Showing papers on "Point source published in 1991"

Frequency domain implementation of the three-dimensional geometric point response correction in SPECT imaging

Abstract: Analytical expressions exist for the geometric point response functions for parallel, fan, and cone beam collimators. The geometric point response functions are shown to be shift-invariant for these three collimators if the distance between the point source and the detection plane is constant. For single photon emission computed tomography (SPECT), this property is used in the projector/backprojector to correct for geometric distortion using the iterative EM reconstruction algorithm. In the implementation, the system geometric response, the crystal intrinsic response and the attenuation were compensated. The attenuation compensation was accomplished in the spatial domain. The point response corrections were accomplished using frequency domain filtering, taking advantage of the shift-invariance of the system point response function at a fixed depth. The frequency domain implementation significantly speeded up the algorithm. Phantom studies showed the increase of resolution and the reduction of density nonuniformity artifacts which were due to the spatially varying system response function and the attenuation effect. >

X-ray halos as diagnostics of interstellar grains

Abstract: Interstellar grains scatter X-rays from point sources into an observable halo surrounding the source image. Attention is presently given to the scattering of various grain-size distributions, both for halos around point sources and the cases of a source occulted by the moon. The limitations associated with using the X-ray halo to ascertain the size distribution of grains. The distribution of dust along the line-of-sight is noted to affect the inner part of the halo, rendering it more peaked toward the central point source if the dust is concentrated near the source. 30 refs.

Cross-plane scattering correction-point source deconvolution in PET

Abstract: The authors propose a new 2-D point source scattering deconvolution method. The cross-plane scattering is incorporated into the algorithm by modeling a scattering point source function. In the model, the scattering dependence on axial and transaxial directions is reflected in the exponential fitting parameters, and these parameters are directly estimated from a limited number of measured point response functions. The results comparing the standard in-plane line source deconvolution to the cross-plane point source deconvolution show that for a small source the former technique overestimates the scatter fraction in the plane of the source and underestimates the scatter fraction in adjacent planes. In addition, the authors also propose a simple approximation technique for deconvolution. >

Monte Carlo study of light propagation in optically thick media: point source case

Abstract: The results of a Monte Carlo study on light propagation in dense turbid media are presented. The calculations refer to the radiation emerging from a spherical scattering cell containing the diffusing medium (no diffusers outside the cell are considered) in whose center a point source is placed. Both the total scattered power emerging from the sphere and the impulse response were evaluated for a large range of optical depths and different types of diffuser. The results pertaining to both the radiance at the surface of the scattering cell and the impulse response are described by simple empirical relations. The results suggest a method of measuring the albedo and the asymmetry factor of the diffusing medium. A comparison with some results of the diffusion approximation is also presented.

The detection of X-rays from the hot interstellar medium of the Large Magellanic Cloud

Abstract: A comprehensive reanalysis is presented of the Einstein IPC data base on the LMC, and a number of new algorithms are used to improve the reliability of the point source detection in this crowded region and to produce the first large-scale map of diffuse emission free from the effects of solar scattered X-rays and cosmic-ray particles. Algorithms described in detail include a technique to decontaminate fields containing solar scattered flux, a mechanism for obtaining a spectrum-weighted vignetting function, and a source excision and smoothing algorithm which results in a diffuse map of uniform statistical quality. A catalog of discrete X-rays sources in the direction of the LMC is presented which contains 33 new sources and eliminates a number of spurious and/or marginal directions from previous lists. A possible detection of the long-sought shadowing effect on the cosmic X-ray background produced by cold LMC gas is reported.

Protostellar candidates in a sample of bright far-infrared IRAS sources

Abstract: CO and radio continuum emission has been observed in the direction of 39 bright 100-micron sources from the IRAS point source catalog in an attempt to better understand the early stages of star formation. All 39 sources are found to be associated with molecular clouds and are generally characterized by high temperatures (15-40 K) and wide wings in the profiles (up to 34 km/s). Twenty-six of the 39 sources show wide (C-12)O lines, which are interpreted as a result of gas outflow associated with star formation processes. Of the 24 sources for which distances and 6-cm flux densities can be assigned, 11 are premain sequence objects, and 4 others are possibly in this category. Six of the eleven premain sequence sources have associated continuum emission. 30 refs.

Modeling of the subsurface interface radar

Abstract: A finite-difference time-domain (FDTD) method is used to solve the 21 2 D problem of the response of an arbitrary source, in particular, an impulsive point source, in a two-dimensional isotropic inhomogeneous medium. Cosine and sine transforms are used to reduce the three-dimensional problem to two dimensions. The complete solution is obtained by linearly superimposing several transformed field components. This provides great savings in terms of computer storage and run time over the three-dimensional FDTD model. A criterion is given to ensure the stability of this finite difference scheme. Examples of application of this analysis to actual problems such as the subsurface interface radar are illustrated.

A broadband seismological investigation of the 1989 Loma Prieta, California, earthquake: Evidence for deep slow slip?

Abstract: Analysis of regional and teleseismic broadband body waves and long-period surface waves from the 1989 Loma Prieta earthquake demonstrates that a fairly simple average fault model explains most characteristics of seismic wave radiation over a broad period range (1 to 300 sec), but there are some systematic period-dependent characteristics that are not yet understood. Determinations of the preferred point source focal mechanism are consistent for different seismic waves, as long as optimal source-time functions and depths are chosen for each wavetype. The major double couple orientation compatible with the entire period range has a strike = 128 +_ 3 °, dip = 67 _+ 5 ° , and rake = 133 _+ 8% While this source mechanism consistency is encouraging and indicates that the average source process is quite simple, there are some systematic discrepancies in the source parameter estimates from different period waves. The source duration estimated from the principal body-wave ground motions is 8 + 2 sec, compatible with the duration of near-field strong-motion vibrations, but the surface waves indicate total durations of 18 to 30 sec, depending on the earth model selected for computing propagation corrections. The model dependence of the surface-wave results suggests that it may be possible to reconcile the duration estimates with improved earth models, but the discrepancy is quite large. There is some complexity in the source-time function, comprised of three subevents at different depths, but this complexity is only manifested in short-period signals and does not explain the duration discrepancy. Changes of fault geometry or slip direction during rupture, as suggested by recent finite-fault inversions, are unlikely to cause a bias in source duration estimates. The centroid depth determined from our body-wave analysis is 10 to 12 km, consistent with the finite-fault models, which indicate rupture extending from 5 to 18 km. Surface-wave centroid depth estimates (15 to 30 km) vary substantially for different choices of global Q model and lithospheric structure in the source region, but they do tend to be deeper than the body-wave results. It is again not clear whether this depth discrepancy is due to the source process or inadequate earth models. The body-wave and surface-wave seismic moment estimates are also not fully consistent; the moment from our body-wave inversions is 2.4 _+ 0.3 x 1019 N-m, while our surface-wave analysis gives 3.2 _+ 0.5 x 1019 N-m. It is not known whether reasonable changes of the earth models can bring the moment estimates into full agreement. The systematic discrepancies in source duration, centroid depth, and seismic moment, despite the consistency in source orientation, raise the possibility of a deep, relatively slow co-seismic slip component, but trade-offs with modeling assumptions preclude us from confirming this hypothesis at present.

A Search of the Northern Sky for Ultra--High-Energy Point Sources

Abstract: A search has been made for steady emission or ultra-high-energy radiation from individual point sources over most or the northern sky. Over 2×10 8 air showers induced by primary particles or energy greater than about 10 TeV were recorded from 1986 April until 1991 May by the CYGNUS air shower array. No statistically significant excess above the background from the isotropic flux or cosmic rays was found for any direction in the sky from 0° to 80° declination. In addition, 49 specific potential sources were examined, and none showed a statistically significant excess or events

A Numerical Procedure for Calculating the Effect of a Turbid Medium on the MTF of an Optical System

Abstract: A Monte Carlo based method of calculation of the effect of a turbid medium on the modulation transfer function (MTF) of an optical system is presented. A positive comparison of the results was made with published experimental data of Kuga and Ishimaru both for small and large size particulate. The comparison also showed that one limitation of the small angle approximation (SAA) scheme of calculating the MTF can come from the choice of the object point source. Our numerical data obtained by using a Lambertian source agree well with all the experimental data, while the data obtained for an isotropic point source agree with the theoretical data based on the SAA scheme in the case of a particulate not small in comparison with the wavelength.

Inversion of seismograms to determine simultaneously the moment tensor components and source time function for a point source buried in a horizontally layered medium

Abstract: A method of determining simultaneously the moment tensor and source time function in the point source approximation is presented. For trial values of the moment tensor components and of the source time function, parametrized by the sum of overlapping triangles delayed in time, theoretical seismograms can be synthetized and compared with the recorded ones. The iterative procedure determines the adjustment of source parameters until a good correlation of both synthetic and observed records is reached. The Green functions in a horizontally stratified medium are constructed with the use of a modal summation method. The limits of applicability of the algorithm are illustrated by the inversion of four synthetic seismograms constructed for two horizontally stratified models of the structure in Friuli area, Italy. The records constructed for the same structural model as for which the Green functions were computed can be inverted even in the high-frequency range. In the opposite case, when the records and Green functions used corresponded to different structural models, a good correlation of the input records with the final synthetics was obtained for low - pass filtered data only. Additional tests performed with input seismograms contaminated with random noise yielded good resolution of the moment tensor and the duration of the source time function even for a high noise to signal ratio.

Piezomagnetic Field Associated with the Mogi Model Revisited

Abstract: The piezomagnetic field associated with the Mogi model was reexamined for the point source and the finite spherical source problems. In the point source case the question was how to deal with divergent stresses near the pressure source. In SASAI's (1979) method, the magnetized crust was divided into two layers, i.e. the upper half shallower than the source (0

The Infrared Properties of Compact Galactic Radio Sources: The Young and the Restless

Abstract: The IRAS Point Source Catalog has been compared with the 20 cm Galactic plane survey (GPS) of Zoonematkermani S. et al. (1990, Astrophys. J. Sup. Ser., 74). A new technique for distinguishing chance from true coincidences using the bands at which the IRAS sources are detected is developed. For each match, an estimate of the probability that the match represents a true association is derived. Of the 1992 radio sources, 330 have high probability associations, with most having reliabilities above 90%. The GPS/IRAS sources are clearly Galactic: they have a very restricted distribution in both Galactic latitude and longitude.

Stationary MHD waves generated by a source in a moving plasma

Abstract: The inductive interaction between a conducting body and a magnetized plasma in relative motion is formulated in terms of a source driving stationary magnetohydrodynamic waves in the frame of the body. Of particular interest is the existence of analytic solutions which describe the stationary wave pattern associated with a point source current. Such solutions are significant because not only do they, in principle, facilitate the construction of more general solutions by serving as Green's functions but also they provide an asymptotic description of the far field of the stationary wave pattern generated from each point of a source of finite size. The total pressure perturbation exhibits a Laplacian type behavior in the case of “subsonic” flow and a Mach cone like disturbance for “supersonic” flow. On the other hand, the associated electric potential response is mixed in nature in that it consists of compressive magnetoacoustic perturbations as well as the well-known Alfven wings corresponding to the shear Alfven mode. In the subsonic case it is shown that although there is a strong disturbance in the potential on the Alfven wings it is not narrowly confined to these wings by virtue of the two-dimensional dipolelike contributions from the fast magnetoacoustic mode. In supersonic flow the potential disturbance is swept downstream and confined to the fast Mach cone, on which it is singular and within which lies the other singularity on the Alfven wings.

Earthquake Ground Motion Modeling. I: Deterministic Point Source

Abstract: A model for earthquake ground motion is developed using principles of geophysics and stochastics in a sequence of two papers. In the first paper, the earth is idealized as being composed of horizontally stratified layers, with uniform physical properties for each layer. The seismic source is modeled as a concentrated seismic moment located within one of the layers. The partial differential equations for the seismic motion in each layer are solved using a Fourier finite-Hankel transformation approach, and solutions in terms of state vectors of displacements and forces are converted to wave vectors composed of up-going and down-going waves. Transmission and reflection matrices are obtained for each layer, for each layer-to-layer interface, and for the free ground-surface boundary to characterize their roles in transmitting and reflecting a wave motion. The use of these matrices allows the numerical calculation to be channeled in the directions of wave propagation, resulting in better numerical accuracy. An example is included for illustration.

Some parameters and characteristics of an off-axis paraboloid

Abstract: A reflecting off-axis paraboloid is frequently used either to collimate the light from a point source or to concentrate in a point the light from a collimated beam. Because these paraboloids are expensive and difficult to manufacture, a spherical mirror is frequently used instead. The aberrations arising with this substitution are analyzed and evaluated.

Wavefield extrapolation of body waves for 3-D imaging of earthquake sources

Abstract: Summary Reverse-time imaging of earthquake source parameters is extended from two to three spatial dimensions and from two- to three-component recordings. Provided that the recording aperture is sufficiently large, and the data are not spatially aliased, source radiation patterns, in both time and space, can be reconstructed by elastic reverse-time propagation of body waves; such reconstructions are necessarily partial as only the energy that was recorded is available for reconstruction. For a point source, the origin time and the (3-D) spatial location of an event can be reconstructed by extracting the time and position, of the best focused energy from the backward propagating wavefield. For a spatially and temporally extended source, biased estimates of fault position and rupture/slip time history can be estimated if the recording aperture is sufficient; for smaller recording apertures, the time history can still be estimated if the fault location and geometry are known a priori. The latter is viable for a reasonable number (<200) of three-component recordings, and so is potentially applicable to real data. All these processes assume that a smoothed representation of the 3-D velocity distribution in the volume containing the source and receivers is available.

Improving projection lithography image illumination by using sources far from the optical axis

Abstract: In a projection lithography system having fly‐eye elements, a virtual source is created as an array of approximately mutually incoherent point sources. This paper describes simulated results of the light amplitude and phase of the mask‐projected image for a point source and discusses the dependence on the point source location on a plane situated perpendicular to the optical axis. We showed that the projected image illuminated by point sources far from the optical axis was improved by the effect of interference between multiple apertures. Resolution of the 0.4 μm lines and spaces was improved theoretically and experimentally at a wavelength of 435.8 nm and a numerical aperture of 0.45.

Enhanced backscattering from a plane mirror viewed through a turbulent phase screen

Abstract: The distribution of average intensity scattered by a plane mirror placed behind a turbulent phase screen is experimentally investigated for a point source of light. It was found that the average intensity distribution in the source plane coincides with the correlation function of intensity fluctuations of a virtual source located at the mirror and observed from the real source plane. Nonmonotonic dependence between the enhancement factor and the diameter of the mirror is observed in experiments. Experimental results are in good agreement with theoretical predictions.

A Review of the Kelvin Ship Wave Pattern

Abstract: This paper finds that the Kelvin ship wave induced by a moving pressure point consists of a single system of divergent waves but that there are infinitely many different Kelvin wave patterns for a submerged point source.

Far-field radiation of a vibrating point source in anisotropic media

Abstract: In this paper, Lamb's technique,(1) which was used to study acoustic radiation due to an external force acting in an infinite isotropic solid or over the surface of an isotropic solid, has been extended to the case of general anisotropic media. The far-field radiation is numerically calculated for a vibrating point source in an infinite anisotropic solid or on the free surface of a semi-infinite anisotropic solid. The directivity patterns of a point source are obtained for the three different elastic modes, and a brief discussion of the main features of the numerical results is presented.

Experimentally determining a parametric model for the point source response of a gamma camera (SPECT)

Abstract: The authors experimentally investigate the response of a gamma camera to a source as a function of depth of the source in water and of distance from the gamma camera. They compute a least-squares estimate of the FWHM (full width at half maximum) of the camera line spread function as an affine function of these two parameters. They demonstrate a method for experimentally estimating the parameters of a camera response model and use this approach to estimate the parameters of a 2-D Gaussian model for the camera point source response in a true 3-D formulation. The 2-D Gaussian model was shown to yield good agreement with experimental data. >

The modified diffusion dipole model

Abstract: Being able to predict how light interacts with tissue is important in photodynamic therapy (PDT). Mathematical models have been constructed to predict light dose within a tissue, the nature of the model being dependent upon the illumination geometry. One of the more complex models to develop involves a superficial point source, and to date, existing models for this illumination geometry have involved simplifying assumptions or have been mathematically complicated. A modified diffusion dipole model for predicting the light fluence rate (or space irradiance) due to a point source of light on a tissue surface is derived. Based on the diffusion dipole model of Fretterd and Longini (1973), the modified diffusion dipole model incorporates a more realistic boundary condition, obviates the need to approximate the monopole separation, and is mathematically simple. Space irradiance isodoses within optical phantoms illuminated by a superficial point source were measured using an isotropic detector.

The Near-Field Singularity Predicted by the Spiral Green's Function in Acoustics and Electrodynamics

Abstract: The retarded solution of the wave equation for a point source in circular motion, whose speed exceeds the wave speed, is singular on a spiralling tube-like surface that is at rest in the rest frame of the point source. When solving the wave equation for a corresponding extended source, therefore, we are faced with integrals over the volume of the source which are improper and need to be handled either with the aid of the theory of generalized functions or by Hadamard’s method of finite parts. In this paper, after isolating the finite part of the gradient of the retarded potential due to a rotating extended source, we calculate the asymptotic values of the coefficients in its Fourier representation and show that the radial component of this gradient does not remain finite at those points within the source which move with the wave speed, and so lie on the boundary of the domain of hyperbolicity of the equation of the mixed type to which the wave equation in this case reduces. This latter singularity arises because the problem in question, though well posed physically, is in fact mathematically ill posed.