Biomedical Engineering Reference
In-Depth Information
7.5.2
The Brainstem and Cerebellum
The brainstem also presents continuous and predictable boundaries,
although the cerebellar folia show a more complex, but still regular, pattern.
The signal intensity gradients associated with the cerebellar folia sensitize
them to shifts in the superior-inferior and postero-anterior directions.
7.5.3
The Cerebral Cortex
In contrast to the ventricular system and brainstem, the cerebral cortex is con-
voluted and the signal intensity gradients are highly variable in both magni-
tude and direction. The sulci are generally wider in the frontal lobes than in
the occipital lobes. The major fissures (central and lateral) are wider again.
Because of the many different gradient directions, the cortex is sensitive to
shifts in many different directions. It can be divided into (a) a superficial part
representing the external surface and including the tips of the gyri and (b) a
deeper part consisting of infolded cortex below the external surface. The
superficial gyri form part of the general external shape of the brain and have
a reasonable degree of regularity about their position. Their signal intensity
gradients reflect this, and it is possible to recognize shifts of the superficial
cortex using the simple plateau model, although the plateaus are not so well
defined as with the ventricular system.
The deeper infolded cortex is notable for the fact that there are frequently two
border zones in close proximity, with signal intensity gradients that are opposed
(''reverse slopes'') or obliqued. The cortex is subject to shifts which may move
both of these border zones in the same direction producing biphasic changes
(i.e., Figure 7.2, right hemisphere). There are also other processes which move
the sulcal border zones in opposite directions, either towards one another (as in
generalized brain swelling) or apart (as in cortical atrophy, i.e., Figure 7.13).
Combinations of the different signal intensity gradients and different inplane
shifts in both border zones within sulci (and fissures) of different widths result
in a variety of appearances, some of which are illustrated in Figure 7.14.
Adjacent to the cortex are blood vessels as well as meninges; shifts which
involve these may result in more complex multiphasic changes.
7.5.4
Global Change in Brain Size
When there is a global change in brain size, the matching process effectively
aligns the central portion of the two volume images of the brain. As a result,
the more peripheral regions are progressively displaced from their original
positions. If slices are imaged perpendicular to a radius from the center out
to the periphery, there will be a progressive throughplane shift which reaches
a maximum at the outer surface. This effect will generally be greatest for
transverse slices at the superior aspect of the brain and at the inferior aspect
of the cerebellum. For parasagittal slices, the largest changes are at the lateral
extremities of the hemisphere, since these are furthest from the center of the
brain. The effect is reversed for generalized reduction in size. To obtain more
