gms | German Medical Science

State of the art stereotaxy is characterized by, (i) accurate localization of the target volume and structures at risk, (ii) image guided treatment planning, (iii) precise guidance of instruments and beams, and (iv) intraoperative verification of the probe position. The aim of this work is the development of an accurate, simple, hygienic and fast method of intraoperative control of the position of an implant with respect to its planned position. Two-plane x-ray imaging meets all these requirements.

We have developed software (STVX V2) for intraoperative verification of probe position in stereotactic space. A frontal and lateral base image of the patient's head and the frame with stereotactic localizer is scanned and calibrated to stereotactic space. The stereotactic treatment is imported and the projected trajectory of the catheters are calculated and displayed. The base images with the expected trajectories are printed on transparencies in scale 1:1. This allows easy control of the actual probe on intraoperative x-ray control images simply by adjusting the transparencies to the control images at the light box. If necessary, the control images can also be scanned and evaluated at the computer. Since no stereotactic localizers can be used during operation, the control images have to be co-registered to the base images. This is done by maximizing the mutual information of base and control images. After co-registration, coordinates and distances in stereotactic space can be measured on the control images. This enables the quantitative evaluation of intraoperative control images.

The software developed, STVX V1, has been in clinical use since 2/2002 for all stereotactic operations (biopsies, brachytherapy, deep brain stimulation, etc.) in our hospital, and since 6/2002 in version 2. The use of control transparencies has proven to be especially efficient and reliable. The accuracy of coordinate measurements on control images has been determined from phantom experiments to be -0.1 mm ±0.13 mm and conformity of x-ray and stereotactic CT is 0.49 mm ±0.23 mm.

The developed method for intraoperative verification of stereotactically implanted instruments and seeds is superior to classical tele-x-ray, since no time consuming and critical alignment of x-ray tubes, film and head frame is required, and short film-focus distances can be used. The good visibility of probes - compared to MR - and the high accuracy - better than 0.5 mm - justify the characterization of the developed method as a gold standard for intraoperative determination of probe position.