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 the echoes from microbubbles. Because spatial resolution is particularly important in applications, including mapping vessel density in tumors, we explore the use of wideband signals in contrast imaging. It is first demonstrated that microspheres can be destroyed using one or two pulses of ultrasound. Thus, temporal signal processing strategies that use the change in the echo over time can be used to differentiate echoes from bubbles and echoes from tissue. Echo parameters, including intensity and spectral shape for narrowband and wideband transmission, are then evaluated. Through these experiments, the echo intensity received from bubbles after wideband transmission is shown to be at least as large as that for narrowband transmission, and can be larger. In each case, the echo intensity increases in a nonlinear fashion in comparison with the transmitted signal intensity. Although the echo intensity at harmonic multiples of the transmitted wave center frequency can be larger for narrowband insonation, echoes received after wideband insonation demonstrate a broadband spectrum with significant amplitude over a very wide range of frequencies.