Magnetic susceptibility is a physical quantity that describes the degree of magnetization of a material in response to an applied magnetic field. The magnetic susceptibility of tissue is determined mainly by water, but substances with different magnetic properties cause the susceptibility to change slightly, which can be measured with MRI techniques. Examples of substances that have appreciable effects are myelin in white matter brain connections (susceptibility becomes more diamagnetic) or iron in different forms such as the ferric iron contained in ferritin in tissue or ferrous iron bound to deoxygenated hemoglobin in blood that make the susceptibility more paramagnetic. Thus, susceptibility-based imaging such as quantitative susceptibility mapping (QSM) can in principle have biomarker information about these tissue constituents.
In addition, the orientation of the white matter connections affects the magnetic susceptibility and has to be described by a susceptibility tensor (1-4). In principle, susceptibility tensor imaging (STI) can be used to perform fiber tracking similar to what is done with diffusion tensor imaging (DTI), but at much higher spatial resolution (order of magnitude or more in image voxel volume). However, many issues have to be resolved to make that possible in humans and current results still differ from diffusion methods (see figure). Our goal is to make such methods applicable in humans.
(1) C. Liu, Susceptibility tensor imaging. Magn Reson Med, 2010, 63, 1471-1477
(2) X. Li, D. S. Vikram, I. A. Lim, C. K. Jones, J. A. Farrell and P. C. van Zijl, Mapping magnetic susceptibility anisotropies of white matter in vivo in the human brain at 7 T. Neuroimage, 2012, 62, 314-330
(3) C. Liu, W. Li, B. Wu, Y. Jiang and G. A. Johnson, 3D fiber tractography with susceptibility tensor imaging. Neuroimage, 2012, 59, 1290-1298
(4) W. Li, C. Liu, T. Q. Duong, P. C. van Zijl and X. Li, Susceptibility tensor imaging (STI) of the brain. NMR Biomed, 2017;30(4):10.1002/nbm.3540.