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Figure 11. Diagram shows the process with which a 3D diffusion probability density function is obtained for one voxel (one brain position). In A, a 3D grid that represents q-space, each yellow dot corresponds to an MR signal sampling point. The signal is sampled at each point by varying the direction and strength of the diffusion gradient (q vector) of the pulsed gradient SE sequence. With a single application of the pulsed gradient SE sequence, one point in q-space is sampled for each brain position simultaneously, and the result is one diffusion-weighted image. In B, the left panel shows sections through the MR signal sampled in q-space for a specific brain position (one voxel), and the right panel shows the diffusion probability density function in the same voxel after a 3D Fourier transform of the MR signal in q-space is performed. The cross-shaped appearance of the diffusion probability density function is often seen in voxels in the brainstem, where axons of the corticospinal tract cross with axons of the middle cerebellar peduncle.
