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Proceedings Paper

Evaluation of dynamic electromagnetic tracking deviation
Author(s): Johann Hummel; Michael Figl; Michael Bax; Ramin Shahidi; Helmar Bergmann; Wolfgang Birkfellner
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Paper Abstract

Electromagnetic tracking systems (EMTS's) are widely used in clinical applications. Many reports have evaluated their static behavior and errors caused by metallic objects were examined. Although there exist some publications concerning the dynamic behavior of EMTS's the measurement protocols are either difficult to reproduce with respect of the movement path or only accomplished at high technical effort. Because dynamic behavior is of major interest with respect to clinical applications we established a simple but effective modal measurement easy to repeat at other laboratories. We built a simple pendulum where the sensor of our EMTS (Aurora, NDI, CA) could be mounted. The pendulum was mounted on a special bearing to guarantee that the pendulum path is planar. This assumption was tested before starting the measurements. All relevant parameters defining the pendulum motion such as rotation center and length are determined by static measurement at satisfactory accuracy. Then position and orientation data were gathered over a time period of 8 seconds and timestamps were recorded. Data analysis provided a positioning error and an overall error combining both position and orientation. All errors were calculated by means of the well know equations concerning pendulum movement. Additionally, latency - the elapsed time from input motion until the immediate consequences of that input are available - was calculated using well-known equations for mechanical pendulums for different velocities. We repeated the measurements with different metal objects (rods made of stainless steel type 303 and 416) between field generator and pendulum. We found a root mean square error (eRMS) of 1.02mm with respect to the distance of the sensor position to the fit plane (maximum error emax = 2.31mm, minimum error emin = -2.36mm). The eRMS for positional error amounted to 1.32mm while the overall error was 3.24 mm. The latency at a pendulum angle of 0° (vertical) was 7.8ms.

Paper Details

Date Published: 13 March 2009
PDF: 7 pages
Proc. SPIE 7261, Medical Imaging 2009: Visualization, Image-Guided Procedures, and Modeling, 72612U (13 March 2009); doi: 10.1117/12.813645
Show Author Affiliations
Johann Hummel, Wilhelminenspital (Austria)
Medical Univ. of Vienna (Austria)
Michael Figl, Medical Univ. of Vienna (Austria)
Michael Bax, Stanford Univ. School of Medicine (United States)
Ramin Shahidi, Stanford Univ. School of Medicine (United States)
Helmar Bergmann, Medical Univ. of Vienna (Austria)
Wolfgang Birkfellner, Medical Univ. of Vienna (Austria)

Published in SPIE Proceedings Vol. 7261:
Medical Imaging 2009: Visualization, Image-Guided Procedures, and Modeling
Michael I. Miga; Kenneth H. Wong, Editor(s)

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