Data Acquisition and Corrections

  • Gopal B. Saha


PET is based on the detection in coincidence of the two 511-keV annihilation photons that originate from β+ emitting sources (e.g., the patient). The two photons are detected within an electronic time window (e.g., 12 ns) set for the scanner and must be along the straight line connecting the centers of the two detectors called the line of response (LOR). Since the two photons are detected in coincidence along the straight line in the absence of an absorptive collimator, this technique is called electronic collimation. All coincident events are collectively called prompts, which include true, random, scattered, and multicoincidence events. Examples of these events are illustrated in Fig. 3.1. A true coincident event arises when two 511-keV photons from a single annihilation event are detected by a detector pair along the LOR (Fig. 3.1a). Random coincidences occur when two unrelated 511-keV photons from two separate annihilation events are detected by a detector pair within the same time window (Fig. 3.1b). These events raise the background causing loss of image contrast. Scatter coincidences occur due to Compton scattering of annihilation photons in the patient in the forward direction without much loss of energy [Hoffman and Phelps,(1986)]. One 511-keV photon and one scattered photon or both scattered photons arising from the same annihilation event may fall within the 511-keV energy window and be detected by a detector pair within the coincidence time window (Fig. 3.1c). Scatter coincidences raises the background of the image and degrades the image contrast. Multicoincidence occurs when more than two unrelated 511-keV photons are detected in coincidence in different detectors within the time window, but these events are discarded because of the difficulty in positioning them.


Attenuation Correction Energy Window Coincidence Event Mass Attenuation Coefficient Random Coincidence 
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Copyright information

© Springer Science+Business Media, LLC 2010

Authors and Affiliations

  • Gopal B. Saha
    • 1
  1. 1.Department of Nuclear MedicineThe Cleveland Clinic FoundationClevelandUSA

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