TIme of Flight in PET

Time of Flight (TOF)

While PET data is determined with LORs, the main problem with this approach is that you cannot determine the exact location of the coincident event (LORs only give you the line within 3D space). In theory, if one could determine the actual location of coincident event then spatial resolution would improve and to some degree this is possible. The question then remains, how can we do this?
1. In theory (and technically) it depends on the arrival of the two 511 keV photons to each specific detector or element
  1. Usually the time window is set to between 4 to 12 nsec
  2. When the two gammas arrive within the window the event is recorded
  3. Distance can then determined by defining which one arrives first, which one arrives second
  4. By having this data you should be able to extrapolate back the origin/location of the annihilation event
2. In actuality TOF is being applied with current PET technology, however, it has its limitation. Let us discuss the TOF process and its limitations
  1. Given the right software/hardware the timing window must be set to 0.6 nsec. Question - how does this effect the system's sensitivity?
  2. At tissue with this technology is the inability to place the exact point of the annihilation event, however, we can get close and it's within 9cm
  3. Initially one might think that 9cm is just too much to make a real imaging difference. But this type of thinking will get you into trouble!
  4. While spatial resolution of a PET systems is in the range of 5 to 10 mm, the fact that we can pinpoint the event, via TOF, to 9cm means that we can more accurate place the LOR within a very limited area
  5. Without TOF all LORs must be back-projected through their entire path
  6. With TOF all LORs can be backprojected to length that is no greater than 9cm
  7. Furthermore, because a patient's transaxial slice is significantly greater than 9cm
  8. The end result is that smaller LORs improve spatial resolution
  9. Let us also think about the size of the body habitus, application of TOF, and scatter
    1. The larger the patient and the lack of TOF results in increased scatter. The fact remains, the larger the diameter these gammas travel through the greater probability of Compton, further increasing the scatter events
    2. If TOF is applied this improves image quality via the reduction of scatter. In fact, the larger the patient the more effective TOF will be, as this relates knowing, within 9cm, where the annihilation event occurred
    3. How does scatter and size relate? Without the application of TOF a 120 kg patient's S/N ratio would be 6 times greater when compared to someone who weighed 50 kg. In addition the larger patient, because of attenuation, requires an increase in scanning time to acquire acceptable count density. It is suggested that when TOF is applied to the larger patient image quality improves and the concern of the 6 to 1, S/N ratio, no longer applies
3. What components does a PET Scanner require to apply the TOF?