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Results for “parameter optimization”

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At least 127 records · Page 7Linked to original sources

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↗

PubMed1991

Automatic classification of cells in cell cycle phases based on Ki-67 antigen quantification by fluorescence microscopy.

Ki-67 antigen is thought to be a marker of cell proliferation, as it can be detected in cycling cells, i.e. cells in G1, S, G2 and M phases, but not in resting cells. The immunocytochemical staining pattern obtained by the Ki-67 monoclonal antibody varies, depending on the cell cycle phases. Analysis of double staining of Ki-67 antigen and DNA in the MCF-7 cell line by videomicrofluorometry allows the description of both the level and the pattern of Ki-67 staining in the form of a set of parameters defining each cell. These parameters were measured in MCF-7 cell populations characterized according to their position in the cell cycle. They were submitted to a statistical analysis (principal component and discriminant analysis) which allowed the determination of the optimal parameters to characterize a given cellular group and permitted the use of these parameters for an automatic classification of cells in the different cell cycle phases. In G1, S, G2, prophase + metaphase and anaphase + telophase cells, these parameters allowed a classification of cells with a good-classification rate of 94.37%. A comparison of this method with methods based on the DNA histogram and bromodeoxyuridine uptake was performed. The classification coefficients stemming from the discriminant analysis were introduced into a program to obtain, automatically, the Ki-67 labelling index and the percentages of cells in each phase. This method, which allows a quick evaluation of the proliferation and the phase indices, may be more widely applicable.

Breast Neoplasms↗

PubMedDate not supplied

The significance of positive controls in testing the oncogenic potential of cell material for virus vaccine production.

For all the parts involved in the production and control of virus vaccine it is important that controls of the cell material should be carried out under optimal conditions for the detection of any abnormal characteristics of the cells. The experiments reported here attempt to establish some of the optimal parameters for testing out the tumor-inducing properties of a human diploid cell culture. All experiments were carried out on 6-week old Syrian hamsters and the main objectives were: (i) to devise an optimal immunosupression scheme and (ii) to find a more suitable positive control for testing the oncogenic potential of the human diploid cells. The anti-hamster lymphocyte serum proved to be a more potent immunosuppresser than hydrocortisone succinate as seen from the development of tumors following injections with KB cells; this scheme was subsequently used in assessing the usefulness of KB and Swiss-12 cells as positive controls. The experiments lend support to the suggestion that a heteroploid fibroblastic culture (such as Swiss-12) should become a standard positive control since not only are the tumors so obtained larger and less prone to early necrosis but, more important, they bear a closer resemblance to the kind of tumors likely to be induced by the diploid cell cultures being tested.

Animals↗

PubMed1992

Pharmacokinetic analysis of the perivascular distribution of bifunctional antibodies and haptens: comparison with experimental data.

A mathematical model is developed to describe the concentration profiles around individual tumor blood vessels for two-step approaches to cancer treatment. The model incorporates plasma pharmacokinetics, interstitial diffusion, reversible binding between antibody and hapten and between antibody and tumor-associated antigens, and physiological parameters to evaluate present experimental approaches and to suggest new guidelines for the effective use of two-step approaches. Results show considerable interaction between the binding kinetics, initial drug doses, and antigen density, with optimal parameter ranges depending on the desired goal: treatment or detection. The hapten concentration in tumors was found to be nonuniform because of specific binding to antibodies. While binding of the hapten to the bifunctional antibody is necessary for improved retention, too large a binding affinity may lead to very poor penetration of the hapten into regions far away from blood vessels. The time delay between antibody and hapten injection was found to be an important parameter. Longer time delays were found to be advantageous, subject to constraints such as internalization of the antibody and tumor growth during treatment. A proper combination of initial doses for the two species was also seen to be crucial for maximum effectiveness. Comparison of the model with the experimental data of Le Doussal et al. (Cancer Res., 51: 6650-6655, 1991) and Stickney et al. (Cancer Res., 50: 3445-3452, 1990) suggests two novel, yet testable, hypotheses: (a) the early pharmacokinetics of low molecular weight agents can have an important effect on later concentrations using two-step approaches; and (b) metabolism may play an important role in reducing concentrations in the tumor and tumor:plasma concentration ratios. These results should help in the effective design of two-step strategies.

Antibodies, Neoplasm↗

PubMed1989

[Basic investigation on hyperthermia by low-frequency ultrasonic].

Ultrasonic heating methods for hyperthermia have been developed because of several advantage i.e., its easiness in steering the applicator for the target, and its non-interference with other electronic equipment. Most of them make hot spots by focusing the ultrasonic beam, using acoustic lens or many transmitters. Therefore, these methods are based on analogous idea to the ray focusing, and higher frequency, i.e., from 500 kHz to 5 MHz, is used. However, there are some problems; hot spots are generated before the focus, and it is difficult to heat the region beyond gas and bones owing to the attenuation and scattering of the ultrasonic beam. Then, we propose a new method for heating the depths and local regions of the body. In this method, to heat the depths of the body low-frequency ultrasound is used since it has larger penetration depth and it is less scattered than higher-frequency ultrasound. To heat only tumours, hot spots are generated by synthesizing acoustic fields resulted from several incident waves. The heat generation and temperature distribution was analyzed using the models with properties similar to tissues for different values of parameters such as frequency, position and the number of sources. The results show that hot spots can be generated at the depths of the body. and that desirable temperature distribution can be obtained by selecting optimal parameters and cooling condition. To determine the temperature distribution more accurately it would be required to introduce the more complicated model of heat removal effect by blood flow and the experimental results. These problems are left for the future investigation.

Humans↗

PubMed1990

Optimization of filtration for reduction of lung dose from Rn decay products: Part I--Theoretical.

A theoretical model was developed for the optimization of filter characteristics that would minimize the dose from the inhalation of Rn decay products. Modified forms of the Jacobi-Porstendorfer room model and the Jacobi-Eisfeld lung dose model were chosen for use in the mathematical simulation. Optimized parameters of the filter were the thickness, solidity, and fiber diameter. For purposes of the calculations, the room dimensions, air exchange rate, particle-size distribution and concentration, and the Rn concentration were specified. The resulting computer-aided optimal design was a thin filter (the minimum thickness used in the computer model was 0.1 mm) having low solidity (the minimum solidity used was 0.5%) and large diameter fibers (the maximum diameter used was 100 microns). The simulation implies that a significant reduction in the dose rate can be achieved using a well-designed recirculating filter system. The theoretical model, using the assumption of ideal mixing, predicts an 80% reduction in the dose rate, although inherent in this assumption is the movement of 230 room volumes per hour through the fan.

Air Pollutants↗

PubMed1991

Application of non parametric methods to pharmacokinetic models of the Michaelis-Menten type.

Non linear parametric regression methods for data fitting as applied to complex pharmacokinetic models give rise to convergence difficulties which have not been completely solved. The Cornich-Bowden non parametric method was applied by Shelver to the one compartment open model with first order absorption to simulated data with different error structures. The application of his procedure was extended to a pharmacokinetic model with Michaelis-Menten elimination. Simulated concentration/time data were generated by a microcomputer program which imposed the appropriate error structure:constant and proportional standard deviation with or without one outlier of different magnitude. Parameter optimization was performed using either a Newton-Raphson algorithm based on the integrated form of the Michaelis-Menten equation and a standard non linear regression program: NONLIN. In the case of the non parametric method, the median of the parameter values obtained for each combination of data points was used as best estimate of the parameters. One hundred data sets were used and performance of the two methods was assessed by comparing bias and standard deviation of the mean parameter value thus obtained, given the true values of the parameters used to generate the data. The non parametric performed well with homoscedastic data and was less sensitive to the presence of outliers than the non linear regression technique. In the case of heterosocedastic data both methods performed poorly but the non parametric method was more sensitive to the presence of outliers.

Algorithms↗

PubMed1992

A fully automated adaptive unsharp masking technique in digital chest radiograph.

The authors are developing a fully automated adaptive unsharp masking technique with parameters that depend on regional image features of a digital chest radiograph. A chest radiograph includes various regions such as lung fields, retrocardiac area, and spine. These areas have very different texture patterns and optical densities. Therefore, for best evaluation, it is necessary to enhance the image contrast of each region by an optimal parameter. In the current study, a chest radiograph was automatically divided into three segments (lung field, retrocardiac area, and spine) by using a histogram analysis of pixel values. The lung fields and retrocardiac area were selectively enhanced with a small mask size and mild weighting factors that had been previously determined to be optimal. The spine was enhanced with a large mask size and adequate weighting factors. An observer performance test indicated that this technique provides excellent diagnostic accuracy for simulated nodules in chest radiographs.

Humans↗

PubMed1991

The use of a computer to select optimized conditions for high-performance liquid chromatography separation.

Computer simulation allows the convenient prediction and optimization of HPLC separation as a function of various separation conditions. The use of retention and bandwidth relationships that have been validated for a broad range of chromatographic systems minimizes the number of experimental runs needed, especially for the new technique of restricted multi-parameter optimization. The chromatographer is free to use these procedures in a trial-and-error mode, or alternatively use can be made of resolution maps and other data summaries. "Gridding" experiments, based on the automated collection of chromatographic data, can be used to supplement predictions obtained from computer simulation.

Chemistry Techniques, Analytical↗

PubMed1986

Optimizing the pentetrazol infusion test for seizure threshold measurement.

Seizure thresholds in mice were determined using the pentetrazol infusion method. Concentration of infusate and rate of infusion were varied to assess the optimal parameters for seizure threshold detection. Seizure threshold elevations were produced by flurazepam and threshold decreases by FG 7142. An infusion rate of 1.1 ml min-1 was best for detecting both increases and decreases in threshold. However a concentration of 2.5 mg ml-1 gave optimal measurement of elevations in threshold whereas decreased thresholds were best detected with a concentration of 10 mg ml-1.

Animals↗

PubMed1976

Parameters of electrical stimulation of the medial preoptic area for release of gonadotrophins in male rats.

The release of LH and FSH after the application of an electrical stimulus to the anterior diencephalon of male rats has been studied. The stimulus was applied through either platinum or steel electrodes implanted stereotaxically in animals anaesthetized with urethane. The efficacy of various parameters of stimulation by means of a current consisting of balanced biphasic square waves, was tested by systematically changing the frequency, amplitude and duration of the pulses. The effect of direct current (d.c.) stimulation on hormone release was also examined. The concentrations of the hormones in blood withdrawn from the femoral vein before and at frequent intervals up to 80 min after application of the stimulus were determined by radioimmunoassay. The optimal parameters for the release of LH by square wave stimulation of the medial preoptic area were: frequency, 60 Hz; pulse amplitude, 0-50 mA; pulse duration, 1-00 ms. This stimulus was more effective when applied through steel than through platinum electrodes. Direct current stimulation (15 muA for 10 s) through steel electrodes was most effective of all. When applied through platinum electrodes to the medial preoptic and anterior hypothalamic areas, the optimal square wave stimulus produced significant increases in the concentration of LH after 5 and 10 min respectively. The concentration of plasma FSH in these animals also increased, but the increments were much less than the increments in LH. The magnitude of the respective increases of the gonadotrophins after stimulation of the two brain areas did not differ significantly. Measurement of the milk ejection response to stimulation of the hypothalamo-hypophysial tract in the lactating rat indicated that the spread of the square wave stimulus was no more than 1-5 mm from the electrode tip. The significance of these findings is discussed with respect to the importance of the medial preoptic area in the male rat, the neurones which may be involved in the regulation of gonadotrophin secretion, and the parameters of stimulation used in studying the hypothalamo-hypophysial system.

Animals↗

PubMed1992

Comprehensive model of transport and metabolism of adenosine and S-adenosylhomocysteine in the guinea pig heart.

Regulation of blood flow and mitochondrial respiration in the heart would be clarified by improved knowledge of interstitial concentrations and cellular production rates of adenosine; however, these variables cannot be measured directly. To interpret indexes that are available, a comprehensive mathematical model was developed, based on a large body of experimental data. The model describes most of the important pathways of capillary-tissue transport and cellular metabolism of adenosine in the guinea pig heart. It includes capillary flow, solute transport between tissue regions, nonlinear enzyme kinetics for adenosine kinase and adenosine deaminase, and reversible biunireactant kinetics for S-adenosylhomocysteine hydrolase in cardiomyocytes and endothelial cells, intracellular production of adenosine via AMP hydrolysis and transmethylation, and extracellular production of adenosine. A single set of parameter values for the model was obtained in the first stage of the analysis by taking certain values directly from published sources, other values were subject to specific constraints, and other values were determined by parameter optimization. The effects of flow and endothelial metabolism on the relation between interstitial and venous adenosine concentrations were determined. The relation between myocardial adenosine production rate and S-adenosylhomocysteine accumulation in the presence of excess homocysteine was estimated. In the second stage of the analysis, the model was used to investigate the mechanism of myocardial adenosine production, without changing the parameter values. Cellular adenosine production rates were estimated by fitting measurements of venous adenosine release obtained during altered energetic conditions in experiments by different investigators. The original results showed a dissociation between measurements of cytosolic AMP concentrations and venous adenosine release. It is concluded that 1) it is essential to account for the effect of flow on interstitial and venous adenosine concentrations, since decreased flow may produce effects outwardly resembling inhibition of the enzyme 5'-nucleotidase, 2) adenosine concentrations in epicardial transudate are not in equilibrium with interstitial fluid, and 3) the rate of cellular adenosine production increases monotonically with free cytosolic concentrations of AMP during a variety of alterations in energy balance of the guinea pig heart.

Adenosine↗

PubMed1989

Fitting piecewise linear regression functions to biological responses.

An iterative approach was achieved for fitting piecewise linear functions to nonrectilinear responses of biological variables. This algorithm is used to estimate the parameters of the two (or more) regression functions and the separation point(s) (thresholds, sensitivities) by statistical approximation. Although it is often unknown whether the response of a biological variable is adequately described by one rectilinear regression function or by piecewise linear regression function(s) with separation point(s), an F test is proposed to determine whether one regression line is the optimal fitted function. A FORTRAN-77 program has been developed for estimating the optimal parameters and the coordinates of the separation point(s). A few sets of data illustrating this kind of problem in the analysis of thermoregulation, osmoregulation, and the neuronal responses are discussed.

Algorithms↗

PubMed1991

Towards a neural transplantation therapy for Parkinson's disease: experimental principles from animal studies.

Animal experimentation is necessary to identify the optimal parameters and procedures for clinical application of neural tissue transplantation. Experiments in animal models of Parkinson's disease are briefly reviewed. Embryonic nigral tissues survive transplantation well and yield good functional recovery on a variety of motor tests. The age of the donor, the implantation technique, and the site of placement in the host brain all influence graft efficacy. Nevertheless there are theoretical limits to the power of current procedures, and even under optimal circumstances full recovery cannot be expected.

Adrenal Glands↗

PubMed1989

[Visually evoked responses as follow-up parameters in multiple sclerosis].

VEP- and MRI-changes were compared in 93 patients with definite or probable multiple sclerosis. A high correlation between slowing of visual conduction time and volume of peri-ventricular white matter lesions was found. The finding suggests a retrogenicular site of the pathogenic process being frequently responsible for VEP-abnormalities. Long-standing clinical courses show better VEP/MRI-correlations even without clinical symptoms, whereas acute cases more often have related visual disabilities but no convincing correlations. VEP represent one function of those regions in the brain which are frequently impaired by the demyelinating process in multiple sclerosis. They provide an optimal parameter of clinical course.

Adolescent↗

PubMed1977

Optimal conditions for the separation of rat T lymphocytes on anti-immunoglobulin--immunoglobulin affinity columns.

The separation of rat T lymphocytes was investigated on anti-Ig--Ig columns. A simple and efficient method for the purification of rat Ig by precipitation of rat serum with sodium sulfate is presented. Protein binding characteristics of glass and plastic beads, as solid support of affinity columns, are described, as well as optimal parameters for coating beads with rat Ig (with BSA, ribonuclease and lysozyme, as comparison). Binding of Ig was primarily dependent on the concentration of the Ig solution. Maximal strong binding of Ig (6.2 X 10(3) molecules per micron2 of bead surface) was reached a 400 microng per ml concentration of purified Ig solution during 20 min of incubation. Higher concentrations increased only the amount of loosely bound Ig on the surface of beads whereas the amount of firmly bound Ig remained unchanged. Fractionation of lymphoid cell suspensions on anti-Ig--Ig affinity columns prepared at optimal conditions resulted in highly purified T-cell suspensions containing less than 1% of lymphocytes bearing surface Ig receptors.

Animals↗