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Results for “parameter optimization”

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At least 109 records · Page 6Linked to original sources

PubMed1990

Fatigue life improvement of nitrogen-ion-implanted pedicle screws.

An experimental study of the fatigue life of pedicle screws and their nitrogen-ion-implanted counterparts was conducted. Nitrogen-ion implantation parameters were varied in the first part of the study to obtain the best implantation characteristics. Using this optimal parameter for N(+)-implantation, the performance of two matched groups were compared at two bending moments near the "knee" of the S-N curves. At 3.96 N-m and 5.09 N-m of applied bending moments, the increase in the mean fatigue life for the former was 98% and for the latter 20%. The 98% prolongation was statistically significant at P less than 0.005; however, the 20% increase was statistically insignificant (P less than 0.1). The implantation depth is about 0.1 mu in the near surface of the screw. This appears to be thick enough to inhibit crack initiation at 3.96 N-m, but only marginally at 5.09 N-m. Increasing the implantation depth has the potential of increasing the fatigue life at the higher bending moments.

Bone Screws↗

PubMed1992

An optimal reference plane to detect glaucomatous nerve fiber layer abnormalities with computerized image analysis.

Nerve fiber layer height measured with respect to a standardized retinal reference plane is diminished by glaucoma. The definition of the reference plane influences the nerve fiber layer measurements. We empirically determined the best reference plane for measurement of nerve fiber layer height. Optimal parameters for measurement reproducibility were determined for a group of 6 normal and 6 glaucomatous eyes each imaged nine times. Optimal ability to distinguish normal from glaucomatous eyes was determined for a group of 33 normal eyes and 36 glaucomatous eyes each imaged once. Measurements with the smallest variability (root mean square error = 32 microns) and the highest sensitivity (83%) and specificity (88%) were achieved when the reference plane is defined by portions of the image from a peripheral temporal area 32 degrees wide, and for two peripheral nasal areas of 55 degrees width centered 30 degrees above and below horizontal. These parameters for the definition of the reference plane should provide measurements of nerve fiber layer height with the least variability and the greatest ability to discriminate between eyes with early glaucomatous damage and normal eyes.

Glaucoma↗

PubMed1991

Quantitative structure-activity relationships of antitumor guanidinothiazolecarboxamides with survival enhancement for therapy in the 3LL Lewis lung carcinoma model.

Guanidinothiazolecarboxamides (GTCs) are a novel class of antitumor agents found to be systemically active against experimental pulmonary metastases of 3LL Lewis lung carcinoma. A series of substituted benzothiazole GTCs were found to produce enhancement of survival in this model by using 8 days of intraperitoneal dosing initiated 2 days after intravenous tumor challenge. Quantitative structure-activity relationships have been discovered in the GTC series with survival enhancement correlated to substituent parameters. Optimal correlations were found between the probit transform of the drug-induced increased lifespan (ILS) and field and pi parameters. Among the most effective analogues in this series was N-(5-fluorobenzothiazol-2-yl)-2-guanidinothiazole-4-carboxam ide.

Adenocarcinoma↗

PubMed1978

The mechanism of reaction of fully reduced membrane-bound cytochrome oxidase with oxygen at 176K.

1. The results of non-linear optimization studies on the mechanism of reaction of fully reduced cytochrome oxidase with O2 at 176K are presented. The analysis is carried out on data obtained by means of dual-wavelength multi-channel spectroscopy at three wavelength pairs (604-630, 608-630 and 830-940 nm) and at three O2 concentrations (60, 200 and 1180 micron). The only model that satisfies the triple requirement of a standard deviation within the standard error of the experimental data, good determination of the optimized parameters and a random distribution of residuals is a three-species sequential mechanism. 2. On the basis of the optimized values of the relative absorption coefficients of the intermediates at each wavelength obtained from the present paper together with data from low-temperature trapping, e.p.r. and magnetic-susceptibility studies, the possible valence states of the metal centres in each of the intermediates are discussed.

Chemical Phenomena↗

PubMed1990

Model-based optimization of infectivity parameters: a study of the early epidemic in San Francisco.

By combining dynamic modeling of human immunodeficiency virus (HIV) transmission with mathematical optimization techniques, we calculate values of epidemiological parameters characterizing the early epidemic (1978-1986) among homosexual and bisexual men in San Francisco. The seroconversion fraction data reported by the San Francisco hepatitis B vaccine trials cohort study for this period is accurately simulated by a model assuming varying infectivity among three stages of HIV infection (early antigen stage, HIV antibody-positive stage, and AIDS stage). Using optimization techniques, we generate curves of the annual number of new partners and the annual number of risk contacts as functions of time. We project future case rates using optimized parameter values, and we study the sensitivity of these projections to variations in parameters, including the population size and the incubation period.

Bisexuality↗

PubMed1989

Processing of myoelectric signals for estimating the location of innervation zones in the skeletal muscles.

An electrical activity associated with the excitement of muscle fibers arises from the myoneural junctions located at the middle length of the fibers and propagates to the both ends. This propagation can be detected with a multi-electrode array placed on the skin surface. Therefore, the position of the myoneural junctions, or of the innervation zones as a collection of myoneural junctions, can be estimated from the source of the propagation. The authors propose a method for determining the position of the propagation sources automatically with a computer. The method was based on the line-source model of muscle fibers. The positions of the propagation sources were treated as parameters in the model. When some values are assigned to the parameters, the surface potentials are calculated from the currents assumed to propagate on the muscle fibers. The computer searched for the optimal parameters which gave the best fit between the calculated and the measured potentials on the skin surface. Using this method some separate sources of propagation were detected at the middle length of the biceps brachii. The activity of the individual innervation zones was quantitatively described by the average amplitude of the potentials arising from the corresponding sources. This description clarified the changes in the activity of innervation zones when the contraction forces were varied.

Action Potentials↗

PubMed1988

[Transcranial electroanalgesia in rats: an optimal regimen of electrical stimuli].

According to earlier experimental results in rabbits the maximal analgesic effect in rats as determined by vocalization test was elicited by transcranial application of combination of direct current and rectangular pulses (70 Hz, 3.0-3.5 ms) with optimal ratio of amplitudes of direct and mean pulse currents being about 2:1. In anesthetized rats during the transcranial application of the current with parameters optimal for analgesia the most prominent inhibition of the nociceptive blood pressure reflexes were also shown. The similar results obtained in different species of animals confirmed our earlier conclusion that the most pronounced transcranial analgesia may be elicited by the narrow band current parameters only. Directions of further investigation on the electroanalgesia mechanisms using transcranial current application in rats as an experimental model are discussed.

Animals↗

PubMed1989

Optimization of electroporation for transfection of mammalian cell lines.

Electroporation can be a highly efficient method for introducing DNA molecules into cultured cells for transient expression of genes or for permanent genetic modification. However, effective transformation by electroporation requires careful optimization of electric field strength and pulse characteristics. We have used the transient expression of the firefly luciferase gene as a rapid and sensitive indicator of gene expression to describe the effects on transfection efficiency of altering electroporation field strength and shape. Using the luciferase assay, we investigated the correlation of cell viability with optimal transfection efficiency and determined the optimal parameters for a number of phenotypically distinct mammalian cell lines derived from the nervous and immune systems. The efficiency of electroporation under optimal conditions was compared with that obtained using DEAE-dextran or calcium phosphate-mediated transformation. Transfection by electroporation using square wave pulses, as opposed to exponentially decaying pulses, was found to be significantly increased by repetitive pulses. These methods improve the ability to obtain high efficiency gene transfer into many mammalian cell types.

Animals↗

PubMedDate not supplied

Maximization of contrast-to-noise ratio to distinguish diffusion and microcirculatory flow.

Optimization of the contrast-to-noise ratio (CNR) is described for microcirculation magnetic resonance (MR) imaging techniques based on flow-compensated/flow-dephased sequences, both with and without even-echo rephasing. The authors present the most advantageous manner of applying flow-dephased gradients, such that dephasing is maximal while diffusion losses are minimal. The theoretical considerations include phase, diffusion, echo time, and bandwidth in the determination of the optimal parameters for microcirculation imaging. Studies in phantoms consisting of stationary and flowing copper sulfate in Sephadex columns demonstrate the validity of the calculations. Optimized in vivo images of a rat stroke model demonstrate the potential of the flow-compensated/flow-dephased technique and the importance of optimizing CNR.

Animals↗

PubMed1991

Invited review: electrodiagnostic assessment and monitoring of motor unit changes in disease.

The motor unit characteristics change dynamically with disease. This is the basis for the use of neurophysiologic methods for diagnostic purposes and for long-term monitoring. A great variety of nerve conduction and EMG parameters are available to study the motor unit. They reflect different aspects of motor unit function. Therefore, the electromyographer must choose his technique and optimal parameters depending on the situation, type of condition, whether the purpose is diagnosis or monitoring, and so on. In this presentation the relationship between EMG parameters and physiologic and morphologic counterparts in some pathologic conditions will be discussed.

Electromyography↗

PubMed1988

Application of the molecular replacement method to multidomain proteins. 1. Determination of the orientation of an immunoglobulin Fab fragment.

Multidomain proteins provide special problems in the application of the molecular replacement method of structure determination. The structure of the Fab fragment from the autoimmune poly(dT)-specific antibody HED10 has been determined using molecular replacement. An analysis of the effects of varying the model and the parameters used in the rotation function indicates that dividing the molecule into individual relatively rigid domains simplifies interpretation of the results, and that the optimal parameters depend on the molecule under study.

Antibodies↗

PubMed1988

Depth of intraspinal wire penetration during passage of sublaminar wires.

A major concern with the use of sublaminar wires for segmental spinal instrumentation is the risk of neurologic compromise associated with repeated passage of wires through the epidural space. Because of the inability to visualize the wire tip during its sublaminar passage, the surgeon is unable to appreciate the depth of wire penetration (DOWP). The purpose of this investigation was to determine, through direct measurement, the depth of intraspinal penetration during the passage of sublaminar wires. Using their model, the authors have been able to define the optimal parameters for safe wire passage. Careful attention to maintaining contact between the wire tip and the under-surface of the lamina, using a wire of optimal configuration, will result in minimal DOWP and reduce the possibility of neurologic compromise.

Adult↗

PubMed1977

[Image processing in pathology. III. Structure and use of an automatic morphometry system for recording morphometric features of nuclei (author's transl)].

UNLABELLED: An image processing system for morphological investigation of liver cell nuclei in biopsy specimens is demonstrated. The present study deals with the age dependence of normal hepatic nuclear areas. The results were compared with those of conventional methods. MATERIAL AND METHODS: The image processing system consists of a microscope, an image dissector and a computer (central processing unit, display, teletype, magnetic tape devices and line printer). 74 liver biopsies were investigated. They sshowed no signs of acute or chronic diseases. Paraffin sections of 4 micron in thickness were prepared and stained by Feulgen method. 8-10 biopsies were available for each decade from 1-8. 18,000 liver cell nuclei from all specimens were isolated and measured automatically. Using the image processing system and adequate computer programs 20 parameters of all isolated nuclei were gained (parameters of shape, size and structure of the nuclei). For sampling and processing of the information the following programs are developed: PROBE Program for investigation of optimal parameters of scanning and processing. SAMPL Program for scanning, object isolation, feature extraction and storage of data records. KORRL Program for investigation of correlations between the features of objects. LISTE Program for listing of data records. ULTRA Program for search of objects with extremal features. CODER Program for correction od data records. ADAPT Program for training of classificators. SCHAU Program for demonstration and checking of the classificator. MORPH Program for output of feature distributions and statistical parameters. Following object features were calculated and investigated: features of size: KOFL area within the object contour; KKFL minimal covex area of the object; ZKFL object area without holes; KONL length of the object contour.

Adolescent↗

PubMed1987

PEDA: a microcomputer program for parameter estimation and dosage adjustment in clinical practice.

PEDA, an integrated program in BASIC for implementation on microcomputers, has been developed for use in clinical practice to assist dosage adjustment for individual patients. A parameter optimization for individual patients is based on the principle of Bayes' theory and Maximum Likelihood Estimation, and utilizes a prior information on the distribution of population pharmacokinetic parameters, means and variances, as well as serum drug concentrations. The program can accommodate a one-compartment open linear model and a non-linear model at steady state (Michaelis-Menten model) and handle both uniform and non-uniform multiple dosage regimens mostly arising from clinical settings. Clinical examples which demonstrate the ability and the flexibility of the program are provided. The program may also be used as an aid for instruction in clinical pharmacokinetics.

Adult↗

PubMed1991

Receiver operating characteristic curves of fetal, umbilical and uteroplacental blood velocity waveforms as predictors of fetal outcome.

The maximum blood flow velocity waveforms (FVW) were recorded by a pulsed Doppler ultrasound from the fetal aorta, umbilical artery and arcuate artery in 142 pregnancies suspected of intrauterine growth retardation (IUGR) according to ultrasound fetometry. The FVW were analysed for pulsatility index (PI) and Blood Flow Class (BFC). The aim of the study was to find the optimal parameter and cut-off level for predicting the outcome of pregnancy. Receiver operating characteristic curves were used for the analysis. The umbilical artery PI and BFC shows the strongest relationship to IUGR. The optimal cut-off point was found at 2 SD above the mean PI of the normal population. The aortic PI had a relatively low predictive value for IUGR, but the absence of aortic end-diastolic velocities (BFC 2 and 3) was the best parameter for predicting fetal distress. The arcuate artery PI had low predictive capacity for fetal outcome. The data indicate that the absence of diastolic velocities in the fetal aorta and the umbilical artery PI, with the cut-off level of 2 SD above the mean for the normal population, are the best and comparable blood flow parameters, which can be used for monitoring fetuses suspected of IUGR.

Blood Flow Velocity↗

PubMed1977

The effect of intravaginal electrical stimulation on the feline urethra and urinary bladder. Electrical parameters.

Intravaginal electrical stimulation (IVS) in cats caused urethral closure and bladder inhibition. The aim was to ascertain the electrical parameters most appropriate for these effects. Minimum voltage was used as main criterion to select an effective, non-destructive stimulation when the shape, frequency and duration of the pulses were systematically varied. Urethral closure was achieved at minimum voltage (3 V) with alternating pulses at a frequency of 50 Hz and a pulse duration of 1.5 ms. Corresponding optimal parameters for bladder inhibition were: alternating pulses, 10 Hz and 1.5 ms. Minimum voltage was 1 V. Moreover, the positions of the electrodes were of significance for the responses. The clinical implications of these findings are that: 1)more differentiated treatment of incontinence could be achieved by adapting the stimulation parameters to the cause of incontinence, and 2) specific electrode positions would probably give optimal responses.

Animals↗

PubMed1991

Two-parameter mobile phase optimization for the simultaneous high-performance liquid chromatographic determination of dopamine, serotonin and related compounds in microdissected rat brain nuclei.

A new high-pressure liquid chromatography method with electrochemical detection is described that allows the simultaneous determination of dopamine, serotonin, 3,4-dihydroxyphenylacetic acid, 3-methoxy-4-hydroxyphenylacetic acid, 5-hydroxytryptophan and 5-hydroxyindoleacetic acid in microdissected nuclei from individual rat brains. No sample pre-treatment steps are required. Resolution and analysis time were optimized by a simple limited optimization procedure, involving two-parameter factorial design.

3,4-Dihydroxyphenylacetic Acid↗

PubMed1992

Exact likelihood evaluation in a Markov mixture model for time series of seizure counts.

This paper provides an alternative to Albert's (1991), Biometrics 47, 1371-1381) approximation to the E-step when using the EM algorithm for parameter estimation in Markov mixture models. Use of a recursive algorithm of Baum et al. (1970, Annals of Mathematical Statistics 41, 164-171) results in exact evaluation of the likelihood, optimal parameter estimates, and very efficient computation. Applications to time series of seizure counts and fetal movements clearly show the advantages of this exact approach.

Algorithms↗