Combined real time Ultrasound-thermometry and Elastography imaging during HIFU therapy: ex-vivo results

National audienceThe use of High intensity Focused Ultrasound (HIFU) for non invasive therapy requires improving real-time monitoring of the lesion formation during treatment, to avoid damage of the surrounding healthy tissues. The goal of this study is to show the feasibility of a full ultrasound approach that relies on the assessment of the changes in both tissue elasticity and temperature implemented on conventional ultrasonic imaging probes. HIFU treatment and monitoring were performed using a confocal set made up of a 8MHz ultrasound diagnostic probe (Vermon) and a 2.5MHz single element transducer focused at 30mm (Imasonic) on ex-vivo porcine samples. 2D temperature estimation was combined with Supersonic Shear Imaging (SSI) on the same device (Aixplorer, SuperSonic Imagine). To lower the thermo-acoustic lens effect, the ultrasound thermometry was performed using plane wave emissions steered at 13 angles (+/-12 degrees). The SSI sequence consisted in 2 successive shear waves induced at different lateral positions. Each wave was created thanks to pushing beams of 100?s at 3 depths. The shear wave propagation was acquired at 17000 frames/s, from which the elasticity map was recovered. HIFU sonications were interleaved with fast imaging acquisitions allowing a duty cycle of more than 90%. Elasticity and temperature mapping was achieved every 5 seconds during treatment. Tissue stiffness was found to decrease at the focal zone with temperature up to 50°C. Ultrasound-based temperature estimation was highly-correlated to stiffness variation maps. The lesion formation comes with a strong increase (up to four-fold) of the elastic modulus. These combined methods provide complementary measurements of tissue properties and overcome the limitations of US-thermometry alone as SSI acquisitions are motion-insensitive. A full ultrasoundbased monitoring technique was developed to achieve elasticity and temperature mapping of soft tissues under HIFU treatment. It demonstrated the feasibility of monitoring and detecting the HIFU thermal lesion