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Figure 2: In vitro ultrasonographic images of HPMO NPs of harmonic mode at different concentrations ((a)–(d): 20 mg/mL; (e)–(h): 10 mg/mL; (i)–(l): 5 mg/mL) and different mechanical indexes ((a), (e), and (i): 1.2; (b), (f), and (j): 1.0; (c), (g), and (k): 0.8; (d), (h), and (l): 0.6). (m) The in vitro acoustic signal intensities (gray value) of HPMOs solutions under different imaging modes, concentrations, and mechanical indexes.