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What is PET?
The name "PET" comes from Positron Emission Tomography. It is a new scanning technique in medical research. PET allows us, for the first time, to measure in detail the functioning of distinct areas of the human brain while the patient is comfortable, conscious and alert. We can now study the chemical process involved in the working of healthy or diseased human brains in a way previously impossible. Before the advent of the PET scanner, we could only infer what went on within the brain from post-mortems (dissections after death) or animal studies.

PET represents a new step forward in the way scientists and doctors look at the brain and how it functions. An X-ray or a CT scan shows only structural details within the brain. The PET scanner gives us a picture of the brain at work.

Positron: Antimatter equivalent of the electron
A positron is an anti-electron. Positrons are given off during the decay of the nuclei of specific radioisotopes. A type of radioactive fluorine produced at TRIUMF for the PET programme is a positron emitter. When matter collides with its corresponding antimatter, both are annihilated. When a positron meets an electron, the collision produces two gamma rays having the same energy, but going in opposite directions. The gamma rays leave the patient’s body and are detected by the PET scanner. The information is then fed into a computer to be converted into a complex picture of the patient’s working brain.

How does it work?
A conventional "X-ray" is taken by firing X-rays through a person and onto a film. This "shadow" image shows some structures in the body, such as cartilage and bone. A CT scanner uses fine streams of X-rays. By firing them through the body from several directions, the CT scanner is able to build up a composite picture of anatomical details within a "slice" through the person. Magnetic Resonance Imaging (MRI) does much the same thing, but using magnetic and radiowave fields. In contrast, the PET scanner utilizes radiation emitted from the patient to develop images. Each patient is given a minute amount of a radioactive pharmaceutical that closely resembles a natural substance used by the body. One example of such a pharmaceutical produced at TRIUMF is 2-fluoro-2-deoxy-D-glucose (FDG), which is similar to a naturally occurring sugar, glucose, with the addition of a radioactive fluorine atom. Gamma radiation produced from the positron-emitting fluorine is detected by the PET scanner and shows in fine detail the metabolism of glucose in the brain.

How does it feel to have a PET scan?
During a scan the patient reclines on a comfortable couch with his or her head inside the large, doughnut-shaped Positron Emission Tomograph. While the patient’s head must be kept very still, the only real discomfort involved may be the pinprick of a hypodermic needle as a minute amount of radiopharmaceutical is injected. The radiopharmaceutical could be administered as an intravenous injection or inhaled as a gas. How it is administered depends on the radiopharmaceutical. Which one is chosen depends on what function the scientist wants to study.

What does a PET scan show?
The brain function being studied during a PET scan determines which radiopharmaceutical is used. Oxygen-15 can be used to label oxygen gas for the study of oxygen metabolism, carbon monoxide for the study of blood volume, or water for the study of blood flow in the brain. Similarly, fluorine-18 is attached to a glucose molecule to produce FDG for use in the observation of the brain’s sugar metabolism. Many more PET radiopharmaceuticals exist, and research is under way to develop still more to assist in the exploration of the working human brain. For example, dopa, a chemical active in brain cells, is labelled with positron-emitting fluorine or carbon and applied in research on the communication between certain brain cells which are diseased, as in dystonia, Parkinson’s disease, or schizophrenia.

PET radioisotopes produced at TRIUMF
Labelling agent	Half-life
carbon-11	20.3 minutes
oxygen-15	2.03 minutes
fluorine-18	109.8 minutes
bromine-75	98.0 minutes

How much radiation does a patient get?
PET scans using radioactive fluorine in FDG would result in patients receiving exposures comparable to (or less than) those from other medical procedures, such as the taking of X-rays. Other scanning agents - for instance, 6-F-dopa or radioactive water - normally cause even less exposure.

More information on the PETTVI scanner

PET: Research tool of the future
The PET programme at TRIUMF and UBC is in a unique position among world medical research centres in having many of the expensive major facilities required to mount a powerful PET programme. These include the cyclotrons and radiochemistry laboratories at the TRIUMF project that are the source of the PET scanning agents. The laboratories are linked to the UBC Health Sciences Centre Hospital by the world’s longest "pea shooter", a 2.4 km pneumatic pipeline used for the delivery of PET scanning agents in the shortest possible time. These facilities have a capacity that has not been equaled by any other university/health sciences centre in the world.

For more information see the TRIUMF/UBC PET and PET Group at UBC pages.