High-accuracy ultrasound target localization for hand-eye calibration between optical tracking systems and three-dimensional ultrasound

Real-time target localization in ultrasound is useful in many clinical and scientific areas. For example in radiation therapy tumors can be localized in real-time and irradiated with a high accuracy. To measure the position of an ultrasound target in a global coordinate system or to extend the tracking volume by moving the ultrasound transducer an optical marker is attached to it and observed by an optical tracking system. The necessary calibration matrices from marker to ultrasound volume are obtained using hand-eye calibration algorithms which take sets of corresponding observations of the optical marker and an ultrasound target as input. The quality of these calibration matrices is highly dependent on the measured observations. While the accuracy of optical tracking systems is very high, accurate tracking in ultrasound is difficult because of the low resolution of the ultrasound volume, artifacts and noise. Therefore accurate hand-eye calibration is difficult between ultrasound and optical tracking systems. We have tested different phantoms, matchingand sub-pixel strategies to provide highly accurate tracking results in 3D ultrasound volumes as basis for hand-eye calibration. Tests have shown that – using the described methods calibration results with RMS errors of less than 1mm between observed and calibrated targets can be reached.