B(1)(+)/actual flip angle and reception sensitivity mapping methods: simulation and comparison

Knowledge of the spatial distribution of transmission field B₁⁺ and reception sensitivity maps is important in high-field (≥3 T) human magnetic resonance (MR) imaging for several reasons: these include post-acquisition correction of intensity inhomogeneities, which may affect the quality of images; modeling and design of radiofrequency (RF) coils and pulses; validating theoretical models for electromagnetic field calculations; testing the compatibility with MR environment of biomedical implants. Moreover, inhomogeneities in the RF field are an essential source of error for quantitative MR spectroscopy. Recent studies have also shown that B₁⁺ and reception sensitivity maps can be used for direct calculation of tissue electrical parameters and for estimating the local specific absorption rate (SAR) in vivo.Several B₁⁺ mapping techniques have been introduced in the past few years based on actual flip angle (FA) mapping, but, to date, none has emerged as a standard. For reception sensitivity calculation, the signal intensity equation can be used where the nominal FA distribution must be replaced with the actual FA distribution calculated by one of the B₁⁺ mapping techniques.This study introduces a quantitative comparison between two known methods for B₁⁺/actual FA and reception sensitivity mapping: the double-angle method (DAM) and the fitting (FIT) method. Experimental data obtained using DAM and FIT methods are also compared with numerical simulation results.     

Depth and orientational dependencies of MRI T(2) and T(1ρ) sensitivities towards trypsin degradation and Gd-DTPA(2-) presence in articular cartilage at microscopic resolution

Depth and orientational dependencies of microscopic magnetic resonance imaging (MRI) T(2) and T(1ρ) sensitivities were studied in native and trypsin-degraded articular cartilage before and after being soaked in 1 mM Gd-DTPA(2-) solution. When the cartilage surface was perpendicular to B(0), a typical laminar appearance was visible in T(2)-weighted images but not in T(1ρ)-weighted images, especially when the spin-lock field was high (2 kHz). At the magic angle (55°) orientation, neither T(2)- nor T(1ρ)-weighted image had a laminar appearance. Trypsin degradation caused a depth- and orientational-dependent T(2) increase (4%-64%) and a more uniform T(1ρ) increase at a sufficiently high spin-lock field (55%-81%). The presence of the Gd ions caused both T(2) and T(1ρ) to decrease significantly in the degraded tissue (6%-38% and 44%-49%, respectively) but less notably in the native tissue (5%-10% and 16%-28%, respectively). A quantity Sensitivity was introduced that combined both the percentage change and the absolute change in the relaxation analysis. An MRI experimental protocol based on two T(1ρ) measurements (without and with the presence of the Gd ions) was proposed to be a new imaging marker for cartilage degradation.     

Sensitive and automated detection of iron-oxide-labeled cells using phase image cross-correlation analysis

Superparamagnetic iron oxide (SPIO) nanoparticles are increasingly being used to noninvasively track cells, target specific molecules and monitor gene expression in vivo. Contrast changes that are subtle relative to intrinsic sources of contrast present a significant detection challenge. Here, we describe a postprocessing algorithm, called Phase map cross-correlation Detection and Quantification (PDQ), with the purpose of automating identification and quantification of localized accumulations of SPIO agents. The method is designed to sacrifice little flexibility - it works on previously acquired data and allows the use of conventional high-SNR pulse sequences with no extra scan time. We first investigated the theoretical detection limits of PDQ using a simulated dipole field. This method was then applied to three-dimensional (3D) MRI data sets of agarose gel containing isolated dipoles and ex vivo transplanted allogenic rat hearts infiltrated by numerous iron-oxide-labeled macrophages as a result of organ rejection. A simulated dipole field showed this method to be robust in very low signal-to-noise ratio images. Analysis of agarose gel and allogenic rat heart shows that this method can automatically identify and count dipoles while visualizing their biodistribution in 3D renderings. In the heart, this information was used to calculate a quantitative index that may indicate its degree of cellular infiltration.     

First-principles study of electronic structure and elasticity of UAlx (x=1,2,3) system

A detailed theoretical study of structural, electronic, and elastic properties of cubic UAl_x (x=1,2,3) is presented employing the pseudopotential plane-wave method based on density-functional theory. The structure parameters of these three compounds have been calculated within generalized gradient approximation (GGA) and local density approximation (LDA). The calculated results were compared with the experimental data and previous research. With the GGA approximation, the elastic constants, shear modulus, Young's modulus, and Poisson's ratio of UAl_x (x=1,2,3) are derived. According to the generalized mechanical stability criteria for cubic crystals, our calculation suggested that C15 UAl₂ and L12 UAl₃ are stable substance under hydrostatic pressures, but B2 UAl might be expected as a metastable compound, which is not reported in previous literature, and future experimental confirmation is needed. Furthermore, the calculated energy band structure and density of state (DOS) are found to be in good agreement with the theoretical values. Additionally, the charge density of these compounds have also been worked out and analyzed.     

Quantification of slow flow using FAIR

Phase contrast (PC)-based MRI methods are considered to be the most accurate approach for spatially resolved flow quantification, but the measurement of very slow velocities requires signal detection at long echo times and the application of strong field gradients. On the other hand, measurements based on time-of-flight or inflow effects can be conducted at short echo times and without flow-encoding gradients. A method for imaging flow at velocities of the order of 0.1 mm/s is presented and validated here. It consists of measuring the apparent spin-lattice relation rate (R₁*) of the flowing fluid using magnetization preparation by alternating slice-selective and nonselective inversion pulses (FAIR or flow-sensitive alternating inversion recovery) and a fast gradient-echo detection sequence. This method is appropriate for the quantitative imaging of slow flow at low Reynolds numbers in fluids where the T₂ values are too short to allow sensitive flow measurements by phase contrast-based methods.     

Intense violet-blue photoluminescence in BaZrO3 powders: A theoretical and experimental investigation of structural order-disorder

Intense violet-blue photoluminescence (PL) emission at room temperature was verified in BaZrO₃ (BZO) powders with structural order-disorder. Ab-initio calculations, ultraviolet-visible absorption spectroscopy and PL were performed. Theoretical results showed that the local disorder in the network-formed Zr clusters present an important role in the formation of hole-electron pair. The experimental data and theoretical results are in agreement, indicating that the PL emission in BZO powders can be related to the structural order-disorder degree in the lattice.     

Fabrication, characterization, and measurement of viscosity of α-Fe2O3-glycerol nanofluids

Magnetic nanofluids, ferrofluids, are a special category of smart nanomaterials, consisting of stable dispersion of magnetic nanoparticles in different fluids. In this study, magnetic nanoparticles of hematite, α-Fe₂O₃, were prepared by solvothermal method using Fe(NO₃)₃ as a starting material. The nanoparticles were characterized by X-ray diffraction (XRD) and transmission electronic microscope (TEM).To the best of our knowledge, this is the first research on the rheological properties of nanofluids of α-Fe₂O₃ nanoparticles and glycerol. The experimental results showed that the viscosity of α-Fe₂O₃-glycerol nanofluids increases with increasing the particle volume fraction and decreases with increasing temperature. Our results clearly showed that the α-Fe₂O₃-glycerol nanofluids are non-Newtonian shear-thinning and their shear viscosity depends strongly on temperature. The experimental data were compared with some theoretical models. The measured values of the effective viscosity of nanofluids are underestimated by the theoretical models.     

Equilibrium signal intensity mapping, an MRI method for fast mapping of longitudinal relaxation rates and for image enhancement

INTRODUCTION: Inhomogeneity of magnetic fields, both B(0) and B(1), has been a major challenge in magnetic resonance imaging (MRI). Field inhomogeneity leads to image artifacts and unreliability of signal intensity (SI) measurements. This work proposes and shows the feasibility of generating equilibrium signal intensity (SI(Eq)) maps that can be utilized either to speed up relaxation-rate measurement or to enhance image quality and relaxation-rate-based weighting in various applications. METHODS: A 1.5-T MRI scanner was used. In canines (n=4), myocardial infarction was induced, and 48 h after the administration of 0.05 mmol kg(-1) Gd(ABE-DTTA), a contrast agent with slow tissue kinetics, in vivo R(1) mapping was carried out using an inversion recovery (IR)-prepared, fast gradient-echo sequence with varying inversion times (TIs). To test the SI(Eq) mapping method without the confounding effects of motion and blood flow, we carried out ex vivo R(1) mapping after the administration of 0.2 mmol kg(-1) Gd(DTPA) using an IR-prepared, fast spin-echo sequence in another group of dogs (n=2). R(1,full) maps and SI(Eq) maps were generated from the data from both sequences by three-parameter nonlinear curve fitting of the SI versus TI dependence. R(1,full) maps served as the reference standard. Raw IR images were then divided by the SI(Eq) maps, yielding corrected SI maps (COSIMs). Additionally, R(1) values were calculated from each single-TI image separately, using the SI(Eq) value and a one-parameter curve-fitting procedure (R(1,single)). Voxelwise correlation analysis was carried out for the COSIMs and the R(1,single) maps, both versus the standard R(1,full) maps. Deviations of R(1,single) from R(1,full) were statistically evaluated. RESULTS: In vivo, COSIM versus R(1,full) showed significantly (P<.05) better correlation [correlation coefficient (CC)=0.95] than SI versus R(1,full) with a TI=700-800 ms, which is 200-300 ms longer than the tau(null) (500 ms) of viable myocardium. With such TIs, SI versus R(1,full) yielded CCs of 0.86-0.88. R(1,single) versus R(1,full) yielded a peak CC of 0.96 at TI=700-900 ms. Mean deviations of R(1,single) from R(1,full) were below 5% for TIs between 500 and 1000 ms. Ex vivo, where tau(null) was 300 ms, the advantage of correction with SI(Eq) was not in the improvement of linear correlation but more in the reduction of scatter. Peak CCs for SI versus R(1,full) and COSIM versus R(1,full) at TI=500 ms were 0.96 for both. The ex vivo CC for R(1,single) versus R(1,full) at TI=500 ms was 0.98. Mean deviations of R(1,single) from R(1,full) were below 5% for TIs between 400 and 700 ms. CONCLUSIONS: Once the corresponding SI(Eq) map is obtained from a control stack, R(1) can be obtained accurately, using only a single IR image and without the need for a stack of TI-varied images. This approach could be applied in various dynamic MRI studies where short measurement time, once the dynamics has started, is of essence. When using this method with IR-prepared T(1)-weighted images, it is essential that the single TI be chosen such that the longitudinal relaxation in all voxels of interest would have passed tau(null). SI(Eq) maps are also useful in eliminating confounders from MR images to allow obtaining SI values that reflect more faithfully the relaxation parameter (R(1)) sought.     

Thermal effects of the diode end-pumped Nd:YVO4 slab

The thermal effects of the diode end-pumped Nd:YVO₄ slab laser crystal are investigated both analytically and numerically. The theoretical model is established with considering the divergence of the pump beam in the slab. A detailed theoretical analysis of the temperature, stress and the focal length of thermal lens are presented. Using 3D finite element analysis, an accurate numerical solution based on the theoretical model is achieved. Our analysis includes the thermal fracture damage of the thermally and optically anisotropic slab and the predicated values are compared with the experimental results.     

Whole-body T2* mapping at 1.5 T

This study investigated the feasibility of an MRI protocol providing whole-body T2* maps at 1.5 T. Seven healthy volunteers (mean age=30.1+/-3.7, three women and four men) and two patients (both male, 53 and 46 years old) affected by transfusion-dependent anemias participated in the study. Coronally oriented images of five subsequent body levels were acquired using a fat-suppressed multiecho 2D gradient-echo sequence (12 echo times ranging from 4.8 to 76.3 ms were selected) and afterwards composed. Parametrical T2* maps of the whole body were reconstructed on a pixel-by-pixel basis. For both, healthy volunteers and patients, representative T2* values were computed from extended regions of interest (ROIs). Good-quality whole-body T2* maps were computed in all volunteers and patients. In healthy volunteers, T2* values were assessed in the cerebral white (58.5+/-4.2 ms) and gray (81.4+/-5.5 ms) matter, liver (34.3+/-7.0 ms), spleen (63.5+/-3.3 ms), kidneys (65.4+/-10.3 ms) and skeletal muscles (~30 ms). The liver presented faster relaxation rates in males as compared to females. One patient (serum ferritin concentration=927 microg/dl) showed shortened T2* values in liver (3.6+/-5.5 ms), spleen (3.1+/-4.8 ms), kidneys (11.1+/-7.1 ms) and muscles (25.1+/-3.4 ms). The second patient (serum ferritin concentration=346 microg/dl) presented reduced T2* values in liver (3.9+/-7.3 ms), spleen (20.1+/-9.8 ms) and kidneys (24.6+/-7.7 ms). The presented technique may find clinical application in the assessment of the iron burden in the entire body, and in monitoring of chelation therapies in patients treated with frequent blood transfusions.     

(110) Facet of rutile-structured GeO2: an ab initio investigation

GeO₂ is an enticing candidate to replace SiO₂ in electronics industry. To date, no work has appeared in the literature specifically focusing on the (110) facet of rutile GeO₂. Density functional theory was applied along with plane-wave approximation to describe the structure and energetics of the (110) plane of tetragonal germanium dioxide. Surface energies were estimated using dispersion-free Perdew–Burke–Ernzerhof (PBE) method as well as dispersion-corrected PBE-D3, D3-corrected revised version of PBE (RPBE-D3), optB88-vdW and optB86b-vdW methods. Atomic displacements with respect to the bulk structure were also reported for slabs relaxed using the aforementioned exchange–correlation functionals. Adsorption energies for weakly bound CO on rutile GeO₂(110) surface were also computed using the above-mentioned methods. Vibrational frequency blue-shift of CO as adsorbed onto rutile GeO₂(110) surface was also predicted. All the calculated values were comparable to the pertinent literature data, either theoretical or experimental, when available.     

Fermi edge singularity evidence from photoluminescence spectroscopy of AlGaAs/InGaAs/GaAs pseudomorphic HEMTs grown on (3 1 1)A GaAs substrates

InGaAs/AlGaAs/GaAs pseudomorphic high electron mobility transistor (P-HEMT) structures were grown by Molecular Beam Epitaxy (MBE) on (3 1 1)A GaAs substrates with different well widths, and studied by photoluminescence (PL) spectroscopy as a function of temperature and excitation density.The PL spectra are dominated by one or two spectral bands, corresponding, respectively, to one or two populated electron sub-bands in the InGaAs quantum well. An enhancement of PL intensity at the Fermi level energy (E_F) in the high-energy tail of the PL peak is clearly observed and associated with the Fermi edge singularity (FES). This is practically detected at the same energy for all samples, in contrast with energy transitions in the InGaAs channel, which are shifted to lower energy with increasing channel thickness. PL spectra at low temperature and low excitation density are used to optically determine the density of the two-dimensional electron gas (2DEG) in the InGaAs channel for different thicknesses. The results show an enhancement of the 2DEG density when the well width increases, in good agreement with our previous theoretical study.     

Multi-scale theoretical study of support effect on sintering dynamics of Pt

The capability of theoretical durability studies to offer an efficient alternative methodology for predicting the potential performance of catalysts has improved in recent years. In this regard, multi-scale theoretical methods for predicting sintering behavior of Pt on various catalyst supports are being developed. Various types of Pt diffusions depending on support were confirmed by the micro-scale ultra accelerated quantum chemical molecular dynamics (UA-QCMD) method. Moreover, macro-scale sintering behavior of Pt/γ-Al₂O₃, Pt/ZrO₂ and Pt/CeO₂ catalysts were studied using a developed 3D sintering simulator. Experimental results were well reproduced. While Pt on γ-Al₂O₃ sintered significantly, Pt on ZrO₂ sintered slightly and Pt on CeO₂ demonstrated the highest stability against sintering.     

Observation of the K2 4Δg state by cw perturbation-facilitated optical-optical double resonance spectroscopy

This paper reports for the first time both, an experimental observation and theoretical calculations of the K₂ 4³Δ_g state. For the experiment we used cw perturbation-facilitated optical-optical double resonance (PFOODR) spectroscopy. A single mode Ti-sapphire laser and a dye laser served as the pump and probe lasers, respectively. A total of 55 PFOODR signals have been assigned to the 4³Δ_g ← b³Π_u transitions. Absolute vibrational numbering was determined by using quantum defect analysis combined with comparing observed intensities with calculated Franck-Condon factors (FCF). For the former we used known parameters from the 2³Δ_g state since the 2³Δ_g and the 4³Δ_g states belong to the same Rydberg series. We report here our experimental and calculated spectroscopic constants, the corresponding RKR potential energy curve, the Franck-Condon table for the 4³Δ_g ↔ b³ Π_u system, as well as a comparison with the theoretical potential energy curve. The T_e value is found to be 28408.938(52) cm⁻¹.     

EPR study of some rare-earth ions (Dy3+, Tb3+, and Nd3+) in YBa2Cu3O6-compound

We investigate the low temperature X-band electron paramagnetic resonance (EPR) of YBa(2)Cu(3)O(x) compounds with x congruent with 6.0 doped with Dy(3+), Tb(3+), and Nd(3). The EPR spectra of Dy(3+) and Tb(3+) have been identified. The EPR of Tb(3+) is used also to study the effect of suppression of high T(c) superconductivity by doping with Tb(3+). The EPR of Nd(3+) is probably masked by the intense resonance of Cu(2+). All experimental EPR results compare well with theoretical estimations.     

MRI measurements of CO2 hydrate dissociation rate in a porous medium

After obtaining experimental data of CO₂ hydrate formation and dissociation in a porous medium using magnetic resonance imaging (MRI), the purpose of this study was to analyze the different dissociation rate of CO₂ hydrate using two heating rates. Images were obtained by using a fast spin-echo sequence, and the field of view was set to 40×40×40 mm. The vessel pressure was monitored during hydrate formation and dissociation, which was used to compare with MRI mean intensity. The result indicated that the MRI could visualize hydrate formation and dissociation, and the MRI mean intensity of water was in good agreement with the vessel pressure changes. The hydrate formation and dissociation rates were also quantified using the MRI mean intensity of water. The experimental results showed that the higher heating rate caused the rapid hydrate dissociation.     

METHODS OF HEAT LOSS MEASUREMENT FOR A THERMOHYDRAULIC FACILITY

Three methods, using different thermal transients, are proposed to measure the heat loss of a thermohydraulic facility. They can he used for loop-type facilities, with circulation pumps, and for vessel-type facilities, with fluid at saturation. The analysis of uncertainties of the measurement results can help to design a heat loss experiment for a specific application. Examples are presented of heat loss measurements at the LOBI-MOD2 facility, which is an integral system thermohydraulic test facility for safety investigations of pressurized water reactor (PWR) nuclear power plants.     

Observation of the transition from diffusive regime to ballistic regime of the 2DEG transport property in AlxGa1−xN/GaN heterostructures

Electron–electron interaction effect of the two-dimensional electron gas (2DEG) in Al_xGa_1−xN/GaN heterostructures has been investigated by means of magnetotransport measurements at low temperatures. From the temperature dependence of the longitudinal conductivity of the heterostructures, a clear transition region has been observed. Based on the theoretical analysis, we conclude that this region corresponds to the transition from the diffusive regime to the ballistic regime of the 2DEG transport property. The interaction constant is determined to be −0.423, which is consistent with the theoretical prediction. However, the critical temperature for the transition, which is 8 K in Al_xGa_1−xN/GaN heterostructures, is much higher than the theoretical prediction.     

Nuclear spin resonance of (129)Xe doped with O(2)

Spin-lattice relaxation of (129)Xe nuclei in solid natural xenon has been investigated in detail over a large range of paramagnetic O(2) impurity concentrations. Direct measurements of the ground state magnetic properties of the O(2) are difficult because the ESR (electron spin resonance) lines of O(2) are rather unstructured, but NMR measurements in the liquid helium temperature region (1.4-4 K) are very sensitive to the effective magnetic moments associated with the spin 1 Zeeman levels of the O(2) molecules and to the O(2) magnetic relaxation. From these measurements, the value of the D[Sz(2)-(1/3)S(2)] spin-Hamiltonian term of the triplet spin ground state of O(2) can be determined. The temperature and magnetic field dependence of the measured paramagnetic O(2)-induced excess line width of the (129)Xe NMR signal agree well with the theoretical model with the spin-Hamiltonian D=0.19 meV (2.3 K), and with the reasonable assumption that the E[S(x)(2)-S(y)(2)] spin-Hamiltonian term is close to 0 meV. An anomalous temperature dependence between 1.4 K and 4.2K of the (129)Xe spin-lattice relaxation rate, T(1n)(-1)(T), is also accounted for by our model. Using an independent determination of the true O(2) concentration in the Xe-O(2) solid, the effective spin lattice relaxation time (which will be seen to be transition dependent) of the O(2) at 2.3 K and 0.96 T is determined to be approximately 1.4 x 10(-8)s. The experimental results, taken together with the relaxation model, suggest routes for bringing highly spin-polarized (129)Xe from the low temperature condensed phase to higher temperatures without excessive depolarization.     

Quantitative discrimination of water and hydrocarbons in porous media by magnetization prepared centric-scan SPRITE

MRI has considerable potential as a non-destructive probe of porous media, offering the possibility of rapid quantification of local oil and water content. This potential has not yet, however, been completely realized. In this paper, we explore a general magnetization preparation approach to the discrimination of water and oil in a model, representative, porous medium. These measurements have, as a common element, a centric scan pure phase encode readout based on the SPRITE methodology. Magnetization preparation permits facile T1, T2 and diffusion coefficient mapping as the basis for oil and water discrimination. Diffusion coefficient mapping proved to be the most robust approach to discrimination of oil and water. These methods are illustrated through static experiments and a dynamic immiscible fluid displacement experiment.