SEARCH · PubMed Health

Results for “parameter optimization”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 541 records · Page 30Linked to original sources

PubMed2000

Interactions between hydroxypropylcelluloses and vapour/liquid water.

Understanding of the uptake of water vapour or liquid water by cellulose-based polymers is important because of the influence of these processes on many of the biologically or technologically relevant properties of these polymers. In this work we studied these processes in the cases of twelve hydroxypropylcelluloses with low or medium-high degrees of substitution (L-HPCs and HPCs, respectively), characterization of which showed significant differences in structural and physical parameters (substitution pattern, crystallinity, particle size, specific surface area, and intraparticular porosity). Water vapour sorption-desorption isotherms determined to characterize the uptake of water vapour were fitted well by the Young-Nelson model, the optimized parameters of which indicated that at all relative humidities the capacity to bind water vapour as a surface monolayer is greater for HPCs than L-HPCs, but the capacity to absorb water vapour internally is greater for L-HPCs than HPCs. Guggenheim-Anderson-deBoer (GAB) models fitted the sorption-desorption isotherms less well. Differential scanning calorimetry (DSC) experiments showed all sorbed water vapour to be held as non-freezing water. Isoperibol microcalorimetry experiments carried out to investigate interactions with liquid water showed enthalpies of hydration/dissolution of between -62.86 and -71.35 J g(-1) for L-HPCs and between -82.95 and -99.80 J g(-1) for HPCs, and DSC showed average numbers of non-freezing water molecules per polymer repeat unit of 2.65-4.19 for L-HPCs and 18.10-22.42 for HPCs. DSC characterization of the kinetics of the water uptake by 10 mg compacts obtained by direct compression of hydroxypropylcelluloses showed faster uptake by L-HPC compacts than by HPC compacts, among which there were significant differences in capacity for diffusive uptake. The explanations of the above differences in terms of the different substituent contents, particle sizes and porosities of the HPCs is supported by multiple linear regression analyses.

Cellulose↗

PubMed2006

Quantitative measurement of speech sound distortions due to inadequate dental mounting.

In this paper, the most adequate quantitative parameters are sought that reflect the dependence of speech sound distortions on the correct position of dental mountings. We suggest the Hurst fractal exponent and some parameters calculated from the Paul wavelet transform of speech sound are useful as quantitative parameters. The investigations are focussed on the alterations of the first /s/ phoneme when the patient produces the Sisyphe sound. The results will be useful to obtain a rigorous, computer assisted method to establish (design) the optimal parameters of prosthetic mounts in order to ensure best speech quality.

Computer Simulation↗

PubMed1999

Separation of natural pyrethrum extracts using micellar electrokinetic chromatography.

The separation of the six pyrethrin esters in a technical pyrethrum extract (Riedel-de-Haën, Cresent Chemical Co. Inc. Hauppauge, NY, USA) by micellar electrokinetic chromatography (MEKC) using both sodium dodecyl sulfate (SDS) and a polymerized surfactant as pseudo-stationary phases has been investigated and optimized. Parameters such as pH, SDS and polymerized sodium N-undecyl sulfate (poly-SUS) concentration, type and concentration of background electrolyte and organic modifier, as well as the acetonitrile/water ratio in the sample were studied to optimize the resolution, efficiency, and analysis time. An optimized separation of the six pyrethrin esters was achieved in 25 min with 25 mM Tris, buffered at pH 9, containing 30 mM SDS, 25% (v/v) acetonitrile, and an equal volume ratio of acetonitrile/water sample matrix at a voltage of 25 kV. The use of 0.5% (w/v) poly-SUS enhanced resolution of the pyrethrin esters and shortened the total analysis time from 25 to 20 min, compared to the SDS mediated separation. The optimized MEKC results are compared to the HPLC separation of these esters and show an improvement in efficiency and total analysis time.

Chromatography, High Pressure Liquid↗

PubMed2000

Nonlinear multivariate regression outperforms several concisely designed neural networks on three QSPR data sets

Neural networks (NNs) are accepted as the most powerful nonlinear technique in QSAR and QSPR modeling. However, the NN models are often very robust, containing a large number of parameters optimized during the training procedure. We have recently found (J. Chem. Inf. Comput. Sci. 1999, 39, 121-132) that the simpler nonlinear multiregression (MR) models are significantly better than the robust NNs, according to the same statistical parameters. In the present paper we investigated whether the nonlinear MR models are also better than the concisely designed NN models. Nonlinear MR models were generated in the following way. First, nonlinear terms, the 2-fold and 3-fold cross-products of initial descriptors, were calculated and added to initial descriptors. Then, the combination of two powerful techniques for descriptor selection (CROMRsel for "the best" selection and CROMRiisel for approximative, "i by i" stepwise selection) were used to detect the most important descriptors in MR models. For boiling points (BPs) of 150 alkanes the 20-descriptor MR model produced the cross-validated (CV) standard error of 2.88 K, and the best NN model (with 70-80 adjusted weights) had 3.60 K. Prediction of BPs of 50 compounds using the 17-descriptor MR model (obtained on 100 compounds) gave the standard error of 3.58 K. In the case of modeling of 243 chemical shifts CV standard errors were (in ppm) 0.89 and 1.19 with 15- and 9-descriptor MR models, respectively. The best NN models adjusted 60-90 weights and achieved 1.42 ppm. The standard error in predicting the 83 chemical shifts using the 10-descriptor MR model obtained on 160 samples was 1.25 ppm. It is also shown in this data set that the model quality depends on the scaling procedure used for transformation of the initial descriptors. In modeling the sublimation enthalpy the CV correlation coefficient was 0.97 using the best 4-descriptor MR model versus 0.93 obtained using NN with approximately 50 adjusted weights. The CV correlation coefficient in predicting the sublimation enthalpies for 21 compounds using the 4-descriptor MR model was 0.98. This is, to our knowledge, the first unambiguous result which shows a way for obtaining nonlinear MR models having better fitted, cross-validated, and predictive performances than the corresponding NN models. Moreover, the nonlinear MR models are significantly simpler than the NN models, which allows one to establish the functional relationships between the modeled property/activity and descriptors.

Journal Article↗

PubMedDate not supplied

Cryopreservation and xenotransplantation studies of microencapsulated rat pancreatic islets.

Islets of Langerhans were isolated from the Sprague Dawley rat pancreas digested by injected collagenase, and purified by Ficoll density gradient centrifugation. In order to make smaller and more uniform microencapsulated islets, we designed a special high-voltage electrostatic microcapsule generator. The effects of operational parameters of the generator on the size and the uniformity of microcapsules were analyzed, such as the voltage, the plunger speed of suspension delivery to the needle tip, the distance between needle tip and solution surface. The optimal parameter combinations for making microcapsules are: 5 kV of voltage, 50 mm/h of the plunger speed, and 20 mm distance. The high-voltage electric system can produce uniform microcapsules with diameters ranging from 0.3 to approximately 0.5 mm, which are smaller and more uniform than those produced by air-jet system. A comparison of the cryopreservation effects between microencapsulated islets and unencapsulated islets showed that the microcapsules can protect the fragile islets from freezing damage, and increase the retrieval rate from 68.5% to 92.6%. Xenotransplantation of the cryopreserved rat islets resulted in the normalization of the metabolic blood glucose of the diabetic mice for 90 days, whereas the unencapsulated islets were easily fragmented and lost during the freezing process. They only reversed hyperglycemia for less than 3-5 days.

Animals↗

PubMed1995

Genetic-algorithm selection of a regulatory structure that directs flux in a simple metabolic model.

A genetic algorithm (GA) is used to optimize parameters for allosteric regulation of enzymes in a model of a metabolic futile cycle, in which two metabolites are interconverted by a pair of irreversible enzymatic reactions. The cycle is regulated by end products of the surrounding pathway. The optimization criterion for the GA is the proper direction of chemical flux in the regulated cycle toward one or the other end product in response to a simple, time-dependent model of biochemical "need" based on externally imposed variation of the end product concentrations. An energetic cost, to be held to a minimum, is also imposed on the operation of the cycle. The best-performing individuals selected by the GA are found to switch rapidly the direction of net flux according to need. In different "environments" (specific time courses of end product concentrations), the GA produces better- or poorer-performing individuals. In some cases "generalists" and "specialists" are produced. The present approach provides, purely as a consequence of formally specifying the task of flux direction, the new result of numerical confirmation, in a simple model, of the intuition that negative feedback and reciprocal regulation are important for good flux direction in arbitrary environments, and gives rise to a diversity of structures, suggestive of the results of biological evolution.

Algorithms↗

PubMed1993

Iontophoresis of methylene blue for gonioscopic pulsed dye laser sclerostomy.

Iontophoresis of methylene blue using pipette tip probes is an effective means of scleral staining in preparation for gonioscopic pulsed dye laser sclerostomy. The effects of the surface area of the pipette tip and of the duration of iontophoresis on the concentration and distribution of stain were investigated in rabbit eyes. Iontophoresis of 1% methylene blue was done using currents from 0.2 to 2.0 mAmp, pipette tip probes with surface areas from 0.02 to 7.1 mm2, and durations of iontophoresis from 0.5 to 10 minutes. Optimum iontophoresis parameters for delivering methylene blue to the eye were a current of 0.4 mA, a probe tip surface area of 0.19 mm2, and a duration of five minutes. The maximal scleral stain concentration, approximately 0.4%, was achieved using these optimal parameters. Microscopic analysis of frozen tissue sections demonstrated the stain to have penetrated the full thickness of the sclera after five minutes. Tissue samples were analyzed for dye 0.5, 2, 5, 12, and 24 hours after iontophoresis was completed. Decreased dye concentration by over 50% within 2 hours and complete disappearance of dye within 24 hours were demonstrated. Methylene blue concentration thresholds for laser ablation were also examined; the threshold of ablation of methylene blue was 0.0625% when an energy level of 75 mJ was used. We propose that iontophoresis of methylene blue is a useful adjunct to pulsed dye laser sclerostomy formation.

Animals↗

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↗

PubMed1998

[Effect of the evaluation parameter on sensitivity of resonance thrombography of thrombocytopenia in dogs].

Based on 109 blood samples taken from 36 dogs suffering from thrombocytopenia resonance thrombography with the resonance thrombograph RTG 801 (von Hoerner und Sulger Electronic GmbH, Schwetzingen; manufacturer: Fresenius AG, Bad Homburg) was distinctly more sensitive and more closely correlated to the platelet count using an optimized parameter of the resonance thrombogramm (RTG) in comparison to usual parameters. Nevertheless, clinical requirements regarding samples with platelet counts > 25,000/microliter were not fulfilled. Out of 13 samples with reduced platelet count and simultanous extended capillary bleeding time, depending on the used parameter a maximum of 9 samples could be detected as pathological by the RTG. The normal RTG in part of the cases with clearly altered primary haemostasis contrasts to the exclusive use of RTG in the screening of thrombocytopenia in dogs.

Animals↗

PubMed2005

Auto-tuning of PID controller parameters with supervised receding horizon optimization.

In this paper, a novel two-layer online auto-tuning algorithm is presented for a nonlinear time-varying system. The lower layer consists of a conventional proportional-integral-derivative (PID) controller and a plant process, while the upper layer is composed of identification and tuning modules. The purpose of the upper layer is to find a set of optimal PID parameters for the lower layer via an online receding horizon optimization approach, which result in a time-varying PID controller. Through mathematical analysis, the proposed system performance is equivalent to that of a standard generalized predictive control. Simulation and experiment demonstrate that the new method has a better control system performance compared with conventional PID controllers.

Journal Article↗

PubMed1995

[The diagnosis of acute pulmonary embolism: a comparison between spiral CT and DSA in an animal experiment].

PURPOSE: The value of helical computed tomography (CT) for diagnosis of acute pulmonary embolism was assessed and compared with digital subtraction angiography (DSA) as a reference method. MATERIAL AND METHODS: In 11 dogs, lobar, segmental, and subsegmental occlusions of the pulmonary arteries were produced. Subsequent to selective pulmonary angiography, the animals were examined with contrast-enhanced helical CT. RESULTS: In the main and lobar pulmonary arteries there was a complete correlation between CT and DSA in documentation of total and partial embolic occlusions. Identification of segmental and subsegmental pulmonary emboli by CT required a second run with optimized parameters in 7 of 11 cases. Nevertheless, 18% of the peripheral arteries could not be classified. CONCLUSION: Helical CT as less invasive modality is competitive with DSA in demonstration of central pulmonary emboli. However, CT imaging of peripheral pulmonary emboli requires optimal bolus timing.

Acute Disease↗

PubMed2003

Experimental robotic milling in skull-base surgery.

OBJECTIVE: In the field of otorhinolaryngology, a variety of skull implants have been developed to assist hearing-impaired or even deaf patients. The first step in the implantation procedure, and also in lateral skull-base surgery, is to drill the calvarian or mastoid bone. We intended to investigate the hitherto unknown parameters for performing this procedure and to establish the first set-up for robotic milling of the lateral skull base. MATERIALS AND METHODS: Experimental milling of the skull base was conducted on two human specimens using a hexapod robot. RESULTS: Optimized parameters were determined with a drill speed of 30,000 revolutions/min and a form feed rate of 5 mm/s for the calvarium and 1 mm/s for mastoid bone, respectively, in a spiral-path fashion. While using a cutting burr, mean force levels were 4.81 N for calvarian bone and 6.12 N for mastoid bone, respectively--well below our empirical limit of 10 N. However, maximum levels easily surpassed these limits, reaching 27.7 N. CONCLUSION: The prerequisites for robotic skull-base surgery were fulfilled. With further work to implement feedback of sensory input, robots may increase precision for various tasks in skull-base surgery.

Humans↗

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↗

PubMed1995

Concentration-dependent metabolism of diazepam in mouse liver.

Previous mouse liver studies with diazepam (DZ), N-desmethyldiazepam (NZ), and temazepam (TZ) confirmed that under first-order conditions, DZ formed NZ and TZ in parallel. Oxazepam (OZ) was generated via NZ and not TZ despite that preformed NZ and TZ were both capable of forming OZ. In the present studies, the concentration-dependent sequential metabolism of DZ was studied in perfused mouse livers and microsomes, with the aim of distinguishing the relative importance of NZ and TZ as precursors of OZ. In microsomal studies, the Kms and Vmaxs, corrected for binding to microsomal proteins, were 34 microM and 3.6 nmole/min per mg and 239 microM and 18 nmole/min per mg, respectively, for N-demethylation and C3-hydroxylation of DZ. The Kms and Vmaxs for N-demethylation and C3-hydroxylation of TZ and NZ, respectively, to form OZ, were 58 microM and 2.5 nmole/min per mg and 311 microM and 2 nmole/min per mg, respectively. The constants suggest that at low DZ concentrations, NZ formation predominates and is a major source of OZ, whereas at higher DZ concentrations, TZ is the important source of OZ. In livers perfused with DZ at input concentrations of 13 to 35 microM, the extraction ratio of DZ (E[DZ]) decreased from 0.83 to 0.60. NZ was the major metabolite formed although its appearance was less than proportionate with increasing DZ input concentration. By contrast, the formation of TZ increased disproportionately with increasing DZ concentration, whereas that for OZ decreased and paralleled the behavior of NZ. Computer simulations based on a tubular flow model and the in vitro enzymatic parameters provided a poor in vitro-organ correlation. The E[DZ], appearance rates of the metabolites, and the extraction ratio of formed NZ (E[NZ, DZ]) were poorly predicted; TZ was incorrectly identified as the major precursor of OZ. Simulations with optimized parameters improved the correlations and identified NZ as the major contributor of OZ. Saturation of DZ N-demethylation at higher DZ concentrations increased the role of TZ in the formation of OZ. The poor aqueous solubility (limiting the concentration range of substrates used in vitro), avid tissue binding and the coupling of enzymatic reactions in liver, favoring sequential metabolism, are possible explanations for the poor in vitro-organ correlation. This work emphasizes the complexity of the hepatic intracellular milieu for drug metabolism and the need for additional modeling efforts to adequately describe metabolite kinetics.

Animals↗

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↗

PubMed1995

Distortion product otoacoustic emissions in rhesus (Macaca mulatta) monkey ears: normative findings.

Distortion product otoacoustic emissions (DPOAEs) in rhesus monkeys were characterized and the optimal parameters for their generation were determined. Robust DPOAEs were readily measurable from the ear canals of six rhesus monkeys (n = 12 ears). The nonmonotonic behavior of the f2/f1 ratio functions in rhesus monkeys was found to be similar to other animals and humans. The optimal mean f2/f1 ratio of 1.21 and the effect of the primary frequency and level on the optimal f2/f1 ratios were also similar to human measurements. The contour of the rhesus monkey DPOAE 'audiograms' and their behavior were also comparable to human measurements with slight differences in peak frequencies. The rhesus monkey DPOAE input/output (I/O) functions were generally monotonic with a slope approaching unity with increasing frequency. Therefore, our study shows that many basic DPOAE characteristics are remarkably similar in the two species and emphasizes the appropriateness of the rhesus monkey as a model for DPOAE research. Detailed studies of the behavior of DPOAEs can be carried out in a model that is phylogenetically close to human both in hearing and in the gross structure and histology of the inner ear.

Acoustic Stimulation↗

PubMed1980

An evaluation of the clinical potential of a comprehensive model of human respiration in artificially ventilated patients.

1. We have investigated the feasibility of accurately simulating the respiratory function of artificially ventilated patients, using a computer model of the respiratory system. Twelve patients artificially ventilated after uncomplicated cardiac bypass surgery was studied. 2. The basic information required to simulate individual ventilated subjects was measured or derived. A program was written to enable key model parameters to be adjusted automatically to match model predictions to these clinical measurements. On completion of this matching procedure all the variables computed by the model were compared with patient values (measured or derived) and their accuracy was assessed. 3. The matching algorithm successfully optimized parameters of the model representing metabolic activity, tissue respiratory quotient, venous admixture, physiological dead space and total body bicarbonate to match measured values for oxygen consumption, carbon dioxide production, Pa, O2, PaCO2, and arterial HCO3- respectively. Other variables compared arise from the solution of equations within the model and correlation between model and patient values is generally good (r greater than 0.9). However, values of Pv-, O2 correlate less well (r = 0.85). Factors affecting the accuracy of patient simulation are discussed and some deficiencies analysed. 4. The creation of an accurate, steady-state representation of a patient by the model opens up the possibility of using it interactively as an aid to clinical management. Some possible future developments of the technique are discussed.

Blood↗

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↗