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IntroductionFilmless computer X ray apparatuses and even digi tal computer X ray apparatuses are being widely intro duced into domestic medical practice. Moreover, there are many approaches to digital X ray imaging described in the literature at length [8,12].The goal of this work was to describe a widely used approach to digital X ray imaging: luminescent screenoptical system-charge coupled device (CCD) matrixamplifier-analog to digital converter. CCD cooling should increase the signal/noise ratio of resulting image, thereby increasing the diagnostic capacity of the system without increasing radiation load. The goal of this work was to substantiate the necessity of CCD cooling, which improves characteristics of X ray image. Formation of Useful Signal and CCD NoiseThe incident X ray γ quanta received during expo sure time at input screen element constitute the useful signal S in :(1)The variance ∆ 2 S in of input signal or variance of the number of γ quanta ∆ 2 N γ in Poisson statistics is:Transformation of X ray signal gives resulting digital signals for each pixel (image element) I m . The statistical characteristics of the signal are: mean value I 0 and vari ance ∆ 2 I. Because each stage of image transformation introduces additional noise, the relationship between mean value and variance is:where ∆ 2 CCD contains both intrinsic CCD noise and noise of amplifier and analog to digital converter. The number α > 1 is associated with the quantum efficiency of the device [7]. The numerical value of α is determined by the number of preliminary amplification stages and signal transformation system.It follows from Eq. (3) that intrinsic CCD noise sig nificantly deteriorates image quality at low signal ampli tude. Therefore, suppression of intrinsic CCD noise is very important. The resulting useful signal from a CCD matrix is additionally corrupted by the following factors: dark charge (spontaneous generation of electron-hole pairs in accumulation and transfer modes), the tempera ture dependence of the process being described by the fol lowing equation:where Т 0 is initial temperature; ∆T is temperature change corresponding to dark charge doubling; q Т tr (Т 0 ) is dark charge generated during transfer; i Т acc (Т 0 ) is dark current during signal accumulation. In addition, transformation chain noise also contributes substantially to final discrete signal noises as a function of the number of forming elec trons. This noise or output reading noise (∆ 2 N re ) 1/2 is determined by the device design.It follows from Eq. (4) that contribution of the CCD to useful signal variance is:The statistical characteristics of the system are deter mined by mean value I signal and variance of output sig nal ∆ 2 I calculated from Eq. (3).

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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