Onset delay of acoustic second harmonic backscatter from bubbles or microspheres

A well-known method of enhancing blood detectability in ultrasound imaging of tissues detects the second harmonic of the incident radiation, which is generated by ultrasound contrast agents in the form of bubbles or microspheres that may have been injected into the blood. We report here a delay in the onset of the backscattered second harmonic with respect to the backscattered first harmonic for these agents. This effect, which should limit the axial resolution attainable with harmonic imaging, is investigated by simulation as well as experiment, and its dependence on the incident ultrasonic amplitude and microsphere parameters is established.