A contrast element tracking method comprises obtaining a sequence of frames each comprising ultrasound or other medical imaging data representing an anatomical region of a human or animal subject at a respective different time; for each frame, identifying one or more portions of the ultrasound or other medical imaging data as single or multiple contrast element signal portions representative of a contrast element or plurality of contrast elements; assigning respective position data to each of the single contrast element signal portions and each of the multiple contrast element signal portions; and using a linking model that uses at least said assigned position data to link single or multiple contrast element signal portions represented in at least one of the frames to single or multiple contrast element signal portions represented in at least one other of the frames thereby to track movement of contrast elements through said region of the subject.