<s>
Magnetic	B-Algorithm
resonance	I-Algorithm
angiography	I-Algorithm
(	O
MRA	O
)	O
is	O
a	O
group	O
of	O
techniques	O
based	O
on	O
magnetic	O
resonance	O
imaging	O
(	O
MRI	O
)	O
to	O
image	O
blood	O
vessels	O
.	O
</s>
<s>
Magnetic	B-Algorithm
resonance	I-Algorithm
angiography	I-Algorithm
is	O
used	O
to	O
generate	O
images	O
of	O
arteries	O
(	O
and	O
less	O
commonly	O
veins	O
)	O
in	O
order	O
to	O
evaluate	O
them	O
for	O
stenosis	O
(	O
abnormal	O
narrowing	O
)	O
,	O
occlusions	O
,	O
aneurysms	O
(	O
vessel	O
wall	O
dilatations	O
,	O
at	O
risk	O
of	O
rupture	O
)	O
or	O
other	O
abnormalities	O
.	O
</s>
<s>
The	O
most	O
frequently	O
applied	O
MRA	O
methods	O
involve	O
the	O
use	O
intravenous	O
contrast	B-Algorithm
agents	I-Algorithm
,	O
particularly	O
those	O
containing	O
gadolinium	O
to	O
shorten	O
the	O
T1	O
of	O
blood	O
to	O
about	O
250	O
ms	O
,	O
shorter	O
than	O
the	O
T1	O
of	O
all	O
other	O
tissues	O
(	O
except	O
fat	O
)	O
.	O
</s>
<s>
Flow	O
dependent	O
MRA	O
can	O
be	O
divided	O
into	O
different	O
categories	O
:	O
There	O
is	O
phase-contrast	B-Algorithm
MRA	I-Algorithm
(	O
PC-MRA	B-Algorithm
)	O
which	O
utilizes	O
phase	O
differences	O
to	O
distinguish	O
blood	O
from	O
static	O
tissue	O
and	O
time-of-flight	O
MRA	O
(	O
TOF	O
MRA	O
)	O
which	O
exploits	O
that	O
moving	O
spins	O
of	O
the	O
blood	O
experience	O
fewer	O
excitation	O
pulses	O
than	O
static	O
tissue	O
,	O
e.g.	O
</s>
<s>
Phase-contrast	O
(	O
PC-MRA	B-Algorithm
)	O
can	O
be	O
used	O
to	O
encode	O
the	O
velocity	O
of	O
moving	O
blood	O
in	O
the	O
magnetic	O
resonance	O
signal	O
's	O
phase	O
.	O
</s>
<s>
Whereas	O
most	O
of	O
techniques	O
in	O
MRA	O
rely	O
on	O
contrast	B-Algorithm
agents	I-Algorithm
or	O
flow	O
into	O
blood	O
to	O
generate	O
contrast	O
(	O
Contrast	O
Enhanced	O
techniques	O
)	O
,	O
there	O
are	O
also	O
non-contrast	O
enhanced	O
flow-independent	O
methods	O
.	O
</s>
<s>
Contrast-enhanced	O
magnetic	B-Algorithm
resonance	I-Algorithm
angiography	I-Algorithm
uses	O
injection	O
of	O
MRI	B-Algorithm
contrast	I-Algorithm
agents	I-Algorithm
and	O
is	O
currently	O
the	O
most	O
common	O
method	O
of	O
performing	O
MRA	O
.	O
</s>
<s>
An	O
alternative	O
is	O
to	O
use	O
a	O
contrast	O
agent	O
that	O
does	O
not	O
,	O
as	O
most	O
agents	O
,	O
leave	O
the	O
vascular	O
system	O
within	O
a	O
few	O
minutes	O
,	O
but	O
remains	O
in	O
the	O
circulation	O
up	O
to	O
an	O
hour	O
(	O
a	O
"	O
blood-pool	B-Algorithm
agent	I-Algorithm
"	O
)	O
.	O
</s>
<s>
Subtractionless	O
contrast-enhanced	O
magnetic	B-Algorithm
resonance	I-Algorithm
angiography	I-Algorithm
:	O
recent	O
developments	O
in	O
MRA	O
technology	O
have	O
made	O
it	O
possible	O
to	O
create	O
high	O
quality	O
contrast-enhanced	O
MRA	O
images	O
without	O
subtraction	O
of	O
a	O
non-contrast	O
enhanced	O
mask	O
image	O
.	O
</s>
<s>
This	O
approach	O
has	O
been	O
shown	O
to	O
improve	O
diagnostic	O
quality	O
,	O
because	O
it	O
prevents	O
motion	O
subtraction	O
artifacts	B-Algorithm
as	O
well	O
as	O
an	O
increase	O
of	O
image	O
background	O
noise	O
,	O
both	O
direct	O
results	O
of	O
the	O
image	O
subtraction	O
.	O
</s>
<s>
Non-enhanced	O
magnetic	B-Algorithm
resonance	I-Algorithm
angiography	I-Algorithm
:	O
Since	O
the	O
injection	O
of	O
contrast	B-Algorithm
agents	I-Algorithm
may	O
be	O
dangerous	O
for	O
patients	O
with	O
poor	O
kidney	O
function	O
,	O
others	O
techniques	O
have	O
been	O
developed	O
,	O
which	O
do	O
not	O
require	O
any	O
injection	O
.	O
</s>
<s>
Both	O
PC-MRA	B-Algorithm
and	O
TOF-MRA	O
have	O
advantages	O
and	O
disadvantages	O
.	O
</s>
<s>
PC-MRA	B-Algorithm
has	O
fewer	O
difficulties	O
with	O
slow	O
flow	O
than	O
TOF-MRA	O
and	O
also	O
allows	O
quantitative	O
measurements	O
of	O
flow	O
.	O
</s>
<s>
PC-MRA	B-Algorithm
shows	O
low	O
sensitivity	O
when	O
imaging	O
pulsating	O
and	O
non-uniform	O
flow	O
.	O
</s>
<s>
This	O
either	O
applies	O
to	O
PC-MRA	B-Algorithm
where	O
the	O
phase	O
difference	O
between	O
blood	O
and	O
static	O
tissue	O
is	O
reduced	O
compared	O
to	O
faster	O
flow	O
and	O
to	O
TOF-MRA	O
where	O
the	O
transverse	O
blood	O
magnetization	O
and	O
thus	O
the	O
blood	O
signal	O
are	O
reduced	O
.	O
</s>
<s>
Contrast	B-Algorithm
agents	I-Algorithm
may	O
be	O
used	O
to	O
increase	O
blood	O
signal	O
–	O
this	O
is	O
especially	O
important	O
for	O
very	O
small	O
vessels	O
and	O
vessels	O
with	O
very	O
small	O
flow	O
velocities	O
that	O
normally	O
show	O
accordingly	O
weak	O
signal	O
.	O
</s>
<s>
4D	O
dynamic	O
MR	B-Algorithm
angiography	I-Algorithm
(	O
4D-MRA	O
)	O
:	O
The	O
first	O
images	O
,	O
before	O
enhancement	O
,	O
serve	O
as	O
a	O
subtraction	O
mask	O
to	O
extract	O
the	O
vascular	O
tree	O
in	O
the	O
succeeding	O
images	O
.	O
</s>
<s>
BOLD	O
venography	O
or	O
susceptibility	B-Algorithm
weighted	I-Algorithm
imaging	I-Algorithm
(	O
SWI	O
)	O
:	O
This	O
method	O
exploits	O
the	O
susceptibility	O
differences	O
between	O
tissues	O
and	O
uses	O
the	O
phase	O
image	O
to	O
detect	O
these	O
differences	O
.	O
</s>
<s>
The	O
imaging	O
of	O
venous	O
blood	O
with	O
SWI	O
is	O
a	O
blood-oxygen-level	B-Algorithm
dependent	I-Algorithm
(	O
BOLD	O
)	O
technique	O
which	O
is	O
why	O
it	O
was	O
(	O
and	O
is	O
sometimes	O
still	O
)	O
referred	O
to	O
as	O
BOLD	O
venography	O
.	O
</s>
<s>
Other	O
artifacts	B-Algorithm
observed	O
in	O
MRA	O
include	O
:	O
</s>
<s>
Phase-contrast	B-Algorithm
MRA	I-Algorithm
:	O
Phase	O
wrapping	O
caused	O
by	O
the	O
underestimation	O
of	O
maximum	O
blood	O
velocity	O
in	O
the	O
image	O
.	O
</s>
<s>
The	O
fast-moving	O
blood	O
about	O
maximum	O
set	O
velocity	O
for	O
phase-contrast	B-Algorithm
MRA	I-Algorithm
gets	O
aliased	O
and	O
the	O
signal	O
wraps	O
from	O
pi	O
to	O
-pi	O
instead	O
,	O
making	O
flow	O
information	O
unreliable	O
.	O
</s>
<s>
Saturation	O
artifact	B-Algorithm
due	O
to	O
laminar	O
flow	O
:	O
In	O
many	O
vessels	O
,	O
blood	O
flow	O
is	O
slower	O
near	O
the	O
vessel	O
walls	O
than	O
near	O
the	O
center	O
of	O
the	O
vessel	O
.	O
</s>
<s>
Venetian	O
blind	O
artifact	B-Algorithm
:	O
Because	O
the	O
technique	O
acquires	O
images	O
in	O
slabs	O
(	O
as	O
in	O
Multiple	O
overlapping	O
thin-slab	O
acquisition	O
,	O
MOTSA	O
)	O
,	O
a	O
non-uniform	O
flip	O
angle	O
across	O
the	O
slab	O
can	O
appear	O
as	O
horizontal	O
stripe	O
in	O
the	O
composed	O
images	O
.	O
</s>
<s>
The	O
most	O
common	O
method	O
is	O
maximum	B-Application
intensity	I-Application
projection	I-Application
(	O
MIP	O
)	O
,	O
where	O
the	O
computer	O
simulates	O
rays	O
through	O
the	O
volume	O
and	O
selects	O
the	O
highest	O
value	O
for	O
display	O
on	O
the	O
screen	O
.	O
</s>
<s>
If	O
several	O
such	O
projections	O
are	O
combined	O
into	O
a	O
cine	O
loop	O
or	O
QuickTime	B-Application
VR	I-Application
object	O
,	O
the	O
depth	O
impression	O
is	O
improved	O
,	O
and	O
the	O
observer	O
can	O
get	O
a	O
good	O
perception	O
of	O
3D	O
structure	O
.	O
</s>
