MTF of columnar phosphors with a homogenous part: an analytical approach
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ORIGINAL ARTICLE
MTF of columnar phosphors with a homogenous part: an analytical approach Konstantinos Psichis 1 & Nektarios Kalyvas 2 & Ioannis Kandarakis 2 & George Panayiotakis 1 Received: 30 July 2019 / Accepted: 30 July 2020 # International Federation for Medical and Biological Engineering 2020
Abstract A method for the theoretical estimation of the MTF of columnar phosphors with a homogeneous part at the end used in X-ray imaging has been developed. This method considers the light transport inside the scintillator through an analytical modelling, the optical photon beams distribution on the scintillator–optical sensor interface, and uses the definition of the PSF and a Gauss fitted LSF to estimate the MTF of an indirect detector. This method was applied to a columnar CsI:Tl scintillator and validated against experimental results found in literature, and a good agreement was observed. It was found that, by increasing the pixel size of the optical detector and the thickness of the scintillator, the MTF decreased as expected. This method may be used in evaluating the performance of the columnar phosphors used in medical imaging, given their physical and geometrical characteristics. Keywords Digital detectors . Columnar phosphors . CsI:Tl . Analytical model . MTF
1 Introduction Columnar phosphors are employed in indirect digital detectors used in medical X-ray imaging for the conversion of the impinging X-ray photons to optical photons. Their main optical characteristic is the suppression of the lateral spreading of the optical photon beams produced, due to their columnar structure. Thus, they may be constructed in long forms for increased absorption of the impinging X-rays without sacrificing the spatial resolution. The most commonly used columnar phosphor in digital X-ray imaging is the CsI:Tl employed in Active Matrix Flat Panel Imagers (AMFPI) [41, 44] or coupled with a CCD camera [26] or CMOS sensors [2, 34, 48, 49]. Other columnar phosphors used in digital X-ray imaging are the RbBr:Tl+ [28] and CsBr:Eu2+ [37] employed in computed radiography (CR). The concept of light transport in medical imaging scintillators [38] has been implemented in the derivation of Modulation Transfer Function (MTF) either through Monte
* George Panayiotakis [email protected] 1
Department of Medical Physics, School of Medicine, University of Patras, 26500 Patras, Greece
2
Department of Biomedical Engineering, University of West Attica, 12210 Athens, Greece
Carlo modelling for granular phosphors [33] and CsI:Tl [2–6, 18, 42, 43] or through the analytical modelling for granular [1, 15, 22] and columnar phosphors, e.g. CsI:Tl [24, 31]. MTF is a parameter widely used to evaluate the performance of digital X-ray detectors [9, 34, 38]. From our knowledge of the current literature, little work has been done on the analytical modelling of light transport in columnar phosphors leading to the derivation of MTF. In a previously published analytical method, the light propagation process in pure columnar scintillators has been describ
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