3D breast ultrasound elastography provides volumetric lesion stiffness details in real time. 3D acquisition is compared to breast MRI in order to evaluate volume measurements calculated in ultrasound. Lesion elasticity is calculated in 3 planes with immediate reconstruction. The purpose of this prospective study is to evaluate the feasibility of this technique and compare the results to those of Breast MRI. For women receiving preoperative chemotherapy, this technique will be evaluated with regard to tumour shrinkage and lesion elasticity modifications under treatment.