Generating Susceptibility Weighted Images Using Susceptibility Maps

Karen Mok¹, Jaladhar Neelavalli², Saifeng Liu³, E. Mark Haacke, ⁴

SWI is a widely used clinical tool for venography in the brain [1]. It is a unique imaging methodology which combines the magnitude and phase information, where phase is assumed to be directly related or proportional to the magnetic susceptibility of the structure. While this is true, the actual phase effects are dependent on tissue orientation, as well as the size and shape of the vessels and paramagnetic structures inside the brain. The phase images are also affected by local and global susceptibility effects arising from tissue interfaces between air and bone. At typical imaging resolutions of high in-plane and low through-plane resolution (i.e. voxel aspect ratios from in-plane to slice direction varying from 1:2 to 1:5), due to phase integration effects, the sign of the venous vessel phase turns out to be quite robustly consistent, independent of the vessel orientation. This is partly the reason why SWI typically acquired axially is so successful in depicting the veins exquisitely. However, at isotropic resolutions, when there are minimal phase integration effects, veins have different signs in the phase images depending on their orientation. This creates a problem in generating a proper phase based mask for susceptibility weighting. There have been attempts to use bi-directional masks which use both positive and negative signed phase values in their mask creation [2]. Although this may provide good venograms, it can also lead to artifacts due to remnant phase wraps. In this abstract, we evaluate the possibility of using the susceptibility map, instead of phase, to create the mask for susceptibility weighting. Since susceptibility maps are orientation independent, they provide more accurate representation of the brain as well as reducing the enlargement effect of the vessels in the brain due dipole effects.