PET scanning is a gamma-based imaging technique that allows doctors to detect early signs of cancer, brain disorders and heart diseases by identifying changes in organs and tissues at cellular level. In a PET scan, the patient is injected with a radioactive substance and placed on a flat table that moves into a donut-shaped housing called gantry. 
The gantry of a PET system contains several gamma ray detectors that require very precise temperature control to one another in order to generate a proper image. If one detector fails it will have a significant impact on the final image result.