477 research outputs found
An apodized-aperture x-ray detector design for improved image quality in mammography
X-ray imaging for early cancer detection, such as screening mammography, requires images with high signal-to-noise ratio (SNR) using low levels of radiation exposure. Conventional detectors consist of a matrix of sensor elements, producing images where each pixel corresponds to a single sensor element. This imposes a fundamental limitation on image contrast and SNR for imaging fine detail for a given exposure. The work presented here reconsiders x-ray image formation using a new x-ray detector design that synthesizes image pixels from a large number of very small sensor elements with the goal of optimizing contrast and SNR.
Our new detector design, called apodized-aperture pixel (AAP), makes use of recent technology developments to produce images from an “over-sampled” sensor signal while suppressing both signal and noise aliasing to improve the modulation transfer function (MTF) and detective quantum efficiency (DQE).
Signal and noise performance of the AAP approach is described theoretically using a cascaded-systems analysis. This approach preserves the MTF of the small sensor elements up to the image sampling cut-off frequency where the MTF is increased by up to 53%. Frequencies above the cut-off are suppressed, eliminating both signal and noise aliasing artifacts and corresponding to a high-frequency DQE increase by 2.5x. X-ray interactions in a scintillator introduce signal and noise correlations, including x-ray reabsorption and converter blur, resulting in reduced aliasing and decreased improvement in DQE. Best results with the AAP design were obtained using a high-resolution converter, such as selenium (Se), with little impact from reabsorption.
Implementation on a Se/CMOS micro-sensor prototype with 7.8\mum element size with image pixel size approximately 50\mum showed a flat DQE curve (ideal) up to 10cycles/mm. AAP images of resolution test patterns, mammography phantoms, and specimen imaging of micro-calcifications from biopsies showed the expected improvements in SNR and visibility of fine-detail.
It is concluded that synthesizing image pixels from small physical sensor elements can increase MTF and DQE, and eliminate aliasing artifacts, for a desired image pixel size. The resulting increase in SNR may benefit all forms of radiography, and in particular mammography, where accurate visualization of fine detail is important for early cancer detection
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Symmetry breaking and electrostatic attraction between two identical surfaces
This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.By allowing the surface charge of one surface to affect the adsorption equilibrium of the other, we establish the existence of a long-range attractive interaction between two identical surfaces in an electrolyte containing polyvalent counter ions with a mean-field Poisson-Boltzmann approach. A Stern electrostatic condition from linearization of the mass-action adsorption isotherm is used to capture how polyvalent ion condensation affects and reverses the surface charge. We furthermore establish a direct mapping between this Stern layer conditions and previously derived modified Mean-field formulations associated with correlated fluctuations theory. For a sufficiently potential-sensitive isotherm, antisymmetric charge inversion can occur to produce an attractive force that increases with decreasing ionic strengths. Analyses of a mass-action isotherm produce force-separation relations, including an exponential far-field force decay distinct but consistent with previously proposed correlated fluctuation theories, and in quantitative agreement with experimental data
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Blood pressure distribution in microvascular networks
This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Blood rheology is complex and nonlinear. The effective viscosity variations are important due to red blood cells packing inside capillaries, the socalled FåhræusLindquist
effect, whilst concomitantly phase segregation appears in bifurcations. We have performed direct numerical simulations of different nonlinear rheological models of the blood on realistic threedimensional
microvascular networks. These simulations
point out two significant results. First, various rheological models lead to very similar pressure distributions over the whole range of physiologically relevant hematocrits. Secondly, different models for phase segregation lead to very distinct hematocrit distributions in the microvascular
network. Moreover, for all the investigated rheological models, the hematocrit distribution very weakly affects the pressure distribution, when prescribing uniform pressure boundary conditions.The research was supported by GDR n° 2760 Biomécanique des fluides et des transferts Interaction fluide/structure biologique, the
ASUPS A03 and A05 of Paul Sabatier University, Toulouse, France and the ANR project ANR06BLAN023801
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Effective Transport Template for Particle Separation in Microfluidic Bumper Arrays
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Microfluidic bumper arrays are increasingly being used for the size-based sorting of particle
suspensions. The separation mechanism is based on the interaction between a spatially periodic array of obstacles
and the suspended particles as they are driven through the obstacle lattice either by volume forces or
by the Stokesian drag of the surrounding fluid. By this mechanism, a focused stream of suspended particles
of different sizes entering the lattice splits into different currents, each entraining assigned ranges of particle
dimensions, and each characterized by a specific angle with respect to the main device axis. In this work, we
build up on recent results stemming from macrotransport process theory to derive a closed-form solution for
the steady-state distribution of advecting-diffusing particles in the presence of anisotropic dispersion, which
typically characterizes large-scale behavior of particle motion through the periodic lattice. Attention is focused
on separation resolution, that ultimately controls the feasibility of the separation in specific applications
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Experimental investigation on self-similar heat sinks for liquid cooled electronics
This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.The high heat transfer coefficients in microchannels are attractive for direct cooling of electronic systems requiring high heat-flux removal. In this work we are presenting the results of a study on self-similar heat sinks for liquid cooled electronics, made from copper, designed for industrial application and for large scale production. The internal structures, where the most part of the active cooling takes place, have been designed in order to achieve high heat transfer coefficients. As it is almost impossible to validate the design and describe the flow characteristics inside the device via analytical solutions, a well known numerical code was employed to have an insight of the thermal-fluid
distributions. It is clear from the simulation that even if copper is characterized by a high thermal conductivity, most of the heat is removed in the overflow-structure, on the side of the device adjacent to the source of heat. This paper attempts to critically analyse a comprehensive list of data as well as plots in order to illustrate the significant characteristics of this type of device
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Transport properties of fluids in nanochannels: bridging nano to macro
This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.A method of calculating transport properties in nanochannels is presented in this work. The Molecular Dynamics simulation of a system of liquid argon flowing in a nanochannel formed by krypton walls was the basis for our analysis concerning transport properties and specifically diffusion coefficient, shear viscosity and thermal conductivity. It is shown that for confined systems, such as nanochannels, if one of the transport properties is known, then the others can be estimated. The simulation results reveal that all properties approach bulk values at relatively small channel widths, at about 6-7nm. Below this critical point, the wall effect on fluid atoms is strong and the transport properties change dramatically. In order to extend the calculations over rough-wall nanochannels, we apply the relation extracted for flat wall channels to channels with walls consisted of successive rectangular protrusions and cavities
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Dispersion phenomena in microchannels: Transition from Taylor-Aris to convection-dominated regime
This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.This article addresses the qualitative and quantitative properties of solute transport and dispersion in microchannel of finite-length. As the Peclet number increases a transition from the Taylor-Aris to a new
regime referred as convection dominated dispersion occurs, which is controlled by the velocity profile near the stagnation points at the solid walls. The properties characterizing dispersion dominated regime can be used for analytical purposes as a chromatographic-based velocimetry and for determining the eventual occurrence of slip at the solid walls of microchannels
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Transport properties and structure of fluids in hydrophobic/hydrophilic nanochannels
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.When downsizing towards the nanoscale, system dimensions have been found to affect channel flows mainly
because of the presence of the walls that interact strongly with fluid particles. Parameters which are not taken into account at the classical theory continuum theory at the macroscale, should be taken into account at the nano or even micro-scale where the surface to volume ratio increases significantly. Such property is the wall/fluid interaction which determines the wetting (hydrophilic behavior) or not (hydrophobic behavior) of a surface. We first investigate the effect of wall/fluid interaction on fluid atom distribution near the wall through the radial distribution function and, next, we calculate the three most important fluid transport properties, i.e., the diffusion coefficient, shear viscosity and thermal conductivity. Transport properties seem to be affected significantly in the channel region adjacent to the wall
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Reversion scheme for droplet parameters with rainbow refractometry based on Debye theory
This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Rainbow refractometry is a non-intrusive technology for determining the refractive index and diameter of droplet simultaneously. Most of the present schemes for the refractive index and diameter of droplet are based on empirical formulas with Airy theory. However, the anti-noise capability and the generality of the empirical method are weak. In the paper, an objective function was designed to quantify the deviation between the low frequency component of the captured rainbow and the simulated rainbow with Debye (p=2) theory. Further, a novel inversion scheme for single droplet based on Debye (p=2) theory and the objective function was proposed. Experiments were carried out to evaluate the performance of the scheme. Results indicate that the relative error of the radius is less than 8%, the absolute error of the refractive index is better than 5×10-4.Research Award Program for Outstanding Young
Teachers in Southeast University (No.3203001202) and QingLan Project (No.1103000126)
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