Study on Contrast Effect of Microbubbles as Ultrasound Contrast Agents

In this study, the contrast effect from different sized microbubbles is investigated both theoretically and experimentally. Theoretically, the reference backscattered power is defined to evaluate the contrast effect of different sized microbubbles dispersed in the heart. In this study, the calculation results of two kinds of microbubbles of sizes 35 µm and 2.2 µm are discussed. From the calculation results obtained, it is found that the contrast effect from microbubbles of size 35 µm is better than that from microbubbles of size 2.2 µm. The reason being that as the size of microbubbles decreases, a large portion of ultrasonic energy is lost due to absorption. Based on the same reason, microbubbles less than 10 µm will cause an acoustic shadowing effect at a higher concentration of microbubbles, according to the calculation results. Experimentally, microbubbles of an average size of 35 µm are produced, and by filtration, microbubbles of an average size of 2.1 µm are collected. Then, the contrast effect produced from microbubbles before and after filtration are compared. It is observed that for both kinds of microbubbles, a good contrast effect is observed at a relatively low concentration of microbubble dispersion. However, distortion of images occurs at high concentrations. The acoustic shadowing effect is observed for microbubbles of an average size of 2.1 µm which can also be expected from the calculation results. On the other hand, the multiple-scattering effect which results in blurred images, occurs for microbubbles of an average size of 35 µm.