Virtual Source Concept Research Articles

Enabling the autofocus approach for parameter optimization in planar measurement geometry clinical optoacoustic imaging.

In optoacoustic (photoacoustic) tomography, several parameters related to tissue and detector features are needed for image formation, but they may not be known a priori. An autofocus (AF) algorithm is generally used to estimate these parameters. However, the algorithm works iteratively and is therefore impractical for clinical imaging with planar geometry systems due to the long reconstruction times. We have developed a fast autofocus (FAF) algorithm for 3D optoacoustic systems with planar geometry. Such an algorithm exploits the symmetries of the planar geometry and a virtual source concept to reduce the dimensionality of the parameter estimation problem. The dimensionality reduction makes FAF much simpler computationally than the conventional AF algorithm. We show that the FAF algorithm required about 5s to provide accurate estimates of the speed of sound in simulated data and experimental data obtained using an imaging system that is poised to enter the clinic. The applicability of FAF for estimating other image formation parameters is discussed. We expect the FAF algorithm to contribute decisively to the clinical use of optoacoustic tomography systems with planar geometry.

A Semi-Analytic, Numerically Efficient Model for Low-Frequency Sound Scattering by an Infinite Cylinder Located Near a Boundary

Scattering of sound by a target can be described as a wave radiated by virtual point sources inside the target. In the Rayleigh scattering regime, the strength of the virtual sources can be calculated analytically. When a target is located close to the ocean surface or another reflecting boundary, reflections of the incident and single-scattered waves from the boundary lead to multiple scattering from the target, with the target being insonified by nearby virtual sources. At low frequencies and for shallow targets, the distance from a virtual source to the target is not necessarily large compared to the acoustic wavelength or the target’s dimensions. Then, multiple orders of scattering make significant contributions and incident wave fields that cannot be approximated as a plane wave. This paper takes advantage of the virtual source concept and recently derived explicit analytic representations of 2D acoustic Green’s functions in unbounded fluids with inclusions of a circular cross-section, to develop a simple, intuitive, and numerically efficient model of multiple scattering. Scattering from soft, hard, and fluid targets is considered. The model is used to study the acoustic field in the vicinity of cylindrical targets near a pressure release surface or a hard bottom.

Modeling organic thin-film transistors based on the virtual source concept: A case study

We apply the virtual source concept to describe the DC current-voltage characteristics of organic thin-film transistors. The draining of charge carriers from the virtual source is calculated using emission-diffusion theory. The electrical characteristics of organic thin-film transistors are commonly analyzed employing a drift mobility that is enhanced by the transistor overdrive. One possible origin of such a mobility model is charge trapping. We show that the direct parametrization of the fraction of mobile to total charges including trapped ones allows for a straight forward adaptation of the virtual-source emission-diffusion theory to organic electronics capable of describing measured transfer and output curves with a small number of parameters. The resulting model offers an alternative parametrization of well-known bias and temperature dependences. Beyond others, a limited charge injection does not arise from the high-field bulk saturation velocity but from the unidirectional thermal velocity and the charge carrier mean free path is a critical model parameter. Moreover, diffusivity replaces the drift mobility as the current scaling factor, which would lead to slightly different predictions for the current-voltage curves in the case that the Einstein relation is not valid.

Scattering of low-frequency sound by shallow underwater targets

Rayleigh scattering of sound by a target can be described as a wave radiated by virtual point sources inside the target. When a target is located close to the ocean surface or another reflecting boundary, reflections of the incident and single-scattered waves from the boundary lead to multiple scattering from the target, with the target being insonified by nearby virtual sources. At low frequencies and for shallow targets, the distance from a virtual source to the target is not necessarily large compared to acoustic wavelength or the target’s dimensions. This paper takes advantage of the virtual source concept and recently derived explicit analytic representations of 2-D and 3-D acoustic Green’s functions in unbounded fluids with inclusions of a circular cross-section, to develop a simple and numerically efficient model of multiple scattering. Scattering from soft, hard, fluid, and solid objects is considered. The model is used to study the acoustic field in the vicinity of shallow targets, calculate normal-mode compositions of the multiple-scattered field, examine conditions of applicability of the single-scattering approximation, and investigate implications of multiple scattering for target detection and classification.

The upgrade of the cold neutron three-axis spectrometer IN12 at the ILL

After nearly 40years of successful operation the cold three-axis spectrometer IN12 at the Institut Laue-Langevin, Grenoble, France, has been relocated to a new position and the primary spectrometer has been upgraded. Latest modern optical components are employed. A new guide in combination with a virtual source concept and a double focusing monochromator guarantee highest flux. With its high unpolarized and polarized neutron flux IN12 allows for demanding experiments. A velocity selector in the guide ensures a clean beam and a very low background. A gain in flux of about an order of magnitude at the sample position has been achieved compared to the previous instrument and IN12׳s wavelength range now extends far into the warmish region.

MIT virtual source GaNFET‐high voltage (MVSG‐HV) model: A physics based compact model for HV‐GaN HEMTs

AbstractA new physics based transport and charge model for long channel GaN HEMTs is proposed. The model is based on the concept of virtual source (VS) carrier transport (A. Khakifirooz et al., IEEE‐TED 56(8), 1674 (2009) [1]) and is extended to drift diffusion regime. The model is fully scalable from non‐velocity saturation to velocity saturation regime. In addition, non‐linear access regions are modeled as implicit‐gated transistors. Self‐heating effects and self‐consistent charge models are included. The model is validated against state of the art industry devices. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Upgrade of the primary spectrometer of the cold triple-axis spectrometer FLEX at the BER II reactor

Monte Carlo simulation results for the upgrade of the FLEX/V2 cold neutron triple-axis spectrometer are presented. The main goals are increased flux and energy resolution, while keeping a low background. In the same time the energy transfer range is significantly increased, bridging the gap between cold and thermal neutrons. We aim towards a flexible instrument, by employing the virtual source concept defining a beam before a double-focusing monochromator. This enables fine-tuning according to energy resolution, intensity and sample size requirements. A velocity selector adds the extra flexibility of tuning the spectrometer to any desired fixed wavevector. Neutron ray-tracing programs McStas and VITESS were extensively used and tested in this process.

Effects Influencing Focusing in Synthetic Aperture Vector Flow Imaging

Previously, a synthetic aperture vector velocity estimation method was proposed. Data are beamformed at different directions through a point, where the velocity is estimated. The flow direction is estimated by a search for the direction where the normalized cross-correlation peaks and the velocity magnitude along this direction are found. In this paper, different effects that influence the focusing in this method are investigated. These include the effect of phase errors in the emitted spherical waves, motion effects, and the effect of various interpolation methods in beam-forming. A model based on amplitude drop and phase error for spherical waves created using the virtual source concept is derived. This model can be used to determine the opening angle of a virtual source. Simulations for different virtual source placements are made, and it is recommended that the virtual sources be placed behind the aperture when shallow structures are imaged, and when deeper-lying structures are imaged the virtual sources be placed in front of the aperture. Synthetic aperture methods involve summation of data from numerous emissions. Motion between these emissions results in incoherence and affects resolution, contrast, and the signal-to-noise ratio. The effects of motion on the synthetic aperture vector velocity estimation method are investigated, and it is shown that for both axial and lateral motion, the contrast and signal-to-noise ratio can be seriously affected. A compensation method using the previous vector velocity estimate, when new data are beamformed, is implemented and tested. It is shown from a number of flow phantom experiments that a significant improvement with respect to bias and standard deviation of the velocity estimates can be obtained by using this compensation. Increased performance is gained at the expense of computation time. Different interpolation methods can be used for beam-forming the data. In this paper, the velocity estimation performance using various more complex interpolation schemes are compared to that using linear interpolation. No significant difference in the performance of the method is seen when other interpolation methods are used.

Output-only modal analysis using blind source separation techniques

The present study carries out output-only modal analysis using two blind source separation (BSS) techniques, namely independent component analysis and second-order blind identification. The concept of virtual source is exploited and renders the application of these BSS techniques possible. The proposed modal analysis method is illustrated using numerical and experimental examples.

Physical interpretation of independent component analysis in structural dynamics

This paper focuses on the relation between the vibration modes of mechanical systems and the modes computed through a blind source separation technique called independent component analysis (ICA). For free and random vibrations of weakly damped systems, a one-to-one relationship between the vibration modes and the ICA modes is demonstrated using the concept of virtual source. Based on this theoretical link, a time-domain structural system identification technique is proposed and is illustrated using numerical examples.

Shortest Path Auction Algorithm Without Contractions Using Virtual Source Concept

In this paper, the problem of finding a shortest path tree rooted at a given source node on a directed graph (SPT) is considered. A new efficient algorithm based on a primal-dual approach is presented, which improves both the convergence and the complexity of the best known auction-like algorithm. It uses the virtual source (VS) concept based on the following consideration: when a node i is visited for the first time by any algorithm which preserves verified the dual admissibility conditions, then the shortest path (SP) from the source node to i is found. Therefore, the SP from the source to the remaining nodes may be computed by considering i as a “virtual source”. We propose a very efficient implementation of an auction-like algorithm that uses this concept and enables us to obtain a computational cost of O(n2), where n is the number of nodes. Numerical experiments are reported showing that the new method outdoes previously proposed auction-like algorithms and is highly competitive with other state-of-art SP approaches.

Novel in situ method for locating virtual source in high-rate electron-beam evaporation

The concept of virtual source simplifies calculation of thickness distribution on extended substrates in high rate vacuum coating employing electron-beam heating. The height of the point (virtual source), from which vapor can be assumed to emanate in accordance with Knudsen’s cosine law, to yield the experimentally obtained thickness distribution, is calculated and this establishes the position of virtual source. Such as post facto determination is cumbersome as it is valid for the prescribed material evaporating at a certain rate in a specified geometry. A change in any of these entails a fresh measurement. Experimenters who use a large number of materials and deposit at different rates therefore have to carry out a number of trials before they can locate the virtual source at the desired deposition parameters. An in situ method for obtaining virtual source position can go a long way in reducing the labor of these experiments. A novel in situ method is described to locate the virtual source.

Is the 'fictitious' virtual source a redundant concept for scanned therapeutic electron beams?

Fermi-Eyges small-angle multiple scattering theory has excellent predictive power in air for magnetically scanned therapeutic electron beams. Modelling such beams by a collimated isotropic source of electrons allows the variation of beam output due to lateral scatter disequilibrium and beam profile full width at half maximum (FWHM) to be accurately predicted with changes in beam energy and beam geometry. A limitation of the theory for beam output is that backscattered or forward-scattered radiation from the photon collimators is not modelled by Fermi-Eyges theory and therefore empirical data must be obtained for normalisation purposes with changes in field size. Incorporating this empirical data into the model obviates the need for the fictitious virtual source concept with magnetically scanned beams.

Deterministic Properties of Turbulent Diffusion Flames from Low Q* Fires

General geometry and height of diffusion flames and the height-dependence of centerline temperature above propane burners of relatively low heat release rate for the fuel size are described. The flame behavior is classified into 3 regimes according to the difference in the height-dependence of centerline temperature and in the general behavior of the flame. Flame height is represented as a function of Q∗ = Q/ρ0CpT0g1/2D5/2. The criterion for the adequacy of the concept of virtual source for the prediction of centerline temperature from the point-source turbulent plume theory is discussed.

On the finite line source problem in diffusion theory

A simple formula for calculating dispersion from a continuous finite line source, placed at right angles to the mean wind direction, is derived on the basis of statistical theory. Comparison is made with the virtual source concept usually used and this is shown to be correct only in the limit where the virtual time lag T v is small compared to the timescale of the turbulence t 1.

Design Criteria for Cooling-Water Outlet Structures

This paper describes some laboratory studies utilizing buoyant and nonbuoyant tracers to investigate the effects of the initial conditions on the lateral mixing of cooling water or wastewater discharges into rivers. The paper begins with a short phenomenological description of the dispersion of cooling-water side discharges into rivers from which a concept for quantifying the effect of the outfall parameters on the near-field mixing is then deduced. By means of a virtual-source concept and two bulk parameters describing the stability of the near-field density stratification and the strength of an assumed reference source, the results can be presented in a straightforward graphical form. These graphs provide a first estimate of design criteria for an outlet structure which, e.g., is to be designed to maximize the rate of near-field mixing for a given cooling-water discharge.