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A digital X-ray detectorconverter converts X-ray radiation energy passing through a patients body into digital signal. The device can use direct or indirect conversion. In the former case, the electric charge induced in the sensitive area of the detector as a result of radiation absorption is measured. This method is used in gas or solid-state ionization chambers and selenium-based detectors. In the latter case, the energy of gamma quanta is converted to energy of another type (for example, light energy), which induces charge on the detecting elements (CCD matrices, photodiodes, etc.).The detection system can operate in counting or integrating mode. In the counting mode, each photon is detected separately. Thus, the number of detected electrons is counted by the detecting system (more sophisticated detectors also measure photon energy). The detector produces a digital signal representing the number of gamma quanta detected per given time interval or the detected spectrum of X-ray radiation. The detector used in the MTsRU Sibir low-dose photoroentgenograph is based on a multiwire proportional camera. The main disadvantages of counting systems are their complicated structure, large size, and comparatively low operating speed. The majority of commercially available detecting systems operate in the integrating mode. In this mode, the total photon signal received in a given time interval is measured. The main disadvantage of such systems is that it is impossible to determine the number of detected photons and the energy of each photon. Fluctuations of signal from each photon lead to additional noise. Nevertheless, low intrinsic noise and high density of detection channels make integrating systems very useful [2].The parameter of detection quantum efficiency (DQE) is now widely used for quantitative assessment of the efficiency of various X-ray detectors. The DQE is defined as the coefficient of conversion of squared signal-tonoise ratio by the detector [3]:(1)where SNR out and SNR in are the signal-to-noise ratios at the detector output and input, respectively.Because the photon flux q in incident on the detector is described by the Poisson distribution, we obtain that SNR 2 in = q in . Thus, the output signal-to-noise ratio is determined by the following equation:(2) where α is the conversion coefficient of the detector; σ S is the mean-square signal fluctuation at the detector output.The dependence of DQE on the spatial frequency can be effectively used to assess the quality of conversion provide by a digital X-ray detector [4]. In the case of a detector consisting of elements of the same type spaced at equal distances, the output signal of each element depends on the radiation fluxes at the input of this element and the adjacent elements, as well as the distances to them. In the general case, the signal at a given element is affected by all elements within a certain vicinity. The effect of each of these elements depends on the distance to it. Thus, the signal transmission coefficient of a multielement (multichannel...

In recent years, magnetic-resonance tomography or magnetic-resonance imaging (MRI) has become a leading diagnostic method [4]. Its speedy introduction into clinical practice was facilitated by such an advantage as the possibility of imaging of parts of the human body which are hardly accessible to other diagnostic methods (e.g., posterior cranial fossa). Although spatial resolution of X-ray tomography is better than in MRI, the latter provides much higher image contrast without injection of potentially toxic contrasting agents. The radiation background created by MRI is virtually safe because the frequency of the monitoring radiation used in MRI examination is 9-10 orders of magnitude less than the frequency of X-ray radiation, and it is regarded as biologically harmless.Dozens of magnetic resonance (MR) tomograph models are presently commercially available from foreign and domestic manufacturers. The problem of selection of the most appropriate model is rather difficult. Advertising brochures and leaflets provided by manufacturers of MR tomographs cannot solve this problem because they do not necessarily contain required technical information. It should also be taken into consideration that advertizing materials always contain only favorable information, which should be treated with caution.In this work we consider only technical and utility characteristics of MR tomographs. This does not mean that medical characteristics (e.g., availability of angiographic detection mode, synchronization with ECG or pulse signal, etc.) are less significant; they may have a decisive significance for the selection of a given model. However, the comparison of MR tomographs by medical characteristics is beyond the scope of this work and will be discussed elsewhere.A number of general aspects of the problem should be discussed before considering technical and utility characteristics of MR tomographs.There were several stages in the history of the development of MR tomography. These stages were distinguished by the methodology of data acquisition and processing, and thereby image quality. In the early stages, the method of reconstruction of projections (reverse projection) was used. Later, it was replaced by the method of measurement of phasing gradient (duration or amplitude of gradient pulse) and then, by the method of simple frequency coding. The method of two-dimensional Fourier transformation (two-dimensional spin tomography) followed. The models considered further in this work are based on this method.The majority of contemporary models of MR tomographs are based on the same technical principles, although their technological and circuitry implementations may differ in different models. For example, they may differ by the radio-frequency coil design. However, we assume that the technological capacity of the tomograph models considered in this work either have been implemented in full measure or will be implemented during further upgrade.In our opinion, tomographic image quality is not necessarily the main criterion of comparison...

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