Optical hyperlens: far-field imaging beyond the diffraction limit
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We propose an approach to far-field optical imaging beyond the diffraction limit. The proposed system allows image magnification, is robust with respect to material losses and can be fabricated by adapting existing metamaterial technologies in a cylindrical geometry.From the computer simulation study of blood vessel imaging in cerebral angiography it is found that, with proper choice of the focal-spot size and magnification ratio, radiographic magnification in conjunction with fast screens can give blood vessel images "equivalent" to or better than those from slower screens without magnification. However, this result is highly dependent on blood vessel diameter. If a sufficiently small focal spot is not available so that the proper choice of focal-spot size and magnification cannot be made, then the vessel image from slower screens without magnification can be better than that from fast screens with magnification.
Computed radiography
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The resolution characteristics of a digital subtraction angiographic (DSA) system have been evaluated under two different magnification techniques, namely; the geometrical magnification, and the electro-optical magnification. It has been found that the DSA system resolution is less dependent on the focal spot size (or, modulation transfer function, MTF) than that of routine radiographic angiographic studies. The evaluation results have been utilized to re-establish the DSA examination procedure.
Optical transfer function
Digital subtraction angiography
Subtraction
Digital radiography
Image subtraction
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An x-ray unit designed for conventional nonmagnification and magnification mammography has been evaluated in terms of image quality and corresponding radiation exposure levels. The technical advantages of the radiographic magnification technique can result in improved image quality and reduction of the recording-system noise. The microfocal spot allows 1.5 x magnification mammograms with minimal geometric unsharpness. However, the magnification technique requires an increased radiation dose to the breast, compared to conventional nonmagnification techniques. An additional radiation dose may be required for screen-film magnification views because of reciprocity law failure due to long exposure times. The increased-dose limitation and the small dimensions of the recording-system cassettes have precluded the use of magnification in place of nonmagnified images for routine mammographic examination. The magnification technique has proved to be beneficial in selected cases.
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Many types of magnification described by different authors have dissimilar meanings. Basic principles of magnification such as relative size magnification, relative distance magnification and angular magnification used commonly in low vision are clarified in this paper. The three frequently-used terms of magnification, including apparent magnification, two relative magnification formulae, namely, M = F/4 and M = F/2.5, and iso-accommodative magnification are described. The concepts of equivalent viewing power (EVP) and equivalent viewing distance (EVD) are reviewed as an alternative means of specifying magnifying power. They represent an intrinsic property of an optical system that corresponds to the resolution afforded by the system in terms of dioptric power.
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Magnification selective renal arteriograms were performed on 24 patients, 12 of whom were hypertensive, and compared with non-magnification arteriograms by two observers independently. The magnification angiograms were performed on a Siemens Microfocus Bi 125/3/50 RG tube with a 0.1 mm focal spot. Of the 24 patients examined, information crucial to the diagnosis was found only on the magnification films in three patients (12.5%). Extra information compared with the nonmagnification films was found in the magnification films in 12 patients (50%). No additional information was discovered in the remaining nine patients (37.5%). The magnification angiograms enabled the interlobular vessels to be visualized—this was not possible on the non-magnification films. Against the additional information gained must be weighed the disadvantages of magnification arteriography which include increased radiation dose and lengthening of procedure time plus additional injections of contrast. In conclusion, there is a place for magnification renal arteriography and the advantages seem to outweigh the disadvantages.
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It has been shown that the quality of blood vessel images in cerebral angiograms can be improved either by employing radiographic magnification in conjunction with fast screens or by using slower screens and fast film without magnification. It is not known which of these methods results in better images, and under what conditions magnification used with slow screens might result in further improvement. By means of a computer, the dependence of image contrast and sharpness on magnification with various screen-film systems has been studied. One result indicates that, for blood vessel diameters less than 100 mu m and focal spot sizes of 0.3 mm or larger, the use of slow screens without magnification results in better images than those obtained with fast screens and magnification. For blood vessels larger than 300 mu m in diameter and focal spot sizes of 0.3 mm or less, fast screens with magnification can give better images than those obtained using slow screens without magnification.
Image contrast
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Two techniques of direct-magnification radiography are compared : (a) standard magnification using a 40 cm air gap to reduce scatter, and (b) short-target-film (STF) magnification using a 55 cm target-film distance and a rotating grid to reduce scatter. Both techniques provide nearly equal resolution, although STF magnification is slightly superior in this respect. The advantages and disadvantages of STF magnification are discussed.
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