Dependence of the reversibility of focused- ultrasound-induced blood-brain barrier opening on pressure and pulse length in vivo

The most challenging aspect of intravenously-administered drugs currently developed to treat central nervous system (CNS) diseases is their impermeability through the blood-brain barrier (BBB), a specialized vasculature system protecting the brain microenvironment. Focused ultrasound (FUS) in conjunction with systemically administered microbubbles has been shown to open the BBB locally, noninvasively, and reversibly. The objective of this study was to investigate the effect of FUS (center frequency: 1.5 MHz) pulse length (PL), ranging here from 67 μs to 6.7 ms, on the physiology of the FUS-induced BBB opening. Dynamic contrastenhanced (DCE) and T1-weighted magnetic resonance imaging (MRI) were used to quantify the permeability changes using transfer rate (K_trans) mapping, the volume of BBB opening (VBBB) and the reversibility timeline of the FUS-induced BBB opening, with the systemic administration of microbubbles at different acoustic pressures, ranging from 0.30 to 0.60 MPa. Permeability and volume of opening were both found to increase with the acoustic pressure and pulse length. At 67-μs PL, the opening pressure threshold was 0.45 MPa, with BBB opening characteristics similar to those induced with 0.60 MPa at the same PL, as well as with 0.67-ms PL/0.30 MPa. On average, these cases had Ktrans = 0.0049 ± 0.0014 min-1 and V_BBB = 3.7 ± 4.3 mm³, and closing occurred within 8 h. The 6.7-ms PL/0.30 MPa induced similar opening with 0.67-ms PL/0.45 MPa, and a closing timeline of 24 to 48 h. On average, K_trans was 0.0091 ± 0.0029 min-1 and V_BBB was 14.13 ± 7.7 mm³ in these cases. Also, there were no significant differences between the 6.7-ms PL/0.45 MPa, 0.67-ms PL/0.60 MPa and 6.7-ms PL/0.60 MPa cases, yielding on average a K_trans of 0.0100 ± 0.0023 min-1 and V_BBB equal to 20.1 ± 5.7 mm³. Closing occurred wit- in 48 to 72 h in these cases. Stacked histograms of the K_trans provided further insight to the nonuniform spatial distribution of permeability changes and revealed a correlation with the closing timeline. These results also suggest a beneficial complementary relationship between the elongation of the PL and the decrease of the peak negative acoustic pressures, and vice versa. Linear regression between K_trans and V_BBB showed a good correlation fit. Also, the time required for closing linearly increased with V_BBB. The volume rate of decrease was measured to be 11.4 ± 4.0 mm³ per day, suggesting that the closing timeline could be predicted from the initial volume of opening. Finally, no histological damage was detected in any of the cases 7 d post-FUS, indicating the safety of the methodology and parameters used.