Line Source Measurements Research Articles

Theory and application of laser ultrasonic shear wave birefringence measurements to the determination of microstructure orientation in transversely isotropic, polycrystalline graphite materials

Laser ultrasonic line source methods have been used to study elastic anisotropy in nuclear graphites by measuring shear wave birefringence. Depending on the manufacturing processes used during production, nuclear graphites can exhibit various degrees of material anisotropy related to preferred crystallite orientation and to microcracking. In this study, laser ultrasonic line source measurements of shear wave birefringence on NBG-25 have been performed to assess elastic anisotropy. Laser line sources allow specific polarizations for shear waves to be transmitted – the corresponding wavespeeds can be used to compute bulk, elastic moduli that serve to quantify anisotropy. These modulus values can be interpreted using physical property models based on orientation distribution coefficients and microcrack-modified, single crystal moduli to represent the combined effects of crystallite orientation and microcracking on material anisotropy. Ultrasonic results are compared to and contrasted with measurements of anisotropy based on the coefficient of thermal expansion to show the relationship of results from these techniques.

Line source representations for shear wave birefringence measurements in transversely isotropic materials using laser ultrasonics

Line source representations for thermoelastic, laser excitation of transversely isotropic materials are developed to model shear wave birefringence effects that can occur in these materials. Continuum descriptions for elastic wave propagation are used to generate equations of motion that are solved using transform methods. Analytical solutions are presented for epicentral waveforms for propagation parallel and perpendicular to the axis of symmetry. These solutions are particularly simple and allow for rapid examination of the influence of various components of the stiffness tensor on wave propagation. In particular, for propagation perpendicular to the axis of symmetry, line source orientations parallel and perpendicular to this axis yield two cases that demonstrate shear wave birefringence. This effect is computed for titanium as well as for a nearly isotropic material–both Class I–to highlight the influence of diagonal as well as off-diagonal components of the stiffness tensor on the character of laser ultrasonic waveforms when the tensor is referenced to the principal coordinate system. The results presented in this work should be used to guide the interpretation of laser ultrasonic, line source measurements in transversely isotropic materials when birefringence effects are present.

A study of the effects of vehicle emissions on the atmosphere of Sultan Qaboos University in Oman

Traffic jams have become a serious matter at Sultan Qaboos University (SQU) in Muscat, Oman due to an increase in student numbers. Therefore, the main objective of this research study is to apply the information gained by running CALPUFF software to model and investigate the concentration dispersion of the three most dominant vehicle emissions carbon monoxide (CO), mononitrogen oxides (NOx), and carbon dioxide (CO2) at SQU. The study was divided into two main parts: a line source study focused on investigating the dispersion of CO, NOx, and CO2 for a street located within the university's boundaries, and a study of the university domain as an area source. The study was based on statistical data collected on April 2, 2014, between 00h00 and 23h00 Local Standard Time (LST). Information related to both surface and upper air quality was obtained and processed into data files which were used to activate CALMET. For selecting vehicle emission factors, a comparison between different studies was conducted. CALPUFF line source case study simulation results showed that CO levels were within the allowable concentrations assigned by the United States' Environmental Protection Agency (EPA) standards. On the other hand, the NOx and CO2 simulated results were higher than the allowable amounts according to the EPA emission standards. Moving to the area source study, results demonstrated that the three pollutants significantly exceeded the allowable standards. The top five maximum concentrations were located near the origin of the study domain for both line and area source measurements. At the times that maximum concentrations were recorded, the wind was blowing from the sea toward the land. As a result, the greatest accumulation of pollutants was located close to the domain of the area of study.

Implementation of Cascade Gamma and Positron Range Corrections for I-124 Small Animal PET

Small animal Positron Emission Tomography (PET) should provide accurate quantification of regional radiotracer concentrations and high spatial resolution. This is challenging for non-pure positron emitters with high positron endpoint energies, such as I-124: On the one hand the cascade gammas emitted from this isotope can produce coincidence events with the 511 keV annihilation photons leading to quantification errors. On the other hand the long range of the high energy positron degrades spatial resolution. This paper presents the implementation of a comprehensive correction technique for both of these effects. The established corrections include a modified sinogram-based tail-fitting approach to correct for scatter, random and cascade gamma coincidences and a compensation for resolution degradation effects during the image reconstruction. Resolution losses were compensated for by an iterative algorithm which incorporates a convolution kernel derived from line source measurements for the microPET Focus 120 system. The entire processing chain for these corrections was implemented, whereas previous work has only addressed parts of this process. Monte Carlo simulations with GATE and measurements of mice with the microPET Focus 120 show that the proposed method reduces absolute quantification errors on average to 2.6% compared to 15.6% for the ordinary Maximum Likelihood Expectation Maximization algorithm. Furthermore resolution was improved in the order of 11-29% depending on the number of convolution iterations. In summary, a comprehensive, fast and robust algorithm for the correction of small animal PET studies with I-124 was developed which improves quantitative accuracy and spatial resolution.

Measurement of Thermal Conductivity of Liquid Based on Transient Double-Hot Wire Technique

A new experimental apparatus has been developed for measuring the thermal conductivity of liquid by the two transient hot-wire method, which is based on the theory of one-dimensional unsteady heat conduction in infinite medium. The glass tubes, the heating line source and temperature measurement devices are designed in this experiment. As the temperature controller, low-temperature bath provides a constant water bath environment and current calibrator is the power source. Wheatstone bridge is used to test the relation between the temperature changes and resistance. Data acquisition and control unit is data acquisition devices of the Wheatstone bridge. As the temperature coefficient of platinum wire resistance is calibrated. It can be used to measure 32 groups of data from 10°C to 40°C at the every 10°C according to the experimental steps.

Performance evaluation of small-animal multipinhole μSPECT scanners for mouse imaging

We compared the performance of three commercial small-animal μSPECT scanners equipped with multipinhole general purpose (GP) and multipinhole high-resolution (HR) collimators designed for imaging mice. Spatial resolution, image uniformity, point source sensitivity and contrast recovery were determined for the U-SPECT-II (MILabs), the NanoSPECT-NSO (BioScan) and the X-SPECT (GE) scanners. The pinhole diameters of the HR collimator were 0.35 mm, 0.6 mm and 0.5 mm for these three systems respectively. A pinhole diameter of 1 mm was used for the GP collimator. To cover a broad field of imaging applications three isotopes were used with various photon energies: (99m)Tc (140 keV), (111)In (171 and 245 keV) and (125)I (27 keV). Spatial resolution and reconstructed image uniformity were evaluated in both HR and a GP mode with hot rod phantoms, line sources and a uniform phantom. Point source sensitivity and contrast recovery measures were additionally obtained in the GP mode with a novel contrast recovery phantom developed in-house containing hot and cold submillimetre capillaries on a warm background. In hot rod phantom images, capillaries as small as 0.4 mm with the U-SPECT-II, 0.75 mm with the X-SPECT and 0.6 mm with the NanoSPECT-NSO could be resolved with the HR collimators for (99m)Tc. The NanoSPECT-NSO achieved this resolution in a smaller field-of-view (FOV) and line source measurements showed that this device had a lower axial than transaxial resolution. For all systems, the degradation in image resolution was only minor when acquiring the more challenging isotopes (111)In and (125)I. The point source sensitivity with (99m)Tc and GP collimators was 3,984 cps/MBq for the U-SPECT-II, 620 cps/MBq for the X-SPECT and 751 cps/MBq for the NanoSPECT-NSO. The effects of volume sensitivity over a larger object were evaluated by measuring the contrast recovery phantom in a realistic FOV and acquisition time. For 1.5-mm rods at a noise level of 8 %, the contrast recovery coefficient (CRC) was 42 %, 37 % and 34 % for the U-SPECT-II, X-SPECT and NanoSPECT-NSO, respectively. At maximal noise levels of 10 %, a CRCcold of 70 %, 52 % and 42 % were obtained for the U-SPECT-II, X-SPECT and NanoSPECT-NSO, respectively. When acquiring (99m)Tc with the GP collimators, the integral/differential uniformity values were 30 %/14 % for the U-SPECT-II, 50 %/30 % for the X-SPECT and 38 %/25 % for the NanoSPECT-NSO. When using the HR collimators, these uniformity values remained similar for U-SPECT-II and X-SPECT, but not for the NanoSPECT-NSO for which the uniformity deteriorated with larger volumes. We compared three μSPECT systems by acquiring and analysing mouse-sized phantoms including a contrast recovery phantom built in-house offering the ability to measure the hot contrast on a warm background in the submillimetre resolution range. We believe our evaluation addressed the differences in imaging potential for each system to realistically image tracer distributions in mouse-sized objects.

In Situ Thermal Conductivity Measurements of Building Materials with a Thermal Probe

Abstract Transient line source measurements have been employed in various industries, e.g., plastics, foodstuffs, and refractory bricks, to measure thermal conductivity and sometimes thermal diffusivity. Measurements have traditionally been carried out in carefully controlled thermal environments. In pursuance of better data to inform energy efficiency calculations for building envelopes, a transient line source using thermal probe technology is assessed for in situ measurements, where materials may be subject to varied moisture content under diverse environmental conditions. A robust stand alone field apparatus has been developed. Laboratory based measurements have been undertaken, and results are reported for agar immobilized water and polytetrafluoroethylene, as well as aerated concrete and oak at specific moisture contents. The field apparatus was used to measure in situ the thermal properties of walls in real buildings, one of aerated concrete and one of mass earth construction. A new and simplified method of identifying appropriate analysis time windows has been developed, which allows an assessment of confidence levels in thermal conductivity results. Thermal diffusivity results were discounted as the effects of contact resistance between the probe and sample could not be differentiated from the heat capacity effects of the sample material. Valid thermal conductivity results were achieved for reference materials containing moisture and in situ measurements. The in situ thermal conductivity values were shown to be at significant variance with design values. Consistent thermal conductivity results were achieved for common building materials above 0.15 W⋅m −1⋅K−1, and indicative results were achieved for values below this despite measurements being taken under varying environmental conditions. The principal cause of error in the measurement of materials with higher thermal resistance was identified.

Characterizing the MTF in 3D for a Quantized SPECT Camera Having Arbitrary Trajectories.

The emergence of application-specific 3D tomographic small animal and dedicated breast imaging systems has stimulated the development of simple methods to quantify the spatial resolution or Modulation Transfer Function (MTF) of the system in three dimensions. Locally determined MTFs, obtained from line source measurements at specific locations, can characterize spatial variations in the system resolution and can help correct for such variations. In this study, a method is described to measure the MTF in 3D for a compact SPECT system that uses a 16 × 20 cm(2) CZT-based compact gamma camera and 3D positioning gantry capable of moving in different trajectories. Image data are acquired for a novel phantom consisting of three radioactivity-filled capillary tubes, positioned nearly orthogonally to each other. These images provide simultaneous measurements of the local MTF along three dimensions of the reconstructed imaged volume. The usefulness of this approach is shown by characterizing the MTF at different locations in the reconstructed imaged 3D volume using various (1) energy windows; (2) iterative reconstruction parameters including number of iterations, voxel size, and number of projection views; (3) simple and complex 3D orbital trajectories including simple vertical axis of rotation, simple tilt, complex circle-plus-arc, and complex sinusoids projected onto a hemisphere; and (4) object shapes in the camera's field of view. Results indicate that the method using the novel phantom can provide information on spatial resolution effects caused by system design, sampling, energy windows, reconstruction parameters, novel 3D orbital trajectories, and object shapes. Based on these measurements that are useful for dedicated tomographic breast imaging, it was shown that there were small variations in the MTF in 3D for various energy windows and reconstruction parameters. However, complex trajectories that uniformly sample the breast volume of interest were quantitatively shown to have slightly better spatial resolution performance than more simple orbits.

Finite element modeling of acoustic field induced by laser line source near surface defect

A numerical model of acoustic field induced by laser line source near the surface defect is established by finite element method (FEM), where a surface notch of rectangular shape has been introduced to represent the fatigue defect for the convenience of modeling. After calculating numerically the transient displacement distributions, which are generated by the laser irradiation, the ultrasonic wave modes on the surface and in the body of the plate material are presented in details. The longitudinal, transverse and surface acoustic waves (SAWs) excited by laser pulses near surface notch are compared under the situations that the notch depths are different. As the notch depth increases, the directivity of the bulk waves generation changes greatly. The amplitude of the reflected SAW rises observably at the same time, which is observed experimentally when the laser source is shifted near the surface notch in scanning laser line source (SLLS) measurement. Another effect induced by the surface notch is the time lag of the transmitted SAW pulse with respect to the original incident pulse. These phenomena can be explained from the results. The conclusions can be used to surface notch depth evaluation.

Uranium pinhole collimators for 511-keV photon SPECT imaging of small volumes

Depleted uranium (DU) pinhole collimators are evaluated for small-volume SPECT imaging of 511-keV gammas, with a view toward small-animal imaging. Tungsten (W) pinholes are examined for comparison. Line source measurements show planar full-width at half-maximum (FWHM) as low as 6.5 mm for a 1-mm DU pinhole, with SPECT system FWHM of 5.0 mm at 43-mm ROR and 6.7 mm at 69-mm ROR. Planar FWHM is smaller for DU than for W pinholes but is insensitive to aperture size for pinholes of 1-3 mm diameter at this energy for the collimators studied. These trends indicate penetration at pinhole edges, as confirmed by LSF shape and full-width at tenth-maximum trends. In SPECT scans of rod phantoms at 71-mm ROR, DU pinholes were only barely able to resolve cold rods as large as 4.7 mm (center-to-center spacing=2/spl times/diameter). For both F-18 alone and F-18 imaged along with Tc-99m, 3.9-mm hot rods were resolved. Tc-99m scans resolved 2.3- mm rods with F-18 present and 1.5-mm rods without F-18. DU pinhole SPECT at 511 keV does not currently match the spatial resolution of high-resolution positron emission tomography (PET) but is open to improvement and, unlike PET, is capable of simultaneous multiple-isotope acquisitions.

Estimation of the depth-dependent component of the point spread function of SPECT.

The point spread function (PSF) of a gamma camera describes the photon count density distribution at the detector surface when a point source is imaged. Knowledge of the PSF is important for computer simulation and accurate image reconstruction of single photon emission computed tomography (SPECT) images. To reduce the number of measurements required for PSF characterization and the amount of computer memory to store PSF tables, and to enable generalization of the PSF to different collimator-to-source distances, the PSF may be modeled as the two-dimensional (2D) convolution of the depth-dependent component which is free of detector blurring (PSF(ideal)) and the distance-dependent detector response. Owing to limitations imposed by the radioactive strength of point sources, extended sources have to be used for measurements. Therefore, if PSF(ideal) is estimated from measured responses, corrections have to be made for both the detector blurring and for the extent of the source. In this paper, an approach based on maximum likelihood expectation-maximization (ML-EM) is used to estimate PSF(ideal). In addition, a practical measurement procedure which avoids problems associated with commonly used line-source measurements is proposed. To decrease noise and to prevent nonphysical solutions, shape constraints are applied during the estimation of PSF(ideal). The estimates are generalized to depths other than those which have been measured and are incorporated in a SPECT simulator. The method is validated for Tc-99m and T1-201 by means of measurements on physical phantoms. The corrected responses have the desired shapes and simulated responses closely resemble measured responses. The proposed methodology may, consequently, serve as a basis for accurate three-dimensional (3D) SPECT reconstruction.

Energy dependence of nonstationary scatter subtraction--restoration in high resolution PET.

In previous works, the determination of object and detector scatter kernels from line source measurements was described and their application in scatter correction was investigated. It was also shown that low energy data contains a large fraction of useful events (true and detector scatter events). In the present work, data acquired in multispectral mode was summed in single broad energy windows of lower energy thresholds varying from 129 to 516 keV in steps of 43 keV and a constant upper energy threshold of 645 keV. Line-source projections were fitted by extracting the object and detector scatter kernels as a function of energy threshold. These kernels were then used to process scatter by the nonstationary convolution subtraction-restoration method in phantom images. After scatter correction, the sensitivity is found to increase by up to 64% at the lower threshold of 129 keV, relative to the conventional photopeak energy window (344-645 keV). Whereas contrast and spatial resolution are degraded as the energy discriminator is lowered, such degradation is fully recovered by the scatter correction. As a result of scatter correction, the noise increases insignificantly in hot regions but substantially in cold regions, in proportion of the amount of scatter.

Object and detector scatter-function dependence on energy and position in high resolution PET

The authors have shown in previous works that distinct non-stationary analytical scatter kernels can be extracted from line source measurements and used to independently subtract object scatter and subtract or restore detector scatter in high resolution PET. In this work, the dependence of the scatter components on energy threshold and source position was investigated. Line source measurements were acquired in multispectral mode using the Sherbrooke PET simulator. Scatter parameters were extracted from data summed in energy windows with a lower threshold varying from 129 keV to 516 keV in steps of 42 keV, and a fixed upper threshold of 644 keV. Decreasing the lower threshold from 344 keV to 129 keV increases the trues by only 25%, but increases object scatter by 136% and almost triples detector scatter. A gain in efficiency by a factor of 2 or more would result from recovering the latter by restoration in the broad window. The intensity and shape of the scatter functions for both object and detector are shown to have a significant dependence on energy and position. This dependence needs to be taken into account in the design of kernels for accurate scatter correction over a broad energy range. >

Determination of thermal conductivity for deep boreholes

Two methods for thermal conductivity determinations on rock cores and fragments were tested on a suite of samples from the Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland (KTB) superdeep drill hole in Germany. They were also compared with estimates of thermal conductivity using the mineral composition of the rock and physical well logs and with in situ thermal conductivity measurements. Laboratory methods provide reasonably precise determinations of the thermal conductivities of both solid core (±5%) and drill cuttings (±10%) at room temperature and pressure. The most common methods presently used for crystalline rocks are the steady state “divided‐bar” (DB) technique and the transient “half‐space” line source (LS). Sample preparation and measurement times are comparable for the DB and LS, with sample preparation being more time consuming on average. For isotropic rocks there is little to choose from between the two methods, which both give reliable values of conductivity in the vertical direction. The LS is easier to set up and use in field laboratory situations, which renders it the preferred method for field reconnaissance. The gneissic crystalline rocks penetrated by the KTB boreholes typically have anisotropy of the order of 10–20%. The DB provides unambiguous values of conductivity in a given direction, so its use is preferable for obtaining both principal conductivities and the vertical component. Anisotropy can be estimated using LS measurements in many different directions, but the potential for large random errors is much greater than with the more straightforward DB approach. For deep research wells the difficulties of extrapolating laboratory results to in situ conditions (particularly temperature) present additional obstacles to determining heat flow. Laboratory measurements of water‐saturated samples under in situ conditions, combined with in situ measurements and judicious use of calculations based on mineralogy and well log derived physical properties, can aid in the accurate characterization of thermal conductivity in deep wells. The application of different methods helped to link variations of heat flow with depth in the KTB hole to the anisotropy of thermal conductivity or thermal refraction and thus allowed the calculation of background heat flux in this geologically complex area.

Fast SPECT simulation including object shape dependent scatter

A fast simulator of SPECT projection data taking into account attenuation, distance dependent detector response, and scatter has been developed, based on an analytical point spread function model. The parameters of the scatter response are obtained from a single line source measurement with a triangular phantom. The simulator is able to include effects of object curvature on the scatter response to a high accuracy. The simulator has been evaluated for homogeneous media by measurements of (99m)Tc point sources placed at different locations in a water-filled cylinder at energy windows of 15% and 20%. The asymmetrical shapes of measured projections of point sources are In excellent agreement with simulations for both energy windows. Scatter-to-primary ratio (SPR) calculations of point sources at different positions in a cylindrical phantom differ not more than a few percent from measurements. The simulator uses just a few megabytes of memory for storing the tables representing the forward model; furthermore, simulation of 60 SPECT projections from a three-dimensional digital brain phantom with 6-mm cubic voxels takes only ten minutes on a standard workstation. Therefore, the simulator could serve as a projector in iterative true 3-D SPECT reconstruction.

An analytical model for Compton scatter in a homogeneously attenuating medium

Accurate reconstruction of SPECT images is hampered by four nonlinear effects in the acquisition process: attenuation, scatter, collimator acceptance angle, and statistical noise. A good mathematical description of these effects is obviously crucial for the reconstruction. Poisson noise, attenuation, and collimator acceptance angle are relatively easy to model. Scattering, however, is a very complex process, and is mostly described using empirical models. A new model for the scatter point spread function in a homogeneous medium that is based on physical considerations and is capable of predicting scatter contributions of point sources as a function of depth in homogeneous media is presented. The model is verified with Monte Carlo simulations and line source measurements, and is compared to an existing empirical model.

Knudsen-effect errors with transient line-source measurement in fluids

This paper decribes the Knudsen-effect errors of the transient line-source method used for accurate measurements of the thermal conductivity and thermal diffusivity of fluids. The analysis demonstrates that the instrument can be used with a good accuracy (>0.5%) to lower densities than previously thought. The principal errors are illustrated by measurements on propane in the temperature range 250–300 K at densities less than 9 kg · m−3.

Shape-resonance-induced non-Franck–Condon effects in the valence-shell photoionization of O2

Non-Franck–Condon effects in the valence-shell photoionization spectrum of O2 are studied using the multiple scattering model. Calculations for the 3σg, 1πu, and 1πg levels indicate that the enhanced sensitivity of dipole strengths to internuclear separation, induced by shape resonances, reduces and broadens the resonant cross section; nonresonant channels are negligibly affected. The resulting cross sections and asymmetry parameters show fair overall agreement with line and continuum source measurements.

Vibrational effects in the shape-resonant photoionisation of CO2

Multiple scattering model calculations of vibrationally averaged asymmetry parameters (B) for the 1 pi g, 1 pi u, 3 sigma u and 4 sigma g levels of CO2 are compared to fixed-nuclei calculations. Vibrational branching ratios and vibrationally resolved asymmetry parameters beta nu for the 1 pi g and 1 pi u levels are also presented. The latter are compared to recent He I line source measurements. The resonant features of the vibrationally averaged beta is less pronounced than those calculated with fixed nuclei, while the resolved spectra show shape-resonant features which are not apparent in the unresolved spectra.

Evaluation of focused collimator performance II. Digital recording of line-source response

A scheme has been developed for presenting the geometrical specifications of focused collimators and their physical properties for scanning extended sources. The performance of a collimator is described by its plane-source sensitivity (which is nearly independent of the source-to-collimator distance), its central axis depth response, and its spatial resolution as a function of depth for a given γ-ray energy. All three parameters are derived from a set of line-source measurements in air. An analysis of the line-source response determines the septum penetration for the selected γ-ray energy. For sources in an absorbing medium, the depth response of the collimator must be modified by an experimentally determined γ-ray attenuation coefficient while the resolution of the collimator remains unchanged.