Material Investigation for the Development of Non-rigid Phantoms for CT-MRI Image Registration

Abstract

In radiotherapy, deformable image registration (DIR) has been frequently used in different imaging examinations in recent years. However, no phantom has been established for quality assurance for DIR. In order to develop a non-rigid phantom for accuracy control between CT and MRI images, we investigated the suitability of 3D printing materials and gel materials in this study. We measured CT values, T1 values, T2 values, and the proton densities of 31 3D printer materials-purchased from three manufacturers-and one gel material. The dice coefficient after DIR was calculated for the CT-MRI images using a prototype phantom made of a gel material compatible with CT-MRI. The CT number of the 3D printing materials ranged from -6.8 to 146.4 HU. On MRI, T1 values were not measurable in most cases, whereas T2 values were not measurable in all cases; proton density (PD) ranged from 2.51% to 4.9%. The gel material had a CT number of 111.16 HU, T1 value of 813.65 ms, and T2 value of 27.19 ms. The prototype phantom was flexible, and the usefulness of DIR with CT and MRI images was demonstrated using this phantom. The CT number and T1 and T2 values of the gel material are close to those of the human body and may therefore be developed as a DIR verification phantom between CT and MRI. These findings may contribute to the development of non-rigid phantoms for DIR in the future.