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A Machine-Vision Based Radiotherapy Gating Device
El-Hakim, Sabry; MacPherson, M.; Crljenko, T.; Vo, D.; Gerig, L.
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A Machine-Vision Based Radiotherapy Gating
Device*
El-Hakim, S., MacPherson, M., Crljenko, T., Vo, D., and Gerig, L. July 2002
* published Proceedings of the 44th Annual Meeting of the American Association of Physicists in Medicine (AAPM 2002), July 14-18, 2002. Montreal, Quebec . NRC 44994.
Copyright 2002 by
National Research Council of Canada
Permission is granted to quote short excerpts and to reproduce figures and tables from this report, provided that the source of such material is fully acknowledged.
A Machine-Vision Based Radiotherapy Gating Device
M MacPherson*1, T Crljenko1, D Vo1, S El-Hakim2, L Gerig1, (1)Ottawa Regional Cancer Centre, Ottawa, CANADA, (2)IIT, National Research Council of Canada, Ottawa, CANADA
Patient respiration during radiotherapy can cause significant motion of the tumour volume, which can be mitigated by gating the accelerator beam to patient respiration. This work describes a machine-vision respiratory-gating system, based on our reported photogrammetric patient position measurement technique1. The system comprises two CCD cameras permanently mounted approximately two meters from the linac or simulator isocentre. Inherent contrast of retro-reflective markers on the patient’s skin is obtained by infrared illumination combined with infrared filters on the cameras. Emphasis on image processing (automatic positional calibration, corrections for radial and decentering lens distortion, and image deformation) allows absolute marker positions to be reported in the linac (IEC) frame of reference with an accuracy of +/- 0.5 mm, a precision of 0.2 mm and a temporal resolution of less than 150 ms. Hence, the system is simultaneously capable of monitoring mean patient position and real-time variations due to motion and respiration. Digital capture of fluoroscopy video is time-synchronized to the vector position of the reflective markers with respect to the isocentre, and the clinically relevant parameter of motion in the beam’s eye view is used to set gating thresholds. With this approach, we have demonstrated under fluoroscopy that respiratory motion can be reduced from more than 2 cm to less than 2 mm. Experimental details and system performance will be reviewed, and new applications discussed.
1
LH Gerig, et al. “The Development and Clinical Application of a Patient Position Monitoring System.” Videometrics III. Proc. SPIE 2350, Nov. 1994.
