1,720,991 research outputs found
Production of quasi-monochromatic X-rays via crystal array for mammography
No full textQuasi-monochromatic X-ray beams have been produced in the mammographic energy range. The source is based on a conventional tungsten anode X-ray tube and an array of graphite mosaic crystals as monochromator. An optical system consisting of an array of three crystals (2.8 x 3.0 cm2) has been assembled so as to produce in the image plane an irradiation field obtained with adjacent reflected beams. At 18 keV the beam has a field size of about 6.0 X 8.0 cm2, with a percentage energy resolution of 13% (FWHM). The field size is limited by the crystal dimension along the vertical direction and by the energy spread and the number of crystals along the other one. Radiographic images of a test object have been obtained both with a screen/film combination and a digital detector. Techniques to eliminate the spatial non- uniformities have been applied. Field non-uniformities have been removed with a proper correction procedure: 'flat fielding' for a digital imaging system, or scanning technique for screen/film combination
Production of quasi-monochromatic X-rays via crystal array for mammography
No full textQuasi-monochromatic X-ray beams have been produced in the mammographic energy range. The source is based on a conventional tungsten anode X-ray tube and an array of graphite mosaic crystals as monochromator. An optical system consisting of an array of three crystals (2.8 x 3.0 cm(2)) has been assembled so as to produce in the image plane an irradiation field obtained with adjacent reflected beams. At 18 keV the beam has a field size of about 6.0 x 8.0 cm(2), with a percentage energy resolution of 13% (FWHM). The field size is limited by the crystal dimension along the vertical direction and by the energy spread and the number of crystals along the other one. Radiographic images of a test object have been obtained both with a screen/film combination and a digital detector. Techniques to eliminate the spatial non-uniformities have been applied. Field non-uniformities have been removed with a proper correction procedure: "flat fielding" for a digital imaging system, or scanning technique for screen/film combination
Evaluation of a dichromatic X-ray source for dual-energy imaging in mammography
No full textA novel X-ray system, providing dichromatic beams for dual-energy radiography, has been assembled. The source generates pairs of superimposed quasi-monochromatic beams, having energies E and 2E; with E tuneable in the 15–20 keV range. In this paper the characteristics of the radiation field in terms of energy resolution and fluence, for three dichromatic X-ray beams are reported. A study of the spectra attenuated by a 5 cm-thick phantom of breast equivalent tissue demonstrates that the optimal energy of the dichromatic beam for dual-energy application may be set as a function of the thickness of investigated tissue. A detailed topographic study of mean energy and flux shows the spatial superposition of the first and the second diffraction order beam, that is the main requirement for the application of a single exposure dual-energy radiography. The bidimensional mapping of the irradiated beam is also reported, showing the presence of energy and intensity gradients. We estimate that the observed gradients do not affect the results of dual-energy technique application in an appreciable way
Quality control in mammography: A pilot programme for first level activity
No full textQuality control programmes are the best tools for maintaining optimal performances of mammographic units. Programmes to spread quality control activities have been organised in various countries. This study presents the results of an educational programme having three main objectives: quality assurance promotion, operator training with simple instruments and procedures, and operator training in quality control data analysis. Special kits have been distributed to several mammographic centres. In each centre the kit has been used to execute the following tests: AEC performance, processing conditions, entrance exposure and image quality. The data gathered in 53 centres for a period of 4 weeks are compared and discussed. Several centres were found not to satisfy completely the technical recommendations of the italian protocol, implying training for radiographers and radiologists in QC procedures is desirable
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Spatial resolution measurements in quasimonochromatic x rays with mosaic crystals for mammography application
No full textA novel x-ray source for mammography application is being investigated. Quasimonochromatic x rays have been produced via Bragg diffraction with a W-anode x-ray tube and a graphite mosaic crystal array. The system provides 18 keV x rays with an energy resolution DeltaE/E approximate to0.12. A thorough analysis of the spatial resolution of the system has been performed in order to understand and quantify the effect of the introduction of an active optical element such as a mosaic crystal in the x-ray path. The focal spot of the source and its emission properties have been studied by using the slit camera method. Experiments have shown that the introduction of a mosaic crystal in the optical path modifies the resolution properties of the Bragg diffraction-based radiography system. Along the direction perpendicular to the diffraction plane the resolution properties of the imaging system mainly depend on the x-ray tube focal spot size and position. Along the diffraction plane the focal spot size depends on mosaic characteristics and on the geometrical. setup. Hence, it could be modified by setting the appropriate experimental conditions. (C) 2001 American Association of Physicists in Medicine
A comparative characterisation of Highly Oriented Pyrolytic Graphite by means of diffraction topography
No full textThe Highly Oriented Pyrolytic Graphite was examined to study the crystal internal structure and correlate it with the crystal optical behavior. Different samples were studied using x-ray diffraction topography. It was found that some samples are quite perfect and the results agree with ideally imperfect crystal models. Other samples present well defined granular macrostructures superposed to the well-known Gaussian-like crystallite distribution
Bragg diffraction-based quasi-monochromatic source for mammography using mosaic crystals
No full textThe existence of an optimal energy range for mammography has been demonstrated by several authors. Improvement in image contrast, and reduction of patient dose can be achieved using narrow energy band X ray beams in the 16-24 keV range. Quasi-monochromatic X rays in the mammographic energy range have been produced via Bragg diffraction by making use of a conventional W-anode, Be-window X ray tube and a monochromator optical system based on a set of mosaic crystals. The mosaic crystals are high oriented pyrolytic graphite (002) which provide an interesting choice for monochromators because of their high integrated reflectivity compared to perfect crystals. The monochromator optical system consists of an array of ten crystals (2.8 × 6.0 cm2 of size) which are assembled so as to produce in the image plane an irradiation field obtained with adjacent reflected beams. A scanning technique of the optical system has been applied in order to remove the spatial non-uniformities of the entire irradiation field. The source has been characterized in terms of beam size and monochromaticity, photon flux and exposure rate, field uniformity, capability in low contrast detection, dose reduction, and spatial resolution properties. The system provides a large field (10.5 × 12.0 cm2) of quasi-monochromatic X rays (ΔE/E = 12%) at the energy of 18 keV. The spatial resolution capabilities of the sources are affected by the introduction of an active optical element such as a mosaic crystal monochromator. They may be optimized by choosing the proper irradiation geometry. The mean glandular dose delivered to the standard breast by the quasi-monochromatic source is about an half of those delivered by the conventional mammography units
- …
