Quality control of the stereo calibration of a visual tracking system (VTS) for patient motion detection in SPECT

We have previously demonstrated the feasibility of a stereo visual tracking system (VTS) for patient motion detection in SPECT, and presented the most recent hardware solutions. The stereo optical approach requires careful calibration of the VTS relative to the SPECT system. The calibration step is intended to determine the calibration matrix needed to express the 3D location of reflective spheres detected by the optical cameras in the SPECT coordinate system. A physical phantom including 7 hollow non-coplanar retro-reflective spheres whose center can receive a drop of radioactive tracer was used to determine the calibration parameters. Radioactive sources and reflective spheres were both used to track the motion of a phantom, and motions were identical, thereby validating the calibration. The optical cameras employed have pan, tilt and zoom (PTZ) capabilities. A very small change in PTZ parameters (due to bumps, slow drifts...) since the last calibration could lead to a large error in the markers' location. For that reason, and as part of the VTS quality control assessment, a correction matrix accounting for any PTZ change since the last calibration was successfully used to adjust the calibration matrix. This correction matrix was determined using the location of retro-reflective markers stuck to the wall as seen in images acquired before each clinical acquisition and in images acquired during the most recent VTS/SPECT calibration