Simulations of contrast effects from free microbubbles in relation to their size, concentration and acoustic properties

To gain a deeper insight into the diagnostic images produced by microbubbles as ultrasound contrast agents, the basic acoustical properties such as the scattering cross section, extinction cross section and the expected backscattered power from different sized microbubbles are calculated. The results proved that the ratio of scattering to extinction cross section, size and concentration of microbubbles are the factors determining the maximum backscattered power received from microbubble dispersions. When microbubbles decrease in size, the energy loss by microbubbles upon ultrasound exposure gradually changes from loss by scattering to loss by absorption. As a result, the concentration of microbubbles required to produce a B mode image of the comparable brightness is increased. However, an increased concentration of microbubbles does not necessarily produce a clearer image. In this paper, it is further established that an optimum concentration exists at which the microbubbles give maximum backscattered power. The optimum microbubble concentration and the maximum backscattered power also depend on the depth of the microbubbles in the microbubble dispersion. In addition, the concentration of microbubbles necessary to produce the maximum backscattered power during resonance is less than that at other frequencies.