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Theoretical Studies of Image Artifacts and Counting Losses for Different Photon Fluence Rates and Pulse-Height Distributions in Single-Crystal NaI(TI) Scintillation Cameras

Department of Radiation Physics, Lund University Hospital, Lund, Sweden

Correspondence: For reprints contact: Sven-Erik Strand, PhD, Radiation Physics Dept., Lasarettet, S-221 85 Lund, Sweden.

ABSTRACT

Using computer simulations, we have developed a theoretical model to explain the correlation between counting losses and image artifacts in single-crystal NaI(TI) scintillation cameras. The theory, valid for scintillation cameras of the Anger type, is based on the physical properties of the NaI(TI) crystal. Based on a statistical model using random numbers, pulse trains of the light pulses from scintillations were simulated. Pulse-height distributions for different event rates were calculated, with various Compton distributions. Images of point sources and line sources were generated. Counting losses and image artifacts were dependent on the shape of the pulse-height distribution. The calculated counting losses decreased with larger Compton distributions, due to increasing numbers of pileup events in the energy window; this also caused severe image distortion. The improvement of the spatial resolution with pileup rejection was demonstrated. The theoretical results are in good agreement with experimental results obtained previously.

It is concluded that, in modern cameras, the decay time of the scintillation determines the amount of pileup, and the resolving time of the electronics governs the count rates. The results indicate that in some modern cameras the limits of the count-rate capacity in Anger cameras may be reached.