Ultrasound attenuation in encapsulated microbubble suspensions: The multiple scattering effects

The phenomenon of image distortions caused by the multiple scattering (MS) effects of encapsulated microbubbles in ultrasoniqc imaging was experimentally found in previous studies (Soetanto and Chan 2000a), but its mechanism has not been fully understood. To study the MS effects of microbubbles in contrast imaging, two approaches are employed in this article-the effective medium approach initialized by Kargl (2002), which includes all the high-order rescattering of free bubbles, and the classic lowest-order approximation approach of Commander and Prosperetti (1989), which ignores the higher-order rescattering between bubbles. In this work, they are modified to model encapsulated microbubble suspensions, and the discrepancies in attenuation coefficients calculated by these two approaches, i.e., the higher-order rescattering of bubbles are defined as the measure of the MS effects of microbubbles. The intrinsic relations between the MS effects of microbubbles in suspensions and physical properties of the microbubbles, such as the bubble concentrations, sizes, and the shell thicknesses etc., are simulated and discussed. It is found that in suspensions for identical microbubbles >12 μm in size, the MS effects come to be significant when the bubble concentrations exceed 1 × 105 bubbles/mL. The MS effects of microbubbles with broad size spectrums are examined by simulating Soetanto and Chan's experiments. Also, the MS effects of UCAs in current ultrasonic imaging practice are discussed. The STARs and extinction cross-sections of different-sized individual encapsulated microbubbles are calculated for further investigations on the mechanism of the MS effects of UCAs. (E-mail: zhudu@nju.edu.cn)