Decrease In Relaxation Time Research Articles

Pancreatic receptors: initial feasibility studies with a targeted contrast agent for MR imaging.

To evaluate cholecystokinin (CCK) as a target-specific vector for magnetic resonance (MR) receptor imaging of rat pancreas. Monocrystalline iron oxide (MION) was labeled with CCK by noncovalent attachment. Receptor specificity of the conjugate was determined with competitive binding studies. Pharmacologic determinations were blood half-lives, biodistribution, time responses, dose responses, and limited toxicity. Specific cell binding of MION-20-CCK was saturable and inhibitable by a CCK antagonist. Blood half-life of MION-20-CCK was 20 minutes, which was shorter than that of unlabeled MION. Biodistribution studies showed a statistically significant decrease in relaxation times in pancreatic tissues from 42.7 msec +/- 2.0 to 33.8 msec +/- 1.4 (P < or = .05) but not in tumor after administration of MION-20-CCK. The half-life of MION-20-CCK in the pancreas was 3 weeks; no signs of toxicity were shown at the level tested. Target-specific MR imaging of pancreatic receptors is feasible. Additional studies are necessary to perfect binding strategies, optimize preparations, and scale up synthesis for imaging in other species.

Relaxivity and magnetization transfer of white matter lipids at MR imaging: importance of cerebrosides and pH.

To measure the effect of the major lipid components of white matter on the relaxation times and magnetization transfer (MT) of water protons. Multilamellar vesicle (MLV) suspensions of pure phosphatidylcholine (PC) and PC with varying proportions of cholesterol (CHOL), sphingomyelin (SPH), and galactocerebroside (GC) were produced at varying pH. T1, T2, and MT were measured. Suspensions consisting of PC with CHOL or SPH caused greater shortening of T1 and T2 and slightly greater MT effects than did PC alone. Suspensions consisting of GC and PC caused the greatest decrease in relaxation times, and the MT effect was two to three times greater than with either CHOL-PC or SPH-PC suspensions. At acidic pH, the relaxation time changes and the MT effects with CHOL-PC, SPH-PC, and GC-PC suspensions were greater than the effects of the same MLVs at physiologic pH. GC has the strongest effect on relaxivity and MT of all the major white matter lipids. The relaxivity and MT observed with white matter lipids is pH dependent, indicating that chemical exchange of protons likely contributes to relaxivity and MT.

Frequency dependence of the complex dielectric constant of poly(ethylene oxide) under hydrostatic pressure

Electrical-impedance measurements have been made in the frequency range 5 Hz to10 MHz in pure poly(ethylene oxide) having a molecular weight of 600 000 from 254 K nearly up to the melting point of the crystalline phase (about 330 K). As the temperature approaches the melting point there are large increases in the reale′ and imaginarye″ parts of the dielectric constant. The frequency dependence ofe′ is characterized by a primary-relaxation process, whose frequency increases with increasing temperature as a consequence of decrease of the average structural relaxation time. There is strong evidence that this low-frequency dispersion rises mainly from the diffusive transport of localised charge carriers rather than a purely orientation relaxation process. In addition the effects of hydrostatic pressure (0–25 Gpa) on the frequency dependences of the reale′ and imaginarye″ parts of the dielectric constant have been measured in the same temperature range.

Reduction in myocardial high energy phosphate spin lattice relaxation times using manganese.

Paramagnetic contrast agents are being widely used in proton magnetic resonance imaging. The present study demonstrates the potential usefulness of paramagnetic contrast agents in 31P NMR spectroscopy. Using the Langendorff perfused rat heart, manganese chloride (Mn2+) at a concentration of 50 microM/L was added to the perfusate for 10 or 30 min. 31P NMR T1 relaxation times were subsequently measured at 121.5 MHz. After the 10 min exposure to Mn2+, 31P NMR T1 measurements of phosphocreatine and ATP were reduced by ca 25 and 50%, respectively, with no significant change in linewidths. A small additional decrease in T1 relaxation times after infusion of Mn2+ for 30 min was not significantly different from the values at 10 min. Potential uses of Mn2+ in 31P NMR spectroscopy include improving the S/N ratio of spectra and assessment of calcium ion channel activity.

Spin-dependent photoconductivity in hydrogenated amorphous germanium and silicon-germanium alloys.

In this work we use spin-dependent photoconductivity (SDPC) to study the recombination process of photoexcited carriers in hydrogenated amorphous germanium (a-Ge:H) and silicon-germanium alloys (a-${\mathrm{Si}}_{\mathit{x}}$${\mathrm{Ge}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$:H). The a-Ge:H SDPC signal is found to be strongly affected by the larger spin-orbit coupling \ensuremath{\lambda}, when compared to a-Si:H (${\ensuremath{\lambda}}_{\mathrm{Ge}}$\ensuremath{\approxeq}7${\ensuremath{\lambda}}_{\mathrm{Si}}$), which results in a reduced spin-lattice relaxation time. The decrease in the spin-lattice relaxation time gives the following characteristics to the a-Ge:H SDPC signal: (i) small amplitudes (-\ensuremath{\Delta}\ensuremath{\sigma}/\ensuremath{\sigma}\ensuremath{\le}${10}^{\mathrm{\ensuremath{-}}6}$); (ii) a linear dependence on microwave power, and strong temperature dependence. In a-${\mathrm{Si}}_{\mathit{x}}$${\mathrm{Ge}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$:H alloys, the incorporation of Ge is marked by a sudden change in the SDPC signal from Si-like to Ge-like, for x0.9. The origin of the spin-dependent recombination in a-Ge:H and a-${\mathrm{Si}}_{\mathit{x}}$${\mathrm{Ge}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$:H is discussed.

Theoretical investigation of gain enhancements in strained In/sub 0.35/Ga/sub 0.65/As/GaAs MQW lasers via p-doping

We present a systematic theoretical investigation of the influence of p-doping on the gain characteristics of strained In/sub 0.35/Ga/sub 0.65/As/GaAs multiple-quantum-well (MQW) lasers, and compare the results with those obtained experimentally from devices with record 30 GHz modulation bandwidths. Experimentally, the combination of p-doping and strain has been found to lead to only a small increase in the differential gain, /spl part/g//spl part/n, but a large decrease in the non-linear gain coefficient, /spl epsiv/; this behaviour has been theoretically accounted for by a doping-induced decrease in the intraband relaxation time, /spl tau//sub in/. The theoretical investigations reveal that the assumption of a constant intraband relaxation time is not sufficient to describe the role of p-doping in the above devices, and highlight the importance of utilizing an appropriate lineshape function for the modeling of high speed laser modulation behaviour.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Reorientational Dynamics of a Nematogenic Liquid in Porous Sol-Gel-Glasses: Surface and Pore Size Effects

ABSTRACTTime-resolved transient grating optical Kerr effect experiments on a nanosecond timescale were used to investigate the reorientational dynamics of pentylcyanobiphenyl (5CB) confined in the nanometer lengthscale pores of silica glasses. Pore size dependent studies (25 Å ≤ d ≤ 100 Å) over a large temperature range from the bulk phase transition temperature Tni, = 35.2 ° C up to 115 ° C demonstrate a drastic influence of geometrical restriction on the cooperative reorientational motion in the nematogenic liquid. At temperatures far above the phase transition (T ≥ 70 °C) a bulk phase and a distinct surface layer with different dynamic properties are observed. With pore size reduction to 25 Å, a decrease of the surface layer thickness and an increase of the surface layer relaxation time are found. This is associated with the modification of the surface layer structure by geometrical factors, e.g., the pore curvature. Measurements at lower temperature in the range Tni ≤ T ≤ Tni = 30 ° C show a drastic decrease of the mean relaxation time τ of the order parameter fluctuations compared to the bulk. The relaxation dynamics exhibit a pronounced pore size dependence which can be explained by a crude correlation length cutoff model as well as a modified LandaudeGennes ansatz. However, these approaches cannot reproduce the nonexponentiality of the observed decays suggesting the importance of interporous interactions.

Intracellular labeling of T-cells with superparamagnetic contrast agents.

Isolated rat T-cells have been labeled intracellularly, using endocytosis uptake of two superparamagnetic contrast agents, AquaMag100 and BMS180549, which are both iron-oxide particles coated with dextran. No deterioration of cell proliferation response to mitogen stimulation was observed after labeling with either superparamagnetic contrast agent. AquaMag100 particles show aggregation and co-precipitation in culture media for T-cells. BMS180549 particles not only produce no observable aggregation or co-precipitation, but also have a higher efficiency for labeling T-cells than AquaMag100. The efficiency of cell labeling was determined by measuring the decrease in the spin-spin relaxation time of the water proton in cell samples containing 1 x 10(7) labeled T-cells/milliliter of 2% w/w gelatin. After optimization of the labeling procedures, a shortening of the spin-spin relaxation time by a factor of approximately 7 to 10 has been demonstrated. Under the present experimental conditions, the up-regulation of low density lipoprotein receptor does not increase the labeling efficiency by endocytosis. Our results suggest that intracellular labeling of specific cell types can be achieved with good efficiency and the labeled cells can be detected by magnetic resonance imaging in rat testicles in vivo.

Fluid saturation-dependent nuclear magnetic resonance spin-lattice relaxation in porous media and pore structure analysis

Nuclear-magnetic-resonance spin-lattice relaxation measurements were conducted in a Bentheimer sandstone sample for drainage experiments involving gas-liquid fluid phases with ten different saturation levels ranging from complete water saturation (Sw=1) down to Sw=0.14. A monotonic decrease in relaxation times was observed as the water saturation was lowered over this broad saturation range. This phenomenon is explained by considering that for a drainage process the liquid (wetting) phase was drained from different-sized pores at different saturations with larger pores being drained first. The relaxation decay curves corresponding to each saturation state were analyzed using both stretched exponential and discrete multiexponential functions. In particular, the effect due to bulk fluid relaxation was eliminated so that a more appropriate relationship between pore size distribution and the relaxation rate is obtained. From these analyses, the relative variation of pore size distributions corresponding to different saturation levels was obtained. A power-law dependence of saturation with relaxation times is observed which indicates that the relaxation analysis can be used to characterize fluid saturations.

Magnetic resonance imaging investigation of blood-brain barrier damage in adoptive transfer experimental autoimmune encephalomyelitis

Recent advances in fast magnetic resonance imaging (MRI) techniques have allowed quantification of parameters such as T1 relaxation time, which can be modified by changes in the water content of a tissue. We have used this new method to study the evolution of blood-brain barrier (BBB) changes after adoptive transfer of MBP-specific (AT-EAE) and ovalbumin-specific T cell lines in Lewis rats. Measurable changes in T1 relaxation time suggesting widespread increase in BBB permeability were found, starting on day 3 post inoculation (p.i)., in the midbrain and brainstem of AT-EAE rats. In addition, we noted a significant decrease in T1 relaxation time before injection of a paramagnetic agent, in the cisternal cerebrospinal fluid (CSF) of diseased animals, starting on day 5 p.i. In vitro measurement of T1 in CSF containing various concentrations of albumin, IgM and glucose showed that, at physiological concentrations, a T1 decrease is mainly associated with an increase in albumin concentration. A moderate increase in BBB and blood-CSF barrier permeability was found as early as 4–8 h p.i., in rats injected with MBP-specific as in animals injected with ovalbumin-specific T cell lines, suggesting a non-specific mechanism. Experimental MRI may become a powerful tool to sequentially analyse changes in barrier dynamics, for example following pharmacological intervention.

Magnetic resonance imaging and different levels of iron overload in chronic liver disease

The need for accurate and noninvasive evaluation of liver iron stores prompted us to evaluate the reliability of high-field magnetic resonance imaging equipment in liver patients with low or moderate siderosis, given the poor results obtained using systems operating at low field strength in such cases. Twenty patients with sporadic porphyria cutanea tarda and 28 with comparable chronic liver diseases (chronic hepatitis or cirrhosis) and moderate siderosis were compared with 10 patients with idiopathic or secondary hemochromatosis and 10 healthy controls. Plasma iron profile, ferritin concentration and liver iron concentration, determined with atomic absorption spectroscopy, were matched with the magnetic resonance parameters - namely, transverse relaxation time and the signal intensity for a given proton amount, obtained with equipment operating at a field strength of 1.5 T. Hemochromatosis patients with mean liver iron concentrations of 550 μmol/gm dry wt (vs. 10 μmol of controls) exhibited an impressive reduction in the signal intensity with respect to the other three groups, and this reduction prevented any further comparison with the same porphyria cutanea tarda and chronic liver disease groups, whose liver iron level was twice that of the controls. The signal intensity remained almost unchanged in the latter groups, whereas the transverse relaxation time was significantly reduced. Moreover, correlation with liver iron was significantly inverse in the case of the transverse relaxation time (n = 17, r = 0.62, p = 0.008) and direct in the case of the transverse relaxation rate. The transverse relaxation time values returned to normal in five patients who had completed an iron-depletion program. No significant relationships between porphyrinuria (in porphyria cutanea tarda patients) or associated histological lesions (steatosis or inflammation) and the signal intensity or the transverse relaxation time were observed. We conclude that a magnetic resonance imaging system operating at higher field strength (with respect to previous studies) represents a useful, non-invasive diagnostic tool for the evaluation of even modest liver iron overload. Under these conditions, the decrease in the transverse relaxation time is proportional to the decrease in iron. Hemochromatosis cannot be investigated under the same conditions. (HEPATOLOGY 1993; 17:997–1002.)

On the relaxation of small magnetic interacting particles and the consequences in quantum tunnelling of magnetization

We use the mean field approximation to discuss the magnetization relaxation times of coupled small magnetic particles. It is shown, in particular for antiferromagnetic interactions, that the relaxation time decreases for a given temperature in comparison with the noninteracting case. This effect is maximum around the “ordering temperature” where the inverse of the logarithm of the relaxation time saturates to a constant value. In this context we discuss also the influence of antiferromagnetic coupling between particles on possible experimental data and its relation to quantum tunneling of the magnetization.

Effects of Dry Etching and Hydrogen Passivation on Transport Properties and Photoluminescence of GaAs/AlGaAs Heterostructures

We present an investigation of electron cyclotron resonance plasma etching induced damage of the 2-dimensional electron gas (2DEG) in GaAs/AlGaAs heterostructures using low temperature photoluminescence (PL), electron paramagnetic resonance (EPR) and the Shubnikov-de Haas (SdH) effect. Dry etching of half of the 20 nm top layer of GaAs results in a decrease of the single-particle relaxation time by 20 - 50%, while the concentration of a surface-related paramagnetic defect increases by about one order of magnitude. At the same time, the PL intensity decreases by a factor of 5 - 10. Plasma hydrogenation experiments, annealing and wet etching experiments have been carried out, and the different characterization results are related to each other. We find that passivation in a hydrogen plasma, which leads to a strong increase of the PL intensity of etched as well as unetched samples, causes damage to the transport properties of the 2DEG. The defect concentration is not related in a simple way to the changes of optical and transport properties.

2H n.m.r. study of high pressure effects on the molecular dynamics in polystyrene: 3. Spin relaxation studies

The effect of high pressure on 2H n.m.r. spin relaxation in polystyrene is studied. Deuteron spin-lattice relaxation behaves non-exponentially both in chain- and ring-deuterated polystyrene at temperatures up to 20 K above the glass transition. The non-exponentiality is caused by the broad distribution of correlation times and the slowness of the spin diffusion process. The fast flipping phenyl rings are an effective relaxation mechanism both for the chain and for the phenyl deuterons. The distribution of correlation times is either due to a heterogeneity of local environments or to a change in time-scale of the motion over long time intervals. Denser packing of the polymer caused by high pressure results in a decrease in the spin-lattice relaxation time. The apparent transverse relaxation time can be used as a monitor of the glass transition.

Persistence of regions with high segment density in polyethylene melts

AbstractProton nuclear magnetic resonance (NMR) spin‐spin relaxation measurements were made on three commercial‐grade polyethylenes in the melt state, free of solvent. All samples exhibit a three‐component relaxation behavior, with components being assigned to amorphous low‐molecular weight material (non‐network fraction), amorphous entangled network fraction, and an ordered or high‐segmental‐density fraction, in order of decreasing relaxation times. Sample thermal history is shown to have a considerable effect on the overall relaxation behavior, and therefore on the relative amounts of each of the three components in the melt. An adequate thermal treatment of samples produces an equilibrium melt with invariant composition of the three fractions. The effects of thermal history on the relative amount of high‐segment‐density regions in the melt parallels its effect on the fraction of crystalline material in the solid polymer. These results are evidence for the persistence of ordered regions in polyethylene at temperatures well above the crystalline melting point of the polymer. We further comment on the nature of the two slower relaxing components and present examples of how the components manifest themselves in other polymer characterization techniques. © 1992 John Wiley &amp; Sons, Inc.

Molecular Orientation in Injection-Molded Polypropylene Copolymers with Ethylene

Abstract The molecular orientation process in injection molding of polypropylene copolymers with ethylene was theoretically analyzed from a view point of growth of melt orientation (recoverable shear strain) at the gate and its relaxation in the mold cavity. Regardless of the characteristics of raw resin (copolymerization type and ethylene content) and molding conditions (cylinder temperature), there exist unitary correlations between calculated degree of molecular orientation and experimentally measured degrees of molecular orientation such as the thickness of skin layer and crystalline c-axis orientation function. According to this analysis, the fact that molecular orientation is decreased by copolymerization with ethylene is due to decrease in melt orientation (recoverable shear strain), decrease in the relaxation time and reduction of crystallization temperature.

Hot-carrier transport in p-GaAs

Minority-electron transport properties in p-GaAs have been investigated using time-of-flight (TOF) and time-integrated/time-resolved photoluminescence (PL) measurements. It was found that with an increase in hole concentration the drift velocity decreases, a thermal nonequilibrium energy state exists between electrons and holes, and the energy relaxation time decreases. By combining these experimental data with a Monte Carlo calculation the scattering mechanisms in highly doped semiconductors were clarified. It was also found that energy and momentum transfers during the electron-hole interaction play an important role in the minority-electron transport for p-GaAs. This investigation revealed that the combination of an experimental approach using optical techniques, such at TOF or PL, with a theoretical approach such as a Monte Carlo calculation is indispensable in clarifying hot-carrier transport.

Effects of processing and valence states of metal oxides on electrical properties of ZnO varistors

The electrical characteristics of ZnO varistors with various valence states of manganese and cobalt were studied. The electrical properties of varistors are discussed in terms of defects produced by the additions. Compositions with lower trap densities have the characteristics of higher non-linear coefficient, higher voltage ratio and lower leakage current. The electrical properties are improved by increasing the sintering temperature and time due to a better tunnelling effect. It is also found that the depression angle and relaxation time decrease as a function of increasing sintering time, and the depression angle decreases and relaxation time increases with increasing sintering temperature. This indicates an enhancement in the degree of uniformity in trapping conductance and the trap emission/capture rate when the sintering time increases. The dependence of the depression angle and relaxation time on the composition are also presented.

Dipolar ferromagnets and glasses (invited)

What is the ground state and what are the dynamics of 1023 randomly distributed Ising spins? We have attempted to answer these questions through magnetic susceptibility, calorimetric, and neutron scattering studies of the randomly diluted dipolar-coupled Ising magnet LiHoxY1−xF4. The material is ferromagnetic for dipole concentrations at least as low as x=0.46, with a Curie temperature obeying mean-field scaling relative to that of pure LiHoF4. In the dilute spin limit, an x=0.045 crystal shows very unusual glassy properties characterized by decreasing barriers to relaxation as T→0. Its properties are consistent with a single low degeneracy ground state with a large gap for excitations. A slightly more concentrated x=0.167 sample, however, supports a complex ground state with no appreciable gap, in accordance with prevailing theories of spin glasses. The underlying causes of such disparate behavior are discussed in terms of random clusters as probed by neutron studies of the x=0.167 sample. In addition to tracing the evolution of the glassy and ferromagnetic states with dipole concentration, we investigate the effects of a transverse magnetic field on the Ising spin glass, LiHo0.167Y0.833F4. The transverse field mixes the eigenfunctions of the ground-state Ising doublet with the otherwise inaccessible excited-state levels. We observe a rapid decrease in the characteristic relaxation times, large changes in the spectral form of the relaxation, and a depression of the spin-glass transition temperature with the addition of quantum fluctuations.

From classical to quantum glass.

We study the effects of a transverse magnetic field on the dynamics of the randomly diluted, dipolar coupled, Ising magnet ${\mathrm{LiHo}}_{0.167}$${\mathrm{Y}}_{0.833}$${\mathrm{F}}_{4}$. The transverse field mixes the eigenfunctions of the ground-state Ising doublet with the otherwise inaccessible excited-state levels. We observe a rapid decrease in the characteristic relaxation times, large changes in the spectral form of the relaxation, and a depression of the spin-glass transition temperature with the introduction of quantum fluctuations.