Nuclear scans

Other names include nuclear imaging, radionuclide imaging, and nuclear medicine scans.

What do they show?

Nuclear scans make pictures based on the body’s chemistry rather than on physical shapes and forms (as is the case with other imaging tests). These scans use substances called radionuclides (also called tracers or radiopharmaceuticals) that release low levels of radiation. The amount of radioactivity used is very small and not known to cause harm.

Body tissues affected by certain diseases, such as cancer, may absorb more or less of the tracer than normal tissues. Special cameras pick up the pattern of radioactivity to create pictures that show where the material travels and where it collects. These scans show some internal organ and tissue problems better than standard x-ray images.

If cancer is present, the tumor may show up on the picture as a “hot spot” − an area of increased tracer uptake. Depending on the type of scan done, the tumor may instead be a “cold spot” − a site of decreased uptake.

Nuclear scans are used to find tumors. They are also used to study a cancer’s stage (the extent of its spread) and to decide if treatment is working.

Nuclear scans may not find very small tumors, and cannot always tell the difference between benign (not cancer) and malignant (cancer) tumors. They are often used along with other imaging tests to give a more complete picture of what is going on. For example, bone scans that show “hot spots” on the skeleton are usually followed by x-rays of the affected bones, which are better at showing details of the bone structure.

Nuclear scans have different names, depending on the organ involved. Some of the more commonly used nuclear scans (described in more detail further on) are:

• Bone scans
• Gallium scans
• PET scans
• Thyroid scans
• MUGA scans

How do they work?

The type of scan done depends on what organ or tissue the doctor wishes to study. In most cases you are given a substance that sends out small doses of radiation. Some are swallowed while others are given into a vein or inhaled as a gas.

Radionuclide scans: Because they look at more than just the shape of a tumor, radionuclide scans are used for more than just creating pictures. Here are some of the more common radionuclides now in use:

• Gallium-67 is used to look for cancer in certain organs. It can also be used for a whole body scan. This may be called a gallium scan.
• Technetium-99 is used in whole body scans, especially in bone scans. Bone scans look for cancers that may have spread (metastasized) from other places to the bones. Technetium-99 is also used in heart scans, including the MUGA scan which looks at heart function.
• Thallium-201 scans, more often used in cardiology (the study of heart disease), are sometimes used to look at how well treatment is working for certain kinds of tumors and may be used to find some types of cancer.
• Radioactive iodine (iodine-123 or iodine-131) can be used to find and treat thyroid cancers.

Radionuclides send out gamma rays which are picked up by a special camera (known as a gamma camera, rectilinear scanner, or scintiscan). The signals are processed by a computer, which turns them into 2- or 3-dimensional (3-D) pictures, sometimes with color added for extra clarity. A radiologist or a doctor who specializes in nuclear medicine interprets the pictures and sends a report to your doctor.

Positron emission tomography scans: Positron emission tomography (PET) is a scan that uses a form of radioactive sugar. Body cells take in different amounts of the radioactive sugar, depending on how fast they are growing. Cancer cells, which grow quickly, are more likely to take up larger amounts of the sugar than normal cells. The radioactive sugar gives off tiny atomic particles called positrons, which run into electrons in the body, giving off gamma rays. A special camera picks up these rays as they leave the body and turns them into pictures.

PET scans are used to find cancer and to see if it is responding to treatment. The chemical changes they show can also help doctors look at the effects of cancer treatment. Because PET scans look at body function, they may show changes that suggest disease before the changes can be seen on other imaging tests.

PET/CT scans: Some machines combine a PET scan with a CT scan. PET/CT scanners give more detailed information on the location of any increased cell activity, helping doctors pinpoint tumors.

Use of monoclonal antibodies in nuclear scans: A special type of antibody produced in the lab, called a monoclonal antibody, can be designed to stick to substances found only on the surface of cancer cells. A radioactive substance can be attached to the monoclonal antibody, which is then given into a vein. It travels in the bloodstream until it gets to the tumor and sticks to it. This causes the tumor to “light up” when seen through a special scanner. Examples of monoclonal antibody scanning used to look at cancers are the ProstaScint® scan for prostate cancer, the OncoScint® scan for ovarian cancer, and the CEA-Scan® for colon cancer.

How do I get ready for the test?

The steps needed to prepare for a nuclear scan depend on the type of test and the tissue that will be studied. Some scans require that you don’t eat or drink for 2 to 12 hours before the test. For others, you may be asked to take a laxative or use an enema. Be sure your doctor or nurse knows everything you take, even over-the-counter drugs, vitamins, and herbs. You may need to avoid some medicines (prescription and over-the-counter) before the test. Your health care team will give you instructions.

The radioactive material can be given by mouth, put into a vein (IV), or inhaled as a gas. You may get it anywhere from a few minutes to many hours before the test. For example, in a bone scan, the tracer is put into a vein in your arm about 2 hours before the test begins. For gallium scans, the tracer is given a few days before the test.

What is it like having the test?

In most cases, a nuclear scan done is done as an outpatient. Because of the special materials and equipment needed, the scans are usually done in the radiology or nuclear medicine department of a hospital.

The scanner has a hole in the middle and looks like a large doughnut. You lie on a padded table which fits through the hole and the scanner moves back and forth. The technician may ask you to change positions to allow different views to be taken. The test is not painful. But you may get uncomfortable after lying on the table for a while.

If you are having a brain scan, many sets of pictures may be needed. The first scans are taken as the radioactive material moves through the arteries into the brain. The second set, taken a few hours later, shows the material in the brain itself. You will be asked to move your head into different positions. Likewise, a thyroid scan may require 2 sets of scans using doses of radioactive iodine that you swallow.

How long does it take?

A nuclear scan takes about 30 to 60 minutes, plus the waiting time after the radioactive material is given. For bone scans, the material takes 2 to 3 hours to be absorbed, and the scan itself takes another hour or so. Gallium scans take several days between the injection and the actual scan. Results of nuclear scans are usually available within a few days.

What are the possible complications?

For the most part, nuclear scans are safe tests. The doses of radiation are very small, and the radionuclides have a low risk of being toxic or causing an allergic reaction. Some people may have pain or swelling at the site where the material is injected. Rarely, some people will develop a fever or allergic reaction when given a monoclonal antibody.

What else should I know about these tests?

• The amount of radiation exposure from a nuclear scan is about the same as that from standard x-rays. The scanner itself does not put out radiation. The body gets rid of the radionuclide used for the test within a few hours or a few days. Talk to your health care team about having sex, or being close to children or pregnant women during this time.
• You will be asked to drink a lot of water to flush out any of the radioactive material that is not absorbed.
• To reduce the risk of exposure to radioactive material in your urine after a scan, you should flush the toilet as soon as you use it.
• Nuclear scans are rarely recommended for pregnant women, so let your doctor know if you are or might be pregnant.
• If you are breast-feeding, be sure to tell the doctor ahead of time. You may need to pump breast milk and discard it until the radionuclide is gone from your system.