Imaging feedback of tissue liquefaction (histotripsy) in ultrasound surgery

Abstract

High intensity pulsed ultrasound was used to disrupt liver tissue through cavitation while monitoring with 8 MHz ultrasound imaging. Application of 2000 pulses (sufficient to liquefy the target tissue) resulted in an average 22.4 dB reduction in speckle amplitude. This method may provide effective feedback in ultrasound surgery using cavitation. INTRODUCTION Previous work has shown that under certain conditions, high-intensity pulsed ultrasound can be used to liquefy tissue (1). We are exploring this method as an alternative and complement to traditional thermal therapy techniques. The process of tissue breakdown (we refer to as "histotripsy") is thought to occur as a result of repeated subdivision of tissue structure through cavitation. We hypothesize that cavitation induced liquefaction of tissue should produce a significant reduction in standard B-scan imaging speckle amplitude as tissue structure is broken down and the underlying source of speckle backscatter in the MHz regime is reduced. This may provide an effective feedback mechanism for treatment efficacy in ultrasound surgery using cavitation. METHODS Freshly harvested liver samples (less than 6 hours) were placed in degassed saline and then vacuum sealed in thin plastic bags. The samples were placed in the nominal focus of a 1 MHz 512 channel therapeutic array transducer (Imasonic, Besancon, FR). An 8 MHz linear imaging transducer (Siemens Elegra) was placed opposite to the therapeutic transducer very close to the liver sample in order to monitor ultrasound treatments (Fig. 1).