1,721,098 research outputs found
Digital Mammography: physical priciples and future applications
No full textMammography is currently considered the best tool for the detection of breast cancer, pathology with a rate of incidence in constant increase. To produce the radiological picture a screen film combination is conventionally used. One of the inherent limitations of screen- film combination is the fact that the detection, display and storage processes are one and the same, making it impossible to separately optimize each stage. These limitations can be overcome with digital systems. In this work we evaluate the main characteristics of digital detectors available on the market and we compare the performance of digital and conventional systems. Digital mammography, due to the possibility to process images, offers many potential advantages, among these the possibility to introduce the dual-energy technique which employs the composition of two digital images obtained with two different energies to enhance the inherent contrast of pathologies by removing the uniform background. This technique ..
Abstract ID: 176 Geant4 implementation of inter-atomic interference effect in small-angle coherent X-ray scattering for materials of medical interest
No full textAdvanced applications of digital mammography such as dual-energy and tomosynthesis require multiple exposures and thus deliver higher dose compared to standard mammograms. A straightforward manner to reduce patient dose without affecting image quality would be removal of the anti-scatter grid, provided that the involved reconstruction algorithms are able to take the scatter figure into account [1]. Monte Carlo simulations are very well suited for the calculation of X-ray scatter distribution and can be used to integrate such information within the reconstruction software. Geant4 is an open source C++ particle tracking code widely used in several physical fields, including medical physics [2,3]. However, the coherent scattering cross section used by the standard Geant4 code does not take into account the influence of molecular interference. According to the independent atomic scattering approximation (the so-called free-atom model), coherent radiation is indistinguishable from primary radiation because its angular distribution is peaked in the forward direction. Since interference effects occur between x-rays scattered by neighbouring atoms in matter, it was shown experimentally that the scatter distribution is affected by the molecular structure of the target, even in amorphous materials. The most important consequence is that the coherent scatter distribution is not peaked in the forward direction, and the position of the maximum is strongly material-dependent [4]. In this contribution, we present the implementation of a method to take into account inter-atomic interference in small-angle coherent scattering in Geant4, including a dedicated data set of suitable molecular form factor values for several materials of clinical interest. Furthermore, we present scatter images of simple geometric phantoms in which the Rayleigh contribution is rigorously evaluated
Concentratore di raggi x duri per radioterapia
No full textConcentratore di raggi x, comprendente un supporto e una pluralità di elementi diffrattivi posizionati sul supporto e dispo-sti ad anello circolare attorno a un asse centrale del concentratore, gli elementi diffrattivi essendo atti a diffrangere raggi x in attraversamento verso un punto focale del concentratore. Il supporto comprende una pluralità di super-fici anulari concentriche aventi un pro-filo poligonale, ciascuna superficie anulare comprendendo una pluralità di facce piane di allineamento estendentisi parallela-mente all’asse centrale del concentratore e rivolte radialmente verso l’esterno, in cui ciascuna faccia di allineamento (11sa, 12sa) presenta un rispettivo elemento diffrattivo disposto a contatto con essa
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
Real-time digital radiography system with variable magnification:MTFs and noise evaluation
No full textHigh Z and medium Z scintillators in ultra high resolution small animal PET
No full textAs small animal PET scanners are continuously improving in their performances, one is lead to the question of how far can spatial resolution go. In this paper we address the limiting effects to spatial resolution and whether the photoelectric interaction, and therefore high Z materials, outperform medium Z scintillators. In particular, with a Monte Carlo simulation, we compare the ultimate performances, in spatial resolution, of three scintillators: BGO, NaI(Tl) and YAP:Ce. BGO is the PET scintillator which has the highest photofraction whereas YAP:Ce has the lowest. NaI(Tl), instead is a relatively high Z but low density scintillator. There are three principle contributions to the degradation of spatial resolution: multiple Compton scattering electron range after a gamma interaction and K-shell fluorescence emission. We present the results of simulations of crystals with different thicknesses, with and without K-shell fluorescence emission and electron transport. We conclude that the effect of multiple scattering, electron range and fluorescence emission to the spatial resolution are smaller for low Z, high density materials like YAP:Ce. The fraction of misplaced events, defined here as F = NWrong/NTot, is F0.5mm = 52% for BGO in the case of 0.5 mm binning, increasing to F0.1mm = 80% for the 0.1 mm binning. In the case of YAP:Ce, the scatter fractions are respectively F0.5mm = 27% and F0.1mm = 44%. We conclude that for ultra high resolution PET detectors, medium Z scintillators, such as YAP:Ce, may outperform high Z materials
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
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