Accuracy verification of a PSD-equipped camera-based photostereometric system developed for measuring cranial movements in six degrees of freedom.

: The overall objective of this research is to develop a new type of X-ray television that allows quantitative analysis of jaw movement. Previous X-ray televisions require the patient's head to be held immobile in a normalized orientation by means of a positioning jig. This immobilization, however, is somewhat time consuming and also restricts natural jaw movements. In order to avoid these problems, we have been developing an automatic head-positioning system, consisting of two subsystems: a robotized chair and a head-position sensor. This paper describes the latter sensing subsystem and focuses on its measurement accuracy. To obtain the position and orientation of the head, two video cameras, equipped with a position-sensitive device (PSD), detect the three-dimensional positions of four non-coplanar LED markers mounted on the head. The position data obtained is then utilized to operate the robotized chair so as to hold the head in the same orientation. Accuracy verification study revealed that the overall error of the LED position lay within 1.56 mm (2.8% of total range of the motion. 55 mm) and the spatial resolution at all LEDs was 0.19 mm when using an averaging filter.