Effect of Acoustic Power on In Vivo Molecular Imaging with Targeted Microbubbles: Implications for Low-Mechanical Index Real-Time Imaging

Background: The aim of this study was to evaluate the influence of acoustic power on ultrasound molecular imaging data with targeted microbubbles. Methods: Imaging was performed with a contrast-specific multipulse method at mechanical indexes (MIs) of 0.18 and 0.97. In vitro imaging was used to measure concentration-intensity relationships and to assess whether damping from microbubble attachment to cultured endothelial cells affected signal enhancement. Power-related differences in signal enhancement were evaluated in vivo by P-selectin-targeted and control microbubble imaging in a murine model of hind-limb ischemia-reperfusion injury. Results: During in vitro experiments, there was minimal acoustic damping from microbubble-cell attachment at either MI. Signal enhancement in the in vitro and in vivo experiments was 2-fold to 3-fold higher for high-MI imaging compared with low-MI imaging, which was due to greater pixel intensity, the detection of a greater number of retained microbubbles, and increased point-spread function. Yet there was a linear relationship between high-MI and low-MI data indicating that the relative degree of enhancement was similar. Conclusion: During molecular imaging, high-MI protocols produce more robust targeted signal enhancement than low-MI protocols, although differences in relative enhancement caused by condition or agent are similar.