1. A method of tracking fiducial markers on a surface of images of body tissue with a digital image-based elasto-tomography system, said method comprising the steps of: placing a plurality of fiducial markers on a tissue surface; actuating the tissue surface to move the tissue and the fiducial markers on the tissue surface; imaging the tissue surface with an array of calibrated digital cameras; choosing motion invariant properties of the fiducial markers to form a global motion invariant signature: wherein there are a plurality of subclasses of fiducial markers on the tissue surface and each subclass comprises markers with a unique characteristic different from unique characteristics of other fiducial markers of other subclasses; tracking the fiducial markers on the actuated tissue surface from image to image in each calibrated digital camera using the global motion invariant signature; and using the motion of each fiducial marker as tracked from image to image in each calibrated camera and each camera's calibration to measure tissue surface motion.