Radiocontrast agent
Radiocontrast agents are substances used to enhance the visibility of internal structures in
Magnetic resonance imaging (MRI) functions through different principles and thus MRI contrast agents have a different mode of action. These compounds work by altering the magnetic properties of nearby hydrogen nuclei.
Types and uses
Radiocontrast agents used in X-ray examinations can be grouped in positive (iodinated agents, barium sulfate), and negative agents (air, carbon dioxide, methylcellulose).[1]
Iodine (circulatory system)
- Contrast CTs
- Angiography (arterial investigations)
- Venography (venous investigations)
- VCUG (voiding cystourethrography)
- HSG (hysterosalpingogram)
- IVU (intravenous urography)
Organic iodine molecules used for contrast include iohexol, iodixanol and ioversol.
Barium sulfate (digestive system)
Barium sulfate is mainly used in the imaging of the digestive system. The substance exists as a water-insoluble white powder that is made into a slurry with water and administered directly into the gastrointestinal tract.[citation needed]
- Upper gastrointestinal series
- Barium enema (large bowel investigation) and DCBE (double contrast barium enema).
- Barium swallow (oesophageal investigation)
- Barium meal (stomach investigation) and double contrast barium meal
- Barium follow through (stomach and small bowel investigation)
- CT pneumocolon / virtual colonoscopy
Barium sulfate, an insoluble white powder, is typically used for enhancing contrast in the GI tract. Depending on how it is to be administered the compound is mixed with water, thickeners, de-clumping agents, and flavourings to make the contrast agent. As the barium sulfate doesn't dissolve, this type of contrast agent is an opaque white mixture. It is only used in the digestive tract; it is usually swallowed as a barium sulfate suspension or administered as an enema. After the examination, it leaves the body with the feces.
Air
As in the picture on the right where both air and barium are used together (hence the term "double-contrast" barium enema) air can be used as a contrast material because it is less radio-opaque than the tissues it is defining. In the picture it highlights the interior of the colon. An example of a technique using purely air for the contrast medium is an air arthrogram where the injection of air into a joint cavity allows the cartilage covering the ends of the bones to be visualized.
Before the advent of modern neuroimaging techniques, air or other gases were used as contrast agents employed to displace the cerebrospinal fluid in the brain while performing a pneumoencephalography. Sometimes called an "air study", this once common yet highly-unpleasant procedure was used to enhance the outline of structures in the brain, looking for shape distortions caused by the presence of lesions.
Carbon dioxide
Carbon dioxide also has a role in angioplasty. It is low-risk as it is a natural product with no risk of allergic potential. However, it can be used only below the diaphragm as there is a risk of embolism in neurovascular procedures. It must be used carefully to avoid contamination with room air when injected. It is a negative contrast agent in that it displaces blood when injected intravascularly.
Discontinued agents
Thorotrast
Nonsoluble substances
In the past, some non water-soluble contrast agents were used. One such substance was iofendylate (trade names: Pantopaque, Myodil) which was an iodinated oil-based substance that was commonly used in myelography. Due to it being oil-based, it was recommended that the physician remove it from the patient at the end of the procedure. This was a painful and difficult step and because complete removal could not always be achieved, iofendylate's persistence in the body might sometimes lead to arachnoiditis, a potentially painful and debilitating lifelong disorder of the spine.[3][4] Iofendylate's use ceased when water-soluble agents (such as metrizamide) became available in the late 1970s. Also, with the advent of MRI, myelography became much less-commonly performed.
Adverse effects
Modern iodinated contrast agents – especially non-ionic compounds – are generally well tolerated.[5] The adverse effects of radiocontrast can be subdivided into type A reactions (e.g. thyrotoxicosis), and type B reactions (hypersensitivity reactions: allergy and non-allergy reactions [formerly called 'anaphylactoid reactions']).[6]
Patients receiving contrast via IV typically experience a hot feeling around the throat, and this hot sensation gradually moves down to the pelvic area.
The documentation of adverse drug reactions to contrast media should be documented precisely so that the patient receives adequate prophylaxis if contrast medium is administered again. [7]
Contrast induced nephropathy
Iodinated contrast may be
Thyroid dysfunction
Iodinated radiocontrast can induce overactivity (hyperthyroidism) and underactivity (hypothyroidism) of the thyroid gland. The risk of either condition developing after a single examination is 2–3 times that of those who have not undergone a scan with iodinated contrast. Thyroid underactivity is mediated by two phenomena called the Plummer and Wolff–Chaikoff effect, where iodine suppresses the production of thyroid hormones; this is usually temporary but there is an association with longer-term thyroid underactivity. Some other people show the opposite effect, called Jod-Basedow phenomenon, where the iodine induces overproduction of thyroid hormone; this may be the result of underlying thyroid disease (such as nodules or Graves' disease) or previous iodine deficiency. Children exposed to iodinated contrast during pregnancy may develop hypothyroidism after birth and monitoring of the thyroid function is recommended.[9]
See also
References
- S2CID 233676619.
- S2CID 45053720.
- ^ Dunlevy, Sue (10 December 2016). "Australians crippled and in chronic pain from dye used in toxic X-rays". The Daily Telegraph (Sydney). Retrieved 27 October 2017.
- ^ William P. Dillon; Christopher F. Dowd (2014). "Chapter 53 – Neurologic Complications of Imaging Procedures". Aminoff's Neurology and General Medicine (5th ed.). pp. 1089–1105.
- ISBN 978-3-85200-196-8.
- S2CID 205452727.
- PMID 32142634.
- PMID 25203000.
- PMID 25375985.
External links
- Media related to Radiocontrast agents at Wikimedia Commons