Lipid-shelled microbubbles (MBs) are contrast agents often used as intravascular stress sources to the blood-brain barrier (BBB). Although MB stability under ultrasound imaging sequences has been studied, their behavior under therapeutic exposure is not well-understood. As a result, MB acoustic stability must be established in order to safely and efficiently penetrate the BBB. Thus, we investigated their acoustic stability when exposed to different acoustic pressures on a weekly scale.
C4F10 MBs with a 9:1 molar ratio of 1,2-distearoyl-sn-glycerol-3- phosphocholine (DSPC) and 1,2-distearoyl-sn-glycero-phosphoethanolamine-PEG2000 (DSPE- PEG2000) were manufactured in-house. MBs were diluted to 107 microbubbles/mL and exposed to therapeutic pulses (fc: 0.5 MHz, pulse length: 1 ms, pulse repetition frequency: 1 Hz, n=10) at acoustic pressures ranging from 200 kPapk-neg to 400 kPapk-neg while flowing through 4-mm vessel mimicking phantom (5% w/v gelatin) on day 0, 7, 14 post-activation. A 7.5 MHz passive cavitation detector was used to record MB acoustic emissions.
Acoustic emissions were quantified for cavitation levels and t80, time for which 80% of acoustic energy was released, over a period of 14 days for all acoustic pressures. Harmonic stable cavitation (SCDh) increased significantly for 300kPapk-neg and 400kPapk-neg on day 7 while increasing significantly on day 14 for when exposed to 200kPapk-neg. Inertial cavitation (ICD) showed a significant increase for each pressure throughout. t80 decreased significantly for 300kPapk-neg and 400kPapk-neg but not for 200kPapk-neg (p-value: 0.36, one-way ANOVA; n=10 pulses).
MB cavitation levels increase significantly over 14 days for all pressures while significantly decreasing t80 with the exception of MBs exposed to 200kPapk-neg. Perhaps, the fragmentation threshold decreases as a result of changes in size and/or lipid distribution of the MB shell. Thus, MBs may be used for a two-week period while taking into account their natural decay with respect to concentration.