1 WIMRC 3rd International Cavitation Forum 2011 University of Warwick, UK, 4 th -6 th July 2011 Laser-nucleated acoustic cavitation: size matters 1 Institute for Medical Science and Technology, University of Dundee, Dundee, DD2 1FD, UK 2 InSightec Ltd, 5 Nahum Heth Street, Tirat Carmel, 30120, Haifa, Israel *corresponding author: p.prentice@dundee.ac.uk ABSTRACT We report on the development of an instrument for hybrid ‘sonoptic’ cavitation studies. A focused ultrasound transducer is housed in a custom built chamber, which permits optical access to the focal volume, without perturbing the propagating acoustic field. This configuration allows pulsed-laser irradiation of the fluid at the focus, and simultaneous high speed observation of cavitation activity in this region. In this paper we provide a brief description of the apparatus and present preliminary data on distinct cavitation regimes we have observed. Specifically, laser- induced cavitation in an established field, and a new phenomenon that we refer to as laser-nucleated acoustic cavitation. The former involves a laser pulse of energy above the threshold value for optical breakdown for the medium, in a pre- established ultrasound field. Here, a cavity rapidly expands to a maximum diameter of a few 100μms, from the plasma generated on absorption of the optical energy, and collapses to form debris that is subsequently driven by the ultrasound radiation. By contrast, laser-nucleated acoustic cavitation is initiated by a pulse of energy below the ambient breakdown threshold, in a pre-established field. For this regime, either form of radiation does not result in cavitation activity without the other. In combination, the role of the laser pulse is to initiate activity which is dominated by the ultrasound exposure from the outset. Crucially, the spatial and temporal precision afforded to the occurrence of cavitation by laser-nucleation, allows the use of high speed micro-photography to resolve cavitation cloud evolution and behavior. This allows us to consolidate our assertion of acoustic cavitation, with close-up ultra-high speed images of the clouds, and observation of constituent cavity sizes. It is expected that such observations will contribute to a greater understanding of cavitation in focused ultrasound, including for potential future therapeutic applications. NOMENCLATURE f c transducer centre frequency FOV Field of View FUS Focused Ultrasound Surgery HIFU High Intensity Focused Ultrasound LIC Laser-Induced Cavitation LNAC Laser-Nucleated Acoustic Cavitation Mfps Megaframes per second (10 6 fps) MHz Megahertz (10 6 Hz) mJ millijoule PNP Peak Negative Pressure (amplitude) R f Resonant bubble radius (to frequency, f) R max Maximum bubble radius R min Minimum (recorded) bubble radius UCA Ultrasound contrast agent (shelled microbubble suspensions) US Ultrasound 1. INTRODUCTION 1.1 Acoustic cavitation Acoustic cavitation refers specifically to the formation and subsequent activity of bubbles, or cavities in the pressure fluctuations of an acoustic field, to distinguish from its mechanical or hydro-dynamical counterparts. The phenomenon is a common occurrence in the application of HIFU fields to the ablation of diseased tissue, known as FUS. In this emerging technique, ultrasound is focused transcutaneously to the site of pathology, thus alleviating the need for conventional surgical intervention, and associated risk of infection and extended patient recovery times. In current clinical procedure, the heating Bjoern Gerold 1,2 , Joyce Joy 1 , Oleg Prus 2 , Javier Grinfeld 2 , Alexander Volovyk 2 , Yaov Medan 2 , Andreas Melzer 1 , Sandy Cochran 1 , Paul Prentice 1 *