Diffusion is one of the basic phenomena in nature,and can be quantified with MRI in terms of signal attenuation.Due to the fact that diffusivity of water molecules varies in different directions,and that the probability density function (PDF) of the molecular diffusion faithfully reflects micro-environment of the diffusing molecules,diffusion MRI has been developed to infer the geometry of microstructures.The theory of NMR shows that PDF is the Fourier transform of the MRI signal in the acquisition space (q-space).Based on this theory,a technique called diffusion spectrum imaging (DSI),is proposed to resolve complex axonal fiber geometry at each location in the brain,and consequently more accurate depiction of white matter tractography and its integrity.