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PubMed1992

Abstract processes in texture discrimination.

In this study some experiments on texture segmentation are reported using the local Gabor power spectrum. The techniques applied are: (1) supervised pixel classification; (2) boundary detection by spectral dissimilarity estimation; (3) region-based segmentation based on Gaussian spectral estimation; and (4) the same as (3) but based on central moments of the local spectrum. It is shown that very-acceptable-to-excellent results can be obtained. It is argued, however, that the shortcomings of region-based and boundary-based approaches require that both processes should act in parallel, not only in digital image processing but also in the modelling of visual perception.

Form Perception↗
PubMed1999

Spontaneous slow and fast MEG activity in male schizophrenics treated with clozapine.

RATIONALE: The atypical neuroleptic clozapine induces specific electroencephalogram changes, which have not been investigated using the technique of magnetoencephalography (MEG). OBJECTIVE: The present study investigated whether spontaneous magnetoencephalographic (MEG) activity in patients treated with clozapine differs from that in patients treated with haloperidol and untreated control subjects. METHODS: A 2 x 37 channel biomagnetic system was used to record spontaneous magnetic activity for the frequency ranges (2-6 Hz), (7.5-12 Hz), (12.5-30 Hz) in schizophrenic patients and controls in two trials within 3 weeks. After data acquisition, the processed data were digitally filtered and the spatial distribution of dipoles was determined by a 3-D convolution with a Gaussian envelope. The dipole localisation was calculated by the dipole density plot and the principal component analysis. The target parameters were absolute dipole values and the dipole localisations. The relationship between absolute dipole values, dipole localisations and psychopathological findings (documented by the use of the PANSS, BPRS-scale) during a 3 week period with constant doses of clozapine and haloperidol was investigated using correlation analysis. RESULTS: Our results lend strong support to the assumption of a significant elevation of absolute dipole values [dipole density maximum (Dmax), dipole number (Dtotal), absolute and relative dipole density] in the fast frequency range (12.5-30 Hz) over the left hemisphere, especially in the temporoparietal region by clozapine. In this area, we found a dipole concentration effect only in patients treated with the atypical neuroleptic, whereas the dipole distribution in patients treated with haloperidol and healthy controls was concentrated in the central region. With regard to the absolute dipole values in the frequency ranges 2-6 Hz (delta, theta) and 7.5-12 Hz (alpha), we found no statistically significant differences between the groups investigated. In the slow frequency range (2-6 Hz) no difference was found between the clozapine and haloperidol group for the dipole localisation, which predominated in the temporoparietal region, in contrast to the central dipole distribution in control subjects. CONCLUSIONS: The results of an increase in beta activity under clozapine demonstrate a smaller reduction in activity in terms of unspecific sensory and motor paradigms in comparison with typical neuroleptics. The temporoparietal concentration of dipoles, in particular over the left half of the brain, might illustrate either their special role in the disease process, or the effects of the medication. The latter possibility was supported by the differing dipole distribution in the clozapine group with a left temporoparietal centre in both frequency ranges, and a deviating central dipole localisation in the fast activity range in the haloperidol group.

Adult↗
PubMed2005

Accelerated degradation models for failure based on geometric Brownian motion and gamma processes.

Based on a generalized cumulative damage approach with a stochastic process describing degradation, new accelerated life test models are presented in which both observed failures and degradation measures can be considered for parametric inference of system lifetime. Incorporating an accelerated test variable, we provide several new accelerated degradation models for failure based on the geometric Brownian motion or gamma process. It is shown that in most cases, our models for failure can be approximated closely by accelerated test versions of Birnbaum-Saunders and inverse Gaussian distributions. Estimation of model parameters and a model selection procedure are discussed, and two illustrative examples using real data for carbon-film resistors and fatigue crack size are presented.

Equipment Failure Analysis↗
PubMed2003

Vicious walks with a wall, noncolliding meanders, and chiral and Bogoliubov-de Gennes random matrices.

Spatially and temporally inhomogeneous evolution of one-dimensional vicious walkers with wall restriction is studied. We show that its continuum version is equivalent with a noncolliding system of stochastic processes called Brownian meanders. Here the Brownian meander is a temporally inhomogeneous process introduced by Yor as a transform of the Bessel process that is the motion of radial coordinate of the three-dimensional Brownian motion represented in spherical coordinates. It is proved that the spatial distribution of vicious walkers with a wall at the origin can be described by the eigenvalue statistics of Gaussian ensembles of Bogoliubov-de Gennes Hamiltonians of the mean-field theory of superconductivity, which have a particle-hole symmetry. We report that a time evolution of the present stochastic process is fully characterized by the change of symmetry classes from type C to type CI in the nonstandard classes of random matrix theory of Altland and Zirnbauer. The relation between the noncolliding systems of the generalized meanders of Yor, which are associated with the even-dimensional Bessel processes, and the chiral random matrix theory is also clarified.

Journal Article↗
PubMed1986

The myocardial signature: absolute backscatter, cyclical variation, frequency variation, and statistics.

This paper studies the absolute myocardial backscatter as a function of the frequency and phase of the cardiac cycle. This was achieved by calibration of the ultrasonic instrumentation and the random diffraction process. We have discovered a first-order model in which the scattering from the myocardium is Rayleigh scattering with a cardiac cycle variation in the scattering cross section. Furthermore, the statistics are approximately those of a radio frequency waveform with two independent Gaussian components (Rayleigh envelope). Deviations from the first-order model suggest measurable fine structure related to myocardial ultrastructure. This model has profound effects on the choice of optimal radiation patterns and signal processing schemes for preparing diagnostic parameters (e.g., integrated backscatter).

Animals↗
PubMed2006

Filter for biomedical imaging and image processing.

Image filtering techniques have numerous potential applications in biomedical imaging and image processing. The design of filters largely depends on the a priori, knowledge about the type of noise corrupting the image. This makes the standard filters application specific. Widely used filters such as average, Gaussian, and Wiener reduce noisy artifacts by smoothing. However, this operation normally results in smoothing of the edges as well. On the other hand, sharpening filters enhance the high-frequency details, making the image nonsmooth. An integrated general approach to design a finite impulse response filter based on Hebbian learning is proposed for optimal image filtering. This algorithm exploits the interpixel correlation by updating the filter coefficients using Hebbian learning. The algorithm is made iterative for achieving efficient learning from the neighborhood pixels. This algorithm performs optimal smoothing of the noisy image by preserving high-frequency as well as low-frequency features. Evaluation results show that the proposed finite impulse response filter is robust under various noise distributions such as Gaussian noise, salt-and-pepper noise, and speckle noise. Furthermore, the proposed approach does not require any a priori knowledge about the type of noise. The number of unknown parameters is few, and most of these parameters are adaptively obtained from the processed image. The proposed filter is successfully applied for image reconstruction in a positron emission tomography imaging modality. The images reconstructed by the proposed algorithm are found to be superior in quality compared with those reconstructed by existing PET image reconstruction methodologies.

Algorithms↗
PubMed2003

Simple reaction times in color space: the influence of chromaticity, contrast, and cone opponency.

PURPOSE: This study examined the influence of stimulus chromaticity on simple reaction times (RTs) to determine the stage of chromatic processing that is most influential in their generation. METHODS: Simple RTs were measured in response to the cosinusoidally ramped onset of small, equiluminant, colored Gaussian spots. The chromaticity of these stimuli was varied, to modulate along a series of vectors in color space that included red-green (L-M) and blue-yellow (S-[L+M]) opponent axes. RESULTS: RTs are highly sensitive to small departures from subjective equiluminance. They are also dependent on stimulus chromaticity. The longest RTs are generated in response to equiluminant stimuli that isolate S-cone activity, whereas the shortest are generated by stimuli that modulate the L-M opponent axis. However, temporal processing differences are highly dependent on how the chromatic stimuli are scaled in relation to one another. The differences are reduced when scaling is based on detection threshold. The relationship between chromatic contrast and RT can be described by the modified Piéron equation RT = RT(0) + k x C(-1). CONCLUSIONS: Simple RTs generated in this study conform to the idea that they are largely determined by cone-opponency mechanisms. The use of cone contrast as a metric for scaling chromatic stimuli exaggerates differences between the temporal responsiveness of L-M and S-(L+M) opponency mechanisms.

Adult↗
PubMed1990

Determination of physiological levels of glucose in an aqueous matrix with digitally filtered Fourier transform near-infrared spectra.

A procedure is described for the measurement of clinically relevant concentrations of glucose in aqueous solutions with near-infrared (NIR) absorbance spectroscopy. A glucose band centered at 4400 cm-1 is used for this analysis. NIR spectra are collected over the frequency range 5000-4000 cm-1 with a Fourier transform spectrometer. A narrow-band-pass optical interference filter is placed in the optical path of the spectrometer to eliminate light outside this restricted range. This configuration provides a 2.9-fold reduction in spectral noise by utilizing the dynamic range of the detector solely for light transmitted through the filter. In addition, a novel spectral processing scheme is described for extracting glucose concentration information from the resulting absorbance spectra. The key component of this scheme is a digital Fourier filter that removes both high-frequency noise and low-frequency base-line variations from the spectra. Numerical optimization procedures are used to identify the best location and width of a Gaussian-shaped frequency response function for this Fourier filter. A dynamic area calculation, coupled with a simple linear base-line correction, provides an integrated area from the processed spectra that is linearly related to glucose concentrations over the range 1-20 mM. The linear calibration model accurately predicted glucose levels in a series of test solutions with an overall mean percent error of 2.5%. Based on the uncertainty in the parameters defining the calibration model and the variability of the magnitudes of the integrated areas, an overall uncertainty of 7.8% was estimated for predicted glucose concentrations.

Calibration↗
PubMed2006

Automatic identification of Mycobacterium tuberculosis by Gaussian mixture models.

Tuberculosis and other kinds of mycobacteriosis are serious illnesses for which early diagnosis is critical for disease control. Sputum sample analysis is a common manual technique employed for bacillus detection but current sample-analysis techniques are time-consuming, very tedious, subject to poor specificity and require highly trained personnel. Image-processing and pattern-recognition techniques are appropriate tools for improving the manual screening of samples. Here we present a new technique for sputum image analysis that combines invariant shape features and chromatic channel thresholding. Some feature descriptors were extracted from an edited bacillus data set to characterize their shape. They were statistically represented by using a Gaussian mixture model representation and a minimal error Bayesian classification procedure was employed for the last identification stage. This technique constitutes a step towards automating the process and providing a high specificity.

Bacteriological Techniques↗
PubMed2000

Predicting the time to quasi-extinction for populations far below their carrying capacity.

Populations threatened by extinction are often far below their carrying capacity. A population collapse or quasi-extinction is defined to occur when the population size reaches some given lower density. If this density is chosen to be large enough for the demographic stochasticity to be ignored compared to environmental stochasticity, then the logarithm of the population size may be modelled by a Brownian motion until quasi-extinction occurs. The normal-gamma mixture of inverse Gaussian distributions can then be applied to define prediction intervals for the time to quasi-extinction in such processes. A similar mixture is used to predict the population size at a finite time for the same process provided that quasi-extinction has not occurred before that time. Stochastic simulations indicate that the coverage of the prediction interval is very close to the probability calculated theoretically. As an illustration, the method is applied to predict the time to extinction of a declining population of white stork in southwestern Germany.

Animals↗
PubMed1999

Identification by microscopically controlled comet assay of peritoneal macrophages in a mixture of peritoneal exudate for DNA strand break analysis.

A simple modification of the alkaline comet assay allows the study of DNA damage in a specific cell type in a mixture of primary cells. Peritoneal macrophages from mice are selected from other peritoneal exudate cells without complex preparation and separation steps by their size and shape of the nuclei and their comets. The DNA damage can be well characterised by the manually monitored parameter 'tail length'. Complex measurement of the 'tail moment', often used for characterising DNA damage is not required, a fact which further simplifies the protocol. The distribution of tail length within one sample is symmetric and can be described by a Gaussian distribution and the mean tail length. As a first application, UV-A sensitivity of resident and stimulated macrophages was studied. The resident macrophages were more sensitive to UV-A than the stimulated ones. DNA damage repair follows the same simple monoexponential time course for both cell types. The simplicity of results, i.e., applicability of tail lengths and Gaussian statistics as well as monoexponential kinetics, suggest that microscopically controlled comet assay is well suited to study elementary processes of DNA damage induction and repair.

Animals↗
PubMed2001

Observation of a topological relaxation mode in microemulsions.

We report the observation of two diffusive relaxation modes in a very swollen microemulsion, measured by quasielastic light scattering experiments. In addition to a short-time diffusion process, we observe a long-time diffusive relaxation mode with unusual scaling behavior: the diffusion constant D is an exponential function of the characteristic length scale xi, D~exp(-xi). This observation provides experimental evidence for thermally activated topological relaxation of random fluid phases, as predicted by Milner et al. [J. Phys. (Paris) 51, 2629 (1990)]. We describe the activation energy of such a process by some simple microscopic model for the surfactant layer and give an estimation of the membrane elastic and Gaussian moduli kappa and kappa;, and its spontaneous curvature.

Journal Article↗
PubMed2001

Intensity-weighted phase-derivative statistics.

It is shown that amplitude weighting can improve the accuracy of measurements of the frequency offset of a signal contaminated by multiplicative Gaussian noise. The more general non-Gaussian case is investigated through study of the statistics of a simple phase-screen scattering model. Formulas are derived for the low-order moments of the intensity-weighted phase derivative. Numerical simulation is tested against these results and is used to generate full probability densities that are analytically intractable and to determine the optimum weighting for the non-Gaussian regime of the model. The results are relevant to a variety of remote-sensing and signal-processing problems.

Journal Article↗
PubMed1999

[A robust method for automated detection and grading of retinal venous beading in diabetes mellitus].

Diabetes mellitus is a high prevalence disease in the world and retinal vessel changes such as venous beading are common findings in patients suffering from long-standing diabetes. Three methods (1. Combined use of the SED, ROSE, Modified LoG algorithm; 2. Gaussian fitting-based method; 3. Gradient-based method) for detection venous caliber were used in the first processing stage, then the magnitude of fast Fourier transform of variation function of venous caliber was used as a detective index of venous beading. The reliability of the results, justified by their low variability, makes feasible the standardization of quantitative parameters to be used both in the diagnosis and in the prognosis of diabetes, and also allows prospective studies based upon them.

Algorithms↗
PubMed2006

Feasibility of ultra-low-dose multi-detector-row CT-colonography: detection of artificial endoluminal lesions in an in-vitro-model with optimization of image quality using a noise reduction filter algorithm.

PURPOSE: To assess the most favorable slice thickness in Multi-Detector-Row CT-colonography (MDCTC), and the feasibility of dose reduction in an in-vitro-setting as well as the possibility of optimization of image quality using a noise reduction filter algorithm. - MATERIALS AND METHODS: 18 artificial lesions with sizes from 1 to 8 mm were randomly positioned in two cleansed pig colons. At a "Somatom Plus 4 Volume Zoom", six scanning protocols using a slice collimation of 2.5, 1, and 1 mm with a reconstructed slice thickness of 3, 3, and 1.25 mm were performed with tube currents of 100, and 10 mAs, respectively. Using a non-commercial software, a non-linear Gaussian filter was used to minimize image noise. Image noise was assessed before and after application of the filtering process. Using a threshold of -750 HU, two blinded readers analyzed the virtual colonography in respect to lesion location, size, and shape. Artifacts were noted. An automated detection system was evaluated. - RESULTS: Using 10 mAs, a ten-fold dose reduction was achieved. After application of the mathematical filter, image noise was reduced by 45-80% for 100 mAs, and by 50-70% for 10 mAs scans. Only with a slice thickness of 1.25 mm, all lesions could be detected. The definition of lesion size and shape was more accurate with higher mAs. Only minor noise artifacts were noted on low-dose images. The automated polyp detector marked not more than 60% of artificial lesions. - CONCLUSION: MDCTC benefits from narrow slice collimation. In an in-vitro-model, a significant dose reduction is achievable with preservation of a high lesion detection rate. The noise reduction filter algorithm improved image quality substantially.

Algorithms↗
PubMed1988

A nonparametric analysis method for evaluation of survey results. Application to ligand assay survey.

A nonparametric analysis method that does not depend on gaussian data distributions is proposed for evaluation of proficiency survey results. The proposed method also provides a mechanism for processing results qualitatively reported as "less than" or "greater than" pre-established limits of quantitation. The evaluation limits for acceptability are calculated to include the central 95% of all test results from all methods. Systematic differences in the levels of analyte concentrations caused by differences in analytical measurement systems are normalized by mathematically transforming the test results from each analytic method by dividing them by their method-specific group median. This nonparametric method was compared with the traditional mean +/- 2 SD limits using data collected for digoxin, free thyroxine, and quantitative chorionic gonadotropin measurement in the College of American Pathologists Ligand Assay Survey. The nonparametric method more accurately classified 5% of the results as "unacceptable." When more than 2.5% of the test results for a method were designated as "less than," no lower limit was used for evaluation and only 2.5% of results were classified as "unacceptably high." The current College of American Pathologists procedure favors analytical methods with larger coefficients of variation by setting wider limits of acceptability, while the proposed procedure favors methods with smaller coefficients of variation.

Chorionic Gonadotropin↗
PubMed1990

Non-stationary analysis of electromyographic activity prior to a ballistic voluntary contraction.

The electromyogram (EMG) in a ballistic voluntary action of muscle after a slight sustained contraction shows a short period of decrease or disappearance of the activity prior to the onset of phasic discharge. The EMG of elbow extensor (triceps brachii) prior to phasic discharge was analyzed as a non-stationary stochastic process. The distribution of the EMG amplitude began to change from 100 ms before the phasic discharge. The statistical test showed that the EMG activity became non-Gaussian from 30 to 50 ms before the phasic discharge. The standard deviation of the EMG showed a decreasing tendency from 80 to 100 ms prior to the phasic discharge. The statistical evidence for the EMG depression was also confirmed in fourth and sixth order coefficients of the orthogonal expansion of the probability density function. These results suggested that the EMG process profoundly changed its statistical characteristics just before the phasic discharge.

Action Potentials↗