<s>
Radiocontrast	B-Application
agents	I-Application
are	O
substances	O
used	O
to	O
enhance	O
the	O
visibility	O
of	O
internal	O
structures	O
in	O
X-ray-based	O
imaging	O
techniques	O
such	O
as	O
computed	O
tomography	O
(	O
contrast	O
CT	O
)	O
,	O
projectional	O
radiography	O
,	O
and	O
fluoroscopy	O
.	O
</s>
<s>
Radiocontrast	B-Application
agents	I-Application
are	O
typically	O
iodine	O
,	O
or	O
more	O
rarely	O
barium	O
sulfate	O
.	O
</s>
<s>
The	O
contrast	B-Algorithm
agents	I-Algorithm
absorb	O
external	O
X-rays	B-Library
,	O
resulting	O
in	O
decreased	O
exposure	O
on	O
the	O
X-ray	B-Algorithm
detector	I-Algorithm
.	O
</s>
<s>
This	O
is	O
different	O
from	O
radiopharmaceuticals	B-Algorithm
used	O
in	O
nuclear	O
medicine	O
which	O
emit	O
radiation	O
.	O
</s>
<s>
Magnetic	O
resonance	O
imaging	O
(	O
MRI	O
)	O
functions	O
through	O
different	O
principles	O
and	O
thus	O
MRI	B-Algorithm
contrast	I-Algorithm
agents	I-Algorithm
have	O
a	O
different	O
mode	O
of	O
action	O
.	O
</s>
<s>
Radiocontrast	B-Application
agents	I-Application
used	O
in	O
X-ray	B-Library
examinations	O
can	O
be	O
grouped	O
in	O
positive	O
(	O
iodinated	O
agents	O
,	O
barium	O
sulfate	O
)	O
,	O
and	O
negative	O
agents	O
(	O
air	O
,	O
carbon	O
dioxide	O
,	O
methylcellulose	O
)	O
.	O
</s>
<s>
Iodinated	B-Algorithm
contrast	I-Algorithm
contains	O
iodine	O
.	O
</s>
<s>
It	O
is	O
the	O
main	O
type	O
of	O
radiocontrast	B-Application
used	O
for	O
intravenous	O
administration	O
.	O
</s>
<s>
Iodine	O
has	O
a	O
particular	O
advantage	O
as	O
a	O
contrast	B-Algorithm
agent	I-Algorithm
for	O
radiography	O
because	O
its	O
innermost	O
electron	O
(	O
"	O
k-shell	O
"	O
)	O
binding	O
energy	O
is	O
33.2	O
keV	O
,	O
similar	O
to	O
the	O
average	O
energy	O
of	O
x-rays	B-Library
used	O
in	O
diagnostic	O
radiography	O
.	O
</s>
<s>
When	O
the	O
incident	O
x-ray	B-Library
energy	O
is	O
closer	O
to	O
the	O
k-edge	O
of	O
the	O
atom	O
it	O
encounters	O
,	O
photoelectric	O
absorption	O
is	O
more	O
likely	O
to	O
occur	O
.	O
</s>
<s>
Depending	O
on	O
how	O
it	O
is	O
to	O
be	O
administered	O
the	O
compound	O
is	O
mixed	O
with	O
water	O
,	O
thickeners	O
,	O
de-clumping	O
agents	O
,	O
and	O
flavourings	O
to	O
make	O
the	O
contrast	B-Algorithm
agent	I-Algorithm
.	O
</s>
<s>
As	O
the	O
barium	O
sulfate	O
does	O
n't	O
dissolve	O
,	O
this	O
type	O
of	O
contrast	B-Algorithm
agent	I-Algorithm
is	O
an	O
opaque	O
white	O
mixture	O
.	O
</s>
<s>
It	O
is	O
only	O
used	O
in	O
the	O
digestive	O
tract	O
;	O
it	O
is	O
usually	O
swallowed	O
as	O
a	O
barium	B-Application
sulfate	I-Application
suspension	I-Application
or	O
administered	O
as	O
an	O
enema	O
.	O
</s>
<s>
As	O
in	O
the	O
picture	O
on	O
the	O
right	O
where	O
both	O
air	O
and	O
barium	O
are	O
used	O
together	O
(	O
hence	O
the	O
term	O
"	O
double-contrast	O
"	O
barium	O
enema	O
)	O
air	O
can	O
be	O
used	O
as	O
a	O
contrast	B-Algorithm
material	I-Algorithm
because	O
it	O
is	O
less	O
radio-opaque	O
than	O
the	O
tissues	O
it	O
is	O
defining	O
.	O
</s>
<s>
An	O
example	O
of	O
a	O
technique	O
using	O
purely	O
air	O
for	O
the	O
contrast	B-Algorithm
medium	I-Algorithm
is	O
an	O
air	O
arthrogram	O
where	O
the	O
injection	O
of	O
air	O
into	O
a	O
joint	O
cavity	O
allows	O
the	O
cartilage	O
covering	O
the	O
ends	O
of	O
the	O
bones	O
to	O
be	O
visualized	O
.	O
</s>
<s>
Before	O
the	O
advent	O
of	O
modern	O
neuroimaging	B-Algorithm
techniques	O
,	O
air	O
or	O
other	O
gases	O
were	O
used	O
as	O
contrast	B-Algorithm
agents	I-Algorithm
employed	O
to	O
displace	O
the	O
cerebrospinal	O
fluid	O
in	O
the	O
brain	O
while	O
performing	O
a	O
pneumoencephalography	O
.	O
</s>
<s>
It	O
is	O
a	O
negative	O
contrast	B-Algorithm
agent	I-Algorithm
in	O
that	O
it	O
displaces	O
blood	O
when	O
injected	O
intravascularly	O
.	O
</s>
<s>
Thorotrast	O
was	O
a	O
contrast	B-Algorithm
agent	I-Algorithm
based	O
on	O
thorium	O
dioxide	O
,	O
which	O
is	O
radioactive	O
.	O
</s>
<s>
In	O
the	O
past	O
,	O
some	O
non	O
water-soluble	O
contrast	B-Algorithm
agents	I-Algorithm
were	O
used	O
.	O
</s>
<s>
Modern	O
iodinated	B-Algorithm
contrast	I-Algorithm
agents	O
–	O
especially	O
non-ionic	O
compounds	O
–	O
are	O
generally	O
well	O
tolerated	O
.	O
</s>
<s>
The	O
adverse	O
effects	O
of	O
radiocontrast	B-Application
can	O
be	O
subdivided	O
into	O
type	O
A	O
reactions	O
(	O
e.g.	O
</s>
<s>
The	O
documentation	O
of	O
adverse	O
drug	O
reactions	O
to	O
contrast	B-Algorithm
media	I-Algorithm
should	O
be	O
documented	O
precisely	O
so	O
that	O
the	O
patient	O
receives	O
adequate	O
prophylaxis	O
if	O
contrast	B-Algorithm
medium	I-Algorithm
is	O
administered	O
again	O
.	O
</s>
<s>
Iodinated	B-Algorithm
contrast	I-Algorithm
may	O
be	O
toxic	O
to	O
the	O
kidneys	O
,	O
especially	O
when	O
given	O
via	O
the	O
arteries	O
prior	O
to	O
studies	O
such	O
as	O
catheter	O
coronary	O
angiography	O
.	O
</s>
<s>
Iodinated	O
radiocontrast	B-Application
can	O
induce	O
overactivity	O
(	O
hyperthyroidism	O
)	O
and	O
underactivity	O
(	O
hypothyroidism	O
)	O
of	O
the	O
thyroid	O
gland	O
.	O
</s>
<s>
The	O
risk	O
of	O
either	O
condition	O
developing	O
after	O
a	O
single	O
examination	O
is	O
2	O
–	O
3	O
times	O
that	O
of	O
those	O
who	O
have	O
not	O
undergone	O
a	O
scan	O
with	O
iodinated	B-Algorithm
contrast	I-Algorithm
.	O
</s>
<s>
Children	O
exposed	O
to	O
iodinated	B-Algorithm
contrast	I-Algorithm
during	O
pregnancy	O
may	O
develop	O
hypothyroidism	O
after	O
birth	O
and	O
monitoring	O
of	O
the	O
thyroid	O
function	O
is	O
recommended	O
.	O
</s>
