A positron is a positively charged electron (B+) that is emitted from the nucleus of some radioisotopes that are unstable due to an excessive number of protons and a positive charge.
When the nucleus under goes positron decay, a positron and a neutrino are emitted. The result is a new nuclide with one fewer proton and one more neutron.
The emitted positron moves a short distance and annihilates with an electron. The annihilation reaction
results in two 511 keV gamma rays emmitted in coincidence 180 degress from one another.
For a PET scan, a positron-decaying radiopharmaceutical is injected into the body.
When the patient is positioned in the PET camera, the scintillator picks up the emitted gamma rays. The emission is traced back to where the annihilation happens in the body. Computer reconstruction creates images, based upon these emitted gamma rays.
There are several radiopharmaceuticals that are currently used for PET imaging. The PET radiopharmaceuticals approved by Medicare are listed in Table A.
TABLE A: Radiophamaceuticals Used in PET Imaging
|Radiopharmaceuticals||Half Life||Physiologic Parameter||Source||Applications|
|13 NH 3 (Ammonia)||10 min||Blood flow||Cyclotron||Cardiology|
|82 RbCl (Rubidium)||76 sec||Blood Flow||Generator||Cardiology|
|F 18 FDG||109 min||Metabolism||Cyclotron||Oncology