An EPID based method for performing high accuracy calibration between an optical external marker tracking device and the LINAC reference frame

Abstract

Purpose: With the increasing use of external 3D optical tracking cameras to guide modern radiation therapy procedures, it has become vitally important to have an accurate camera to linear accelerator (LINAC) reference frame calibration. To eliminate errors present in current calibration procedures based on the manual hand alignment of a device using the light field crosshairs and in room guidance lasers, a semiautomated quantitative calibration approach requiring only use of an electronic portal imaging device (EPID) was developed. Methods: A phantom comprised of seven highly IR reflective plastic BBs was placed on the LINAC treatment couch and imaged with both a 3D stereoscopic IR imager and the on board megavoltage (MV) EPID imager. Having knowledge of the optically determined 3D positions and projected EPID images of the BBs, simulated annealing was used to optimize the location of the BBs in the LINAC frame using four different optimization functions. Singular value decomposition was then used to calculate the transformation matrix between the camera and LINAC reference frames. Results were then compared to a traditional camera calibration method for overall accuracy. Results: Using modeled data, the simulated annealing process was able to determine the actual locations of the BBs with a RMSE of 0.23 mm. Using projection images acquired with an MV imager, the process was able to determine locations of BBs within .26 mm. The results depend on the choice of optimization function. Conclusions: Results show that the method can be used to provide highly accurate spatial registration between an external 3D imaging reference frame and the LINAC frame. The experimental MV imager results, while not as precise as the simulated results, exceed 1 mm accuracy and the current accepted AAPM TG-142 standard of 2 mm positioning accuracy.

Towards frameless maskless SRS through real-time 6DoF robotic motion compensation

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Robotic real-time translational and rotational head motion correction during frameless stereotactic radiosurgery

“…To take into account camera uncertainty, a precise calibration between the camera frame to Linac reference frame is required. 17,18 …”

Development of a 6DOF robotic motion phantom for radiation therapy

“…To ensure proper alignment of the device coordinate system, a transformation was performed between the Polaris coordinate frame and the IR tool frame, which was previously established as the Stewart platform frame of Ref. 32. To perform this transformation, an 8-point calibration technique was performed, wherein the robotic device moved across a 3D trajectory to the corners of a cube and remained stationary for 90 s (∼1000 data points at a Polaris frame rate of 12 Hz) to provide good statistics for averaging.…”