Feasibility of a Direct-Conversion Method from Magnetic Susceptibility to Relative Electron Density for Radiation Therapy Treatment Planning

Recently, several institutions have been developing magnetic resonance imaging (MRI)-guided radiotherapy treatment systems. In this study, we examine whether it is possible to perform radiation therapy planning (RTP) using a magnetic susceptibility map obtained using MRI. The head of a healthy volunteer was scanned using dual-energy computed tomography (CT) and MRI. A T2-star-weighted 3D gradient echo-based sequence (GRE) with images taken at four different echo times was acquired using the MRI scanner. The CT images were converted to relative electron density (rED) using a predefined ΔCT-rED conversion table. ΔCT was derived using the energy-subtraction method. The rED map was obtained from a single-linear relationship with the ΔCT-rED conversion table, whereas the magnetic susceptibility map was obtained from quantitative susceptibility mapping (QSM) via MRI. Subsequently, to obtain the relationship between the magnetic susceptibility and the rED, the rED map was rigidly aligned to the susceptibility map and resampled at the susceptibility map’s resolution. Finally, the magnetic susceptibility rED conversion table was obtained via voxel-by-voxel mapping between the two maps. No strong relationship between magnetic susceptibility and rED was obtained in the healthy volunteer’s head or in this study. The coefficient correlation between these parameters was 0.0145. Magnetic susceptibility values may be not able to convert to rED using our proposed method in How to cite this paper: Ito, K., Kadoya, N., Nakajima, Y., Saito, M., Sato, K., Nagasaka, T., Yamanaka, K., Dobashi, S., Takeda, K., Matsushita, H. and Jingu, K. (2017) Feasibility of a Direct-Conversion Method from Magnetic Susceptibility to Relative Electron Density for Radiation Therapy Treatment Planning. International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 6, 252-265. https://doi.org/10.4236/ijmpcero.2017.63023 Received: May 23, 2017 Accepted: July 15, 2017 Published: July 18, 2017 Copyright © 2017 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution-NonCommercial International License (CC BY-NC 4.0). http://creativecommons.org/licenses/by-nc/4.0/ Open Access DOI: 10.4236/ijmpcero.2017.63023 July 18, 2017