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X-ray tests show bones and certain other tissues.

X-rays are a type of high-energy radiation. An X-ray machine can produce short bursts of X-rays. The rays pass easily through fluids and soft tissues of the body. However, dense tissue such as bone will block some of the X-rays. (Density means how much of something there is in a certain amount of space.) The more dense the tissue, the fewer X-rays pass through. Air and water are less dense because the particles which make them are not held closely together. They let more X-rays pass through them.

In an X-ray picture bones show as light/white areas, whereas air shows as black or darker areas. The other lighter area in the middle of the image is the heart. The muscle which makes up the heart is also quite dense and stops X-rays passing through.

Posteroanterior chest X-ray

Chest X-ray PA

By Mikael Häggström (Own work) via Wikimedia Commons

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A film, similar to a photographic film, is placed behind the part of the body being X-rayed. The X-ray machine fires a short burst of X-rays through part of your body. The X-rays hit the film, which is then developed. The more X-rays that hit the film, the blacker it develops. So, dense parts of the body that block many of the X-rays show up as white (such as bones). Hollow or air-filled parts of the body show up as black (such as parts of the lung). Soft tissues (such as muscle and body organs) show up as various shades of grey, depending on how dense they are.

The developed film is studied by an X-ray doctor (radiologist) who sends a report to the doctor who requested the test.

An ordinary X-ray test is painless. You cannot see or feel X-rays. You should stay still when the X-ray beam is 'fired', as otherwise the picture may be blurred. 

  • Bones, teeth, bone fractures and other abnormalities of bone.
  • Joint spaces and some abnormalities of joints, such as osteoarthritis.
  • The size and shape of the heart. So, certain heart conditions can be detected.
  • Changes in the density of some softer tissues. For example, a lung tumour is more dense than air-filled lung and will show as a 'shadow' on a chest X-ray. A breast tumour is more dense than ordinary breast tissue and shows as a 'shadow' on an X-ray of the breast. An X-ray of the breast is also known as a mammogram.
  • Collections of fluid - for example, in the lung or gut - may show as grey 'shadows' against the normal black of the air-filled chest, or hollow gut.

An ordinary X-ray is a quick, easy and relatively cheap test. It may be all that is needed to diagnose or assess various problems. However, an ordinary X-ray has limited uses. More sophisticated 'contrast' X-rays, CT scans, or other imaging techniques may be needed for accurate or further assessment of certain body parts, particularly of soft tissues (eg, to diagnose inflammation) and organs such as the brain or liver.

How do I get an X-ray?

Your GP or consultant may order an X-ray to help diagnose a medical problem you have been discussing with them. X-rays are usually taken in your local hospital department, although some larger health centres also now take certain simple X-rays.

A report on the X-ray is sent to the doctor who ordered the test. The report will come from a specialist in X-rays and their interpretation - a radiologist. The pictures obtained from the X-ray are not usually seen by the requesting doctor, unless it is an emergency situation such as in an A&E department.

There is very little risk with having one X-ray test. However, with repeated tests there is a risk that the X-rays may damage some cells in the body, possibly leading to cancer in the future. The dose of X-ray radiation is always kept to the minimum needed to obtain a good picture of the particular body part being checked. (Also, radiographers who take the X-ray pictures always wear lead aprons or go behind a protective screen when the X-rays are fired, to avoid repeated exposure to X-rays.)

Pregnant women
If possible, pregnant women should not have an X-ray test, as there is a small risk that X-rays may cause an abnormality to the unborn child. This is why women are asked if they are, or might be, pregnant, before having an X-ray.

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