SPRITE MRI with Prepared Magnetization and Centrick-Space Sampling

A technique for imaging materials with short transverse relaxation times and prepared longitudinal magnetization is proposed. The technique is single-point ramped imaging withT₁-enhancement (SPRITE) MRI with centrick-space sampling. The effects of transient state behavior on image resolution and signal/noise are estimated. Centric sampling in the basic SPRITE sequence gives increased signal-to-noise and permits a quantitative determination of the MR parameters associated with longitudinal spin preparation. Spin-lock and inversion recovery preparation experiments are presented.     

Magnetic Resonance Imaging of Gases: A Single-Point Ramped Imaging with T1 Enhancement (SPRITE) Study

A pure phase-encoding MRI technique, single-point ramped imaging withT₁enhancement, SPRITE, is introduced for the purpose of gas phase imaging. The technique utilizes broadband RF pulses and stepped phase encode gradients to produce images, substantially free of artifacts, which are sensitive to the gasT₁andT^*:₂relaxation times. Images may be acquired from gas phase species with transverse relaxation times substantially less than 1 ms. Methane gas images,¹H, were acquired in a phantom study. Sulfur hexafluoride,¹⁹F, images were acquired from a gas-filled porous coral sample. High porosity regions of the coral are observed in both the MRI image and an X-ray image. Sensitivity and resolution effects due to signal modulation during the time-efficient acquisition are discussed. A method to increase the image sensitivity is discussed, and the predicted improvement is shown through 1D images of the methane gas phantom.     

Centric scan SPRITE magnetic resonance imaging

Two rapid, pure phase encode, centric scan, Single Point Ramped Imaging with T₁-Enhancement (SPRITE) MRI methods are described. Each retains the benefits of the standard SPRITE method, most notably the ability to image short T₂^* systems, while increasing the sensitivity and generality of the technique. The Spiral-SPRITE method utilizes a modified Archimedean spiral k-space trajectory. The Conical-SPRITE method utilizes a system of spirals mapped to conical surfaces to sample the k-space cube. The sampled k-space points are naturally Cartesian grid points, eliminating the requirement of a re-gridding procedure prior to image reconstruction. The effects of transient state behaviour on image resolution and signal/noise are explored.     

Spin echo SPI methods for quantitative analysis of fluids in porous media

Fluid density imaging is highly desirable in a wide variety of porous media measurements. The SPRITE class of MRI methods has proven to be robust and general in their ability to generate density images in porous media, however the short encoding times required, with correspondingly high magnetic field gradient strengths and filter widths, and low flip angle RF pulses, yield sub-optimal S/N images, especially at low static field strength. This paper explores two implementations of pure phase encode spin echo 1D imaging, with application to a proposed new petroleum reservoir core analysis measurement.In the first implementation of the pulse sequence, we modify the spin echo single point imaging (SE-SPI) technique to acquire the k-space origin data point, with a near zero evolution time, from the free induction decay (FID) following a 90° excitation pulse. Subsequent k-space data points are acquired by separately phase encoding individual echoes in a multi-echo acquisition. T₂ attenuation of the echo train yields an image convolution which causes blurring. The T₂ blur effect is moderate for porous media with T₂ lifetime distributions longer than 5 ms. As a robust, high S/N, and fast 1D imaging method, this method will be highly complementary to SPRITE techniques for the quantitative analysis of fluid content in porous media.In the second implementation of the SE-SPI pulse sequence, modification of the basic measurement permits fast determination of spatially resolved T₂ distributions in porous media through separately phase encoding each echo in a multi-echo CPMG pulse train. An individual T₂ weighted image may be acquired from each echo. The echo time (TE) of each T₂ weighted image may be reduced to 500 μs or less. These profiles can be fit to extract a T₂ distribution from each pixel employing a variety of standard inverse Laplace transform methods. Fluid content 1D images are produced as an essential by product of determining the spatially resolved T₂ distribution. These 1D images do not suffer from a T₂ related blurring.The above SE-SPI measurements are combined to generate 1D images of the local saturation and T₂ distribution as a function of saturation, upon centrifugation of petroleum reservoir core samples. The logarithm mean T₂ is observed to shift linearly with water saturation. This new reservoir core analysis measurement may provide a valuable calibration of the Coates equation for irreducible water saturation, which has been widely implemented in NMR well logging measurements.     

Combination of magic-echo and single-point imaging techniques for solid-state MRI

Due to reduced molecular motion the transverse relaxation timeT ₂ in solid materials is typically shorter by a factor of 10³ to 10⁵ in comparison to those in liquids, resulting in a large intrinsic nuclear magnetic resonance line-width that can be well above 20 kHz. Therefore high-resolution solid-state magnetic resonance imaging requires either very strong gradients or special line-narrowing techniques. Single-point imaging (SPI) is a successful pure phase encoding sequence in imaging soft-solid materials; however, when used to study rigid solid materials it still suffers from a very long acquisition time and large gradients. On the other hand, magic echo is a technique that can be used to effectively refocus dipolar interaction, thus achieving a line narrowing. Therefore, the aim of this work is to improve the signal intensity with the combination of the magic echo technique and the SPI sequence. In this paper first applications and a comparison of the SPI sequence with a combination of the magic echo and the SPI sequence to image structures of solid-state materials are presented.     

Three-dimensional adiabatic inversion recovery prepared ultrashort echo time cones (3D IR-UTE-Cones) imaging of cortical bone in the hip

PurposeWe present three-dimensional adiabatic inversion recovery prepared ultrashort echo time Cones (3D IR-UTE-Cones) imaging of cortical bone in the hip of healthy volunteers using a clinical 3T scanner.MethodsA 3D IR-UTE-Cones sequence, based on a short pulse excitation followed by a 3D Cones trajectory, with a nominal TE of 32 μs, was employed for high contrast morphological imaging of cortical bone in the hip of heathy volunteers. Signals from soft tissues such as muscle and marrow fat were suppressed via adiabatic inversion and signal nulling. T₂^⁎ value of the cortical bone was also calculated based on 3D IR-UTE-Cones acquisitions with a series of TEs ranging from 0.032 to 0.8 ms. A total of four healthy volunteers were recruited for this study. Average T₂^⁎ values and the standard deviation for four regions of interests (ROIs) at the greater trochanter, the femoral neck, the femoral head and the lesser trochanter were calculated.ResultsThe 3D IR-UTE-Cones sequence provided efficient suppression of soft tissues with excellent image contrast for cortical bone visualization in all volunteer hips. Exponential single component decay was observed for all ROIs, with averaged T₂^⁎ values ranging from 0.33 to 0.45 ms, largely consistent with previously reported T₂^⁎ values of cortical bone in the tibial midshaft.ConclusionsThe 3D IR-UTE-Cones sequence allows in vivo volumetric imaging and quantitative T₂^⁎ measurement of cortical bone in the hip using a clinical 3T scanner.     

Nonlinear dielectric susceptibility in SrTiO3 and possible nature of the low-temperature phase

The nonlinear dielectric susceptibility of SrTiO₃ is studied experimentally. Intense higher-order harmonics of the polarization, whose amplitudes grow with decreasing temperature and saturate at T<4 K, are observed in the temperature range T<50 K. Hysteresis phenomena, differences in the zero-field cooling (ZFS) and field cooling (FC) regimes for the higher-order harmonics, are observed when a constant electric field is applied. The existence in SrTiO₃ of a strongly frustrated state, which arises when the interaction of the structural and ferroelectric order parameters is taken into account, is proposed. The low-temperature state of SrTiO₃ is analyzed on the basis of a phase diagram which is similar to that of a spin glass [S. Kirkpatrick and D. Sherington, Phys. Rev. B 17, 4384 (1978); J. R. L. De Almeida and D. J. Thouless, J. Phys. A 11, 983 (1978)]. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 634–639 (25 April 1996)     

Optical absorption spectrum of Ni2+ ions doped in sodium potassium sulphate single crystal

Single crystals of nickel-doped sodium potassium sulphate were grown by slow evaporation method at room temperature. From the nature and position of the bands observed, a successful interpretation of all the observed bands could be made assuming the octahedral symmetry for the Ni^{2 +} ion in the crystal. The bands have been ascribed to transitions from the ground³A_2g(F) state to the excited³T_2g(F),¹E_g(D),³T_1g(F),¹T_2g(D) and³T_1g(P) states. The experimental and calculated energies are in good agreement. The crystal field parameters derived areDq= =880 cm^–1,B= 900cm^–1 andC=3600 cm^–1.One of the authors, Sujatha John expresses her thanks to the Secretary and the Principal, R. B. V. R. Reddy College, Hyderabad for according her permission to pursue the M. Phil. course.     

Temperature variation of the fundamental absorption edge in AgGaSe2

Optical absorption measurements were made in the temperature range 9–300 K on the chalcopyrite semiconductor compound AgGaSe₂ and the optical energy gap E_G determined as a function of temperature T. In order to obtain the values of E_G as a function of T, the Elliot-Toyozawa model [R.J. Elliot, J. Phys. Rev. 108 (1957) 1384; D.D. Sell, P. Lawaets, Phys. Rev. Lett. 26 (1971) 311] was employed to perform the analysis of the optical absorption spectra. The resulting E_G vs. T curve was fitted to a semi-empirical model that takes into account both the thermal expansion and the electron–phonon interaction contributions. The results have been used to estimate values of the deformation potentials of the valence and conduction bands of the compound.     

Electrocaloric response in a relaxor ferroelectric polymer at temperatures far below the dielectric maximum

ABSTRACT

Relaxor ferroelectric polymer poly(vinylidene fluoride–trifluoroethylene–chlorofluoroethylene) (P(VDF-TrFE-CFE)) and its blends have been shown to exhibit a giant electrocaloric effect (ECE) over a broad temperature range, e.g. from 0 to 50 °C. Here, a special calorimeter was designed to directly characterize the heat absorbed Q_ECE due to ECE cooling of the blend of P(VDF-TrFE-CFE) with P(VDF-TrFE) 65/35 mol% copolymer down to ?15 °C, which covers the temperature range for many refrigeration applications. From Q_ECE, the isothermal entropy change ΔS and adiabatic temperature change ΔT are deduced. The data reveal that at ?15 °C the relaxor terpolymer/copolymer blend generates ΔS = 23.0J kg^?1 K^?1 and ΔT = 5.1K under 100 MV/m, which are still more than 65% of the ECE at room temperature. This temperature is far below the dielectric peak temperature of the relaxor blend and the results reveal the promise of the relaxor polymers for a broad range of EC cooling applications.     

The growth of Al x Ga1−x As-GaAs heterostructure lasers by metalorganic chemical vapor deposition

Al _x Ga_1–x As-GaAs double-heterostructure lasers have been grown by the metalorganic chemical vapor deposition process. Conventional DH lasers have exhibited excellent performance characteristics that are equal to or better than comparable devices grown by liquid-phase epitaxy. Other unique laser structures have been grown that demonstrate the flexibility of the metalorganic chemical vapor deposition materials technology. This paper will describe the preparation and properties of Ga_1–x Al_xAs-GaAs lasers grown by this process.The author thanks J. J. Coleman, J. E. Cooper, L. L. Gray, R. G. Imerson, R. E., Johnson, F. F. Kinoshita, N. L. Lind, D. Medellin, L. A. Moudy, C. D. Sallee, and R. D. Yingling for assistance in the work described here. Helpful discussions with H. M. Manasevit and W. I. Simpson are greatly appreciated. The Auger profilling studies performed by C. M. Garner and T. J. Raab are gratefully acknowledged. The collaboration on quantum-well lasers with the research group of Prof. N. Holonyak, Jr. (University of Illinois at Urbana-Champaign) has been extremely beneficial. Discussions with and the support of P. D. Dapkus was essential to the achievement of the results described here. In addition, the support of J. E. Mee is greatly appreciated. This work has been partially supported by the Office of Naval Research under Contract N 00014-78-C-0711.     

Multinuclear MRI of Solids: from Structure to Transport

Direct multinuclear imaging of rigid solids has been performed using the conventional two-pulse spin-echo pulse sequence and liquids magnetic resonance imaging (MRI) hardware. Two-dimensional ²⁷Al and ⁵¹V images of an Al₂O₃-V₂O₅-glass composite sample and ¹¹B, ²³Na, ²⁷Al and ²⁹Si images of glass have been detected, extending the range of nuclei and solid materials that can be studied by this approach. For a spinning cylinder packed with Al₂O₃ powder, quantitative velocity maps have been obtained by directly detecting the ²⁷Al nuclear magnetic resonance signal of the solid phase. The two velocity components in the imaging plane transverse to the rotation axis have been mapped using the three-pulse stimulated echo sequence. Some possibilities to improve sensitivity in the MRI experiments on rigid solids have been considered. In particular, inversion of the satellite transitions by a double frequency sweep adiabatic passage has led to a signal enhancement by a factor of two in ²⁷Al MRI of a glass sample despite a short repetition time (0.5 s) of the imaging pulse sequence. Authors' address: Igor V. Koptyug, International Tomography Center, Russian Academy of Sciences, 3A Institutskaya ulitsa, Novosibirsk 630090, Russian Federation     

Dynamic spin relaxation processes of μ+ in an antiferromagnet

A new mechanism for positive muon spin-lattice relaxation (SLR) in an antiferromagnet is described. The temperature dependence of the relaxation rate (1/T ₁) is calculated in a simplified case, and SLR of muon in MnF₂ is studied in the context of this model.The author wishes to thank: S. R. Kreitzman and J. Shaham for their encouragement and stimulating ideas; L. P. Le, G. M. Luke, W. D. Wu, and Y. J. Uemura for helpful discussions.     

Strongly anisotropic s-wave gaps in exotic superconductors

The exotic superconductors, defined as those that follow the phenomenological trend reported by Uemura (T _c approximately proportional to λ_L ^?2), at present constitute the most broad and general class of superconductors which can reasonably be considered 'similar to the high-T _c cuprates'. It is therefore of much interest to determine the forms of their pairing gap functions. We examine evidence for the gap forms in non-cuprate exotics and demonstrate some general features. The cubic materials often have highly anisotropic gaps. The planar materials tend to have an even stronger gap anisotropy, to the extent that they often have gap nodes, but nevertheless they usually have an s-wave-like gap symmetry. There is good evidence for the latter even in the controversial cases of planar organics and nickel borocarbides. Exceptions to these generalizations are also pointed out and discussed.     

Biexponential analysis of intravoxel incoherent motion in calf muscle before and after exercise: Comparisons with arterial spin labeling perfusion and T2

PurposeThis study aimed to clarify exercise-induced changes in intravoxel incoherent motion (IVIM) parameters obtained from diffusion-weighted imaging (DWI) of the calf muscle, as well as the relationships between IVIM parameters, perfusion, and water content in muscle tissue.Materials and methodsThirteen healthy volunteers underwent IVIM-DWI, arterial spin labeling (ASL), and multi-echo spin-echo T₂ mapping of the right calf on a 3.0-T magnetic resonance imaging scanner before and after performing dorsiflexion exercise. From the data, we derived the perfusion-related diffusion coefficient (D^⁎), perfusion component fraction (F), blood flow parameter (FD^⁎), and restricted diffusion coefficient (D) in the tibialis anterior muscle. The muscle blood flow (MBF) and transverse relaxation time (T₂) were also calculated from the ASL and multi-echo spin-echo data, respectively. We compared the parameters measured before and after exercise and assessed the relationship of each IVIM-derived perfusion parameter (D^⁎, F, and FD^⁎) with MBF and each diffusion parameter (D and ADC) or F with T₂.ResultsNotably, all these parameters were significantly increased after exercise. Before exercise, the FD^⁎ exhibited a significant positive correlation with the MBF, whereas no significant correlation was observed between D^⁎ or F and MBF. After exercise, both D^⁎ and FD^⁎ exhibited significant positive correlations with MBF, whereas F was not significantly correlated with MBF. Additionally, D was significantly correlated with T₂ after exercise, but not before exercise. No significant correlations were found between ADC and T₂ either before or after exercise.ConclusionsThe IVIM analyses before and after exercise enable the simultaneous evaluation of exercise-induced changes in perfusion and water diffusion in the muscle and increases the body of information on muscle physiology.     

Kähler metrics and gauge kinetic functions for intersecting D6‐branes on toroidal orbifolds – The complete perturbative story

We systematically derive the perturbatively exact holomorphic gauge kinetic function, the open string Kähler metrics and closed string Kähler potential on intersecting D6‐branes by matching open string one‐loop computations of gauge thresholds with field theoretical gauge couplings in 𝒩 = 1 supergravity. We consider all cases of bulk, fractional and rigid D6‐branes on T⁶/Ω ℛ and the orbifolds T⁶/(ℤ_N × Ω ℛ) and T⁶/(ℤ₂ × ℤ_2M × Ω ℛ) without and with discrete torsion, which differ in the number of bulk complex structures and in the bulk Kähler potential. Our analysis includes all supersymmetric configurations of vanishing and non‐vanishing angles among D6‐branes and O6‐planes, and all possible Wilson line and displacement moduli are taken into account. The shape of the Kähler moduli turns out to be orbifold independent but angle dependent, whereas the holomorphic gauge kinetic functions obtain three different kinds of one‐loop corrections: a Kähler moduli dependent one for some vanishing angle independently of the orbifold background, another one depending on complex structure moduli only for fractional and rigid D6‐branes, and finally a constant term from intersections with O6‐planes. These results are of essential importance for the construction of the related effective field theory of phenomenologically appealing D‐brane models. As first examples, we compute the complete perturbative gauge kinetic functions and Kähler metrics for some T⁶/ℤ₂ × ℤ₂ examples with rigid D‐branes of [1]. As a second class of examples, the Kähler metrics and gauge kinetic functions for the fractional QCD and leptonic D6‐brane stacks of the Standard Model on T⁶/ℤ₆^′T⁶/ℤ₆^′ from [2] are given.     

On the electronic structure of N,N′-ethylenebis(acetylacetonatiminato)Co(II), Co(II)acacen

Single crystal E.P.R. and cobalt ENDOR measurements on N,N′-ethylenebis(acetylacetonatiminato)Co(II), Coacacen, diluted in Niacacen·1/2 H₂O are reported. Forbidden Δm _Co = 1,2 transitions in the E.P.R. spectra have been observed. The g-tensor (E.P.R.) and the cobalt hyperfine and quadrupole tensors (ENDOR) have been determined. The g- and cobalt hyperfine tensors are discussed. They support the |² A ₂,yz? groundstate proposed for four coordinated low-spin Schiff base complexes of cobalt(II). The measured quadrupole coupling is also compatible with a |² A ₂,yz? groundstate, if anisotropic contraction of the cobalt 3d orbitals is taken into consideration.     

A simulation based method to assess inversion algorithms for transverse relaxation data

NMR relaxometry is a very useful tool for understanding various chemical and physical phenomena in complex multiphase systems. A Carr-Purcell-Meiboom-Gill (CPMG) [P.T. Callaghan, Principles of Nuclear Magnetic Resonance Microscopy, Clarendon Press, Oxford, 1991] experiment is an easy and quick way to obtain transverse relaxation constant (T₂) in low field. Most of the samples usually have a distribution of T₂ values. Extraction of this distribution of T₂s from the noisy decay data is essentially an ill-posed inverse problem. Various inversion approaches have been used to solve this problem, to date. A major issue in using an inversion algorithm is determining how accurate the computed distribution is. A systematic analysis of an inversion algorithm, UPEN [G.C. Borgia, R.J.S. Brown, P. Fantazzini, Uniform-penalty inversion of multiexponential decay data, Journal of Magnetic Resonance 132 (1998) 65–77; G.C. Borgia, R.J.S. Brown, P. Fantazzini, Uniform-penalty inversion of multiexponential decay data II. Data spacing, T₂ data, systematic data errors, and diagnostics, Journal of Magnetic Resonance 147 (2000) 273–285] was performed by means of simulated CPMG data generation. Through our simulation technique and statistical analyses, the effects of various experimental parameters on the computed distribution were evaluated. We converged to the true distribution by matching up the inversion results from a series of true decay data and a noisy simulated data. In addition to simulation studies, the same approach was also applied on real experimental data to support the simulation results.     

Studies of parity and time reversal symmetries in neutron scattering from165Ho

We describe searches for parity and time reversal violations in the scattering of polarized neutrons from polarized and aligned¹⁶⁵Ho targets. We have completed a search with 7.1 and 11.0 MeV neutrons for P_oddT_odd terms in the elastic scattering forward amplitude of the form s. (I×K), wheres is the neutron spin,I is the target spin andk is the neutron momentum vector. The target was a single crystal of holmium, polarized horizontally along itsb axis by a 1 Tesla magnetic field. The neutrons were polarized vertically. Differences in the neutron transmission were measured for neutrons with spins parallel (antiparallel) toI×k. The P,T violating analyzing powers were found to be consistent with zero at the few 10⁻³ level: ρ_P,T(7.1 MeV)=−0.88 (±2.02) x 10⁻³, ρ_P,T(11.0 MeV)=−0.4 (±2.88) x 10⁻³. We have also attempted to find enhancements with MeV neutrons in P-violation due to the term s\k. We are preparing an aligned target cryostat for investigations of P_evenT_odd terms {bd(I\k)(I×k)\s} in neutron scattering. The target will be a single crystal cylinder of¹⁶⁵Ho cooled to 100 mK in a bath of liquid helium and rotated by a shaft from a room temperature stepping motor. The cylinder will be oriented vertically and the alignment (c) axis oriented horizontally. Warming or rotation of the sample allows one to separate effects that mimic the sought-after time reversal violating term.     

Specific heat of the non-centrosymmetric superconductor Re3W

The alloys of non-centrosymmetric superconductor, Re 3 W, which were reported to have an α-Mn structure [P. Greenfield and P. A. Beck, J. Metals, N. Y. 8, 265 (1959)] with T c = 9 K, are prepared by arc melting. The values of ac susceptibility and the low-temperature specific heat of these alloys are measured. It is found that there are two superconducting phases coexisting in the samples with T c1 ≈ 9 K and T c2 ≈ 7 K, which are both non-centrosymmetric in structure as reported previously. By analysing the specific heat data measured in various magnetic fields down to a temperature of 1.8 K, we find that the absence of the inversion symmetry does not lead to an obvious deviation from an s-wave pairing symmetry in Re 3 W.