SU-F-J-70: Monte Carlo Study On a Cone-Beam Computed Tomography Using a Cross-Type Carbon Fiber Antiscatter Grid.

PURPOSE: If a cone-beam computed tomography (CBCT) image can be used for adaptive radiotherapy (ART), an irradiated actual dose distribution can be verified on every treatment day. However, a CBCT image is commonly degraded by scattered photons and thus cannot be directly used for dose calculation. The purpose of the present study is to evaluate whether a cross-type carbon fiber antiscatter grid improves CBCT image quality using a Monte Carlo (MC) method for achieving CBCT-based ART.

METHODS: We designed a new carbon fiber (C6 H10 O5 ) interspaced grid, which allows the grid ratio to be increased while maintaining the transmission of primary photons. The grid ratio was 20:1, and the grid frequency was 20 lp/cm, with lead strips oriented along the cone beam projections. The MC method was used to simulate a projection data acquisition system of a material phantom (water, lung, bone, cartilage, and adipose) with a bow-tie filter. The parameters used to evaluate the image quality were uniformity, contrast, and contrast-to-noise ratio (CNR). Moreover, to evaluate the accuracy of the CT number, we compared CT numbers between images reconstructed by only primary photons and those obtained using grids.

RESULTS: The proposed grid had a beneficial impact on the image quality and accuracy of the CT number. The uniformity was improved by approximately 70% compared to the case without the grid. The average contrast and CNR improved by 14.2% and 22.6%, respectively, when using the proposed grid. CT numbers using the proposed grid were most consistent with nominal CT numbers obtained with only primary photons.

CONCLUSION: The proposed cross-type carbon fiber grid improves CBCT image quality and achieves an accurate CT number, without increasing the imaging doses. Therefore, the cross-type carbon fiber antiscatter grid should be considered to achieve CBCT-based ART. This work was supported by Japan Society for the Promotion of Science KAKENHI. Grant Number 15K19211.