<s>
In	O
neuroscience	O
,	O
tractography	B-Algorithm
is	O
a	O
3D	O
modeling	O
technique	O
used	O
to	O
visually	O
represent	O
nerve	O
tracts	O
using	O
data	O
collected	O
by	O
diffusion	B-Algorithm
MRI	I-Algorithm
.	O
</s>
<s>
It	O
uses	O
special	O
techniques	O
of	O
magnetic	O
resonance	O
imaging	O
(	O
MRI	B-Algorithm
)	O
and	O
computer-based	O
diffusion	B-Algorithm
MRI	I-Algorithm
.	O
</s>
<s>
The	O
results	O
are	O
presented	O
in	O
two	O
-	O
and	O
three-dimensional	O
images	O
called	O
tractograms	B-Algorithm
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
long	O
tracts	O
that	O
connect	O
the	O
brain	O
to	O
the	O
rest	O
of	O
the	O
body	O
,	O
there	O
are	O
complicated	O
neural	B-General_Concept
circuits	I-General_Concept
formed	O
by	O
short	O
connections	O
among	O
different	O
cortical	O
and	O
subcortical	O
regions	O
.	O
</s>
<s>
Nerve	O
tracts	O
are	O
not	O
identifiable	O
by	O
direct	O
exam	O
,	O
CT	O
,	O
or	O
MRI	B-Algorithm
scans	O
.	O
</s>
<s>
The	O
most	O
advanced	O
tractography	B-Algorithm
algorithm	O
can	O
produce	O
90%	O
of	O
the	O
ground	O
truth	O
bundles	O
,	O
but	O
it	O
still	O
contains	O
a	O
substantial	O
amount	O
of	O
invalid	O
results	O
.	O
</s>
<s>
Tractography	B-Algorithm
is	O
performed	O
using	O
data	O
from	O
diffusion	B-Algorithm
MRI	I-Algorithm
.	O
</s>
<s>
This	O
shape	O
is	O
often	O
modeled	O
as	O
an	O
ellipsoid	O
,	O
and	O
the	O
technique	O
is	O
then	O
called	O
diffusion	B-Algorithm
tensor	I-Algorithm
imaging	I-Algorithm
.	O
</s>
<s>
Barriers	O
can	O
be	O
many	O
things	O
:	O
cell	O
membranes	O
,	O
axons	B-Algorithm
,	O
myelin	O
,	O
etc	O
.	O
</s>
<s>
;	O
but	O
in	O
white	O
matter	O
the	O
principal	O
barrier	O
is	O
the	O
myelin	O
sheath	O
of	O
axons	B-Algorithm
.	O
</s>
<s>
Bundles	O
of	O
axons	B-Algorithm
provide	O
a	O
barrier	O
to	O
perpendicular	O
diffusion	O
and	O
a	O
path	O
for	O
parallel	O
diffusion	O
along	O
the	O
orientation	O
of	O
the	O
fibers	O
.	O
</s>
<s>
Conditions	O
where	O
the	O
myelin	O
or	O
the	O
structure	O
of	O
the	O
axon	B-Algorithm
are	O
disrupted	O
,	O
such	O
as	O
trauma	O
,	O
tumors	O
,	O
and	O
inflammation	O
reduce	O
anisotropy	O
,	O
as	O
the	O
barriers	O
are	O
affected	O
by	O
destruction	O
or	O
disorganization	O
.	O
</s>
<s>
One	O
way	O
is	O
by	O
a	O
ratio	O
called	O
fractional	B-Algorithm
anisotropy	I-Algorithm
(	O
FA	O
)	O
.	O
</s>
<s>
Using	O
diffusion	B-Algorithm
tensor	I-Algorithm
MRI	I-Algorithm
,	O
one	O
can	O
measure	O
the	O
apparent	O
diffusion	O
coefficient	O
at	O
each	O
voxel	B-Algorithm
in	O
the	O
image	O
,	O
and	O
after	O
multilinear	B-General_Concept
regression	I-General_Concept
across	O
multiple	O
images	O
,	O
the	O
whole	O
diffusion	B-Algorithm
tensor	I-Algorithm
can	O
be	O
reconstructed	O
.	O
</s>
<s>
The	O
reconstructed	O
diffusion	B-Algorithm
tensor	I-Algorithm
can	O
be	O
treated	O
as	O
a	O
matrix	O
,	O
and	O
we	O
can	O
compute	O
its	O
eigenvalues	O
and	O
eigenvectors	O
.	O
</s>
<s>
This	O
can	O
be	O
done	O
using	O
numerical	O
integration	O
,	O
e.g.	O
,	O
using	O
Runge	B-Algorithm
–	I-Algorithm
Kutta	I-Algorithm
,	O
and	O
by	O
interpolating	O
the	O
principal	O
eigenvectors	O
.	O
</s>
