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---------- REPLY AT END > Actually, it is quite similar to an MRI. BUT, I had to be motionless for 2.5 > HOURS, my head was TIED down, and they inject a chemical into a vein in one > arm while taking blood out of an artery on the other. This is also a > continuous process. > > And all of this happens AFTER you've been without food or medication for over > 12 hours. > > I had severe diarrhea and dizziness after it was over. NOT a fun-filled day. >Jim Finn REPLY I have had 2 PET scans at Johns Hopkins in Baltimore. The PET scan is a bit slow compared to a high power MRI machine. The 'chemical" they inject can be flourodopa or any one of several others or combinations. The "chemical" has many of its atoms treated with a cyclotron [atom smasher] to change the isotope of some element to a highly unstable one which will emit gamma rays detectable by the PET scan detectors. The studies I participated in used the carbon atom [normally carbon 12] changed to carbon 11...This is usually written with the "11" as a superscript IN FRONT of the C for carbon. The carbon 11 used in my study had a half life of about 22 minutes. This means that one-half of the "chemical" has been used up in producing gamma rays in 22 minutes. In 44 minutes 75% is used up, in 66 minutes 87.5% has been used up, etc. The amount of gamma rays sent out per time period is falling... The initial gamma ray count per second drops 50% in the 1st 22 minutes, drops to 50% of the 22 minute rate at the end of 44 minutes, etc. In theory I am still giving off gama rays fom my first PET scan which was over three years ago. [like to story of walking 1/2 of the distance to an object every minute.. you never get all the way there but you get very very close]. There is a big problem with the detectors. There are lots of gamma rays around us all the time,but the detectors want to count only the ones from the "chemical." THe gamma rays they want to count come from very tiny explosions when the atoms are reverting from one stage to another. The explosions are in some ways like a shot gun. [With a shot gun, the explosion inside the shotgun shell sends projectiles out one end of the barrel. There is a "kick" from the gun in the opposite direction.] The tiny explosions in the "chemical" send out two gamma rays with each explosion. The gamma rays are going in opposite directions [remember the equal and opposite direction we learned in science so many years ago?] The PET detectors are set up in pairs, one on either side of the location to be studdied. The PET computer does not count a gamma ray particle unless there are two at exactly the same time moving in opposite directions, thus leaving out random ambient radiation. Knowing something about what is going on makes the experience more interesting. Knowing why makes it more of an adventure. The "taking out of the other arm" part is to measure general concentration in the body as opposed to the particular part being studied. The operators at Johns Hopkins gave me a choice classical, soft rock, or elevator music. I chose the classical. Will Johnston Jim Finn