X-rays, like light rays, are a form of electromagnetic radiation. They are produced in a special vacuum tube after accelerated electrons (which are negatively-charged particles) have hit a tungsten target. Like light rays, X-rays are absorbed more by some substances than by others. Air and gases absorb least X-rays, whereas materials of a high density, such as lead, absorb most. When the rays are directed onto a photographic plate placed behind the part of the

Angiography makes blood human body being investigated, an image is revealed after development of the film. In examining the human body with X-rays, the image or picture that results depends on the different densities of body tissues. Those containing air (the lungs and bowel) allow most X-rays to pass through to the film. These tissues are said to be ‘radiolucent’ and create black or dark portions on the image. Fatty tissue is almost as radiolucent as air-containing tissue. Muscles and large blood vessels absorb more X-rays and show up as greyer areas on the film. Bone, which contains a high level of calcium, absorbs most X-rays and is said to be ‘radiopaque’ or ‘radiodense’, and casts a white ‘shadow’ on the film.

Plain X-rays

Plain X-rays are useful for diagnosis in many cases. They are used to examine bones and joints, for fractures and other abnormalities (their most obvious and early application). Often, an X-ray of the chest is made to visualize areas of infection within the lungs -these may show as white patches because they contain fluid that is more radiopaque than the surrounding air-filled spongy tissue. (Solid tumours show up in the same way.) On the same image the heart can be checked, which with its powerful muscle and rich blood supply, shows up well enough to enable easy assessment of its overall size and shape. An abdominal X-ray is taken when checking for swallowed foreign bodies (metal items such as needles show up very clearly); distension of the bowel with fluid caused by obstruction or gas under the diaphragm; and also when a stomach ulcer has perforated. It is possible to obtain an accurate picture of the nasal sinuses, by taking a number of X-rays with the patient’s head at a different angle to the film each time. Infection of the sinuses is present when radiopaque mucus replaces the translucent air the nasal passages normally contain.

Another application of X-rays is examining the breasts of a woman for early detection of a lump or tumour, which are more radiodense than the surrounding fatty, glandular tissue. This technique is called mammography.

X-rays made using a contrast agent

The limitations of plain X-rays mean that they cannot reveal the internal lining of hollow organs such as the stomach, intestines, gall bladder, blood vessels and urinary organs. This problem has been overcome by the use of contrast agents that are introduced into the structures to be imaged. The contrast agent coats or lines the inner surface of the organ and, because the agent is radiopaque, casts a clear shadow on film once the X-ray is taken.

Barium is highly dense to X-rays and is used to image the digestive tract. It may be swallowed as a liquid or introduced via the rectum as an enema. The barium outlines the oesophagus, stomach and duodenum. As a person drinks the barium solution, its progress through the body can be monitored ‘live’ on a screen. Narrowing of the oesophagus caused by inflammation, stricture or a tumour retards the rate at which it becomes filled.

At the same time as the stomach receives the barium solution, the patient is tilted on a moveable table so that the internal surface of the stomach is coated. The outline of a tumour or ulcer then shows up, if present. Ulcers in the duodenum may also be identified as the barium continues its journey. After this the barium solution is too dilute to outline the rest of the intestine. If, however, it is introduced as an enema, the whole of the large bowel can be imaged in a similar way.

Large blood vessels can be seen on a plain X-ray, but smaller ones need a contrast agent, injected into the blood, to make them show up. The internal shape of a vessel and whether the lining is curved may reveal blockage, or an aneurysm. This technique is called arteriography (for arteries) or angiography (for vessels in general). The contrast is provided by an iodine solution injected through a needle or fine tube into the area being investigated. Angiography is used particularly for the heart. The tube is fed from a large vein into the heart chambers and then near to the coronary arteries that supply the heart muscle with blood. The released contrast agent outlines any narrowing or blockage in these arteries that may be causing angina or be responsible for a heart attack.

Iodine-containing contrast agents are also used to visualize the urinary system (kidneys, ureters and bladder). The technique produces a set of film images called an intravenous pyelogram (IVP) or urogram. The agent is injected into the bloodstream; about 20 minutes later the kidneys are filtering it from the blood and so their outlines become visible on the X-ray image; next, the ureters, the tubes passing urine from the kidneys to the bladder, become visible; and finally the bladder is outlined. Such images allow diagnosis of stones in the urinary system, kidney tumours, abnormality or absence of a kidney, bladder tumours, and (in men) obstructed flow of urine from the bladder by an enlarged prostate gland. Contrast media are also used to image the lymph system (lymphangiogram) and various other hollow vessels and body parts.