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PubMed2000

Reconstruction and visualization of irregularly sampled three- and four-dimensional ultrasound data for cerebrovascular applications.

Although recent studies have demonstrated the potential value of compounded data for improvement in signal-to-noise ratio and speckle contrast for three-dimensional (3-D) ultrasonography, clinical applications are lacking. We investigated the potential of six degrees-of-freedom (6-DOF) scanhead position and orientation measurement (POM) devices for registration of in vivo multiplanar, irregularly sampled ultrasound (US) images to a regular 3-D volume space. The results demonstrate that accurate spatial and temporal registration of four-dimensional (4-D) US data can be achieved using a 6-DOF scanhead tracking system. For reconstruction of arbitrary, irregularly sampled US data, we introduce a technique based upon a weighted, ellipsoid Gaussian convolution kernel. Volume renderings of 3-D and 4-D compounded in vivo US data are presented. The results, although restricted to the field of cerebrovascular disease, will be of value to other applications of 3-D sonography, particularly those in which compounding of data through irregular sampling may provide superior information on tissue or vessel structure.

Blood Flow Velocity↗

PubMed2006

Fast IIR isotropic 2-D complex Gabor filters with boundary initialization.

Gabor filters are widely applied in image analysis and computer vision applications. This paper describes a fast algorithm for isotropic complex Gabor filtering that outperforms existing implementations. The main computational improvement arises from the decomposition of Gabor filtering into more efficient Gaussian filtering and sinusoidal modulations. Appropriate filter initial conditions are derived to avoid boundary transients, without requiring explicit image border extension. Our proposal reduces up to 39% the number of required operations with respect to state-of-the-art approaches. A full C++ implementation of the method is publicly available.

Algorithms↗

PubMed1999

[The method of wavelet transformation used for extracting the characteristic of the exercise ECG].

This article introduces the basic rationale of wavelet transformation and the algorithm of typical dyadic wavelet transformation with mother wavelet of the first derivative of a Gaussian smoothing function, this wavelet transformation is used to recognize the characteristic parameters of the exercise ECG, it is concluded that this method make significant effects on restricting base line drifting and lessening noisy component in the exercise ECG, it provides a new way to do further research on characteristic recognition of the exercise ECG in the exercise test.

Algorithms↗

PubMed1991

Sensitivity enhancement and reduction of Gibbs artifact in T2-weighted imaging using variable tip angle excitation.

Introduction of a Gaussian weighting function into the phase encoding of the spin-echo imaging method via variable tip angle excitation, with concomitant optimization of the recycle delay, is shown to increase the sensitivity of T2-weighted imaging by up to 40%. In addition, the Gibbs artifact associated with sharp boundaries is reduced considerably without the need for postprocessing of the data.

Artifacts↗

PubMed1995

Dynamical analysis of neuromuscular transmission jitter.

Utilizing prolonged axonal stimulation single fiber EMG, neuromuscular transmission becomes a time-series of interpotential intervals (IPIs). In this form, the underlying processes of neuromuscular transmission can be studied using standard numerical techniques to determine whether these processes can be described by a simple mathematical model. In particular, neuromuscular transmission jitter can be examined in this way. In this article, we attempt to determine whether healthy jitter is noise or deterministic chaos. The presence of deterministic chaos was assessed by analysis of the IPI time-series using visual inspection of both phase-space plots and their principal component dimensions, and using the Grassberger-Procaccia algorithm to determine the correlation dimension of the time-series dynamics. These graphical and mathematical techniques provided little evidence for the existence of deterministic chaos. Linear autoregression time-series prediction also failed to account for the variability of the data and IPI histograms exhibited simple gaussian distributions. These results suggest normal neuromuscular transmission jitter is the result of intrinsic noise.

Action Potentials↗

PubMed2000

Spatial color contrast matching: broad-bandpass functions and the flattening effect.

The contrast matching function (CMF) is the reciprocal of test contrast that perceptually matches the contrast of standard pattern, measured as a function of test spatial frequency (SF). Achromatic CMFs usually flatten as the contrast of the standard is raised, and are broader than the achromatic, bandpass, contrast sensitivity function (CSF). This report investigates whether chromatic CMFs have similar characteristics. For this purpose, the red-green color channel was defined using minimum flicker and hue cancellation techniques. Spatially localized (D6), vertical, equiluminant patterns (SFs: 0.063-8 cpd; contrast: 3-80%) were used to measure the CSF and CMF of isoluminant patterns presented with a temporal Gaussian envelope. CMFs were measured using a randomized double-staircase procedure and the two-interval forced choice technique. Two color-normal observers, whose task was to select the interval that had higher color contrast, participated in experiments. Results show that: (a) the color CMFs are lowpass functions of SF at low standard contrasts (3-12.5%), broad-bandpass at intermediate contrasts (6.25-60%), and near-flat at high contrasts (80%); and (b) isoluminant CMFs have higher upper cut-off frequencies than isoluminant CSFs. It is concluded that: (i) color-contrast-constancy (CMF independent of SF) is partly achieved at high contrasts because color CMFs flatten as contrast increases; (ii) the information processing at suprathreshold levels is different from that at the threshold levels; and (iii) the model that explained achromatic CMFs using achromatic threshold mechanisms could not explain chromatic CMFs using chromatic threshold mechanisms.

Adult↗

PubMed2006

Separating structural heterogeneities from stochastic variations in fluorescence resonance energy transfer distributions via photon distribution analysis.

We establish a probability distribution analysis (PDA) method for the analysis of fluorescence resonance energy transfer (FRET) signals to determine with high precision the originating value of a shot-noise-limited signal distribution. PDA theoretical distributions are calculated explicitly including crosstalk, stochastic variations, and background and represent the minimum width that a FRET distribution must have. In this way an unambiguous distinction is made between shot-noise distributions and distributions broadened by heterogeneities. This method simultaneously and effectively extracts highly resolved information from FRET distributions. The theoretical histograms match the exact profile of histograms generated from constant transfer efficiency experimental data with a chi2 near unity. The chi2 surface suggests an ultimate level of precision with FRET of < 1% of the Förster radius. Distributions of FRET signals in donor-acceptor-labeled DNA were unambiguously identified as being broader than shot-noise variations could explain. A model describing a Gaussian distribution of distances was tested with the PDA method and demonstrated 5 A inhomogeneities due to dye motions. The capability of this method to recover quantitative information from FRET distributions has potential applications for studying molecular conformations and dynamics. Potential sources for artifacts such as acceptor photobleaching, spectrally different observation volumes, and fluctuations of the Förster radius are ruled out.

DNA↗

PubMed2006

Configurational statistics of macromolecules in solution from correlations of liquid molecules.

We address the relevant quest for a simple formalism describing the microstructure of liquid solutions of polymer chains. On the basis of a recent relativistic-type picture of self-diffusion in (simple) liquids named Brownian relativity (BWR), a covariant van Hove's distribution function in a Vineyard-like convolution approximation is proposed to relate the statistical features of liquid and chain molecules forming a dilute polymer solution. It provides an extension of the Gaussian statistics of ideal chains to correlated systems, allowing an analysis of macromolecular configurations in solution by the only statistical properties of the liquid units (and vice versa). However, the mathematical solution to this issue is not straightforward because, when the liquid and polymer van Hove's functions are equated, an inverse problem takes place. It presents some conceptual analogies with a scattering experiment in which the correlation of the liquid molecules acts as the radiation source and the macromolecule as the scatterer. After inverting the equation by a theorem coming from the Tikhonov's approach, it turns out that the probability distribution function of a real polymer can be expressed from a static Ornstein-Uhlenbeck process, modified by correlations. This result is used to show that the probability distribution of a true self-avoiding walk polymer (TSWP) can be modeled as a universal Percus-Yevick hard-sphere solution for the total correlation function of the liquid units. This method suits in particular the configurational analysis of single macromolecules. The analytical study of arbitrary many-polymer systems may require further mathematical investigation.

Journal Article↗

PubMed2005

Normative values for intima-media thickness and distensibility of large arteries in healthy adolescents.

OBJECTIVE: Sonographic evaluation of arterial wall morphology and elasticity is increasingly accepted as a non-invasive tool in cardiovascular assessment. Several studies suggest that intima-media thickness (IMT) and arterial elasticity indices may sensitively reflect different vasculopathic processes in children. However, normative values and the impact of adolescent growth are largely unknown. METHODS: We assessed the IMT of the common carotid (cIMT) and femoral arteries (fIMT), carotid elasticity indices and interacting anthropometric factors in 247 healthy subjects aged 10-20 years. RESULTS: cIMT, fIMT, incremental elastic modulus (Einc) and circumferential wall stress (CWS) were positively, and distensibility coefficient (DC) inversely, correlated with age, height, body mass index (BMI), systolic blood pressure (BP) and brachial pulse pressure (r = 0.56 to -0.45, P < 0.05 to 0.0001). DC (r = -0.29, P < 0.0001) and stiffness index beta (r = 0.25, P < 0.0001), but not Einc, were significantly associated with cIMT independently of age. All vascular parameters showed non-Gaussian distributions. Excessively high IMT was associated with BMI and pulse pressure above the 90th percentile, and elevated Einc with high-normal BMI. Multivariate analysis identified independent positive effects of standardized BMI and brachial pulse pressure on normalized cIMT, negative effects of systolic BP and cIMT on DC, a positive effect of cIMT on stiffness, and positive effects of systolic BP and BMI on Einc and CWS. CONCLUSIONS: Morphological and functional measures of large arteries should be normalized to take account of changes during adolescence and skewed distributions. Relative body mass, systolic blood pressure and/or pulse pressure are determinants of IMT and elasticity.

Adolescent↗

PubMed2004

Interspike interval statistics of neurons driven by colored noise.

A perfect integrate-and-fire model driven by colored noise is studied by means of the interspike interval (ISI) density and the serial correlation coefficient. Exact and approximate expressions for these functions are derived for weak dichotomous or Gaussian noise, respectively. It is shown that correlations in the input result in positive correlations in the ISI sequence and in a reduction of ISI variability. The results also indicate that for weak noise, the noise distribution only shapes the ISI density but not the ISI correlations which are determined by the noise's correlation function.

Action Potentials↗

PubMed2005

EM in high-dimensional spaces.

This paper considers fitting a mixture of Gaussians model to high-dimensional data in scenarios where there are fewer data samples than feature dimensions. Issues that arise when using principal component analysis (PCA) to represent Gaussian distributions inside Expectation-Maximization (EM) are addressed, and a practical algorithm results. Unlike other algorithms that have been proposed, this algorithm does not try to compress the data to fit low-dimensional models. Instead, it models Gaussian distributions in the (N - 1)-dimensional space spanned by the N data samples. We are able to show that this algorithm converges on data sets where low-dimensional techniques do not.

Algorithms↗

PubMed1979

A time series approach to forecasting Australian total live-births.

The relationship between classical demographic deterministic forecasting models, stochastic structural econometric models and time series models is discussed. Final equation autoregressive moving average (ARMA) models for Australian total live-births are constructed. Particular attention is given to the problem of transforming the time series to stationarity (and Gaussianity) and the properties of the forecasts are analyzed. Final form transfer function models linking births to females in the reproductive age groups are also constructed and a comparison of actual forecast performance using the various models is made. Long-run future forecasts are generated and compared with available projections based on the deterministic cohort model after which some policy implications of the analysis are considered.

Australia↗

PubMed2003

Peak localization of sparsely sampled luminance patterns is based on interpolated 3D surface representation.

Objects in the world are typically defined by contours and local features separated by extended featureless regions. Sparsely sampled profiles were therefore used to evaluate the cues involved in localizing objects defined by such separated features (as opposed to typical Vernier acuity or other line-based localization tasks). Objects, in the form of Gaussian blobs, were defined at the sample positions by luminance cues, binocular disparity cues or both together. Remarkably, the luminance information in the sampled profiles was unable to support localization for objects requiring interpolation when the perceived depth from the luminance cue was cancelled by a disparity cue. Disparity cues, on the other hand, improved localization substantially over that for luminance cues alone. These data indicate that it is only through the interpolated depth representation that the position of the sampled object can be recognized. The dominance of a depth representation in the performance of such tasks shows that the depth information is not just an overlay to the 2D sketch of the positional information, but a core process that must be completed before the position of the object can be recognized.

Cues↗

PubMed2004

Defect dynamics observed by NMR of quadrupolar nuclei in gallium nitride.

Hexagonal and cubic polytypes of bulk gallium nitride powders are characterized by 69,71Ga and 14N MAS NMR at 11.7 T. The (corrected) 71Ga chemical shifts are 333.0 and 357.5 ppm, respectively; the corresponding 14N chemical shifts are -301.8 and -297.0 ppm (all shifts referenced to 1 M gallium nitrate). The 69,71Ga nuclear quadrupole coupling constants (NQCC) in the hexagonal form are axially symmetric and agree with previous single-crystal determinations. The 71Ga MAS NMR satellite pattern envelope of the cubic form has a large Gaussian half-height width of 297 kHz, due to nonzero NQCC values induced by defects. The 14N MAS NMR spinning sideband pattern of the cubic form has a Lorentzian envelope half-height width of 17.5 kHz for the same reason. A sample containing both phases shows an unexpected marked loss of the 71Ga MAS NMR satellite transition intensity expected for the hexagonal phase. Static 71Ga-selective Hahn spin-echo measurements at the perpendicular edge of the powder pattern for the hexagonal form in this sample show a large reduction in T2, especially at higher temperatures. The partial destruction of both spin-echoes and rotational echoes is due to a chemical-exchange type process involving sites having different NQCC values.

Journal Article↗

PubMed1999

The normalized interaural correlation: accounting for NoS pi thresholds obtained with Gaussian and "low-noise" masking noise.

Recently, Eddins and Barber [J. Acoust. Soc. Am. 103, 2578-2589 (1998)] and Hall et al. [J. Acoust. Soc. Am. 103, 2573-2577 (1998)] independently reported that greater masking of interaurally phase-reversed (S pi) tones was produced by diotic low-noise noise than by diotic Gaussian noise. Based on quantitative analyses, Eddins and Barber suggested that their results could not be accounted for by assuming that listeners' judgments were based on constant-criterion changes in the normalized interaural correlation produced by adding the S pi signal to the diotic masker. In particular, they showed that a model like the one previously employed by Bernstein and Trahiotis [J. Acoust. Soc. Am. 100, 3774-3784 (1996)] predicted an ordering of thresholds between the conditions of interest that was opposite to that observed. Bernstein and Trahiotis computed the normalized interaural correlation subsequent to half-wave, square-law rectification and low-pass filtering, the parameters of which were chosen to mimic peripheral auditory processing. In this report, it is demonstrated that augmenting the model by adding a physiologically valid stage of "envelope compression" prior to rectification and low-pass filtering provides a remedy. The new model not only accounts for the data obtained by Eddins and Barber (and the similar data obtained by Hall et al.), but also does not diminish the highly successful account of the comprehensive set of data that gave rise to the original form of the model. Therefore, models based on the computation of the normalized interaural correlation appear to remain valid because they can account, both quantitatively and qualitatively, for a wide variety of binaural detection and discrimination data.

Auditory Perception↗

PubMed1994

Data compression by linear prediction for storage and transmission of EEG signals.

The EEG time series has been subjected to various formalisms of analysis to extract meaningful information regarding the underlying neural events. In this paper the linear prediction (LP) method has been used for analysis and presentation of spectral array data for the better visualisation of background EEG activity. It has also been used for signal generation, efficient data storage and transmission of EEG. The LP method is compared with the standard Fourier method of compressed spectral array (CSA) of the multichannel EEG data. The autocorrelation autoregressive (AR) technique is used for obtaining the LP coefficients with a model order of 15. While the Fourier method reduces the data only by half, the LP method just requires the storage of signal variance and LP coefficients. The signal generated using white Gaussian noise as the input to the LP filter has a high correlation coefficient of 0.97 with that of original signal, thus making LP as a useful tool for storage and transmission of EEG. The biological significance of Fourier method and the LP method in respect to the microstructure of neuronal events in the generation of EEG is discussed.

Algorithms↗

PubMed2003

A theoretical framework to three-dimensional ultrasound reconstruction from irregularly sampled data.

Several techniques have been described in the literature in recent years for the reconstruction of a regular volume out of a series of ultrasound (US) slices with arbitrary orientations, typically scanned by means of US freehand systems. However, a systematic approach to such a problem is still missing. This paper focuses on proposing a theoretical framework for the 3-D US volume reconstruction problem. We introduce a statistical method for the construction and trimming of the sampling grid where the reconstruction will be carried out. The results using in vivo US data demonstrate that the computed reconstruction grid that encloses the region-of-interest (ROI) is smaller than those obtained from other reconstruction methods in those cases where the scanning trajectory deviates from a pure straight line. In addition, an adaptive Gaussian interpolation technique is studied and compared with well-known interpolation methods that have been applied to the reconstruction problem in the past. We find that the proposed method numerically outperforms former proposals in several control studies; subjective visual results also support this conclusion and highlight some potential deficiencies of methods previously proposed.

Algorithms↗

PubMed1989

Strategies and techniques for testing the precision, reliability and reproducibility of computerized two-dimensional gel electrophoresis analysis systems.

A set of test procedures is described for evaluating the reliability, precision and reproducibility of computer systems designed to analyze two-dimensional gel electrophoretograms. The Elsie 4 gel analysis system (Analyt. Biochem., 169, 49-70 (1988) is used to demonstrate the use of these tests. Three major groups of tests are described: analysis of the scanner; analysis of mathematically constructed model gels; and analysis of real gel images. Scanner tests involve evaluating the stability and reproducibility of the scanner. The tests consist primarily of measuring the output of the scanner over a time period to determine its stability, and evaluating the consistency of the scanner at different points in the scanning field. Commonly encountered real gel situations are simulated and analyzed by arranging computer-generated 'ideal spots' in different ways. With such gels we can determine such things as the ability of the computer system to separate closely resolved and/or shoulder spots; whether or not streaks and/or smears are handled correctly; how random noise affects measurements; and, in idealized situation, the accuracy of the quantitation. The idealized spots are generated using the two-dimensional Gaussian as a model of density distribution; other spot models can be used. The Elsie 4 system is capable of finding virtually any spot (if detection parameters are set low enough), and of resolving different-sized spots whose peaks are separated by one and one-half their mean half width). The most significant tests are done on a set of actual gels.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoradiography↗