In vivo bubble nucleation probability in sheep brain tissue.

Area of Science:

• Biomedical Engineering
• Ultrasound Physics
• In Vivo Imaging

Background:

• Gas nuclei are naturally present in living tissues and can initiate cavitation when exposed to high-amplitude ultrasound.
• Understanding the in vivo nuclei population is crucial for determining the necessary rarefaction pressure for bubble formation in biological tissues.
• Current knowledge regarding the in vivo nucleation threshold for bubble formation remains limited.

Purpose of the Study:

• To investigate the in vivo bubble nucleation process in brain tissue.
• To determine the threshold of peak negative pressure required for bubble formation in vivo.
• To evaluate the nucleation probability as a function of peak negative pressure.

Main Methods:

• A novel in vivo method combining passive and active cavitation detection with a multi-element linear ultrasound probe (4-7 MHz).
• Induction of bubble nucleation using a focused single-element transducer (660 kHz) driven by a high-power electric burst.
• Experiments conducted in vivo on the brains of trepanated sheep.

Main Results:

• Successfully detected single nucleation events in brain tissue in vivo using passive recording and ultrafast active imaging.
• Determined that no nucleation events were detected below a peak negative pressure of -12.7 MPa.
• Established that bubble nucleation in vivo in brain tissues is a random phenomenon below this pressure threshold.

Conclusions:

• The study identified a significantly higher threshold for in vivo bubble nucleation (-12.7 MPa) compared to recommendations based on the mechanical index.
• The findings highlight the random nature of bubble nucleation in brain tissue below the identified pressure threshold.
• This research provides critical insights into the in vivo cavitation threshold, essential for the safe application of ultrasound therapies.