Radionuclide Scanning

A technique in which a radioactive substance is introduced into the body to assess structure and function of tissues

The technique of radionuclide scanning produces images using radiation emitted from a substance within the body. The radioactive substance, called a radionuclide, is introduced into the body and taken up by the organ or tissue to be imaged. A counter positioned outside the body detects the radiation that is emitted by the radionuclide and transmits this information to a computer. The computer then converts the information into images. Radionuclide scanning is used both to image the structure of many internal organs and to provide a measure of their function. SPECT scanning and PET scanning are two specialized forms of radionuclide scanning.

How does it work?

Radionuclides are normally introduced into the body by a single intravenous injection and are then carried in the bloodstream to the tissues (see Having a radionuclide scan). Xenon gas, one particular type of radionuclide used during a lung scan, is inhaled (see Radionuclide lung scanning).

Different tissues take up different radionuclides. A specific radionuclide is chosen because it will accumulate in the particular tissues to be investigated. For example, iodine is taken up by the thyroid gland, and radioactive iodine is therefore injected intravenously to produce a radionuclide scan of the gland.

A radionuclide emits radiation in the form of gamma rays, which are similar to X-rays. The gamma rays are detected outside the body by a device called a gamma camera. Within the camera are detectors that pick up the radiation and convert information about its quantity and location into a form that can be analysed by a computer. The computer then builds up an image and displays it on a monitor. The image is built up dot by dot, with each dot representing a certain amount of radiation.

Radionuclide scans show parts of the body as areas of colour of varying intensity. Areas of intense colour are called “hot spots”; these are areas where there is a high uptake of radionuclide. Areas of less intense colour, called “cold spots”, are areas where the radionuclide uptake is low. The greater the amount of tissue activity, the greater the uptake.

What is it used for?

An important feature of radionuclide scanning is that it can produce a “map” of an organ based on the activity of its tissues, therefore providing information about how that organ or tissue is functioning. This imaging technique may be used to detect abnormal levels of activity in organs such as the thyroid gland and kidneys, and it is useful for detecting some tumours of these organs. A radionuclide scan of bone will indicate areas that have increased activity, which might be due to problems such as Paget’s disease of the bone or cancer. Changes in the function of a tissue or an organ often develop before structural changes occur, and radionuclide scanning can detect some diseases at a significantly earlier stage than most other imaging techniques. For example, a radionuclide scan of bone can detect infection of bone tissue (see Osteomyelitis) weeks before it would become apparent on an ordinary X-ray.

Radionuclide scans are particularly useful for assessing how well a treatment has worked. Scans may be done before and after a particular treatment to compare the function of an organ.

Two particular types of radionuclide scanning may be used to look at the function of the heart. Thallium scanning reveals areas of the heart muscle with a poor blood supply and is used to look at the activity of the heart muscle during exercise (see Exercise testing). MUGA (multiple-gated acquisition scanning) is a technique in which the blood flow into and out of the heart is measured to assess how efficiently the heart pumps blood around the body.

What are the risks?

Radionuclide scanning has no immediate risks, but radiation-based imaging may damage the body cells, leading to an increased risk of cancer in later life. However, radionuclides are always administered in very small amounts, and they break down quickly in the body.

Procedure: Having a Radionuclide Scan

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

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