Application of the optoacoustic technique to visualization of lesions induced by high-intensity focused ultrasound

Abstract

Today several techniques are being applied to monitoring of high-intensity focused ultrasound (HIFU) therapy, including MRI, conventional ultrasound, and elastography. In this work a new method for noninvasive monitoring of HIFU therapy is proposed: the optoacoustic method. The optoacoustic technique is based on the excitation of wideband ultrasonic pulses through the absorption of pulsed laser radiation in tissue and subsequent expansion of the heated volume. The excited optoacoustic (OA) pulse contains information on the distribution of optical properties within the tissue—light scattering and absorption coefficients. Therefore, if thermal lesions have different optical properties than the untreated tissue, they will be detectable on the OA waveform. The considerable change in light scattering and absorption coefficients after tissue coagulation was measured using techniques previously developed by our group. Heating induced by HIFU also influences the OA signal waveform due to the rise of thermal expansion coefficient of tissue with temperature. This dependence was measured in order to evaluate the feasibility of the OA technique in temperature monitoring. An OA image of HIFU lesion induced by a 1.1 MHz focused transducer in a liver sample was reconstructed using a 64-element wideband array transducer for OA signal detection.