MARKER TRACKS POST-PROCESSING FOR ACCURATE FIDUCIAL MARKER POSITION ESTIMATION IN CONE BEAM CT PROJECTION IMAGES

Abstract

This paper describes details of a method for robust and accurate marker position estimation in projection CB images. The method is based on previously proposed tracking algorithms which can cope with multiple proximate markers and image clutter. The algorithm described in this paper can be seen as a post processing algorithm which uses all the calculated hypothetical marker positions, from the tracking algorithm, for all the markers and all projection images in a single combinatorial optimisation process. The algorithm has been design to estimate intra fraction motion during image guided radiation therapy. The results from the algorithm can be used in treatment planning, subsequent treatment monitoring and correction of motion artefacts in cone beam CT. The proposed post processing algorithm reduced the maximum marker position error from 5.6 pixels, using tracker alone, to 2.6 pixels after post processing. This should be compared to estimated 2.5 pixels maximum error present in the ground truth data. For the total number of 3,840 tracked markers after post processing 1.61% and 0.02% of their positional errors were respectively above three and six standard deviation of the ground truth, estimated separately for each marker and each projection image, whereas corresponding results after using tracker alone were 2.86% and 0.23%.