Concept and Preliminary Experiments of Navigated Imaging with an Electromagnetic Position Measurement System

Purpose In cardiology the angiography is a method to detect vascular stenosis by using X-ray radiation and contrast medium. There is special equipment, so called C-arms, to produce radioscopic images. An angiography system consists of an X-ray image intensifier (XRII), an X-ray source, and the mechanical structure to position the system around the patient. In general, the measurement and 3-d-reconstruction of cardiovascular structures is limited and difficult to use. The usual diagnostic methods in angiography produce a high X-ray radiation exposure to patients and medical staff. Preliminary work and valuable results described in [1] and [2] using optical measuring system. In [3] the catheter tip position was determined in a dynamic heart phantom with electromagnetic tracking. Similar to that the catheter tip was tracked in a static model with an electromagnetic measuring system in [4]. The feasibility of magnetic tracking devices for image-guided intervention was showed in [5] and [6]. In this work a concept and preliminary tests are presented, which combines the concept of measurement using navigated imaging with direct position measuring. The aim is the visualization of the position and movement of the catheter tip in digital X-ray images without constant fluoroscopy. Material and Methods To reach the specified goal the electromagnetic position measuring system Aurora (NDI, Ontario, Canada) is used with the angiography system Integris BH 5000 (Philips Medical, Best, The Netherlands). Kirschstein, U.; K. Kronberg, A. Hein (2005): Concept and Preliminary Experiments of Navigated Imaging with an Electromagnetic Position Measurement System, CURAC 2005, Berlin, als Vortrag angenommen 2/5 Different useful positions of placing the field generator were determined (figure 1, 2). Three aspects were important to find the correct position: 1. Clinical application as position in sterile/ non-sterile area without disturbing the intervention process 2. Position has to allow typical rotational positions of the C-arm 3. Patient’s heart must be inside the measurement volume of the position measuring system Several metallic equipments disturbed the measurement results and capabilities, such as X-ray source, image-intensifier, patient table, C-arm, monitors, other medical equipment used in the catheter lab. Probably the main interfering part, the image-intensifier, could be tracked, too. For typical imaging positions of the C-arms special error-correction functions can be calculated. After finding a feasible position, measurement with different positions of the C-arm were carried out to determine the influence of image-intensifier and C-arm on the measurement results. Similar to the planned clinical application the relative error is most important. Therefore different positions of two probes were measured using a geometrically known phantom. One probe was fixed and the other turned to pre-defined positions to simulate motion during pulsation (figure 3). The distance of both probes was calculated and compared to their real distance to resolve a relative error. This experiment was carried out twice per C-arm position. After performing the experiment for the first time, the C-arm was rotated and put to the same position again. Another test was carried out to determine the absolute position during C-arm movement. While rotating the C-arm in all possible directions the position of a probe and its distance to the measurement system was measured.