PET is an acronym for “Positron Emission Tomography.” It refers to a technique for imaging the inside of a patient’s body using radiation emitted by special drugs called radiopharmaceuticals.
A radiopharmaceutical is a drug containing small amounts of radioactivity. Its radioactive atoms are attached to molecules designed to collect in certain body tissues and organs, depending on what the doctor wants to look at. A PET scanner detects the radiation emitted by radiopharmaceuticals injected in the patient, and the scanner uses this information to construct a 3D distribution of radioactivity in the patient.
PET imaging relies on a specific radioactive process called positron emission. In this process, a radioactive atom decays and emits a positively charged electron, or positron. After the positron travels only a short distance in the patient, it combines with an electron and the two particles annihilate each other, converting into two photons of equal energy (511 keV) traveling in opposite directions. The photons that escape the body can be detected at the same time by two detectors on opposite sides of the PET scanner.
Examples of PET images (colored) superimposed on MRI images (grayscale).
Thyroid cancer
University Hospital, Tuebingen, Germany
Breast cancer with bone metastases
mMR Munich (TUM/LMU)
Funded by the DFG
Liver cancer
mMR Munich (TUM/LMU)
Funded by the DFG
Images courtesy of Siemens Healthcare GmbH.
What is PET imaging used for?
PET scanners have better radiation sensitivity than other nuclear medicine imaging techniques. Advanced PET electronics can detect when two photons with the energy of 511 keV each have arrived at detectors at the same time, and these data are run through sophisticated computer algorithms to produce an image of where a radiopharmaceutical has concentrated in the patient’s body.
PET imaging has demonstrated significant value in diagnosing, describing, staging and monitoring the treatment response of diseases, especially cancer. PET/CT machines combine PET and CT scanners to achieve enhanced imaging accuracy: the CT data are used to correct the PET data for photon attenuation as well as to provide complementary images of the patient’s anatomy.