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Proceedings Paper

Reduction of ring artifacts caused by 2D anti-scatter grids in flat-panel CBCT
Author(s): Santiago F. Cobos; Hristo N. Nikolov; Steven I. Pollmann; David W. Holdsworth
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Paper Abstract

Two-dimensional anti-scatter grids (2D-ASGs) have been developed to selectively capture scattered photons while preserving image signal in flat-panel cone-beam CT systems (CBCT). However, 2D-ASGs affect the response of detector elements underneath grid-septa, producing grid-line artifacts (GLA), which render traditional gain-and-offset corrections ineffective. GLA in the projection images lead to ring-artifacts in CBCT reconstructions, which undermine the improvements in image quality associated with 2D-ASGs. We propose a novel implementation of an exposuredependent gain-correction and notch-Fourier filtering of the projection data to minimize GLA-related ring-artifacts in CBCT. A pixel-by-pixel gain-factor was calculated by dividing the intensities of a flat-field image (with no-ASG) by the intensities of a flat-field image with added ASG, at different exposure levels. Exposure levels were modified using copper filtration of the x-ray beam at six-different thicknesses (0 to 2.5 mm, 0.5 mm increments). Exposure-dependent gain-factors were stored in a multidimensional array and pixel-by-pixel exposure response was characterized using nonlinear curve-fitting. The exposure-dependent gain-correction was applied to 215 projection images of a 14 cm water phantom using a cobalt-chrome 2D-ASG. Residual faint grid-lines were removed using a customized Fourier-notch filter prior to Parker-weighted FDK reconstruction. Traditional gain-and-offset correction produced severe ring-artifacts (i.e., σ = 833.64 HU) when compared to the exposure-dependent gain correction (i.e., σ = 76.16 HU). Additionally, Fouriernotch filtering improved CT number accuracy by 43 HU. Our results suggest that characterization of the exposuredependent response of GLA-affected pixels can minimize ring-artifacts and improve CT-number accuracy, thus eliminating some of the difficulties of 2D-ASG implementation in CBCT systems.

Paper Details

Date Published: 16 March 2020
PDF: 7 pages
Proc. SPIE 11312, Medical Imaging 2020: Physics of Medical Imaging, 1131228 (16 March 2020); doi: 10.1117/12.2548778
Show Author Affiliations
Santiago F. Cobos, Robarts Research Institute, Bone and Joint Institute (Canada)
Schulich School of Medicine & Dentistry (Canada)
Hristo N. Nikolov, Robarts Research Institute, Bone and Joint Institute (Canada)
Steven I. Pollmann, Robarts Research Institute, Bone and Joint Institute (Canada)
David W. Holdsworth, Robarts Research Institute, Bone and Joint Institute (Canada)
Schulich School of Medicine & Dentistry (Canada)
Univ. of Western Ontario (Canada)


Published in SPIE Proceedings Vol. 11312:
Medical Imaging 2020: Physics of Medical Imaging
Guang-Hong Chen; Hilde Bosmans, Editor(s)

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