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Radionuclide Scanning

by Jon A. Jacobson, MD

In radionuclide scanning, radionuclides are used to produce images. A radionuclide is an unstable atom that becomes more stable by releasing energy as radiation. Most radionuclides release high-energy photons as gamma rays (which are similar to x-rays). Radionuclides are also used to treat certain disorders, such as thyroid disorders.

Procedure

For scanning, a radionuclide is combined (or labeled) with 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) them, 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:

  • Red blood cells accumulate in the intestine when the intestine is bleeding rapidly.

  • White blood cells accumulate in areas that are inflamed or infected.

The combination of the radionuclide and the substance used to label it 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.

Because the body metabolizes many of the substances used, radionuclide scanning can sometimes provide information about how a tissue is functioning, as well as what it looks like.

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 need to be removed. Sometimes the person lies on a motorized table, and the camera rotates around the person. 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.

Uses

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. 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 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.

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.

Variations

Single-Photon Emission Computed Tomography (SPECT)

SPECT is similar to computed tomography but uses radionuclide emissions rather than x-rays. 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.


Disadvantages

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 75 chest x-rays. Other scans may involve even more radiation.

Radionuclide scanning can take hours to do because of the need to wait between injection and scan. Sometimes the images are not very clear.

Because the radiation can affect a fetus, women who are pregnant or may be pregnant should tell their doctor.

Positron Emission Tomography

Positron emission tomography (PET) is a type of radionuclide scanning. In PET, a substance that the body uses (metabolizes), such as glucose or oxygen, is labeled with an atom (called a radionuclide) that releases positively charged particles of radiation called positrons. Positrons collect in a specific area of the body. The more active the tissue, the more positrons it collects and uses and the more radiation it gives off.

The PET scanner contains several rings of detectors that record the radiation released and produce color tomographic images of the area. The intensity of the color indicates how active the tissue is. The resulting scan shows different levels of activity in different intensities of color. Thus, PET can provide information about a tissue’s function and can identify abnormal tissues, which may be more or less active than normal tissues. However, PET does not show anatomic and structural detail of tissues and organs as well as most other types of imaging tests.

Procedure

Before the procedure, people may be asked not to consume alcohol, caffeine, tobacco products, or any drugs that might affect mental function (such as sedatives).

For PET, a person lies on a table, and the labeled substance is injected in the person’s vein. The substances take about 30 to 60 minutes to reach the area being evaluated. The table is then positioned so that area being evaluated is within the large circular opening of the PET scanner.

The person is asked to lie flat during most of the test, which may take 45 to 60 minutes. Depending on the area of the body being evaluated, the person may be asked to do certain activities, such as mental tasks to stimulate activity in the brain.

Uses

PET is used to evaluate blood flow and activity in the heart and brain. PET can show how well the heart functions, which can help determine whether a person is a candidate for coronary artery bypass graft surgery or a heart transplant. A PET scan of the brain can also show which areas of the brain are most active during certain activities—for example, during mathematical calculations.

PET can show where a cancer is and where it has spread. PET helps doctors evaluate lung cancer, colorectal cancer, esophageal cancer, head and neck cancer, lymphoma, and melanoma. It helps doctors determine whether enlarged lymph nodes in people with cancer are due to the spread (metastasis) of the cancer or to another abnormality.

PET is used in research to provide information about seizure disorders and to help doctors diagnose Alzheimer’s disease, Parkinson’s disease, transient ischemic attacks, and strokes.

Variations

Single-Photon Emission Computed Tomography (SPECT)

SPECT is similar to computed tomography but uses radionuclide emissions rather than x-rays. 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.


Disadvantages

The amount of radiation exposure from PET is similar to that from CT.

Because radionuclides used in PET give off radiation for only a short time, PET can be done only if the radionuclide is produced at a nearby location and can be obtained quickly. PET is relatively expensive and not widely available.

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