Standardized phantoms exist for X-ray machines, CT scanners, and other imaging modalities to calibrate devices regularly. X-ray phantoms, for example, absorb X-rays in predictable ways, consisting of materials that mimic how the human body attenuates such electromagnetic radiation. But, making a calibration phantom for positron emission tomography (PET) scanners is very different from other phantoms because it has to replicate how radioactive tracers look like after being injected into a patient. Germanium is loaded into a special vessel and a PET scanner is used to observe the changing radioactive glow from day to day, providing a value against which to adjust the scanner. These exist, but there is no standard phantom that can be used in PET scanners from different manufacturers. Read more.