This is a PET scan you are talking about? Or is it a CAT scan? MRI? I
lost which test is being examined.
sol
Marshall Dudley wrote:
I believe they use an isotope of tecknesium (sp?). When atoms of this
isotope decay they produce a positron, which finds an electron and they
annilate each other. An electron and a positron both contain 511 MeV of rest
mass energy, so they will produce two gamma rays of 511 MeV that are directed
almost exactly directly away from each other. Sodium Iodide scintillating
material surrounds the patient, and then multiple phototubes look at the
sodium iodide. The configuration is such that the gamma is absorbed by the
sodium iodide, which produces a flash of light, and the phototubes record the
light and send the pulse to some pulse analysis hardware and computer. The
pulse analysis determines exactly where in the sodium iodide the flash
occured, and with two flashes on opposite sides, the computer figures out a
straight line between the two, where the original source of the positron will
be on this line. When another one occurs, if it occurs at the same spot, a
second line will be computed that will intersect the first line, and that
will be the spot that the activity is coming from. By analyzing tens of
thousands of lines, the computer can come up with a pretty good 3 d view of
where all the activity is.
It is not the positron that causes damage to material, after all the loss of
an electron is rather non consequential. It is the 511 MeV gamma rays, which
is an extremely hard x-ray, like cosmic rays. that can cause damage if
absorbed before they get out of the body.
Marshall
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