|Fig. 1: PET Scanner (Source: Wikimedia Commons.)|
A Positron Emission Tomography (PET) Scan is a medical imaging technique that uses radioactive tracers to construct a 3D representation of the body's functional activity.  The first large-scale PET scanner was developed in the 1950s by neurosurgeon William Sweet and physicist Gordon Bowell. [1,2] Sweet and Bowell's PET scanner was used to detect brain tumors with sodium iodine, but now there are many uses for PET imaging.  Currently, PET scans are used for diagnosis, management, and treatment of conditions such as epilepsy, Parkinson's disease, dementia, and various cancers. 
Patients who are undergoing a PET scan first receive radioactive tracers through an intravenous injection.  The injected tracers consist of a positron-emitting isotope attached to a biological carrier molecule, which can interact with proteins or sugars in the body.  The biological carrier molecule used, depends on what the doctor or researcher is looking for. For instance, the sugar fluorodeoxyglucose (FDG) is often used as the carrier molecule during PET scans that examine cancer cells.  After a few minutes, the radioactive isotope builds up in regions of the body where there is an affinity for the carrier molecule.  The isotope then begins to decay, releasing positrons (positive electrons) that interact with surrounding electrons. This collision results in the complete destruction of the positron and the electron (electron-positron annihilation), and the release of 2 photons in opposite directions.  The PET scanner then detects these gamma-rays and creates a 3-D image that indicates where the radio-tracer accumulated in the body.  It is now a very common practice to combine PET scans with CT scans in order to provide patients with both metabolic and anatomical data related to their condition. [2,4]
Although PET scans have various potential applications in the medical field, oncology accounts for about 90% of the PET scans currently performed.  If a doctor suspects new cancer or cancer metastasis in a patient, they will use FMG (a glucose analog) as the biological carrier molecule for a PET scan. [3,4] FMG can be used to detect tumors because these fast growing cells use high levels of glucose for their metabolism. Thus, if a PET scan shows that a tissue is using a lot of FMG it could indicate the presence of a cancerous tumor. 
© Zoe White. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 L. Portnow, D. Vaillancourt, and M. Okun, "The History of Cerebral PET Scanning: From Physiology to Cutting-Edge Technology," Neurology 81, 1275 (2013).
 G. Bernal, "History of PET Scanners," Physics 241, Stanford University, Winter 2014.
 M. Analoui, J. D. Bronzino, and D. R. Peterson, Medical Imaging: Principles and Practices, 1st Ed. (CRC Press, 2012).
 G. Krishnamurthi, "Positron Emission Tomography: Functions and Uses," Physics 241, Stanford University, Winter 2015.