A study of cavitation activity in ex vivo tissue exposed to high intensity focused ultrasound

Cavitation is often avoided in Focused Ultrasound Surgery (FUS or HIFU) because it can render lesion formation unpredictable. However, cavitation is known to enhance heating. Emissions used as indicators for cavitation activity in ex vivo tissue are not fully understood. This study investigates a wide range of simultaneous acoustic emissions and other potential indicators of cavitation activity. A high frequency (<= 50MHz) data acquisition system is used to detect cavitation ex vivo. The passive cavitation detector (PCD) used is a broadband (0.1-10MHz) cavitation sensor. Its broadband nature allows simultaneous measurement of subharmonics, superharmonics and broadband emissions, all potential indicators of either inertial or both types of cavitation. The electrical impedance change of the transducer (1.69MHz, 15cm focal length, 1.79f-number), caused by backscattered ultrasound, has been monitored. Low frequency acoustic signals (< 100kHz) have been recorded using a hydrophone (Reson TC4013, I Hz-170kHz). The ultimate aim of this work is to investigate the possibility of detecting cavitation signals from HIFU during clinical treatments. Results of monitoring multiple cavitation signals during ex vivo HIFU exposure are presented. The relationship between impedance change and superharmonic emissions, indicating discrete acoustic emissions or scattering of ultrasound from bubbles, are discussed. Artefacts in B-mode ultrasound scans taken during HIM exposures have been seen to correlate with impedance change and acoustic emissions. This is still under investigation. Cavitation thresholds in degassed water and ex vivo tissue have been investigated. This work paves the way for investigation of the enhancement of lesion formation from HIFU exposures by exploiting cavitation activity.