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

ABSTRACT

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 (40-180ml) were scanned using a 3.5MHz probe at fundamental frequency and using a 2.5MHz probe with and without harmonic imaging. RESULTS: Scanning at 3.5MHz, the long axis (B) scans did not significantly underestimate the balloon volume but the 'short axis' (C) scans did (mean difference from actual volumes -0.7-1.4ml, P=0.14 - 3.9 +/- 1.2 ml,P < 0.0001 for B and C scans, respectively). Scanning at 2.5MHz both B and C scans significantly underestimated even more the true volume, C scans to a greater extent (mean difference -6.9 +/- 2.4ml and -11.2 +/- 4.0ml for B and C scans respectively,P < 0.0001 in both cases). However with harmonic imaging, transmitting at 1.7MHz and receiving at 2-4MHz, there was no significant difference of either B or C scans from the reference values (mean difference of B scans -1.2 +/- 1.9ml, P=0.06 and C scans -0.6 +/- 2.2ml, P=0.4). CONCLUSION: The enhanced resolution provided by harmonic imaging improves accuracy of volume analysis by real-time three-dimensional echocardiography.