Institute of Fundamental Technological Research

Objective: Focused ultrasound (FUS) with intravenously administered microbubbles (MBs) enables different therapeutic effects, e.g. localized opening of the blood-brain barrier (BBB). Acoustic activation of MBs under FUS induces mechanical effects---primarily stable or inertial cavitation - that can reversibly disrupt endothelial tight junctions without permanent tissue damage. MB acoustic emissions are widely used as indicators of cavitation activity and, by extension, treatment efficacy and safety. While some aspects of microbubble behavior under different exposure conditions are known, the overall influence of various parameter combinations on cavitation dose remains incompletely described. Approach: This study examined how MB concentration (0.0008-0.4% V/V), peak negative pressure (61.5-2600 kPa), pulse duration (95-952 µs), and effective sonication time affect cavitation activity in a flow setup. Cavitation was quantified as a cavitation dose which was divided into three types: stable harmonic (SCD_har), ultraharmonic (SCD_ultra), and broadband (ICD) emissions. Results: SCD_har and ICD increased mostly monotonically with pressure and MB concentration, while SCD_ultra peaked at intermediate values suggesting optimal parameters for the control of the ultrasound BBB opening procedure. Cavitation metrics showed 10% reproducibility. Critically, we found that for fixed effective sonication times, increasing the number of pulses led to significantly change the response of cavitation dose in time. To our knowledge, this relationship has not been studied before, change of pulse length was always related to effective sonication time. Our results suggests that pulse number is an important factor of how MB oscillate, introducing a potentially pivotal control parameter for therapeutic ultrasound.Significance: These findings provide new insights into MB dynamics and highlight pulse count as an underrecognized yet potentially important factor in protocol design. This perspective may inform refinements of FUS treatments, contributing to greater safety, consistency, and efficacy, and represents a step toward optimizing ultrasonic BBB opening strategies.

Microbubbles, Nonlinear oscillations, Ultrasound

In ultrasound-mediated blood–brain barrier (BBB) opening procedures, intravenously injected microbubbles (MBs)—gas-filled agents encapsulated by lipid or protein shells—play a central role. Originally developed as ultrasound contrast agents, MBs have demonstrated considerable potential for modulating BBB permeability. Upon exposure to focused ultrasound within cerebral vasculature, MBs undergo oscillations that transiently disrupt the tight junctions of endothelial cells, facilitating the temporary passage of macromolecules exceeding 400 Da. This bioeffect can be harnessed to improve the delivery and therapeutic efficacy of drugs targeting, among others, neurodegenerative disorders such as Alzheimer’s disease.
This study investigates the influence of various parameters on the acoustic emissions of microbubbles (MBs; SonoVue, Bracco), including MB concentration (0.0008%, 0.004%, 0.016%, 0.08%, 0.4% [V/V]), peak negative pressure (61.5 ± 8, 121 ± 15.5, 252.5 ± 33, 600 ± 80, 1300 ± 165, and 2600 ± 340 kPa), and ultrasound pulse duration (100, 200, and 1000 μs). Experiments were conducted in a flow setup equipped with a focused transducer (H101, Sonic Concepts, f₀ = 1.05 MHz) and a passive acoustic receiver.
Three cavitation metrics were derived from the recorded acoustic signals: stable cavitation dose from harmonics (SCDₕₐᵣ), from ultraharmonics (SCDᵤₗₜᵣₐ), and inertial cavitation dose (ICD) based on broadband emissions.
The results indicate that SCDₕₐᵣ generally increases with pressure, reaching a maximum at 600 kPa before declining at higher pressures. SCDᵤₗₜᵣₐ peaked at MB concentrations of 0.004% and 0.016%, while ICD remained relatively uniform across concentrations, with no substantial variations. A pronounced ICD response was observed at pressures ≥ 600 kPa, with the highest values recorded at the maximum MB concentration (0.4%).
Currently, no standardized methodology exists for quantifying cavitation dose, making it a subject of ongoing research. The presented findings highlight key trends in MB behavior under varying experimental conditions. The selected parameter ranges provide a broad perspective on MB acoustic responses within this experimental setup.