Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Spectral estimation”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

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

At least 1,279 records · Page 71Linked to original sources
PubMed1990

Spectral analysis and acoustic transmission of mitral and aortic valve closure sounds in dogs. Part 1. Modelling the heart/thorax acoustic system.

A system model based on the simultaneous recording and analysis of the intracardiac and thoracic phonocardiograms to estimate the time-varying properties of the heart/thorax acoustic system of the dog is described. The presence of instrumental noise in the recording of intracardiac phonocardiograms is characterised, and it is demonstrated that its effect on the estimate of the transfer and coherence functions of the system can be quantified and corrected. Application of the model to study the spectral characteristics and the acoustic transmission properties of the mitral component M1 of the first heart sound and of the aortic component A2 of the second heart sound in the dog shows that the heart/thorax acoustic system acts like a bandpass filter having a higher attenuation for A2 than for M1. Between 20 and 100 Hz, the mean attenuation of M1 is 30 dB while that of A2 is 46 dB. Above 100 Hz, the attenuation slope is -12 dB per octave for M1 and -6 dB per octave for A2.

Animals↗
PubMedDate not supplied

Simulation studies of 4-MV x-ray spectral reconstruction by numerical analysis of transmission data.

The use of transmission measurements with combinations of attenuating materials has been investigated as a method of determining the x-ray energy spectrum from a 4-MV linear accelerator. Simulation studies have been made with three 4-MV x-ray spectra of slightly different shapes. From each of these spectra, the narrow beam transmission curve was calculated, using lead to attenuate the beam for the first several points. Aluminum was then added to attenuate the beam to 0.001 of its initial intensity. A computer program using an iterative least-squares technique has been developed to analyze the transmission data and determine the spectrum by applying some physically reasonable sensible constraints. The calculated transmission values agreed with the simulated data within the estimated experimental uncertainties, and it is concluded that the technique is reliable for the spectral reconstruction and is sensitive to small spectral changes.

Computers↗
PubMed1985

Aortic stenosis in adults. Non-invasive estimation of pressure differences by continuous wave Doppler echocardiography.

The peak and mean aortic transvalvar pressure differences measured invasively and non-invasively by continuous wave Doppler echocardiography were compared in 87 consecutive patients with aortic stenosis. The mean values were calculated from the maximal velocities of the aortic jet recorded with a spectral display of the Doppler frequency shifts and by applying a modified Bernoulli equation. Technically satisfactory velocity curves for estimating the mean pressure differences could not be obtained in three patients and invasive measurements were not obtained in two. In all patients the peak transvalvar pressure difference was calculated since the aortic jet was identified non-invasively. The peak and mean pressure differences measured invasively and non-invasively correlated well--with only minor underestimation of the pressure differences measured with the Doppler technique--regardless of age, sex, and the presence or absence of aortic valvar regurgitation, or other valvar lesions. With a systematic search for the highest velocities in the aortic jet and with on line spectral analysis of the Doppler frequencies the peak and the mean aortic pressure differences can be determined non-invasively with a high degree of precision in almost all patients.

Adult↗
PubMed2003

Independent spectral representations of images for recognition.

In recent years, studies have shown that independent components of local windows of natural images resemble the receptive fields of cells in the early stages of the mammalian visual pathway. However, the role of the independence in visual recognition is not well understood. We argue that the independence resolves the curse of dimensionality by reducing the complexity of probability models to the linear order of the dimension. In addition, we show empirically that the complexity reduction does not degrade the recognition performance on all the data sets that we have used with an independent spectral representation. In this representation, an input image is first decomposed into independent channels given by the estimated independent components from training images, and each channel's response is then summarised by using its histogram as an estimate of the underlying probability model along that dimension. We demonstrate the sufficiency of the proposed representation for image characterization by synthesizing textures and objects through sampling and for recognition by applying it to large data sets. Our comparisons show that the independent spectral representation often gives improved recognition performance.

Journal Article↗
PubMed2006

Anterior-posterior biphonation in a finite element model of vocal fold vibration.

In this paper, a finite-element model is used to simulate anterior-posterior biphonation [Neubauer et al., J. Acoust. Soc. Am. 110(6), 3179-3192 (2001)]. The anterior-posterior stiffness asymmetric factor and the anterior-posterior shape asymmetric factor describe the asymmetry properties of vocal folds. Spatiotemporal plot, spectral analysis, anterior-posterior fundamental frequency ratio, cross covariation function, and correlation length quantitatively estimate the spatial asymmetry of vocal fold oscillations. Calculation results show that the anterior-posterior stiffness asymmetry decreases the spatial coherence of vocal fold vibration. When the stiffness asymmetry reaches a certain level, the drop in spatial coherence desynchronizes the vibration modes. The anterior and posterior sides of the vocal fold oscillate with two independent fundamental frequencies (f(a) and f(p)). The complex spectral characteristics of vocal fold vibration under biphonation conditions can be explained by the linear combination of f(a) and f(p). Empirical orthogonal eigenfunctions prove the existence of higher-order anterior-posterior modes when anterior-posterior biphonation occurs. Then, it is found that the anterior-posterior shape asymmetry also decreases the spatial coherence of vocal fold vibration, and shape asymmetry is a possible reason for anterior-posterior biphonation.

Humans↗
PubMed2006

Entropy indices vs the bispectral index for estimating nociception during sevoflurane anaesthesia.

BACKGROUND: It is now possible to acquire and process raw EEG and frontal EMG signals to produce two spectral-entropy-based indices (response entropy and state entropy) reflective of analgesic and hypnotic levels during general anaesthesia (with the Datex-Ohmeda S/5 Entropy Module, Datex-Ohmeda, Helsinki, Finland). However, there are no data available on the accuracy of the Entropy Module in estimating nociception during sevoflurane anaesthesia. METHODS: Forty female patients were enrolled in the present study. Each patient was allocated randomly to one of four end-tidal sevoflurane concentration (ET(sev)) groups (1.3, 1.7, 2.1 or 2.5%). A BIS Sensor (Aspect Medical Systems, Newton, MA) and an Entropy Sensor (Datex-Ohmeda) were applied side-by-side to the forehead. The bispectral index (A-2000 BIS Monitor, version 3.4, Aspect Medical Systems), response entropy, state entropy and patient movement were observed after electrical stimulation (20, 40, 60 and 80 mA, 100 Hz, 5 s) and after skin incision during sevoflurane anaesthesia (1.3, 1.7, 2.1 or 2.5%). Accuracy of the EEG variables in differentiating the intensity of electrical stimulation was estimated by the prediction probability (P(K)) values. RESULTS: Response entropy and state entropy [median, (range)] before skin incision were significantly lower in patients who did not move [29 (15-41) and 24 (14-41)] than in those that did [38 (24-53) and 37 (24-52)], but there was no significant difference in BIS. All EEG variables increased significantly (P<0.0001 for all) with increases in the intensity of electrical stimulation. The difference between response entropy and state entropy increased with increases in the electrical stimulation (P<0.0001). However, no EEG variables could differentiate the intensity of the electrical stimulations accurately because of low P(K)-values (P(K)<0.8). CONCLUSION: Noxious stimulation increased the difference between response entropy and state entropy. However, an increase in the difference does not always indicate inadequate analgesia and should be interpreted carefully during anaesthesia.

Adult↗
PubMed1981

Validation of a radioimmunoassay for indole-3-acetic Acid using gas chromatography-selected ion monitoring-mass spectrometry.

A radioimmunoassay for indole-3-acetic acid (IAA) has been validated by comparison with a physico-chemical assay utilizing gas chromatography-selected ion monitoring-mass spectrometry and 4,5,6,7-tetradeutero-indole-3-acetic acid as an internal standard. The radioimmunoassay provided a reliable estimate of the free IAA content of etiolated corn shoots. However, base hydrolysis of extracts for determination of ester IAA released substances which interfered with the radioimmunoassay. Interference was detected by internal controls and by lack of agreement with the mass spectral method. Interfering compounds could be removed from extracts by chromatography on diethylaminoethyl- and hydroxypropylated (lipophilic) Sephadex G-25. Following such purification the radioimmunoassay estimate of the total (free + ester) IAA content of etiolated corn shoots agreed with the mass spectral method within 2% on the average.

Journal Article↗
PubMed1991

Underestimation of visual texture slant by human observers: a model.

The perspective image of an obliquely inclined textured surface exhibits shape and density distortions of texture elements which allow a human observer to estimate the inclination angle of the surface. However, it has been known since the work of Gibson (1950) that, in the absence of other cues, humans tend to underestimate the slant angle of the surface, particularly when the texture is perceived as being "irregular." The perspective distortions which affect texture elements also shift the projected spatial frequencies of the texture in systematic ways. Using a suitable local spectral filter to measure these frequency gradients, the inclination angle of the surface may be estimated. A computational model has been developed which performs this task using distributions of outputs from filters found to be a good description of simple-cell receptive fields. However, for "irregular" textures the filter output distributions are more like those of "regular" textures at shallower angles of slant, leading the computational algorithm to underestimate the slant angle. This behavioral similarity between human and algorithm suggests the possibility that a similar visual computation is performed in cortex.

Algorithms↗
PubMed2004

Single motor unit and spectral surface EMG analysis during low-force, sustained contractions of the upper trapezius muscle.

Intramuscular and surface electromyographic (EMG) activities were recorded from the left and right upper trapezius muscle of eight healthy male subjects during 5-min long static contractions at 2% and 5% of the maximal voluntary contraction (MVC) force. Intramuscular signals were detected by wire electrodes while surface EMG signals were recorded with linear adhesive electrode arrays. The surface EMG signals were averaged using the potentials extracted from the intramuscular EMG decomposition as triggers. The conduction velocity of single motor units (MUs) was estimated over time from the averaged surface potentials while average rectified value and mean power spectral frequency were computed over time from 0.5 s epochs of surface EMG signal. It was found that (1) MUs were progressively recruited after the beginning of sustained contractions of the upper trapezius muscle at 2% and 5% MVC, (2) the conduction velocity of the MUs active since the beginning of the contraction significantly decreased over time, and (3) although the CV of single MUs significantly decreased, the mean power spectral frequency of the surface EMG did not show a consistent trend over time. It was concluded that spectral surface EMG analysis, being affected by many physiological mechanisms, may show limitations for the objective assessment of localized muscle fatigue during low force, sustained contractions. On the contrary, single motor unit conduction velocity may provide an early indication of changes in muscle fiber membrane properties with sustained activity.

Action Potentials↗
PubMed2005

Cardiac autonomic modulation in hypertensive patients with Chagas' disease.

BACKGROUND: Arterial hypertension and Chagas' disease are prevalent pathologies in Latin America. It has been demonstrated that each one of them may cause cardiac autonomic dysfunction. This study aimed to investigate the pattern of cardiac autonomic modulation in chagasic-hypertensive patients. METHODS: Subjects (n=120) without left ventricular dysfunction were distributed in four groups: healthy control (n=30); hypertensive (n=30); chagasic (n=30) and chagasic-hypertensive (n=30). Patients were evaluated by autoregressive spectral analysis of heart rate variability in three different conditions: baseline, cold face and passive tilt tests. Power spectral densities in low (0.04-0.15 Hz) and high (0.15-0.50 Hz) frequency bands were estimated in both absolute and normalized units. RESULTS: Baseline median values (percentile 25 to percentile 75) of mean arterial pressure (in mmHg) were 93.3 (85.0-96.7), 116.7 (*, #) (110.0-129.2), 86.7 (83.3-92.5) and 106.7 (*, #) (106.7-110.0) for healthy control, hypertensive, chagasic and chagasic-hypertensive patients, respectively (*p<0.05 versus healthy control, #p<0.05 against chagasic group). Heart rate at rest did not differ among groups. Regarding to spectral parameters in baseline conditions, the absolute power of high frequency component of heart rate variability of the chagasic-hypertensive group was significantly lower than that found in healthy control and hypertensive patients. There were no differences in spectral parameters responses during cold face test. After passive tilt test, however, decreases in high frequency oscillations and increases in sympathovagal balance (low and high frequency ratio) were significantly lower in hypertensive, chagasic and chagasic-hypertensive patients as compared with healthy control. CONCLUSIONS: These data indicate that chagasic-hypertensive patients presented an impairment of cardiac parasympathetic modulation at baseline conditions as well as in response to passive orthostatic stress.

Adult↗
PubMed1997

Application of factor analysis of infrared spectra for quantitative determination of beta-tricalcium phosphate in calcium hydroxylapatite.

This work represents a method, which allows to determine phase composition of calcium hydroxylapatite basing on its infrared spectrum. The method uses factor analysis of the spectral data of calibration set of samples to determine minimal number of factors required to reproduce the spectra within experimental error. Multiple linear regression is applied to establish correlation between factor scores of calibration standards and their properties. The regression equations can be used to predict the property value of unknown sample. The regression model was built for determination of beta-tricalcium phosphate content in hydroxylapatite. Statistical estimation of quality of the model was carried out. Application of the factor analysis on spectral data allows to increase accuracy of beta-tricalcium phosphate determination and expand the range of determination towards its less concentration. Reproducibility of results is retained.

Biocompatible Materials↗
PubMed1983

Resonance energy transfer between the active sites of myocardial-type creatine kinase (isozyme MB).

The single reactive sulfhydryl group, located in the active site of each subunit of dimeric creatine kinase from rabbit muscle (isozyme MM), was selectively labeled with 3-(4-maleimidylphenyl)-7-(diethylamino)-4-methylcoumarin (CPM). Isozyme BB, purified to homogeneity from rabbit brain, was conjugated with the sulfhydryl-specific reagent 5'-(iodoacetamido)fluorescein (5'-IAF). Spectral analyses demonstrated that 1.8 mol of CPM and 1.9 mol of 5'-IAF had reacted per mol of protein. Labeled isozymes were combined, denatured in 8 M urea, and renatured by dialysis, producing the parent labeled homodimers and forming the heterolabeled hybrid dimer, creatine kinase MB. Similar hybridizations were performed to prepare singly labeled hybrids, starting with labeled and unlabeled homodimers. The hybrid isozymes were isolated by ion-exchange chromatography, and spectral analyses of singly labeled heterodimers revealed overlap between the absorption spectrum of MB labeled with acetamidofluorescein on the B subunit and the corrected fluorescence emission spectrum of MB labeled with CPM on the M subunit. Analyses included evaluation of the quantum yield of the CPM-labeled hybrid, estimation of the range of the orientation factor K2 from fluorescence polarization and anisotropy studies, and determination of J, the spectral overlap integral for the fluorescence donor (CPM-labeled MB) and acceptor (acetamidofluorescein-labeled MB). Results of these experiments permitted an estimation of R0, the distance between the donor and the acceptor at which energy transfer is 50% efficient. Comparison of the relative fluorescence of the donor in the presence (heterolabeled hybrid) and absence (hybrid conjugated with CPM on the M subunit) of the acceptor or determination of the normalized sensitization of the acceptor fluorescence led to an evaluation of the transfer efficiency and the actual transfer distance of between 27 and 52 A.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2004

A self-learning coupled map lattice for vortex shedding in cable and cylinder wakes.

A coupled map lattice (CML) with self-learning features is developed to model flow over freely vibrating cables and stationary cylinders at low Reynolds numbers. Coupled map lattices that combine a series of low-dimensional circle maps with a diffusion model have been used previously to predict qualitative features of these flows. However, the simple nature of these CML models implies that there will be unmodeled wake features if a detailed, quantitative comparison is made with laboratory or simulated wake flows. Motivated by a desire to develop an improved CML model, we incorporate self-learning features into a new CML that is first trained to precisely estimate wake patterns from a target numerical simulation. A new convective-diffusive map that includes additional wake dynamics is developed. The new self-learning CML uses an adaptive estimation scheme (multivariable least-squares algorithm). Studies of this approach are conducted using wake patterns from a Navier-Stokes solution (spectral element-based NEKTAR simulation) of freely vibrating cable wakes at Reynolds numbers Re=100. It is shown that the self-learning model accurately and efficiently estimates the simulated wake patterns. The self-learning scheme is then successfully applied to vortex shedding patterns obtained from experiments on stationary cylinders. This constitutes a first step toward the use of the self-learning CML as a wake model in flow control studies of laboratory wake flows.

Journal Article↗
PubMed2004

Design of a mechanical-tunable filter spectrometer for noninvasive glucose measurement.

The development of an accurate and reliable noninvasive near-infrared (NIR) glucose sensor hinges on the success in addressing the sensitivity and the specificity problems associated with the weak glucose signals and the overlapping NIR spectra. Spectroscopic hardware parameters most relevant to noninvasive blood glucose measurement are discussed, which include the optical throughput, integration time, spectral range, and the spectral resolution. We propose a unique spectroscopic system using a continuously rotating interference filter, which produces a signal-to-noise ratio of the order of 10(5) and is estimated to be the minimum required for successful in vivo glucose sensing. Using a classical least-squares algorithm and a spectral range between 2180 and 2312 nm, we extracted clinically relevant glucose concentrations in multicomponent solutions containing bovine serum albumin, triacetin, lactate, and urea.

Algorithms↗
PubMed2000

Algorithms for estimating blood velocities using ultrasound.

Ultrasound has been used intensively for the last 15 years for studying the hemodynamics of the human body. Systems for determining both the velocity distribution at one point of interest (spectral systems) and for displaying a map of velocity in real time have been constructed. A number of schemes have been developed for performing the estimation, and the various approaches are described. The current systems only display the velocity along the ultrasound beam direction and a velocity transverse to the beam is not detected. This is a major problem in these systems, since most blood vessels are parallel to the skin surface. Angling the transducer will often disturb the flow, and new techniques for finding transverse velocities are needed. The various approaches for determining transverse velocities will be explained. This includes techniques using two-dimensional correlation (speckle tracking), multiple beams, and the new transverse modulation technique. The different advantages and disadvantages of the approaches are explained.

Algorithms↗
PubMed2001

Blood pressure and heart rate variability and baroreflex sensitivity before and after brain death.

OBJECTIVES: To evaluate spontaneous blood pressure and heart rate variability and spontaneous baroreflex sensitivity before and after brain death. METHODS: Spontaneous variability of arterial blood pressure and heart rate-estimated by power spectra of systolic (SBP) and diastolic blood pressure (DBP) and pulse interval (PI)-and spontaneous baroreflex sensitivity (BRS)-estimated by the alpha index and the sequence technique-were evaluated in 11 patients twice: shortly before and 1 hour after the onset of brain death. RESULTS: Significant spectral changes occurred after brain death: a general power reduction in PI spectra; a shift of SBP, DBP and PI powers toward the lower frequencies, resulting in a greater slope of the "1/f" spectral trends; and a marked reduction of SBP and DBP powers (-93%) and of SBP-PI coherence (-63%) at 0.1 Hz. The estimated average BRS was relatively high before brain death (around 11 ms/mm Hg), and fell close to 0 or even was not detectable at all after brain death. CONCLUSIONS: Parameters describing spontaneous blood pressure and heart rate variability and indexes reflecting the baroreflex function, which were relatively normal up to a few hours before brain death, underwent marked changes with the onset of brain death. All the changes found are likely to reflect the cessation of activity of the cardiovascular brain stem centres. These findings indicate that techniques of blood pressure and heart rate spectral analysis and of dynamic assessment of baroreflex sensitivity may be useful to complement the diagnosis of brain stem death.

Adult↗
PubMed1975

Measurement of spectral bandwidth, as exemplified with the Beckman "Enzyme Analyzer System TR spectrophotometer".

This paper describes simple methods for determining spectral bandwidth by measurement and by calculation. We measured the spectral bandwidth of the monochromator from the Beckman Enzyme Analyzer TR, and found it to be 3.5-4.0 nm (av 3.8) by two different methods. We visually estimated it to be 4 nm. Using the specifications for the diffraction grafting, focal length, and fixed slit width, we calculated the theoretical spectral bandwidth to be 3.6 nm.

Autoanalysis↗
PubMed2005

Experimental design for the estimation of photophysical parameters of the two-state excited-state proton-exchange reaction in the presence of pH buffer.

The excited-state proton-exchange reaction of commonly used fluorescent pH probes at physiological pH becomes reversible upon addition of pH buffer. Using computer-generated fluorescence decay surfaces, we investigated under which experimental conditions (pH, buffer concentration, and excitation and emission wavelengths) the rate constants describing the excited-state processes (k(ij)) and the spectral parameters related to excitation ((~)b(1)) and emission ((~)c(1)) can be accurately and precisely estimated by global compartmental curve fitting. It was found that a minimum of three fluorescence decay traces should be collected for the pH probe in the presence of buffer. These three decays should be characterized by at least two different pH values and at least two different buffer concentrations. In addition to these three traces, a minimum of one trace corresponding to the pH probe without buffer has to be recorded. Furthermore, for the accurate estimation of k(ij), (~)b(1), and (~)c(1), at least two of these traces should be collected at the same pH and excitation and emission wavelengths. The experimental conditions should be chosen in such a way that decays with unambiguous biexponential character are obtained. For fluorescent pH probes with pK(a) approximately equal to 7 that are responsive in the near-neutral pH range, it is advisable to use buffers with pK(B)(a) values comparable to or higher than the pK(a) of the probe. Because the changes in the decay times are already apparent with a small quantity of buffer, there is no need to use excessively high buffer concentrations. From a practical point of view, the best experimental design is attained when one combines in a single fluorescence decay surface traces originating from samples characterized by different pH values at the same buffer concentration with traces characterized by different buffer concentrations at the same pH and decays of samples without buffer measured at several pH values.

Buffers↗