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 bub ble radius when insonified. The detection of contrast microbubbles dur ing a medical examination can indicate whether a region is perfused wi th blood. In this paper, we consider the effect of the phase of sonifi cation signal on the backscatter by the bubble echo. By transmitting t wo short pulses of ultrasound with opposite phases, we demonstrate tha t a unique pair of echoes can be generated by a single microbubble, an d that the properties of these echoes may be useful in the discriminat ion of bubble and tissue echoes. Specifically, the significant echo am plitude begins coincident with each transmitted rarefactional half-cyc le, and the mean frequency of this echo depends on the transmitted pha se. When rarefaction is transmitted first for a 2.25 MHz signal, the m ean 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.