RADIOGRAPHIC CONTRAST MEDIA

A contrast medium is any substance introduced into the body in order to make an organ, or the surface of an organ, or materials within the lumen of an organ visible on imaging, for example, barium coated on gastric mucosa make it visible on x-ray. Usually, the medium is of greater radiographic density than the structure it outlines; occasionally lower densities are introduced, the usual example being air.

BARIUM

This is a suspension of powdered barium sulphate in a water. Barium sulphate is insoluble and chemically quite inert. Soluble salts of barium are very poisonous and only pharmaceutical quality barium sulphate should be used. Barium depends for its radiopacity on its electron density (reflected indirectly by its atomic number) which is much greater than the radio-opacity of soft-tissue and greater than the radio-opacity of bone.

Should barium leak from the G.I. tract into tissues or into a body cavity eg. mediastinum or peritoneum, it can cause a fibrogranulomatous reaction. Spill into the bronchial tree is a manageable problem unless it is gross, when death may ensue; weak barium preparations have been used for bronchography. After oral administration, it may compact in the large bowel causing constipation and occasionally may precipitate obstruction if there is a predisposing pathology.

WATER SOLUBLE MEDIA

These are all soluble organic compounds containing iodine and have four sub-groups.

i) Ionic monomers - Sodium and/or meglumine salts, eg. Urografin, Conray and Gastrograffin. These solutions are fully ionised, being soluble salts giving two particles in solution for each set of three iodine atoms. Solutions which have sufficient iodine to be useful radiographic contrast agents are all significantly hypertonic to serum. These are rarely used in Australia now, except in dilute form for micturating cystourethrography and as contrast medium in the gut for abdominal CT.

ii) Non-ionic monomers - where the acid radical of the salt has been replaced by a non-ionising but still water-soluble radical. Osmolarity is reduced because the molecule containing 3 atoms of iodine does not ionise. They are expensive costing at least four times as much as the simple ionic media. Examples - iopamidol (Iopamiro), iohexol (Omnipaque).

iii) Ionic dimers - where two molecules of the simple acid form a dimer with a single ionic radical containing 6 atoms of iodine. Ioxaglate (Hexabrix) is the only example. Osmolarity is reduced as the dimer only dissociates into two ions, one of which bears the six iodine atoms.

iv) Non ionic dimers - these have just been released on the market. Osmolarity is quite low as the molecule forms only one particle for six iodine atoms, and useful solutions will probably be isotonic with blood and CSF. However, they will probably be too viscous for anything but very specialised use.

The water soluble group will clear from just about any body cavity, either by virtue of their solubility or simply by being fluid. They do not cross the intact gastrointestinal mucosa or the intact blood-brain barrier. They are eliminated by passive glomerular filtration with no tubular secretion or reabsorption.

Hypersensitivity reactions to all groups have been documented, including urticaria, laryngeal oedema, hypotension, bronchospasm, anaphylactic shock and sudden death. There is no test that can predict a patient's reaction to contrast - it seems to be a cellular type of hypersensitivity with no convenient precipitin test available. There is no correlation with cutaneous iodine patch test. Testing by subcutaneous injection is as likely to be as risky as the full diagnostic study. Major reactions are unpredictable; even in a patient known to have reacted before, the chances of a second reaction are less than 50%. The non-ionic media have a much lower rate of complications, and for all groups, intravenous injection carries a much higher risk than intra-arterial injection. The only disadvantage of non-ionic media is their cost.

Other major problems with the early ionic water soluble media such as Urografin and Conray stem mainly from their hyperosmolarity and, where applicable, ionicity. Large volumes cause a solute load and shifts in extracellular fluid volume and some cases of sudden death probably relate to cardiac catastrophe rather than anaphylaxis. They are all irritant to mucosal surfaces, particularly the bronchial mucosa. Intra-arterial injection can be painful. Nausea is quite common, and vomiting may occur, with intravenous injection. Passage through the kidneys may exacerbate compromised renal function particularly in myelomatosis and diabetes. Sickle cell anaemia is also a significant risk factor. All the risks related to hyperosmolarity, particularly the renal aspects, are increased by vigorous prior dehydration.

PRECAUTIONS FOR THE USE OF WATER SOLUBLE MEDIA

a) Patients with known sensitivity to contrast media, or a clear allergy to food or drugs, or with a history of atopy (asthmatic or otherwise) should be premedicated with steroids in the preceding 24 hours (eg. two doses of Prednisone 25 mgm for adults with appropriate reduction in dose for children).

b) Patients with renal failure, myelomatosis and diabetes should not be vigorously dehydrated or purged.

c) Those in actual or incipient heart failure, with electrolyte disturbances or with serious cardiac arrhythmia should be stabilised and/or discussed.

d) There is always a risk of vomiting and possibly aspiration following injection of water soluble media, including myelography. The stomach should be reasonably empty; however this does not mean that the patient has to be starved for any great period. Free fluids, including sugar drinks, can be given up to two hours before injection, after which nil by mouth is appropriate.

EMERGENCY OESOPHAGEAL STUDIES

Diagnostic swallows for presumed oesophageal rupture should be done with a low osmolality water soluble medium. If this is unavailable dilute barium is a reasonable alternative. The risk of respiratory complications from aspiration of hyperosmolar media is greater than that of the consequences of leakage of barium into the mediastinum.

OTHER ASPECTS OF PATIENT PREPARATION

1. Bowel studies usually require clean mucosa, ie. no food or faeces present therefore preparation is needed. This will be discussed when the booking is made.

2. If an arterial puncture is to be performed then pre-catheter bleeding/clotting studies are indicated.

3. Renal function (creatinine) should be known prior to injection of the large amount of intravascular contrast.

4. Antibiotic prophylaxis - indicated if there is a possibility of bacterial dissemination, particularly in a patient with valvar or congenital heart disease or if a prosthetic graft is to be punctured.

5. Hepatitis B and HIV status.

If in doubt - DISCUSS with the radiologist who will do the procedure.