Continuous-wave magnetic resonance imaging of short T(2) materials

There is growing interest in the use of magnetic resonance imaging (MRI) to examine solid materials where the restricted motion of the probed spins leads to broad lines and short T(2) values, rendering many interesting systems invisible to conventional 2DFT pulsed imaging methods. In EPR T(2) seldom exceeds 0.1 mus and continuous-wave methods are adopted for spectroscopy and imaging. In this paper we demonstrate the use of continuous-wave MRI to obtain 2-dimensional images of short T(2) samples. The prototype system can image samples up to 50 mm in diameter by 60 mm long and has been used to image polymers and water penetration in porous media. Typical acquisition times range between 10 and 40 min. Resolution of 1 to 2 mm has been achieved for samples with T(2) values ranging from 38 to 750 mus. There is the possibility of producing image contrast that is determined by the material properties of the sample.     

On complex singularities of the 2D Euler equation at short times

We present a study of complex singularities of a two-parameter family of solutions for the two-dimensional Euler equation with periodic boundary conditions and initial conditions in the short-time asymptotic régime. As has been shown numerically in Pauls et al. [W. Pauls, T. Matsumoto, U. Frisch, J. Bec, Nature of complex singularities for the 2D Euler equation, Physica D 219 (2006) 40-59], the type of the singularities depends on the angle ? between the modes p and q. Thus, the Fourier coefficients of the solutions decrease as with the exponent α depending on ?. Here we show for the two particular cases of ? going to zero and to π that the type of the singularities can be determined very accurately, being characterised by α=5/2 and α=3 respectively. In these two cases we are also able to determine the subdominant corrections. Furthermore, we find that the geometry of the singularities in these two cases is completely different, the singular manifold being located “over” different points in the real domain.     

Ultra-low field NMR measurements of liquids and gases with short relaxation times

Interest in nuclear magnetic resonance measurements at ultra-low magnetic fields (ULF, approximately microT fields) has been motivated by various benefits and novel applications including narrow NMR peak-width, negligible susceptibility artifacts, imaging of samples inside metal containers, and possibility of directly imaging neuronal currents. ULF NMR/MRI is also compatible with simultaneous measurements of biomagnetic signals. However the most widely used technique in ULF NMR-prepolarization at high field and measurement at lower field-results in large transient signals which distort the free induction decay signal. This is especially severe for the measurement of signals from samples and materials with short T1 time. We have devised an approach that largely cancels the transient signals. The technique was successfully used to measure NMR signals from liquids and gases with T1 in the range 1-4 ms.     

Determination of 27Al-29Si connectivities in zeolites with 2D 27Al-->29Si RAPT-CPMG-HETCOR NMR

The recently introduced concept of the combined use of rotor assisted population transfer (RAPT) and Carr-Purcell-Meiboom-Gill (CPMG) techniques to boost the sensitivity of cross polarization (CP) based NMR experiments is applied to a synthetic zeolite (ZSM-4). The sensitivity was increased by a factor of approximately 4, which enabled acquisition of a high quality two-dimensional 27Al-29Si HETCOR (heteronuclear correlation) spectrum. By separating the resonances in two dimensions, through-space connectivities between spins were revealed and the effective resolution was improved in both dimensions, which allowed determination of the existing ambiguities in spectral assignments in this material. The spectra provided clear indication of random distribution of aluminum and silicon within the ZSM-4 network. Additionally, unexpected correlations were observed between different components of inhomogeneously broadened 29Si and 27Al lines, which are most likely due to differences in the second coordination sphere environments.     

Spatially inhomogeneous development of antiferromagnetism in URu2Si2: evidence from 29Si NMR under pressure

From 29Si NMR study, we present evidence for spatially inhomogeneous development of antiferromagnetic (AF) ordering below T(o) = 17.5 K in URu2Si2. In the pressure range between 3.0 and 8.3 kbar, we have observed the 29Si NMR lines arising from the AF region as well as the previously observed 29Si NMR line which correspond to the nonmagnetic region in the sample. The AF volume fraction is enhanced by applied pressure, whereas the magnitude of internal field at the Si site remains constant (910 Oe) up to 8.3 kbar. In the AF region, the ordered moment is about an order of magnitude larger than 0.03 mu(B)/U.     

29Si and 19F MAS NMR spectra of isolated 29Si(19F)2 and 29Si(19F)3 spin systems: experiments and simulations

We consider ²⁹Si and ¹⁹F MAS NMR spectra of isolated ²⁹Si(¹⁹F)₂ and ²⁹Si(¹⁹F)₃ spin systems in two organosilicon compounds of the type RR’SiF₂ and RSiF₃(R,R′=organic ligands). Experimental spectra are analysed by means of numerical simulations. It is found that the SiF₃ group in RSiF₃ is reorienting rapidly around the molecular Si–C bond direction in the solid state. The two ¹⁹F shielding tensors in RR’SiF₂ have strongly differing orientations relative to the two Si–F bond directions in the molecule. Possibilities and limitations of straightforward MAS NMR approaches for the full characterisation of ²⁹Si(¹⁹F)₂ and ²⁹Si(¹⁹F)₃ spin systems and other dipolar coupled two and three-spin systems are discussed.     

T2 relaxation time measurements in osteoarthritis

Quantification of changes in T(2) relaxation time, in human cartilage, with progression of osteoarthritis (OA), and evaluation of qualitative correlations with clinical evaluation, histology and polarized light microscopy (PLM). Cartilage-bone plugs were harvested from fresh cadaveric knees (n = 10) and specimens after surgical knee replacement (n = 2) at 12 locations, including lateral and medial sides of tibia, femora and patella. Magnetic resonance imaging was performed at 1.5 Tesla using a.2D spin echo sequence. Histological slices were assessed for OA severity through a grading scale based on combined histological and PLM results. T(2) values in clinically moderate OA were generally higher than in severe OA and normal cartilage. Significant association was established between normal and early OA subjects and T(2) variation, in the medial compartment of the knee (p < 0.05) but especially in the medial tibial cartilage (p < 0.00005). As expected, medial and lateral tibio-femoral compartments underwent more severe degeneration. Additionally, there were intracompartmental variation of the relaxation times and histological patterns, which demonstrate the underlying focal involvement of OA in the knee. Furthermore, T(2) values reflected OA pathogenesis with a positive correlation with histology grading scale. Finally, increased T(2) is correlated to histological degeneration of cartilage and may be a good marker for early OA in tibial articular cartilage.     

In vivo measurements of T1 relaxation times of 31P-metabolites in human skeletal muscle

The T1 relaxation times were estimated for 31P-metabolites in human skeletal muscle. Five healthy volunteers were examined in a 1.5 Tesla wholebody imaging system using an inversion recovery pulse sequence. The calculated T1 relaxation times ranged from 5.517 sec for phosphocreatine to 3.603 sec for -ATP.     

Photoneutron cross section of 29Si

The photoneutron cross section of ²⁹Si has been measured over the energy range 8 to 28 MeV using the bremsstrahlung yield curve technique. The giant dipole resonance is observed to be centered at 21.2 MeV and is 5.5 MeV wide. In addition, a pygmy resonance containing much fine structure is observed below 17 MeV. An attempt is made to interpret these observations in terms of the weak coupling of the extra neutron to a ²⁸Si core. However, detailed comparisons of the structure and relative strengths of the ²⁸Si and ²⁹Si cross sections reveal that the coupling of the extra-core nucleon is large and significantly modifies the core wave function.     

Isomer ratio measurements for the reaction 29Si(18O,p2n)44m, 44gSc

Isomer ratios for the reaction ²⁹Si(¹⁸O, p2n)^44gSc, ^44gSc have been deduced from activity measurements for projectile energies between 30 and 99 MeV. Statistical model calculations show that the isomer ratio dependence on projectile energy up to about 80 MeV can be adequately described by assuming a fixed ratio of quadrupole to dipole γ-ray strengths. Such a ratio of E2/E1 strengths agrees with corresponding values deduced from the literature. The values of the γ-ray strength ratios needed to fit the experimental isomer ratios are extremely sensitive to the relative amounts of quadrupole γ-ray admixture and to the presence of discrete levels other than those which conform to the yrast line.     

Spectroscopy of highly excited states in29Si

Particleγ-ray coincidences have been measured in the²⁸Si (d,pγ) reaction at 6.5 and 7 MeV bombarding energy, in the²⁶Mg (α,nγ) reaction at 12, 14 and 15 MeV, and in the²⁷A1 (τ,pγ) reaction at 9 MeV. Theγ-decay has been observed for all bound states of²⁹Si and for 56 unbound states up to 12,960 KeV excitation energy. Particleγ-ray angular correlations were measured in the²⁸Si (d,pγ) reaction at 6.5 MeV and in the²⁶Mg (α,nγ) reaction at 12 MeV. Spin (-parity) assignments or restrictions were obtained for nearly all bound states and some high-spin states above the binding energy. The assignment of mirror levels in²⁹Si and²⁹P has been extended to 8.2 MeV excitation energy. The excitation energies of 41 positive-parity states are reproduced by shell model calculations. The possible existence of aK ^π=5/2⁺ band with prolate deformation is discussed.     

Properties of Heusler-type materials Fe2TSi and FeCo2Si

A number of 3d, 4d and 5d elements are substituted into Fe₃Si in an attempt to form Heusler-like alloys. It is found that with the heat treatment followed, only the 3d elements V, Mn and Co form Heusler alloys. Substitution of the other elements produces polyphase materials. For such elements the limits of solubility in Fe₃Si are determined. For the Heusler alloys the data on degrees of disorder, hyperfine fields and magnetization are presented. These studies, in addition, test the already known site preferences of the transition metal impurities in Fe₃Si at high impurity concentrations.     

On some fundamental features of optically stimulated luminescence measurements

The influence of electron–phonon interaction on the shape of the optically stimulated luminescence decay curve has been examined. The formula describing the dependence of optical cross-section on stimulation energy and temperature has been applied in a computer simulation of optically stimulated luminescence process. Computer simulations have been carried out for different temperatures using both one-trap and two-trap models. The ideas of an OSL measurement with continuously increasing energy of stimulation have been tested via computer simulations. The outcomes of this technique have been presented for a wide range of material parameters.     

Magnetic resonance T21 measurements of the normal human lung in vivo with ultra-short echo times

The objective of this study was to measure T₂1 values of the normal human lung in vivo during breathhold using a rapid gradient-echo sequence with ultra-short echo times (TE). A sagittal slice of the right lung was imaged in six volunteers with various TE ranging from 0.5 ms to 5 ms using a clinical 1.5 Tesla MR scanner. T₂1 values were calculated in a region of interest in the dependent and non-dependent lung. In the dependent lung, T₂1 values of 1.1 ms ± 0.15 ms were measured, and in the non-dependent lung, 0.86 ms ± 0.11 (p < 0.01). T₂1 measurements of the normal human lung during breathhold are feasible with a clinical MR unit. The short T₂1 values require the use of very short TE times (<2.5 ms) in gradient-echo sequences to obtain adequate signal intensity from lung tissue.