What calculation uses the difference in arrival time of two annihilation photons to determine the position of the annihilation event?

The calculation that utilizes the difference in arrival time of two annihilation photons to determine the position of the annihilation event is Time of Flight (ToF) imaging. This technique plays a critical role in enhancing the spatial resolution of PET images.

In PET imaging, when a positron emitted from a radiotracer encounters an electron, they annihilate, producing two gamma photons that are emitted in nearly opposite directions. Time of Flight assesses when these two photons arrive at the detector array. By calculating the time difference between their arrivals, the system can effectively pinpoint the location of the annihilation event along the line connecting the two detection points. This leads to improved accuracy in localizing the source of the emitted radiation and allows for clearer imaging of the underlying biological processes.

In contrast, other methods such as energy window calculation focus on the energy level of the detected photons rather than their timing. Positron range estimation deals with the decay distance of the emitted positrons before they annihilate, and attenuation correction addresses the loss of signal strength as the photons pass through various tissues but does not utilize timing information. Thus, Time of Flight is distinctly relevant to the timing aspect of photon detection in PET imaging.