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## Khadem et al. PosterAbstract
Comparative Tracking Error Analysis of Five Different Optical Tracking Systems
Rasool Khadem, Ph.D., Clement C. Yeh, M.S., Mohammad Sadeghi-Tehrani, M.S., Michael R. Bax, M.S., Jeremy A. Johnson, M.S., Jacqueline Nerney Welch, M.S., Eric P. Wilkinson, B.S.E., Ramin Shahidi, Ph.D. California Institute of Computer-Assisted Surgery, Stanford University School of Medicine, Stanford, CA ## Abstract
The positional and angular precision of five different optical tracking system (OTS) configurations are measured. The dependence of the two precision measurements on position and within the digitizing volume and angle between the dynamic reference frame (DRF) and camera are examined. The maximum positional and angular error for all measurements and for 95% of all measurements are also presented. ## Introduction
The OTS is responsible for reporting the location and orientation of a dynamic reference frame (DRF) in a three-dimensional space. Ideally, the OTS would report the true location and orientation of the DRF and would be constant for multiple readings of a stationary DRF. However, errors do arise in OTS measurements for several reasons, including quantization due to a finite number of the pixels in the image sensor, imperfect optics, and inaccuracies due to triangulating the position of each emitter. If the average of multiple readings does not converge to the correct location and orientation, the system is biased; if multiple readings are not closely grouped, the system is imprecise. This study measures the precision of OTS position and angle measurements. Since precision is a qualitative term, ## Materials and Methods
## Results
For the following results summary, refer to Figure 3, Figure 4, and Figure 5 and Tables 3-4. ## Positional jitter for all systems
- Dominated by the
*z*component (camera look direction). - Relatively constant over single
*z*-plane (independent of*x*,*y*, and q). - Increases with increasing
*z*. - Relatively constant for varying angles up to some cutoff angle.
- Best jitter obtained with 300 mm FlashPoint due to proximity of digitizing volume to OTS camera.
## Angular jitter for all systems
- Relatively constant over single
*z*-plane (independent of*x*,*y*, and q). - Relatively constant for a given depth up to some angle (60 degrees for active configurations, 40 degrees for passive).
## Differences between systems
- For IGT systems, positional jitter increases with
*z*; for NDI systems, it remains relatively constant over a given range of*z*and q. - Both passive and active configurations of the Polaris camera have much larger outliers for both positional and angular measurements than do any of the FlashPoint systems (Figure 5).
- When considering all data, the maximum error for the NDI cameras is far larger than the error for any of the IGT configurations; when the worst 5% of outliers are ignored, the performance of the NDI configurations significantly improve and nearly reach that of the IGT systems.
- Both positional and angular jitter of the IGT systems were more predictable and well-behaved than that of either NDI configuration.
- Passive NDI behaves differently than the four active OTS configurations. Positional and angular jitters increase dramatically for orientations larger than 40 degrees. The variation in jitter for the NDI passive configuration is also much larger than for the active configurations.
## Conclusion
The precision of position and angle measurements made by five commercially available optical tracking systems has been quantified throughout a volume. The easiest way to reduce both positional and angular jitter of measurements made by an optical tracking system is to minimize the distance between the camera and the tracked instrument while staying in the camera's digitizing volume. The method presented for jitter measurement and analysis is independent of the tracking technology, and can be used for investigating the precision of future tracking systems. |

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