Trans-cranial opening of the blood-brain barrier in targeted regions using astereotaxic brain atlas and focused ultrasound energy

Abstract

Objective: The blood-brain barrier (BBB) protects the brain by preventing the entry of largemolecules; this poses a major obstacle for the delivery of drugs to the brain. Anovel technique using focused ultrasound (FUS) energy combined with microbubblecontrast agents has been widely used for non-invasive trans-cranial BBB opening. Traditionally, FUS research is conducted with magnetic resonance imaging (MRI)guidance, which is expensive and poses physical limitations due to the magneticfield. A system that could allow researchers to test brain therapies without MRintervention could facilitate and accelerate translational research. Methods: In this study, we present a novel FUS system that uses a custom-built FUSgenerator mounted on a motorized stereotaxic apparatus with embedded brain atlasto locally open the BBB in rodents. The system was initially characterized using atissue-mimicking phantom. Rodent studies were also performed to evaluate whethernon-invasive, localized BBB opening could be achieved using brain atlas-basedtargeting. Brains were exposed to pulsed focused ultrasound energy at1.06 MHz in rats and 3.23 MHz in mice, with the focal pressure estimatedto be 0.5-0.6 MPa through the skull. BBB opening was confirmed in grosstissue sections by the presence of Evans blue leakage in the exposed region of thebrain and by histological assessment. Results: The targeting accuracy of the stereotaxic system was better than 0.5 mm inthe tissue-mimicking phantom. Reproducible localized BBB opening was verified withEvans blue dye leakage in 32/33 rats and had a targeting accuracy of±0.3 mm. The use of higher frequency exposures in mice enabled a similarprecision of localized BBB opening as was observed with the low frequency in therat model. Conclusions: With this dedicated small-animal motorized stereotaxic-FUS system, we achievedaccurate targeting of focused ultrasound exposures in the brain for non-invasiveopening of the BBB. This system can be used as an alternative to MR-guided FUS andoffers researchers the ability to perform efficient studies (30 min perexperiment including preparation) at a reduced cost in a conventional laboratoryenvironment.