SU-E-T-409: A Comparison of 3D Optical Surface and IR Marker Tracking for Head Motion Compensated Frameless SRS

Both infrared marker and 3D facial surface tracking were performed simultaneously on both a head phantom and healthy volunteers. Accuracies of the two 6D real-time tracking systems were quantitatively assessed both spatially and temporally. The suitability of 3D facial surface tracking for providing real-time head positioning to guide a robotic head motion compensation stage was investigated.

A xyz stage was programmed to move a head phantom to approximately 200 points in 10×10×10 mm³ and 100×100×100 mm³ cubic volumes, respectively. Both 3D surface and IR marker tracking were performed simultaneously on the head phantom to allow direct comparison of spatial accuracy. Sinusoidal motion was performed along all three axis directions and recorded under the same clock cycle. Spatial and temporal accuracies on actual deformable facial surfaces were assessed by simultaneous 3D surface and IR marker tracking on human volunteers. 3D surface tracking both with and without the use of robotic head motion compensation was evaluated.

For 10 samples at each position, the 10 mm cube had RMSE of 0.30 mm and 0.08 mm for the AlignRT and Polaris, respectively. Temporal lag of the AlignRT system was found to be approximately twice the fps time (t_{lag} ∼ 2/fps). The fps value was found to be strongly correlated with region-of-interest (ROI) surface size and could vary from 0.5 - 1.6 fps, leading to a lag time of 1 - 4 s. The Polaris output lagged behind the reference by 0.06 s in all cases.

Both 3D surface and IR marker devices demonstrate high spatial accuracy for phantom based studies, however, a slight dimensional warping along the SI direction was observed to occur with the AlignRT. The current ROI dependent lag time of the AlignRT could also pose an issue for image guidance techniques requiring quick response times.