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Positron Emission Tomography (PET)

Introduction

Positron emission tomography (PET) is a highly sensitive non-invasive, 3D medical imaging technique that measures biochemical function in living subjects. PET is often merged with X-ray CT or magnetic resonance imaging (MRI) so the functional information can be overlaid onto a high-resolution anatomical reference. To perform a PET scan, the subject is injected with a tiny amount of a short-lived radioactive tracer. This tracer is an analog of a biological molecule that is labeled with a positron-emitting isotope such as fluorine-18, nitrogen-13, carbon-11, oxygen-15, gallium-68, copper-64, etc. PET can be used to monitor metabolic rate, cell proliferation, gene expression, density of receptors or cell surface markers, and many other processes, depending on what type of tracer is injected.

The injected tracer circulates throughout the body, and accumulate in certain cells/tissues depending on the biochemistry of the tracer. When the positron-emitting isotope decays, a positron is emitted which travels a tiny distance through nearby tissue and combines with an electron, releasing two opposing gamma rays as a result of an electron-positron annihilation. It is these gamma rays that are detected by the PET scanner.  Because gamma rays can easily penetrate body tissues, PET is applicable in all situations from  cells to small animals to humans.

Applications

PET has applications in areas ranging from basic scientific research to preclinical  and clinical research to patient care. The following are some example applications of PET imaging:
  • diagnosis and staging of cancer
  • monitoring response (e.g. of tumor) to therapy
  • searching for cancer metastases
  • measuring the density of brain receptors
  • diagnosis of neurological disorders (can distinguish among several conditions that cause cognitive decline)
  • measuring the density of cell surface markers (to predict response to cancer therapy)
  • measuring the activity of an enzyme (to predict response to a drug)
  • tracking the fate of a drug throughout the body after injection
  • monitoring plant metabolism (for biofuel or environmental remediation research)

Suggested Resources

The following videos give a good introduction to PET and the PET scan process:
  • Ken Rikard-Bell, The PET Scan - A Beginner's Guide (YouTube link)
  • ANSTO, PET Scan animation (YouTube link)
  • NIBIB, How Does a PET Scan Work? (YouTube link)
  • NCRI Cancer Conference,  Patient information on PET scans in cancer clinical trials (YouTube link)
The following articles and papers give a good overview of PET, its history, and its current applications:
  • Wikipedia entry on Positron Emission Tomography (Link)
  • S. S. Gambhir, Molecular imaging of cancer with positron emission tomography, Nature Reviews Cancer 2: 683-693, 2002. doi:10.1038/nrc882. (Journal Link)
  • Czernin, J., Phelps, M.E., 2002. Positron emission tomography scanning: current and future applications. Annu. Rev. Med. 53, 89–112. doi:10.1146/annurev.med.53.082901.104028 (Journal Link)