Re: What is the underlying principle of a PET scan machine.

A Positron Emission Tomography (PET) scan is a technique that is used both 
for medical diagnosis involving the detections of certain types of tumors 
and for study of the functioning the the brain and other organs. It is a 
type of Nuclear Medicine study (others are bone scans, lung perfusion 
scans, thyroid scans) that uses the radioactive isotope Fluorine-18 
attached to a glucose molecule. The combination is called 18-F-FDG, which 
stands for 18-Fluorine-fluorodeoxyglucose. The molecule is taken up by 
cells as glucose is, so it localizes most strongly in cells, such as 
tumors and the brain, that are metabolically active. The F-18 decays by 
emitting a positron, which can travel up to 2 mm before bumping into an 
electron and emitting two 0.5ll Mev photons.

PET Can produce very detailed images. It does this by taking advantage of 
the fact that when the positron and electron annihilate, the two photons 
produced travel away from each other at 180 degrees. The detector is in 
the shape of a donut with the patient in the center of the donut hole. 
Since each F-18 decay produces two produces two photons, since the photons 
travel 180 degrees away from each other, and since the two photons from a 
decay  strike the detector array virtually simultaneously, the detector: 
1) assumes that any two photons hitting within 30 nanoseconds of each 
other come from a single event (it is right 70% of the time in this 
assumption), 2) draws a line between the two photon "hits" in order to 
locate the site of emission somewhere along that line, and 3) repeats the 
procedure for a huge number of decays. The points where the greatest 
number of lines intersect are the areas of greatest cellular activity.

The F-18 has a half-life of 111 minutes. It is produced in a cyclotron 
(usually at a site within a few hours of the PET scanner), combined with 
glucose, and shipped to the PET. One shipment of 18-F-FDG can last a PET 
scanner an entire day; a large enough dose is delivered to provide for all 
scheduled patients when the half-life is taken into account.