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PET Imaging P ositron E mission T omography

PET Imaging P ositron E mission T omography. The Positron. Positron is an elementary particle Has same mass as electron The charge is equal but opposite to electron. +. -. Positron. Electron. Gamma 1. +. -. Gamma 2. Positron is an Anti-particle.

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PET Imaging P ositron E mission T omography

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  1. PET ImagingPositron Emission Tomography

  2. The Positron • Positron is an elementary particle • Has same mass as electron • The charge is equal but opposite to electron + - Positron Electron

  3. Gamma 1 + - Gamma 2 Positron is an Anti-particle • When a particle and an antiparticle interact they annihilate • Both particles are destroyed • Two photons(Gamma-rays) are created • Photons are emitted along a straight line (180 degrees angled)

  4. Image and its negative demonstrate the idea of an electron and a positron

  5. Photon Energy • Each photon has the energy of the particle that is destroyed • For positron-electron annihilation • 511 keV (for each gamma-ray)

  6. Where do positrons come from? • Cosmic rays • High energy photon interactions (pair production) • Radioactive decay • Radioactive material can be incorporated into radiopharmaceuticals

  7. Imaging Concepts

  8. In coincidence counting an event is ONLY registered if a signal is received from two detectors within a window. The smaller the window the more accurate the coincidence measurement. A few nanoseconds is usually used. Annihilation Detection Coincidence

  9. Where was the event? Coincidence

  10. Where was the event?

  11. PET Scanner • Ring (or rings) with lots of little detectors • Detector should be fast for accurate coincidence measurements

  12. PET scanner • PET scanners lack conventional collimation so they have a high geometric efficiency • They do have septal rings to reduce cross walk from ring to ring • With septal rings in 2D • Without septal rings 3D • 3D works best in head

  13. Septa

  14. PET camera components • Ring of crystals. • Photomultiplier tubes. • Pulse height analyzers. • Timing discriminators. • Coincidence circuits. • Septal rings.

  15. Main problems in PET • Attenuation correction. • Photon emission occurring at other than 180 degrees • Reconstruction of the image.

  16. Corrections • Scanners use a window system like the gamma camera to help eliminate scatter • Randoms are corrected for by measuring coincidence rates with a delay of time.

  17. Image Reconstruction • Filtered back projection - like CT • Iterative reconstruction Good Enough Trial Image Done YES NO

  18. Attenuation Correction • Like all radionuclide imaging there is a problem due to attenuation.

  19. Radioactive Rod Sources Ge -68 or Cs-137 Data reconstructed to make a density map of the body Density map information is used in iterative reconstruction Attenuation Correction

  20. Advantages of PET Imaging Higher sensitivity because no collimator used. Higher resolution because of coincidence detection. Less image errors because of using time of flight calculations

  21. Replace rod source with CT scanner Better attenuation maps Image fusion PET CT

  22. Exposure Causes • Main sources of radiation exposure: • Handling doses and injecting patients • Radiation from patients • Radiation from sources • Contamination

  23. Patients • Patients usually rest about 30 min before scanning to enable radiopharmaceutical to spread within the body • Scan takes about 40 min • Exposure is about 1.5 mR/hr per 10 mCi

  24. Individuals of Concern • Technologists • Pregnant Technologists • Other Workers • Visitors • Companions

  25. PET: The use of the positron annihilation products in imaging of internal body organs. • True coincidence events: Events from a single positron annihilation that was produced by emission of a positron. • Single events: Events produced from unpaired photon. • Line Of Response (LOR): an imaginary line drawn between two detectors registering a signal.

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