A study of digital X-ray detection systems with optical image transfer

IntroductionDigital X ray image detectors are widely used for mass scale prophylactic X ray chest survey of popula tions. Several variant models of digital X ray image detec tors are now used in clinics. There are advantages and dis advantages of each model [1]. Comparative analysis of X ray image detectors includes many parameters: spatial resolution, threshold contrast, dynamic range, etc.[2]. Requirements for some parameters are sometimes overes timated. For instance, an X ray image detector with too high spatial resolution requires enhanced X ray radiation dose to obtain high quality X ray image. Such an increase in the X ray radiation dose in diagnostic examination is appropriate, while in prophylactic examination it is unac ceptable. High spatial resolution is not required in case of an X ray tube with large focal spot size because of image unsharpness [3]. Optimal requirements should be satis fied in digital X ray image detector construction [4].Development of multifunctional digital X ray image detectors provides a promising approach to this problem. Parameters of multifunctional digital X ray image detec tors can be adapted for X ray survey use. A system pro viding variable spatial resolution and different threshold contrast is protected by a Russian Federation Patent [5]. This system provides a methodological basis for a two mode X ray apparatus. The diagnostic mode requires enhanced spatial resolution. The prophylactic mode with reduced X ray radiation dose and decreased spatial reso lution can be used for mass scale prophylactic X ray chest survey of the population The goal of this work was to describe a widely used scheme of X ray image detector: shielding raster -lumi nescence screen -optical system -CCD matrix of pho tosensitive detector -amplifier -analog to digital con verter (ADC). Both theoretical and experimental sub stantiation of this scheme is given. Formation of Useful SignalLet us consider signal transformation in this system. Let the number of X ray γ quanta N γ per image element (pixel) of an X ray luminescence screen be input signal S in :(1)According to Poisson statistics, input signal variance ∆ 2 S in is equal to the number of incident γ quanta:(2)Let the number of electrons N e generated as a result of exposure per CCD pixel be output signal:Correlation between the number of electrons and the number of γ quanta is determined by signal transforma tion [6,7]:The variance ∆ 2 N e of the number of generated elec trons is:where λ is probability of absorption of γ quantum in X ray luminescence screen; q = ηpn is the number of elec trons generated in CCD pixel per γ quantum of initial X ray radiation per pixel of X ray luminescence screen; n is mean number of visible light photons generated in X ray luminescence screen per absorbed X ray γ quantum; p is probability of absorption of a photon emitted by the X ray luminescence screen pixel by the CCD matrix pixel; η is CCD quantum efficiency. CCD Matrix NoiseFormed useful signal of the CCD matrix is corrupt ed by some interference: dark char...

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“…Correlation between the number of electrons and the number of γ quanta is determined by signal transforma tion [6,7]:…”

Section: Formation Of Useful Signalmentioning

confidence: 99%

“…In the first analog video systems this parameter was used for determining X ray image intensifier quality [6]. This parameter is now used for determining threshold contrast of digital X ray image detectors [8 10].…”

Section: Contrast Sensitivity As An Objective Characteristic Of Visuamentioning

confidence: 99%