Shockwave-induced Reversible Blood-Brain Barrier Opening

Evans blue (EB) was used as an indicator to show the location and extent of BBB opening in rat brains since EB cannot penetrate an intact BBB. Immediately after the injection of 0.5 ml of 3% EB via the rats’ tail vein, 100 shockwave iterations were applied (intensity level 5, pulse repetition frequency (PRF) 5 Hz, N = 8). The following process was modified from a previous study^{27 (link)}. The original two hour interval time for euthanization was extended to three hours. Three hours after the shockwave application, the rats were sacrificed and the brains were harvested for histological analysis^{27 (link)}. The intensity levels for each parameter are shown in Table 1.Table 1

Major shockwave parameters of the intensity levels.

Intensity level	0.1	1	2	3	4	5	10	20
Negative peak pressure (MPa)	−4.2	−7.3	−7.92	−8.54	−9.17	−9.79	−12.9	−18.7
Positive peak pressure (MPa)	5.4	11.8	13.66	15.51	17.37	19.22	28.5	77.7
Energy flux density (Total) (mJ/mm2)	0.03	0.1	0.13	0.16	0.18	0.21	0.35	0.82

To evaluate the performance of shockwave applications, the successful (visible) BBB opening rate is defined as number of rats with BBB opening after shockwave treatment over the total number of rats receiving shockwave treatment. To define BBB opening, the Evans blue stained area of the histology sections were further analyzed using a color histogram of Image J. The delta blue and delta red in the proposed study were the differences in pixels of blue and red color in RGB-image system that were used to evaluate the staining level of Evans blue and the red blood cell extravasation level between the shockwave applied side and untreated side. Both of them were assessed using student’s t-test and one-way ANOVA and post-hoc analysis, wherein p < 0.05 for the delta blue between the shockwave applied side and untreated side.
To confirm the reversibility of the shockwave-induced BBB opening, 0.5 ml of 3% EB was injected via the rats’ tail vein 24 hrs after applying 100 shockwave iterations with all other conditions remaining constant (N = 5). The rats were then sacrificed for brain sectioning 3 hours after the EB injection.
Traditionally, ultrasound contrast agent (UCA) is used together with HIFU to induce BBB opening in the brain by enhancing cavitation^{28 (link),29 (link)}. The effect of UCA on shockwaves was investigated by adding various concentrations of SonoVue UCA from 2.5 × 10² to 2.5 × 10⁸ microbubbles/kg body weight (the clinical concentration is 3–15 × 10⁶ microbubbles/kg) through the tail vein immediately before shockwave treatment (N = 5).