Effect of Mechanical Index on Repeated Sparse Activation of Nanodroplets in Vivo

Current localization-based super-resolution ultrasound imaging requires a low concentration of flowing microbubbles to visualize microvasculature beyond the diffraction limit and acquisition is slow. Previous studies have shown that sparse activation of nanodroplets can be used for generating fast or even real-time super-resolution imaging. However, the optimal experimental conditions to activate the droplets for generating the super-resolution images are still unknown, especially the activation ultrasound amplitude or mechanical index (MI). An in vivo super-resolution image of a rabbit kidney is obtained in 1.1 seconds using AWSALM pulse sequence. The aim of this study is to investigate the effect of the activation MI on the repeated activation of nanodroplets in the rabbit kidney. It was found that the droplet activation was not observed in the rabbit kidney at a MI of 1.1. The activation of droplet started at an activation MI of 1.3. The contrast of activated droplet signals is maximized at an MI of 1.5 and decreased when the activation MI was increased above 1.5. The possible explanation might be that, at high MIs, the activated droplets were destroyed. Such understanding of the effects of activation MI could help improve droplet-base fast super-resolution imaging.