TITLE:
Feasibility of a Direct-Conversion Method from Magnetic Susceptibility to Relative Electron Density for Radiation Therapy Treatment Planning
AUTHORS:
Kengo Ito, Noriyuki Kadoya, Yujiro Nakajima, Masahide Saito, Kiyokazu Sato, Tatsuo Nagasaka, Kazuomi Yamanaka, Suguru Dobashi, Ken Takeda, Haruo Matsushita, Keiichi Jingu
KEYWORDS:
Magnetic Susceptibility, Quantitative Susceptibility Mapping, MRI-Based Radiation Therapy Planning
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.6 No.3,
July
18,
2017
ABSTRACT: 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 healthy volunteer’s head. In contrast to the
magnetic-susceptibility values obtained from the QSM algorithm, which were
strongly affected by calcification and iron content, the rED or CT number was
not considerably affected by such materials.