Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Design and Principles Of OperationAnnihilationCoincidence DetectionPositron emission is a mode of radioactive transformation and was discussed inChapter 18. Positrons emitted in matter lose most of their kinetic energy by causingionization and excitation. When a positron has lost most of its kinetic energy,it interacts with an electron by annihilation (Fig. 22-14, left). The entire mass of theelectron-positron pair is converted into two 511-keV photons, which are emitted innearly opposite directions. In solids and liquids, positrons travel only very short distancesbefore annihilation.If both of these annihilation photons interact with detectors, the annihilationoccurred close to the line connecting the two interactions (Fig. 22-14, right). Circuitrywithin the scanner identifies interactions occurring at nearly the same time,a process called annihilation coincidence detection (ACD). The circuitry of thescanner then determines the line in space connecting the locations of the two interactions.Thus, ACD establishes the trajectories of detected photons, a function performedby collimation in SPECT systems. However, the ACD method is much lesswasteful of photons than collimation. Additionally, ACD avoids the degradation ofspatial resolution with distance from the detector that occurs when collimation isused to form projection images.A true coincidence is the simultaneous interaction of emissions resulting from a singlenuclear transformation. A random coincidence (also called an accidental or chancecoincidence), which mimics a true coincidence, occurs when emissions from differentnuclear transformations interact simultaneously with the detectors (Fig. 22-15).A scatter coincidence occurs when one or both of the photons from a single annihilationare scattered, but both are detected (Fig. 22-15). A scatter coincidence is a511 keYphotonAnnihilatione--- of positronand electron511 keYphotonFIGURE 22-14. Annihilation coincidence detection (ACD). When a positron is emitted bya nuclear transformation, it scatters through matter losing energy. After it loses most ofits energy, it annihilates with an electron, resulting in two 511-keV photons that are emittedin nearly opposite directions (left). When two interactions are simultaneouslydetected within a ring of detectors surrounding the patient (right), it is assumed that anannihilation occurred on the line connecting the interactions. Thus, ACD, by determiningthe path of the detected photons, performs the same function for the PET scanner asdoes the collimator of a scintillation camera.