Editing Positron emission tomography (section)

=== Localization of the positron annihilation event ===
The most significant fraction of electron–positron annihilations results in two 511&nbsp;keV gamma photons being emitted at almost 180 degrees to each other. Hence, it is possible to localize their source along a straight line of coincidence (also called the '''line of response''', or '''LOR'''). In practice, the LOR has a non-zero width as the emitted photons are not exactly 180 degrees apart. If the resolving time of the detectors is less than 500 [[picoseconds]] rather than about 10 [[nanoseconds]], it is possible to localize the event to a segment of a [[Chord (geometry)|chord]], whose length is determined by the detector timing resolution. As the timing resolution improves, the [[signal-to-noise ratio]] (SNR) of the image will improve, requiring fewer events to achieve the same image quality. This technology is not yet common, but it is available on some new systems.<ref>{{cite web |url=http://www.uphs.upenn.edu/news/News_Releases/jun06/PETCTITC.htm |archive-url=https://web.archive.org/web/20060628052004/http://www.uphs.upenn.edu/news/News_Releases/jun06/PETCTITC.htm |url-status=dead |archive-date=June 28, 2006 |title=Invitation to Cover: Advancements in "Time-of-Flight" Technology Make New PET/CT Scanner at Penn a First in the World |date=June 15, 2006 |access-date=February 22, 2010 |publisher=University of Pennsylvania }}</ref>