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 847 records · Page 47Linked to original sources
PubMed2002

Time and frequency analysis of beat-to-beat R-T interval variability in patients with ischaemic left ventricular dysfunction providing evidence for non-neural control of ventricular repolarisation.

BACKGROUND: Determinants of temporal lability in ventricular repolarisation are not fully recognised. We aimed to analyse the sources of RT variability by comparing normal subjects and patients after myocardial infarction (MI) with either depressed or preserved left ventricular (LV) function. METHODS: One hundred and nine patients (27 women, 82 men, aged 51 +/- 9 years) were divided into three groups: 24 patients (pts) with an uncomplicated angiographically proven coronary heart disease (CHD-group), 59 post-MI pts with preserved LV function (LVEF > 40%, PMI-N-group) and 26 post-MI pts with depressed LV function (LVEF < 40%, PMI-L-group). An ECG signal of low-noise 512 heartbeats was recorded using a computer-assisted amplifier (16 bit, 2 kHz). The onset and offset of the R-wave and T-wave were determined automatically. The magnitude of R-R and R-T variability was measured as the standard deviation of all intervals (SD-RR and SD-RT, ms, respectively). Their relationship was quantified by the correlation coefficient r(RT/RR). Power spectral density of RR or RT variability was estimated with the FFT (Welch's averaged periodogram, Hanning window) and frequency relation was quantified using a squared coherence spectrum (SCS). For all spectral and cross-spectral measurements two frequency ranges were considered: high (0.15-0.50 Hz, HF) and low (0.04-0.15 Hz, LF). Spectral power and SCS of RR and RT variability for both ranges (HF(RR), LF(RR), HF(RT), LF(RT), SCS(HF), SCS(LF)), and the ratios LF/HF(RR) and LF/HF(RT) were drawn for comparisons. The central frequency of HF(RR) was considered as the frequency of respiration (f(resp), Hz). RESULTS: In the PMI-L group the SD-RT was significantly greater compared to the remaining groups and accounted for almost 10% of the SDRR. Also, the coefficient r(RT/RR) was weakest in this group. The spectral indices of RR variability were similar in all groups, while the greatest value of the HFRT was observed in the PMI-L group. The SCS(LF) was insignificant in this group, contrary to the CHD and PMI-N groups. Additionally, there were significant negative relationships between f(resp) and spectral indices of RT variability in PMI-patients with depressed LV function. CONCLUSION: A greater beat-to-beat variation in RT interval duration along with increased power of its HF component indicates an important role of respiration in ventricular repolarisation control, while reduced time- and frequency RT-RR relationships seem to relate to an impaired process of ventricular duration adaptation.

Adult↗
PubMed2001

Fibrillation power, an alternative method of ECG spectral analysis for prediction of countershock success in a porcine model of ventricular fibrillation.

BACKGROUND: Noninvasive prediction of defibrillation success after cardiac arrest and cardiopulmonary resuscitation (CPR) may help in determining the optimal time for a countershock, and thus increase the chance for survival. METHODS: In a porcine model (n=25) of prolonged cardiac arrest, advanced cardiac life support was provided by administration of two or three doses of either vasopressin or epinephrine after 3 or 8 min of basic life support. After 4 min of ventricular fibrillation and 18 min of life support, defibrillation was attempted. The denoised power spectral density of 10 s intervals of the ventricular fibrillation electrocardiogram (ECG) was estimated from averaged and smoothed Fourier transforms. We have eliminated the spectral contribution of artifacts from manual chest compressions and provide a definition for the contribution of ventricular fibrillation to the power spectral density. This contribution is quantified and termed "fibrillation power". RESULTS: We tested fibrillation power and two established methods in their discrimination of survivors (n=16) vs. non-survivors (n=9) in the last minute before the countershock. A fibrillation power > or =79 dB predicted successful defibrillation with sensitivity, specificity, positive predictive value and negative predictive value of 98%, 98%, 99% and 97% while a mean fibrillation frequency > or =7.7 Hz was predictive with 85%, 83%, 90% and 77% and a mean amplitude > or =0.49 mV was predictive with 95%, 90%, 94% and 91%. CONCLUSIONS: We suggest that fibrillation power is an alternative source of information on the status of a fibrillating heart and that it may match the established mean frequency and amplitude analysis of ECG in predicting successful countershock during CPR.

Animals↗
PubMed2001

Cramér-Rao bounds: an evaluation tool for quantitation.

The Cramér-Rao lower bounds (CRBs) are the lowest possible standard deviations of all unbiased model parameter estimates obtained from the data. Consequently they give insight into the potential performance of quantitation estimators. Using analytical CRB expressions for spectral parameters of singlets and doublets in noise, one is able to judge the precision as a function of spectral and experimental parameters. We point out the usefulness of these expressions for experimental design. The influence of constraints (chemical prior knowledge) on spectral parameters of the peaks of doublets is demonstrated and the inherent benefits for quantitation are shown. Abbreviations used: CRB Cramér-Rao lower bounds

Animals↗
PubMed2003

Applications of Fourier transform infrared microspectroscopy in studies of benign prostate and prostate cancer. A pilot study.

Fourier transform infrared (FTIR) microspectroscopy has been applied to a study of prostate cancer cell lines derived from different metastatic sites and to tissue from benign prostate and Gleason-graded malignant prostate tissue. Paraffin-embedded tissue samples were analysed by FTIR, after mounting onto a BaF(2) plate and subsequent removal of wax using Citroclear followed by acetone. Cell lines were analysed as aliquots of cell suspension held between two BaF(2) plates. It was found that the ratio of peak areas at 1030 and 1080 cm(-1), corresponding to the glycogen and phosphate vibrations respectively, suggests a potential method for the differentiation of benign from malignant cells. The use of this ratio in association with FTIR spectral imaging provides a basis for estimating areas of malignant tissue within defined regions of a specimen. Initial chemometric treatment of FTIR spectra, using the linear discriminant algorithm, demonstrates a promising method for the classification of benign and malignant tissue and the separation of Gleason-graded CaP spectra. Using the principle component analysis, this study has achieved for the first time the separation of FTIR spectra of prostate cancer cell lines derived from different metastatic sites.

Adenocarcinoma↗
PubMed2006

Study of taurine and tauret content in the compound eye of locust with light and dark adaptation.

Taurine as well as tauret (retinyliden taurine) levels were measured in locust Locusta migratoria compound eyes. HPLC measurements revealed relatively low taurine levels (1.9 +/- 0.16 mM) in dark-adapted eyes. Glutamate, aspartate and glycine levels were 2.0 +/- 0.2, 2.7 +/- 0.4 and 3.0 +/- 0.37 mM, respectively, while GABA was present only in trace amounts. After about 4 h of light adaptation at 1500-2000 lx, amino acid levels in the compound eye were as follows: taurine, 1.8 +/- 0.17 mM; glutamate, no change at 2.1 +/- 0.2 mM; aspartate sharply increased to 4.7 +/- 0.7 mM; glycine slightly decreased to 2.8 +/- 0.3 mM; and GABA trace levels. In the compound eye of locust Locusta migratoria, the existence of endogenous tauret in micro-molar range was established. In the dark, levels were several times higher compared with compound eye after light adaptation 1500 lx for 3 h, as estimated by TLC in combination with spectral measurements. Existence of tauret in compound eye is of special interest because in the compound eye, rhodopsin regeneration is based on photoregeneration.

Animals↗
PubMed1985

Computerized EEG pattern classification by adaptive segmentation and probability-density-function classification. Description of the method.

A phenomenological model for the representation of clinical EEGs is proposed. It assumes each individual record to consist of a few repetitive patterns which are described sufficiently by their power spectra. An algorithm for automatic EEG evaluation is described. It consists of two steps, a segmentation process which isolates the elementary patterns, and a clustering procedure which groups similar patterns with each other. Results are represented in graphical form. Diagnostic classification is not attempted. An appendix highlights the advantages of autoregressive modelling for EEG spectral analysis and, in particular, the estimation of the power contained in the various "rhythms".

Biometry↗
PubMed1987

Macular pigment and reduced foveal short-wavelength sensitivity in retinitis pigmentosa.

Some patients with retinitis pigmentosa (RP) show a reduced foveal short-wavelength sensitivity that cannot be attributed to a reduction in the sensitivity of the short-wavelength cone system. To determine whether an increased amount of macular pigment (xanthophyll) might account for this finding, we derived estimates of the two-way optical density of the macular pigment of five such RP patients as well as of five normals. The spectral reflectance of the foveal region of each subject was obtained from digitized images of the bleached fundus provided by a television-based reflectometer. The density spectra of the macular pigment, melanin, and oxygenated hemoglobin were fit by a least-squares procedure to the log of the ratio of parafoveal to foveal spectral reflectance in order to obtain a quantitative estimate of the contribution of each of these ocular pigments to foveal short-wavelength reflectance. By this analysis, the two-way densities of the macular pigment, melanin, and oxyhemoglobin of the RP patients were not significantly different from those of the normals. Therefore, the reduced foveal short-wavelength sensitivity of these patients was not due to an increased amount of macular pigment, but may result instead from morphological abnormalities in the foveal cones such that a normal amount of macular pigment screens the cones more effectively.

Adult↗
PubMed1993

Light scattering in the eye of the blowfly chalky mutant: the effect on spectral sensitivity of photoreceptors R1-6.

In non-pigmented insect eyes, stray light causes the narrowing of the spectral sensitivity functions of the visual cells, and this effect is shown to be a sensitive tool for estimation of the stray light absorption by a photoreceptor. Spectral sensitivity functions of single cells of Calliphora chalky mutants were obtained in three sets of experiments with different contribution of stray light in total light absorption by a visual cell, and the relative parasitic absorption due to stray light appeared to be as high as 70% of total absorption even with a point light source. The effect of light scattering may be of some functional significance in non-pigmented parts of the wild type eyes as, for instance, in the dorsal rim area of the crickets.

Animals↗
PubMed2005

Human cervical lymphadenopathy: evaluation with in vivo 1H-MRS at 1.5 T.

AIM: To evaluate the feasibility of performing in vivo proton magnetic resonance spectroscopy ((1)H-MRS) of cervical lymph nodes, and the clinical usefulness of the technique in the characterization of cervical lymphadenopathy. MATERIALS AND METHODS: Cervical lymphadenopathy was examined in 20 individuals with malignant disease, i.e. 10 with squamous cell carcinoma (SCC), 6 with undifferentiated carcinoma (UDC) and 4 with non-Hodgkin's lymphoma (NHL). Cervical lymphadenopathy was also examined in 4 individuals with benign disease, i.e. 3 with tuberculosis (TB) and 1 with Castleman's disease. A point-resolved spectroscopic sequence with echo times (TE) of 136 and 272 ms and a time-domain spectral fitting procedure were used to estimate peak amplitude of choline (Cho), creatine (Cr) and unsuppressed water. Cho/Cr and Cho/water ratios were measured for each lesion. The mean ratio for each lesion group was obtained and results were compared statistically. RESULTS: At TE of 136 ms, spectra were interpretable in all 24 cases and a Cr peak was identified with post-processing in 15 cases. The Cho/Cr and Cho/water ratios for NHL were 9.1 +/- 5.2 and 1.7 +/- 0.2 x 10(-3), for UDC were 4.4 +/- 0.9 and 1.2 +/- 0.4 x 10(-3), and for SCC were 2.1 +/- 0.6 and 0.5 +/- 0.3 x 10(-3), respectively. Both Cho/Cr and Cho/water ratios for UDC were significantly higher than SCC (p = 0.002 and 0.026, respectively). At TE of 272 ms, spectra were interpretable in 22 of 24 cases and a Cr peak was identified with post-processing in 11 cases. Cho/Cr and Cho/water ratios for NHL were 5.4 and 4.6 +/- 1.4 x 10(-3), for UDC were 4.2 +/- 1.5 and 2.6 +/- 1.0 x 10(-3) and for SCC were 2.5 +/- 1.1 and 1.3 +/- 0.6 x 10(-3), respectively. The Cho/water ratio for UDC was significantly higher than for SCC (p = 0.04). The Cho/Cr ratio for UDC was also higher than for SCC, but this difference was not statistically significant (p = 0.07). Neither Cho nor Cr was detected in the 3 cases of TB. CONCLUSION: In vivo (1)H-MRS is a feasible technique for the evaluation of cervical lymph nodes and it offers potential as a clinical tool in the investigation of cervical lymphadenopathy. However, further studies with larger patient cohorts are needed to validate the findings of this initial report.

Adult↗
PubMed2006

High serum high-sensitivity C-reactive protein concentrations are associated with relative cardiac sympathetic overactivity during the early morning period in type 2 diabetic patients with metabolic syndrome.

Sympathetic activation is associated with metabolic syndrome (MS) and increased risk of cardiovascular disease. The aim of this study was to investigate whether cardiac autonomic activity or sympathovagal balance, as estimated by a 24-hour power spectral analysis of heart rate variation, is associated with serum concentrations of high-sensitivity C-reactive protein (hs-CRP), a sensitive predictor for cardiovascular events, in type 2 diabetic patients with and without MS. We studied 104 type 2 diabetic patients (50 female and 54 male). The diagnosis of MS was based on the National Cholesterol Education Program Adult Treatment Panel III criteria. Based on the serum hs-CRP, diabetic patients were also divided into 3 groups: low risk (CRP < 1.0 mg/L), moderate risk (1.0 < or = CRP < or = 3.0), and high risk (CRP > 3.0). Heart rate variation was determined automatically every 5 minutes over 24 hours using an ambulatory Holter electrocardiographic recording. Power spectral analysis of the R-R intervals was performed by fast Fourier transformation. Low frequency (LF, both sympathetic and parasympathetic activities), high frequency (HF, pure parasympathetic activity), and the ratio of LF to HF, an index of sympathovagal balance, were used as indices of cardiac autonomic activity. Blood concentrations of hs-CRP, interleukin 6, and plasminogen activator inhibitor 1 were higher in diabetic patients with than in those without MS (P < .0001, P = .0056, and P < .0001, respectively). Both the 24-hour mean LF and the LF-to-HF ratio were also significantly higher in diabetic patients with than in those without MS (P = .0397 and P = .0483, respectively). The LF-to-HF ratio at 6:00 am was significantly higher in diabetic patients with a high CRP concentration than in those with a low or moderate CRP concentration (P < .001 and P < .01, respectively). Only urinary albumin and hs-CRP were independent factors predicting the LF-to-HF ratio at 6:00 am in diabetic patients. In conclusion, type 2 diabetic patients with MS have elevated markers of inflammation and evidence of cardiac sympathetic predominance. High serum concentrations of hs-CRP are associated with relative cardiac sympathetic overactivity during the early morning in type 2 diabetic patients.

Adult↗
PubMed2004

Detection of pseudoperiodic patterns using partial acquisition of magnetic resonance images.

Improving the resolution of magnetic resonance imaging (MRI), or, alternatively, reducing the acquisition time, can be quite beneficial for many applications. The main motivation of this work is the assumption that any information that is a priori available on the target image could be used to achieve this goal. In order to demonstrate this approach, we present a novel partial acquisition strategy and reconstruction algorithm, suitable for the special case of detection of pseudoperiodic patterns. Pseudoperiodic patterns are frequently encountered in the cerebral cortex due to its columnar functional organization (best exemplified by orientation columns and ocular dominance columns of the visual cortex). We present a new MRI research methodology, in which we seek an activity pattern, and a pattern-specific experiment is devised to detect it. Such specialized experiments extend the limits of conventional MRI experiments by substantially reducing the scan time. Using the fact that pseudoperiodic patterns are localized in the Fourier domain, we present an optimality criterion for partial acquisition of the MR signal and a strategy for obtaining the optimal discrete Fourier transform (DFT) coefficients. A by-product of this strategy is an optimal linear extrapolation estimate. We also present a nonlinear spectral extrapolation algorithm, based on projections onto convex sets (POCSs), used to perform the actual reconstruction. The proposed strategy was tested and analyzed on simulated signals and in MRI phantom experiments.

Algorithms↗
PubMed2006

Doppler ultrasound signals simulation from vessels with various stenosis degrees.

As a non-invasive method, the Doppler ultrasound technique is used to detect the vessel stenosis. To search for characteristics of Doppler ultrasound signals sensitive to the stenosis, a computer simulation approach is proposed in this paper to generate Doppler ultrasound signals from vessels with various stenosis degrees. The blood flow velocity distribution in a stenosed vessel is firstly calculated using the transient finite element method (FEM). Then the power spectral density of Doppler signals is estimated using the overall-distribution nonparametric estimation method. Finally Doppler signals are generated using the cosine-superposed method. The proposed approach is proved to be useful for simulating Doppler ultrasound signals from vessels with various stenosis degrees. It is also shown that characteristics of Doppler ultrasound signals may be used to estimate the vessel's stenosis degree.

Algorithms↗
PubMed2003

EPR studies of phospholipid bilayers after lipoperoxidation. 1. Inner molecular order and fluidity gradient.

The molecular order of fatty acyl chains in oriented lipid bilayers on solid support (SPB), made of either natural or synthetic phospholipids oxidized by Fenton reagent and probed with spin labeled lecithin (5-DSPC) was studied by means of EPR spectrometry. Phospholipids (ASPC, EYPC, mitochondrial extract) were oxidized as either aqueous buffer/methanol dispersions or reverse-phase evaporation vesicles (REV) suspensions. Oxidation was preliminarily revealed both by assaying MDA and by detecting conjugated dienes. Oxidized phospholipid species was quantified by preparative TLC. The degree of order in oriented lipid bilayers of samples containing oxidized phospholipids was estimated by the loss of EPR spectral anisotropy, and an empirical index of the related bilayer disorder was calculated from the second derivative spectra. Bilayers made of each non-oxidized phospholipid species from either ethanol solutions or REV suspensions showed the highest anisotropy, while the increasing presence of oxidized lipids in the samples resulted in progressive loss of EPR spectral anisotropy. In contrast, vesicles containing 40% of the oxidized species maintained an unaltered fluidity gradient, while REV formation was hindered by oxidized phospholipid percentages higher than 45% for ASPC and EYPC, and 35% for Mitochondrial lipids (MtL). It is concluded that the early stages of lipoperoxidation bring about disordering of the phospholipid bilayer interior rather than fluidity alterations, and that prolonged oxidation may result in loss of structural and chemical properties of the bilayer until the structure no longer holds.

Animals↗
PubMed1986

Digital signal processing of stridor and snoring in children.

Stridor and snoring are common signs of upper airway obstruction. The nature and characteristics of the stridor and snoring depend upon the site of obstruction. Sophisticated analysis of these sounds may provide important information concerning the source of the sound helping to assess the patient more objectively. The preliminary results of computerized digital analysis of stridor and snoring sounds are presented in 5 children. Two main programs were applied to analyse the signal: the Power Spectral Density (PSD) function and the Estimated Cross-sectional Area (ECSA). A consistent pattern according to the site of the produced sound was seen. Further acoustical analyses are needed to standardize this method and to program the computer to indicate the various sites of lesions.

Child, Preschool↗
PubMed1999

Quantitative relationship between fluctuations of blood pressure and sympathetic nerve activity in pentobarbital anesthetized rats.

Transfer function analysis was used to examine the coupling between the sympathetic nerve activity (SNA) and the blood pressure (BP) fluctuations. In pentobarbital anesthetized Wistar rats, linear regression of the relationship between frequency (X-axis) and the logarithmic transfer magnitude, i.e. log (BP power density/renal SNA power density) (Y-axis), in the low frequency range (0.016-0.85 Hz) revealed an excellent fit (r = 0.97-0.98). Comparing the regression lines, rats under large dose of pentobarbital anesthesia (40 mg/kg, i.v. single dose) had significantly smaller intercept and slope values compared to rats under small dose of pentobarbital anesthesia (12.5 mg/kg). When intercept and slope values were compared between intact and acutely sinoaortic denervated rats, no significant difference were found. The results suggest that sympathetic modulation of vasomotor tone may be a major factor in generating BP fluctuations between 0.016 and 0.85 Hz in rats. Furthermore, these results support the possibility of using low frequency spectral power of BP to quantitatively estimate the fluctuations of SNA for rats under pentobarbital anesthesia if anesthetic depth is controlled.

Anesthesia, Intravenous↗
PubMed1996

Watching proteins move using site-directed spin labeling.

Site-directed spin labeling of proteins has proven to be a practical means for determining secondary structure and its orientation; surfaces of tertiary interactions; inter-residue distances; chain topology and depth of a given side chain from the membrane/aqueous surface in membrane proteins; and local electrostatic potentials at solvent-exposed sites. Moreover, the mobility of a side chain together with its solvent-accessibility may serve to uniquely identify the topographical location of specific residues in the protein fold. Future spectral analysis should permit a quantitative estimation of the contribution of backbone flexibility to the overall side-chain dynamics. The ability to time-resolve the structural features mentioned above makes SDSL a powerful approach for exploring the evolution of structure on the millisecond time scale. We anticipate future applications to the study of protein folding both in solution and in chaperone-mediated systems.

Electron Spin Resonance Spectroscopy↗
PubMed2004

Low temperature FTIR spectroscopy and hydrogen bonding in cytosine polycrystals.

The FTIR spectra of both the pure NH and isotopically substituted ND (<10% and >90% D) polycrystalline cytosine were recorded in the range 400-4000 cm(-1) as a function of temperature (10-300 K). For the first time, uncoupled NH(D) stretching mode bands of amine and imine groups were observed in the spectra of isotopically diluted cytosine at low temperatures. These bands correspond to the three distinct H-bonds that are present in the crystal, in agreement with the available data obtained by structural methods. At least nine bands were observed below 1000 cm(-1) and, in consonance with their temperature and isotopic exchange behavior, were assigned to the NH proton out-of-the-plane bending modes. Six of these bands were found to correspond to additional "disordered" H-bonds, which could not be observed by structural methods. Empirical correlations of spectral and thermodynamic parameters enabled to estimate the contribution of the H-bonds to the sublimation enthalpy of the crystal, in agreement with independent experimental data.

Crystallization↗
PubMed2001

Sources of uncertainty in isotope ratio measurements by inductively coupled plasma mass spectrometry.

A model is presented describing the effects of dead time and mass bias correction factor uncertainties, flicker noise, and counting statistics on isotope ratio measurement precision using inductively coupled plasma mass spectrometry (ICPMS) with a single collector. Noise spectral analysis is exploited to enable estimation of the flicker noise parameters. For the instrument used, the flicker noise component exhibited a fairly weak frequency (t) dependence (is proportional to f -0.33+/-0.12), but was directly proportional to the total number of counts, Q. As white noise, determined by counting statistics, is given by Q0.5, the isotope ratio measurement uncertainties will actually cease to improve when Q exceeds a certain threshold. This would suggest that flicker noise could become the limiting factor for the precision with which isotope ratios can be determined by ICPMS. However, under most experimental conditions, uncertainties associated with mass discrimination and dead time correction factors are decisive. For ratios up to approximately 22 (115In/113In), optimum major isotope count rates are generally below 0.3 MHz, for which precision in the mass discrimination factor is limiting. The model derived could be used as a starting point for determining optimum conditions and understanding the limitations of single-collector ICPMS for precise isotope ratio measurements.

Journal Article↗