부산대학교 도서관

Optimization of the bubble performance requires size isolation and accurate shell characterization using models that are not limited by linear assumptions. MBs and NBs with 2 different shell compositions (crosslinked (C) and non-crosslinked (NC)) were made in-house. NC shell is made with 4 different lipids including DBPC, DPPA,DPPE and DSPE-PEG2000 . C shell bubbles have additional ingredients that produce a UV polymerized crosslinked shell. Using the method of multiple differential centrifugations, two distinct size populations were separated with mean diameters of 2.9 µm for NC-MB and 3.3 µm for C-MB. The attenuation and sound speed of the diluted solutions were measured through transmission and reception method using one pair of PVDF transducers with center frequencies of 10 MHz and 100% BW at acoustic pressures of approximately 5 to 40 kPa. Our nonlinear model accounting for large amplitude MB oscillations was used to fit the measured attenuation and sound speed data at each pressure. As the pressure increased from 5 kPa to ≈ 50kPa, resonance frequency (f r ) of the NC-MBs with a mean diameter (MD) of 2.9 µm decreased from 9.1 to 5.9 MHz and f r of the C-MBs with (MD) of 3.3 µm decreased from 8 to 4.8 MHz. NC-NB solutions did not display any attenuation peak in the frequency range of 2-10 MHz, additionally; the measured attenuation was 5-10 times smaller than the MBs with the same shell composition. Fitting of the shell parameters suggests that crosslinking the shell results in ≈ 37% increase in stiffness and 50 % decrease in shell viscosity. The lower attenuation of the NBs even at very high concentrations may explain the enhancement in NB contrast ultrasound.