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Most other
contrast media are iodine containing compounds, and most of them are water
soluble. Iodine is also a ‘heavy’ element (atomic mass 127). Different iodine
compounds behave differently in the body.
For
example, an iodine compound that is specifically excreted by the kidney is
used to study the urinary system. 20 to 40 ml of this fluid, when injected in
the bloodstream through a vein (intravenous) is diluted in the (approximately)
4 litres of blood, so blood vessels are not visible on an X-ray. But when
excreted by the kidney in a small amount of urine, it is concentrated and the
cavities of the urinary system are outlined. This is called intravenous
urography. Tha fact that it is excreted by the kidneys also means that the
kidneys are functioning! The compound can also be injected through a tube
passed in the urinary bladder – it can reach as high as the calyces of the
kidneys. This is ‘ascending’ urography.
Another
iodine compound is given by mouth, is absorbed by the digestive system,
reaches the liver via the portal vein and is excreted in bile, outlining the
gall bladder and bile ducts.
Arteries
and veins, the heart, and cavities in the brain can also be studied in this
manner with other compounds.
Some of
these procedures can be potentially hazardous and are done only if there are
specific reasons for doing them.
Contrast Media - 2
Key Points :
The choice and the route of
administration of a contrast medium depends on the physiology of the structure being
investigated!
Barium
sulphate for the GI Tract, iodine containing compounds for most other structures.