<s>
Fractional	B-Algorithm
anisotropy	I-Algorithm
(	O
FA	O
)	O
is	O
a	O
scalar	O
value	O
between	O
zero	O
and	O
one	O
that	O
describes	O
the	O
degree	O
of	O
anisotropy	O
of	O
a	O
diffusion	O
process	O
.	O
</s>
<s>
FA	O
is	O
a	O
measure	O
often	O
used	O
in	O
diffusion	B-Algorithm
imaging	I-Algorithm
where	O
it	O
is	O
thought	O
to	O
reflect	O
fiber	O
density	O
,	O
axonal	O
diameter	O
,	O
and	O
myelination	O
in	O
white	O
matter	O
.	O
</s>
<s>
A	O
Diffusion	O
Ellipsoid	O
is	O
completely	O
represented	O
by	O
the	O
Diffusion	B-Algorithm
Tensor	I-Algorithm
,	O
D	O
.	O
FA	O
is	O
calculated	O
from	O
the	O
eigenvalues	O
(	O
)	O
of	O
the	O
diffusion	B-Algorithm
tensor	I-Algorithm
.	O
</s>
<s>
where	O
R	O
is	O
the	O
"	O
normalized	O
"	O
diffusion	B-Algorithm
tensor	I-Algorithm
:	O
</s>
<s>
The	O
eigenvectors	O
and	O
eigenvalues	O
of	O
the	O
Diffusion	B-Algorithm
Tensor	I-Algorithm
give	O
a	O
complete	O
representation	O
of	O
the	O
diffusion	O
process	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
brain	O
can	O
be	O
thought	O
of	O
as	O
a	O
fluid	O
permeated	O
by	O
many	O
fibers	O
(	O
nerve	O
axons	B-Algorithm
)	O
.	O
</s>
<s>
Liquid	B-Device
crystals	I-Device
can	O
also	O
exhibit	O
anisotropic	O
diffusion	O
because	O
the	O
needle	O
or	O
plate-like	O
shapes	O
of	O
their	O
molecules	O
affect	O
how	O
they	O
slide	O
over	O
one	O
another	O
.	O
</s>
<s>
When	O
the	O
FA	O
is	O
0	O
the	O
tensor	B-Device
nature	O
of	O
D	O
is	O
often	O
ignored	O
,	O
and	O
it	O
is	O
called	O
the	O
diffusion	O
constant	O
.	O
</s>
<s>
One	O
drawback	O
of	O
the	O
Diffusion	B-Algorithm
Tensor	I-Algorithm
model	O
is	O
that	O
it	O
can	O
account	O
only	O
for	O
Gaussian	B-Application
diffusion	O
processes	O
,	O
which	O
has	O
been	O
found	O
to	O
be	O
inadequate	O
in	O
accurately	O
representing	O
the	O
true	O
diffusion	O
process	O
in	O
the	O
human	O
brain	O
.	O
</s>
<s>
Due	O
to	O
this	O
,	O
higher	O
order	O
models	O
using	O
spherical	O
harmonics	O
and	O
Orientation	O
Distribution	O
Functions	O
(	O
ODF	O
)	O
have	O
been	O
used	O
to	O
define	O
newer	O
and	O
richer	O
estimates	O
of	O
the	O
anisotropy	O
,	O
called	O
Generalized	O
Fractional	B-Algorithm
Anisotropy	I-Algorithm
.	O
</s>
