Increasing the endothelial layer permeability through ultrasound-activated microbubbles

The mechanism of interaction between focused ultrasound and microbubbles in blood-brain barrier opening in mice

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. Sixty-seven mice were injected intravenously with bubbles of 1-2, 4-5, or 6-8 μm 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.

Sonoporation of endothelial cells by vibrating targeted microbubbles

Molecular imaging using ultrasound makes use of targeted microbubbles. In this study we investigated whether these microbubbles could also be used to induce sonoporation in endothelial cells. Lipid-coated microbubbles were targeted to CD31 and insonified at 1 MHz at low peak negative acoustic pressures at six sequences of 10 cycle sine-wave bursts. Vibration of the targeted microbubbles was recorded with the Brandaris-128 high-speed camera (~13 million frames per second). In total, 31 cells were studied that all had one microbubble (1.2-4.2 micron in diameter) attached per cell. After insonification at 80 kPa, 30% of the cells (n=6) had taken up propidium iodide, while this was 20% (n=1) at 120 kPa and 83% (n=5) at 200 kPa. Irrespective of the peak negative acoustic pressure, uptake of propidium iodide was observed when the relative vibration amplitude of targeted microbubbles was greater than 0.5. No relationship was found between the position of the microbubble on the cell and induction of sonoporation. This study shows that targeted microbubbles can also be used to induce sonoporation, thus making it possible to combine molecular imaging and drug delivery.

In vivo transcranial cavitation threshold detection during ultrasound-induced blood-brain barrier opening in mice

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.

Molecular sonography with targeted microbubbles: current investigations and potential applications

Sonography using targeted microbubbles affords a variety of diagnostic and potentially therapeutic clinical applications. It provides a whole new world of functional information at the cellular and molecular level. This information can then be used to diagnose and possibly prevent diseases at early stages as well as devise therapeutic strategies at the molecular level. It is also useful in monitoring tumor response to therapy and devising treatment timing and plans based on the molecular state of an individual's health. Moreover, targeted microbubble-enhanced sonography has several advantages over other imaging modalities, including widespread availability, low cost, fast acquisition times, and lack of radiation risk. These traits are likely to advance it as one of the imaging methods of choice in future clinical trials examining the impact of molecular imaging on treatment outcome. This review describes the fundamental concepts of targeted microbubble-enhanced sonography as well as its potential clinical applications.