Hybrid tracking system for flexible endoscopes

With the miniaturization of electromagnetic tracking systems (EMTS) the range of possible applications in image guided therapy was extending. A diameter smaller than 1 mm allows for mounting these sensors into the working channel of flexible endoscopes for navigation within the body. Knowing the exact position of the instrument with respect to the patient’s position preoperative CT or MR images can simplify and ease navigation during various interventions. The Aurora EMTS seems to be an ideal choice for this purpose. However, using this system exhibits an important limitation: the sensor offers just 5 degrees of freedom (DOF) which means that rotations round the axis of the sensor cannot be measured. To overcome this restriction we used an additional optical tracking system (OTS) which is calibrated to deliver the missing DOF. To evaluate the suitability of our new navigation system we measured the Fiducial Registration Error (FRE) of the diverse registrations and the Target Registration Error (TRE) for the complete transformation from the US space to the CT space. The FRE for the ultrasound calibration amounted to 3.2 mm±2.2 mm, resulting from 10 calibration procedures. For the transformation from the OTS reference system to the EMTS emitter space we found an average FRE of 0.8 mm±0.2 mm. The FRE for the CT registration was 1.0 mm±0.3 mm. The TRE was found to be 5.5 mm± 3.2 mm.