Effects 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. Recent studies have shown that Acoustic Wave Sparsely Activated Localization Microscopy (AWSALM) using phase change nanodroplets can generate 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). The aim of this study is to investigate the effect of the activation MI on the repeated activation of decafluorobutane nanodroplets in vivo. Noninvasive ultrasound data acquisition on a rabbit kidney using the AWSALM pulse sequence was repeated for different activation MIs. It was found that the droplet activation was not observed in the rabbit kidney at an 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. Such understanding of the effects of activation MI could help improve droplet-base fast super-resolution imaging.