Compensating for nonstationary blurring by further blurring and deconvolution

In many imaging systems, the point spread function (PSF) is nonstationary. Usually, a computation‐intensive iterative algorithm is used to deblur the nonstationary PSF. This article presents a new idea of using a noniterative method to compensate for the spatially variant PSF. This method first further blurs the image with a nonstationary kernel so that the resultant image has a stationary PSF, then deblurs the resultant image using an efficient decovolution technique. The proposed method is illustrated and implemented by single photon emission computed tomography applications. © 2009 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 19, 221–226, 2009     

An iterative reconstruction algorithm for single photon emission computed tomography with cone beam geometry

An iterative EM reconstruction algorithm for single photon emission computed tomography is implemented for cone beam geometry that uses a ray-driven projector-backprojector. The cone beam projector-backprojector models the attenuated Radon transform of a source distributed within an attenuator as line integrals of discrete voxels representing the source and attenuation distributions. The attenuation coefficient distribution for each voxel is assumed to be equal to the average value over a cubical region, and the integral of the source concentration through each voxel is obtained by interpolating between the source values at eight neighboring voxels. The calculation of the attenuation factors requires a specification of the attenuation distribution, estimated either from an assumed constant distribution with an estimated body outline or from transmission measurements. The distribution of attenuation coefficients is stored in memory, and the attenuation factors are calculated for each voxel during the projection and backprojection operations instead of using precalculated values. Computer simulations of half-scan and full-scan reconstructions show that the algorithm is able to compensate for attentuation and to suppress the propagation of artifacts that can result with limited angular sampling. The simulations verify that the algorithm has important application to cardiac SPECT imaging; because of variable attentuation through the thorax, it is necessary to use an iterative algorithm with an attenuation map in order to quantify radiopharmaceutical distributions in the heart.     

Three-dimensional fluorescence enhanced optical tomography using referenced frequency-domain photon migration measurements at emission and excitation wavelengths

The ultimate success of near-infrared optical tomography rests on the precise measurement of light propagation within tissues or random media, the accurate prediction of these measurements from a light propagation model, and an efficient three-dimensional solution of the inverse imaging problem. To date, optical tomography algorithms have focused on frequency-domain photon migration (FDPM) measurements of phase-delay and amplitude attenuation, which are reported relative to the incident light, even though phase-delay and amplitude of incident light are nearly impossible to measure directly. In this contribution, we examine referenced, fluorescence-enhanced frequency-domain photon migration measured at excitation and/or emission wavelengths and report on a measurement strategy to minimize measurement and calibration error for efficient coupling of data to a distorted Born iterative imaging algorithm. We examine three referencing approaches and develop associated inversion algorithms for (1) normalizing detected emission FDPM data to the predicted emission wave arising from a homogeneous medium, (2) referencing detected emission FDPM data to that detected at a reference point, and (3) referencing detected emission FDPM data to detected excitation FDPM data detected at a reference point. Our results show the latter approach to be practical while reducing the nonlinearity of the inverse problem. Finally, in light of our results, we demonstrate the method for eliminating the influence of source strength and instrument functions for effective fluorescence-enhanced optical tomography using FDPM.     

一种声发射源的新型平面定位方法研究

声发射现象中产生的弹性波在传播时声压遵从指数衰减规律,文中提出一种利用声衰减特性对声发射源进行平面定位的能量定位新方法。从理论上导出了该方法的定位计算公式,并用AE21C型声发射仪在高分子合成纤维平板上以铅笔芯折断产生信号为模拟源进行了实验测量,证明这种新的源定位方法可行,并具有无须测量传声媒质的声衰减和声速等优点。     

声发射检测中利用能量进行定位的新方法

声发射检测过程中,对缺陷的定位是重要的一个步骤,传统的声发射检测利用时差定位方法,而时差定位里重要的参数是声速。通过推导,得到一种利用声衰减特性和能量参数对声发射源进行定位的新方法,并用声发射仪对普通钢件以铅笔芯折断作为模拟源进行测试,证明这种无需声速测量的新方法可行,且准确性可以得到保证。该方法的提出为声发射定位技术拓展了新方向,对提高声发射检测质量有一定的帮助。     

Quantitative photoacoustic tomography from boundary pressure measurements: noniterative recovery of optical absorption coefficient from the reconstructed absorbed energy map

We describe a noniterative method for recovering optical absorption coefficient distribution from the absorbed energy map reconstructed using simulated and noisy boundary pressure measurements. The source reconstruction problem is first solved for the absorbed energy map corresponding to single- and multiple-source illuminations from the side of the imaging plane. It is shown that the absorbed energy map and the absorption coefficient distribution, recovered from the single-source illumination with a large variation in photon flux distribution, have signal-to-noise ratios comparable to those of the reconstructed parameters from a more uniform photon density distribution corresponding to multiple-source illuminations. The absorbed energy map is input as absorption coefficient times photon flux in the time-independent diffusion equation (DE) governing photon transport to recover the photon flux in a single step. The recovered photon flux is used to compute the optical absorption coefficient distribution from the absorbed energy map. In the absence of experimental data, we obtain the boundary measurements through Monte Carlo simulations, and we attempt to address the possible limitations of the DE model in the overall reconstruction procedure.     

Wavelet‐based denoising and its impact on analytical SPECT reconstruction with nonuniform attenuation compensation

In single photon emission computed tomography (SPECT), the nonstationary Poisson noise in projection data (sinogram) is a major cause of compromising the quality of reconstructed images. To improve the quality, we must suppress the Poisson noise in the sinogram before or during image reconstruction. However, the conventional space or frequency domain denoising methods will likely remove some information that is very important for accurate image reconstruction, especially for analytical SPECT reconstruction with compensation for nonuniform attenuation. As a time‐frequency analysis tool, wavelet transform has been widely used in the signal and image processing fields and demonstrated its powerful functions in the application of denoising. In this article, we studied the denoising abilities of wavelet‐based denoising method and the impact of the denoising on analytical SPECT reconstruction with nonuniform attenuation. Six popular wavelet‐based denoising methods were tested. The reconstruction results showed that the Revised BivaShrink method with complex wavelet is better than others in analytical SPECT reconstruction with nonuniform attenuation compensation. Meanwhile, we found that the effect of the Anscombe transform for denoising is not significant on the wavelet‐based denoising methods, and the wavelet‐based de‐noise methods can obtain good denoising result even if we do not use Anscombe transform. The wavelet‐based denoising methods are the good choice for analytical SPECT reconstruction with compensation for nonuniform attenuation. © 2013 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 23, 36–43, 2013     

Dosimetry at 28 keV synchrotron radiation for cell irradiations

Accurate dosimetry is a prerequisite for reliable comparisons between radiobiological irradiation experiments. Parameters affecting the determination of absorbed dose to cells in the shape of a small cell pellet in a centrifuge tube, irradiated by 28 keV mono-energetic photons from a synchrotron, were investigated. Thermoluminescent dosimeter (TLD), diode and ion chambers were utilized to monitor the irradiations. The distribution of the absorbed dose and such parameters as scatter, attenuation and interface dosimetry in the target, which influence the dose, were studied. A method for inter-calibrations of two different calibration sources by using TLD and TLD readers is given. Characteristics of the TLD, that is, fading, supralinearity, energy response, self-attenuation and mini-dosimetry were considered for the dosimetry. A method for correcting photon fluence attenuation in cylindrical TLDs is presented. The study shows that the absorbed dose to cells irradiated at low photon energy at a synchrotron irradiation facility can, using accurate dosimetry protocol, be correctly and reproducibly determined.     

Simultaneous estimation of the 3-D soot temperature and volume fraction distributions in asymmetric flames using high-speed stereoscopic images

An inverse radiation analysis using soot emission measured by a high-speed stereoscopic imaging system is described for simultaneous estimation of the 3-D soot temperature and volume fraction distributions in unsteady sooty flames. A new iterative reconstruction method taking self attenuation into account is developed based on the least squares minimum-residual algorithm. Numerical assessment and experimental measurement results of an ethylene/air diffusive flame show that the proposed method is efficient and capable of reconstructing the soot temperature and volume fraction distributions in unsteady flames. The accuracy is improved when self attenuation is considered.     

Highly directional emission and photon beaming from nanocrystal quantum dots embedded in metallic nanoslit arrays

We demonstrate a directional beaming of photons emitted from nanocrystal quantum dots that are embedded in a subwavelength metallic nanoslit array with a divergence angle of less than 4°. We show that the eigenmodes of the structure result in localized electromagnetic field enhancements at the Bragg cavity resonances, which could be controlled and engineered in both real and momentum space. The photon beaming is achieved using the enhanced resonant coupling of the quantum dots to these Bragg cavity modes, which dominates the emission properties of the quantum dots. We show that the emission probability of a quantum dot into the narrow angular mode is 20 times larger than the emission probability to all other modes. Engineering nanocrystal quantum dots with subwavelength metallic nanostructures is a promising way for a range of new types of active optical devices, where spatial control of the optical properties of nanoemitters is essential, on both the single and many photons level.     

Scanning fluorescence correlation spectroscopy: a tool for probing microsecond dynamics of surface-bound fluorescent species

In this report, a combined imaging and fluorescence correlation spectroscopy (FCS) method is described and its ability to characterize microsecond fluctuations in the fluorescence emission of a sample is demonstrated. A sample scanning laser confocal microscope is operated in the customary way while recording the time that each photon is detected with a time resolution of 50 ns using a low-cost counting board. The serial data stream of photon detection times allows access to fluorescence signal fluctuations that can be used to characterize dynamics using correlation methods. The same data stream is used to generate images of the sample. Using the technique, we demonstrate that it is possible to characterize the kinetics of transitions to and from nonemitting or "dark" states of the fluorescent dyes DiIC16 and ATTO 520. Results are similar to, but deviate slightly from, a model that has been frequently used for extracting singlet-triplet: conversion rates using conventional solution-based FCS. Like conventional FCS, the concentration, or in our case the areal density of coverage, of fluorescent species can also be obtained. Many single-molecule fluorescence experiments aim to extract kinetics from intensity trajectories; this method may be used as a rapid and convenient technique for characterization of surface-linked or thin-film samples prior to performing the more time and effort intensive single-molecule measurements. Besides the capacity to measure photophysical phenomena, the surface-sensitive FCS method could also be applied for measuring conformational changes or interaction kinetics for species immobilized on a surface. One possible scenario is measurements of the frequency and duration of association of ligand-receptor pairs where a fluorescently labeled component is freely diffusing and the other is surface immobilized. Given that microarrays of custom-designed, surface-immobilized peptides and nucleic acids are now readily available, the ability to sensitively measure association and dissociation rates of the surface-linked species with a freely diffusing species could be a useful extension to what has already become an extremely important tool for characterizing the interactions of biomolecules.     

Enhancing radiographic imaging of cementitious materials in composite structures with photon attenuating inclusions

This article investigates the use of photon attenuating inclusions (PAI) for enhancing radiographic imaging of cementitious materials in composite structures. By targeting characteristic high attenuation regions in the photon energy spectrum, PAI can improve void detection in the parent material by increasing contrast in radiographic images. Radiographic imaging of cementitious grout specimens with 5% and 15% weight fractions of candidate PAI materials (Fe, BaCO₃, BaSO₄) was performed to characterize X-ray attenuation. Virtual radiography simulations were then conducted to evaluate the application of PAI to grout void detection in post-tensioned concrete construction. The simulations demonstrate that grout void detection is most effective when the high energy region of the emission spectrum is near the K-edge of the PAI material.     

Origine spatiale des électrons dans la structure à photoémission interne or-silicium

The photoelectric yield per incident photon for internal emission in a gold-silicon structure is calculated by a simulation process (the Monte Carlo method) with the classical hypothesis that only the volume effect is involved. The spatial distribution of photoexcited electrons as a result of multiple reflections of light inside the metallic film is introduced in the calculation. It is shown that the experimental yield may be correctly explained if in addition to the classical volume effect a photoelectric effect localized inside the metal near the metal-semiconductor interface is assumed.     

Photon emission as a tool for esd failure localization and as a technique for studying ESD phenomena

Photon emission microscopy gives the opportunity for non-destructive localization of failure sites on VLSI chips. Failures that can be detected using photon emission microscopy are gate-oxide breakdown, latch-up, junction breakdown and intermetal oxide failures. This makes it a valuable evaluation technique of failures in ESD protection circuits. Real-time photon emission observations of reverse-biased transistors showed that this technique is also an important tool in the evaluation and development of ESD protection circuits.     

Development of lung and soft tissue substitutes for photons

The use of solid tissue substitutes is a well-accepted and common practice in dosimetric studies and in the production of counting standards for radiological protection. However, only a few solid tissue substitutes simulating a particular body tissue with respect to a set of physical characteristics are commercially available. Hence, we have developed polyurethane-based tissue substitutes simulating soft tissue, muscle, muscle-adipose mixture tissue (90% muscle + 10% adipose), brain, cartilage, larynx, thyroid, trachea, liver, kidney, skin and lungs. Tissue substitutes for photons were formulated using the basic data method together with an equation for calculating the optimum relative mass of corrective additives. The tissue substitutes were formulated to be phantom materials in the photon energy range of at least 8 keV-10 MeV. In particular, they were designed to match the body tissues with linear attenuation coefficients for low photon energy (13.6, 17.2 and 20.2 keV from 239Pu) and to have the same mass densities as the tissues. The tissue substitutes developed in the present study were examined for the photon transmissions using 16.6 keV KX rays from 93Nb(m). The experimental transmission curves of the tissue substitutes were found to be consistent with those derived from data on the body tissues in ICRP Publication 23. It was found that the developed tissue substitutes are suitable to the corresponding body tissues defined by ICRP.     

Frequency-domain fluorescent diffusion tomography: a finite-element-based algorithm and simulations

We present a finite-element-based algorithm for reconstruction of fluorescence lifetime and yield in turbid media, using frequency-domain data. The algorithm is based on a set of coupled diffusion equations that describe the propagation of both excitation and fluorescent emission light in multiply scattering media. Centered on Newton's iterative method, we implemented our algorithm by using a synthesized scheme of Marquardt and Tikhonov regularizations. A low-pass spatial filter is also incorporated into the algorithm for enhancing image reconstruction. Simulation studies using both noise-free and noisy data have been performed with the nonzero photon density boundary conditions. Our results suggest that quantitative images can be produced in terms of fluorescent lifetime and yield values and location, size, and shape of heterogeneities within a circular background region.     

Calculation of radiation attenuation coefficients, effective atomic numbers and electron densities for some building materials

Some building materials, regularly used in Turkey, such as sand, cement, gas concrete (lightweight, aerated concrete), tile and brick, have been investigated in terms of mass attenuation coefficient (μ/ρ), effective atomic, numbers (Z(eff)), effective electron densities (N(e)) and photon interaction cross section (σ(a)) at 14 different energies from 81- to 1332-keV gamma-ray energies. The gamma rays were detected by using gamma-ray spectroscopy, a High Purity Germanium (HPGe) detector. The elemental compositions of samples were analysed using an energy dispersive X-ray fluorescence spectrometer. Mass attenuation coefficients of these samples have been compared with tabulations based upon the results of WinXcom. The theoretical mass attenuation coefficients were estimated using the mixture rule and the experimental values of investigated parameters were compared with the calculated values. The agreement of measured values of mass attenuation coefficient, effective atomic numbers, effective electron densities and photon interaction cross section with the theory has been found to be quite satisfactory.     

Correction of detector nonlinearity for the balloonborne Michelson Interferometer for Passive Atmospheric Sounding

The detectors used in the cryogenic limb-emission sounder MIPAS-B2 (Michelson Interferometer for Passive Atmospheric Sounding) show a nonlinear response, which leads to radiometric errors in the calibrated spectra if the nonlinearity is not taken into account. In the case of emission measurements, the dominant error that arises from the nonlinearity is the changing detector responsivity as the incident photon load changes. The effect of the distortion of a single interferogram can be neglected. A method to characterize the variable responsivity and to correct for this effect is proposed. Furthermore, a detailed error estimation is presented.     

A novel ultra-light structure for radiation shielding

A new ultra-light structure based on the application of open-cell metal foams has been designed and investigated to determine its ability for attenuation of γ-rays and thermal neutrons. Open-cell metal foam, a unique class of material, has been employed in the structure and is studied in this work where radiation attenuation abilities of foams and foams filled with water and borated water have been compared with bulk Aluminum. The γ-ray attenuation measurements were performed using γ-ray at 0.662, 1.173 and 1.332 MeV photon energies and thermal neutron attenuation measurements were conducted using a polyenergetic thermal neutron beam. The results show that the metallic foam by itself attenuates less γ-ray as compared to bulk material, while the mass attenuation coefficients of foams filled with water is higher than that of bulk metals. The thermal neutron experiment, on the other hand, has shown a dramatic attenuation improvement in foams filled with water and particularly with borated water as compared to bulk metal and foam.     

一种量测松散介质对应力波衰减效应的实验方法及其在珊瑚砂中的应用

砂土等松散介质对在其中传播的应力波有非常明显的衰减作用,因此,松散介质常常作为爆炸波消波材料被广泛应用于防护工程中。为了准确地量测松散介质对应力波的衰减效应,基于并改进了传统SHPB装置,提出了一种定量研究应力波在砂土介质中衰减规律的新方法。该文方法适用于所有在冲击荷载的应变率范围内（约1~102 s-1）应变率效应不明显的松散介质。方法基于拟合的透射系数T₂,通过杆中的三波（入射波、反射波和透射波）计算得到试件两端真实的峰值应力,还可以计算试件的弹性波速、峰值应力速度、试件端部应力波的前沿升时等关键参数,简单实用,可操作性强。采用提出的方法,对干燥珊瑚砂进行了应力波衰减实验,得出了应力波荷载峰值随传播距离的衰减规律。经对比实验与参数讨论发现,拟合透射系数引起的结果误差不超过2.83%,具有很好的可靠性与实用性。