Nonlinear corruption of ultasound transmission by microbubble contrast agents

Current techniques for detection of microbubble contrast agents make use of their nonlinear behavior under ultrasound insonation. For these techniques there is a problem which has been largely ignored so far; the bubbles along the ultrasound transmission path between transducer and target bubbles alter the ultrasound signal nonlinearly and contribute to the nonlinearity of the echoes. This process can lead to imaging artifacts, especially in regions at depth. In this paper the nonlinear alteration of ultrasound transmission due to microbubbles is referred to as corruption. The aim of this work is to provide insight into the nonlinear corruption caused by microbubble contrast agents and the effects on current nonlinear imaging techniques. The corruption is investigated through both simulation and experimental measurements. Results show that the imaging artifact due to this corruption is significant when detection techniques deploying multiple insonation pulses, such as power modulation and combined phase and amplitude modulation are used. Further investigation reveals that the corruption affects both amplitude and phase of ultrasound transmission and is dependent on both the initial amplitude and phase of the insonation pulse. Our results also suggest that forward scattering may play a significant role in the nonlinear corruption process.