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At least 307 records · Page 17Linked to original sources
PubMed1986

Measurement of the dynamics of arteriolar diameter.

The diameter of the arteriolar vessels of the microcirculation undergoes a continuous variation as a consequence of vasomotion. The quantification of this process requires the implementation of spectral analysis techniques that model short data records of a finite number of superposed sinusoidal waveforms. The following techniques were tested with artificially synthetized records and actual data: the fast Fourier transform, the high-resolution autoregressive method, the maximum entropy method, and the Prony Spectral Line Estimator (PSLE). It was found that the PSLE provides the most accurate estimation of the spectral components of the dynamics of diameter changes because it does not require any assumption on the nature of the data outside the interval under analysis.

Animals↗
PubMed1997

Autonomic nervous system activity during tilt testing in syncopal patients, estimated by power spectral analysis of heart rate variability.

Spectral analysis of heart rate variability (HRV) was used to assess changes in autonomic function before and during postural tilt in 28 syncopal patients: 14 (group A) with positive and 14 (group B) with negative tilting test, and 14 normal controls (group C). Frequency-domain measurements of the high (HF) and low (LF) frequency bands and the ratio LF/HF were derived from Holter recordings, computed by Fast Fourier analysis for 4-minute intervals immediately before tilt testing, immediately after tilting, and just before the end of the test. In group A, the mean values of LF and HF decreased slightly in response to tilting while the LF/HF ratio increased, though these changes were not statistically significant. All parameters showed a statistically significant increase just before the onset of syncope. In group B, there were no significant changes in the parameters measured throughout the test. In group C, there was an increase in the LF and LF/HF ratio and a decrease in the HF immediately after tilting. There were no further significant changes in any of the parameters during the test. Syncopal patients have a different pattern of response to the orthostatic stimulus, in that they do not show the increase in sympathetic tone observed in normal individuals immediately after tilting. In the patients with a positive tilt test, there is a shift in the balance of ANS activity towards the sympathetic system shortly before the onset of syncope.

Autonomic Nervous System↗
PubMed2005

Voice abnormality in adults with congenital and adult-acquired growth hormone deficiency.

CONTEXT: Adult males with congenital, untreated, severe GH deficiency (GHD) due to genetic GHRH receptor deficiency exhibit distinctive, high-pitched, and raspy voice characteristics. OBJECTIVE/DESIGN: To determine the physical underpinning of this phenomenon, we performed voice recordings, translarynx impedance measurements, spectral analysis, and estimates of spectral complexity [approximate entropy (ApEn)] in four affected men. Results were compared with those obtained in four men with untreated adult-onset GHD and a normal male population. RESULTS: Congenital GHD subjects had a high-pitched voice with a fundamental frequency typical of normal females (174-266 Hz). Their frequency spectra were characterized by abnormal harmonics, with reversal/interruption of the normal amplitude decay among higher-order harmonics, findings consistent with a creaky quality of the voice. Patients with adult-onset GHD, acquired at ages 31, 38, and 40 yr, had a normal male pitch (fundamental frequency, 117-154 Hz) but pathologically low ApEn values, corresponding to a breathy quality of the voice and suggesting abnormal vocal fold function. A fourth patient who acquired GHD at age 22 yr had a pitch intermediate between male and female, high ApEn, and a spectral pattern similar to the congenital GHD patients. CONCLUSIONS: This study demonstrates an effect of GH on laryngeal size and vocal fold compliance that results in a high pitch and disordered spectral quality. The time of onset of GHD determines which type of abnormality predominates.

Adult↗
PubMed1997

Acetyl-L-carnitine flux to lipids in cells estimated using isotopomer spectral analysis.

Acetyl-L-carnitine is known as a reservoir of activated acetyl units and as a modulator of metabolic function. The objective of this study was to quantify the fate of the acetyl moiety of acetyl-L-carnitine in lipogenic pathways. Lipogenesis was studied in an adipocyte model, differentiated 3T3-L1 cells, and a hepatoma cell, HepG2 cells. Lipogenesis and ketogenesis were examined in rat hepatocytes. Both de novo synthesis and elongation of fatty acids were investigated using gas chromatography/mass spectrometry and [1,2-(13)C]acetyl-L-carnitine. Comparisons were performed with [13C]glucose and [13C]acetate. Isotopomer Spectral Analysis, a stable isotope method for differentiating between the enrichment of the precursor and the amount of synthesis was used to analyze the data. Acetyl-L-carnitine was generally less effective than acetate as a precursor for de novo lipogenesis. The effects of acetyl-L-carnitine were not identical to those of acetate plus carnitine as expected if acetyl-L-carnitine flux to acetyl CoA is controlled by carnitine acetyl transferase. Acetyl-L-carnitine (2 mM) contributed approximately 10% of the lipogenic acetyl-CoA used for synthesis and elongation as well as 6% of the ketogenic acetyl-CoA. No differences were found between the precursor enrichment for de novo lipogenesis and for elongation of saturated fatty acids. Flux of acetyl-L-carnitine to lipid was increased, not decreased, by the ATP citrate lyase inhibitor, -hydroxycitrate. In contrast, flux of glucose to lipid was dramatically decreased by this inhibitor. These results indicate that flux of acetyl-L-carnitine to lipid can bypass citrate and utilize cytosolic acetyl-CoA synthesis.

3T3 Cells↗
PubMed1980

[Evaluation of estimation method of X-ray spectral distribution by attenuation data (author's transl)].

This paper describes a method for estimating spectral distributions from X-ray attenuation data obtained by inserting metal filters in the beam. First, the principle of the estimation method is briefly outlined. the spectral distribution is approximated by a histogram with an appropriate number of cells whose heights are estimated by recursive algorithm. The algorithm is based on a gradient method that seeks the minimum of a least-square criterion function. Next, the following two problems to which special attention should be paid in practical applications are discussed: 1) appearance of discontinuity in estimated distribution; and 2) termination criterion of iterative computation. The problem 1) is investigated theoretically and the improved algorithm is devised. Computer simulation is performed for the problem 2), and a proper termination criterion based on the decreasing curve of the least-square criterion function is given. The validity of estimation results is illustrated about the spectral distribution of 140 kV constant potential X-ray source.

Computers↗
PubMed1993

Effect of method and parameters of spectral analysis on selected indices of simulated Doppler spectra.

The sensitivity of Doppler spectral indices (mean frequency, maximum frequency, spectral broadening index and turbulence intensity) to the conditions of spectral analysis (estimation method, data window, smoothing window or model order) increases with decreasing signal bandwidth and growing index complexity. The bias of spectral estimate has a more important effect on these indices than its variance. A too low order, in the case of autoregressive modeling and minimum variance methods, and excessive smoothing, in the case of the FFT method, result in increased errors of Doppler spectral indices. There is a trade-off between the errors resulting from a short data window and those due to insufficient temporal resolution.

Arteries↗
PubMed2000

Lipari-Szabo mapping: A graphical approach to Lipari-Szabo analysis of NMR relaxation data using reduced spectral density mapping.

In this paper, we explore connections between the Lipari-Szabo formalism and reduced spectral density mapping, and show how spectral density estimates can be associated with Lipari-Szabo parameters via a simple geometric construction which we call Lipari-Szabo mapping. This relationship can be used to estimate Lipari-Szabo parameters from spectral density estimates without the need for nonlinear optimization, and to perform 'model selection' in a graphical manner. The Lipari-Szabo map also provides insight into the Lipari-Szabo model, and allows us to determine when a given set of experimental spectral densities are inconsistent with the Lipari-Szabo formalism. Practical applications of Lipari-Szabo mapping in conjunction with more traditional analysis methods are discussed.

Anisotropy↗
PubMed1993

On-line estimation of myoelectric signal spectral parameters and nonstationarities detection.

In this communication, we present a method for detecting nonstationarities of random time series with an approximately Gaussian distribution of amplitudes. This method is suitable for real time implementation. Here we report some results obtained by applying them to a time series of spectral parameters of surface myoelectric signals, collected during voluntary isometric contractions of human muscles. Moreover, we describe the computerized system that we used to implement our detector of nonstationarity. This system is based on the TMS 320C25 DSP chip and realizes on-line estimation and display of spectral parameters, as well as detection of their nonstationarities, featuring a sampling frequency up to 20 k samples/s. A friendly user interface, fully menu driven, allows the user to select different options during the system's operation by means of hot keys. The accuracy of the system was tested by comparing its estimates with those of an off-line system, previously characterized, which we took as a reference. The estimates of spectral parameters obtained by means of the two systems were always consistent. The on-line stationarity detector was able to recognize rates of variation of the spectral parameters as small as 1%/s during contractions lasting 10-15 s. This sensitivity makes it suitable for clinical application. The set of results herein presented has been selected to highlight the main characteristics of the system.

Algorithms↗
PubMed1998

Precision in estimating the frequency separation between spectral lines.

It is common to estimate the frequency separation between peaks in a digitized frequency-domain spectrum by fitting an appropriate function to the experimental spectrum using least-squares procedures. In this paper, we assess from first principles the precision associated with such measurements of frequency separation. In addition to the frequency separation between the peaks, other parameters involved in fitting the spectrum are the peak widths, the lineshape functions (Gaussian, Lorentzian, etc.) for the peaks, and the peak amplitudes. The precision also depends on the signal-to-noise ratio and the spacing between adjacent data points in the digitized spectrum. It is assumed that the residuals considered in the least-squares fitting procedure are the differences between the intensities of corresponding digitized data points in the experimental and fitted spectra. Under these conditions, analytical expressions for the precision in peak separation are derived for the following cases: (i) when the amplitudes of two peaks are known and the two peaks have known equal widths; (ii) when the ratio of the amplitudes of two peaks is known, and the widths of the two peaks are known to be equal, but the actual value of the peak width is not known. In each case, the situation with two Gaussian peaks and the situation with two Lorentzian peaks are considered. In all cases, the absolute precision P(eta) in the estimated frequency separation eta between the two peaks is approximated by an equation of the type P(eta) approximately F(eta/Delta, alpha)SK, where Delta is the peak width, alpha is the ratio A2/A1 of amplitudes of the two peaks, S is the signal-to-noise ratio, and K is the density of data points in the frequency-domain spectrum. The form of the function F(eta/Delta, alpha) depends on the type of lineshape (Gaussian or Lorentzian), and depends on which of the parameters A1, A2, and Delta are known independently of the fitting procedure. Attempts to extend our first-principles approach to assess the precision in least-squares estimates of frequency separation between peaks in more complex situations than those discussed above generally lead to analytical expressions that are formidably complicated. In such cases, numerical approaches based on the theoretical framework developed here may be employed to assess the precision in estimating the frequency separation.

Magnetic Resonance Spectroscopy↗
PubMed2000

Differential change in cardiac baroreflex sensitivity estimated by sequence and spectral analysis during etomidate anesthesia.

Spontaneous baroreflex sensitivity, high-frequency gain, (0.15-0.35 Hz), and mid-frequency gain (0.07-0.14 Hz) are noninvasive measures of cardiac baroreflex function derived by spontaneous sequence and cross-spectral analysis. To demonstrate the difference between these baroreflex estimates, 14 patients received etomidate (0.3 mg/kg bolus and 0.9 mg/kg/h infusion), lidocaine (60 mg), and vecuronium (0.1 mg/kg) by intravenous injection. The authors found that spontaneous baroreflex sensitivity and high-frequency gain were decreased (p <0.05) after etomidate anesthesia, whereas mid-frequency gain was maintained. Spontaneous baroreflex sensitivity, high-frequency gain, and mid-frequency gain, although compared simultaneously, did not change in a parallel manner. In another 5 patients, who received normal saline only, measures were unchanged. The authors conclude that spontaneous baroreflex sensitivity, high-frequency gain, and mid-frequency gain are not interchangeable. Experimental results on baroreflex control depend on the parameter selected.

Adult↗
PubMed2004

Spectral characterization of a color scanner by adaptive estimation.

A new method of spectral characterization for color scanners by the use of adaptive estimation is proposed. It deals with estimation of high-dimensional reflectance vectors from low-dimensional scanner response vectors when the scanner departs from linearity. We first investigate the spectral linearity of the scanner, and then estimate the spectral reflectance adaptively based on the local statistics of a set of neighboring training samples. As the proposed characterization method does not utilize the mathematically recovered spectral responsivity, its inherent inaccuracy is not critical to the spectral characterization. Experimental results showed significant advantage of adaptive estimation when compared with other methods.

Journal Article↗
PubMed2006

An improved spectral width Doppler method for estimating Doppler angles in flows with existence of velocity gradients.

Doppler angle (i.e., beam-to-flow angle) is an important parameter for quantitative flow measurements. With known Doppler angles, volumetric flows can be obtained by the mean flow velocity times the cross-section area of the vessel. The differences or changes between prestenotic and poststenotic volumetric flows have been quantified as an indicator for assessing the clinical severity of the stenosis. Therefore, several research groups have dedicated themselves to developing user-independent methods to determine automatically the Doppler angle. Nevertheless, most of these methods were developed for narrow ultrasound beam measurements. For small vessels, where the beam width is a significant fraction of the diameter of the vessel, the effect of velocity gradients plays an important role and should not be ignored in the Doppler angle estimations. Accordingly, this paper is concerned with a method for improving the estimation of Doppler angles from spectral width Doppler (SWD) method, but correcting for velocity-gradient broadening that may arise when the beam has a nonzero width. In our method, Doppler angles were firstly calculated by SWD and then were corrected by an artificial neural network (ANN) method to neutralize the contribution of velocity gradient broadening (VGB). This SWD and ANN conjoint method has been successfully applied to estimate Doppler angles from 50 degrees to 80 degrees for constant flows in 10 mm, 4 mm and 1 mm diameter tubes, whose mean flow velocities were 15.3, 19.9 and 25.5 cm/s, respectively, and the achieved mean absolute errors of the estimated Doppler angles were 1.46 degrees , 1.01 degrees and 1.3 degrees.

Blood Flow Velocity↗