Design and analysis of a calibration-method for stereo-optical motion tracking in MRI using a virtual calibration

Motion tracking for head motion compensation in MRI has been a research topic for several years. However, literature is not giving much attention to the calibration of such setups. We present a method to calibrate the coordinate systems of a stereo-optical camera setup mounted to the MRI head coil. Though using a simple setup and visible instead of infrared light for tracking, it is possible to achieve a sub-millimeter tracking precision. Blue water-filled spheres are positioned throughout the whole MRI imaging volume and detected in images of the tracking cameras as well as MRI scans. In order to register the coordinate systems of both camera system and MRI scanner, a heuristic-enhanced brute-force approach is used to match detected spheres in the different images. Then, a rigid transformation is calculated and applied to the cameras' external parameters to align the coordinate systems. The precision of our setup was evaluated using leave-one-out cross validation both for the camera calibration and the scanner coordinate system registration. We found that the cameras' locations and orientations are correct within 0.03 mm and 0.03 degrees, using a number of 45 spheres. Evaluation of the MRI coordinate system registration showed an average reprojection error of 1.1 mm. Influence of a feature point jitter of 0.5 px is 0.03 mm for a point close to the cameras and 0.3 mm for a point close to the back of the patient's head. Tracked poses are correct within 0.17 mm and 0.001 degrees.