Accuracy assessment and interpretation for optical tracking systems

Highly accurate spatial measurement systems are among the enabling technologies that have made image-guided surgery possible in modern operating theaters. Assessing the accuracies of such systems is subject to much ambiguity, though. The underlying mathematical models that convert raw sensor data into position and orientation measurements of sufficient accuracy complicate matters by providing measurements having non-uniform error distributions throughout their measurement volumes. Users are typically unaware of these issues, as they are usually presented with only a few specifications based on some "representative" statistics that were themselves derived using various data reduction methods. As a result, much of the important underlying information is lost. Further, manufacturers of spatial measurement systems often choose protocols and statistical measures that emphasize the strengths of their systems and diminish their limitations. Such protocols often do not reflect the end users' intended applications very well. Users and integrators thus need to understand many aspects of spatial metrology in choosing spatial measurement systems that are appropriate for their intended applications. We examine the issues by discussing some of the protocols and their statistical measures typically used by manufacturers. The statistical measures for a given protocol can be affected by many factors, including the volume size, region of interest, and the amount and type of data collected. We also discuss how different system configurations can affect the accuracy. Single-marker and rigid body calibration results are presented, along with a discussion of some of the various factors that affect their accuracy. Although the findings presented here were obtained using the NDI Polaris optical tracking systems, many are applicable to spatial measurement systems in general.