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At least 1,621 records · Page 90Linked to original sources
PubMed2002

Positron emission tomography compartmental models: a basis pursuit strategy for kinetic modeling.

A kinetic modeling approach for the quantification of in vivo tracer studies with dynamic positron emission tomography (PET) is presented. The approach is based on a general compartmental description of the tracer's fate in vivo and determines a parsimonious model consistent with the measured data. The technique involves the determination of a sparse selection of kinetic basis functions from an overcomplete dictionary using the method of basis pursuit denoising. This enables the characterization of the systems impulse response function from which values of the systems macro parameters can be estimated. These parameter estimates can be obtained from a region of interest analysis or as parametric images from a voxel-based analysis. In addition, model order estimates are returned that correspond to the number of compartments in the estimated compartmental model. Validation studies evaluate the methods performance against two preexisting data led techniques, namely, graphical analysis and spectral analysis. Application of this technique to measured PET data is demonstrated using [11C]diprenorphine (opiate receptor) and [11C]WAY-100635 (5-HT1A receptor). Although the method is presented in the context of PET neuroreceptor binding studies, it has general applicability to the quantification of PET/SPECT radiotracer studies in neurology, oncology, and cardiology.

Artifacts↗
PubMed1994

Single motor unit myoelectric signal analysis with nonstationary data.

The information content of the myoelectric signal (MES) is commonly revealed by statistical measures in the time or frequency domain. Empirical analyses of the MES from a single motor unit have generally assumed that features are invariant with time. Theoretical and experimental work has been done to demonstrate how nonstationary behavior in the discharge statistics of a motor neuron may affect estimates of features extracted from the motor unit's contribution to the MES. Specifically, it has been shown that nonstationary behavior can markedly influence estimates of features describing motor neuron firing behavior and consequently, the low-frequency portion of the MES power spectral density. These results may help to explain the discrepancies in the literature which report empirical models of motor neuron firing statistics.

Algorithms↗
PubMed2005

Spatiotemporal blind source separation approach to atrial activity estimation in atrial tachyarrhythmias.

The analysis and characterization of atrial tachyarrhythmias requires, in a previous step, the extraction of the atrial activity (AA) free from ventricular activity and other artefacts. This contribution adopts the blind source separation (BSS) approach to AA estimation from multilead electrocardiograms (ECGs). Previously proposed BSS methods for AA extraction--e.g., independent component analysis (ICA)--exploit only the spatial diversity introduced by the multiple spatially-separated electrodes. However, AA typically shows certain degree of temporal correlation, with a narrowband spectrum featuring a main frequency peak around 3.5-9 Hz. Taking advantage of this observation, we put forward a novel two-step BSS-based technique which exploits both spatial and temporal information contained in the recorded ECG signals. The spatiotemporal BSS algorithm is validated on simulated and real ECGs from a significant number of atrial fibrillation (AF) and atrial flutter (AFL) episodes, and proves consistently superior to a spatial-only ICA method. In simulated ECGs, a new methodology for the synthetic generation of realistic AF episodes is proposed, which includes a judicious comparison between the known AA content and the estimated AA sources. Using this methodology, the ICA technique obtains correlation indexes of 0.751, whereas the proposed approach obtains a correlation of 0.830 and an error in the estimated signal reduced by a factor of 40%. In real ECG recordings, we propose to measure performance by the spectral concentration (SC) around the main frequency peak. The spatiotemporal algorithm outperforms the ICA method, obtaining a SC of 58.8% and 44.7%, respectively.

Algorithms↗
PubMed1990

Evaluation of a spectrum target prediction model in speech perception.

A model of a spectrum target prediction mechanism is proposed and evaluated by comparing predicted values with results of psychoacoustic experiments. When the trajectory of the cepstrally smoothed LPC spectrum is approximated by a second-order critically damped system, the proposed model can estimate target values using short-period spectrum sequences (50 ms) without being given the onset positions of the spectral transition. Additionally, this model decreases the length of transitional sounds and recovers vowel characteristics neutralized by coarticulation. Moreover, this model compensates for the transitions of syllables and extracts stable characteristics from syllable transitions. This model is applicable to coarticulation recovery in speech signal processing.

Humans↗
PubMed1997

Increment and decrement detection in sinusoids as a measure of temporal resolution.

Measuring thresholds for the detection of brief decrements in the level of a sinusoid is an established method of estimating auditory temporal resolution. Generally, a background noise is added to the stimulus to avoid the detection of the "spectral splatter" introduced by the decrement. Results are often described in terms of a temporal-window model, comprising a band-pass filter, a compressive nonlinearity, a sliding temporal integrator, and a decision device. In this study, thresholds for increments, as well as decrements, in the level of a 55 dB SPL, 4-kHz sinusoidal pedestal were measured as function of increment and decrement duration in the presence of a broadband background noise ranging in spectrum level from -20 to +20 dB SPL. Thresholds were also measured using a 55-dB, 8-kHz pedestal in the absence of background noise. Thresholds for decrements, in terms of the dB change in level (delta L), were found to be more dependent on duration than those for increments. Also, performance was found to be dependent on background-noise level over most levels tested. Neither finding is consistent with the predictions of the temporal-window model or other similar models of temporal resolution. The difference between increment and decrement detection was more successfully simulated by using a decision criterion based on the maximum slope of the temporal-window output.

Adult↗
PubMed1999

Nonstationarity and 1/f noise characteristics in heart rate.

The power spectrum of human heart rate (HR) measured over 24 h exhibits "power-law" 1/f alpha-type spectral behavior with alpha approximately 1. We investigate possible nonstationarity in time of the exponent alpha using maximum likelihood estimation, which allows relatively short data segments to be used. Examination of 24-h HR records from ambulatory normal and congestive heart failure (CHF) subjects indicates that the power-law structure of HR is nonstationary. In addition, alpha varies with time scale and is different for normal (alpha approximately 1) and CHF (alpha approximately 1.5) subjects. Simulations suggest that a possible mechanism underlying the observed power-law spectrum may be a switching between values of alpha near zero (white noise) and near two (Brownian motion). This mechanism generates power-law forms quantitatively similar to CHF subjects when the switching occurs very rapidly and similar to normal subjects when the switching is less rapid.

Artifacts↗
PubMed1976

Photoreceptor thresholds and visual pigment levels in normal and vitamin A-deprived Xenopus tadpoles.

1. Measurements of the aspartate-isolated PIII component of the electroretinogram (ERG) were used to estimate photoreceptor threshold in dark-adapted tadpoles of the clawed toad, Xenopus laevis raised on a normal diet. Spectral sensitivity functions established that the rod mechanism governed absolute dark thresholds from stage 40 to the end of premetamorphic development. 2. Parallel measures of rod outer-segment dimensions and visual pigment levels demonstrated a) that visual pigment concentration remained constant at all tadpole stages, and b) that the fall in threshold over the course of premetamorphic development depended exclusively on the increased probability of quantal absorption that accompanied the growth of the rod outer segments. 3. When tadpoles were obtained from vitamin A-deficient females and raised subsequently on a vitamin A-free diet, the first appearance of the ERG was delayed and its absolute threshold raised, with respect to controls. 4. Histological examination of vitamin A-deprived retinas indicated that rod outer segments retained their structural integrity in spite of up to a 75% loss of visual pigment. 5. The threshold rise which accompanied a fall in visual pigment levels, whether effected by dietary deficiency or by bleaching, was greater than could be attributed solely to a reduction in the probability of quantal absorption.

Age Factors↗
PubMed1988

Computer-aided sleep staging in clinical environment.

In the present paper we will comment on how we have been implementing computer-aided sleep staging (CAS) in the Department of Psychiatry, Erasmus Hospital (Free University of Brussels). Major features of our CAS include real-time stage estimation, full report and hypnogram available in the morning, and evolution of state variables throughout the night (e.g. electroencephalographic spectral characteristics, eye movement density, myographic activity). Parallel, we report an extensive validation study of this system on 50 subjects divided in two groups of 25 patients with psychiatric diseases, and 25 normal subjects.

Adult↗
PubMed1995

Importance of time-window overlap in the detection and analysis of embolic signals.

BACKGROUND AND PURPOSE: The detection of asymptomatic embolic signals by Doppler ultrasound may offer a powerful investigational tool in the management of cerebrovascular disease. However, early studies, particularly in patients with carotid artery disease, have reported very different frequencies of embolic signals. While this may reflect differences in patient groups and the criteria used for embolic signal identification, the degree of time-window overlap may be important. If this is insufficient, some embolic signals may fall between two time windows and not appear on the spectral display. Furthermore, the use of nonrectangular time windows, such as the Hanning window, may result in variation of the intensity of an embolic signal depending on where it is detected within the time window. METHODS: To test the importance of this potential problem, the same 25 embolic signals recorded as the audio signal on digital audiotape were each played repeatedly through a transcranial Doppler ultrasound (TCD) system using fast Fourier transform analysis. An older system with no time-window overlap was used, and a more modern system was also used in which three different degrees of overlap were used: -9%, 27%, and 57%. The number of signals audible but not appearing on the spectral display was recorded. The variability in the relative intensity increase for the same embolic signal played repeatedly was estimated by calculating the coefficient of variation of the relative intensity increase. RESULTS: With the older system, 39/500 (7.8%) of embolic signals were missed. With the newer system, the number of embolic signals missed was fewer and decreased with increasing degrees of overlap (10/500 for -9% overlap, 1/500 for 27% overlap, and 0/500 for 57% overlap). For those setups in which embolic signals were missed, there was a highly significant relationship between duration of embolic signal and number of signals missed. In parallel with these results, the coefficient of variation of the relative intensity increase became progressively less with increasing degrees of time-window overlap. For all processing setups, the coefficient of variation was greater for the less intense and shorter duration signals, but this dependence, as estimated by the slope of the regression line, became less strong with higher degrees of overlap. CONCLUSIONS: Inadequate degrees of fast Fourier transform time-window overlap will result in the failure of current TCD machines to detect embolic signals. Furthermore, this and the time windowing currently usually used may result in variability in the relative intensity increase of identical embolic signals. These factors need to be taken into account when comparing data on the frequencies of embolic signals recorded by different researchers and in the design of future TCD equipment.

Humans↗
PubMed1989

The performance of mechanically scanned color flow mapping.

The performance of mechanically scanned Color Flow Mapping is analysed. It is shown that the Doppler spectrum is convolved with a scaled version of the Fourier transform of the two-way point spread function of the transducer. This spectral broadening is no larger than the inherent limit of the method if the point spread function shows smooth amplitude variation and little phase variation. The spectral broadening can cause clutter from stationary objects to fall outside the MTI filter stopband and, thereby, alter the estimates of the mean velocity and turbulence. Mechanically scanned and electronically stepped Color Flow Mapping are compared.

Color↗
PubMed2005

Effect of bio-optical parameter variability on the remote estimation of chlorophyll-a concentration in turbid productive waters: experimental results.

The analytical development and underlying hypothesis of a three-band algorithm for estimating chlorophyll-a concentration ([Chla]) in turbid productive waters are presented. The sensitivity of the algorithm to the spectral location of the bands used is analyzed. A large set of experimental observations ([Chla] varied between 4 and 217 mg m(-3) and turbidity between 2 and 78 nephelometric turbidity units) was used to calibrate and validate the algorithm. It was found that the variability of the chlorophyll-a fluorescence quantum yield and of the chlorophyll-a specific absorption coefficient can reduce considerably the accuracy of remote predictions of [Chla]. Instead of parameterizing these interferences, their effects were minimized by tuning the spectral regions used in the algorithm. This allowed us to predict [Chla] with a relative root-mean-square error of less than 30%.

Journal Article↗
PubMed2006

Spectral simulation study on the influence of the principal component analysis step on principal component regression.

Principal component regression (PCR) is unique in that the principal component analysis (PCA) step is explicitly involved in the central part of the method. In the present paper, the PCA part is examined in order to study the influence of noise in spectra on PCR by spectral simulation. It has been suggested, as a result, that PCR calibration would have a large inaccuracy when the estimated number of basis factors analyzed by the eigenvalue method is less than that by cross-validation, which was studied by use of synthesized spectra. This instability is because the minute noise is largely enhanced by the PCA calculation via the normalization of loadings. At the same time, the noise enhancement by PCA has also been characterized to influence the estimation of basis factors.

Algorithms↗
PubMed2006

Studying bacterial metabolic states using Raman spectroscopy.

Natural metabolic variability expected during characteristic growth phases in batch cultures of Escherichia coli and Staphylococcus epidermidis were studied by Raman spectroscopy. Spectral changes induced by metabolic changes found in the growth phases (i.e., lag, exponential, stationary, and decay) were identified. Maximum intensity of bands assigned to DNA and RNA bases are seen at the beginning of the exponential phase, when cells are metabolically active, and minimum intensities are seen when cells are decaying. High agreement in spectral variation due to growth phases was seen for all the trials that were performed, four growth cycles for E. coli and two for S. epidermidis. Batch cultures were monitored by standard plate counts to identify all growth phases, including decay. Spectral data were analyzed by principal component analysis (PCA) and discriminant analysis to identify similarities and differences and to estimate a classification performance based on growth phases. For the species evaluated, spectra during decay are grouped closer to each other and separated from lag, exponential, and stationary cells. These results suggest that Raman spectroscopy can be used to study metabolic states in bacteria and in particular cell viability.

DNA, Bacterial↗
PubMedDate not supplied

[The variability characteristics of the arterial pressure in man and animals].

The development of computer monitoring methods over haemodynamic parameters has made possible on objective quantitative estimation of blood pressure (BP) variability in humans and animals which can have both periodic and aperiodic feachers. The spectral analysis of spontaneous BP fluctuations reveals presence of several power peaks main of which, as well as in heart rate variability, reflect sympathetic and parasympathetic activity. Both in hypertensive patients and animal models of arterial hyper tension BP variability is higher, but its relationship with BP level is not realized in a simple causal scheme. Observations in prehypertensive period demonstrate an opportunity of dissociation between rates of BP level and variability increase. The pharmacological analysis permisses to suggest the sympathetic activation as the main cause of BP destabilization. Despite a leading role of baroreceptor reflex (BR) in damping of evoked BP fluctuations, its role in regulation of spontaneous variability seems to be ambiguous. The studies excluding episodes of locomotor and other activities from the curve analysis do not confirm a correlation between BP variability and BR gain. The spectral analysis of BP lability in animals with denervated mechanoreceptor zones has revealed BR effective control only of a low and superlow frequency areas (less than 0.07 Hz). The study of the regional blood flows in sinoartic denervated rats has concluded that the increase of their autonomy is the main reason of BP lability. The offset of BR efferent limb by ganglionic blocking agents or desympathization leads to analogical BP lability. Thus, BP variability seems to be a reflection of functional efficiency of BP stabilization mechanisms. Considering that on its origin, connections with other haemodynamic parameters, amplitude and frequency characteristics is a complex phenomenon, no one of mechanisms is an universal instrument of its regulation.

Animals↗
PubMed2000

A comparison of different time series techniques to analyze phasic coupling: a case study of cardiac and electrodermal activity.

This study examined the efficiency of different time series analysis techniques to extract information on the coupling of spontaneous phasic physiological responses. We compared four bivariate approaches, cross-spectral, cross-covariance, cross-covariance with prewhitening, and dynamic factor analysis, in their ability to yield unbiased estimates of (a) shared variance, (b) covariance, (c) strength of relationship, and (d) interchannel time-lag in empirical and simulated interbeat interval-electrodermal activity (IBI-EDA) time series. All methods produced similar estimates of the grand-averaged IBI-EDA dynamics, but only the measures of covariance produced reliable and unbiased estimates of the interindividual distribution of IBI-EDA coupling. We conclude that the extraction of phasic response patterns during continuous and unrestricted experimental situations may considerably facilitate psychophysiological research.

Adult↗
PubMed1985

Purification and partial characterization of the b-type cytochrome from human polymorphonuclear leukocytes.

Polymorphonuclear leukocytes contain an oxidase system that can be activated to produce superoxide radicals and hydrogen peroxide. A nonmitochondrial b cytochrome, functioning in the generation of these oxygen species, has been purified to apparent homogeneity from human polymorphonuclear phagocytes. After solubilization of the cytochrome with Triton X-100, the cell extract was subsequently chromatographed on Blue Sepharose and Sephacryl S-300. The final preparation was maximally purified 170-fold with a specific content of 5.33 +/- 2.03 nmol mg-1 of protein (mean +/- S.D.; n = 7) and a yield of 21 +/- 13% (n = 5). The apparent molecular mass of the nondenatured cytochrome was estimated by gel filtration to be 235 kDa. Upon polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, a single polypeptide was found with a molecular mass of 127 kDa. From the pyridine hemochrome spectrum 1 protoheme IX/polypeptide was calculated. The light absorbance bands of the dithionite-reduced cytochrome were found to be at 558.5 (alpha), 529 (beta), and 426 nm (Soret), and that of the oxidized cytochrome at 413.5 nm. The difference absorbance coefficients are delta epsilon (426.5 - 440 nm) = 160.6 +/- 11 mM-1 cm-1 and delta epsilon (558.5 - 542 nm) = 29.3 +/- 2 mM-1 cm-1 (mean +/- S.D.; n = 5). Carbon monoxide binds to the cytochrome in a time-dependent fashion (maximum binding after 50-60 min). The midpoint potential of the solubilized nonpurified cytochrome is identical to the cytochrome in situ (Em7.0 = -218 +/- 7 mV (mean +/- S.D.; n = 5)). However, purified cytochrome b shows a significantly decreased midpoint potential, estimated at -407 +/- 18 mV (n = 4). The protein does not contain noncovalently bound FAD or FMN, and no spectral evidence was obtained for the presence of covalently bound flavin. Preliminary amino acid analysis of the cytochrome shows a high content of hydrophilic residues.

Amino Acids↗
PubMed1980

Psychophysiological monitoring of operator's emotional stress in aviation and astronautics.

The level of emotional stress depending on the power of motivation and the estimation by the subject of the probability (possibility) of goal achievement, largely influences the operator's skill performance (that of a pilot, controller, astronaut). A decrease in the emotional tonus leads to drowsiness, lack of vigilance, missing of significant signals, and to slower reactions. The extremely high stress level disorganizes the activity, complicates it with a trend toward untimely acts and reactions to the insignificant signals (false alarms). The best methods to monitor the degree of the operator's emotional state during his skill performance are the integral estimation of the changes in heart-rate and T-peak amplitude, as well as the analysis of spectral and intonational characteristics of the human voice during radio conversation. These methods were tested on paratroopers, pilots in civil aviation, and airport controllers.

Aerospace Medicine↗
PubMedDate not supplied

[Use of electron paramagnetic resonance in assessing viscosity of blood lipids in diabetes mellitus in children].

Viscosity of blood serum lipids was studied in children with insulin-dependent diabetes involving nitroxyl containing probes-registered by means of radiospectrometer RE 1307. The following parameters were studied: the ratios between values exhibiting the probe distribution in polar and hydrophobic phases [formula: see text] as well as between content of low polar (I+1) and high polar (I-1) spectral components. The "f' value showed the state of lipid density and the ratio [formula: see text] allowed to estimate the lipid microviscosity. 34 children of 7-15 years old with insulin-dependent diabetes were examined in dynamics of treatment under stationary conditions. Distinct metabolic impairments and aggravation of the lipid peroxidation syndrome were mainly responsible for pronounced increase in lipid viscosity. The procedure of spin probes may contribute to study of diabetes pathogenesis as well as to serve as a diagnostic means in evaluation of the disease complications at early stages.

Adolescent↗