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71.
Mørkenborg J Pedersen M Jensen FT Stødkilde-Jørgensen H Djurhuus JC Frøkiaer J 《Magnetic resonance imaging》2003,21(6):637-643
Quantitative determination of in-vivo gadolinium diethylenetriamine-pentaacid (Gd-DTPA) concentration is attractive in various studies involving perfusion, tracer kinetics and permeability constants. Using a 1.5 T clinical system and a 7 T small-bore system, we evaluated a method for absolute determination of Gd-DTPA concentrations in plasma solutions. Different solutions of Gd-DTPA and (99m)Tc-DTPA were mixed in human plasma and concentrations in the range of 0-5.0 mmol/l (1.5 T system) or 0-3.0 mmol/l (7 T system) of Gd-DTPA were divided into thirteen tubes. All MRI measurements were carried out using conventional sequences (SE, FLASH and GRASS). The MR measured intensity was converted to Gd-DTPA concentration by mathematical interpretation of the sequences. All MRI sequences showed, that the measured concentrations of Gd-DTPA revealed a slight non-linear difference compared with the calculated Gd-DTPA concentrations determined by the plasma (99m)Tc-DTPA using gamma counting. This non-linearity was most pronounced at high Gd-DTPA concentrations, suggesting that the discrepancy could be a result of an increased plasma relaxivity at higher concentrations. Adjustment of measured Gd-DTPA concentration was therefore performed using a selected power function, A[Gd-DTPA](a), which yielded the best linear relationship. Regression analysis showed that the scaling constant (A) varied from 0.11 to 97.45 and the power constant (a) varied from 0.83 to 1.6. Based on these constants, the MRI measured concentrations of Gd-DTPA did not differ from the calculated concentrations of Gd-DTPA obtained from reference measurements of (99m)Tc-DTPA. In the 1.5 T system, a linear relationship (r(2) > or = 0.95) was demonstrated in the range of 0-5.0 mmol/l Gd-DTPA, and in the 7 T system, a linear relationship (r(2) > or = 0.92) was demonstrated in the range of 0-3.0 mmol/l Gd-DTPA. Additionally, the effect of signal-to-noise on measured concentrations of Gd-DTPA was simulated using MR data of the mixed solutions of Gd-DTPA in plasma and the analytical expression of the pulse sequences. The simulations showed that the concentrations were most sensitive to noise in the GRASS sequence. In conclusion, this study demonstrates a novel approach to quantify accurately the Gd-DTPA concentration directly from MRI signal data using different routine sequences. 相似文献
72.
Analogues of MKC-442 capable of undergoing Michael addition reactions were synthesised in order to investigate the activity against the HIV-1 mutant (Y181C). An improved activity was postulated on the basis of a possible covalent binding to the mercapto group of Cys181. Lithiation of the C-6 position of 1-ethoxymethyl-5-ethyl-1H-pyrimidine-2,4-dione (5) was followed by reaction with alpha,beta-unsaturated aldehydes and oxidation of the alcohols formed to give the alkenoyl analogues 1a-3a. Analogues 1b-3b containing an allyloxymethyl group in the N-1 position instead of the ethoxymethyl group could not be synthesised due to isomerisation of the allylic group during the metallation reaction. The NMR data for compounds 1a-3a showed a hindered rotation, which was more pronounced for the 6-cyclohexenylcarbonyl derivative 3a than for the propenyl derivatives 1a and 2a. Moderate activity against wild type HIV-1 was observed for the alcohol 8 and the ketones 2a-3a. However, no activity was observed against the Y181C mutant. 相似文献
73.
A novel method for assessing structural diversity is presented. Maximum common subgraph identity is used as the measure of similarity between two chemical structures. A conditional probability treatment of similarity distributions for libraries of chemical structures is used to define diversity. This evaluation method together with the evaluation of traditional physicochemical properties is used to assess a large number of chemical libraries and to understand structural differences between these. 相似文献
74.
Asger Hobolth Jan Pedersen Eva B. Vedel Jensen 《Annals of the Institute of Statistical Mathematics》2003,55(2):227-242
In this paper we propose a flexible continuous parametric shape model for star-shaped planar objects. The model is based on
a polar Fourier expansion of the normalized radius-vector function. The expected phase amplitudes are modelled by a simple
regression with parameters having nice geometric interpretations. The suggestedgeneralized p-order model is an extension of first- and second-order Gaussian shape models, and in particular the Gaussian assumption is relaxed. The
statistical analysis is straightforward, as demonstrated by an application concerning shape discrimination of two cell nuclei
populations. 相似文献
75.
Hansen K Andersen JU Hvelplund P Møller SP Pedersen UV Petrunin VV 《Physical review letters》2001,87(12):123401
The exponential law is valid both for decay from a single quantum state into a continuum and for an ensemble maintained in thermal equilibrium. For statistical decay of an ensemble of isolated systems with a broad energy distribution, the exponential decay is replaced by a 1/t distribution. We present confirmation of this decay law by experiments with cluster anions in a small electrostatic storage ring. Deviations from the 1/t law for such an ensemble give important information on the dynamics of the systems. As examples, we present measurements revealing strong radiative cooling of anions of both metal clusters and fullerenes. 相似文献
76.
An experimental implementation of a global sound equalization method in a rectangular room using active control is described in this paper. The main purpose of the work has been to provide experimental evidence that sound can be equalized in a continuous three-dimensional region, the listening zone, which occupies a considerable part of the complete volume of the room. The equalization method, based on the simulation of a progressive plane wave, was implemented in a room with inner dimensions of 2.70 m × 2.74 m × 2.40 m. With this method, the sound was reproduced by a matrix of 4 × 5 loudspeakers in one of the walls. After traveling through the room, the sound wave was absorbed on the opposite wall, which had a similar arrangement of loudspeakers, by means of active control. A set of 40 digital FIR filters was used to modify the original input signal before it was fed to the loudspeakers, one filter for each transducer. The optimal arrangement of the loudspeakers and the maximum frequency that can be equalized is analyzed theoretically in this paper. The presented experimental results show that sound equalization was possible from 10 Hz to approximately 425 Hz in the listening zone. A flat frequency response with deviations within ±5 decibels from the desired value was achieved. A higher demanding performance with deviations within ±1.5 decibels from a flat frequency response was attained in the interval between 20 Hz and 280 Hz. At the same time, the impulse response was quite well approximated to a delayed delta function in the listening zone. Examples of the spatial distribution of the sound field are also shown. 相似文献
77.
In this paper, a novel inertia-capacitance (IC) beam substructure formulation based on the IC-field presentation from the bond graph method is developed. The IC beam provides a modular, systematic and graphical approach to beam modeling. These features allow the modeler to focus more on the modeling and less on the mathematics. As such, the IC beam is proposed as an alternative to the many existing types of beam models available in the literature. The IC beam is formulated in the center of mass body fixed coordinate system allowing for easy interfacing in a multibody system setting. This floating frame approach is also computationally cheap. Elastic deformations in the IC beam are assumed to be small and described by modal superposition. The formulation couples rigid body and elastic deformations in a nonlinear fashion. The formulation is also compact and efficient. Detailed derivations for a two-dimensional planar IC beam with bending modes are presented. A modal acceleration method based on the decoupling of bending modes is proposed for use in the IC beam. The rotating beam spin-up maneuver problem is solved. The Karnopp-Margolis method is applied to ensure complete integral causality for an efficient numerical system. Geometric substructuring technique is applied to model large deflections. The IC beam is shown to be capable of solving the rotating beam problem accurately and efficiently. 相似文献
78.
T.P. Leervad Pedersen J. Skov Jensen J. Chevallier O. Hansen J.M. Jensen B. Bech Nielsen A. Nylandsted Larsen 《Applied Physics A: Materials Science & Processing》2005,81(8):1591-1593
The synthesis of evenly distributed Ge nanoclusters in plasma-enhanced chemical-vapour-deposited (PE-CVD) SiO2 thin films containing 8 at. % Ge is reported. This is of importance for the application of nanoclusters in semiconductor
technology. The average diameter of the Ge nanoclusters can be controlled in the range of 3–6 nm by variation of the annealing
parameters. Using a combination of transmission electron microscopy and Raman-scattering spectroscopy, the nanoclusters were
shown to be crystalline. However, photoluminescence measurements showed no light emission that could be definitively correlated
to the presence of the nanoclusters.
PACS 61.82.Rx; 78.67.Bf; 81.07.Bc 相似文献
79.
An experimental technique is presented for measurement of contact stress distribution using Fuji Pressensor film. The development
of packets of Pressensor disks for use in small areas or surfaces of complex curvature is explained. A digital-image-scanning
procedure has been developed to substantially increase the spatial resolution with which stress distributions can be constructed
from stained Fuji Pressensor film. The technique is developed for small, discrete disks as well as for larger, continuous
sheets of Pressensor. As an illustration, sealed packets of small Pressensor disks are used to measure the stress distribution
across a cadaveric juvenile femoral head under load. 相似文献
80.
Pedersen CM Figueroa-Perez I Boruwa J Lindner B Ulmer AJ Zähringer U Schmidt RR 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(42):12627-12641
Streptococcus pneumoniae LTA is a highly complex glycophospholipid that consists of nine carbohydrate residues: three glucose, two galactosamine and two 2‐acetamino‐4‐amino‐2,4,6‐trideoxygalactose (AATDgal) residues that are each differently linked, one ribitol and one diacylated glycerol (DAG) residue. Suitable building blocks for the glucose and the AATDgal residues were designed and their synthesis is described in this paper. These building blocks permitted the successful synthesis of the core structure Glcβ(1‐3)AATDgalβ(1‐3)Glcα(1‐O)DAG in a suitably protected form for further chain extension ( 1 b , 1 c ) and as unprotected glycolipid ( 1 a ) that was employed in biological studies. These studies revealed that 1 a as well as 1 lead to interleukin‐8 release, however not via TLR2 or TLR4 as receptor. 相似文献