Abbreviations: CC, corpus callosum CR, corona radiata CG, cingulate gyrus SLF, superior longitudinal fasciculus. In contrast, voxels containing intersecting fibers such as between the callosal projections and the corona radiata exhibit multimodal diffusion. Note how voxels containing a single fiber orientation such as the corpus callosum exhibit unimodal diffusion. The color channel mapping is ( red, green, blue) T = ψ( u)| u|.
The brightness is scaled by the ODF amplitude. The ODFs are color-coded according to the red-green-blue color sphere shown at right, with red indicating medial-lateral, green showing anterior-posterior, and blue showing superior-inferior (Douek et al., 1991). For visualization purposes, each ODF has been min-max normalized. The ODF for each voxel is depicted as a color-coded spherical polar plot. The region includes the intersection of the CR (blue) with the projections from CC (red). The region of interest and anteroposterior level are shown in the structural images at bottom right. Results and Discussion Diffusion MRI of Intravoxel Fiber CrossingĬoronal ODF maps from (A) DSI at q max = 1050 cm −1, b max = 1.7 × 10 4 s/mm 2 (B) QBI at q = 670 cm −1, b = 4 × 10 3 s/mm 2 and (C) QBI at q = 950 cm −1, b = 1.2 × 10 4 s/mm 2. Further, by varying the magnitude of the applied diffusion-sensitizing magnetic field gradient, it is possible to sensitize the diffusion signal to length scales on the order of tens of microns.
We show that the QBI technique can resolve complex subvoxel histoarchitecture including white matter fiber crossing and divergence within individual imaging voxels. In particular, the ODF can describe the complex diffusion patterns that arise from intravoxel fiber crossing. The ODF captures the relevant angular contrast of the diffusion function and, unlike the diffusion tensor, is capable of describing multimodal diffusion. The radial projection is comparable to projecting the three-dimensional distribution of stars onto the celestial sphere information on the distance to the stars is lost, but the angular distribution of the stars is still retained. To understand the radial projection, it is helpful to think of the following analogy.
The ability of q-ball imaging to resolve complex intravoxel fiber architecture eliminates a key obstacle to mapping neural connectivity in the human brain noninvasively. The technique, called q-ball imaging, can resolve intravoxel white matter fiber crossing as well as white matter insertions into cortex. Here, we present a novel magnetic resonance imaging technique that can resolve multiple axon directions within a single voxel. Accurate reconstruction of neural connectivity patterns from DTI has been hindered, however, by the inability of DTI to resolve more than a single axon direction within each imaging voxel. DTI measures the molecular diffusion of water along neural pathways. Recently, investigators have proposed a method to image neural connectivity noninvasively using a magnetic resonance imaging method called diffusion tensor imaging (DTI). While functional brain imaging methods can locate the cortical regions subserving particular cognitive functions, the connectivity between the functional areas of the human brain remains poorly understood.
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