Spatial-frequency-dependent pulse height spectroscopy of x-ray scintillators using single x-ray imaging

Indirect flat panel imager (I-FPI) performance is limited by noise in the scintillator's response to single x-rays, i.e. random variation of gain and blur. Single x-ray imaging (SXI) is an experimental methodology to record the response of scintillators to single x-rays using an image intensifier that is lens coupled to an EMCCD camera. In this work we developed a framework for using ensembles of SXI images to compute the spatial-frequency-dependent pulse height spectrum, Pr[g(f)], a novel detector performance metric whose moments may be used to compute scintillator MTF(f), qNNPS(f), and DQE(f). The approach is demonstrated using cesium iodide (CsI:Tl) samples of various thicknesses and screen-optical properties. SXI was conducted on each sample using ∼60 keV i3-rays emitted by Am-241. Each image in an SXI ensemble was summed in 1D, and Fourier transformed to form an ensemble of frequencydependent responses. The histogram at each frequency f forms the Pr[g(f)]. To validate this approach against standard methods, each sample was also evaluated using the standard method by coupling it to a photodiode-TFT array as in a conventional I-FPI. MTF(f) and qNNPS(f) were measured using a slanted edge and flood field images, respectively, using an 85 kVp x-ray spectrum filtered to achieve an average energy of 60 keV. MTF(f) and qNNPS(f) results agreed well between both methods, and discrepancies were consistent with the differing experimental conditions. The Pr[g(f)] may be used to help optimize CsI:Tl in I-FPIs, and to improve the computational efficiency of Monte Carlo simulations of I-FPI performance.