Most fMRI applications utilize a BOLD contrast, whose mechanism is attributable to some basic biophysical properties of hemoglobin and its susceptibility effects in nuclear magnetic resonance (NMR). Deoxyhemoglobin is paramagnetic and its concentration inversely depends on the blood oxygenation level [28]. The paramagnetic property of the blood causes the bulk susceptibility difference between a blood vessel and its surrounding neural tissue. This effect, in turn, gives rise to the inhomogeneity of the local magnetic field and introduces additional resonance frequency shifts and phase dispersion for extra-vessel molecules. The BOLD contrast is pronounced in gradient echo (i.e. T2*-weighted) MR images where higher oxygenation level leads to larger MR signals [16]. In addition, the echo-planar imaging (EPI) technique further provides remarkable temporal resolution for recording dynamic BOLD signals in realtime [29].
