Objectives: We introduced a harmonic-to-fundamental ratio (HFR) of the radiofrequency (RF) signals that reduces confounding effects of attenuation. We studied whether HFR analysis of RF signals received from contrast microbubbles allows accurate measurement of the left ventricular (LV) cavity area under varying levels of attenuation.
Background: Attenuation is a fundamental problem in ultrasound imaging and limits the use of clinical echocardiography.
Methods: RF data from short axis systolic and diastolic scans were obtained from 14 open-chest dogs following left-atrial bolus of Optison. Attenuation was induced by interposed silicone pads calibrated to induce 7dB or 14dB reductions of the backscattered RF signal. RF images were reconstructed from the RF signals, HFR values calculated for each image pixel for 0dB, 7dB and 14dB attenuation conditions, and LV area obtained by summation of "LV cavity pixels". A reference LV cavity area was obtained from endocardial border tracings in enhanced scans by experts.
Results: Correlation of the HFR-defined and reference areas at systole was R=0.95, R=0.94, and R=0.91 for 0dB, 7dB and 14dB levels of attenuation, respectively, and at diastole was R=0.95 for 0dB, 7dB and 14dB levels of attenuation. The mean difference from both systolic and diastolic values was <1.45 cm(2) (i.e. negligible) in all attenuation settings.
Conclusion: Our novel HFR method supports precise measurement of the LV cavity area in contrast images with simulated high attenuation of ultrasound signals.