Investigation of the Lag Effect in X-Ray Flat-Panel Detector for Cone-Beam Computed Tomography
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Investigation of the Lag Effect in XRay FlatPanel Detector for ConeBeam Computed Tomography A. K. Avakyan*, I. L. Dergacheva, A. A. Elanchik, T. A. Krylova, T. K. Lobzhanidze, S. A. Polikhov, and V. P. Smirnov
The purpose of this work is to investigate the factors determining the magnitude and nature of the lag effect (resid ual signal) in the Varian PaxScan 4343CB flatpanel detector. Depending on the detector mode, typical value of residual signal is 12% after single exposed frame, increasing up to 3% after multiple exposures. The lag effect is the cause of up to 3.6% error of average flatfield signal measurement if used for averaging series of continu ously exposed frames, which leads to incorrect assessment of Xray attenuation coefficients. Spatial dependence of the lag effect is confirmed; the results of correction of projection images using spatially localized impulse response functions are demonstrated.
Introduction Major tasks in radiotherapy are ensuring high dose delivery accuracy and minimizing dose loads applied to radiationsensitive organs and tissues (critical organs and structures). These objectives require the shapes and posi tions of both the target and critical structures to be mon itored throughout radiotherapy courses. The main moni toring device in the majority of remote radiotherapy sys tems is conebeam computed tomography (CBCT), based on an Xray source and an Xray flatpanel detec tor (FPD). Such solutions were selected during develop ment of the Onyx radiotherapy system by the Research Institute of Technical Physics and Automation under contract from the Russian Ministry of Education and Science and Rusatom Healthcare [1]. Lag causes the presence of a residual component of the initial signal in frames following the frame in which the initial signal was generated. The lag effect has the fol lowing explanations [26]: − incomplete readout of the charge from capacitors; − the effect of charge trapping with subsequent release in semiconductor photodiodes; − scintillator afterglow; − variation in the transfer characteristic of channels during irradiation. Research Institute of Technical Physics and Automation, Moscow, Russia; Email: [email protected] * To whom correspondence should be addressed.
If not properly controlled, the lag effect creates sig nificant artifacts both in projection (2D) and tomograph ic (3D) images [7]. Compliance with technical specifica tions for the characteristics of Xray images requires com plex methods to be developed for measuring and com pensating for lag. This report demonstrates the following main results: − relationship between lag value and the number of frames read after exposure, the frame reading rate, bin ning, the intensity of the individual exposure, and multi ple subsequent exposures; − spatial nonuniformity of lag value across detector area; − lag influence on the accuracy of measurement of the light field (the result of irradiation of the FPD in the absence of an object in the irradiation field) and, as a result, on meas
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