Transmit aperture processing for nonlinear contrast agent imaging.

Nonlinear contrast agents, such as bubbles, are used in ultrasound to enhance backscatter from blood. To increase contrast between these agents and tissue, nonlinear imaging methods, such as harmonic imaging or difference frequency imaging, can be used. For these, power is transmitted at one frequency and received at a different frequency. Nonlinear imaging methods, however, suffer from reduced contrast because of signal transmitted by the array at the receive frequency that linearly reflects off tissue. In this paper, transmit aperture processing is proposed to improve contrast in nonlinear imaging by suppressing signal transmission at the desired receive frequency. A specific method, known as alternate phasing, is presented as an example of this approach. Simulation results show that alternate phasing greatly improves contrast between tissue and bubbles.