PET scanners may be regarded as a series of subsystems, each of which requires a minimum amount of time to elapse between successive events for them to be registered as separate. Since radioactive decay is a random process, there is always a finite probability that successive events will occur within this minimum time, and at high count-rates, the fraction of events falling in this category can become very significant. The principle effect of this phenomenon is to reduce the number of coincidence events counted by the PET scanner, and since the effect becomes stronger as the photon flux increases, the net result is that the linear response of the system is compromised at high count-rates. The parameter that characterises the counting behaviour of the system at high event rates is known as the "dead time". The fractional dead time of a system at a given count-rate is defined as the ratio of the measured count-rate and the count-rate that would have been obtained if the system behaved in a linear manner.
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