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Positron Emission Tomography (PET) imaging at the UCI Brain Imaging Center

The key to understanding and treating brain disorders is to obtain accurate information about how the brain functions.

** What is PET?

PET stands for Positron Emission Tomography

Positron, type of radioactive particle
Emission, from the radioactive atom diagram of crystals
Tomography, an x-ray photograph of a plane of the body. In our case, the plane of the body and area of interest is a "slice" of the brain.

Our current PET scanner, also called a positron camera, cost $1.3 million. It has 2048 detecting crystals in the ring of the dome of the camera and a resolution of 4+mm, one of the best in the world. It is a head scanner only and is not used for other parts of the body.

The radioactive isotopes must be manufactured by a sophisticated machine called a cyclotron. Each of the four radioactive isotopes produced by the medical cyclotron on our premises has a short half-life, from 2 - 120 minutes. Once produced, it is immediately administered to the patient. In 1988, the Brain Imaging Center bought its own cyclotron and until 2000 produced the radioisotopes required to serve both clinical and research needs. The cyclotron, which continues to produce radioisotopes, is now operated through a collaborative agreement with PETNet, a private firm.

The PET scan

The subject's head is cradled by a mask taped to the head-rest of the scanner bed. The scanner bed movement is precision controlled, giving extreme accuracy in the placement of the subject. Blood samples are taken during the scan for evaluation and testing, and all data are fed into the computers.

For a Fluor-deoxy-glucose (FDG) scan, the injected radioactive isotope is "fixed" in the areas of the subject's brain activated during the cognitive task they have just completed in the uptake room. For other types of scans, the scan procedure sequence may be different.

As the radioactive isotope disintegrates, positrons are emitted. The positrons travel a short distance in the brain before striking a nearby electron. When this collision occurs, two gamma rays are simultaneously produced and travel away from each other at 180 degrees apart, toward the edges of the ring in the dome of the camera. Each time two detectors detect a gamma ray simultaneously, the annihilation is recorded. The scanner will acquire images of fifteen slices of the brain at once. Two sets of images are normally taken in 40 minutes (20 minutes each) yielding thirty slices of the brain.

The scan itself is not painful. In unusual circumstances sedatives can be given to calm the subject. Further cognitive activity or sedatives administered after the assigned cognitive task is completed do not effect the scan. The radioactive isotope is "fixed" in the cell where it was metabolized through the first stage of metabolism.

How a PET Scan Works

The images made with the PET scanner are produced by recording areas of the brain tagged with radioactive isotopes. Intravenously administered to the patient, the isotope is absorbed by the active areas of the brain. As the isotope decays, it emits low-level radiation that is detected by a ring of crystal detectors within the PET scanner.

Sophisticated electronic and computer software programs record the number of events in each section of the brain and calculate the metabolic rates of each section. This information is compared numerically to findings drawn from a control group. Any rates more than two standard deviations above or below the mean are marked on the report by a plus or minus sign (or an asterisk), and are considered abnormal. Using this numerical scale the computer develops a colored picture of the brain by assigning various colors to indicate the level of cognitive activity in each measured section of the brain. Red is assigned to the highest level of activity and purple to the lowest.

Once the positron camera has collected all of the brain activity data from the subject, the PET scan is complete. A computer then reconstructs the images from the raw data into the colored brain images you see on brochures and posters and in the pictures below.

The red areas in the pictures below indicate high levels of activity. Blue or black areas indicate little or no activity in those brain structures. Colors between red and black indicate various levels of activity. By superimposing the PET scan over an MRI scan, researchers can determine the level of activity in specific structures of the brain.

Should I Be Concerned about Radiation Exposure?

* Radiation exposure originates from many sources including the sun. It increases with exposure such as on a beach or at high altitudes or flying in an airplane. The amount of radiation used in PET scanning is low and is called a "tracer dose." The tracer dose is about the same amount of radiation as 3 chest x-ray series. Whole body exposure is about the same as the lungs are exposed to by smoking one package of cigarettes.

* The Journal of Nuclear Medicine Volume 23:613-617, 1982. Letter to Docents from William G. Nabor, Health Physicist, UCI, dated January 23, 1991.

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