Passive elastography monitoring of a high intensity ultrasound treatment: A study of feasibility in in vitro liver

High intensity focused ultrasound (HIFU) is a non-invasive modality of intervention allowing thermal ablation in soft tissues by locally increasing temperature. Thermal lesions can be observed as a change in tissue elasticity properties, and so in shear wave velocity, by elastography. Most of studies based on ultrasound imaging used shear waves created by acoustic radiation force. Some of these studies presented elasticity measurements during a treatment in tissues like livers. They demonstrated that the elasticity increases during the treatment and so that the lesion becomes progressively harder than the original tissue. Acoustic radiation force method still presents some difficulties to obtain elasticity in deep tissues. However, in the human body, a natural noise due to cardiac activity or arterial pulsatility can be used to characterize the elasticity in using noise correlation techniques, it corresponds to passive elastography. This method depends on the imaging technique and so can image deep tissues. The objective is so to study the feasibility of using passive elastography technique during a treatment. Here, experiments are performed in in vitro porcine and bovine livers, heated with an acoustic transducer till 80°C and imaged with a high framerate ultrasound imaging device.