Positron Emission Tomography

A PET scanner uses medical tracers that emit positrons to generate an image of the activity and tissues in the body.

The Scanner:

The Scanner contains gamma cameras used to detect gamma photons emitted when positrons emitted from the medical tracer annihilate electrons. It works just like a CAT scan and rotates around the body producing image slices of the body which can be pieced together to produce a three dimensional image.

The Medical Tracer:

Specific isotopes need to be used with a small half life and emit positrons, for this reason Fluorine-18 is often used as it has a half life of ~110 minutes and releases a positron and gamma photon when it decays. To make the medical tracer the Fluorine-18 atom is attached to glucose in place of an oxygen atom to form (FDG). The FDG can be injected into the body and monitored using the gamma cameras, the FDG accumulates in tissues that respirate a lot e.g. cancer.

When the Fluorine-18 decomposes, it releases a gamma photon and a positron and turns into oxygen-18, the oxygen and the gamma photon are no further interest in the reaction, the positron goes on to react with an electron within 1mm in an annihilation reaction. The result of the annihilation reaction is two gamma photons that travel in opposite directions, the location of the annihilation reaction can be determined by the difference in the arrival times of the photons at the opposite scanners. The concentrations of the signals can be used to create an image of different brightnesses and colours.

Advantages and Disadvantages:

Non-Invasive, Monitor Drugs, Good Diagnosis Tool, Very Expensive, Ionising Radiation, Can’t be used regularly.