This study introduces the use of interventional Magnetic Resonance Elastography (iMRE) for monitoring HIFU ablations, using the acoustic radiation force as a means for generating the shear waves necessary for MRE directly from the HIFU focus. This method allows for monitoring tissue elasticity and temperature in real time during the ablation. Its feasibility is illustrated in vivo on porcine muscle. Tissue was found to stiffen significantly (+125%) when the temperature increased, and changes in tissue stiffness were found to be irreversible. These findings suggest that tissue stiffness may be an interesting biomarker, complementary to thermal dose, to monitor HIFU ablations.