Methane clathrate: CH(4) quantum rotor state dependent rattling potential.

In methane hydrate the dominant peak in the density of states above 3 meV represents a rattling mode of the guest molecule CH(4) in the large ice cages. This mode shifts from 6.7 meV at T=4.5 K to T=30 K to 7.14 meV with conversion of CH(4) guest molecules into the tunneling ground state. The less symmetric angular density distribution PsiPsi(*) in the excited rotational state compared to the ground state allows the methane to fit better in the orientation dependent cage potential surface. This leads to a larger average distance to the cage-forming molecules with a weaker potential and a reduced rattling energy. A two state single particle model with characteristic rattling energies of 5.20 meV for pure T-methane and 7.3 meV for pure A-methane weighted by the population factors can fit the data.

Phonon modes at the 2H-NbSe2 surface observed by grazing incidence inelastic x-ray scattering.

We have determined the dispersion of acoustic and optical surface phonon modes 2H-NbSe2 at the by inelastic x-ray scattering under grazing incidence conditions. Already, at room temperature, an anomaly is observed close to the charge density wave -vector position located at about one-third along the Gamma-M direction of the Brillouin zone. Our results indicate that the anomaly for the surface mode occurs at a lower energy than that measured in bulk sensitive geometry in the same experiment, showing evidence of a modified behavior in the uppermost layers. We demonstrate that inelastic x-ray scattering in grazing incidence conditions provides a unique tool to selectively study either surface or bulk lattice dynamics in a single experiment.

Anharmonic motions of Kr in the clathrate hydrate.

The anomalous glass-like thermal conductivity of crystalline clathrates has been suggested to be the result of the scattering of thermal phonons of the framework by 'rattling' motions of the guests in the clathrate cages. Using the site-specific (83)Kr nuclear resonant inelastic scattering spectroscopy in combination with conventional incoherent inelastic neutron scattering and molecular-dynamics simulations, we provide unambiguous evidence and characterization of the effects on these guest-host interactions in a structure-II Kr clathrate hydrate. The resonant scattering of phonons led to unprecedented large anharmonic motions of the guest atoms. The anharmonic interaction underlies the anomalous thermal transport in this system. Clathrates are prototypical models for a class of crystalline framework materials with glass-like thermal conductivity. The explanation of the unusual molecular dynamics has a wide implication for the understanding of the thermal properties of disordered solids and structural glasses.

Adsorption sites and rotational tunneling of methyl groups in cubic I methyl fluoride water clathrate.

Neutron spectroscopy in the microeV and meV regime and quasielastic scattering is applied to characterize the dynamics of methyl groups of methyl fluoride guest molecules in cubic I CH3F-water clathrate. Only above T approximately 60 K quasielastic spectra are unaffected by quantum effects. They are well described by two Lorentzians representing the CH3F species in the small and large cages of the structure. The intensities show that both cages are completely filled. The linear broadenings with temperature follow the model of rotational diffusion. Two clearly separated tunneling bands were observed at T = 4.2 K and are also assigned to the two types of water cages. Disorder of the environment (H-bonds) is reflected in the shape of the bands. For the less hindered species housing the large cages the tunneling band can be quantitatively converted into a potential distribution function within the model of single particle rotation. Transitions to excited rotational states show the dominance of a sixfold potential term V6 = 13 meV modified by a weak threefold term distributed around a characteristic value V3 = 0.9 meV. The potential distribution of V3 influences the barrier for classical reorientation only weakly in agreement with the results from quasielastic data. Adsorption sites with the guest molecules oriented towards a hydrogen bond along one of twelve local twofold axes of the cage are proposed. Such sites are consistent with the sixfold rotational potential and earlier results from methyl iodide clathrate. Rotation-translation coupling as an alternative dynamical process is excluded.

Phase behaviour of n-hexane/perfluoro-n-hexane binary thin wetting films.

We present X-ray reflectivity investigations of the concentration distribution in binary liquid thin films on silicon substrates. The liquid-vapor coexistence of the binary mixture investigated, hexane and perfluorohexane, is far from criticality. Therefore, a sharp interface separates the liquid film from the vapor. The data reveal a separation of the film in layers parallel to the substrate. A phase diagram is constructed as a projection to the (composition difference, temperature) space, covering a temperature range corresponding to the one-phase and the two-phase regime of the bulk liquid. Although the composition data indicate a mixing gap similar to that of the bulk system, there are two major differences: i) only the near-surface phase changes its composition significantly, and ii) a composition gradient in the film exists also at higher temperatures where in the bulk system the one-phase regime exists.

The structure of methane hydrate under geological conditions a combined Rietveld and maximum entropy analysis.

We present a study of the structure of a fully deuterated methane hydrate under the geological conditions found in the world's oceans. In situ high-resolution neutron diffraction experiments have been performed at temperatures of 220, 275, and 280 K and a pressure of 100 bar, corresponding to the conditions at 1000 m water depth. The data were analyzed with a combination of Rietveld refinement and maximum entropy methods. From the Rietveld refinement, precise atomic parameters of the host lattice could be determined, indicating increasing distortions of the structure of the cages at elevated temperatures and pressures. Debye-Waller factors of the encaged CD(4) molecules have been found to exceed the values of the Debye-Waller factors of the D(2)O molecules considerably. In the large cage of structure type I the thermal center-of-mass displacements of the guests are 5-10 times larger than those of the water molecules. From the maximum entropy analysis maps of the scattering length density have been obtained, showing details of the vibrational amplitudes of the atoms in methane hydrate. The Debye-Waller factors of all molecules have been found to deviate considerably from a simple spherical geometry.

Near monochromatic X-rays for digital slot-scan mammography: initial findings.

X-ray spectra are composed of a broad bremsspectrum and anode-characteristic emission lines. In mammography typically molybdenum (Mo), rhodium (Rh) or tungsten (W) anodes are used in combination with Mo, Rh or aluminium filters. Only the photons with energies between 17 and 22 keV of the resulting spectrum are suitable for the soft tissue imaging needed for mammography. The aim of this article is to present first results obtained with a monochromator module mounted at the exit of the X-ray tube of a conventional clinical mammography unit. The experimental setup consists of a Siemens Mammomat 300, an X-ray monochromator module and a linear array detector for image acquisition. The technique is similar to the slot-scan technique known from digital mammography. The experimental machine allows to obtain images both with polychromatic and monochromatic X-rays. Initial evaluation of the system was performed by examination of a contrast-detail phantom (CD-MAM-phantom, Nijmegen, The Netherlands). Images done with the new monochromatic technique were compared to images of the phantom done with polychromatic spectra, with film-screen mammography as well as with digital mammography. The new technique with monochromatic slot-scan mammography resulted in correct identification of 93% of the phantom. Digital slot-scan mammography with polychromatic beam resulted in correct identification of 87%, digital full-field mammography in 83% and conventional film-screen mammography in 70% of the phantom. The results suggest that monochromatization has a potential for improving image quality or decreasing dose in X-ray mammography.

X-ray reflectivity study on the surface and bulk glass transition of polystyrene.

The surfaces of polystyrene (PS) films decorated with gold nanoclusters were investigated by x-ray reflectivity measurements. The thicknesses of the films are much larger than the radii of gyration of the different PS samples. By annealing the films above the glass transition temperature T(g) an embedding process of the clusters into the polymer is detected which is accompanied by a substantial increase in the cluster layer thickness due to Brownian motion. These processes start at a sufficiently low viscosity and may be regarded as a probe of the glass transition in the near surface region of the PS films. Simultaneously the thermal expansion of the entire film and hence its approximate bulk behavior were monitored. Two samples of different molecular weight do not show a significant difference between the surface and bulk T(g) values.

A graphical anatomical database of neural connectivity.

We describe a graphical anatomical database program, called XANAT (so named because it was developed under the X window system in UNIX), that allows the results of numerous studies on neuroanatomical connections to be stored, compared and analysed in a standardized format. Data are entered into the database by drawing injection and label sites from a particular tracer study directly onto canonical representations of the neuroanatomical structures of interest, along with providing descriptive text information. Searches may then be performed on the data by querying the database graphically, for example by specifying a region of interest within the brain for which connectivity information is desired, or via text information, such as keywords describing a particular brain region, or an author name or reference. Analyses may also be performed by accumulating data across multiple studies and displaying a colour-coded map that graphically represents the total evidence for connectivity between regions. Thus, data may be studied and compared free of areal boundaries (which often vary from one laboratory to the next), and instead with respect to standard landmarks, such as the position relative to well-known neuro-anatomical substrates or stereotaxic coordinates. If desired, areal boundaries may also be defined by the user to facilitate the interpretation of results. We demonstrate the application of the database to the analysis of pulvinar-cortical connections in the macaque monkey, for which the results of over 120 neuro-anatomical experiments were entered into the database. We show how these techniques can be used to elucidate connectivity trends and patterns that may otherwise go unnoticed.

Visual areas and spatial summation in human visual cortex.

Functional MRI measurements can securely partition the human posterior occipital lobe into retinotopically organized visual areas (V1, V2 and V3) with experiments that last only 30 min. Methods for identifying functional areas in the dorsal and ventral aspect of the human occipital cortex, however, have not achieved this level of precision; in fact, different laboratories have produced inconsistent reports concerning the visual areas in dorsal and ventral occipital lobe. We report four findings concerning the visual representation in dorsal regions of occipital cortex. First, cortex near area V3A contains a central field representation that is distinct from the foveal representation at the confluence of areas V1, V2 and V3. Second, adjacent to V3A there is a second visual area, V3B, which represents both the upper and lower quadrants. The central representation in V3B appears to merge with that of V3A, much as the central representations of V1/2/3 come together on the lateral margin of the posterior pole. Third, there is yet another dorsal representation of the central visual field. This representation falls in area V7, which includes a representation of both the upper and lower quadrants of the visual field. Fourth, based on visual field and spatial summation measurements, it appears that the receptive field properties of neurons in area V7 differ from those in areas V3A and V3B.

Physics for practitioners: the use of radiolabeled monoclonal antibodies in B-cell non-Hodgkin's lymphoma.

The limitations of conventional treatment for non-Hodgkin's lymphoma (NHL) have led to intense investigations of biological therapies for this disease, including monoclonal antibodies. Conjugation of these antibodies with radionuclides results in additional cytotoxic activity with generally acceptable levels of toxicity. Numerous antibodies and radionuclides of widely varying properties are in clinical development. Given the likelihood that one or more radioimmunotherapeutic agents will soon be commercially available, there is a need for oncologists who treat patients with NHL to become more aware of the principles of radiation physics that underlie radioimmunotherapy and the interactions among radiolabeled antibodies, the tumor, and the host.

Perceived speed of colored stimuli.

The influence of contrast and color on perceived motion was measured using a speed-matching task. Observers adjusted the speed of an L cone contrast pattern to match that of a variety of colored test patterns. The dependence of speed on test contrast was the same for all test colors measured, differing only by a sensitivity factor. This result suggests that the reduced apparent speed of low contrast targets and certain colored targets is caused by a common cortical mechanism. The cone contrast levels that equate perceived speed differ substantially from those that equate visibility. This result suggests that the neural mechanisms governing speed perception and visibility differ. Perceived speed differences caused by variations in color can be explained by color responses that are characteristic of motion-selective cortex.

Influence of contrast media on blood coagulation.

RATIONALE AND OBJECTIVES: Contrast agents have been reported to influence the blood clotting system to an extent depending mainly on whether the compounds are ionic or nonionic. The objective of the current series of studies was to determine interspecies differences; the effect of variable incubation times; and the effect on thromboplastin times (TPT) of adding heparin to a number of x-ray and magnetic resonance imaging (MRI) contrast agents. In addition, the stability of clots formed in the presence of iopromide was studied. In a final experiment, the effect of the contrast agents on the bleeding time was studied in rats. METHODS: Nine x-ray and three MRI contrast agents were used in the study. Thromboplastin times was determined in platelet-poor plasma of humans, rats, rabbits, or dogs using calcium (Ca) thromboplastin from human placenta or rabbit brain and lung tissue and incubation times as long as 4 hours. Bleeding times were determined in rats 5 minutes, 4 hours, 24 hours, and 48 hours after intravenous injection of the contrast agents by making a small incision into the tail of the animal, immersing the tail in saline, and measuring the time period during which small blood streaks were visible. RESULTS: Nonionic contrast agents (x-ray and MRI) increased the TPT by a factor of 1.5 to 2, whereas ionic compounds prolonged TPT by a factor of > 3. Thromboplastin times increased in the order of dog < rabbit < rat < human 2- to 4-fold. However, the ranking of different contrast agents remained unchanged. Prolongation of bleeding time lasted as long as 24 hours for some contrast agents. Clots formed in the presence of iopromide were unstable and did not absorb the contrast agent. CONCLUSIONS: The animal models used in the current series of studies seem to be valid for predicting the effect of contrast agents on the blood clotting system in humans.

Ytterbium- and dysprosium-EOB-DTPA. A new prototype of liver-specific contrast agents for computed tomography.

RATIONALE AND OBJECTIVES: A series of studies was conducted to determine whether metal complexes of the EOB-DTPA type are useful as contrast agents for computed tomography (CT). METHODS: Metal complexes using EOB-DTPA as ligand were synthesized with lanthanide metal ions (lanthanum [La], cerium [Ce], praseodyme [Pr], gadolinium [Gd], dysprosium [Dy], ytterbium [Yb], and lutetium [Lu]) and with nonlanthanides (lead [Pb] and bismuth [Bi]). Complex stability was assessed by measuring binding to bone meal. The physicochemical parameters partition coefficient, osmolality, viscosity, and protein binding were determined in vitro. Tolerability was tested both in vitro (thromboplastin time, effect on erythrocytes) and in vivo (acute, neural, and cardiovascular toxicities). Biliary excretion and tissue distribution, especially liver, kidney, and bone concentrations, were measured in rats after intravenous doses of 0.5 mmol/kg. Imaging performance using CT was investigated in vitro in a phantom model and, for Gd-EOB-DTPA, in vivo by injecting doses of 0.5 mmol/kg into healthy or tumor-bearing rats and rabbits. RESULTS: The kinetic stability of M-EOB-DTPA complexes differed widely. Nonlanthanide metals, especially Pb-EOB-DTPA, provided less stable complexes than lanthanides with an optimum of stability for the metals Gd, Dy, Yb, and Lu. Tolerability was good for all compounds, best results were obtained for Gd and Yb. Concentrations in rat liver after administration of Gd-EOB-DTPA, 0.5 mmol/kg intravenous, were approximately 1 mumol/g, resulting in CT enhancement of 16 Hounsfield units (HU). Tumor tissue was not enhanced. In rabbits, at the same dose level 30 HU was found. CONCLUSIONS: Metal complexes of the EOB-DTPA type, especially those of Gd and Yb seem to be useful as iodine-free liver-specific contrast agents for CT.

Pre-clinical evaluation of gadobutrol: a new, neutral, extracellular contrast agent for magnetic resonance imaging.

The Gd(3+)-complex of 10-(2,3-dihydroxy-1-hydroxymethylpropyl)-1,4,7,10-tetraazacyclo dodecane-1,4,7-triacetic acid(gadobutrol) is a new, neutral Gd-chelate for use as an extracellular contrast agent in magnetic resonance imaging (MRI). The blood level in dogs after intravenous (i.v.) injection decreased with a terminal half-life of about 45 min, the clearance was about 3.75 ml/min per kg and the distribution volume of 0.23 l/kg suggested an extracellular distribution. Biodistribution experiments in rats revealed that only a very small amount (0.16%) of the dose was left in the body 7 days after i.v. injection. Measurable amounts of Gd could be detected only in the liver, kidneys and bones. The osmolality (0.57 osmol/kg at 0.5 mol/l and 1.39 osmol/kg at 1 mol/l) is in the range of other low osmolality contrast media for MRI. Only very little interaction with biologically relevant molecules was suggested by a histamine release test and a lysozyme inhibition test. An i.v.-LD50 of 23 mmol/kg in mice combined with a comparatively high T1-relaxivity (5.6 l/mmol per s at 0.47 T and 6.1 l/mmol per s at 2 T) in plasma promises a high margin of safety. In preliminary imaging experiments, gadobutrol caused high enhancement in different lesions (cerebral infarct, brain tumor) of the rat. Tripling of the typical clinical dose of 0.1 mmol/kg was shown to provide additional diagnostic gain in lesions of this type.

Transforming growth factor alpha gene expression in cultured human keratinocytes is unaffected by cellular aging.

BACKGROUND: Cultured human keratinocyte grafts have been shown to stimulate endogenous reepithelialization of both chronic nonhealing and acute partial-thickness wounds. This effect is most likely mediated by cytokines that stimulate keratinocyte growth, such as transforming growth factor alpha. The effect of cellular age on cytokine expression by cultured grafts used for this purpose is presently undefined. In this study, transforming growth factor alpha gene expression in cultured foreskin keratinocytes from donors varying in age from 2 to 82 years was analyzed semiquantitatively by two separate methods, ie, Northern hybridization and competitive polymerase chain reaction. RESULTS: No pattern of decline in transforming growth factor alpha messenger RNA expression with increasing cellular age was observed by either analysis. CONCLUSION: The results indicate that expression of transforming growth factor alpha by cultured grafts may not be significantly affected by increasing cellular age and suggest that, even in the elderly, cultured autografts may be effective as pharmacologic agents for wound treatment.