|Fig. 1: This is a typical PET facility composed of a bed and a PET Scanner. (Source: Wikimedia Commons).|
There are a myriad of imaging techniques in the medical field. These imaging techniques are useful because they are non-invasive techniques which allow physicians a look inside the body to better treat and diagnose patients. Nuclear medicine is the use of radioactivc tracers generally called radiopharmaceuticals in the study of global and regional function, bloodflow, metabolism and morphology of an organ.  Nuclear medical imaging is a branch of medical imaging which utilizes small amounts of radioactive material to diagnose conditions such as a multitude of cancers, heart and neurological conditions along with other abnormalities. Positron Emission Tomography or PET is noteworthy and the most common technique for nuclear medical imaging. PET is ideally suited for monitoring cell/molecular events early in the course of a disease, as well as during pharmacological or radiation therapy.  Positron emission tomography is a result of the creative genius of theoretical and experimental physicists, chemists, biologists, and physicians who did not initially foresee the great benefits the new technology would eventually provide. The origin of contemporary Positron Emission Tomography is in 1948 when Kety and Schmidt first used the nonradioactive tracer, nitrous oxide, to measure cerebral blood flow. Since then, PET has revolutionized medical imaging allowing physicians to better diagnose and analyze many conditions. 
Injected radiolabelled molecular probes (tracers) are used to map out the underlying biochemistry. These tracers help construct 3D images of the anatomy and help understand the underlying biochemistry and physiology. [4,5] This process works by first accelerating charged particles to create relatively short-lived positron-emitting isotopes using a cyclotron. These isotopes are then coupled to a molecule of interest to produce the molecular probe (tracer).  This tracer molecule is then synthesized and intravenously injected in non-pharmacological doses into a subject of interest.  The positron emitter decays by emitting a positron from its nucleus. The positron loses energy and eventually annihilates with a nearby electron to produce two 511,00 eV γ rays emitted in directions 180 degrees apart.  The PET scanner, as seen in Fig. 1, can detect the γ rays in pairs, and images can be reconstructed showing the locations and concentration of the tracer of interest. [2,4,5]
|Fig. 2: Transaxial slice of a male brain taken with Positron Emission Tomography. Patient was injected with a dose of 282 MBq of 18F-FDG and the image was generated from a 20 minutes measurement. Red regions depict great radiotracer concentration (approximately 4 × 104 γ rays per second emitted from each cubic centimeter of brain tissue). Blue regions are regions with not too much radiotracer. (Source: Wikimedia Commons).|
PET is used for advanced images of the body to better analyze cancer, cardiac conditions and brain disorders.  Unlike other methods of medical imaging, PET scans are effective to treat conditions because they can show the condition at a cellular level and help physicians understand and track how diseases metabolize.  In oncology, fluorine-18 (F-18) fluorodeoxyglucose (FDG) is the most common scanning method used. In Fig. 2, the different color regions depict different levels on radiotracer concentration which allows physicians to see how the radiotracer distributes using the brain's biology. In typical body scans, injections of F-18 FDG are injected in doses of 200 to 400 Megabecquerel (MBq) with a half-life of 110 minutes.  Clinical PET imaging is being used in three important areas of clinical diagnosis and management: Cancer diagnosis and management, Cardiology and cardiac surgery and Neurology and psychiatry. PET results alter management in a significant way in more than 25% of patients, with some as high as 40%.  Examples include changing decisions on surgical treatment for non-small cell lung cancer (both avoiding inappropriate surgery and enabling potentially curative resection), the staging and treatment of lymphoma, decisions on surgical resections for metastatic colo-rectal cancer, referral for revascularisation of high-risk coronary artery disease (CAD) patients, neuropsychiatric conditions and many others.  PET has become a staple of medical imaging and demonstrates the value which nuclear medicine holds.
Nuclear medicine is not only about imaging. It is about detecting the ever changing distribution of a radioactive substance in the body measured in quantitative terms, in relation to time. PET excels in this as it is able to provide images of the metabolic distribution of radiotracers in real time. PET allows physicians to study the progress of many conditions and has led to significant findings in the fields of oncology, cardiology and neurology. [7,8] At the moment PET is often combined with non-nuclear methods of medical imaging such as CT scans. The future of PET is going to be largely fused with exploring the possibilities of PET/CT, developing new tracers to target specifically biological properties of cancer cells, exploring in vivo gene expression using PET and further development on microPET to image genetic engineering and disease cell transplants in mice. The future of nuclear medicine is bright and it is evident that PET will continue to propel medicine forward and serve as a top method for medical imaging. 
© Emanuel Pinilla. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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.
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