**3. Targeted Contrast Agents**

The main UCA are MB, NB, and nanodroplets (Figure 3). To enhance their specificity, various ligands were coupled, including antibodies, peptides, and carbohydrates. Depending on their size, UCA have been targeted to intravascular or extravascular markers.

**Figure 3.** Schematic representation of microbubbles, nanobubbles, and nanodroplets.
