carbon 11 used in medical imaging? why is it used and what for?

C-11 is radioactive, and emits positrons. So it makes a good source for positron emission tomography (PET) scans. It has a half-life of only 20.3 minutes, so you don't have the problem of getting rid of long lived radioactive material.

The C-11 is used to make a compound called [C-11]DASB. This compound binds to the serotonin transporters in the brain. The PET machine can detect the special type of radioactivity from the C-11, so the doctors can get a map of the serotonin activity.

One of the tricks is that the C-11 and [C-11]DASB has to be made on the spot, about an hour before you drink the compound for the PET scan.