Radionuclide scanning uses the radiation released by radionuclides (called nuclear decay) to produce images. A radionuclide is an unstable isotope that becomes more stable by releasing energy as radiation. This radiation can include gamma-ray photons or particulate emission (such as positrons, used in PET). Radiation produced by radionuclides may be used for imaging or for treatment of certain disorders (eg, thyroid disorders).

A radionuclide, usually technetium-99m, is combined with different stable, metabolically active compounds to form a radiopharmaceutical that localizes to a particular anatomic or diseased structure (target tissue). The radiopharmaceutical is given by mouth or by injection. After the radionuclide has had time to reach the target tissue, images are taken with a gamma camera. Gamma rays emitted by the radionuclide interact with scintillation crystals in the camera, creating light photons that are converted into electrical signals by photomultiplier tubes. A computer summarizes and analyzes the signals and integrates them into 2-dimensional images. However, only signals near the camera's face can be accurately analyzed; thus, imaging is limited by the range of the camera.

Portable gamma cameras can provide radionuclide imaging at bedside. Generally, radionuclide scanning is considered safe (see Table 1: Principles of Radiologic Imaging: Typical Radiation Doses*).

Uses

The compound labeled with the radionuclide depends on the target tissue or indication:

• For imaging the skeleton, technetium-99m is combined with diphosphonate and used to check for bone metastasis or infection.
• For identifying inflammation, WBCs are labeled and used to identify inflammation.
• For localizing GI bleeding, RBCs are labeled.
• For imaging the liver, spleen, or bone marrow, sulfur colloid is labeled.
• For imaging the biliary tract, iminodiacetic acid derivatives are labeled and used to check for biliary obstruction, bile leaks, and gallbladder disorders.

Radionuclide scanning is also used to image the thyroid gland and the cerebrovascular, cardiovascular, respiratory, and GU systems. For example, in myocardial perfusion imaging, heart tissue takes up radionuclides (eg, thallium) in proportion to perfusion. This technique can be combined with stress testing. Radionuclide scanning is also used to evaluate tumors.

Variations

Single-photon emission CT (SPECT)

SPECT uses a gamma camera that rotates around the patient. The resultant series of images are reconstructed by computer into 2-dimensional tomographic slices in a similar manner to that done in conventional CT. The 2-dimensional images can be used for tomographic reconstruction to yield a 3-dimensional image.

Disadvantages

Radiation exposure depends on the radionuclide and dose used. Effective doses tend to range from 1.5 to 17 mSv—eg, about 1.5 mSv for lung scans, about 3.5 to 4.5 mSv for bone and hepatobiliary scans, and about 17 mSv for a technetium sestimibi heart scans. Reactions to radionuclides are rare.

The area that can be imaged accurately is limited because only signals near the gamma camera's face can be accurately localized. Image detail may also be limited.

Often, imaging must be delayed for up to several hours to give the radionuclide time to reach the target tissue.

Positron Emission Tomography

PET, a type of radionuclide scanning, uses compounds containing radionuclides that decay by releasing a positron (the positively charged antimatter equivalent of an electron). The released positron combines with an electron and produces 2 photons whose paths are 180 ° apart. Ring detector systems encircling the positron-emitting source simultaneously detect the 2 photons to localize the source. Because PET incorporates positron-emitting radionuclides into metabolically active compounds, it can provide information about tissue function.

Fluorine-18 [¹⁸F]–labeled deoxyglucose (FDG) is used most commonly in clinical PET. FDG is an analog of glucose, and its uptake is proportional to glucose metabolic rates. A patient's relative glucose metabolic rate (called the standardized uptake value [SUV]) is calculated: The amount of FDG taken up from the injected dose is divided by the patient's body weight.

Uses

PET has several clinical indications, such as

• Cancer (eg, staging and evaluating specific types of cancer and evaluating response to treatment), which accounts for about 80% of PET usage
• Cardiac function (eg, evaluating myocardial viability, detecting hibernating myocardium)
• Neurologic function (eg, evaluation of dementia and seizures)

PET applications continue to be investigated, although it is important to determine which applications are reimbursable.

Variations

PET-CT

PET-CT

Functional information provided by PET is superimposed on anatomic information provided by CT.

Disadvantages

The typical effective radiation dose during PET is about 7 mSv. The effective radiation dose with PET-CT is 5 to 18 mSv.

Production of FDG requires a cyclotron. FDG has a short half-life (110 min); thus, shipment from the manufacturer and completion of the scan must occur very rapidly. The resulting expense, inconvenience, and impracticality greatly limit the availability of PET.

Last full review/revision July 2008 by Jon A. Jacobson, MD