2F-5 Optimization of Blood-Brain Barrier Opening in Mice using Focused Ultrasound

Abstract

Most neurologically potent drugs cannot be delivered to the brain because of its distinct vasculature: the blood-brain barrier (BBB). Focused ultrasound (FUS) has been shown to produce reversible and localized BBB opening when applied in the presence of microbubbles [Hynynen, K, 2001]. In this study, the dependence of microbubble concentration and vessel type distribution in the targeted brain region on FUS-induced BBB opening was investigated. A therapeutic FUS transducer (1.525 MHz) was confocally combined with a 7.5 MHz diagnostic transducer that allowed for consistent, high precision targeting. To induce BBB opening, eight mice were intravenously injected with microbubbles (Optison; 25-50 muL). The brains were then sonicated (pressure amplitudes: 0.5-1.1 MPa; burst length: 20 ms; duty cycle: 20%; duration: 2-4 times for 30 s per shot with a 30 s delay between shots) through their intact skull and skin. The mice were then injected intraperitoneally (IP) with a gadolinium-based MRI contrast agent (Omniscan; 0.5 ml) and scanned with high-resolution T1-weighted MR imaging (9.4 Tesla; in-plane resolution: 75 mum). The threshold of BBB opening with an injection of 50 muL of Optison was found to be between 0.53 and 0.67 MPa with the area of BBB opening increasing linearly with peak rarefractional pressure. Further reducing the amount of Optison injected to 25 muL decreased the area of BBB opening by approximately a factor of three. High-resolution MRI was able to determine the influence of vessel density and size within the beam on the BBB opening. Using the optimized sonication parameters to induce consistent BBB opening (pressure: 0.8 MPa; Optison: 25 muL) the path of the MRI contrast agent diffusion throughout the BBB-opened region was tracked over the course of two hours. The posterior cerebral artery, because of its larger vessel size (and, therefore, higher Optison concentration interacting with the FUS beam), depicted a greater pixel intensity increase on the T1-weighted MR image at the sonicated region compared to the non-sonicated (control) and homogeneous brain tissue regions. These findings demonstrated the importance of microbubble concentration and vessel characteristics on opening the BBB in the mice. Finally, the use of the optimized parameters for opening at the threshold minimizing probability of irreversible damage, together with slow-diffusion MRI imaging, allowed for temporal analysis of the BBB opening, revealing not only detection of the opening, but also its permeability properties