An algorithm to extract three‐dimensional motion by marker tracking in the kV projections from an on‐board imager: four‐dimensional cone‐beam CT and tumor tracking implications

Imad Ali,Terence Herman,Nesreen Alsbou,Salahuddin Ahmad

doi:10.1120/jacmp.v12i2.3407

Abstract

The purpose of this work is to extract three‐dimensional (3D) motion trajectories of internal implanted and external skin‐attached markers from kV cone‐beam projections and reduce image artifact from patient motion in cone‐beam computed tomography (CBCT) from on‐board imager. Cone beam radiographic projections were acquired for a mobile phantom and liver patients with internal implanted and external skin‐attached markers. An algorithm was developed to automatically find the positions of the markers in the projections. It uses normalized cross‐correlation between a template image of a metal seed marker and the projections to find the marker position. From these positions and time‐tagged angular views, the marker 3D motion trajectory was obtained over a time interval of nearly one minute, which is the time required for scanning. This marker trajectory was used to remap the pixels of the projections to eliminate motion. Then, the motion‐corrected projections were used to reconstruct CBCT. An algorithm was developed to extract 3D motion trajectories of internal and external markers from cone‐beam projections using a kV monoscopic on‐board imager. This algorithm was tested and validated using a mobile phantom and patients with liver masses that had radio‐markers implanted in the tumor and attached to the skin. The extracted motion trajectories were used to investigate motion correlation between internal and external markers in liver patients. Image artifacts from respiratory motion were reduced in CBCT reconstructed from cone‐beam projections that were preprocessed to remove motion shifts obtained from marker tracking. With this method, motion‐related image artifacts such as blurring and spatial distortion were reduced, and contrast and position resolutions were improved significantly in CBCT reconstructed from motion‐corrected projections. Furthermore, correlated internal and external marker 3D‐motion tracks obtained from the kV projections might be useful for 4DCBCT, beam gating and tumor motion monitoring or tracking.PACS numbers: 87.57.Q, 87.57.C‐

Highlights

On-board imaging (OBI)(1,2) has proved clinical vitality for image-guided radiation therapy
Motion extraction from cone-beam projections Figures 6 (c) and (d) show the motion tracks of two internal implanted seed makers and an external metal marker attached to the skin of a liver patient
These 3D motion tracks were extracted from the projections acquired from one CBCT scan using half-fan protocol

Summary

Introduction

On-board imaging (OBI)(1,2) has proved clinical vitality for image-guided radiation therapy. The treatment margins needed to correct for respiratory motion may require large planning target volumes (PTV) that includes normal tissue and critical structures. One approach to minimize motion artifacts in CT is fast scanning using shorter scanner rotation time and multislice technology.[10,14] Several other techniques were advocated to reduce motion artifacts by retrospectively correcting motion in the projections prior to CT reconstruction. The application of these techniques to CBCT from kV OBI systems is limited where the degradation of image quality by motion is stronger in the later one.[20]

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Conclusion