PET/CT > Physics > Positron Emission

Positron Emission

Positron-emitting radionuclides are typically produced in cyclotrons by the bombardment of stable elements with protons, deuterons, or helium nuclei. The radionuclides produced have an excess of protons and thus decay by the emission of positrons. The positron-emitting radionuclide used for clinical whole-body PET/CT imaging is Fluorine-18-deoxyglucose (FDG).

In the FDG decay process, positrons are emitted. When a positron is emitted it travels for a short distance from the site of origin (on the order of 1-3 millimeters) gradually losing energy to the tissue through which it passes. When most of the positrons kinetic energy has been lost, it undergoes a process called annihilation. In annihilation, the positron reacts with an electron in the immediate area and the result is the emission of two very high energy (511 keV each) photons. The two 511 keV photons are emitted in opposite directions at approximately 180 degrees from each other. These two photons interact with the PET detector ring at near opposite sites which define a line within the body along which the annihilation occurred. With computer processing, this line between the two emitted photons permits fairly precise localization of the annihilation reaction and thus defines a tissue site in the body where the positron emission occurred (i.e. an area of FDG activity).

The fact that the positron travels a short distance in tissue before the annihilation reaction occurs results in some degree of uncertainty about the exact location of its origin. In addition, the annihilation photons may actually be emitted at angles slightly different than the theoretical 180 degrees (up to 0.25 degree variation greater or less than 180).These limitations contribute to the inherent degradation of spatial resolution in all PET detector systems. However, even given these limitations, the spatial resolution in PET imaging is still superior to that seen in imaging with a gamma camera for other nuclear medicine examinations. Spatial resolution with PET imaging is on the order of millimeters compared to centimeters with standard gamma camera imaging.