Nano-sized Ultrasound Contrast Agents for Cancer Therapy and Theranostics.

Ultrasound contrast agents (UCA) represented by gas-filled microbubble, can provide simultaneous and co-localized enhancement on image contrast to help disease diagnosis by highlighting tissue borders. Nowadays, Some UCAs (e.g. Levovist®, Optison®, Definity®, and Sonovue®) are commercially available, and have been clinically utilized for enhanced ultrasonography in the US, Canada, Europe, Asia and so on. However, their large diameters (1~10 µm) mainly hinder more precise and deeper applications in the imaging of capillaryabundant organs or tissues (e.g. tumor), and undersized nanoscale UCAs also lack enough backscattering echo intensity to distinguish abnormal distribution of vessels. So novel shapes, structures and materials of nano-sized UCAs are constantly emerging for cancer ultrasonic imaging. Particularly, the cavitation effect of diagnostic ultrasound can accelerate effusion of loaded contents from UCAs, following cellular uptake. This will inevitably contribute to develop other potential applications of nano-sized UCAs towards cancer therapy and theranostics.