Monitoring of thermal ablation therapy based on shear modulus changes: Shear wave thermometry and shear wave lesion imaging

The 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 real-time and quantitative assessment of the changes in tissue elasticity both to map temperature and monitor the lesion formation. HIFU treatment and monitoring was 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 samples. US-temperature estimation based on speckle tracking was combined with Supersonic Shear Wave Imaging (SWI) on the same device (Aixplorer, SuperSonic Imagine). The SWI sequence consisted in successive shear waves induced at different lateral positions. 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%. A low-temperature elevation calibration phase is performed using a dedicated sequence just before the actual treatment. A full elasticity and temperature mapping was achieved every 3 seconds during the treatment. Below 40°C, tissue stiffness was found to reversibly decrease with temperature at the focal zone (-0.86kPa/°C). US-temperature was highly correlated to stiffness variation maps (correlation coefficient: 0.91-0.97). The linear dependence of elasticity changes below 50°C enables to perform thermometry imaging directly from elasticity changes maps. Then, for higher temperatures, lesion formation induced a very strong increase of the elastic modulus in the focal zone. Thus, the same method allowed a complete follow-up of the tissue during treatment in two particular regimes: shear wave thermometry during heating and shear wave lesion imaging when the thermal threshold was reached. Shear wave temperature imaging allows temperature to be estimated up to 50°C. Moreover, SWT was shown to be very low sensitive to motion (for tissue motion less than 2 cm/s) allowing temperature estimation on moving area. Finally, the size of the thermal lesions determined on the stiffness maps correlated strongly with optical contrast of tissue cuts (+/-0.15mm). Shear Wave Thermometry is a novel reliable approach for ultrasound based monitoring of thermal ablation. SWT can be combined with shear wave lesion imaging to achieve a complete follow up of the treatment.