Images produced using radiation and a substance that makes hollow or fluid-filled structures visible
Hollow or fluid-filled body structures, such as the intestines or blood vessels, do not show up well on an ordinary X-ray image (see X-rays). A substance called a contrast medium or “dye” can be introduced into these structures to make them visible. Contrast media are opaque to radiation in the same way as dense body tissues, such as bone. X-rays cannot pass through the media, and areas containing these substances will appear white on an X-ray image.
The contrast medium is injected into the body or introduced orally or rectally, depending on the structure to be imaged. This procedure is generally straightforward, but it may cause discomfort and involve some risks, such as an adverse reaction to the dye. Contrast X-ray procedures are increasingly being replaced by other techniques, in particular CT scanning, MRI, and ultrasound scanning, all of which cause less discomfort and involve fewer risks to health.
What are they used for?
Contrast X-rays can produce images of various hollow or fluid-filled structures that do not show up well on ordinary X-rays. Contrast X-rays may be used to image the blood vessels, urinary system, and digestive tract. Different types of contrast media that may be used include iodine (which is soluble in water) and barium sulphate (which is insoluble).
Water-soluble iodine dyes can be carried around the body in the bloodstream. For this reason, and because they show up well on X-rays, they can be used to check whether blood is flowing normally in the blood vessels. Imaging of blood vessels by using X-rays is known as angiography. In this technique, the dye is injected through a catheter that has been guided into a blood vessel until its tip is near the vessel to be studied. The dye flows into the appropriate vessels, making abnormalities, such as blockages, visible on X-rays.
Angiography is often used to look at arteries that have become narrowed or blocked due to fatty deposits in the vessel walls (see Atherosclerosis). Coronary angiography images blood vessels that supply the heart and femoral angiography is used to image blood vessels in the leg. Better-quality images are produced by using a computer to remove unwanted background information, a procedure called digital subtraction angiography. Newer, noninvasive techniques, such as magnetic resonance angiography, are increasingly being used to obtain the same information without using X-rays.
When injected into a vein, dye circulates in the bloodstream and becomes concentrated in the kidneys before it is excreted in urine. The dye can therefore be used to image various parts of the urinary system. As the dye passes through the urinary tract, it outlines the ureter (the tube linking the kidneys and the bladder) and the bladder. If the bladder and urethra (the tube that takes urine from the bladder out of the body) alone are to be imaged by the X-rays, the contrast medium may be inserted into the bladder through a catheter that is passed up through the urethra. Contrast X-rays may be used to look for suspected kidney disease and obstructions in the urinary system due to kidney stones or tumours (see Intravenous urography).
Barium sulphate is a thick, insoluble, chalky liquid that does not allow X-rays to pass through it and shows up well on X-ray images. It moves slowly through the digestive tract and is not absorbed by the body; therefore it is a good contrast medium for examining the digestive tract. Barium X-ray examinations can also be useful for studying functions such as swallowing. However, endoscopy is often more appropriate than barium investigations.
Barium sulphate is drunk if the upper digestive tract from the oesophagus to the duodenum (the first part of the small intestine) is to be examined. This procedure is known as a barium swallow or a barium meal. Barium meals can be used to investigate most problems of the upper digestive tract, including swallowing difficulties and indigestion (see Nonulcer dyspepsia). The progression of the barium through the digestive tract can be followed by taking X-rays at intervals or recording video images.
Barium sulphate is also used to image the colon. It is given rectally as an enema after a laxative has been taken to empty the bowel. Barium enemas can be used to detect abnormal growths in the colon lining, such as polyps (see Polyps in the colon) and colorectal cancer.
In double-contrast radiography, air is introduced into the digestive tract to replace the barium, which remains only on the digestive tract lining. This technique gives a detailed image of the lining and is useful for detecting changes due to conditions such as Crohn’s disease.
What are the risks?
Radiation can cause damage to body cells, which may rarely lead to cancer later in life. Contrast X-rays expose you to a relatively large quantity of radiation because they require several X-rays.
Injecting contrast media carries a minor risk of serious complications such as anaphylaxis. People who have asthma triggered by an allergy or who have a known sensitivity to iodine are advised not to have contrast X-rays or are pretreated with drugs such as antihistamines or corticosteroids, or other contrast media may be used. Most people experience a flushing sensation as dye is injected. Barium sulphate may also cause constipation.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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