Quantitative susceptibility mapping

[1][2][3][4][5] The voxel intensity in QSM is linearly proportional to the underlying tissue apparent magnetic susceptibility, which is useful for chemical identification and quantification of specific biomarkers including iron, calcium, gadolinium, and super paramagnetic iron oxide (SPIO) nano-particles.

Due to its quantitative nature and sensitivity to certain kinds of material, potential QSM applications include standardized quantitative stratification of cerebral microbleeds and neurodegenerative disease, accurate gadolinium quantification in contrast enhanced MRI, and direct monitoring of targeted theranostic drug biodistribution in nanomedicine.

However, the field to source inverse problem involves division by zero at a pair of cone surfaces at the magic angle with respect to B0 in the Fourier domain.

Consequently, susceptibility is underdetermined at the spatial frequencies on the cone surface, which often leads to severe streaking artifacts in the reconstructed QSM.

Therefore, the non-biological background field needs to be removed for clear visualization on phase images and precise quantification on QSM.

Ideally, the background field can be directly measured with a separate reference scan, where the sample of interest is replaced by a uniform phantom with the same shape while keeping the scanner shimming identical.

[11][12] COSMOS utilizes the fact that the zero cone surface in the Fourier domain is fixed at the magic angle with respect to the B0 field.

COSMOS assumes a model-free susceptibility distribution and keeps full fidelity to the measured data.

This observation is translated into mathematics in MEDI,[13] where edges in a QSM which do not exist in the corresponding magnitude image are sparsified by solving a weighted

In an in vivo human brain, MEDI calculated QSM showed similar results compared to COSMOS without statistically significant difference.

For this region in k-space, spatial-frequencies of the dipole kernel are set to a predetermined non-zero value for the division.

[16] Thresholded k-space division[12][17] only requires a single angle acquisition, and benefits from the ease of implementation as well as the fast calculation speed.

It has been confirmed in in vivo and phantom experiments that cortical bones, whose major composition is calcification, are diamagnetic compared to water.

[19] This may allow QSM to serve as a problem solving tool for the diagnosis of confounding hypointense findings on T2* weighted images.