Biomedical Engineering Reference
In-Depth Information
Fig. 6.1
Major white matter fiber pathways in the brain. (
a
) Commissural tracts—the Corpus
Callosum (
CC
). (
b
) Association tracts—short “U”-fibers connecting neighbouring gyri, the
Superior Longitudinal Fasciculus (
SLF
), the Cingulum and the Inferior Longitudinal Fasciculus
(
ILF
). (
c
) Projection tracts—the Corona Radiata (
CR
) and the Corticospinal Tract (
CST
)(Adapted
from [
74
])
The
commissural tracts
connect a region in one hemisphere to another region of
the opposite hemisphere.
The
association tracts
connect various cortical areas within a given hemisphere.
The
projection tracts
connect the cortex to deep brain regions such as the
thalamus or the spinal cord.
In the spinal cord, it is the grey matter that is located in the center with a typical
H-shaped appearance in transverse sections. It is surrounded by white matter which
contains long ascending and descending pathways.
6.1.2
In Vivo CNS Connectivity
Compared to the understanding of neural circuitry in animals as cats or mice where
the use of invasive tracers is possible [
59
], the knowledge of the human brain
organization is relatively poor. Therefore, how to study the connectivity information
about the CNS anatomy, in particular, about the cerebral and spinal cord white
matter? Anatomical MRI allows us to distinguish and classify grey matter and
white matter. However, with this contrast, white matter retains a homogeneous
aspect, preventing any observation of neural fibers and thus of neuronal connectivity.
Cerebral and spinal dissection used to be the only means of accessing the neural
architecture [
22
,
30
,
74
]. Then, anatomists started using chemical markers to do
neuronography [
55
,
59
]. More recently, neural fiber tractography based on local in-
jection of chemical markers and subsequent observation of the induced propagation
yielded high-quality connectivity mapping in the cat and monkey cerebral cortices
[
59
,
75
].
The relatively recent development of dMRI, which uses the orientational prefer-
ence of water molecules in diffusing along the directions of the fibers has brought in
the last 20 years great hopes for the non-invasive exploration of the neural anatomy
of the CNS. Indeed dMRI provides a non-invasive way of estimating in vivo CNS
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