Wednesday, August 16, 2006

Radioactive man

A few folks are curious to know more about the PET scan, so let me use my wonderkiller powers for good instead of evil for a moment and try to explain what went on.

The injection I was given was fluorodeoxyglucose -- a mix of glucose and the isotope flourine-18. FL-18 is what's known in the trade as a poitron emitter, meaning that the particle it gives off (and thats what makes something radioactive, that it emits some type of sub-atomic particle) is a positron. These are just like electrons, except they have a positive charge, rather than a negative charge, as you may remember from high school chemistry.

These positrons wander around for a few millimeters before meeting an electron and anihilating each other. The result of this little fracas are two gamma ray photons. One of the immutable laws of physics is that on some scale, all forces and energies must balance out to zero. Since the electron was roughly stationary in space, the two emitted photons move off in 180 degree opposite directions, thus having a net momentum of zero.

The PET scanner itself then, is a ring-shaped detector that senses these gamma-ray photons. When it sees two of them 180 degrees apart at the same time, it assumes they are a result of the Fluorine-18 decaying, and that the location of this FL-18 must be somewhere along the line connecting those two gamma rays. Draw a whole bunch of those lines, and voila! You have localized the fluorine in 3-dimensional space.

It is worth noting two things here. First, fluorine-18 has a half-life of only 110 minutes, so it must be made fresh, and nearby. In fact, there is apparently a facility in Sterling, VA, where they have a cyclotron and a bunch of guys with a "time to make the donuts" job who get up early each morning to whip up a batch of this stuff and messenger it out to local radiology clinics.

And we can't talk about radiology and cyclotrons without sharing the story of Albert Swank, Jr, who is trying to run a cyclotron out of his garage in Anchorage, AK. Nuclear medicine is fun, yo!


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