Microbubble dynamics in brain microvessels.

Focused ultrasound stimulation of microbubbles is being tested in clinical trials for its ability to deliver drugs across the blood-brain barrier (BBB). This technique has the potential to treat neurological diseases by preferentially delivering drugs to targeted regions. Yet despite its potential, the physical mechanisms by which microbubbles alter the BBB permeability remain unclear, as direct observations of microbubbles oscillating in brain microvessels have never been previously recorded. The purpose of this study was to reveal how microbubbles respond to ultrasound when within the microvessels of living brain tissue. Microbubbles in acute brain slices acquired from juvenile rats perfused with a concentrated solution of SonoVue® and dye were exposed to ultrasound pulses typically used in BBB disruption (center frequency: 1 MHz, peak-negative pressure: 0.2-1 MPa, pulse length: up to 10 ms) and observed using high-speed microscopy at up to 10 million frames per second. We observed that microbubbles can exert mechanical stresses on a wide region of tissue beyond their initial location and immediate surroundings. A single microbubble can apply mechanical stress to parenchymal tissues several micrometers away from the vessel. Microbubbles can travel at high velocities within the microvessels, extending their influence across tens of micrometers during a single pulse. With longer pulses and higher pressures, microbubbles could penetrate the vessel wall and move through the parenchyma. The probability of extravasation scales approximately with mechanical index, being rare at low pressures, but much more common at a mechanical index ≥ 0.6. These results present the first direct observations of ultrasound-driven microbubbles within brain tissue, and illustrate a range of microbubble behaviors that have the potential to lead to safe drug delivery or tissue damage.

Introduction

Purpose Drug delivery with BBB opening

Study Objective To reveal how microbubbles respond to ultrasound when within the microvessels of living brain tissue.

Animal model / Human subject Rat (Rattus norvegicus), Wistar (wild-type), postnatal day 5-15 (juvenile), sex None

Disease model Healthy

MRI or image guidance method Optical microscopy guidance — focus aligned to the microscope objective focal plane using an in situ needle hydrophone; microbubbles visually targeted/selected via high-speed and color cameras.

Targeted brain region(s) Frontal Cortex

Cargo name and characteristics Evans Blue (small-molecule vascular dye, 4 mg/mL) or Blue India Ink (colloidal dye suspension, 40% v/v) used to visualize vasculature/extravasation; heparin (anticoagulant, 0.05 mg/mL) included in the perfusate (all diluted in 0.9% saline).

Route of administration Transcardial perfusion (into the vasculature via syringe pump)

Outcomes and Safety

Summary of Outcomes Focused ultrasound (1 MHz) drove microbubble oscillations in brain microvessels that produced vessel-wall motion, tissue displacements several micrometers into the parenchyma, bubble translation tens of micrometers, and at higher settings caused microbubble extravasation with associated dye delivery (indicative of vessel puncture and potential tissue damage). Successful parameters: center frequency 1 MHz, peak-negative pressures 0.2–1.0 MPa (pulse lengths up to 10 ms and some short 50 μs pulses); tissue deformation observed at 0.2–0.6 MPa, extravasation was rare at 0.2–0.4 MPa but became common at ≥0.6 MPa (dominant at ~0.8 MI), with larger bubbles (≈4.8–8.4 μm) more likely to extravasate.

Duration of biological effect 10 ms

Safety-related matter The study observed microbubble-induced mechanical stresses including tissue displacement, vessel wall puncture, dye leakage, persistent structural vessel changes and microbubble extravasation (with fragments entering parenchyma), indicating potential tissue damage; these adverse effects were rare or absent at low pressures (no extravasation at 0.2 MPa, only one instance at 0.4 MPa) but became common at higher mechanical index (dominant at MI ≈ 0.8), consistent with MI 0.2–0.4 being generally considered safe while higher pressures increase risk.

Brain Region

Ultrasound Parameters

Ultrasound instrument Immersion transducer A303 S-SU (Olympus); diameter: 13 mm; focal distance: 15.2 mm; center frequency: 1 MHz

FUS Frequency 1 MHz

FUS Pressure 0.2-1 MPa (range used); 0.8 MPa (specific peak-negative pressure reported in figures)

FUS Mode pulsed

Pulse duration 10 ms (long pulses); also 15 μs (0.015 ms, 15 cycles at 1 MHz) and 60 μs (0.06 ms, 60 cycles at 1 MHz)

Focal Characteristics Focal depth: None; Focal length: 15.2 mm; Aperture size: 13 mm

We are open to feedback. If you see a mistake or have a suggestion, please contact us.