Depth of interaction measurement in gamma ray imaging detectors with continuous scintillation crystals

A design for an inexpensive depth of interaction (DOI) detector for gamma rays, suitable for nuclear medical applications, especially positron emission tomography (PET), has been developed, studied by simulations and tested experimentally. The detector consists of a continuous LSO-scintillator of dimensions 42×42×10 mm³ and a new compact large-area (49×49 mm²) position sensitive photo-multiplier (PSPMT) H8500 from Hamamatsu. Since a continuous crystal is used, the scintillation light distribution is not destroyed and its first 3 moments can be used to determine the energy (0th moment), the centroids along the x- and y-direction (1st moments) and the depth of interaction (DOI), which is strongly correlated to the distribution´s width and thus its standard deviation (2nd moment). The simultaneous computation of these moments allows a three-dimensional reconstruction of the position of interaction of the γ-rays within the scintillating crystal and will be realized by a modified position sensitive proportional (PSP) resistor network. No additional photo detectors or scintillating crystals are needed. According to previous Monte Carlo simulations which estimated the influence of Compton scattering for 511 keV γ-rays, the transport of the scintillation light within the detector assembly and also the behavior of the modified PSP resistor network, we expect a spatial resolution of ≲ 2 mm and a DOI resolution of ≈ 5 mm. The first experimental results presented here yield an intrinsic spatial resolution of ≲ 1.8 mm and 2.6 mm for the x- and y-direction respectively and a DOI resolution ≲ 1 cm. Further we measured an energy resolution of 12%-18%.