Modified dead time correction for prompt gamma radiation emitting PET isotopes

Several long-lived positron emitters, such as ⁷⁶Br(t₁2/=16.2 h, 55% β⁺) and ¹²⁴I(t₁2/=4.2 d, 23% β⁺, emit photons simultaneously with positrons. Coincidence detection of these photons with each other or with annihilation photons leads to essentially true coincidences that can not be distinguished from coincidences involving two annihilation photons. These photons also lead to higher singles count rates and thus to a higher dead time. Furthermore, photons that are rejected by the energy discriminator, the number of which is considerably increased for prompt gamma emitting nuclides, are not counted as singles but do contribute to dead time and lead to inaccuracy of the standard dead time correction. The aim of the present study was to assess the effect of additional gamma radiation on dead time correction accuracy and to improve the existing dead time correction for prompt gamma emitting nuclides. Singles life time as a function of singles rate is lower for ⁷⁶Br than for ¹⁸F, whereas coincidence life times are similar. Total dead time for ⁷⁶Br is considerably increased compared to ¹¹C for similar singles count rates. Multiplication of the measured ⁷⁶Br singles rate with a fixed factor of 1.3 leads to a similar dead time as a function of singles rate as for ¹¹C and in an accurate prediction of the total dead time by the standard dead time correction. Multiplication factors for different energy windows correlate well with the relative number of rejected photons.