Nuclear Imaging: PET and SPECT Scans

PET Scans (Positron Emission Tomography) and SPECT Scans (Single Photon Emission Computed Tomography) were first used in the 1970's for research. Now, some 30 years later, these non-invasive techniques have been adapted to diagnose disease in humans.

As part of the family of nuclear imaging techniques, PET and SPECT scans use small amounts of radionuclides (radioactive isotopes) to measure cellular/tissue change. Radionuclides are absorbed by healthy tissue at a different rate than tissue undergoing a disease process. A deviation in normal rates of absorption may be an indication of abnormal metabolic activity, which could lead to structural change (e.g. vertebra). X-rays, CT Scans, and MRI can only image structure (e.g. anatomy), not function or metabolism.

PET Scans
Many physicians in fields including cardiology, neurology, and oncology use PET Scanning. A PET image can map the biological function of an organ, can detect subtle metabolic changes, determine if a disease is active or dormant, may be used to determine if a tumor is benign or malignant (malignant tumors have classic metabolic patterns), and may be used to stage certain types of cancer.

A PET Scan is an expensive test. PET facilities require sophisticated computer equipment, a cyclotron, and highly trained specialists. A cyclotron is a machine - an accelerator that propels charged particles (e.g. protons) using alternating voltage in a magnetic field.

The test begins with the injection of a radionuclide (tracer) specific to the function/metabolism to be investigated. Within a short period of time, the tracer collects in the specific body area. The patient lies comfortably on the scanning table, while a ring-shaped machine is properly positioned over the target body part. Detectors in the 350-degree ring pick up gamma rays emitted from internal body tissues. The computer analyzes this data to produce cross-sectional images on film and/or a video monitor. The images are often color coded according to the concentration of the tracer.

SPECT Scan
A SPECT Scan is capable of providing information about blood flow to tissue. It is a sensitive diagnostic tool used to detect stress fracture, spondylosis, infection (e.g. discitis), and tumor (e.g. osteoid osteoma). Analyzing blood flow to an organ (e.g. bone) may help to determine how well it is functioning.

Similar to a PET Scan, a radionuclide is injected intravenously. Tissues absorb the radionuclide as it is circulated in the blood. As a camera rotates around the patient, it picks ups photons, the radionuclide particles. This information is transferred to a computer that converts the data onto film. The images are vertical and/or horizontal cross-sections of the body part and can be rendered into 3-D format.

Conclusion
Women who are pregnant are not able to undergo PET or SPECT Scanning because of the radioactive isotopes used. The amount of radioactive material injected is minimal. It is quickly broken down by the body and passed within 48 hours. Temporarily increasing fluid intake for a day or two following the test can help facilitate elimination of the tracing substance.