Very Quick Introduction to PET Go to http://en.wikipedia.org/wiki/Positron_emission_tomography Look at the picture of a PET scanner (appears to be a Siemens machine) Injection of radio-isotope. A radioactive tracer isotope which decays by emitting a positron, chemically combined with a metabolically active molecule, is injected into the living subject (usually into blood circulation). There is a waiting period while the metabolically active molecule (usually a sugar) becomes concentrated in tissues of interest, then the subject is placed in the imaging scanner. It happens that only short-lived (< 2 hours) isotopes produce positrons, so a cyclotron must be nearby to produce the isotopes. The most commonly used isotope is F-18, which is substituted for one hydroxyl group (OH) in an ordinary glucose molecule to label flourodeoxyglucose (FDG), which is a sugar that can be metabolized. Positron emission/detection. Look at the picture captioned “Schema of a PET acquisition process”. The short-lived isotope decays, emitting a positron. After traveling up to a few millimeters the positron annihilates with an electron, producing a pair of gamma ray photons moving in opposite directions. These are detected when they reach a scintillator material in the scanning device, creating a burst of light which is detected by photomultiplier tubes. The technique depends on simultaneous or coincident detection of the pair of photons: photons which do not arrive in pairs (i.e., within a few nanoseconds) are ignored. By measuring where the gamma rays end up, their “line of response” (LOR) in the body can be plotted. Image reconstruction. The gamma ray count for each LOR is equivalent to the projection in X-ray CT, and similar reconstruction techniques can be used to produce an image—including filtered back projection. The problem is more difficult than CTreconstruction because there are two factors affecting the projections—amount of positron-emitter on the line and amount of absorption between the emitters and the detector. Only the second factor is there for CT. To produce a good image it is necessary to obtain an absorption map, which is done by passing a radioactive source around the body. The reconstructed image is a map of the density of the isotope in the body, in the form of slice images separated by about 5mm. The resulting map shows the tissues in which the molecular probe has become concentrated, and is read by a nuclear medicine physician or radiologist, who can determine the chemical uptake or activity of certain parts of the body. This information is used for diagnosis. Resolution. Resolution is not so good in comparison to CT and MR—in-plane about 4.6 mm at the center of a slice and about 6mm in the tangential direction and 9 mm in the radial direction at the periphery. Slice thickness 3-5 mm. Matrix is typically 128 x 128. Registration with CT/MRI. PET scans are increasingly read alongside CT or magnetic resonance imaging (MRI) scans, the combination ("co-registration") giving both anatomic and metabolic information (i.e., what the structure is, and what it is doing biochemically). Because PET imaging is most useful in combination with anatomical imaging, such as CT, modern PET scanners are now available with integrated high-end multi-detector-row CT scanners. Because the two scans can be performed in immediate sequence during the same session, with the patient not changing position between the two types of scans, the two sets of images are more-precisely registered, so that areas of abnormality on the PET imaging can be more perfectly correlated with anatomy on the CT images. This is very useful in showing detailed views of moving organs or structures with higher amounts of anatomical variation, such as are more likely to occur outside the brain. An additional advantage is that the CT can be used to produce the absorption map for the PET reconstruction. PET is used heavily in clinical oncology (medical imaging of tumors and the search for metastases). It is also used in brain and heart research. However, with the recent decision of Medicare to cover PET scans for specific patients, there has been a recent trend of increase in clinical use of PET scans throughout the United States. [1] PET scans safety. PET scanning is invasive, in that radioactive material is injected into the subject. However the total dose of radiation is small, usually around 7 mSv. This can be compared to 2.2 mSv average annual background radiation in the UK, 0.02 mSv for a chest X-Ray, up to 8 mSv for a CT scan of the chest, 2-6 mSv per annum for aircrew, and 7.8 mSv per annum background exposure in Cornwall (Data from UK National Radiological Protection Board). The radioactive dose to the patient is small, however the dose to the operators is a limiting factor in the operation of a PET