17-02-2011, 09:45 AM
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What is a PET Scan?
Definition:
A Positron emission tomography (PET) scan is a nuclear medical imaging technique which produces a three-dimensional image of functional processes in the body
History of the P.E.T scan
• The concept of emission and transmission tomography was introduced by David E. Kuhl and Roy Edwards in the late 1950s at the University of Pennsylvania.
• In the 1970s, Tatsuo Ido at the Brookhaven National Laboratory was the first to describe the synthesis of 18F- FDG, the most commonly used PET scanning isotope carrier.
• As you can see there is not one person who developed the PET scan but a whole collection of people have made what it is today.
How it works
1. A short-lived radioactive tracer isotope, is injected into the living subject (usually into blood circulation). The tracer is chemically incorporated into a biologically active molecule.
2. There is a waiting period while the active molecule becomes concentrated in tissues of interest.
3. As the radioisotope undergoes positron emission decay (also known as positive beta decay), it emits a positron, an antiparticle of the electron with opposite charge.
4. After traveling up to a few millimeters the positron encounters an electron.
5. The encounter annihilates them both, producing a pair of (gamma) photons moving in opposite directions.
6. These are detected when they reach a scintillator in the scanning device, creating a burst of light which is detected by photomultiplier tubes.
7. The technicians can then create an image of the parts of your brain, for example which are overactive.
The Tracer
• The radioisotopes used in PET scans are isotopes of carbon, nitrogen, oxygen and 18F used as a substitute for hydrogen.
• These are the only radioactive forms of natural elements that will pass safely through your body and be detected by the scanner.
• The type of tracer used depends on what your doctor wants to measure. For example, if your doctor is looking at a tumor, he might use radio labeled glucose (FDG) and watch how it is metabolized by the tumor.
Comparison with a MRI scan
• When people go in for a ‘scan’ it is most likely a Magnetic Resonance Imaging scan.
So what’s the difference between the two scans?
• A MRI scan does not use nuclear medicine, it creates an image using magnets.
• A MRI scan can assess the size and shape of body organs and tissue. However, they cannot assess function. A PET scan looks at function.
• In other words, MRI scans tell you what something looks like, while a PET scan can tell you how it is working.