The correctness of the needle position produced by the NPD will be ascertained at two levels, before committing the needle to insertion and during insertion. The tracker will be placed in the field of surgery. The tracker will be calibrated to the coordinate space of the template. Then the template is removed and the NPD is placed. The tracker will be calibrated to the coordinate space of the NPD too. Assuming that the tracker does not move during the procedure, this establishes a direct spatial co-registration between the coordinate space of a template and NPD. At any time during the procedure, the current position of the NPD and needle guide can be reported to the responsible physician in template coordinates. Thus the physician will compare the current needle position (reported in template space) to the desired needle position determined by the treatment plan (also in template space). In case of any discrepancy between the reported and desired needle positions, the NPD is removed and replaced by the template, and the procedure continues without delay.