Tissue biomechanical properties are highly promising biomarkers for HIFU ablation monitoring. A quantitative, MR-acoustic radiation force imaging (ARFI)-based method is proposed for measuring both tissue elasticity and viscosity, together with PRFS-derived temperature. This method is based on the identification of the MR-ARFI focal spot pattern. The method was first validated in a phantom with known properties. The method was then evaluated during HIFU heating in a gelatin phantom. Both elastic modulus and viscosity were found to decrease as temperature increased. These results highlight the ability of this method to provide new, quantitative biomarkers of tissue thermal damage in real time.