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An Automated Method of Calibrating EPID, OBI, Couch, and Optical Cameras to the LINAC Reference Frame

R Wiersma*, X Liu , E Pearson , The University of Chicago, Chicago, IL


(Thursday, 7/18/2019) 1:00 PM - 3:00 PM

Room: 304

Purpose: Modern image guided LINACs are highly complex devices that must maintain high spatial accuracies in order to delivery radiation safely and effectively to patients. The overall spatial accuracy of a LINAC is governed by how well its various subsystem coordinate frames are calibrated with respect to each other. Such coordinate frames include the EPID, OBI, couch, and optical tracking systems. Lack of origin coincidence, or axes misalignment, between these various frames can result in patient positioning errors. Currently, calibration is often done independently from each other, and involves manual alignment of tools such as phantoms, rules, levels, and in-room lasers. To overcome these limitations, we present a highly automated calibration method based on simultaneous viewing of a novel IR marker phantom by the optical camera, OBI, and EPID while they are moved through a specific trajectory using the couch.

Methods: A novel phantom was constructed and imaged with both a 3D camera and EPID. Optimization methods were used to solve for the EPID, OBI, and couch coordinate frames with respect to the camera. With this knowledge these can be accurately calibrated to the LINAC reference frame.

Results: Using projection images acquired with the EPID, initial preliminary results show the ability to determine locations of phantom BBs with a 0.23 mm RMSE using back projection. Singular value decomposition was used to calculate the camera-to-LINAC and the camera-to-couch transformation matrices allowing both the EPID and couch to be calibrated to the LINAC frame.

Conclusion: Preliminary results show potential of the method to allow sub-millimeter accuracy calibration of the various EPID, OBI, couch, and camera frames to the LINAC frame. Additional work is required to experimentally validate the method.


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