Water-filled Cavities Research Articles

Voxel size effects in three-dimensional nuclear magnetic resonance microscopy performed for trabecular bone dosimetry.

An important problem in internal dosimetry is the assessment of energy deposition by beta particles within trabecular regions of the skeleton. Recent dosimetry methods for trabecular bone are based on Monte Carlo particle transport simulations within three-dimensional (3D) images of real human bone samples. Nuclear magnetic resonance (NMR) microscopy is a 3D imaging technique of choice due to the large signal differential between bone tissue and the water-filled marrow cavities. Image voxel sizes currently used in NMR microscopy are between 50 microm and 100 microm, but the images are time consuming to acquire and can only be performed at present for in vitro samples. It is therefore important to evaluate what resolution is best suitable in order to properly characterize the trabecular microstructure, to adequately predict the tissue dosimetry, and to minimize imaging time. In this work, a mathematical model of trabecular bone, composed of a distribution of spherical marrow cavities, was constructed. The mathematical model was subsequently voxelized with different voxel sizes (16 microm to 1,000 microm) to simulate 3D NMR images. For each image, voxels are assigned to either bone or marrow according to their enclosed marrow fraction. Next, the images are coupled to the EGS4 electron transport code and absorbed fractions to bone and marrow are calculated for a marrow source of monoenergetic electrons. Radionuclide S values are also determined for the voxelized images with results compared to data calculated for the pure mathematical sample. The comparison shows that for higher energy electrons (>400 keV), good convergence of the results is seen even within images of poor resolution. Above 400 keV, a voxel resolution as large as 300 microm results in dosimetry errors below 5%. For low-energy electrons and high-resolution images, the self-dose to marrow is also determined to within 5% accuracy. Nevertheless, increased voxelization of the image overestimates the surface area of the bone-marrow interface leading to errors in the cross-dose to bone as high as 25% for some low-energy beta emitters.

Cyano-Bridged Re6Q8(Q = S, Se) Cluster-Cobalt(II) Framework Materials: Versatile Solid Chemical Sensors

Face-capped octahedral clusters of the type [Re6Q8(CN)6]4- (Q = S, Se) are used to space apart partially hydrated Co2+ ions in extended solid frameworks, creating porous materials that display dramatic color changes upon exposure to certain organic solvents. The clusters react with cobaltous ions in aqueous solution to precipitate the new solid phases [Co2(H2O)4][Re6S8(CN)6]·10H2O (1), Cs2[Co(H2O)2][Re6S8(CN)6]·2H2O (2), and [Co(H2O)3]4[Co2(H2O)4][Re6Se8(CN)6]3·44H2O (3). The structures of 1·2H2O and 3 were determined by single-crystal X-ray analysis. The former consists of an expanded Prussian blue type framework with [Re6S8]2+ and [Co2(μ-OH2)2]4+ cluster cores occupying alternate metal ion sites, and features cubelike cages enclosing water-filled cavities approximately 258 Å3 in volume. The latter structure exhibits a network of Co2+ ions and [Co2(μ-OH2)2]4+ cores connected through [Re6Se8(CN)6]4- clusters, defining an array of one-dimensional channels with minimum internal diameters of 4.8 Å. A Rietveld refinement against X-ray powder diffraction data established compound 2 as isostructural to an analogous Fe-containing phase with a two-dimensional framework reminiscent of the Hoffman clathrates. Thermogravimetric analyses show that all three compounds are fully dehydrated by ca. 100 °C, with no further significant loss of mass below 500 °C. Upon exposure to diethyl ether vapor, the color of compounds 1 and 3 immediately changes from orange to an intense blue-violet or blue; other polar solvents induce somewhat different colors. These (reversible) changes are associated with the emergence of an envelope of new absorption features at wavelengths between 500 and 650 nm, and the magnitude of the response to a solvent can be estimated by measuring the relative intensity of a band with a maximum near 600 nm. We propose that the vapochromic response is due to solvent molecules entering the pores of the solid, where they disrupt the hydrogen-bonded water network, prompting the release of bound water from the [Co2(H2O)4]4+ clusters and conversion of their Co centers from octahedral to tetrahedral coordination. Significantly, this process does not destroy the three-dimensional connectivity in either structure, but rather creates a much more flexible framework that can expand to accommodate the incoming solvent molecules. Spectroscopic and magnetic data confirm the change in coordination geometry, and the trends in solvent responses (e.g., methanol < ethanol < n-propanol < i-propanol) are consistent with a decreased ability to support the bridging water ligands of the clusters as steric bulk increases. Size-selective sensing is demonstrated with methyl tert-butyl ether, which causes a color change in compound 3, but not in compound 1. X-ray powder diffraction experiments indicate that the vapochromic response in both compounds is affiliated with a reversible change in the bulk crystal structure of the material. Variable-temperature magnetic susceptibility data for compound 1 suggest a weak antiferromagnetic coupling interaction between the water-bridged Co2+ ions of the dinuclear cluster units. Finally, a simple chemical sensing device employing these solids is described, along with some properties relevant to its function.

A method for determining orifice loss coefficients in strongly transient compressible liquid flows

The aim of the current work was to devise a method of establishing orifice loss coefficients in highly transient liquid flows. This was motivated by the fact that (i) many physical systems involve liquid transfer through narrow channels between cavities under unsteady conditions, and (ii) such data does not exist in the literature. A test facility was constructed, consisting of a drop-weight tester which operates in conjunction with a test cell comprising a vertically mounted thick-walled cylinder blanked off at the bottom, and containing upper and lower water-filled cavities separated by a plate with a small, sharp-edged orifice (thickness to diameter ratio of four). The upper cavity is enclosed at its top surface by a piston which protrudes beyond the upper section of the cylinder. When a weight is dropped onto the piston, a variation of pressure (p1) occurs in the upper cavity, approximating the shape of a half sinusoid with time, and with a typical half cycle (T) in the range 1.34–2.73 ms. Mass transfer takes place from the upper to the lower cavity, with a commensurate increase of pressure (p2) in the latter. At a certain time p1 and p2 become equal, at which the latter is at its maximum and the former is close to the end of its sinusoidal half cycle. Thereafter, p1 rapidly diminishes to ambient pressure and p2 approaches ambient pressure over a period of about 5T. During this latter phase, p2 exceeds p1, which is consistent with flow reversal. A comprehensive analysis of the system, in conjunction with pressure measurement, enabled experimental loss coefficients to be determined during that time when mass transfer was taking place between the upper and lower cavities (with p1&amp;gt;p2). Values of cd obtained were in the range 0.85–1.2, increasing for the smaller values of T. These data are viewed in the context of the expected steady flow value of 0.79 for this configuration of orifice, and confirm the hypothesis that flow unsteadiness of a sufficiently small time scale has a significant effect on loss coefficients.

Direct Determination of Hydration in the Interdigitated and Ripple Phases of Dihexadecylphosphatidylcholine: Hydration of a Hydrophobic Cavity at the Membrane/Water Interface

Hydrophobic cavities at the membrane/water interface are stably expressed in interdigitated membranes. The nonsolvent water associated with 1,2-di- O-hexadecyl- sn-glycero-3-phosphocholine (Hxdc 2Gro PCho) in the interdigitated (L β I) and ripple (P β′ ) states and with its ester analogue 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (Pam 2PtdCho) in the gel (L β′ ) and P β′ states are determined directly. In the L β I state at lower temperatures (4–20°C), 16–18 water molecules per phospholipid are bound, consistent with water-filled cavities and hydrated headgroups. At 28°C, the nonsolvent water decreases to 12, consistent with a reduction of the cavity depth by 0.34 nm due to increased chain interpenetration. This geometric lability may be a common feature of hydrophobic cavities. Only 5.4 waters are bound in the noninterdigitated P β′ (40°C), whereas the ester bound 8.1 waters in its P β′ (37°C), a difference of about one water per ester carbonyl. The relative dehydration of the ether linkage is consistent with it promoting more densely packed structures, which in turn, accounts for its ability to interdigitate.

Directed Mutagenesis Studies of the Metal Binding Site at the Subunit Interface of Escherichia coli Inorganic Pyrophosphatase

Recent crystallographic studies on Escherichia coli inorganic pyrophosphatase (E-PPase) have identified three Mg2+ ions/enzyme hexamer in water-filled cavities formed by Asn24, Ala25, and Asp26 at the trimer-trimer interface (Kankare, J., Salminen, T., Lahti, R., Cooperman, B., Baykov, A. A., and Goldman, A. (1996) Biochemistry 35, 4670-4677). Here we show that D26S and D26N substitutions decrease the stoichiometry of tight Mg2+ binding to E-PPase by approximately 0.5 mol/mol monomer and increase hexamer stability in acidic medium. Mg2+ markedly decelerates the dissociation of enzyme hexamer into trimers at pH 5.0 and accelerates hexamer formation from trimers at pH 7.2 with wild type E-PPase and the N24D variant, in contrast to the D26S and D26N variants, when little or no effect is seen. The catalytic parameters describing the dependences of enzyme activity on substrate and Mg2+ concentrations are of the same magnitude for wild type E-PPase and the three variants. The affinity of the intertrimer site for Mg2+ at pH 7.2 is intermediate between those of two Mg2+ binding sites found in the E-PPase active site. It is concluded that the metal ion binding site found at the trimer-trimer interface of E-PPase is a high affinity site whose occupancy by Mg2+ greatly stabilizes the enzyme hexamer but has little effect on catalysis.

Cluster-Expanded Prussian Blue Analogues

A method for expanding the pores in crystalline frameworks is demonstrated with the substitution of face-capped octahedral [Re6Q8(CN)6]4- (Q = Se, Te) clusters into the Prussian blue structure. The clusters react in aqueous solution with Ga3+ and Fe3+, respectively, to precipitate Ga4[Re6Se8(CN)6]3·xH2O and Fe4[Re6Te8(CN)6]3·xH2O. Rietveld analysis of X-ray powder diffraction data for the former compound confirms the expanded crystal structure, which features large water-filled cavities more than twice the volume of those in Prussian blue. The new materials can be dehydrated without loss of integrity, and maintain crystallinity at temperatures up to 250 or 300 °C. Further, the increase in void volume is shown to significantly enhance their capacities as molecular sieves and enable absorption of larger alcohol molecules such as ethanol and n-propanol. A soluble form of the black iron-containing phase exhibits a cluster-to-metal charge-transfer band at 736 nm, slightly lower energy than the metal-to-metal c...

Bacterial S-Layer Protein Coupling to Lipids: X-Ray Reflectivity and Grazing Incidence Diffraction Studies

The coupling of bacterial surface (S)-layer proteins to lipid membranes is studied in molecular detail for proteins from Bacillus sphaericus CCM2177 and B. coagulans E38–66 recrystallized at dipalmitoylphosphatidylethanolamine (DPPE) monolayers on aqueous buffer. A comparison of the monolayer structure before and after protein recrystallization shows minimal reorganization of the lipid chains. By contrast, the lipid headgroups show major rearrangements. For the B. sphaericus CCM2177 protein underneath DPPE monolayers, x-ray reflectivity data suggest that amino acid side chains intercalate the lipid headgroups at least to the phosphate moieties, and probably further beyond. The number of electrons in the headgroup region increases by more than four per lipid. Analysis of the changes of the deduced electron density profiles in terms of a molecular interpretation shows that the phosphatidylethanolamine headgroups must reorient toward the surface normal to accommodate such changes. In terms of the protein structure (which is as yet unknown in three dimensions), the electron density profile reveals a thickness l z ≈ 90 Å of the recrystallized S-layer and shows water-filled cavities near its center. The protein volume fraction reaches maxima of >60% in two horizontal sections of the S-layer, close to the lipid monolayer and close to the free subphase. In between it drops to ∼20%. Four S-layer protein monomers are located within the unit cell of a square lattice with a spacing of ∼131 Å.

Structure of the outer membrane protein A transmembrane domain.

The outer membrane protein A of Escherichia coli (OmpA) is an intensely studied example in the field of membrane protein folding. We have determined the structure of the OmpA transmembrane domain consisting of residues 1-171, by X-ray diffraction analysis, to a resolution of 2.5 A. It consists of a regular, extended eight-stranded beta-barrel and appears to be constructed like an inverse micelle with large water-filled cavities, but does not form a pore. Surprisingly, the cavities seem to be highly conserved during evolution. The structure corroborates the concept that all outer membrane proteins consist of beta-barrels. The structure constitutes a beta-barrel membrane anchor that appears to be the outer membrane equivalent of the single-chain alpha-helix anchor of the inner membrane.

Stick–slip sliding behaviour at the base of a glacier

Measurement of basal sliding is an important component in studying the mechanical and hydrological coupling between a glacier and its bed. During the 1992 summer field season we used a “drag spool” to measure sliding at the ice/bed interface of Trapridge Glacier, a small surge-type glacier in the St Elias Mountains, Yukon Territory, Canada. Measured diurnal variations in sliding appear to be correlated to subglacial water pressure fluctuations. In contrast to other observations where peak subglacial water pressure and glacier motion appear to coincide, our data imply that maximum sliding rates coincide with rises in water pressure. If the growth of water-filled cavities at the glacier bed is associated with these pressure increases, then our observations may correspond to numerical results by Iken (1981) which indicate that the largest sliding velocity occurs during cavity growth and not when the steady-state size of cavitation is attained. However, our data suggest the idea that a localized stick–slip relaxation process is at work. As the water pressure rises, a local strain build-up in the ice is released, resulting in a momentary increase in sliding rate; once the finite relaxation has occurred, further rises in water pressure do not produce additional enhancement of basal sliding, and the stick–slip cycle begins again by accumulation of elastic strain. We have developed a theoretical model for the sliding motion of ice over a surface having a basal drag that varies temporally in response to changes in subglacial water pressure. Our model results support the proposed stick–slip sliding process at the glacier base, whereby accumulated elastic strain in the ice is released as the rising water pressure decouples the ice from the bed.

Stick–slip sliding behaviour at the base of a glacier

Measurement of basal sliding is an important component in studying the mechanical and hydrological coupling between a glacier and its bed. During the 1992 summer field season we used a “drag spool” to measure sliding at the ice/bed interface of Trapridge Glacier, a small surge-type glacier in the St Elias Mountains, Yukon Territory, Canada. Measured diurnal variations in sliding appear to be correlated to subglacial water pressure fluctuations. In contrast to other observations where peak subglacial water pressure and glacier motion appear to coincide, our data imply that maximum sliding rates coincide with rises in water pressure. If the growth of water-filled cavities at the glacier bed is associated with these pressure increases, then our observations may correspond to numerical results by Iken (1981) which indicate that the largest sliding velocity occurs during cavity growth and not when the steady-state size of cavitation is attained. However, our data suggest the idea that a localized stick–slip relaxation process is at work. As the water pressure rises, a local strain build-up in the ice is released, resulting in a momentary increase in sliding rate; once the finite relaxation has occurred, further rises in water pressure do not produce additional enhancement of basal sliding, and the stick–slip cycle begins again by accumulation of elastic strain. We have developed a theoretical model for the sliding motion of ice over a surface having a basal drag that varies temporally in response to changes in subglacial water pressure. Our model results support the proposed stick–slip sliding process at the glacier base, whereby accumulated elastic strain in the ice is released as the rising water pressure decouples the ice from the bed.

Ray theory applied to the internal caustic identification of liquid cavities acoustically insonified

The acoustic field in water-filled cylindrical cavities machined inside a duraluminum matrix is studied. The internal acoustic beam focused near the cusp points within the liquid cavity. An optical process (pulsed schlieren technique) is used to visualize the acoustic wave front as the wave propagates in the fluid cylinder. The axial positions of the p-ray cusp caustics are compared with the theoretical values calculated by the ray theory [J. A. Lock and E. A. Hovenac, J. Opt. Soc. Am. A 8, 1541–1552 (1991)].

The short pulse method of isolation and identification of resonances: Comparison with a quasiharmonic method and application to axisymmetrical scatterers

Several experimental methods using quasiharmonic insonification lead to a direct verification of the resonance scattering theory. They provide, for elastic cylindrical targets, the resonance frequencies and the mode number of each of them. Pulsed techniques already existing only allowed the resonance frequency determination. Recently, a short pulse method of isolation and identification of resonances (short pulse MIIR) has been initiated. It completes previous works and allows a complete comparison with theoretical and quasiharmonic experimental results. This method consists in digitizing the time signals that characterize the scattering from an elastic target insonified by a short pulse for different angular positions of the receiving transducer. For each acquired signal a resonance spectrum is obtained after a spectral amplitude analysis. From these data, computer processing allows the plotting of identification patterns and by the same way the knowledge of the mode of vibration at a given resonance frequency. Even if this method is not restricted to cylindrical scatterers, this paper presents the identification of resonances by means of the short pulse MIIR of targets as cylinders, shells and water-filled shells and cavities. The accuracy and the possibility for separating overlapping resonances are also discussed.

Detecting voids in a 0.6 m coal seam, 7 m deep, using seismic reflection

Surface collapse over abandoned subsurface coal mines is a problem in many parts of the world. High-resolution P-wave reflection seismology was successfully used to evaluate the risk of an active sinkhole to a main north-south railroad line in an undermined area of southeastern Kansas, USA. Water-filled cavities responsible for sinkholes in this area are in a 0.6 m thick coal seam, 7 m deep. Dominant reflection frequencies in excess of 200 Hz enabled reflections from the coal seam to be discerned from the direct wave, refractions, air wave, and ground roll on unprocessed field files. Repetitive void sequences within competent coal on three seismic profiles are consistent with the “room and pillar” mining technique practiced in this area near the turn of the century. The seismic survey showed that the apparent active sinkhole was not the result of reactivated subsidence but probably erosion.

Volume determinations of the encephalic ventricles with CT and MRI

The volume of the encephalic ventricles was determined from magnetic resonance imaging (MRI) scans. Since there are many conditions in which the encephalic ventricles become enlarged such as Alzheimer's disease and hydrocephalus, accurate measurement of the ventricles provides a valuable and safe means of aiding the diagnosis of such conditions and also provides important follow-up information in affected patients. The objective was pursued in a three phase study. This paper presents the data obtained from the first phase. This first phase demonstrated the possibility of measuring fluid filled spaces by MRI in three phantom preparations (small, medium, and large "ventricles"). The results were compared with those obtained from computerized tomography (CT) scans of the same preparations. These volumetric calculations were done with the aid of a Calcomp 9,000 digital analyzer programmed to compensate for the scale factor and slice thickness of the images. The phantom study showed that the results obtained from the MRI scans were better than those obtained from the CT scans in measuring the volume of water-filled cavities (ventricles) in gelatin phantoms. The average percent difference between volumes obtained by an imaging procedure compared to the actual volume as determined by water displacement was 15.8% for CT scanning and a more impressive 8.3% for MRI.

Enhanced Radar Scattering From Water Inclusions In Ice

Abstract It is believed that water-filled cavities on the scale of decimetres are present in a number of glaciers in Spitsbergen. The evidence for this hypothesis comes from airborne radio echo-sounding records collected by the Scott Polar Research Institute in 1983. The size of these cavities prevents the use of Rayleigh scattering theory and requires a more general analysis. Mie scattering theory is used to investigate the scattered power providing some unexpected results, the most significant of which is that for water bodies greater than approximately 0.25 m in radius the returned power from the volume illuminated is greater than that from a perfectly reflecting plane boundary.

Enhanced Radar Scattering From Water Inclusions In Ice

AbstractIt is believed that water-filled cavities on the scale of decimetres are present in a number of glaciers in Spitsbergen. The evidence for this hypothesis comes from airborne radio echo-sounding records collected by the Scott Polar Research Institute in 1983. The size of these cavities prevents the use of Rayleigh scattering theory and requires a more general analysis. Mie scattering theory is used to investigate the scattered power providing some unexpected results, the most significant of which is that for water bodies greater than approximately 0.25 m in radius the returned power from the volume illuminated is greater than that from a perfectly reflecting plane boundary.

Interpretation of Radio-Echo Returns from Internal Water Bodies in Variegated Glacier, Alaska, U.S.A.

AbstractRadio echo-sounding studies were used to detect water-filled cavities on Variegated Glacier, Alaska, during its surge in 1983. Cavity locations were determined by spatial surveys, and changes in cavity size and water content over a 5 week period were inferred from phase and amplitude changes in the echo wave form. Data were taken at both 4 and 8 MHz central frequencies of the impulse transmitter. These bi-frequency results, together with spectral analysis of the digitized wave forms, were used to provide additional information about the cavity dimensions. Cavity response is interpreted in terms of a simplified model of a thin layer of strong dielectric contrast. Computer simulations of radar pulses encountering such layers reproduce the main features of the data.

Interpretation of Radio-Echo Returns from Internal Water Bodies in Variegated Glacier, Alaska, U.S.A.

AbstractRadio echo-sounding studies were used to detect water-filled cavities on Variegated Glacier, Alaska, during its surge in 1983. Cavity locations were determined by spatial surveys, and changes in cavity size and water content over a 5 week period were inferred from phase and amplitude changes in the echo wave form. Data were taken at both 4 and 8 MHz central frequencies of the impulse transmitter. These bi-frequency results, together with spectral analysis of the digitized wave forms, were used to provide additional information about the cavity dimensions. Cavity response is interpreted in terms of a simplified model of a thin layer of strong dielectric contrast. Computer simulations of radar pulses encountering such layers reproduce the main features of the data.

Hydraulics of Subglacial Cavities

AbstractA theoretical model is developed to describe the steady-state behavior of interconnected, water-filled cavities at the glacier bed. Physically plausible cavities should contain constrictions along the flow path, with flow in the wider sections being relatively sluggish. Mean flow rates in cavities may be at least one order of magnitude less than in channels incised into the basal ice (R channels). Melting due to viscous dissipation - the process that allows R channels to exist - probably plays a minor or negligible role, as compared to glacier sliding, in determining the size of cavities. Furthermore, a system of subglacial cavities should not show a tendency for localization of flow in a few main conduits, as does an R-channel system. If water pressure rises to within several bars of overburden pressure, the rate of cavity closure by creep falls below the rate of cavity opening by sliding and melting, with cavities then becoming unstable. Subsequent evolution of the drainage system should depend upon the total melt-water flux. Circumstances may arise in which cavities and channels act as conduits for melt water; such a configuration would probably show unusual transient behavior.

Hydraulics of Subglacial Cavities

AbstractA theoretical model is developed to describe the steady-state behavior of interconnected, water-filled cavities at the glacier bed. Physically plausible cavities should contain constrictions along the flow path, with flow in the wider sections being relatively sluggish. Mean flow rates in cavities may be at least one order of magnitude less than in channels incised into the basal ice (R channels). Melting due to viscous dissipation - the process that allows R channels to exist - probably plays a minor or negligible role, as compared to glacier sliding, in determining the size of cavities. Furthermore, a system of subglacial cavities should not show a tendency for localization of flow in a few main conduits, as does an R-channel system. If water pressure rises to within several bars of overburden pressure, the rate of cavity closure by creep falls below the rate of cavity opening by sliding and melting, with cavities then becoming unstable. Subsequent evolution of the drainage system should depend upon the total melt-water flux. Circumstances may arise in which cavities and channels act as conduits for melt water; such a configuration would probably show unusual transient behavior.