<s>
Perfusion	B-Algorithm
MRI	I-Algorithm
or	O
perfusion-weighted	O
imaging	O
(	O
PWI	O
)	O
is	O
perfusion	O
scanning	O
by	O
the	O
use	O
of	O
a	O
particular	O
MRI	B-Algorithm
sequence	I-Algorithm
.	O
</s>
<s>
Another	O
MRI	B-Algorithm
sequence	I-Algorithm
,	O
diffusion	B-Algorithm
weighted	I-Algorithm
MRI	I-Algorithm
,	O
estimates	O
the	O
amount	O
of	O
tissue	O
that	O
is	O
already	O
necrotic	O
,	O
and	O
the	O
combination	O
of	O
those	O
sequences	O
can	O
therefore	O
be	O
used	O
to	O
estimate	O
the	O
amount	O
of	O
brain	O
tissue	O
that	O
is	O
salvageable	O
by	O
thrombolysis	O
and/or	O
thrombectomy	O
.	O
</s>
<s>
There	O
are	O
3	O
main	O
techniques	O
for	O
perfusion	B-Algorithm
MRI	I-Algorithm
:	O
</s>
<s>
Dynamic	B-Algorithm
susceptibility	I-Algorithm
contrast	I-Algorithm
(	O
DSC	O
)	O
:	O
Gadolinium	O
contrast	O
is	O
injected	O
,	O
and	O
rapid	O
repeated	O
imaging	O
(	O
generally	O
gradient-echo	O
echo-planar	O
T2	O
weighted	O
)	O
quantifies	O
susceptibility-induced	O
signal	O
loss	O
.	O
</s>
<s>
It	O
can	O
also	O
be	O
argued	O
that	O
diffusion	B-Algorithm
MRI	I-Algorithm
models	O
,	O
such	O
as	O
intravoxel	B-Algorithm
incoherent	I-Algorithm
motion	I-Algorithm
,	O
also	O
attempt	O
to	O
capture	O
perfusion	O
.	O
</s>
<s>
In	O
Dynamic	B-Algorithm
susceptibility	I-Algorithm
contrast	I-Algorithm
MR	O
imaging	O
(	O
DSC-MRI	O
,	O
or	O
simply	O
DSC	O
)	O
,	O
Gadolinium	O
contrast	O
agent	O
(	O
Gd	O
)	O
is	O
injected	O
(	O
usually	O
intravenously	O
)	O
and	O
a	O
time	O
series	O
of	O
fast	O
T2*	O
-weighted	O
images	O
is	O
acquired	O
.	O
</s>
<s>
It	O
is	O
important	O
to	O
note	O
that	O
,	O
unlike	O
some	O
techniques	O
such	O
as	O
PET	B-Application
imaging	I-Application
,	O
the	O
contrast	O
agent	O
is	O
not	O
imaged	O
directly	O
,	O
but	O
by	O
an	O
indirect	O
effect	O
on	O
water	O
protons	O
.	O
</s>
<s>
In	O
general	O
,	O
persistent	O
increased	O
signal	O
intensity	O
(	O
corresponding	O
to	O
decreased	O
T1	O
and	O
thus	O
increased	O
Gd	O
interaction	O
)	O
in	O
a	O
DCE-MRI	O
image	O
voxel	B-Algorithm
indicates	O
permeable	O
blood	O
vessels	O
characteristic	O
of	O
tumor	O
tissue	O
,	O
where	O
Gd	O
has	O
leaked	O
into	O
the	O
extravascular	O
extracellular	O
space	O
.	O
</s>
<s>
Arterial	B-Algorithm
spin	I-Algorithm
labelling	I-Algorithm
(	O
ASL	O
)	O
has	O
the	O
advantage	O
of	O
not	O
relying	O
on	O
an	O
injected	O
contrast	O
agent	O
,	O
instead	O
inferring	O
perfusion	O
from	O
a	O
drop	O
in	O
signal	O
observed	O
in	O
the	O
imaging	O
slice	O
arising	O
from	O
inflowing	O
spins	O
(	O
outside	O
the	O
imaging	O
slice	O
)	O
having	O
been	O
selectively	O
inverted	O
or	O
saturated	O
.	O
</s>
