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Results for “parameter optimization”

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PubMed1997

The cerebral response to electrical stimuli in the oesophagus is altered by increasing stimulus frequencies.

Recording of cerebral evoked responses (EP) allows the assessment of visceral afferent pathways and gut-brain communication, but the optimal stimulation parameters remain to be established. The present study determined the optimal stimulation frequency of electrical stimulation of the oesophagus to elicit EP responses. In 13 healthy male volunteers (24.1 +/- 5.9 years), a 5 mm stainless-steel electrode was placed in the distal oesophagus for electrical stimulation (ES). EP were recorded from 21 scalp electrodes placed according to the 10/20 International system. ES (15 mA, 200 microseconds) were delivered in repeated series of 24 stimuli. Stimulus frequency was randomly altered in different series using a pseudologarithmic range (0.1, 0.2, 0.3, 0.5, and 1 Hz). Two series of stimuli were applied using each stimulation frequency. Two-dimensional topographic brain maps were created using interpolation techniques at each stimulation frequency. With increasing stimulus frequency, a significant and progressive decrease of EP amplitudes was observed between frequencies of 0.1 Hz and 1.0 Hz (P1/N2: 7.6 +/- 1.2 vs 1.4 +/- 0.3* microV, N2/P2: 17.2 +/- 1.7 vs 4.6 +/- 0.4* microV, P2/N3: 6.9 +/- 0.7 vs 4.2 +/- 0.5* microV; * = P < 0.05). In addition, there was a significant shortening of the mean peak latency of the intercalated P2 peak (P < 0.0005), with a similar trend for the P3 peak (P < 0.06), with increasing stimulus frequency from 0.1-1.0 Hz. Topographic brain maps localized the maximal early peaks (N1,P1.N2) in the paracentral cortical region (C3, Cz, C4), whereas the later peaks (P2 to P3) were symmetrically spread over the centroparietal and temporal regions (Cz, Pz, T5, T4). There was no difference in the cortical location of maximal EP amplitudes with increasing stimulus frequency. In conclusion, there is a clear relationship between stimulus frequency and amplitude of EP, suggesting rapid attenuation of the cerebral autonomic neural responses with increased electrical stimulation frequency. The effect of increased frequency on peak latencies suggests an alteration of stimulus processing in the thalamocortical region due to an altered perception of stimuli. Early EP peaks originate from basal structures of primarily the dominant hemisphere, while later peaks are localized in centroparietal cortical regions.

Adult↗

PubMed2003

[Effect of laser irradiation on structures of the eye fundus].

Mathematical model of selective laser influence on eye fundus structures is proposed. Mathematical calculations of optimal laser parameters for different eye fundus pathologies are made. They will optimize laser energy influence on retina and chorioida and decrease the risk of possible late side effects.

Choroid↗

PubMed2006

[Evaluation of antibacterial activity of apex-phoresis].

Effects of apex-phoresis on dental root canal microflora were studied and optimal dosage parameters were determined. Microbiological study in vitro has established that optimal antibacterial effect was achieved with the apex-phoresis dosage in the range between 2.5-5 mA/min. Study of the effects of apex-phoresis on dental root canal microflora in vivo disclosed its high antibacterial activity both in cases of pulpitis and destructive forms of chronic periodontitis. For the first time the data are confirmed by molecular-genetic method of detection of hard to cultivate virulent anaerobic bacteria. The results of the study allow to recommend apex-phoresis for wide use in clinical practice as an effective way to sterilize difficult to traverse dental root canals.

Adolescent↗

PubMed2006

Intraoperative electrocortical stimulation of Brodman area 4: a 10-year analysis of 255 cases.

BACKGROUND: Brain tumor surgery is limited by the risk of postoperative neurological deficits. Intraoperative neurophysiological examination techniques, which are based on the electrical excitability of the human brain cortex, are thus still indispensable for surgery in eloquent areas such as the primary motor cortex (Brodman Area 4). METHODS: This study analyzed the data obtained from a total of 255 cerebral interventions for lesions with direct contact to (121) or immediately adjacent to (134) Brodman Area 4 in order to optimize stimulation parameters and to search for direct correlation between intraoperative potential changes and specific surgical maneuvers when using monopolar cortex stimulation (MCS) for electrocortical mapping and continuous intraoperative neurophysiological monitoring. RESULTS: Compound muscle action potentials (CMAPs) were recorded from the thenar muscles and forearm flexors in accordance with the large representational area of the hand and forearm in Brodman Area 4. By optimizing the stimulation parameters in two steps (step 1: stimulation frequency and step 2: train sequence) MCS was successful in 91% (232/255) of the cases. Statistical analysis of the parameters latency, potential width and amplitude showed spontaneous latency prolongations and abrupt amplitude reductions as a reliable warning signal for direct involvement of the motor cortex or motor pathways. CONCLUSION: MCS must be considered a stimulation technique that enables reliable qualitative analysis of the recorded potentials, which may thus be regarded as directly predictive. Nevertheless, like other intraoperative neurophysiological examination techniques, MCS has technical, anatomical and neurophysiological limitations. A variety of surgical and non-surgical influences can be reason for false positive or false negative measurements.

Action Potentials↗

PubMedDate not supplied

Use of sephacryl high resolution (HR) in tetanus anatoxin purification.

The tetanus purified anatoxin is used in the preparation of the tetanus toxoid and multiple vaccines (dT, DT and DTP), all of them strictly following specifications established by the WHO with a minimum antigenic purity equal to 1,000 Lf/mgPN. Aiming to establish more sensitive and accurate methods for purification, samples from four different lots of tetanus anatoxin were submitted to gel filtration in twenty independent trials using the Sephacryl S-100 HR and S-200 HR resins. The Authors were careful to optimize their parameters of performance as to sample volume, elution and selectivity flow for tetanus anatoxin purification, allowing their use in industrial scale. The Sephacryl S-100 HR resin presented the best selectivity, that is, the best separation, allowing a greater linear-flow and, consequently, the best purity index. Satisfactory results were also achieved with the Sephacryl S-200 HR resin after optimization of chromatographic parameters for elution flow and volume of the sample applied. The good results of purification obtained, as well as the high chemical stability, have pointed out both the Sephacryl S-100 HR and S-200 HR resins as equally efficient for industrial production.

Acrylic Resins↗

PubMed2006

Using effusive molecular beams and microcanonical unimolecular rate theory to characterize CH4 dissociation on Pt(111).

The dissociative sticking coefficient for CH4 on Pt(111) has been measured as a function of both gas temperature (Tg) and surface temperature (Ts) using effusive molecular beam and angle-integrated ambient gas dosing methods. The experimental results are used to optimize the three parameters of a microcanonical unimolecular rate theory (MURT) model of the reactive system. The MURT calculations allow us to extract transition state properties from the data as well as to compare our data directly to other molecular beam and thermal equilibrium sticking measurements. We find a threshold energy for dissociation of E0 = 52.5 +/- 3.5 kJ mol(-1). Furthermore, the MURT with an optimized parameter set provides for a predictive understanding of the kinetics of this C-H bond activation reaction, that is, it allows us to predict the dissociative sticking coefficient of CH4 on Pt(111) for any combination of Ts and Tg even if the two are not equal to one another, indeed, the distribution of molecular energy need not even be thermal. Comparison of our results to those from recent thermal equilibrium catalysis studies on CH4 reforming over Pt nanoclusters ( approximately 2 nm diam) dispersed on oxide substrates indicates that the reactivity of Pt(111) exceeds that of the Pt nanocatalysts by several orders of magnitude.

Journal Article↗

PubMed2002

In vivo magnetic resonance imaging of the human cervical spinal cord at 3 Tesla.

PURPOSE: To demonstrate the feasibility of obtaining high-quality magnetic resonance (MR) images of the human cervical spinal cord in vivo at a magnetic field strength of 3 T and to optimize the signal contrast between gray matter, white matter, and cerebrospinal fluid (CSF) on 2D gradient recalled echo (GRE) images of the cervical spinal cord. MATERIALS AND METHODS: Using a custom-built, anatomically molded radio frequency (RF) surface coil, the repetition time and flip angle of a 2D GRE sequence were systematically varied in five volunteers to assess tissue contrast in the cervical spinal cord. RESULTS: The 2D GRE parameters for an optimal balance between gray-white matter and CSF-white matter contrast at 3 T were determined to be a time-to-repetition (TR) of 2000 msec and a flip angle of 45 degrees, with the constant short time-to-echo (TE) of 12 msec used in this study. Excellent tissue contrast and visualization of the internal anatomy of the spinal cord was demonstrated reproducibly in eight subjects using these optimal parameters. CONCLUSION: This study demonstrates that imaging the cervical spinal cord and delineating internal spinal cord structures such as gray and white matter is feasible at 3 T.

Cervical Vertebrae↗

PubMed2006

[Library search of UV spectra of organic environmental pollutants based on neural network].

The effects of optimization of network parameters, noise, and impurity on the network were investigated detailedly. To speed up the convergence of the network and enhance the resolution of the library search of UV spectra, the derivative spectra for BP-ANN library search was proposed. The method has a higher tolerance to noise and impurity levels than using ordinary UV spectra, especially to slop background levels. Finally, the resolutions of library search of UV spectra with ANN with optimized parameters were compared with conventional correlation coefficient method. Results showed that the ANN is superior to conventional correlation coefficient method and is an effective method for library search of UV spectra.

English Abstract↗

PubMed1995

A new segmentation method of electroencephalograms by use of Akaike's information criterion.

Non-stationary EEGs, whose statistical properties change with time, were segmented into stationary segments to closely track the behavior of EEG characteristics. We have developed a new segmentation method of optimizing segmentation parameters by using AIC (Akaike's information criterion) as an objective criterion. We applied the segmentation method to EEGs. The instantaneous power spectra of EEGs estimated with wavelet transform were compared with the segmented EEGs. EEGs were recorded from F3, F4, C3, C4, P3, P4, 01 and 02 in 13 normal subjects. Artifact-free 15-s epochs were taken at each electrode location. Each epoch was divided into stationary segments, consisting of several fixed intervals, by optimizing 2 segmentation parameters (interval length and starting point) so that the sum of AICs for several sequences of segments could be the smallest. The EEG segmentation could represent differences in the power spectra between segments. The average length of segments during relaxed wakefulness was 6.0 +/- 3.8 s. The EEG segmentation during mental arithmetic could detect the start of mental arithmetic.

Adolescent↗

PubMed2000

Optimization of the mass reflector parameters for direct ion extraction

The parameters of a mass reflector combined with direct ion extraction are optimized to reduce the time-of-flight spread caused by the initial velocity distribution of ions. Under the assumption that the initial ion velocity distribution is mass independent, one can find the mass range where the spread reaches its minimum by varying the electric field in the ion mirror. The mass dependence of this particular time-of-flight spread can be diminished when the electric field in the ion mirror changes with time. This was theoretically demonstrated for the mass region 5 x 10(3) < m < 10(5) Da, under conditions where the electric field in the mirror increases by approximately 20% during a time interval of several hundred microseconds. Copyright 2000 John Wiley & Sons, Ltd.

Journal Article↗

PubMed2005

Characteristics of activated carbon prepared from pistachio-nut shell by zinc chloride activation under nitrogen and vacuum conditions.

Activated carbons with well-developed pore structures were prepared from pistachio-nut shells by chemical activation using zinc chloride under both nitrogen atmosphere and vacuum conditions. The effects of preparation parameters on the carbon pore structure were studied in order to optimize these parameters. It was found that under vacuum conditions, the characteristics of the activated carbons produced are better than those under nitrogen atmosphere. The impregnation ratio, the activation temperature, and the activation hold time are the important parameters that influence the characteristics of the activated carbons. The optimum experimental conditions for preparing predominantly microporous activated carbons with high pore surface area and micropore volume are an impregnation ratio of 0.75, an activation temperature of 400 degrees C, and a hold time of 1 h. Under these conditions, the BET surface areas of the carbons activated under nitrogen atmosphere and vacuum conditions were 1635.37 and 1647.16 m2/g, respectively. However, at a ZnCl2 impregnation ratio of 1.5, a furnace temperature of 500 degrees C, and a hold time of 2 h, the predominantly mesoporous activated carbon prepared under vacuum condition had a BET surface area of 2527 m2/g. Fourier transform infrared spectra were used to detect changes in the surface functional groups of the samples during the different preparation stages.

Journal Article↗

PubMed2006

Development of gas chromatography-mass spectrometry with microwave distillation and simultaneous solid-phase microextraction for rapid determination of volatile constituents in ginger.

In this study, gas chromatography-mass spectrometry (GC-MS) following microwave distillation and solid-phase microextraction (MD-SPME) was developed for the analysis of essential oil compounds in fresh ginger. In the proposed method, the isolation, extraction and concentration of volatile components in ginger were carried out in one single step, using the MD-SPME technique, and the analytes on the SPME fiber were analyzed by GC-MS. Some parameters, including SPME fiber coating, microwave power and irradiation time, were optimized. The optimal experiment parameters obtained were: 65 microm PDMS/DVB SPME fiber, a microwave power of 400 W and an irradiation time of 2 min. To demonstrate its feasibility, MD-SPME was compared with conventional SPME for the extraction of essential oil compounds in fresh ginger. Using MD-SPME followed by GC-MS, 54 compounds were separated and identified in ginger, which mainly included geranial (5.25%), zingiberene (15.48%), beta-sesquiphellandrene (5.54%) and beta-phellandrene (22.84%), whereas only 39 compounds were separated and identified by conventional SPME followed by GC-MS. The relative standard deviation (R.S.D.) values of less than 10% show that the proposed method has good repeatability. The result show that MD-SPME, followed by GC-MS, is a simple, rapid, solvent-free method for the determination of volatile compounds in ginger.

Gas Chromatography-Mass Spectrometry↗

PubMed2003

Potts ferromagnets on coexpressed gene networks: identifying maximally stable partitions.

Clustering gene expression data by exploiting phase transitions in granular ferromagnets requires transforming the data to a granular substrate. We present a method using the recently introduced homogeneity order parameter Lambda [H. Agrawal, Phys. Rev. Lett. 89, 268702 (2002)]] for optimizing the parameter controlling the "granularity" and thus the stability of partitions. The model substrates obtained for gene expression data have a highly granular structure. We explore properties of phase transition in high q ferromagnetic Potts models on these substrates and show that the maximum of the width of superparamagnetic domain, corresponding to maximally stable partitions, coincides with the minimum of Lambda.

Colonic Neoplasms↗

PubMed2005

Kernel machine-based one-parameter regularized fisher discriminant method for face recognition.

This paper addresses two problems in linear discriminant analysis (LDA) of face recognition. The first one is the problem of recognition of human faces under pose and illumination variations. It is well known that the distribution of face images with different pose, illumination, and face expression is complex and nonlinear. The traditional linear methods, such as LDA, will not give a satisfactory performance. The second problem is the small sample size (S3) problem. This problem occurs when the number of training samples is smaller than the dimensionality of feature vector. In turn, the within-class scatter matrix will become singular. To overcome these limitations, this paper proposes a new kernel machine-based one-parameter regularized Fisher discriminant (K1PRFD) technique. K1PRFD is developed based on our previously developed one-parameter regularized discriminant analysis method and the well-known kernel approach. Therefore, K1PRFD consists of two parameters, namely the regularization parameter and kernel parameter. This paper further proposes a new method to determine the optimal kernel parameter in RBF kernel and regularized parameter in within-class scatter matrix simultaneously based on the conjugate gradient method. Three databases, namely FERET, Yale Group B, and CMU PIE, are selected for evaluation. The results are encouraging. Comparing with the existing LDA-based methods, the proposed method gives superior results.

Algorithms↗

PubMed1999

Functional MR imaging of the human brain using FLASH: influence of various imaging parameters.

In this study the influence of a large variety of imaging parameters on the signal increase (DeltaS) and the contrast-to-noise ratio (CNR) of functional magnetic resonance imaging experiments was determined using FLASH imaging at 2 T. During visual stimulation of the brain we detected significant variations of DeltaS as a function of the echo time (30 ms: 3.5 +/- 0.4%, 60 ms: 6.8 +/- 0.7%), slice thickness (2.5 mm: 6.8 +/- 0.7%, 10.0 mm: 3.3 +/- 0.3%), and pixel size (4.69 mm: 3.1 +/- 0.3%, 1.88 mm: 5.9 +/- 0.5%). Significant changes of DeltaS with flip angle occurred for TE = 20 ms (15 degrees : 2.1 +/- 0.2%, 60 degrees : 3.2 +/- 0.5%). At TE = 30 ms there still was a slight increase (15 degrees : 3.0 +/- 0.4%, 60 degrees : 3.8 +/- 0.5%), while at TE = 50 ms no changes of DeltaS could be detected with flip angle. Furthermore, DeltaS decreased with the use of first-order flow and motion compensation (off: 5.8 +/- 0.6%, on: 4.5 +/- 0.5%). The purpose of this study was to identify the optimal imaging parameters for blood oxygenation level dependent contrast using FLASH imaging at 2 T. Relying on a time normalized contrast-to-noise ratio (CNR(n)) we found the following parameters to be optimal: TE approximately 40-50 ms, a rather low spatial resolution (slice thickness approximately 5.0-7.5 mm, pixel size approximately 2.3-4.6 mm, matrix size 64 x 48), and flip angles lower than 30 degrees. Flow compensation should not be applied, and a rather low bandwidth of approximately 2.5 kHz is favorable, as it yields a superior signal-to-noise ratio.

Adult↗

PubMed2005

Inferring the structure of latent class models using a genetic algorithm.

Present optimization techniques in latent class analysis apply the expectation maximization algorithm or the Newton-Raphson algorithm for optimizing the parameter values of a prespecified model. These techniques can be used to find maximum likelihood estimates of the parameters, given the specified structure of the model, which is defined by the number of classes and, possibly, fixation and equality constraints. The model structure is usually chosen on theoretical grounds. A large variety of structurally different latent class models can be compared using goodness-of-fit indices of the chi-square family, Akaike's information criterion, the Bayesian information criterion, and various other statistics. However, finding the optimal structure for a given goodness-of-fit index often requires a lengthy search in which all kinds of model structures are tested. Moreover, solutions may depend on the choice of initial values for the parameters. This article presents a new method by which one can simultaneously infer the model structure from the data and optimize the parameter values. The method consists of a genetic algorithm in which any goodness-of-fit index can be used as a fitness criterion. In a number of test cases in which data sets from the literature were used, it is shown that this method provides models that fit equally well as or better than the models suggested in the original articles.

Algorithms↗

PubMed1999

An optimized method for in situ hybridization with signal amplification that allows the detection of rare mRNAs.

In situ hybridization (ISH) using nonradioactive probes enables mRNAs to be detected with improved cell resolution but compromised sensitivity compared to ISH with radiolabeled probes. To detect rare mRNAs, we optimized several parameters for ISH using digoxygenin (DIG)-labeled probes, and adapted tyramide signal amplification (TSA) in combination with alkaline phosphatase (AP)-based visualization. This method, which we term TSA-AP, achieves the high sensitivity normally associated with radioactive probes but with the cell resolution of chromogenic ISH. Unlike published protocols, long RNA probes (up to 2.61 kb) readily permeated cryosections and yielded stronger hybridization signals than hydrolyzed probes of equivalent complexity. RNase digestion after hybridization was unnecessary and led to a substantial loss of signal intensity without significantly reducing nonspecific background. Probe concentration was also a key parameter for improving signal-to-noise ratio in ISH. Using these optimized methods on rat taste tissue, we detected mRNA for mGluR4, a receptor, and transducin, a G-protein, both of which are expressed at very low abundance and are believed to be involved in chemosensory transduction. Because the effect of the tested parameters was similar for ISH on sections of brain and tongue, we believe that these methodological improvements for detecting rare mRNAs may be broadly applicable to other tissues. (J Histochem Cytochem 47:431-445, 1999)

Alkaline Phosphatase↗

PubMed2005

Automated estimator parameter selection for an IBM head/disk assembly.

This paper presents the application of a discrete adaptive observer (DAO) to an IBM head/disk assembly system. Because of the difficulties in tuning, a genetic algorithm is implemented off-line to obtain optimal observer parameters for the DAO. Simulations show that the genetic algorithm is successful in choosing appropriate observer gains. Furthermore, as a result of these optimal gains, the observer state and parameter estimates converge accurately and quickly.

Journal Article↗