Point source

About: Point source is a research topic. Over the lifetime, 5077 publications have been published within this topic receiving 94091 citations.

The experimental study of fatigue crack detection using scanning laser point source technique

Abstract: For the purpose of better understanding the interaction of Rayleigh wave and the fatigue crack in a metallic sample, a set of experimental setups is built, based on the scanning laser source (SLS) technique, utilizing a point source to take place of the line source to generate surface acoustic waves (SAWs), and an interferometer is to detect the SAWs signal. The information of the crack (such as position and length) can be obtained by utilizing a two-dimensional scan of the material surface. This paper focuses on the detection of visible and invisible fatigue crack by using this point-source-based scanning laser source technique, and comparing the results with those of conventional pitch-catch technique. The result shows that with two-dimensional scanning, and analyzing the amplitude of the generated SAWs, not only the visible fatigue can be identified, but also the invisible fatigue crack can be discriminated. As a result, the sensitivity of the scanning point laser source technique is higher than the conventional pitch-catch technique.

In vivo validation of distributed source solutions for the biomagnetic inverse problem.

Abstract: Probabilistic modelling of continuous current sources is applied to the analysis of MEG signals generated by current dipoles implanted in the head of a living human subject. Estimates of the distribution of activity within a circular disk are obtained from signals generated by a single implanted dipole and by a pair of simultaneously active implanted dipoles. The orientation and depth of the disc is determined in advance from the experimental geometry and the measurements. The resulting reconstructions constitute the first in vivo validation of distributed source imaging; they provide a complementary test to earlier works using computer generated data and tests using point source analysis of signals generated by a single implanted dipole. In this work we provide a literal test of spatial resolution by resolving two nearby point-like sources. Temporal resolution is addressed in a de facto manner by imaging at one millisecond intervals. Computer simulations, with controlled amount of noise, are used to demonstrate the robustness of the results, and show the interplay between high spatial accuracy and noise insensitivity.

Refraction of Waves from a Point Source into a Medium of Higher Velocity

Abstract: A point source is placed in one medium and the fields in a second medium, separated from the first by an infinite plane boundary, are calculated for the case that the wave velocity in the second medium exceeds that in the first medium. Absorption is neglected and the problem is solved both by ray methods and by evaluating, by the method of steepest descents, of an exact solution of the wave equation. The agreement between the solutions by different methods permits a ready and expected interpretation of the wave solution. Rays incident on the boundary at angles exceeding the critical angle are totally reflected; however, directly transmitted energy penetrates to all points in the second medium. At points in the second medium outside the critical angle, near the surface and far from the source, the directly transmitted fields are much smaller than the fields, exponentially decaying from the surface, which result from totally reflected rays. This suggests an experiment to measure quantitatively the penetration of the fields into the second medium in total reflection, easily performable for the case of sound waves penetrating from air into water.

The Infrared Imaging Spectrograph (IRIS) for TMT: sensitivities and simulations

Abstract: We present sensitivity estimates for point and resolved astronomical sources for the current design of the InfraRed Imaging Spectrograph (IRIS) on the future Thirty Meter Telescope (TMT). IRIS, with TMT's adaptive optics system, will achieve unprecedented point source sensitivities in the near-infrared (0.84 - 2.45 μm) when compared to systems on current 8-10m ground based telescopes. The IRIS imager, in 5 hours of total integration, will be able to perform a few percent photometry on 26 - 29 magnitude (AB) point sources in the near-infrared broadband filters (Z, Y, J, H, K). The integral field spectrograph, with a range of scales and filters, will achieve good signal-to-noise on 22 - 26 magnitude (AB) point sources with a spectral resolution of R=4,000 in 5 hours of total integration time. We also present simulated 3D IRIS data of resolved high-redshift star forming galaxies (1 < z < 5), illustrating the extraordinary potential of this instrument to probe the dynamics, assembly, and chemical abundances of galaxies in the early universe. With its finest spatial scales, IRIS will be able to study luminous, massive, high-redshift star forming galaxies (star formation rates ~ 10 - 100 M_Θ yr^(-1)) at ~100 pc resolution. Utilizing the coarsest spatial scales, IRIS will be able to observe fainter, less massive high-redshift galaxies, with integrated star formation rates less than 1 MΘsensitivity compared to current integral field spectrographs. The combination of both fine and coarse spatial scales with the diffraction-limit of the TMT will significantly advance our understanding of early galaxy formation processes and their subsequent evolution into presentday galaxies.

Measured spatially variant system response for PET image reconstruction

Abstract: Quantitative accuracy in PET imaging is essential for longitudinal studies and monitoring tumor response to treatment. The goal of this work is to improve the quantitative accuracy of whole-body PET imaging through the use of an accurate, measured system model. Past empirically measured system response functions used line sources positioned at various locations in the imaging field of view. Here, we present a practical method for measuring the detector blurring component of a whole-body PET system with a non-collimated point source. We employ Monte Carlo simulations to show that a non-collimated point source is acceptable for modeling the radial blurring present in a PET tomograph. And, we justify the use of a Na22 point source for collecting these measurements. We measure the system response, simplify it to a two-dimensional function, and incorporate a parameterized version of this response into a modified OSEM algorithm. Reconstructions of measured data from an image quality and line source phantom reveal improved quantitative accuracy and resolution with the modified system model.