1. A method for graphically determining change in position of a medical instrument during a medical procedure, comprising: inserting the medical instrument at least partially into an object under the examination; generating and displaying a temporal sequence of images of the medical instrument for virtual monitoring of instrument positioning during the procedure, wherein each instrument image is vectorized prior to display by transforming each image of the medical instrument into a polyline image formation; and performing, during the procedure, an image subtraction between two of the polyline image formations to generate and display a differential image indicating temporal change in the instrument position.