The experiment described in this chapter was performed in collaboration with Robert Rohling and Dr. John M. Hollerbach from McGill's Biorobotics Laboratory. The part of the experiment involving the OPTOTRAK would not have been possible without their assistance.
The measurement of the physical volume that constitutes the patient's head is performed using a localizing device that allows the spatial position of a probe to be converted into a set of digital signals representing the spatial coordinates and direction of the probe. Consequently, the accuracy of the localizing device is a factor that directly affects the overall accuracy of an procedure, and should be measured objectively.
The experiment described here was designed to provide a quantitative estimate of the error introduced by the measurement of the patient's head geometry by the localizing device. Although our main interest was the accuracy of the FARO surgical arm, an integral part of the ISG Viewing Wand system, the experiment was also performed with the OPTOTRAK, an optical localizing device (used by some other groups), since we had access to it through the Biorobotics Laboratory. The original goal of the experiment was to compare the accuracy of the two localizing devices dynamically, i.e., with the tip of the probe moving along an arbitrary path in space. However it was impossible to synchronize the data acquisition of the OPTOTRAK with the limited sampling rate of the FARO arm, and so a static study of the relative accuracy was performed by comparing the distances between fixed points in space measured by both devices. These measurements were related to a third measurement on a ``gold standard'', consisting of a metal block, with precisely positioned holes. The motivation for the use of this third independent measurement was to be able to measure the accuracy of the two localizing devices in absolute, rather than relative, terms. This was ensured by the fact that the holes in the reference bar were spaced with an accuracy of more than one order of magnitude better than that of both localizing devices.
The OPTOTRAK is a position tracking device that uses three fixed
cameras to track infrared light-emitting-diodes (LED) and determine
their position by triangulation. This device has a stated accuracy of
0.15 mm and a resolution of 0.01 mm at a distance of 2.5 m between the
LED and the cameras. When multiple LEDs are used, the OPTOTRAK
samples them nearly simultaneously (
1000 Hz bandwidth) at a user
selectable sampling rate [31]. On the other hand, the
FARO arm has a stated accuracy of 0.5 mm.