|Fig. 1: Positron Emission Tomography Machine (Source: Wikimedia Commons)|
Positron Emission Tomography (also known as PET) Scans are a type of nuclear medicine and imaging technology that are used to create a 3D image of certain metabolic functions in the body rather than the anatomy. The PET scan machine (shown in Fig. 1) detects gamma rays that are created when a positron, created from positron decay of a tracer molecule, and an electron annihilate each other.  3D images are then generated from the detection of these gamma rays in order to map where in the body the tracer is consumed.
Today, PET scans are used along side CT scans to create PET-CT scans, that can be used to see anatomical structure as well as imaging of different biological functions. PET scans have had huge uses in both oncology as well as neurology, where cancer and brain cell metabolism can be mapped for study.
This paper will explore how PET scans work, and what they are used for in today's medicine.
|Fig. 2: PET Scan Process (Source: Wikimedia Commons)|
PET scans register the gamma rays emitted from positron-electron annihilation. These positrons are given off by a radioactive isotope during positron decay. The scan process starts with the patient being injected with some type of radioactive tracer that has been attached to some metabolized molecule, such as glucose.  Often the sugar used is fluorodeoxyglucose (FDG), where the F-18 molecule decays into O-18. Because glucose is the molecule that is used, PET scans will give a metabolic diagram of wherever glucose is being consumed in the body.
When the radioactive molecule undergoes decay, it emits a positron through beta decay. That positron is then very quickly annihilated by an electron, and this will emit two gamma rays that move in approximately opposite directions.  These pairs can then be picked up with the machine and translated by the computer into an image. This whole process can be seen in Fig. 2.
PET scans are increasingly being used alongside CT or MRI scans in order to give a diagram of a patient that contains both anatomical as well as metabolic data. This allows for more precise locating of where specific metabolic processes are happening in the body, which is great in detecting cancer metastasis or surgical locations for example.
|Fig. 3: PET Scan of the Human Brain (Source: Wikimedia Commons)|
The primary use of PET scans is for oncology. This is because FDG is heavily used in cells with high metabolism, such as liver, brain, and cancer cells. PET scans are used in the diagnosis and treatment of cancers, especially lymphomas and lung cancer. It can also be used in surgery, to see if a primary tumor has been completely removed. Oncology accounts for approximately 90% of current PET scan usage.  PET scans also are used in neurology, especially to show neuroactivity. PET scans follow the assumption that high glucose metabolism is because of high brain activity, and thus active parts of the brain can be mapped. Fig. 3 shows an example of a PET scan used for a human brain. As seen here, the red areas are where there is a large amount of FDG being taken up and used, thus showing where there is high neuron activity.
© Gautam Krishnamurthi. 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.
 E. Lin and A. Alavi, PET and PET/CT: A Clinical Guide, 2nd Ed. (Thieme Medical, 2009).