This Medical Video:
The complex circuitry interconnecting different areas in the brain,
known collectively as white matter, is composed of millions of axons
organized into fascicles and bundles. Upon macroscopic examination
of sections of the brain, it is difficult to discern the orientation
of the fibers. The same is true for conventional imaging modalities.
However, recent advancements in magnetic resonance imaging (MRI)
make such task possible in a live subject. By sensitizing an
otherwise typical MRI sequence to the diffusion of water molecules
it is possible to measure their diffusion coefficient in a given
direction1. Normally, the axonal membrane and myelin sheaths pose
barriers to the movement of water molecules and, thus, they diffuse
preferentially along the axon2. Therefore, the direction of white
matter bundles can be elucidated by determining the principal
diffusivity of water. The three-dimensional representation of the
diffusion coefficient can be given by a tensor and its mathematical
decomposition provides the direction of the tracts3; this MRI
technique is known as diffusion tensor imaging (DTI). By connecting
the information acquired with DTI, three-dimensional depictions of
white matter fascicles are obtained4. The virtual dissection of
white matter bundles is rapidly becoming a valuable tool in clinical
Our journey begins with a transverse section of tightly
packed axons as seen through light microscopy. Although represented
as a two-dimensional slice, we see that these axons in fact resemble
tubes. A simulation of water molecules diffusing randomly inside the
axons demonstrates how the membranes and myelin hinder their
movement across them and shows the preferred diffusion direction
--along the axons. The tracts depicted through DTI slowly blend in
and we ride along with them. As we zoom out even more, we realize
that it is a portion of the corpus callosum connecting the two sides
of the brain we were traveling on and the great difference in
relative scale of the individual axons becomes evident. The surface
of the brain is then shown, as well as the rest of the white matter
bundles--a big, apparently chaotic tangle of wires. Finally, the
skin covers the brain.
With the exception of the simulated water
molecules, all the data presented in the animation is obtained
through microscopy and MRI. Computer algorithms for the extraction
of the cerebral structures and a custom-built graphics engine make
our journey through the brains anatomy possible in a living
Micrograph courtesy of Dr. Christian Beaulieu, University
Music by Mario Mattioli.
E.O., et al., J. Chem. Phys., 1965. 42
2. Beaulieu, C., NMR
Biomed., 2002. 15435-55.
3. Basser, P.J., et al., J. Magn. Reson.
B, 1994. 103247-54.
4. Mori, S., et al., NMR Biomed., 2002.