What is magnetic susceptibility
- ferromagnetic/diamagnetic/paramagnetic tissue
- isotropic/anisotropic susceptibility
Pulse sequences
- Radio-frequency excitation pulse followed by a signal readout block (could be GRE, EPI or spiral readout)
- gradient-recalled-echo sequences with a single (GRE) or multiple (mGRE) echoes
Signal pre-processing
- Phase unwrapping methods: Homodyne filter, Laplacian phase unwrap, HARPERELLA (more tba); Review: Robinson et al., 2017
- Coil combination techniques: Roemer approach, virtual coil, ASPIRE, COMPOSER (more tba); Review: Robinson et al., 2017
- Background field removal methods: Homodyne filter, SHARP, vSHARP, HAPPARELLA, FDM, PDF (more tba); Review: Schweser et al., 2017
Unlike the majority of in vivo MR techtiques which process the magnitude data, susceptibility contrast is mainly using phase data of the signal. Phase data reflects local (=sub-voxel) and non-local magnetic field variations (e.g. RF-related phase offsets or large-length-scale magnetic field inhomogeneities). Local field changes are caused by susceptibility variations within the object, while the non-local field changes originate from sources from outside of the object. Therefore the first challenge is to remove the background field effects.
End products
End products:
- Macroscopic (~averaged across a voxel)
- Susceptibility weighted imaging (SWI);
- Susceptibility tensor imaging (STI);
- Quantitative susceptibility mapping (QSM);
- Microscopic:
- Sub-voxel susceptibility effects extracted from the complex signal evolution; van Gelderen et al., 2012, Wharton & Bowtell 2012, Sati et al., 2013, Alonso-Ortiz et al., 2018, Thapaliya et al., 2018, Tendler et al., 2018
- Myein water imaging (MWI)
Tools
| Tool | Short description | Programming Language | Source code accessibility |
|---|---|---|---|
| STI Suite | Phase data pre-processing tools, SWI&QSM calculation | Matlab | Open source |
