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Figure 5–1. Relative density of vessels in the visual cortex of monkeys.The left panel demonstrates the relative density of vessels in the visual cortex of monkeys. The dense vascular mesh is displayed by perfusing the tissue with barium sulfate and imaging it with synchrotron-based X-ray microtomography (courtesy B. Weber, Max Planck Institute for Biological Cybernetics). The vessel diameter is color coded. Cortical surface without pial vessels is displayed at the top, white matter (WM) at the bottom. At the left of the panel is a Nissl slice from the same area, showing the neural density for layers II through to the WM. Although the density of the vessels appears to be high in this 3-D representation, it is actually less than 3% (see section at the right; white spots are cross-sections of vessels). The average distance between the small vessels (capillaries) is about 50 'µm. This is approximately the distance that oxygen molecules travel by diffusion within the limited transit time of the blood. The dense population of neurons, synapses, and glia occupy the intervascular space, as depicted in the drawing at the top right—a hypothetical distribution of vascular and neural elements in a small section (red rectangle). The drawing in the background shows some of the typical neuronal types (e.g., red, large pyramidal cell; dark blue, inhibitory basket cells; light blue, chandelier inhibitory neurons; and gray, stellate cells) and their processes. As shown in the left panel, this is approximately the gross resolution of magnetic resonance imaging (i.e., mm3).Source. Reprinted from Logothetis NK: "What We Can Do and What We Cannot Do With fMRI." Nature 453:869–878, 2008. Copyright © 2008. Used with permission of Macmillan Publishers, Ltd.

Figure 5–2. Diffusion tensor imaging (DTI) of the corpus callosum.The left panel is a partially rotated left-frontal oblique view of a DTI sequence referred to as B0. DTI sequences have a fuzzy appearance to them and in their unprocessed state lack the anatomical clarity of other magnetic resonance imaging sequences. Major brain landmarks can still be identified, which in this view include the cingulated gyrus (A), corpus callosum (B), and fornix (C). However, from the various DTI sequences, fiber tract directionality can be determined as shown in the panel on the right, which depicts the left hemisphere tracts emanating from the entirety of the length of the corpus callosum. This image is from a normal adult control subject; note the fullness of the projecting tracts. DTI convention depicts anterior-to-posterior projecting tracts as green, vertically oriented tracts as blue, and laterally or back-and-forth projecting tracts as warm colors (red-orange). In this figure only the aggregate fiber tracts of the corpus callosum and fornix are shown, but all regions of the brain and their white matter connections can be shown with DTI.

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