Functional Susceptibility Weighted Magnetic Resonance Imaging

In this chapter we describe the basics of BOLD based fMRI starting from the point already described in previous chapters (the paramagnetism of deoxyhaemolobin and its consequences for the MR signal). This will include the current view on the influence of physiological parameters (CBF, CBV and CMRO2) on the BOLD signal, the link to neuronal activation and its temporal profile. Furthermore, this chapter describes relevant vascular and anatomical features of the brain with an emphasis on the venous side and how these anatomical and physiological properties influence the MR signal. We describe how a fast, high resolution SWI sequence can be used to perform BOLD-based fMRI (= functional SWI) in order to learn more about the contributions of different tissue compartments (gray matter, veins) to the fMRI signal. As the SWI sequence is able to resolve small venous structures the magnitude and phase behavior of veins can be visualized, quantified and veins can be removed from the functional data based on this information. We then describe the phase changes in the (f)MR signal from magnetic field changes in and around vessels quantitatively and use it to detect veins in a standard (i.e. lower spatial resolution) fMRI setup. This is being used in the statistical modeling of complex-valued voxel fMRI measurements in terms of magnitude and phase along with detection of statistically significant changes. We present the results of applying the complex constant phase model to reduce fMRI activation in voxels that have task related phase changes.