<s>
A	O
CT	B-Algorithm
pulmonary	I-Algorithm
angiogram	I-Algorithm
(	O
CTPA	B-Algorithm
)	O
is	O
a	O
medical	O
diagnostic	O
test	O
that	O
employs	O
computed	O
tomography	O
(	O
CT	O
)	O
angiography	O
to	O
obtain	O
an	O
image	O
of	O
the	O
pulmonary	O
arteries	O
.	O
</s>
<s>
Modern	O
MDCT	O
(	O
multi-detector	O
CT	O
)	O
scanners	O
are	O
able	O
to	O
deliver	O
images	O
of	O
sufficient	O
resolution	O
within	O
a	O
short	O
time	O
period	O
,	O
such	O
that	O
CTPA	B-Algorithm
has	O
now	O
supplanted	O
previous	O
methods	O
of	O
testing	O
,	O
such	O
as	O
direct	O
pulmonary	O
angiography	O
,	O
as	O
the	O
gold	O
standard	O
for	O
diagnosis	O
of	O
pulmonary	O
embolism	O
.	O
</s>
<s>
The	O
patient	O
receives	O
an	O
intravenous	O
injection	O
of	O
an	O
iodine-containing	O
contrast	B-Algorithm
agent	I-Algorithm
at	O
a	O
high-rate	O
using	O
an	O
injector	O
pump	O
.	O
</s>
<s>
A	O
normal	O
CTPA	B-Algorithm
scan	O
will	O
show	O
the	O
contrast	O
filling	O
the	O
pulmonary	O
vessels	O
,	O
appearing	O
as	O
bright	O
white	O
.	O
</s>
<s>
CTPA	B-Algorithm
was	O
introduced	O
in	O
the	O
1990s	O
as	O
an	O
alternative	O
to	O
ventilation/perfusion	O
scanning	O
(	O
V/Q	B-Algorithm
scan	I-Algorithm
)	O
,	O
which	O
relies	O
on	O
radionuclide	O
imaging	O
of	O
the	O
blood	O
vessels	O
of	O
the	O
lung	O
.	O
</s>
<s>
CTPA	B-Algorithm
is	O
typically	O
only	O
requested	O
if	O
pulmonary	O
embolism	O
is	O
suspected	O
clinically	O
.	O
</s>
<s>
If	O
this	O
is	O
negative	O
and	O
risk	O
of	O
a	O
PE	O
is	O
considered	O
negligible	O
,	O
then	O
CTPA	B-Algorithm
or	O
other	O
scans	O
are	O
generally	O
not	O
performed	O
.	O
</s>
<s>
Most	O
patients	O
will	O
have	O
undergone	O
a	O
chest	O
X-ray	O
before	O
CTPA	B-Algorithm
is	O
requested	O
.	O
</s>
<s>
After	O
initial	O
concern	O
that	O
CTPA	B-Algorithm
would	O
miss	O
smaller	O
emboli	O
,	O
a	O
2007	O
study	O
comparing	O
CTPA	B-Algorithm
directly	O
with	O
V/Q	O
scanning	O
found	O
that	O
CTPA	B-Algorithm
identified	O
more	O
emboli	O
without	O
increasing	O
the	O
risk	O
of	O
long-term	O
complications	O
compared	O
to	O
V/Q	O
scanning	O
.	O
</s>
<s>
A	O
V/Q	B-Algorithm
scan	I-Algorithm
may	O
still	O
be	O
recommended	O
when	O
a	O
lower	O
radiation	O
dose	O
is	O
required	O
.	O
</s>
<s>
On	O
CTPA	B-Algorithm
,	O
acute	O
emboli	O
have	O
been	O
found	O
at	O
radiodensities	O
ranging	O
between	O
about	O
5	O
and	O
65	O
Hounsfield	O
units	O
(	O
HU	O
)	O
,	O
while	O
chronic	O
emboli	O
have	O
ranged	O
between	O
about	O
30	O
and	O
150	O
.	O
</s>
<s>
CTPA	B-Algorithm
is	O
less	O
desirable	O
in	O
pregnancy	O
due	O
to	O
the	O
amount	O
of	O
ionizing	O
radiation	O
required	O
,	O
which	O
may	O
damage	O
the	O
breasts	O
,	O
which	O
are	O
particularly	O
sensitive	O
during	O
pregnancy	O
,	O
and	O
because	O
of	O
concerns	O
of	O
the	O
effects	O
of	O
iodine	O
on	O
the	O
fetus	O
 '	O
thyroid	O
gland	O
.	O
</s>
<s>
V/Q	B-Algorithm
scans	I-Algorithm
can	O
offer	O
lower	O
radiation	O
doses	O
,	O
and	O
may	O
be	O
adapted	O
to	O
further	O
reduce	O
the	O
dose	O
by	O
omitting	O
the	O
lung	O
ventilation	O
portion	O
of	O
the	O
exam	O
.	O
</s>
<s>
CTPA	B-Algorithm
would	O
then	O
only	O
be	O
performed	O
if	O
exhaustive	O
non-radiation	O
based	O
testing	O
could	O
not	O
make	O
a	O
positive	O
diagnosis	O
.	O
</s>
<s>
CTPA	B-Algorithm
is	O
contraindicated	O
in	O
known	O
or	O
suspected	O
allergy	O
to	O
contrast	B-Algorithm
media	I-Algorithm
or	O
in	O
kidney	O
failure	O
(	O
where	O
contrast	B-Algorithm
agents	I-Algorithm
could	O
worsen	O
the	O
kidney	O
function	O
)	O
.	O
</s>
<s>
An	O
intravenous	O
cannula	O
is	O
required	O
for	O
the	O
administration	O
of	O
iodinated	B-Algorithm
contrast	I-Algorithm
.	O
</s>
<s>
Even	O
though	O
the	O
actual	O
scan	O
may	O
be	O
completed	O
in	O
1	O
second	O
or	O
less	O
,	O
considerable	O
staff	O
and	O
patient	O
time	O
is	O
required	O
for	O
preparation	O
of	O
the	O
contrast	B-Algorithm
agent	I-Algorithm
,	O
positioning	O
on	O
the	O
scanner	O
and	O
planning	O
the	O
scan	O
.	O
</s>
<s>
This	O
is	O
particularly	O
the	O
case	O
,	O
as	O
patients	O
undergoing	O
CTPA	B-Algorithm
are	O
frequently	O
seriously	O
unwell	O
requiring	O
oxygen	O
treatment	O
and/or	O
close	O
monitoring	O
.	O
</s>
<s>
On	O
CTPA	B-Algorithm
,	O
the	O
pulmonary	O
vessels	O
are	O
filled	O
with	O
contrast	O
,	O
and	O
appear	O
white	O
.	O
</s>
<s>
Ideally	O
,	O
the	O
scan	O
should	O
be	O
complete	O
before	O
the	O
contrast	O
reaches	O
the	O
left	O
side	O
of	O
the	O
heart	O
and	O
the	O
aorta	O
,	O
as	O
this	O
may	O
mean	O
contrast	O
has	O
drained	O
from	O
the	O
pulmonary	O
arteries	O
,	O
or	O
require	O
a	O
larger	O
dose	O
of	O
contrast	B-Algorithm
media	I-Algorithm
.	O
</s>
