Radiological contrast agents

Radiological contrast agents are defined as substances that, once introduced by suitable ways in the organism, alter the average atomic number or the electronic density of specific structures and, thus, their X-photon absorption capacity.
The introduction in medical practice of contrast agents has made it possible to extend the radiological study virtually to all organs and apparatuses, overcoming the limitations imposed by natural contrast.
Contrast agents may be divided in:
- opaque contrast agents (which increase the atomic number, therefore inducing greater photon absorption);
- transparent contrast agents (which decrease the electronic density, therefore inducing smaller photon absorption).
"Double contrast" techniques, fundamental for GI study, involve the simultaneous use of both types of agents. Transparent contrast agents are essentially gaseous (CO₂, N₂O, O₂). An aqueous solution of methylcellulose is used in the investigation of the small intestine.
The most commonly used opaque contrast agent for the GI study is barium (BaSO₄). Barium is not absorbed by the gastrointestinal mucosa and thus, in normal conditions, when administered orally, passes freely until it is completely eliminated with faeces. It is easily evacuated when given by enema. Iodized contrast agents used in uroangiography are made up of a benzene ring with three bound iodine atoms. Iodine, in fact, is an atom with great X-photon absorption capacity and is, furthermore, able to form stable bonds with appropriate organic molecules. When these triiodate compounds are introduced in the blood stream, they diffuse reversibly in the interstitial spaces and are then excreted by glomerular filtration; in practice, they have no toxicity from the pharmacological point of view.
Iodized contrast agents for cholecystography, on the other hand, are made up of two triiodized benzene rings linked by an aliphatic bond, therefore these agents are made up of a system with six iodine atoms.
Finally, some iodized oils are employed as contrast agents to make distal subarachnoidal spaces, bronchial tree (bronchography) and lymph (lymphangiography) opaque. They are mono- or bi- iodized esters of vegetal fatty acids in suspension in poppy-seed, olive, sesame and peanut oil with a concentration of iodine that varies between 10 and 40%.
As far as non iodized contrast agents are concerned, there are contraindications to their diagnostic use in patients (Waldenstrom's paraproteinaemia, myeloma, severe liver and renal failure, previous episodes of hypersensitivity to the contrast agent); on the contrary there are no precise limitations to their manipulation and preparation by health workers.
As far as iodized contrast agents are concerned, on the other hand, it must be remembered that they may interact with metallic surfaces containing copper and therefore the employment of tools in which the substance comes into contact with such surfaces must be avoided.

Diagnostic services

Diagnostic services

Radiological contrast agents