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Physics, Techniques and Procedures

Cholegraphic contrast medium

X-ray contrast medium used for noninvasive visualization of the bile ducts and/or gall bladder. There are two major groups; oral cholegraphic media and intravenous cholegraphic media (Table 1). Similar to all iodinated water-soluble contrast media, the contrast media in both groups are derivatives of tri-iodinated benzoic acid (see X-ray contrast medium (I), Fig. 4). The oral contrast media are ionic monomers, derived from one benzene ring. The intravenous contrast media are ionic dimers where two benzene rings are bridged together.

The typical oral cholegraphic medium is a tri-iodinated benzene ring with iodine at positions 2, 4 and 6, a side chain at position 3, no side chain at position 5, and the acid group linked to benzene via a chain at position 1 (Fig.1). The oral cholegraphic media all have some lipid solubility and some water solubility. The vacant position 5 causes the molecule to be bound to protein after its absorption from the small bowel. This protein binding reduces glomerular filtration in the kidneys and causes preferential excretion by the liver. After active transportation into the hepatocytes, the medium is conjugated with glucuronic acid and actively transported into the bile. The conjugates are generally less lipid soluble than the original contrast medium, and this prevents reabsorption from the bile ducts. The conjugated contrast medium is concentrated in the gall bladder by reabsorption of water, and it is eventually excreted in the faeces. A variable amount of the absorbed contrast medium will also be excreted in the urine.

The ionic dimeric intravenous cholegraphic contrast media contain six iodine atoms and have two vacant binding sites at position 5 in each benzene ring (Fig.2). The two vacant binding sites cause them to be protein bound. Both the ionic property and the high protein-binding capacity are causes of their relatively high toxicity. The media are actively excreted into the bile by the hepatocytes but, contrary to the oral media, they are not glucuronised.

Cholegraphic contrast medium, Table 1

Oral and intravenous contrast media

(Generic names with brand names in parenthesis)

ORAL CHOLEGRAPHIC MEDIA

--sodium iopodate (Biloptin)
--calcium iopodate (Solu-Biloptin)
--iopanoic acid (Telepaque, Cistobil)
--iocetamic acid (Cholebrin)
---iopronic acid (Bilimiro)
--iobenzamic acid (Bilibyk)
--sodium tyropanoate (Bilopaque)

INTRAVENOUS CHOLEGRAPHIC MEDIA

--meglumine iotroxate (Biliscopin)
--meglumine iodipamide (Biligrafin)
--meglumine ioglycamate (Biligram)
--meglumine iodoxamate (Endobil)

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Fig.1

The molecular structure of sodium iopodate (Biloptin), an oral cholegraphic contrast medium.
Cholegraphic contrast medium, Fig.1
Cholegraphic contrast medium, Fig.2