Changes in the echoes from ultrasonic contrast agents with imaging parameters

Current harmonic imaging scanners transmit a narrowband signal that limits spatial resolution in order to differentiate the echoes from tissue from th e echoes from microbubbles. Because spatial resolution is particularly impo rtant in applications, including mapping vessel density in tumors, we explo re the use of wideband signals in contrast imaging. It is first demonstrate d that microspheres can be destroyed using one or two purses of ultrasound. Thus, temporal signal processing strategies that use the change in the ech o over time can be used to differentiate echoes from bubbles and echoes fro m tissue. Echo parameters, including intensity and spectral shape for narro wband and wideband transmission, are then evaluated. Through these experime nts, the echo intensity received from bubbles after wideband transmission i s shown to be at least as large as that for narrowband transmission, and ca n be larger. In each case, the echo intensity increases in a nonlinear fash ion in comparison with the transmitted signal intensity. Although the echo intensity at harmonic multiples of the transmitted wave center frequency ca n be larger for narrowband insonation, echoes received after wideband inson ation demonstrate a broadband spectrum with significant amplitude over a ve ry wide range of frequencies.