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Diagnostic frequency ultrasound images of POPC liposomes encapsulating 400 mM ammonium bicabonate as a function of frequency and incubation time in a pH 5 buffer. The images were acquired by employing high-frequency (12–15 MHz; A, B), medium-frequency (8–12 MHz; C, D), and low-frequency (4–8 MHz; E, F) ultrasound transducers.
