On-Board CBCT Imaging Using 2.5 MV Beam on a Radiation Therapy Linac for Metal Artifact Reduction

Abstract

Purpose/Objective(s): Highly attenuating metallic objects such as dental fillings, hip/shoulder prosthesis, fiducial markers, cause severe streaking artifacts in on-board kV CBCT imaging and can lead to errors in imageguided patient setup. The purpose of this work is to reduce metal artifact in on-board CBCT imaging using new 2.5 MV beam available on a radiation therapy Linac. Materials/Methods: A head phantom with a copper bar inserted in the center was scanned with the 2.5 MV beam. With a full gantry rotation, 68 projection views were collected for a total of 20 MU. For comparison, a CBCT scan was also performed using the on-board kV imaging system. The peak voltage for kV scanning was 100 kVp. Due to the sparse projections of MV scanning, we performed iterative image reconstruction on both kV and MV data respectively. Several representative regions of interest were selected for evaluation of metal artifacts and contrast. Results: The mean CT numbers of the bright and dark metal artifacts in kV CBCT are 9000HU and -3000HU. In comparison, there are no bright metal artifacts and the mean CT number of dark metal artifacts is reduced to -460HU in MV CBCT. The mean CT numbers of the bone and metal are 1000HU, 5000HU for kV, and 380HU, 3600HU for MV. The mean CT numbers for three low contrast regions are -800HU, -600HU, -400HU for kV, and -460HU,-300HU, -100HU for MV. Conclusions: Using the new 2.5 MV beam for on-board CBCT imaging, we have significantly reduced metal artifacts compared with kV imaging, although with a decrease in soft-tissue contrast. Combining kV and MV imaging may lead to improved soft-tissue contrast with negligible metal artifact. Author Disclosure: Q. Xu: None. R. Li: None. Y. Yang: None. L. Xing: None.