Noninvasive Blood-Brain Barrier Opening in Live Mice

Most therapeutic agents cannot be delivered to the brain because of brain′s natural defense: the Blood-Brain Barrier (BBB). It has recently been shown that Focused Ultrasound (FUS) can produce reversible and localized BBB opening in the brain when applied in the presence of ultrasound contrast agents post-craniotomy in rabbits [1]. However, a major limitation of ultrasound in the brain is the strong phase aberration and attenuation of the skull bone, and, as a result, no study of trans-cranial ultrasound-targeted drug treatment in the brain in vivo has been reported as of yet. In this study, the feasibility of BBB opening in the hippocampus of wildtype mice using FUS through the intact skull and skin was investigated. In order to investigate the effect of the skull, simulations of ultrasound wave propagation (1.5 MHz) through the skull using μCT data, and needle hydrophone measurements through an exvivo skull were made. The pressure field showed minimal attenuation (18% of the pressure amplitude) and a well-focused pattern through the left and right halves of the parietal bone. In experiments in vivo, the brains of four mice were sonicated through intact skull and skin. Ultrasound sonications (burst length: 20 ms; duty cycle: 20%; acoustic pressure range: 2.0 to 2.7 MPa) was applied 5 times for 30 s per shot with a 30 s delay between shots. Prior to sonication, ultrasound contrast agents (Optison; 10 μL) were injected intravenously. Contrast material enhanced high resolution MR Imaging (9.4 Tesla) was able to distinguish opening of large vessels in the region of the hippocampus. These results demonstrate the feasibility of locally opening the BBB in the mouse hippocampus using focused ultrasound through intact skull and skin. Future investigations will deal with optimization and reproducibility of the technique as well as application on Alzheimer’s-model mice.