The therapeutic efficacy of neurological agents is severely limited, because large compounds do not cross the blood–brain barrier (BBB). Focused ultrasound (FUS) sonication in the presence of microbubbles has been shown to temporarily open the BBB, allowing systemically administered agents into the brain. Until now, polydispersed microbubbles (1–10 μm in diameter) were used, and, therefore, the bubble sizes better suited for inducing the opening remain unknown. Here, the FUS-induced BBB opening dependence on microbubble size is investigated. Bubbles at 1–2 and 4–5 μm in diameter were separately size-isolated using differential centrifugation before being systemically administered in mice (n = 28). The BBB opening pressure threshold was identified by varying the peak-rarefactional pressure amplitude. BBB opening was determined by fluorescence enhancement due to systemically administered, fluorescent-tagged, 3-kDa dextran. The identified threshold fell between 0.30 and 0.46 MPa in the case of 1–2 μm bubbles and between 0.15 and 0.30 MPa in the 4–5 μm case. At every pressure studied, the fluorescence was greater with the 4–5 μm than with the 1–2 μm bubbles. At 0.61 MPa, in the 1–2 μm bubble case, the fluorescence amount and area were greater in the thalamus than in the hippocampus. In conclusion, it was determined that the FUS-induced BBB opening was dependent on both the size distribution in the injected microbubble volume and the brain region targeted.

As a potentially viable method of brain drug delivery, the safety profile of blood-brain barrier (BBB) opening using focused ultrasound (FUS) and ultrasound contrast agents (UCA) needs to be established. In this study, we provide a short-term (30 min or 5h survival) histological assessment of murine brains undergoing FUS-induced BBB opening. Forty-nine mice were intravenously injected with Definity® microbubbles (0.05 μl/kg) and sonicated under the following parameters: frequency of 1.525 MHz, burst length of 20 ms, PRF of 10 Hz, peak rarefactional acoustic pressures of 0.15-0.98 MPa, and two 30-s sonication intervals with an intermittent 30-s delay. The BBB opening threshold was found to be 0.15-0.3 MPa based on fluorescence and MR imaging of systemically-injected tracers. Analysis of three histological measures in H&E-stained sections revealed the safest acoustic pressure to be within the range of 0.3-0.46 MPa in all examined time periods post sonication. Across different pressure amplitudes, only the samples 30 min post-opening showed significant difference (p<0.05) in the average number of distinct damaged sites, microvacuolated sites, dark neurons, and sites with extravasated erythrocytes. Enhanced fluorescence around severed microvessels was also noted and found to be associated with the largest tissue effects while mildly diffuse BBB opening with uniform fluorescence in the parenchyma was associated with no or mild tissue injury. Region-specific areas of the sonicated brain (thalamus, hippocampal fissure, dentate gyrus, and CA3 area of hippocampus) exhibited variation in fluorescence intensity based on the position, orientation, and size of affected vessels. The results of this short-term histological analysis demonstrated the feasibility of a safe FUS-UCA-induced BBB opening under a specific set of sonication parameters and provided new insights on the mechanism of BBB opening.

Ultrasound methods in conjunction with microbubbles have been used for brain drug delivery, treatment of stroke, and imaging of cerebral blood flow. Despite advances in these areas, questions remain regarding the range of ultrasound parameters that disrupt the blood-brain barrier (BBB). In this study, several conditions were investigated to either enhance or reduce the likelihood of BBB disruption. Pulsed focused ultrasound (frequency: 1.5 MHz, pressure: 0.46 MPa, pulse repetition frequency (PRF): 0.1 to 25 Hz, pulse length (PL): 0.03 to 30 milliseconds) was noninvasively and locally administered to a predetermined region in the left hemisphere in the presence of circulating preformed microbubbles (Definity, Lantheus Medical Imaging, N. Billerica, MA, USA; 0.01, 0.05, 0.25 μL/g). Trans-BBB delivery of 3-kDa dextran was observed at PRFs as low as 1 Hz, whereas consistent delivery was observed at 5 Hz and above. Delivery was demonstrated at a PL as low as 33 microseconds. Although the delivered dextran concentration increased with the PL, this also increased the heterogeneity of the resulting distribution. In conclusion, key parameters that disrupt the BBB were identified out of a wide range of conditions. Reducing the total number of emitted acoustic cycles by shortening the PL, or decreasing the PRF, was also found to facilitate a more spatially uniform distribution of delivered dextran.

Over 4 million U.S. men and women suffer from Alzheimer's disease; 1 million from Parkinson's disease; 350,000 from multiple sclerosis (MS); and 20,000 from amyotrophic lateral sclerosis (ALS). Worldwide, these four diseases account for more than 20 million patients. In addition, aging greatly increases the risk of neurodegenerative disease. Although great progress has been made in recent years toward understanding of these diseases, few effective treatments and no cures are currently available. This is mainly due to the impermeability of the blood-brain barrier (BBB) that allows only 5% of the 7000 small-molecule drugs available to treat only a tiny fraction of these diseases. On the other hand, safe and localized opening of the BBB has been proven to present a significant challenge. Of the methods used for BBB disruption shown to be effective, Focused Ultrasound (FUS), in conjunction with microbubbles, is the only technique that can induce localized BBB opening noninvasively and regionally. FUS may thus have a huge impact in trans-BBB brain drug delivery. The primary objective in this paper is to elucidate the interactions between ultrasound, microbubbles and the local microenvironment during BBB opening with FUS, which are responsible for inducing the BBB disruption. The mechanism of the BBB opening in vivo is monitored through the MRI and passive cavitation detection (PCD), and the safety of BBB disruption is assessed using H&E histology at distinct pressures, pulse lengths and microbubble diameters. It is hereby shown that the BBB can be disrupted safely and transiently under specific acoustic pressures (under 0.45 MPa) and microbubble (diameter under 8 μm) conditions.