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Pet Scans

Jason Neal
Physics 191
1 May 2001
Positron Emission Tomography

Positron Emission Tomography is a scanning technique that allows us to measure in detail the functioning of distinct areas of the human brain while the patient is comfortable, conscious and alert.   PET represents a type of functional imaging, unlike X-rays or CT scans, which show only structural details within the brain.   The differences between these types of imaging don't end there.    
In both X-rays and CT scans, a form of radiation is emitted and travels through the body, and a detector receives the unabsorbed rays and transmits them to a computer.   The physics behind PET scanning is quite different.   Basically, a person is injected with a radioactive substance.   This substance begins the process of radioactive decay inside of the person and interacts with the tissue to produce gamma radiation.   These gamma rays are detected by scintillation crystals and transmitted to a computer, where images are produced.   But how does this all take place?
The description of PET scans in detail requires the understanding of the radioactive substance injected into the subject.   First, a small amount of a biochemical substance is tagged with a positron-emitting radioisotope.   A positron is an "anti-electron."   Positrons are given off during the decay of the nuclei of the radioisotope.   When the positron emitted collides with an electron in the tissue of the subject, both the positron and the electron are annihilated.   When this happens, the collision produces two gamma rays having the same energy (511 KeV), but going in opposite directions.
These gamma rays, produced by the annihilation of a positron and an electron,   leave the patient's body and are detected by the PET scanner. The detection of positron-annihilation events forms the heart of any PET scanner. In most systems, the Gamma detector is a BGO (bismuth germinate oxide) crystal, a high-density scintillator.   When it is combined with high...