title: Overestimations in zero frequency DQE of x-ray imaging converters assessed by Monte Carlo techniques based on the study of energy impartation events.
abstract: OBJECTIVE
  The performance of various x-ray converters, employed in medical imaging systems, has been widely examined by several methodologies (experimental, analytical, and Monte Carlo techniques). The x-ray converters most frequently employed in energy integrating digital radiology detectors are the Gd2O2S:Tb granular phosphor, the CsI:TI structured phosphor, and the a-Se photoconductor. The imaging characteristics of an x-ray converter are affected by its x-ray detection properties. However, various definitions of x-ray detection have been used in the literature, leading to different results for the quantum detection efficiency (QDE) for the same type of x-ray converter. For this reason, there is a need for accurate determination of the x-ray detection and, in particular, its relation to detector response.
  
  
  METHODS
  The present article reports on the performance of the three aforementioned x-ray converters in terms of the QDE and the x-ray statistical factor Ix, and examines the effect of the x-ray detection, directly related to converter output signal, on the zero-frequency DQE. For the purposes of this study, Monte Carlo simulation was used to model the x-ray interactions within the x-ray converter. Simulations were carried out in the energy range from 10 keV up to 80 keV and considering two layers of different coating weights (50 and 100 mg/cm2). The prediction and comparison of zero-frequency DQE were based on two different approaches for x-ray detection, i.e., (a) fraction of interacting photons and (b) fraction of photons leading to energy deposition. In addition, the effect of energy deposition through Compton scattering events on the DQE values was estimated.
  
  
  RESULTS
  Our results showed discrepancies between Monte Carlo techniques (based on energy deposition events) and analytical calculations (based on x-ray attenuation) on QDE. Discrepancies were found to range up to 10% for Gd2O2S:Tb (100 mg/cm2), 7.7% for CsI:T1 (50 mg/cm2), and 8.2% for a-Se (50 mg/cm2). Discrepancies were analyzed by examining the scattering effects (elastic and inelastic) within the converters and led to further analysis of scattering events on Ix, as well. Significant overestimations were found for both factors (QDE and I,) on the zero-frequency DQE.
  
  
  CONCLUSIONS
  Considering that the highest overestimation was found in the thin layer (50 mg/cm2), Monte Carlo evaluation showed that the overestimation (%) between DQE values (based on either x-ray interacting events or energy impartation events) was more significant at 20 keV for CsI:T1 (approximately 2.1%), at 40 keV for Gd2O2S:Tb (approximately 8.1%), and finally at 60 and 80 keV for a-Se converter (approximately 4.8 and 8.2%, respectively). Finally, the overestimation due to the effect of Compton scattering on DQE was found more significant for the a-Se converter and especially above 40 keV reaching approximately the value of 8.9% at 80 keV.
authors.affiliation: Department of Medical Instruments Technology, Technological Educational Institute, 122 10 Athens, Greece. liapkin@teiath.gr
authors.forename: P F
authors.initials: PF
authors.lastname: Liaparinos
authors.forename: I S
authors.initials: IS
authors.lastname: Kandarakis
mesh_terms: D017076:Computer-Aided Design; D004735:Energy Transfer; D004867:Equipment Design; D019544:Equipment Failure Analysis; D009010:Monte Carlo Method; D012542:Scattering, Radiation; D014057:Tomography, X-Ray Computed / Q000295:instrumentation*; D014159:Transducers; D014963:X-Ray Intensifying Screens; D014965:X-Rays