Geometrical importance sampling and pulse height tallies in GATE

Monte Carlo simulations are widely used to study the behavior and detection of gamma photons in medical imaging devices. Such simulations are computationally expensive. This is why geometrical importance sampling, a variance reduction technique, was recently incorporated into the GEANT4 Monte Carlo code. In order to use this technique for single photon emission computed tomography (SPECT) imaging, it needed to be made compatible with pulse height tallies. These tallies correspond to the number of detected pulses in distinct energy bins, covering an energy spectrum relevant to SPECT. Since each pulse is the combination of different detector hits, the tally bin is not known until the end of an event. In an analog simulation (without variance reduction) this poses no problems as each detected hit can be stored and the pulse can be calculated at the end of each event. Geometrical importance sampling combined with Russian Roulette however introduces branches into the particle history, which results in a much more complicated pulse calculation. This work describes how pulse height tallies are adjusted to geometrical importance sampling and Russian Roulette within GATE, a medical imaging and simulation application based on GEANT4. The validation of this technique is done through SPECT simulations comparing the analog result with the new method.