Three-dimensional transcranial microbubble imaging for guiding volumetric ultrasound-mediated blood-brain barrier opening.

Focused ultrasound (FUS)-mediated blood-brain barrier (BBB) opening recently entered clinical testing for targeted drug delivery to the brain. Sources of variability exist in the current procedures, motivating the development of real-time monitoring and control techniques to improve treatment safety and efficacy. Here we used three-dimensional (3D) transcranial microbubble imaging to calibrate FUS exposure levels for volumetric BBB opening. Methods: Using a sparse hemispherical transmit/receive ultrasound phased array, pulsed ultrasound was focused transcranially into the thalamus of rabbits during microbubble infusion and multi-channel 3D beamforming was performed online with receiver signals captured at the subharmonic frequency. Pressures were increased pulse-by-pulse until subharmonic activity was detected on acoustic imaging (psub), and tissue volumes surrounding the calibration point were exposed at 50-100%psub via rapid electronic beam steering. Results: Spatially-coherent subharmonic microbubble activity was successfully reconstructed transcranially in vivo during calibration sonications. Multi-point exposures induced volumetric regions of elevated BBB permeability assessed via contrast-enhanced magnetic resonance imaging (MRI). At exposure levels ≥75%psub, MRI and histological examination occasionally revealed tissue damage, whereas sonications at 50%psub were performed safely. Substantial intra-grid variability of FUS-induced bioeffects was observed via MRI, prompting future development of multi-point calibration schemes for improved treatment consistency. Receiver array sparsity and sensor configuration had substantial impacts on subharmonic detection sensitivity, and are factors that should be considered when designing next-generation clinical FUS brain therapy systems. Conclusion: Our findings suggest that 3D subharmonic imaging can be used to calibrate exposure levels for safe FUS-induced volumetric BBB opening, and should be explored further as a method for cavitation-mediated treatment guidance.

Introduction

Purpose Drug delivery with BBB opening

Study Objective To evaluate whether real-time 3D transcranial subharmonic microbubble imaging can be used to calibrate focused ultrasound exposure levels for safe volumetric blood–brain barrier opening.

Animal model / Human subject New Zealand White rabbit (Oryctolagus cuniculus), strain: New Zealand White, age: not reported, sex: male, weight: 2–4 kg

Disease model Healthy

MRI or image guidance method MRI guidance (3.0 T MRI for target selection, treatment planning and post-sonication monitoring) combined with real-time 3D transcranial subharmonic acoustic (microbubble) imaging for calibration/targeting; animal head stabilized with custom ear/bite bars (platform positioning relative to array)

Targeted brain region(s) Thalamus

Cargo name and characteristics Gadovist (gadobutrol) — gadolinium-based MRI contrast agent (small molecule), IV administration (0.1 mL/kg used), used to detect blood-brain barrier opening

Route of administration intravenous

Outcomes and Safety

Summary of Outcomes 3D transcranial subharmonic imaging enabled calibration for volumetric focused‑ultrasound BBB opening in rabbits: exposing grids at 50% of the in‑situ subharmonic pressure threshold (p_sub ≈ 0.68 ± 0.15 MPa) using multi‑point sonications (612 kHz transmit, 6×6 grids with 1.0 mm spacing, 120 pulses/point) produced safe BBB opening with minimal histologic changes, whereas higher exposure levels (≥75% p_sub, tested up to 75–95% p_sub) yielded larger openings but occasionally caused tissue damage/microhemorrhage.

Duration of biological effect 1 week

Safety-related matter Exposures at ≥75% of the subharmonic pressure occasionally produced tissue damage (microhemorrhage/petechiae, extravasated red blood cells and T2* hypointensities). Exposures at 50% p_sub were performed safely with no microhemorrhage on MRI, only rare transient edema, and a single tiny (<50 μm) perivascular RBC extravasation on histology (1 of 20 volumetric sonications) that resolved by 1 week.

Brain Region

Ultrasound Parameters

Ultrasound instrument Clinical-scale prototype focused ultrasound (FUS) brain system: sparse hemispherical transmit/receive ultrasound phased array (hemispherical dome inner diameter = 31.8 cm)

FUS Frequency 612 kHz

FUS Pressure Starting SPTP negative pressure: 10-15 kPa (0.01-0.015 MPa); step size per pulse: ≈10-15 kPa (≈0.01-0.015 MPa). Additional exposures reported as percentages of the calibrated subharmonic pressure: 50-100% p_sub (relative) — safe at 50% p_sub, occasional tissue damage at ≥75% p_sub.

FUS Mode pulsed

Pulse duration 10 ms

Duration of a single FUS session 2 minutes

Focal Characteristics Focal depth: None; Focal length: None; Aperture size: 31.8 cm

Treatment frequency Both single and multiple sessions

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