Positron Emission Tomography (PET) Imaging at the UCI Neuroimaging 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 - A 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, the High Resolution Research Tomography (HRRT), offers the highest resolution PET images available (~2mm isotropic resolution). UCI's scanner is one of only sixteen in the world, and five in the U.S. It has more than 120,000 LSO/LYSO detectors arranged in dual layers to enhance resolution throughout the entire brain image. This enhanced resolution allows movement, emotion, and reward circuits to be distinguished from each other. This is not possible with other scanners, including the newest PET-CT scanners.
The radioactive isotopes must be manufactured by a sophisticated machine called a cyclotron. Each of the three radioactive isotopes produced by the medical cyclotron has a short half-life, 109 minutes. Once produced, it is immediately administered to the patient. In 1988, the Neuroscience Imaging Center bought its own cyclotron and until 2000 produced the radioisotopes required to serve both clinical and research needs.
The PET Scan
After follwing Standard Operating Procedures, 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 affect the scan.
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.