Impact of harmonic imaging and transducer frequency on 'ventricular volume' measurements using real-time three-dimensional echocardiography: Studies in an in vitro model

Aims: Harmonic imaging has increased the yield of quantifiable scans in two-dimensional echocardiography. Although real-time three-dimensional echocardiography avoids geometric assumptions in volume analysis, accurate measurement can be limited by image quality. This study compared volumes from a balloon model mimicking the left ventricle, scanned with and without harmonic imaging, using real time three-dimensional echocardiography. Methods: Two balloons separated by ultrasound gel were suspended in a water bath. To mimic different chamber volumes, 12 volumes of water within the inner balloon (40-180 ml) were scanned using a 3.5 MHz probe at fundamental frequency and using a 2.5 MHz probe with and without harmonic imaging. Results: Scanning at 3.5 MHz, the long axis (B) scans did not significantly underestimate the balloon volume but the 'short axis' (C) scans did (mean difference from actual volumes - 0.7 ± 1.4 ml, P = 0.14 and -3.9 ± 1.2 ml, P<0.0001 for B and C scans, respectively). Scanning at 2.5 MHz both B and C scans significantly underestimated even more the true volume, C scans to a greater extent (mean difference - 6.9 ± 2.4 ml and -11.2 ± 4.0 ml for B and C scans respectively, P<0.0001 in both cases). However with harmonic imaging, transmitting at 1.7 MHz and receiving at 2-4 MHz, there was no significant difference of either B or C scans from the reference values (mean difference of B scans -1.2 ± 1.9 ml, P = 0.06 and C scans -0.6 ± 2.2 ml, P = 0.4). Conclusion: The enhanced resolution provided by harmonic imaging improves accuracy of volume analysis by real-time three-dimensional echocardiography.