NeurotechniqueDiffusion MRI of Complex Neural Architecture

relatively freely along the neural fiber direction but are While functional brain imaging methods can locate the hindered in the fiber-transverse direction. The hindrance cortical regions subserving particular cognitive funcof water diffusion in white matter is putatively due to tions, the connectivity between the functional areas of the diffusion barrier presented by the cell membrane the human brain remains poorly understood. Recently, and the myelin sheath (Beaulieu, 2002; Le Bihan et al., investigators have proposed a method to image neural 1993; Norris, 2001). connectivity noninvasively using a magnetic resoBy measuring the direction of the diffusion anisotropy nance imaging method called diffusion tensor imaging within each voxel, DTI provides an estimate of the neural (DTI). DTI measures the molecular diffusion of water fiber direction within each voxel. The resulting image along neural pathways. Accurate reconstruction of represents a three-dimensional vector field image of neural connectivity patterns from DTI has been hinthe neural fiber orientation. The so-called tractography dered, however, by the inability of DTI to resolve more problem entails computationally reconstructing neural than a single axon direction within each imaging voxel. pathways from the diffusion tensor vector field (Basser Here, we present a novel magnetic resonance imaging et al., 2000; Conturo et al., 1999; Jones et al., 1999; Koch technique that can resolve multiple axon directions et al., 2002; Mangin et al., 2002; Mori et al., 1999; Mori within a single voxel. The technique, called q-ball imand Van Zijl, 2002; Parker et al., 2002; Pierpaoli et al., aging, can resolve intravoxel white matter fiber cross2001; Poupon et al., 2000; Xue et al., 1999). The tractoging as well as white matter insertions into cortex. The raphy problem is analogous to the problem in hydrodyability of q-ball imaging to resolve complex intravoxel namics of reconstructing flow streamlines from a fluidfiber architecture eliminates a key obstacle to mapflow vector field. ping neural connectivity in the human brain noninvaReconstruction of neural pathways from DTI is consively. founded, however, by the limitation that DTI can only