Signal characteristics study of continuous scintillator CCD-based single photon counting (SPC) detector

Detectors with thin continuous scintillators and (charge coupled devices) CCDs have been used for animal SPECT imaging for their high intrinsic resolution. Thicker scintillators augment detection efficiency but also increase the light spread and difficulty in identification of gamma events. The objective of this work was to study the characteristics of the detector composed of a continuous CsI(Tl) scintillator, a fiber optic taper, a light image intensifier (LII), and a CCD image sensor. Continuous scintillator was chosen for its simple optical propagation model. The large variable gain of the LII enables the detector to work in single photon counting (SPC) mode which may recover the light spread of each scintillation event to improve the detector's spatial resolution. The energy of gamma-rays studied was 140 keV. A linear cascade model was used to quantify the photon propagation through each component of the detector. For a continuous scintillator, its modular transfer function (MTF) was modeled by an analytical function. The MTFs of other detector components were obtained from the results of prior reports. With the cascade model, the detector's overall point spread function was derived and mixed with Poisson noise and CCD read-out noise. A local-maxima based peak detection algorithm was used to identify the photon events in each frame acquired. A simulation study with a limited number of photons showed that when the light-spread distance augmented from 0.8 to 1.0 mm, the fraction of noise equivalent count decreased from 0.73 to 0.06. So the thickness limit for a continuous scintillator with the detector configuration studied was about 0.8 mm.