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PET with Silicon Strip Detectors

 

 

  PET with Silicon Strip Detectors  
         
           
  Positron Emission Tomography (PET) can be used to determine the spatial distribution of a radioactive isotope in medical imaging. A PET-isotope emits a positron that annihilates in close proximity thereby emitting two nearly anti-parallel photons with 511 keV each. A PET instrument measures the coordinates of these photons in time-coincidence.   Graph of a silicon PET scanner simulation results
compared with currently existing high resolution small animal scanner results
Silicon PET scanner simulation results compared with currently existing high resolution small animal scanner results. Most instruments use high-Z scintillators with limited spatial resolution. The SiliPET offers the highest sensitivity at the best spatial resolution of 0.5 mm (Courtesy of G. Zavattini, Università di Ferrara).
 
         
  It reconstructs the underlying distribution of the isotope from the tomographic projections. In order to image non-radioactive molecules, a PET isotope is chemically bound to it. One important and widely used application of PET is tracing a radiopharmacon inside small animals for pharmaceutical research. Currently most scanners for small animal PET are scaled-down ersions of human body scanners using pixelated high-Z scintillators to fully absorb 511 keV photons.  
   
  But there is not much gained by fully absorbing the photons because the scatter fraction within the small animal is very small and therefore the photon’s energy is known. In low-Z material like silicon, Compton interaction is dominant which has virtually no spatial extent and can be very accurately localized (down to the range of the recoil electron). The image resolution is then set by the fundamental physical limits of PET like positron range and photon non-collinearity. For proof of principle and to be able to compare with simulations, measurements have been made with the existing silicon tracker of the MEGA Compton camera prototype at MPE (figure on the left). The tracker consists of 11 layers of double sided silicon strip detectors with selftriggering analog readout.
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Photo of the MEGA Compton camera silicon tracker is used for PET measurements. Five top layers were used in time-coincidence to five bottom layers with a PET source placed in-between