Magnetic Model Calibration and Distortion Compensation for Electromagnetic Tracking in a Clinical Environment

Electromagnetic tracking (EMT) of surgical tools is used for image-guided navigation in minimally invasive interventions. Calibration of the tracker position and orientation is a fundamental task to correct systematic errors. This article explores the effect of metallic distortion on the open hardware Anser EMT system and describes a newly developed approach for volume calibration of the magnetic field model. The technique is also applied to effectively map the field in the presence of magnetic shields that can be used to reduce electromagnetic distortions passively. Unlike calibration techniques commonly found in literature, which aim to correct the tracking position error, our method can reconstruct an analytical magnetic field model starting from a reduced amount of scattered data. This way, a faster convergence of the solver algorithm is obtained, and EMT position and orientation errors are inherently corrected. The new modelling technique was applied to generate a new, undistorted magnetic field model for Anser EMT, to demonstrate the effectiveness of passive shielding, and to compensate for the distortion introduced by a commercial fluoroscopy C-arm. Furthermore, the concept of a real-time distortion compensation technique is expressed based on the modelling method presented in this article. For all the experiments, sub-millimeter errors were obtained after calibration, which may meet the needs of most endoscopic and surgical navigation tasks.