The activation of bubbles by an acoustic field has been shown to temporarily open the blood-brain barrier (BBB), but the trigger cause responsible for the physiological effects involved in the process of BBB opening remains unknown. Here, the trigger cause (i.e., physical mechanism) of the focused ultrasound-induced BBB opening with monodispersed microbubbles is identified. Sixtyseven mice were injected intravenously with bubbles of 1-2, 4-5, or 6-8 lm in diameter and the concentration of 10 7 numbers/ml. The right hippocampus of each mouse was then sonicated using focused ultrasound (1.5 MHz frequency, 100 cycles pulse length, 10 Hz pulse repetition frequency, 1 min duration). Peak-rarefactional pressures of 0.15, 0.30, 0.45, or 0.60 MPa were applied to identify the threshold of BBB opening and inertial cavitation (IC). Our results suggest that the BBB opens with nonlinear bubble oscillation when the bubble diameter is similar to the capillary diameter and with inertial cavitation when it is not. The bubble may thus have to be in contact with the capillary wall to induce BBB opening without IC. BBB opening was shown capable of being induced safely with nonlinear bubble oscillation at the pressure threshold and its volume was highly dependent on both the acoustic pressure and bubble diameter.

Focused ultrasound in conjunction with the systemic administration of microbubbles has been shown to open the bloodbrain barrier (BBB) selectively, noninvasively and reversibly. In this study, we investigate the dependence of the BBB opening's reversibility on the peak-rarefactional pressure (0.30-0.60 MPa) as well as the microbubble size (diameters of 1-2, 4-5, or 6-8 mm) in mice using contrast-enhanced T 1 -weighted (CE-T 1 ) MR images (9.4 T). Volumetric measurements of the diffusion of Gd-DTPA-BMA into the brain parenchyma were used for the quantification of the BBB-opened region on the day of sonication and up to 5 days thereafter. The volume of opening was found to increase with both pressure and microbubble diameter. The duration required for closing was found to be proportional to the volume of opening on the day of opening, and ranged from 24 h, for the smaller microbubbles, to 5 days at high peak-rarefactional pressures. Overall, larger bubbles did not show significant differences. Also, the extent of BBB opening decreased radially towards the focal region until the BBB's integrity was restored. In the cases where histological damage was detected, it was found to be highly correlated with hyperintensity on the precontrast T 1 images. Magn Reson Med 67:769-777,

The in vivo cavitation response associated with blood–brain barrier (BBB) opening as induced by transcranial focused ultrasound (FUS) in conjunction with microbubbles was studied in order to better identify the underlying mechanism in its noninvasive application. A cylindrically focused hydrophone, confocal with the FUS transducer, was used as a passive cavitation detector (PCD) to identify the threshold of inertial cavitation (IC) in the presence of Definity® microbubbles (mean diameter range: 1.1–3.3 μm, Lantheus Medical Imaging, MA, USA). A vessel phantom was first used to determine the reliability of the PCD prior to in vivo use. A cerebral blood vessel was simulated by generating a cylindrical channel of 610 μm in diameter inside a polyacrylamide gel and by saturating its volume with microbubbles. The microbubbles were sonicated through an excised mouse skull. Second, the same PCD setup was employed for in vivo noninvasive (i.e. transdermal and transcranial) cavitation detection during BBB opening. After the intravenous administration of Definity® microbubbles, pulsed FUS was applied (frequency: 1.525 or 1.5 MHz, peak-rarefactional pressure: 0.15–0.60 MPa, duty cycle: 20%, PRF: 10 Hz, duration: 1 min with a 30 s interval) to the right hippocampus of twenty-six (n = 26) mice in vivo through intact scalp and skull. T1 and T2-weighted MR images were used to verify the BBB opening. A spectrogram was generated at each pressure in order to detect the IC onset and duration. The threshold of BBB opening was found to be at a 0.30 MPa peak-rarefactional pressure in vivo. Both the phantom and in vivo studies indicated that the IC pressure threshold had a peak-rarefactional amplitude of 0.45 MPa. This indicated that BBB opening may not require IC at or near the threshold. Histological analysis showed that BBB opening could be induced without any cellular damage at 0.30 and 0.45 MPa. In conclusion, the cavitation response could be detected without craniotomy in mice and IC may not be required for BBB opening at relatively low pressures.

Focused ultrasound activation of systemically administered microbubbles is a noninvasive and localized drug delivery method that can increase vascular permeability to large molecular agents. Yet the range of acoustic parameters responsible for drug delivery remains unknown, and, thus, enhancing the delivery characteristics without compromising safety has proven to be difficult. We propose a new basis for ultrasonic pulse design in drug delivery through the blood-brain barrier (BBB) that uses principles of probability of occurrence and spatial distribution of cavitation in contrast to the conventionally applied magnitude of cavitation. The efficacy of using extremely short (2.3 μs) pulses was evaluated in 27 distinct acoustic parameter sets at low peak-rarefactional pressures (0.51 MPa or lower). The left hippocampus and lateral thalamus were noninvasively sonicated after administration of Definity microbubbles. Disruption of the BBB was confirmed by delivery of fluorescently tagged 3-, 10-, or 70-kDa dextrans. Under some conditions, dextrans were distributed homogeneously throughout the targeted region and accumulated at specific hippocampal landmarks and neuronal cells and axons. No histological damage was observed at the most effective parameter set. Our results have broadened the design space of parameters toward a wider safety window that may also increase vascular permeability. The study also uncovered a set of parameters that enhances the dose and distribution of molecular delivery, overcoming standard trade-offs in avoiding associated damage. Given the short pulses used similar to diagnostic ultrasound, new critical parameters were also elucidated to clearly separate therapeutic ultrasound from disruption-free diagnostic ultrasound.F ocused ultrasound (FUS) and microbubble-based drug delivery systems (DDSs) can increase the dose of an agent in a target volume and has potential in applications such as blood-brain barrier (BBB) disruption for the treatment of neurological diseases (1, 2), molecular and viral treatment of tumors (3), gene therapy for treating heart conditions (4), and enhancement of renal ultrafiltration (5). In each method, biologically inert and preformed microbubbles, with a lipid or polymer shell, a stabilized gas core, and a diameter less than 10 μm, are systemically administered and subsequently exposed to noninvasively delivered FUS pulses. Microbubbles within the target volume are "acoustically activated" in a complex range of behaviors known as acoustic cavitation. In stable cavitation, the microbubbles expand and contract with the acoustic pressure rarefaction and compression over several cycles (6). This activity has been associated with a range of bioeffects including displacement of the vessel wall through dilation and contractions (7,8). Large radial bubble expansions may induce inertial cavitation activity, which may lead to bubble collapse due to the inertia of the surrounding media and affect the vascular physiology (8). Each type and magnitude of cavitation activity results...

Blood-brain barrier (BBB) opening using focused ultrasound (FUS) and microbubbles has been experimentally established as a non-invasive and localized brain drug delivery technique. In this study, the permeability of the opening is assessed in the murine hippocampus after the application of FUS at three different acoustic pressures and microbubble sizes. Using DCE-MRI, the transfer rates were estimated, yielding permeability maps and quantitative Ktrans values for a predefined region of interest. The volume of BBB opening according to the Ktrans maps was proportional to both the pressure and the microbubble diameter. A Ktrans plateau of approximately 0.05 min−1 was reached at higher pressures (0.45 and 0.60 MPa) for the larger-sized bubbles (4–5 and 6–8 µm), which was on the same order as the Ktrans of the epicranial muscle (no barrier). Smaller bubbles (1–2 µm) yielded significantly lower permeability values. A small percentage (7.5%) of mice showed signs of damage under histological examination, but no correlation with permeability was established. The assessment of the BBB permeability properties and their dependence on both the pressure and the microbubble diameter suggests that Ktrans maps may constitute an in vivo tool for the quantification of the efficacy of the FUS-induced BBB opening.