Study of ultrasonic contrast agents using a dual-frequency band technique.

We have developed a dual-frequency band technique to study frequency-dependent phenomena associated with ultrasonic contrast agents. Our technique uses a superimposed high-frequency (10 MHz) broad-band ultrasound (US) pulse to investigate contrast agent interaction with a low-frequency (e.g., 0.5 MHz) ultrasonic field. Our digitally controlled system has the ability to produce two colinear, confocal US pulses at different center frequencies, to adjust the relative phasing and pulse repetition frequency of each pulse, and to acquire digital backscatter data. A series of experimental studies demonstrated that the high-frequency backscatter signal responded to several phenomena induced in contrast agent particles by the low-frequency beam. These phenomena included radial pulsations, nonlinear oscillations and depletion. Initial results also demonstrated a relative phase shift between the high- and low-frequency signals; this shift is due to a difference in sound velocity at these frequencies, and it may convey information about the contrast agent concentration.