Improving Image Quality of On-Board Cone-Beam CT in Radiation Therapy Using Image Information Provided by Planning Multi-Detector CT: A Phantom Study

Ching-Ching Yang,Yeh-Chi Lo,Fong-Lin Chen,Qinghui Zhang

doi:10.1371/journal.pone.0157072

Abstract

PurposeThe aim of this study was to improve the image quality of cone-beam computed tomography (CBCT) mounted on the gantry of a linear accelerator used in radiation therapy based on the image information provided by planning multi-detector CT (MDCT).MethodsMDCT-based shading correction for CBCT and virtual monochromatic CT (VMCT) synthesized using the dual-energy method were performed. In VMCT, the high-energy data were obtained from CBCT, while the low-energy data were obtained from MDCT. An electron density phantom was used to investigate the efficacy of shading correction and VMCT on improving the target detectability, Hounsfield unit (HU) accuracy and variation, which were quantified by calculating the contrast-to-noise ratio (CNR), the percent difference (%Diff) and the standard deviation of the CT numbers for tissue equivalent background material, respectively. Treatment plan studies for a chest phantom were conducted to investigate the effects of image quality improvement on dose planning.ResultsFor the electron density phantom, the mean value of CNR was 17.84, 26.78 and 34.31 in CBCT, shading-corrected CBCT and VMCT, respectively. The mean value of %Diff was 152.67%, 11.93% and 7.66% in CBCT, shading-corrected CBCT and VMCT, respectively. The standard deviation within a uniform background of CBCT, shading-corrected CBCT and VMCT was 85, 23 and 15 HU, respectively. With regards to the chest phantom, the monitor unit (MU) difference between the treatment plan calculated using MDCT and those based on CBCT, shading corrected CBCT and VMCT was 6.32%, 1.05% and 0.94%, respectively.ConclusionsEnhancement of image quality in on-board CBCT can contribute to daily patient setup and adaptive dose delivery, thus enabling higher confidence in patient treatment accuracy in radiation therapy. Based on our results, VMCT has the highest image quality, followed by the shading corrected CBCT and the original CBCT. The research results presented in this study should be able to provide a route to reach a high level of image quality for CBCT imaging in radiation oncology.

Highlights

During a course of radiotherapy treatment, any displacement of the target region leads to a lowered dose being delivered to the target
Enhancement of image quality in on-board Cone-beam CT (CBCT) can contribute to daily patient setup and adaptive dose delivery, enabling higher confidence in patient treatment accuracy in radiation therapy
The research results presented in this study should be able to provide a route to reach a high level of image quality for CBCT imaging in radiation oncology

Summary

Introduction

During a course of radiotherapy treatment, any displacement of the target region leads to a lowered dose being delivered to the target. Marchant et al have proposed a shading correction method which enhances CBCT scans by a low spatial frequency grey scale shading function generated with the aid of MDCT scan [14] Their method can effectively increase the accuracy of CBCT density values, but it cannot estimate high-frequency statistical error and scatter noise. The purpose of this study was to improve the image quality of on-board CBCT in terms of target detectability and HU fidelity based on the image information provided by planning MDCT. To reach this goal, a virtual monochromatic CT (VMCT) synthesized using CBCT + MDCT was performed and evaluated using two phantom studies

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