Radionuclide (Isotope) Scan

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How does a radionuclide scan work?

A radionuclide (sometimes called a radioisotope or isotope) is a chemical which emits a type of radioactivity called gamma rays. A tiny amount of radionuclide is put into the body, usually by a injection into a vein. (Sometimes it is breathed in, or swallowed, depending on the test.)

There are different types of radionuclides. Different ones tend to collect or concentrate in different organs or tissues. So, the radionuclide used depends on which part of the body is to be scanned. For example, if radioactive iodine is injected into a vein it is quickly taken up into the tissues of the thyroid gland. So, it is used to scan the thyroid gland.

Cells which are most 'active' in the target tissue or organ will take up more of the radionuclide. So, active parts of the tissue will emit more gamma rays than less active or inactive parts.

Gamma rays are similar to x-rays and are detected by a device called a gamma camera. The gamma rays which are emitted from inside the body are detected by the gamma camera, are converted into an electrical signal, and sent to a computer. The computer builds a picture by converting the differing intensities of radioactivity emitted into different colours or shades of grey.

For example, areas of the target organ or tissue which emit lots of gamma rays may be shown as red spots ('hot spots') on the picture on the computer monitor. Areas which emit low levels of gamma rays may be shown as blue ('cold spots'). Various other colours may be used for 'in between' levels of gamma rays emitted.

What happens during a radionuclide scan?

Depending on the type of scan you have, you either swallow a small quantity of radionuclide, or it is injected into a vein in your arm. It then takes some time, sometimes several hours (depending on what is being scanned) for the radionuclide to travel to the target organ or tissue, and to be 'taken' into the active cells. So, after receiving the radionuclide you may have a wait of a few hours. You may be able to go out and come back to the scanning room later in the day.

When it is time to do the scanning, you lie on a couch while the gamma camera detects the gamma rays coming from your body. The computer turns the information into a picture. You need to lie as still as possible whilst each picture is taken (so it is not blurred). Some pictures can take 20 minutes or more to expose.

The number of pictures taken, and the time interval between each picture, varies depending on what is being scanned. Sometimes only one picture is needed. However, for some scans (such as bone scans or heart scans), two or more pictures are needed. Each picture may be taken several hours apart. So, the whole process can take several hours.

What preparation do I need

Usually very little. For some types of scan, you may be asked to have lots to drink to help to flush the radionuclide from your body. For some types of scan you may also be asked to empty your bladder of urine before the scanning begins.

What is a radionuclide scan used for?

A radionuclide scan may be done for all sorts of reasons.

• A bone scan is a common type. A radionuclide is used which collects in areas where there is a lot of bone activity (where bone cells are breaking down or repairing parts of the bone). So a bone scan is used to detect areas of bone where there is cancer, infection, or damage. These areas of activity are seen as 'hot spots' on the scan picture.
• A kidney scan can assess how well a kidney is working (as the radionuclide chosen is taken up by kidney cells and passes into the urine). So, the scan can detect scars on the kidney, and how well urine drains from the kidney to the bladder.
• Lung Perfusion Scan ('VQ scan') can detect blood clots in the lungs (pulmonary embolus).
• A heart scan can assess blood flow to the heart muscle. Areas of poor blood flow to the heart muscle do not 'take up' the radionuclide very well and this will be shown in the picture.
• Thyroid scan may be done to assess cases of overactive thyroid (hyperthyroidism). For example, some nodules (small 'lumps') are sometimes a focus of overactivity, and will show as 'hot spots' on the picture.

Are there any risks with radioisotope scans

The term 'radioactivity' may sound alarming. But, the radioactive chemicals used in radionuclide scans are considered to be safe, and they leave the body quickly in the urine. The dose of radiation that your body receives is very small. In many cases, the level of radiation involved is not much different to a series of a few normal x-rays. However:

• As with any other types of radiation (such as x-ray), there is a small risk that the gamma rays may affect an unborn child. So, tell your doctor if you are pregnant or if you may be pregnant.
• Rarely, some people have an allergic reaction to the injected chemical. Tell your doctor if you are allergic to iodine.
• Theoretically, it is possible to receive an overdose when the chemical is injected. This is very rare.

© EMIS and PIP 2004   Updated: August 2003   CHIQ Accredited   

The authors and editors of this article are employed to create accurate and up to date content reflecting reliable research evidence, guidance and best clinical practice. They are free from any commercial conflicts of interest. Find out more about updating.

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