acquiring a plurality of pilot images of a target region including a target mass which is moving due to breathing motion, the pilot images containing tissue motion information of a cycle of the breathing motion;

analyzing the pilot images to determine a time of push pulse generation at the focal point in relation to the cycle of breathing motion which results in the generation of a push pulse at a moment during the breathing cycle when the push pulse focal point is aligned with the target mass so that a produced shear wave passes through the target mass; and

setting the time of generation of the push pulse in response to the analyzing.