Temperature effect in high intensity focused ultrasound therapy control using dynamic MR elastography

High intensity focused ultrasound (HIFU) is a method for non-invasive tumor therapy. The high energy density in the focal spot leads to local tissue heating and finally to thermal coagulation in the focal area (Fig.1) [1]. To monitor this treatment, the focus position can be detected online with temperature sensitive MR imaging. However characterization of the induced lesion is difficult. It has been shown that HIFU lesions have a decreased elasticity and that the change in elasticity is larger than change in relaxation times [2]. This new parameter for characterization of the lesions can be measured with MR elastography (MRE). In dynamic MRE [3], images of mechanical wave propagation are acquired using oscillating bipolar gradients. Determination of the local wavelength allows reconstruction of the local elasticity. The problem to generate mechanical waves inside samples can be solved using HIFU itself: The ultrasound absorption in the tissue causes a small but continuous radiation force in the direction of wave propagation. The radiation force has its maximum in the focus of the sound field. Thus a pulsed HIFU sonication induces shear waves with a propagation perpendicular to the sound direction and with the ultrasound focus as the origin point (Fig. 1) [4]. The aim of this work is to improve the significance of the HIFU therapy control by visualization of the generated shear waves using dynamic MRE.