title: Technical Note: modification of the standard gain correction algorithm to compensate for the number of used reference flat frames in detector performance studies.
abstract: OBJECTIVE
  The x-ray performance evaluation of digital x-ray detectors is based on the calculation of the modulation transfer function (MTF), the noise power spectrum (NPS), and the resultant detective quantum efficiency (DQE). The flat images used for the extraction of the NPS should not contain any fixed pattern noise (FPN) to avoid contamination from nonstochastic processes. The "gold standard" method used for the reduction of the FPN (i.e., the different gain between pixels) in linear x-ray detectors is based on normalization with an average reference flat-field. However, the noise in the corrected image depends on the number of flat frames used for the average flat image. The aim of this study is to modify the standard gain correction algorithm to make it independent on the used reference flat frames.
  
  
  METHODS
  Many publications suggest the use of 10-16 reference flat frames, while other studies use higher numbers (e.g., 48 frames) to reduce the propagated noise from the average flat image. This study quantifies experimentally the effect of the number of used reference flat frames on the NPS and DQE values and appropriately modifies the gain correction algorithm to compensate for this effect.
  
  
  RESULTS
  It is shown that using the suggested gain correction algorithm a minimum number of reference flat frames (i.e., down to one frame) can be used to eliminate the FPN from the raw flat image. This saves computer memory and time during the x-ray performance evaluation.
  
  
  CONCLUSIONS
  The authors show that the method presented in the study (a) leads to the maximum DQE value that one would have by using the conventional method and very large number of frames and (b) has been compared to an independent gain correction method based on the subtraction of flat-field images, leading to identical DQE values. They believe this provides robust validation of the proposed method.
authors.affiliation: Department of Medical Physics and Bioengineering, University College London, London WC1E 6BT, United Kingdom.
authors.forename: Anastasios C
authors.initials: AC
authors.lastname: Konstantinidis
authors.forename: Alessandro
authors.initials: A
authors.lastname: Olivo
authors.forename: Robert D
authors.initials: RD
authors.lastname: Speller
mesh_terms: D000465:Algorithms; D016477:Artifacts; D004739:England; D004867:Equipment Design; D019544:Equipment Failure Analysis; D016247:Information Storage and Retrieval / Q000379:methods*; D011856:Radiographic Image Enhancement / Q000295:instrumentation* / Q000379:methods*; D012016:Reference Values; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014963:X-Ray Intensifying Screens