Original Contribution
Quantification of Left Ventricular Size and Function Using Contrast-Enhanced Real-Time 3D Imaging with Power Modulation: Comparison with Cardiac MRI

https://doi.org/10.1016/j.ultrasmedbio.2012.07.001Get rights and content

Abstract

In patients with optimal images, real-time 3-D echocardiography (RT3DE) allows accurate evaluation of left ventricular (LV) volumes and ejection fraction (EF). However, in patients with poor acoustic windows, lower correlations were reported despite the use of contrast. We hypothesized that power modulation (PM) RT3DE imaging that uses low mechanical indices and provides uniform LV opacification could overcome this problem. Accordingly, we sought to: (i) Test the feasibility of quantification of LV volumes and EF from contrast-enhanced (CE) PM RT3DE images, (ii) validate this technique against cardiac magnetic resonance (CMR) reference and (iii) test its clinical value by quantifying the improvement in accuracy and reproducibility. We studied 20 patients who underwent CMR, harmonic nonenhanced RT3DE and CE PM RT3DE imaging on the same day. All images were analyzed to obtain end-systolic and end-diastolic LV volumes (EDV, ESV) and calculate EF. To determine the reproducibility of each RT3DE technique, imaging was repeated in the same setting by a second sonographer. In addition, patients were divided according to the quality of their RT3DE images into two groups, for which agreement with CMR and reproducibility were calculated separately. CE PM RT3DE imaging improved the accuracy of EDV, ESV and EF measurements in patients with poor acoustic windows without significantly affecting those in patients with optimal images. In addition, CE PM RT3DE imaging improved the reproducibility of the measurements, as reflected by a twofold decrease in intermeasurement variability. Importantly, the variability in CE PM RT3DE–derived volumes and EF was under 10%, irrespective of image quality. This methodology may become the new standard for LV size and function, which will be particularly important in patients with poor acoustic windows or contraindications to CMR.

Introduction

Quantification of left ventricular (LV) volumes and ejection fraction (EF) has become an integral part of the echocardiographic evaluation and a cornerstone in the serial evaluation of patients undergoing chemotherapy as well as in decision-making for cardiac resynchronization therapy and implantation of defibrillators. Previous studies have demonstrated that LV volumes and EF measurements using real-time 3-D echocardiography (RT3DE) are accurate when compared with cardiac magnetic resonance (CMR) imaging in patients with optimal image quality (Corsi et al. 2005; Jacobs et al. 2006). However, in patients with poor acoustic windows, relatively low correlations were noted despite the use of contrast enhancement (Caiani et al. 2005b) because volumes were underestimated as a result of microbubble destruction during continuous volume imaging. Dual triggering at end-systole and end-diastole was found to yield higher levels of agreement with CMR reference by effectively reducing microbubble destruction (Caiani et al. 2005b). Nevertheless, this approach is limited because it provides only static images and relies on prospective identification of the timing of end-systole.

Power modulation (PM) is a contrast-targeted imaging mode that is used with low mechanical indices (MIs) to minimize bubble destruction during 2-D imaging and has been shown to provide dynamic images with uniform LV opacification (Caiani et al. 2002; Mor-Avi et al. 2001, 2003). Recently, PM was incorporated into RT3DE technology, allowing dynamic volume imaging of the heart with low MI during contrast enhancement, thus resulting in only minimal bubble destruction and consequently uniform opacification of the entire volume of the LV cavity. We hypothesized that contrast-enhanced PM RT3DE imaging could be useful in the context of volumetric analysis of LV size and function. Accordingly, the aims of this study were: (i) To test the feasibility of quantitative assessment of LV volumes from dynamic, contrast-enhanced RT3DE PM images, (ii) to validate this technique against CMR reference values and (iii) to test the clinical value of this technique by determining to what extent it improves the accuracy and reproducibility of LV volume measurements, especially in patients with poor acoustic windows.

Section snippets

Study design

This study included two separate protocols designed to address the aforementioned goals. We enrolled consecutive patients referred for CMR evaluation of LV volume and function who also underwent RT3DE imaging on the same day, including harmonic imaging without contrast and contrast-enhanced PM imaging. All images were analyzed to obtain end-systolic and end-diastolic volumes (EDV, ESV) and calculate EF.

The study was designed to first address the aforementioned aims in nonselected patients,

Results

Two of the 20 patients were unable to complete their CMR studies because of claustrophobia and so were excluded from analyses that involved comparisons with CMR. In the remaining 18 patients, CMR values of ESV ranged between 62 and 263 mL (mean 137 ± 58), EDV ranged between 125 and 305 mL (mean 211 ± 63) and EF ranged between 18 and 62% (mean 37 ± 11). Mean heart rate during RT3DE imaging was 68 ± 12 bpm, without significant differences between patients with optimal and suboptimal image quality.

Discussion

Although 2-D echocardiography (2DE) is routinely used in clinical practice to measure LV volumes, wall thickness and function (Schiller et al. 1989), this methodology is limited because of the inadvertent use of foreshortened views of the left ventricle and because it relies on geometrical assumptions to calculate quantitative volumetric parameters. These assumptions may introduce considerable measurement errors, particularly in patients with abnormally shaped ventricles and wall motion

Conclusions

Because the technology used in this study is currently widely available in commercial imaging systems, it is important that the measurement technique tested here is optimized and properly validated before its widespread clinical use. This study constitutes the first step in this direction. With additional validation and fine-tuning, this methodology promises to improve the echocardiographic evaluation of LV size and function and may become the new standard for LV size and function in

Acknowledgment

This study was supported by a research grant from Lantheus Medical Imaging.

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