<s>
Arterial	B-Algorithm
spin	I-Algorithm
labeling	I-Algorithm
(	O
ASL	O
)	O
,	O
also	O
known	O
as	O
arterial	B-Algorithm
spin	I-Algorithm
tagging	I-Algorithm
,	O
is	O
a	O
magnetic	B-Algorithm
resonance	I-Algorithm
imaging	I-Algorithm
technique	O
used	O
to	O
quantify	O
cerebral	O
blood	O
perfusion	O
by	O
labelling	O
blood	O
water	O
as	O
it	O
flows	O
throughout	O
the	O
brain	O
.	O
</s>
<s>
Arterial	B-Algorithm
spin	I-Algorithm
labeling	I-Algorithm
utilizes	O
the	O
water	O
molecules	O
circulating	O
with	O
the	O
brain	O
,	O
and	O
using	O
a	O
radiofrequency	O
pulse	O
,	O
tracks	O
the	O
blood	O
water	O
as	O
it	O
circulates	O
throughout	O
the	O
brain	O
.	O
</s>
<s>
There	O
are	O
also	O
other	O
specifications	O
in	O
the	O
MRI	B-Algorithm
that	O
can	O
increase	O
SNR	O
,	O
like	O
the	O
amount	O
of	O
head	B-Algorithm
coils	I-Algorithm
of	O
the	O
MRI	B-Algorithm
,	O
or	O
a	O
stronger	O
field	O
strength	O
(	O
3	O
T	O
is	O
standard	O
,	O
but	O
1.5	O
T	O
is	O
satisfactory	O
)	O
.	O
</s>
<s>
One	O
study	O
has	O
shown	O
suggests	O
that	O
although	O
there	O
are	O
voxel	O
differences	O
when	O
different	O
readout	O
methods	O
are	O
used	O
,	O
average	O
gray	B-Device
matter	I-Device
CBF	O
are	O
still	O
comparable	O
.	O
</s>
<s>
In	O
continuous	O
arterial	B-Algorithm
spin	I-Algorithm
labeling	I-Algorithm
(	O
CASL	O
)	O
,	O
the	O
blood	O
water	O
is	O
inverted	O
as	O
it	O
flows	O
through	O
the	O
brain	O
in	O
one	O
plane	O
.	O
</s>
<s>
This	O
is	O
rectified	O
in	O
pseudo-continuous	O
arterial	B-Algorithm
spin	I-Algorithm
labeling	I-Algorithm
(	O
pCASL	O
)	O
,	O
where	O
a	O
single	O
long	O
pulse	O
is	O
replaced	O
with	O
multiple	O
(	O
up	O
to	O
a	O
thousand	O
)	O
millisecond	O
pulses	O
.	O
</s>
<s>
In	O
pulse	O
arterial	B-Algorithm
spin	I-Algorithm
labeling	I-Algorithm
(	O
PASL	O
)	O
,	O
blood	O
water	O
is	O
inverted	O
as	O
it	O
passes	O
through	O
a	O
labeling	O
slab	O
(	O
of	O
15	O
to	O
20cm	O
)	O
instead	O
of	O
a	O
plane	O
.	O
</s>
<s>
Velocity	O
selective	O
arterial	B-Algorithm
spin	I-Algorithm
labeling	I-Algorithm
is	O
a	O
strategy	O
that	O
still	O
requires	O
validation	O
.	O
</s>
<s>
Velocity	O
selective	O
arterial	B-Algorithm
spin	I-Algorithm
labeling	I-Algorithm
is	O
advantageous	O
in	O
a	O
population	O
where	O
blood	O
flow	O
may	O
be	O
impeded	O
(	O
e.g.	O
</s>
<s>
ASL	O
maps	O
can	O
mainly	O
be	O
analyzed	O
using	O
the	O
same	O
tools	O
to	O
analyze	O
fMRI	B-Algorithm
and	O
VBM	B-Algorithm
.	O
</s>
<s>
Many	O
ASL-specific	O
toolboxes	O
have	O
been	O
developed	O
to	O
assist	O
in	O
ASL	O
analysis	O
,	O
such	O
as	O
BASIL	O
(	O
Bayesian	O
inference	O
for	O
arterial	B-Algorithm
spin	I-Algorithm
labelling	I-Algorithm
MRI	B-Algorithm
)	O
,	O
part	O
of	O
the	O
FSL	B-Algorithm
neuroimaging	O
package	O
and	O
also	O
Ze	O
Wang	O
's	O
ASL	O
toolbox	O
(	O
using	O
MATLAB	O
)	O
to	O
assist	O
in	O
the	O
subtraction	O
and	O
averaging	O
of	O
the	O
tagged/control	O
pairs	O
.	O
</s>
<s>
A	O
region	O
of	O
interest	O
approach	O
can	O
be	O
analyzed	O
by	O
registering	O
the	O
ASL	O
map	O
into	O
a	O
selected	O
cluster	O
,	O
or	O
an	O
atlas	O
,	O
like	O
a	O
standard	O
(	O
such	O
as	O
the	O
Harvard-Oxford	O
Cortical	O
atlas	O
)	O
or	O
an	O
individual	O
atlas	O
developed	O
by	O
software	O
like	O
FreeSurfer	B-Algorithm
.	O
</s>
<s>
The	O
recommended	O
procedure	O
of	O
ASL	O
registration	O
for	O
voxel-wise	O
analysis	O
is	O
to	O
register	O
the	O
perfusion	O
map	O
to	O
a	O
gray	B-Device
matter	I-Device
segmentation	O
of	O
each	O
individual	O
in	O
a	O
non-rigid	O
procedure	O
.	O
</s>
<s>
Gray	B-Device
matter	I-Device
often	O
requires	O
more	O
oxygenation	O
and	O
is	O
the	O
source	O
of	O
more	O
brain	O
activity	O
compared	O
to	O
white	O
matter	O
.	O
</s>
<s>
Therefore	O
,	O
gray	B-Device
matter	I-Device
CBF	O
is	O
often	O
higher	O
than	O
white	O
matter	O
CBF	O
.	O
</s>
<s>
The	O
single	O
value	O
of	O
gray	B-Device
matter	I-Device
CBF	O
is	O
often	O
isolated	O
in	O
order	O
to	O
give	O
a	O
broad	O
overview	O
of	O
CBF	O
differences	O
.	O
</s>
<s>
Gray	B-Device
matter	I-Device
and	O
white	O
matter	O
CBF	O
can	O
be	O
localized	O
using	O
atlases	O
or	O
Freesurfer	B-Algorithm
.	O
</s>
<s>
Studies	O
have	O
suggest	O
that	O
ASL	O
complement	O
resting	O
state	O
fMRI	B-Algorithm
findings	O
well	O
but	O
can	O
differentiate	O
between	O
resting	O
brain	O
networks	O
(	O
such	O
as	O
the	O
default	O
mode	O
network	O
)	O
less	O
.	O
</s>
<s>
Functional	B-Algorithm
MRI	I-Algorithm
(	O
fMRI	B-Algorithm
)	O
has	O
been	O
the	O
modality	O
of	O
choice	O
to	O
visualize	O
brain	O
activity	O
,	O
and	O
takes	O
advantages	O
of	O
a	O
range	O
of	O
techniques	O
that	O
can	O
be	O
used	O
to	O
interpret	O
it	O
.	O
</s>
<s>
However	O
,	O
the	O
signal	O
that	O
fMRI	B-Algorithm
is	O
acquiring	O
is	O
BOLD	B-Algorithm
signal	I-Algorithm
,	O
which	O
does	O
not	O
directly	O
correlate	O
with	O
blood	O
flow	O
.	O
</s>
<s>
Cerebral	O
blood	O
flow	O
on	O
the	O
other	O
hand	O
does	O
,	O
allowing	O
for	O
cardiovascular	O
disease	O
(	O
CVD	O
)	O
and	O
inflammatory	O
risk	O
factor	O
analysis	O
,	O
and	O
disorders	O
(	O
such	O
as	O
schizophrenia	B-Application
and	O
bipolar	O
disorder	O
)	O
that	O
have	O
comorbid	O
effects	O
with	O
CVD	O
.	O
</s>
<s>
ASL	O
imaging	O
can	O
be	O
a	O
useful	O
tool	O
to	O
complement	O
fMRI	B-Algorithm
and	O
vice	O
versa	O
.	O
</s>
<s>
Although	O
the	O
primary	O
form	O
of	O
fMRI	B-Algorithm
uses	O
the	O
blood-oxygen-level	B-Algorithm
dependent	I-Algorithm
(	O
BOLD	B-Algorithm
)	O
contrast	O
,	O
ASL	O
is	O
another	O
method	O
of	O
obtaining	O
contrast	O
.	O
</s>
<s>
ASL	O
is	O
in	O
general	O
a	O
safe	O
technique	O
,	O
although	O
injuries	O
may	O
occur	O
as	O
a	O
result	O
of	O
failed	O
safety	O
procedures	O
or	O
human	O
error	O
like	O
other	O
MRI	B-Algorithm
techniques	O
.	O
</s>
<s>
ASL	O
,	O
like	O
other	O
MRI	B-Algorithm
modalities	O
generate	O
a	O
fair	O
amount	O
of	O
acoustic	O
noise	O
during	O
the	O
scan	O
,	O
so	O
earplugs	O
are	O
advised	O
.	O
</s>
