In radionuclide scanning, radionuclides are used to produce images. A radionuclide is a radioactive form of an element, which means it is an unstable atom that becomes more stable by releasing energy as radiation. Most radionuclides release high-energy photons as gamma rays (which are x-rays that occur in nature, that are not man-made) or particles (such as positrons, which are used in positron emission tomography). (See also Radiation Injury.)
Radionuclides are also used to treat certain disorders, such as thyroid disorders.
For scanning, a radionuclide is used to label a substance that accumulates in a specific part of the body. Different substances are used depending on which part of the body is to be evaluated.
A substance may accumulate because the body uses (metabolizes) it, as for the following:
Iodine is used to make thyroid hormones and thus accumulates in the thyroid gland.
Diphosphonate accumulates where bone is repairing or rebuilding itself.
Or a substance may abnormally accumulate in a specific area, as for the following:
The combination of the radionuclide and the substance it labels is called a radioactive tracer. With imaging, doctors can see where the tracer collects and gives off radiation, which is detected by special scanners or cameras, such as a gamma camera. The camera produces a flat image of where the tracer collects. Sometimes a computer analyzes the radiation to produce a series of 2-dimensional images that look like slices of the body.
Usually, the tracer is injected in a vein, but for some tests, the tracer is swallowed, inhaled, or injected under the skin (subcutaneously) or into the joint. Imaging is done after the tracer has had time to move to the target tissues (which may be almost immediately or take up to several hours).
Before some tests (such as a gallbladder scan), the person is asked to refrain from eating and drinking for several hours. Clothing does not usually need to be removed.
The person must lie still during the scanning, which usually takes about 15 minutes. However, sometimes a scan needs to be repeated after a time, often hours later.
After the test, drinking extra fluids to help the body eliminate the radionuclide is recommended. Normal activities can be resumed immediately.
A radionuclide in the body can sometimes set off radioactivity detectors that are used for security purposes. Detectors may be carried by police or be in place around transportation centers and in other high-security areas. How long the radionuclide can set off detectors varies depending on the radionuclide but is typically a few days or less. To prevent problems with security, a doctor often gives people a note documenting that they have had radionuclide scanning.
Radionuclide scanning can be used to evaluate many parts of the body: thyroid gland, liver and gallbladder, lungs, urinary tract, bone, brain, and certain blood vessels.
Because the body metabolizes many of the substances (such as iodine) used to label the radionuclide, radionuclide scanning can sometimes provide information about how a tissue is functioning, as well as what it looks like.
Various radionuclides are used to image different parts of the body or types of disorders, as for the following:
Blood flow to the heart: Thallium is used to show blood flow through the arteries that carry blood to the heart. Thus, it can help doctors evaluate coronary artery disease. To determine how the heart functions when it is working hard, doctors sometimes use thallium during stress testing, usually by having the person walk or run on a treadmill. This test may also indicate how well the heart is pumping. The test can be done after a heart attack to help doctors estimate prognosis.
Bone: Because technetium tracer collects in bone, it is used to image the skeleton. It is used to check for cancer that has spread (metastasized) to bone and for bone infections.
Inflammation: Technetium or other radionuclides are used to label white blood cells, which gather at sites of inflammation or infection. This test helps doctors identify inflammation and infection.
Bleeding: Technetium is used to label red blood cells. This test helps doctors locate bleeding in the intestine.
Gallbladder and bile ducts: Iminodiacetic acid is labeled. The liver handles this radionuclide as it would bile. Thus, iminodiacetic acid collects where bile collects. This test is used to check for blockages in bile ducts, bile leaks, and gallbladder disorders.
Radionuclide scanning is also used to check for certain cancers, such as lung cancer that has spread to the liver, thyroid cancer, and colorectal cancer.
SPECT is similar to computed tomography but uses radionuclide gamma rays rather than x-rays.
For SPECT, the person lies on a motorized table. A rotating gamma camera takes images from many different angles (tomograms), each representing a slice of the body, and a computer is used to construct them into 2- and 3-dimensional images. These images help doctors more precisely locate structures and abnormalities.
Depending on the area being evaluated, people may be asked to restrict what they eat or drink before the test. The test usually takes 30 to 90 minutes.
The amount of radiation exposure from radionuclide scanning depends on which radionuclide is used and how much is used. For example, with a lung scan, the dose is similar to that used in about 100 single-view chest x-rays. Other scans may involve more or less radiation.
Radionuclide scanning can take hours to do because of the need to wait between injection and scan to give the radionuclide time to reach the target tissue.
Because the radiation can affect a fetus, women who are pregnant or may be pregnant should tell their doctor.