Image-guided ex vivo liver ablation by unfocused ultrasound using passive cavitation detection

Abstract

Ablation therapy is used as an alternative to surgical resection of hepatic tumors. In ablation, tumors are destroyed through heating by RF current, high intensity focused ultrasound (HIFU), or other energy sources. Ablation can be performed with a linear array transducer delivering unfocused intense ultrasound (<10 W/cm²). This allows simultaneous treatment and imaging, a feature uncommon in RF ablation. Unfocused ultrasound can also enable faster bulk tissue ablation than HIFU. In the experiments reported here, a 32-element linear array transducer with a 49 mm aperture delivers 3.1 MHz continuous wave unfocused ultrasound at amplitudes 0.7-1.4 MPa during the therapy cycle. It also operates in pulse-echo mode to capture B-scan images. Ex-vivo fresh bovine liver tissue placed in degassed saline is exposed to continuous wave ultrasound interleaved with brief pulsed ultrasound imaging cycles. Tissue exposures range between 5 to 20 minutes. The following measurements are made at intervals of 1 to 3 seconds: tissue temperature with a needle thermocouple, acoustic emissions with a 1 MHz passive unfocused detector, and tissue echogenicity from image brightness. Passively detected acoustic emissions are used to quantify cavitation activity in the ablation experiments presented here. As severity and extent of tissue ablation are related to temperature, this paper will statistically model temperature as a function of tissue echogenicity and cavitation. The latter two quantities can potentially be monitored noninvasively and used as a surrogate for temperature, enabling improved image guidance and control of ultrasound ablation.