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Development of a 4DOF Robotic Frameless SRS System for Both Translational and Rotational Head Motion Cancellation

R Wiersma

R Wiersma*, A Belcher, Z Grelewicz, The University of Chicago, Chicago, IL

TH-A-137-6 Thursday 8:00AM - 9:55AM Room: 137

Purpose: Frameless techniques have not been able to match or achieve spatial target accuracies better than traditional frame based SRS. This is due to the fact that decreased invasiveness will lead to decreased accuracy. Here, relaxation of the immobilization parameter by replacement of the head ring (strong immobilization) with a more comfortable immobilization device (weak immobilization) will lead to expression of a patient's natural intrinsic small sub-millimeter involuntary motions. Current frameless techniques do not offer a means for correcting this intrinsic motion. To address this issue, we propose a novel non-immobilization SRS approach based on continuous cancellation of a patient's intrinsic head motion throughout the radiation delivery process. The work presents for the first time a system where both translational (XYZ) and rotational (pitch) head motion cancellation is performed.

Methods: A 4D (xyz+pitch) robotic motion cancellation device was constructed using 4 stepper motors and a 4 axis motor controller (PCI-7344, National Instruments). Real-time 6D head motion was optically monitored using either IR markers (Polaris, NDI) or a 3D surface camera (AlignRT, VisionRT). The measured head motion was inputted into a Labview algorithm based on a proportional-integrative-derivative (PID) controller used to guide the motors for performing real-time head motion cancellation.

Results: The real-time (15 fps) 6D head motion of a human volunteer was monitored with and without the use of robotic cancellation. For all axes where motion cancellation was applied (XYZ+pitch), the mean translational and rotational motion never exceeded 0.001mm and 0.01deg., respectively (essentially reduced to small fluctuations around zero). Without cancellation, this increased to 0.8mm and 0.4deg., respectively.

Conclusion: It is found that substantial reductions in both translational and rotational head motion can be realized through real-time robotic motion cancellation. This has potential to allow frameless SRS to be performed at accuracies equal or greater than frame based SRS.

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