In vivobubble nucleation probability in sheep brain tissue

Abstract

Gas nuclei exist naturally in living bodies. Their activation initiates cavitation activity, and is possible using short ultrasonic excitations of high amplitude. However, little is known about the nuclei population in vivo, and therefore about the rarefaction pressure required to form bubbles in tissue. A novel method dedicated to in vivo investigations was used here that combines passive and active cavitation detection with a multi-element linear ultrasound probe (4-7 MHz). Experiments were performed in vivo on the brain of trepanated sheep. Bubble nucleation was induced using a focused single-element transducer (central frequency 660 kHz, f-number = 1) driven by a high power (up to 5 kW) electric burst of two cycles. Successive passive recording and ultrafast active imaging were shown to allow detection of a single nucleation event in brain tissue in vivo. Experiments carried out on eight sheep allowed statistical studies of the bubble nucleation process. The nucleation probability was evaluated as a function of the peak negative pressure. No nucleation event could be detected with a peak negative pressure weaker than -12.7 MPa, i.e. one order of magnitude higher than the recommendations based on the mechanical index. Below this threshold, bubble nucleation in vivo in brain tissues is a random phenomenon.

Three-Dimensional Transcranial Ultrasound Imaging of Microbubble Clouds Using a Sparse Hemispherical Array

“…Passive cavitation detectors (PCDs) are long established for investigating cavitation phenomena [13, 14, 15], and advancement from single element PCDs [16] to arrays of PCDs has added spatial information to cavitation studies [17, 18]. Passive imaging has long been used to localize sources [19, 20, 21] in a number of different fields, but has been of high interest in recent ultrasound research [22, 23, 24, 25] due to its potential to map bubble activity during cavitation-enhanced therapies and thereby enhance treatment safety and assess outcome.…”

A Theoretical Study of Inertial Cavitation from Acoustic Radiation Force Impulse Imaging and Implications for the Mechanical Index1

“… PRP = peak rarefactional pressure; MIE th = the effective value of the mechanical index at threshold (see text for details); Sources for experimental results: a) Hynynen 1991; b) Gateau et al 2011b; c) Hwang et al 2005; d) Hwang et al 2006; e) Miller et al 2011; f) Gateau et al 2011a; g) Vykhodtseva et al 1995; h) Kreider et al 2014. …”

Application of High-Intensity Focused Ultrasound to the Study of Mild Traumatic Brain Injury

“…Gateau et al (2011) found that for 660-kHz pulses, formation of gas bubbles in the sheep brain did not occur when the peak negative pressure was weaker than -12.7 MPa. Fry et al (1995) found the cavitation threshold in dog brain tissue to be around -3.5 MPa for a 1-MHz pulse.…”