Theory: QSM physics

Briefly, there are two main types of magnetic property (a.k.a magnetic susceptibility, \chi) we can measure with MR:

  • Paramagnetism: substances with paramagnetic property generate a secondary magnetic field that enhances the existing magnetic field generated by the MRI scanner. A typical example is iron either in blood or stored in ferritin;

  • Diamagnetism: substances with diamagnetic property generate a secondary magnetic field that reduces the existing magnetic field strength. Examples included myelin and calcification.

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Figure 1: A QSM map. Deep grey matter structures such as globus pallidus and putamen containing high iron concentration is bright (positive magnetic susceptibility) while white matter, which is myelin-rich tissue, is dark (negative magnetic susceptibility). Studies have shown that the susceptibility values in the deep grey matter are highly correlated with iron staining histology result.

Because of the secondary magnetic field generated by (both paramagnetic and diamagnetic) tissues, the overall magnetic field experienced by the water protons will no longer be the same across the whole brain. The strength of this magnetic field inhomogeneity will depend on the local magnetic susceptibility sources: sources with stronger magnetic susceptibility can create a stronger inhomogeneity effect. As a result, the water protons will resonate in different frequencies across the whole brain and the frequency difference at each location is depended on the strength of the neighbouring sources. Measuring the frequency shift can, therefore, compute the magnetic susceptibility of brain tissues and reveal their cellular environment.

Note

Water protons are the main sources of MRI signal. In QSM, they act as our little magnetic field detectors to reveal the local changes of the magnetic field due to their surrounding environment.

Back to (f)MRI Toolkit 2019.