Abstract: This invention relates to a three-dimensional digitizer and a probe means associated with said digitizer that enables a virtual target point to be visualized and depth and range measurements to that target to be determined. In one embodiment, a mechanical articulated arm or other type of 3-D digitizer is used in conjunction with a graphics workstation which displays images from CT, MR, or other scanning means of the anatomy. By registration of the digitizer to the patient's physical anatomy, a representation of the digitizer on the display means of the graphics workstation is possible. The position of the digitizer will be displayed in its quantitative relationship to the patient's anatomy, including internal pathologies such as tumors. The end means of the 3-D digitizer could be a holder or fixation device which can accept a sliding probe. The probe, for example, could be a shaft which slides in a spring-loaded or non-spring-loaded fashion within the fixation means of the digitizer.