Drift Phenomena Research Articles

Non-invasive MR thermometry by 2D spectroscopic imaging of the Pr[MOE-DO3A] complex

Future progress in regional hyperthermia requires a practical method for non-invasive thermometry. In magnetic resonance tomography, spin density, T1 relaxation time, diffusion coefficient and proton resonance frequency are candidates to measure temperature distributions. When used clinically in the pelvic region, all these methods are compromized by artifacts arising from different tissues, tissue alterations under hyperthermia, physiological and random movements, inhomogeneities, drift phenomena, and field instabilities. In this study a paramagnetic complex was evaluated, Pr[MOE-DO3A], with praseodymium as central atom, similar to common gadolinium containing MRI contrast media. The temperature dependence of its methoxy side group ∼ -24ppm downfleld from the water resonance at 25°C was employed to determine 2-D temperature distributions in a cylindrical agar phantom containing 9.5 him of Pr[MOE-DO3A]. The phantom was heated externally through a water jacket creating a stationary temperature distribution throughout the phantom. At first, the correlation between temperature and the chemical shift of the methyl group of the lanthanide complex Pr[MOE-DO3A] was determined. Calibration curves obtained exhibited a linear relationship of 0.12± 0.01 ppm/°C, nearly independent from the surrounding medium. Local temperature distributions were determined employing the spatially resolved method of spectroscopic imaging (SI). 2-D spectroscopic images for three orthogonal slices were obtained by narrow-band excitation and 16 phase encoding steps in two dimensions. The FOV was 180 mm and the slice thickness in all cases was 20 mm for maximal spatial temperature resolution (11.2 × 11.2mm2). The results indicate a measurement time of about 5 s per acquisition under the following conditions: An estimated concentration of 1 mmol/1, a reduced matrix size of 8 × 8, and a reduced repetition time of 3 × T1 (TR ∼ 85 ms). Those SI measurements produced a SNR of ∼ 4 per acquisition, a measurements duration of 10-2. s, equivalent to two to four acquisitions per spectrum, seem sufficient for online temperature monitoring during hyperthermia. The in vitro data suggest the spectroscopic temperature measurement utilizing a temperature-sensitive Pr[MOE-DO3A] complex with a therapeutically realistic concentration of 1 mmol/1 to be suitable for clinical use. Compared to the methods tested so far (ρ, T1, diffusion, proton resonance), the method presented has the unique advantage of being less susceptible to artifacts. The competing methods of non-invasive thermometry employing magnetic resonance imaging are currently being investigated using the same experimental setup.

Separation of polyester oligomers by gradient high-performance liquid chromatography

A highly efficient method for the separation of commercially available polyesters composed of a multitude of individual oligomers by gradient reversed-phase high-performance liquid chromatography (gRP-HPLC) was developed. Oligomers up to a molecular mass of more than 10 000 could be sufficiently resolved on an octadecylsilyl silica (C 18) stationary phase using a ternary gradient consisting of acetonitrile, tetrahydrofuran (THF) and aqueous acetic acid. Detection was performed by measurement of signal responses from either UV detection at 230 nm or evaporative light scattering detection (ELSD). Addition of THF to the mobile phase is essential and omission of the co-solvent provided elution of only a few oligomers, the residual amount being trapped on the highly hydrophobic stationary phase. Due to the substantial UV mismatch invoked by THF exhibiting marked absorbance at the chosen wavelength, ELSD, which is not associated with baseline drift phenomena, is much more suited for identification of individual polyester samples on the basis of the chromatographic fingerprint. In one case about sixty sufficiently resolved peaks each attributable to a single oligomer could be observed. A concentration of about 10 mg/ml of sample was required for an unequivocal distinction of the individual polyesters. The use of a C 18 stationary phase is an ultimate prerequisite for efficient oligomer resolution as impressively evidenced by an almost complete lack of separation into individual oligomers on C 8 or C Phenyl matrices. In the latter cases only few low-molecular-mass oligomers are preceding the broad and unresolved bulk peak envelope of sample constituents.

Thermometry by measuring the chemical shift of lanthanide complex

In the long-term, non-invasive thermometry is vital for the continued clinical and technological development of regional hyperthermia. In magnetic resonance tomography. T1 relaxation time, diffusion and proton resonance frequency are used to measure temperature distributions. When used clinically in the pelvic region, all of these methods are plagued with errors and artefacts on account of the tissue relationships, tissue changes under hyperthermia, physiological and stochastic movements, inhomogeneities, drift phenomena and instabilities. We tested the relationship between the temperature and the chemical shift of a methyl group of a lanthanide complex with central atom praseodymium (Pr-MOE-DO3A. Schering AG). To do this we used cylindrical phantoms containing a 5-mmol-solution of this temperature-sensitive substance. High resolution spectra and relaxation times were determined in a Bruker AMX at 11.5 T. A calibration curve was then recorded by a Siemens Magnetom SP63 at 1.5 T. Local temperature distributions were determined using the chemical shift imaging method, with a matrix size of 16 x 8 and a narrow-band excitation pulse. The temperature distribution was created using a Nd:YAG laser applicator. At a distance of -25.7 ppm from the water line, we found a singlet line with a temperature-dependent chemical shift of 0.13 ppm/C. In the phantom experiment we found that the chemical shift had a linear relationship with a gradient independent of the surroundings, and a temperature resolution of +/-0.6 degree C. With a concentration of 1 mmol/l, a matrix size of 8 x 8 and a measurement period of 5 s per acquisition, phantom measurements using the CSI method produced a signal to noise ratio of 3.5 per acquisition, i.e a measurement period of 10 to 20 s per spectrum. Our in vitro data show that spectroscopic temperature measurement using a temperature-sensitive praseodymium complex with a therapeutically practical concentration of 1 mmol/l already appears to be suitable for clinical use Compared with the methods tested so far (T1, diffusion, proton resonance), this method has the special advantage of not being very susceptible to artefacts. The competing methods of non-invasive thermometry using magnetic resonance tomography/spectroscopy will be investigated next.

Electrical evaluation of InP surface damage by reactive ion etching using mixture of ethane (C 2H 6) and hydrogen (H 2)

The electrical properties of the (100) InP surface were studied based on the fabrication process of a buried heterostructure (BH) laser diode (LD). The current-voltage ( I- V) and capacitance-voltage ( C- V) characteristics were measured for Schottky barriers on the surface at each step of the process. The characteristic variation during the process showed the C 2H 6/H 2-reactive ion etching (RIE) induced damage on the near-surface region: a layer resulting from phosphorus desorption, a low p-concentration layer due to hydrogen passivation, and deep donor defects with electrical drift phenomena. It was found that the damage layers were removed by annealing after the chemical etching which produced almost the same surface as that immediately before the BH regrowth.

Gene diversity analysis in natural populations and cultured stocks of turbot (Scophthalmus maximus L.)

The genetic variability of eight fish‐farm and three natural populations of turbot was studied by electrophoretic analysis of 35 enzymatic loci. The results showed low genetic variability in natural populations of turbot (HT = 0·029 ± 0·013) in comparison with other flatfish species. Great genetic similarity was revealed among the natural populations studied, which indicates high rates of gene flow in this species. The hatchery stocks showed less genetic variation than the wild populations analysed, which suggests genetic drift phenomena involved in the foundation and management of broodstocks. In addition, the heterozygosity differences detected among the hatchery stocks analysed are correlated with inverted levels of fluctuating asymmetry, which supports the existence of inbreeding depression phenomena in turbot culture.

Instabilities in the magnetosphere of millisecond pulsars

A model explaining some observational properties of millisecond pulsars is presented in the framework of the general plasma model for pulsar emission developed by Al. Kazbegi, G. Machabeli, and G. Melikidze in a series of papers. In the region of the open magnetic field lines of the pulsar magnetosphere the existence of a relativistic electron-positron plasma penetrated by a primary particle beam is assumed. Wave excitation due to different plasma instabilities is considered. The main properties of pulsar radiation (e.g., existence of “core” and “cone” types of emission, circular polarization, subpulse drift phenomena, pulse nullings, and mode changing) are explained in the most natural way by this model. The observations show that despite the big difference between the physical parameters of the “typical” and millisecond pulsars their observed properties (pulse profiles, polarization, existence of two different modes in emission, frequency evolution, etc.) are more or less similiar and the physical mechanisms of their radiation do not differ fundamentally from each other. Some particular millisecond pulsars are considered based on the model presented, and predictions are made.

Progress and problems in reliability of Ti:LiNbO<inline-formula><sub><roman>3</roman></sub></inline-formula> optical intensity modulators

The LiNbO3 optical intensity modulator is a promising key device in global optical communication systems which demand high reliability. Although performance of the modulator has been considerably improved to meet practical use, there remain problems related to reliability issues such as unpredictable fluctuation of drift phenomena. In this regard, our recent experimental data on the fabrication of Ti:LiNbO3 modulators are presented here to clarify problems in the device characteristics caused by the device materials. At present, there is little understanding of the origins and mechanisms of the fluctuations, and the estimation and assurance of device quality throughout the long term are difficult. In order to obtain as great a reliability as possible, screening tests for all produced devices are needed.

Dynamical behavior of optical vortices

The dynamical behavior of optical vortices during propagation of an electromagnetic wave is investigated. The propagation of these waves is computed with rigorous scalar diffraction theory. It was observed that a single optical vortex propagates perpendicularly to the wave front. Two optical vortices with the same charges (isopolar vortex pairs) are observed to gyrate around each other during propagation. Two optical vortices with opposite charges (bipolar vortex pairs) are observed to drift laterally away from the general direction of propagation. In cases in which interference between waves is allowed to occur or in which wave fronts are converging to focal points, bipolar vortex pairs are sporadically created and annihilated. An elementary model for the propagation of vortex pairs is proposed to explain the gyration and drift phenomena.

The kinetics of formation of gas-sensitive RGTO-SnO2 films

Recently the rheotaxial growth and thermal oxidation (RGTO) technique has been proposed as a new method for preparing gas-sensitive SnO2 films with a high surface-to-volume ratio. Such films are produced in two stages: in the first step small metallic Sn droplets are formed by evaporating or sputtering Sn onto substrates heated beyond the Sn melting point (Ts >Tm = 232 °C) and, in a second step, these spheres are oxidized by thermal annealing in air (Tox > 500 °C). In this situation, the oxidation kinetics is limited by the diffusion of oxygen through the growing surface SnO2 layer towards the metallic grain interior in incompletely oxidized RGTO-SnO2 films. In the present work, the oxidation kinetics of the Sn spheres has been studied using optical absorption measurements. From our measurements we obtain for the temperature dependence of the oxygen diffusion coefficient in tin oxide D(T) = 1.4 × 10−4 cm2 s−1 exp(−1.67 eV(kT)−1). The consequences of this result are discussed with regard to the preparation of optimized RGTO-SnO2 films and to sensor drift phenomena.

Genetic Diversity in a Threatened Wetland Species, Helonias bullata (Liliaceae)

Genetic variation was examined in Helonias bullata, a threatened perennial plant species that occurs in isolated wetland habitats. Fifteen populations representing the species’ geographic range were sampled. Genetic diversity was low for the species (Hes = 0.053) as well as within populations (Hep = 0.029). Of the 33 allozyme loci examined, 11 (33%) were polymorphic, while on average only 12.8% (4) of the loci were polymorphic within populations. The number of alleles per polymorphic locus was 2.36 for the species and averaged 2.09 across populations. For every genetic parameter calculated, variation in H. bullata was lower than that typically found for narrowly distributed plant species. The lowest levels of genetic diversity were found in northern areas that were colonized following the last glacial epoch. The number of genotypes detected per population ranged from three to 21, with a mean of 13 for this clonally reproducing species. We found a relatively high proportion of total genetic diversity (30.6%) among populations and a significant correlation (p < 0.002) between genetic distance and geographic distance. Genetic drift phenomena appear to play a major role in the population genetics of this species. Anomalously, several populations that appeared most limited in size and vigor were genetically most variable, perhaps because they represent older, relictual populations. Life‐history characteristics of H. bullata coupled with low levels of genetic diversity and the degradation and disappearance of wetlands threaten the existence of this species.

The formation of chemical peculiarities in stellar atmospheres

Abstract Generalized equations of dynamics for plasma components describing the formation of chemical peculiarities in stellar atmospheres due to gravitational, electrostatic, magnetostatic and radiation fields acting on colliding plasma particles have been derived. The equations describing both diffusion and drift phenomena can be treated intercoupled equations of the Fokker–Planck type. It has been shown that in stellar atmospheres there will be generated the electrostatic field which is important for the separation of light elements and their isotopes. A highly anisotropic diffusion in upper atmospheric layers of magnetic stars has been demonstrated. For the interaction of the plasma components with the radiation field, besides a usual upward directed radiation flux generated acceleration, a more complicated light-induced drift due to the selective resonance-like absorption of radiation by thermally moving plasma particles with definite momentum values is shown to be present. The light induced drift ca...

Impurity Evaluations of SiO2 Films Formed on LiNbO3 Substrates

SiO2 films deposited by sputtering (SP) and vacuum evaporation (VE) techniques are commonly used as buffer layers in LiNbO3-based devices. The impurity levels in such films are measured as 1017 to 1022 atoms/cm3 for H, Li and Nb ions, which might affect the electrical properties of the devices. The major differences between the SP and VE films are in the density and the hydrogen contents, which possibly influence the I/V characteristics of the films and leads to different dc drift phenomena of LiNbO3 optical modulators made of such layers.

Noninvasive prediction of SAR distributions with an electro-optical E field sensor.

An integrated electro-optical (eo) E field sensor is developed on the basis of a Ti:LiNbO3 Mach-Zehnder interferometer. A measuring device based on the lock-in principle is introduced to register the E field in phase and amplitude using this E field probe. Segmented electrodes are used to minimize influences from the dielectric surroundings on the base point capacitance of the receiving dipole. The operating point is stabilized against drift phenomena resulting from optical damage and pyroelectric effect. Sensitivity, dynamic range, harmonic distortions and mechanical properties of a prototype of this electro-optical E field sensor are evaluated. A phantom setup in the SIGMA-60 applicator was developed to test this electro-optical sensor for hyperthermia applications. Power deposition patterns of various standard adjustments of the SIGMA ring are visualized in an elliptical lamp phantom. Simultaneously, E field in phase and amplitude is determined on a closed curve in 10 degrees steps around the phantom in a substitute bolus. The numbers are stored and utilized as boundary conditions in a two-dimensional finite elements code which calculates the SAR distribution on an appropriate triangular grid inside the closed curve. An excellent qualitative agreement is obtained between visualized and calculated SAR patterns. This novel measurement method is therefore suitable for noninvasive monitoring of SAR patterns during clinical application of regional radiofrequency hyperthermia.

Inverted n-bar model in descriptior and in state space form

The planar inverted n-bar model and its multibody formulation are presented. The descriptor formulation, which is based on a set of redundant coordinates, results in a differential-algebraic (DAE) system of index 3. A minimum set of coordinates characterizes the state space formulation, which corresponds to an ordinary differential equation (ODE) system. The regular structure of the descriptor form allows a complete formulation of the equations of motion. On this base, by induction arguments the state space form can be derived analytically. We present these equations, since the inverted n-bar model serves as an instructive example for the drift phenomena in numerical simulation, for the design of a controller and for the balancing problem in the context of neural networks.

A high-speed wide dynamic range time-to-digital converter

We describe a time-to-digital converter able to measure intervals as great as 100 ns with a resolution of 4 ps rms. It achieves this large dynamic range by simultaneously sampling four sinusoidal wave forms (sine and cosine waves at 200 and 6.25 MHz) derived from a single quartz oscillator. Twelve-bit analog-to-digital conversion of the 200 MHz waves yields the high time resolution. Eight-bit conversion of the 6.25 MHz samples removes the cycle ambiguity of the 200 MHz data. The digital words are pipelined in a fully parallel data flow architecture. A first-in first-out stage in the pipeline derandomizes the random event arrival times. A subsequent stage in the pipeline uses an arctangent function to convert the sine and cosine pairs into linearized measures of event time. These are subtracted to yield start–stop time interval sizes for individual photoevents. The minimum start–stop interval is 50 ns, set primarily by the cycle time. Because the same processing is employed for the start and stop events, a large class of potential error and drift phenomena are eliminated. The digitizer provides an accurate way to decode the outputs of delay line detectors, offering high event throughput and extremely good long-term timing accuracy. As a side benefit, the pipeline data flow architecture permits simple breadboarding and low-throughput testing of the system stages with the arctangent work implemented in a personal computer. This arrangement is also very convenient for logging diagnostic evaluation data. The same front-end and data flow architecture is directly applicable to very high-speed applications where the event processing is implemented in a digital signal processor.

Analysis and calibration of in situ scanning tunnelling microscopy images with atomic resolution influenced by surface drift phenomena

The influence of surface drift velocities on in situ scanning tunnelling microscopy (STM) experiment with atomic resolution is analysed experimentally and mathematically. Constant drift velocities much smaller than the speed of scanning can in many in situ STM experiments with atomic resolution result in an surface reconstraction. It is shown that a surface atomic structure can be distorted and observed as another atomic structure entirely owing to a velocity in the plane of the surface. The image can be resolved mathematically and the components of the drift velocity as well as the vectors of the non-distorted surface lattice can be determined. The calibration of distances can thus be carried out also when the image is influenced by drift. Results with gold surfaces and graphite surfaces are analysed and discussed.

Adaptive second-order Volterra filtering and its application to second-order drift phenomena

This paper presents an adaptive second-order Volterra filter and its application to model-test data of a prototype tension leg platform (TLP). The least-squares approach of a second-order Volterra model and its adaptive filtering algorithm based on recursive least-squares are introduced. The second-order Volterra filter is applied to identify the linear and quadratically nonlinear relationship between irregular sea wave excitation and the surge response of a tension leg platform. Next, a deconvolution technique, based on the impulse invariance standard Z-transform, is utilized to recover the linear and quadratic forces exerted on the TLP. >

Charging-induced drift and diffusion of aerosol particles in oscillating electric fields

We investigate the recently predicted bipolar charging-induced drift and diffusion phenomena of aerosol particles in rectangular oscillating electric fields. A model system consisting of surface ionization sources is considered. A detailed analysis is made in the continuum regime, but the results are approximately extended to non-continuum regimes. It is found that the induced drift decreases to zero as the oscillation frequency (Ω) is increased to a characteristic value, Ω c = π(θβ/6) 1 2 , where θ and β are the relaxation constants for the ion concentration field and the particle charge. For Ω > Ω c , a small negative drift persists which vanishes for all Ω > 2πθ. The induced diffusivity, D c (Ω) , also decreases monotonically with Ω, attaining much smaller values at Ω c as compared to static field diffusivities. The reduced values of D c can still be rendered orders of magnitude larger than the Brownian diffusivities of particles. The paper discusses the design and operational aspects of the model system, its ideal mass transfer characteristics and a novel application. The phenomena may be more easily observed for small particles (radius < 0.05 μm), at high ion concentrations (> 10 6 cm −3), at moderate fields (0.5–2 kV cm −1) and at low frequencies (0–500 Hz).

Analysis of Optically Induced Drift Effects in Tapered Optical Couplers

A numerical study of the effect of high power levels, at wavelength of 0.85 μm, on the characteristics of LiNbO 3 tapered optical couplers is reported. Some optically induced drift phenomena of the power exchange between the tapered couplers or nonparallel waveguides are revealed and discussed. In addition, we also evaluate the dependence of the photorefractive sensitivity on the slowly tapered function. Based on novel numerical results, the appropriate tapered structure can be used to improve the photorefractive degradation. A further reduction of the undesired cross-talk induced by the optical damage may be achieved by optimizing the shapes of the tapered couplers

A Model for the Drifting-Subpulse Phenomenon

The subpulse drift phenomena—that is, periodic subpulse phase variations in the individual-pulse sequences—was discovered by Drake and Craft (1968) in the beginning of the pulsar era. Many observational programs have been carried out since then, and it has been shown a) that only a minority of pulsars exhibit the phenomenon, and b) that only some of these can be characterized by a definite drift rate (Huguenin, Taylor, and Troland 1970, Backer 1970, Taylor and Huguenin 1971).