Signal transport in Computed Tomography detectors

In Computed Tomography (CT) X-ray intensities are measured by large-scale solid-state detectors. The standard set-up comprises a scintillator pixel array attached to a matrix of photo sensors, which in turn is read out by analog-to-digital conversion electronics. We have developed and validated a three-dimensional system model describing the cascaded system process. The first step comprises a Monte-Carlo (MC) tracking of the primary X-ray quanta energy deposition, taking into account the relevant fluorescence and scattering processes. The second step models the transport of optical photons in the scintillator pixels formed by a solid-state bulk with surrounding back-scattering TiO2 walls. In a third step the individual events are integrated to a read-out signal and analyzed for their statistical properties. The system model is verified by a comparison to optical measurements. A scintillator array is excited by a needle beam X-ray source. The emitted light field is read out by a high-resolution CCD sensor. A good agreement between simulation and experiments is found, with a typical deviation in the range of 5%. The detector response function D(E, E′) is used to quantify the spectral behavior. It yields the probability to measure an energy E′ for an incoming quantum energy E. We calculate the expected energy 〈E′(E)〉 and link the deviations from proportionality in E to properties of the signal transport. Finally the impact of the signal transport statistics on the output signal-to-noise ratio is analyzed.