.

The RT3DE-derived LV volumes correlated highly with cardiac MR values, but were 26 and 29 percent lower consistently across institutions, with the magnitude of the bias being inversely related to the level of experience. The RT3DE measurements also were less reproducible than MR measurements. The analysis software pointed at the important issues inherent to the technique, according to the study.

“We found that the major source of error is that in most patients the spatial resolution of RT3DE imaging is insufficient to provide clear definition of endocardial trabeculae, which are, as a result, lumped together with the myocardium rather than being included in the LV cavity, as during analysis of CMR images,” the authors wrote.

They suggested lessening the error by learning how to identify the true endocardial boundaries beyond the blood-trabeculae interface and that contrast enhancement may specifically allow separating the trabeculae from the myocardium.

Other than that, “to minimize this problem, the spatial resolution of the 3D echo images needs to improve, and that can only be achieved by improvements in imaging technology, which indeed constantly evolves,” Mor-Avi told Cardiovascular Business News.

Researchers also noted that their conclusions apply only to the analysis software by Philips, because alternative software programs were not tested.

“Nevertheless,” they wrote, “it is likely that such alternative analysis would have resulted in similar findings, given the experience-dependent differences in endocardial boundary tracing noted in this study.”

Mor-Avi said that the clinical echocardiography laboratory at his institution is using the technique “more and more when accurate volume measurements are required and image quality is adequate to obtain such measurements.”