1. A method for quantifying tissue density using shear wave information in medical ultrasound scanning, the method comprising: generating a shear wave in a patient's breast; transmitting acoustic energy into the breast while the shear wave propagates within the breast; receiving echoes responsive to the transmitting while the shear wave propagates within the breast; estimating a shear wave propagation property for each of a plurality of locations within the breast as a function of the echoes; determining a density of the breast at each of the locations as an inverse function of the respective shear wave propagation property; and outputting an image of the density for at least one of the locations.