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At least 235 records · Page 13Linked to original sources
PubMed2005

Analysis of microstructural alterations of normal and pathological breast tissue in vivo using the AR cepstrum.

The mean scatterer spacing is considered to be an important parameter for describing ultrasonic scattering and characterization of biological tissue. Autoregressive models are widely used in parametric techniques for spectral estimation. In this paper, we describe the results of a careful examination of the mean scatterer spacing parameter in normal and pathological breast tissues in vivo using the autoregressive cepstrum. Our experimental results carried out at 4.5 MHz using weakly focused pulse-echo single element transducer show that the mean scatterer spacing in normal breast tissues in vivo is 1.25+/-0.21 mm whereas in several pathological breast tissues, it is between 0.82+/-0.10 and 1.09+/-0.07 mm. These results indicate good correlation with microstructure of breast tissue characterization, and hence the AR cepstrum holds promise that it could be used as an effective method for signal analysis of ultrasonic scattering and characterization of breast tissues scatterers.

Adult↗
PubMed2006

Design of a continuous wave blood flow bi-directional Doppler system.

This paper describes the design of a continuous wave blood flow bi-directional Doppler system based on an open, high-resolution architecture that is portable and low cost. The system incorporates the advantages of expensive systems with dedicated hardware. The system is composed of a flow detector probe, a signal conditioning stage, a direction detection module, a signal processing stage and a graphic user interface. The direction detection of the signal is achieved using a fast digital phasing filter. The Doppler signal is processed using a short-time Fourier transform-based algorithm. This is commonly used as a reference. Nevertheless, the system allows us to incorporate alternative high-resolution spectral estimation methods that might offer more precise information to the specialist.

Algorithms↗
PubMed2005

Ultrasonic technique for imaging tissue vibrations: preliminary results.

We propose an ultrasound (US)-based technique for imaging vibrations in the blood vessel walls and surrounding tissue caused by eddies produced during flow through narrowed or punctured arteries. Our approach is to utilize the clutter signal, normally suppressed in conventional color flow imaging, to detect and characterize local tissue vibrations. We demonstrate the feasibility of visualizing the origin and extent of vibrations relative to the underlying anatomy and blood flow in real-time and their quantitative assessment, including measurements of the amplitude, frequency and spatial distribution. We present two signal-processing algorithms, one based on phase decomposition and the other based on spectral estimation using eigen decomposition for isolating vibrations from clutter, blood flow and noise using an ensemble of US echoes. In simulation studies, the computationally efficient phase-decomposition method achieved 96% sensitivity and 98% specificity for vibration detection and was robust to broadband vibrations. Somewhat higher sensitivity (98%) and specificity (99%) could be achieved using the more computationally intensive eigen decomposition-based algorithm. Vibration amplitudes as low as 1 mum were measured accurately in phantom experiments. Real-time tissue vibration imaging at typical color-flow frame rates was implemented on a software-programmable US system. Vibrations were studied in vivo in a stenosed femoral bypass vein graft in a human subject and in a punctured femoral artery and incised spleen in an animal model.

Algorithms↗
PubMed2000

Prefrontal brain electrical asymmetry predicts the evaluation of affective stimuli.

Measures of left-right asymmetry in resting brain activity were derived from spectral estimates of electroencephalogram (EEG) alpha-band power density in 13 homologous scalp electrode pairs from 81 right-handed individuals (43 F) on two occasions separated by 6 weeks. At a third, later session, these individuals completed a cognitive task, comparing word-pairs that systematically differed in affective tone. For an extended series of paired-comparisons, the subject chose the one word-pair that 'went together best'. Objectively, associative strength was comparable for both word-pairs. Individuals with relatively greater left-sided anterior frontal resting activity were more likely to select the more pleasant word-pair. Relations between word-pair selection and asymmetry in resting brain activity at central and posterior sites were not significant.

Adult↗
PubMed1998

Real-time spectral analysis of HRV signals: an interactive and user-friendly PC system.

We present a real-time system, built around a PC and a low-cost data acquisition board, for the spectral analysis of the heart rate variability signal. The Windows-like operating environment on which it is based makes the computer program very user-friendly even for non-specialized personnel. The Power Spectral Density is computed through the use of a hybrid method, in which a classical FFT analysis follows an autoregressive finite-extension of data; the stationarity of the sequence is continuously checked. The use of this algorithm gives a high degree of robustness of the spectral estimation. Moreover, always in real time, the FFT of every data block is computed and displayed in order to corroborate the results as well as to allow the user to interactively choose a proper AR model order.

Calibration↗
PubMed2000

Neuro-cognitive activity during a self-paced visuospatial task: comparative EEG profiles in marksmen and novice shooters.

Log-transformed EEG power spectral estimates (6-7, 9,10-11,18-22, and 36-44 Hz), obtained from skilled marksmen and novice shooters at sites F3, F4, C3, C4,T3, T4, P3, P4, O1, and O2 during the aiming period (6 s) of a target shooting task for each of 40 trials up to the moment of trigger pull, were contrasted to determine regional differences in cortical activation. The EEG power obtained from both groups during the preparatory aiming period was also compared to that observed for a similar time period during the processing of standard verbal and spatial tasks. The marksmen exhibited less activation than the novice shooters at all sites during the aiming period with a pronounced difference in the left central-temporal-parietal area. Fewer group differences in cortical activation were observed during the comparative verbal and spatial tasks with which the groups held equal experience. Additionally, the novice shooters exhibited a cortical activation pattern during target shooting that was similar to that observed during the processing of the comparative verbal and spatial tasks. In contrast, marksmen generally exhibited less cortical activation during the aiming period when contrasted to that during the novel comparative tasks. These results are consistent with the notion of relative economy in the cortical processes of marksmen, relative to controls, during the specific challenge with which they are highly practiced.

Adult↗
PubMed2001

Multigate doppler signal analysis using 3-D regularized long AR modelling.

Autoregressive (AR) modelling has already been proposed as an alternative to fast Fourier transform to process ultrasound (US) Doppler signals. Previous works introduced long AR models, set up under a regularization framework. The latter may be in 1-D (frequency) or 2-D (frequency and space or time). This study generalizes the spectrum regularization in the three dimensions frequency, space and time. The problem of the penalization function is addressed, and a new convex solution is proposed, taking into account possible nonstationarity of the Doppler signal. The parameter tuning is based on simulations using a standard Doppler signal model. The first results show that this processing improves the spectral estimation, and is well suited to flow interpretation.

Algorithms↗
PubMed1999

Human anticipatory eye movements may reflect rhythmic central nervous activity.

To investigate the possibility that rhythmic activity originating in the central nervous system may modulate human eye movements, anticipatory eye movements were generated by tracking an intermittently obscured sinusoidally moving target. Eight subjects tracked intermittently obscured sinusoids of three different frequencies and of two different amplitudes. Eye movements were recorded by an infra-red reflection technique. The eye velocity records were analysed in the frequency domain by power spectral estimates. During periods where the target was obscured, eye movements consisted of a staggered series of anticipatory saccades with intervening smooth anticipatory eye movements or relatively stationary periods. In sections where the intervening smooth components of anticipatory tracking were of high velocity (above 15 deg/s), a superimposed smooth tremulous oscillation at around 10 Hz was sometimes present. Coherence analysis showed that this 10 Hz range oscillation of smooth anticipatory movement was not derived from head tremor and that the same oscillation was present in both eyes. This oscillation was not generally observed during smooth tracking of pseudorandom waveforms. Investigation of anticipatory eye movements has revealed a 10-Hz range oscillation or "tremor" superimposed upon smooth movements that might in other circumstances be inhibited by direct visual feedback. This smooth eye movement oscillation is thought to originate from the central nervous system and may reflect a widespread frequency modulation of motor commands.

Adult↗
PubMed1997

Transcephalic electrical impedance provides a means for quantifying pulsatile cerebral blood volume changes following head-up tilt.

Transcephalic electrical impedance (delta Z) was used to assess pulsatile cerebral blood volume changes while tilting nine premature (30-34 weeks) infants 20 degrees head up. High-frequency (1.50-4.00 Hz) delta Z variability decreased 27% while heart rate did not show any change. We would like to suggest that the variability of transcephalic electrical impedance analysed with spectral estimation seems to provide a means for quantifying alterations in cerebral circulation.

Blood Volume↗
PubMed1997

A Minor Component Analysis Algorithm.

The eigenvectors corresponding to the smallest eigenvalues of the autocorrelation matrix of the input signals are defined as the minor components, which play a very important role in many fields of adaptive signal processing such as spectral estimation, total least squares processing, eigen-based bearing estimation, digital beamforming, moving target indication, and clutter cancellation. This paper proposes a learning algorithm which extracts adaptively the minor component. We will use the Rayleigh quotient as an energy function and prove both analytically and by simulation results that the weight vector provided by the proposed algorithm is guaranteed to converge to the minor component of the input signals. Copyright 1997 Elsevier Science Ltd. All Rights Reserved.

Journal Article↗
PubMed1999

Classification of normal and pathological tremors using a multidimensional electromagnetic system.

A new multidimensional movement analysis system was used to record limb tremor over six degrees-of-freedom, and signal processing techniques were explored to develop a suitable classification method to distinguish between different types of tremor. The specific aims were to investigate the ability of the system to screen for differences between normal subjects and a group of neurological patients, and then to differentiate between three diagnostic groups of patients. Postural tremor at the hand was recorded in normal subjects (n=24) and patients with essential tremor (n=21), multiple sclerosis (n=17) and parkinsonism (n=19). Data were collected using a 3Space Fastrak((R)) (Polhemus, Inc.) over six degrees-of-freedom (three translational directions and three rotations). Spectral estimates produced measures of tremor frequency and amplitude. Mathematical models of the data, using autoregressive modelling and K-nearest neighbour classification, produced parameters used to classify, (1) the normal subjects and 24 patients (using the three rotational movements), and (2) the three patient groups (using all six movement directions). Results were given in terms of the probability of each subject belonging to the groups being classified. 70%). The diagnostic classification produced clear differences between the patient groups (60% for essential tremor, 80% for multiple sclerosis and 60% for parkinsonism). The ability of this assessment technique to distinguish between postural tremor in normal subjects and neurological patients suggests that it could be developed as a screening tool. Classification of tremors between the patients groups, with a high degree of sensitivity, indicates the potential for further development of the system as a diagnostic aid.

Adult↗
PubMed1988

31P NMR studies of the structure of cation-nucleotide complexes bound to porcine muscle adenylate kinase.

The paramagnetic effects on the spin-relaxation rates of 31P nuclei in complexes of porcine muscle adenylate kinase with ATP, GTP, GDP, and AMP were measured in the presence of two dissimilar activating paramagnetic cations, Mn(II) and Co(II), to examine the structures of the enzyme-bound complexes. Experiments were performed exclusively on enzyme-bound complexes to limit contributions to observed relaxation rates to two exchanging complexes (with and without cation). Measurements were made at three frequencies, 81, 121.5, and 190.2 MHz, and as a function of temperature in the range 5-30 degrees C to determine the effect of exchange on the observed relaxation rates. Relaxation rates in the E.MnATP, E.MnGTP, and E.MnGDP complexes were shown to be exchange-limited and therefore without structural information. Relaxation rates for the complexes E.CoATP, E.CoGTP, and E.CoGDP were shown to depend on Co(II)-31P distances. Inability to precisely estimate spectral densities arising from electronic relaxation of Co(II) restricts calculations of Co(II)-31P distances in these complexes to upper and lower limits. At the center of these limits, the Co(II)-31P distances of beta-P and gamma-P in E.CoATP and E.CoGTP, and of beta-P (E.CoGDP), are in the range 3.1-3.5 A appropriate for the first coordination sphere. For all these complexes, the corresponding distance for alpha-P is appreciably larger in the range 3.9-4.5 A.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenine Nucleotides↗
PubMed1995

Evidence for in vivo high-frequency periodicity of plasma beta-endorphin in humans.

OBJECTIVE: To investigate whether high-frequency periodic fluctuation in the release of plasma immunoreactive (ir)-beta-endorphin occurs in humans. DESIGN, PATIENTS AND MEASUREMENTS: Blood was collected at 90-sec intervals for 1 hour from the forearm vein of four healthy women and subsequently analysed for ir-beta-endorphin by radioimmunoassay. RESULTS: In all women circulating beta-endorphin release followed a high-frequency periodic rhythm pattern. The estimated spectral densities within the four subjects varied between 3.6 min to 4.9 min. CONCLUSIONS: Our results provide the first in vivo evidence that fluctuations in the release of beta-endorphin in humans cannot only be characterized by low-frequency pulses, but also by a high frequency periodic rhythm.

Adult↗
PubMed2006

Sparse time-frequency representations.

Auditory neurons preserve exquisite temporal information about sound features, but we do not know how the brain uses this information to process the rapidly changing sounds of the natural world. Simple arguments for effective use of temporal information led us to consider the reassignment class of time-frequency representations as a model of auditory processing. Reassigned time-frequency representations can track isolated simple signals with accuracy unlimited by the time-frequency uncertainty principle, but lack of a general theory has hampered their application to complex sounds. We describe the reassigned representations for white noise and show that even spectrally dense signals produce sparse reassignments: the representation collapses onto a thin set of lines arranged in a froth-like pattern. Preserving phase information allows reconstruction of the original signal. We define a notion of "consensus," based on stability of reassignment to time-scale changes, which produces sharp spectral estimates for a wide class of complex mixed signals. As the only currently known class of time-frequency representations that is always "in focus" this methodology has general utility in signal analysis. It may also help explain the remarkable acuity of auditory perception. Many details of complex sounds that are virtually undetectable in standard sonograms are readily perceptible and visible in reassignment.

Animals↗
PubMed1998

Line spectral analysis for harmonizable processes.

Harmonizable processes with spectral mass concentrated on a number of straight lines are considered. The asymptotic behavior of the bias and covariance of a number of spectral estimates is described. The results generalize those obtained for periodic and almost periodic processes.

Journal Article↗
PubMed2006

Spectral distance for ARMA models applied to electroencephalogram for early detection of hypoxia.

A novel measure of spectral distance is presented, which is inspired by the prediction residual parameter presented by Itakura in 1975, but derived from frequency domain data and extended to include autoregressive moving average (ARMA) models. This new algorithm is applied to electroencephalogram (EEG) data from newborn piglets exposed to hypoxia for the purpose of early detection of hypoxia. The performance is evaluated using parameters relevant for potential clinical use, and is found to outperform the Itakura distance, which has proved to be useful for this application. Additionally, we compare the performance with various algorithms previously used for the detection of hypoxia from EEG. Our results based on EEG from newborn piglets show that some detector statistics divert significantly from a reference period less than 2 min after the start of general hypoxia. Among these successful detectors, the proposed spectral distance is the only spectral-based parameter. It therefore appears that spectral changes due to hypoxia are best described by use of an ARMA- model-based spectral estimate, but the drawback of the presented method is high computational effort.

Algorithms↗
PubMed1991

Use of the BD-100R as a neutron spectrometer through applied pressure variation.

A study was undertaken to demonstrate the feasibility of using the well-characterized BD-100R neutron bubble dosimeter as a neutron spectrometer in low-level radiation fields. The BD-100R dosimeters used in this work consisted of a test tube containing an elastic polymer with interspersed droplets of two types of Freon: Freon-12 and Freon-114. Each superheated liquid droplet is a potential nucleation site, with the minimum energy needed to form a bubble at the nucleation site being inversely proportional to the square of the difference between the applied and the vapor pressure (i.e., Emin alpha(delta P)-2. For a given dose, the number of bubbles formed continually decreases with increasing applied pressure, until a pressure is reached where no bubbles are formed, since the energy transferred can no longer vaporize the Freon. This investigation is intended to demonstrate the feasibility of measuring an unknown spectrum utilizing the dosimeter response (number of bubbles formed) as a function of the neutron energy (applied pressure). A set of 12 dosimeters was exposed under various applied pressures in a well-characterized neutron energy spectrum at the East Beam Port (EBP) of the Maryland University Training Reactor (MUTR). The dosimeters were placed inside a pressure chamber that could accommodate up to 18 dosimeters. Energy response coefficients (cross-sections) were obtained by spectral unfolding techniques on the known spectrum. The same set of dosimeters were then irradiated using a paraffin-moderated Pu/Be source. Measured spectral estimates obtained using the response coefficients were compared with numerical computations generated using the ANISN computer code. The results indicate that further research using the BD-100R as a neutron spectrometer in low radiation fields is warranted.

Neutrons↗
PubMedDate not supplied

A greater reduction in high-frequency heart rate variability to a psychological stressor is associated with subclinical coronary and aortic calcification in postmenopausal women.

OBJECTIVE: Reduced cardiac parasympathetic activity, as indicated by a reduced level of clinic or ambulatory high-frequency heart rate variability (HF-HRV), is associated with an increased risk for atherosclerosis and coronary artery disease. We tested whether the reduction in HF-HRV to a psychological stressor relative to a baseline level is also associated with subclinical coronary or aortic atherosclerosis, as assessed by calcification in these vascular regions. METHOD: Spectral estimates of 0.15 to 0.40 Hz HF-HRV were obtained from 94 postmenopausal women (61-69 years) who engaged in a 3-minute speech-preparation stressor after a 6-minute resting baseline. A median of 282 days later, electron beam tomography (EBT) was used to measure the extent of coronary and aortic calcification. RESULTS: In univariate analyses, a greater reduction in HF-HRV from baseline to speech preparation was associated with having more extensive calcification in the coronary arteries (rho = -0.29, p = .03) and in the aorta (rho = -0.22, p = .06). In multivariate analyses that controlled for age, education level, smoking status, hormone therapy use, fasting glucose, high-density lipoproteins, baseline HF-HRV, and the stressor-induced change in respiration rate, a greater stressor-induced reduction in HF-HRV was associated with more calcification in the coronary arteries (B = -1.21, p < .05), and it was marginally associated with more calcification in the aorta (B = -0.92, p = .09). CONCLUSION: In postmenopausal women, a greater reduction in cardiac parasympathetic activity to a psychological stressor from baseline may be an independent correlate of subclinical atherosclerosis, particularly in the coronary arteries.

Aged↗