Analysis of Position Estimation Techniques in a Surgical EM Tracking System

Electromagnetic tracking systems (EMTSs) are widely used in surgical navigation, allowing to improve the outcome of diagnosis and surgical interventions in terms of shorter hospitalization times, reduction of wound pain and increased accuracy, by providing the surgeon with real-time position of surgical instruments during medical procedures. Particular effort must be made on the development of an efficient and robust algorithm to obtain an accurate estimation of the instrument position, for distances from the magnetic field generator beyond 500 mm, which represents the maximum measurement range of current commercial systems, which strongly affects the freedom of movement of the medical team. In this paper we describe two position estimation techniques, based on a model of the magnetic field, then we analyze the robustness to errors and distortions in a set of simulations for different conditions, discussing their pros and cons in a large measurement volume, and we propose a solution by combining the two techniques. The performance of the new technique is tested on experimental data from a novel EMTS prototype, developed to increase the measurement range of current EMTSs, at a mean distance of 600 mm, and a calibration procedure is performed to reduce systematic errors. A mean position and orientation accuracy of 2.2 mm and 0.5° is achieved, along with mean repeatability errors of 1.5 mm and 0.25°, within the specifications required in many surgical applications.