In vivo soft tissues elasticity during thermal therapy is linked to the thermal dose

Abstract

Increasing interest is given to elasticity to monitor HIFU treatments, since elasticity changes during thermal ablation. Moreover, the shear modulus of in vitro soft tissue samples was proven to be linked to the thermal dose. Hence, the study aims to evaluate the link of in vivo elasticity changes, assessed by supersonic shear imaging, with temperature and thermal dose. Seven male rats (Sprague Dawley, 250 g) were anesthetized. The right leg was depilated and immersed in a thermo-regulated bath. Two optical temperature sensors were placed into the biceps femoris. The leg was warmed at a targeted temperature, from 38°C to 50°C, for a time corresponding to the theoretical threshold of necrosis in muscle (thermal dose ∼ 240 minutes at 43°C). Every 40 seconds, shear plane waves were created in the biceps femoris, in a plane neighboring the thermocouples, using the Supersonic Shear Imaging method based on the radiation force. The shear wave propagation was acquired with 180 images at 10 000 frames/s. The changes of elasticity were assessed for the different temperature-time profiles in order to highlight its dependence with the thermal dose. The leg swelled when the temperature reached 39°C. Then, contraction of the muscular fibers was observed on the Bmode images at 44–45°C (thermal dose from 2 to 140 min), followed by a flow of fluid around the muscle. The effect of the thermal dose on the in vivo elasticity of the muscles was the same for all the rats. The shear modulus exponentially increased at a thermal dose was equal to 220 ± 78 min. The cumulative thermal dose did not significantly change when the shear modulus reached 2.5 fold the initial value or more. Thus, this threshold value (220 min) was assumed to be the threshold of necrosis. This value is consistent with the necrosis of the skin and the theoretical threshold of necrosis of muscular tissues (240 min). This study gives promising prospects for the use of the elasticity to monitor thermal effects on tissues.