Development of a Beam Stop Array System with Dual Scan Mode for Scatter Correction of Cone-beam CT

Abstract

A beam stop array (BSA) is widely used to estimate the scatter distribution of cone-beam computed tomography (CT) experimentally. A rectangular BSA, which is the most generally used, produces a penumbra region at the edge of the grid on the projection image. The penumbra region can be the cause of an inaccurate scatter estimate and loss of image data. In this study, two types of BSAs have been designed and developed, a linear BSA (l-BSA), which is a simple rectangular type, and a curved BSA (c-BSA) to avoid the penumbra effect. A Monte Carlo simulation was performed to determine the thickness and the material of beam stop arrays. The particle tracking technique of MCNP5 code was used to analyze the accuracy of the scatter distribution acquired by the beam stop arrays. The improvement in the image quality was analyzed by using an in-house phantom, a simple type CT phantom, and a humanoid phantom. How the curved beam stop array improved the penumbra effect was also analyzed in a large field of view. The additional dose caused by a dual scan was analyzed by using a Monte Carlo simulation and experiment. The accuracies of the scatter estimates were 83.4% for the c-BSA and 90.4% for the l-BSA. The c-BSA compensated the penumbra effect effectively while the l-BSA had a severe penumbra region in the partial projection image in a large field of view. Contrasts were improved by 24.8 (air-PMMA), 55.8 (Teflon-PMMA), and 81.7% (water-PMMA) for the c-BSA and 11.1 (air-PMMA), 44.1 (Teflon-PMMA), and 82.2% (water-PMMA) for the l-BSA by scatter correction. After noise suppression, the contrast-to-noise ratios were improved by 2.7-4.1 times for the c-BSA and 3.5-5.3 times for the l-BSA. Uniformities were also improved by up to 10 times for the c-BSA and 12 times for the l-BSA. The quality of the humanoid phantom image was also improved after the scatter correction. Consequently, the c-BSA can improve the image quality of cone-beam CT and compensate for the penumbra effect without an additional dose of radiation.