A Theoritical and Simulational Study on the Chemical Shift Effects on the Magnetic Susceptibility of Iron at Ultra-Short Echo Times

Abstract

Magnetic Resonance Imaging(MRI) has been proven to be important for in vivo quantitative characterization of iron deposition, either endogenous or exogenous(Iron Nanoparticles, IONPs). Iron has endogenous paramagnetic characteristics, and causes shortened T1, T2, and T2* relaxation times as well as phase and susceptibility changes by affecting the magnetic environment of water protons [1–2]. Among all the quantitative MRI methods, quantitative susceptibility mapping (QSM) has gained increased interest as iron is paramagnetic, leading to a linear increase in susceptibility with iron concentration. 2017, it has been shown that the iron could be quantified by both QSM and R2* to a range as high as 22 mM by using ultra-short TE(UTE) technics [3]. However, for in vivo study environment, as fat is an essential contents exists everywhere in human body, the chemical shift effects caused by fat will cause severe streaking artifacts in the QSM results and make the quantification failure [4].The H protons of fat, are nestled within long-chain triglycerides and covered by electron clouds. These clouds partially shield the fat protons from the full effects of an externally applied magnetic field. It is still unknown how much the fat will effects the MRI phase signals with UTE acquisitions, as at a very short TE, the chemical shift effects caused phase shift could not be very obvious. In this study, simulation study was performed to evaluate the chemical shift effect on the quantification of the magnetic susceptibility with UTE sequences based on a fat combined susceptibility-phase change model.