Subharmonic analysis using singular-value decomposition of ultrasound contrast agents

Abstract

Ultrasound contrast agents (UCAs) are designed to be used below 10MHz, but interest is growing in studying the response of agents to high-frequency ultrasound. In this study, the subharmonic response of polymer-shelled UCAs with a mean diameter of 1.1μm excited with 40-MHz tone-bursts of 1–20cycles was analyzed. UCAs were diluted in water and streamed through a flow phantom that permitted single-bubble backscatter events to be acquired at peak-negative pressures from 0.75to5.0MPa. At each exposure condition, 1000 single-bubble-backscatter events were digitized. Subharmonic content at 20MHz was screened using a conventional and a singular-value-decomposition (SVD) method. The conventional method evaluated each event spectrum individually while the SVD method treated the 1000-event data set at one time. A subharmonic score (SHS) indicative of how much subharmonic content a 1000-event data set contained was computed for both methods. Empirical-simulation results indicated that SHSs obtained from the two methods were linearly related. Also, experimental data with both methods indicated that subharmonic likelihood increased with pulse duration and peaked near 2MPa. The SVD method also yielded quantitative information about subharmonic events not available with the conventional method.