The effect of the phase of transmission on contrast agent echoes

Ultrasound contrast agents consist of a gas bubble, encapsulated by a shell for stabilization. The shell dampens the fluctuations in the bubble radius when insonified. The detection of contrast microbubbles during a medical examination can indicate whether a region is perfused with blood. In this paper, we consider the effect of the phase of sonification signal on the backscatter by the bubble echo. By transmitting two short pulses of ultrasound with opposite phases, we demonstrate that a unique pair of echoes can be generated by a single microbubble, and that the properties of these echoes may be useful in the discrimination of bubble and tissue echoes. Specifically, the significant echo amplitude begins coincident with each transmitted rarefactional half-cycle, and the mean frequency of this echo depends on the transmitted phase. When rarefaction is transmitted first for a 2.25 MHz signal, the mean frequency is 0.8 MHz higher for an albumin-shelled bubble and 0.9 MHz higher for a lipid-shelled bubble. The experimental results agree with the predictions of the Gilmore-Akulichev equation.