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PubMed2004

Error correction on a tree: an instanton approach.

We introduce a method that allows analytical or semianalytical estimating of the post-error correction bit error rate (BER) when a forward-error correction is utilized for transmitting information through a noisy channel. The generic method that applies to a variety of error-correction schemes in the regimes where the BER is low is illustrated using the example of a finite-size code approximated by a treelike structure. Exploring the statistical physics formulation of the problem we find that the BER decreases with the signal-to-noise ratio nonuniformly, i.e., crossing over through a sequence of phases. The higher the signal-to-noise ratio the lower the symmetry of the phase dominating BER.

Journal Article↗

PubMed2006

First-principles calculation of the single impurity surface Kondo resonance.

We perform first-principles calculations of the surface and bulk wave functions of the Cu(111) surface and their hybridization energies to a Co adatom, including the potential scattering from the Co. By analyzing the calculated hybridization energies, we find the bulk states dominate the contribution to the Kondo temperature, in agreement with recent experiments. Furthermore, we also calculate the tunneling conductance of a scanning tunneling microscope and compare our results with recent experiments of Co impurities in the Cu(111) surface. Good quantitative agreement is found at short parallel impurity-tip distances (<6 A). Our results indicate the need for a new formulation of the problem at larger distances.

Journal Article↗

PubMed1990

An envelope-based approach for direct phase determination of macromolecular structures.

An initial electron density distribution for a crystal structure may be directly derived from observed diffraction data by maximizing the product of the observed and calculated Patterson functions with respect to the electron density values within an envelope. This maximization problem may be formulated as an eigenvalue equation, in which potential electron density distributions are obtained as eigendensities (eigenvectors) of a symmetric matrix. Elements of this matrix depend only on the indices and intensities of the observed reflections, and on the coordinates of grid points inside the envelope. Eigendensities are calculated for a set of small envelopes (enclosing about 20% of the molecular volume) covering a unique region of the unit cell whose points are unrelated by space-group operations, origin shifts or changes in enantiomorph. On the basis of correlation coefficients between the observed and calculated values for both the Patterson function and structure-factor amplitudes, a small set of eigendensities are selected for combination into a final electron density distribution. This electron density distribution may be Fourier transformed to yield calculated structure factors. Test calculations on lysozyme indicate that phase errors of less than 60 degrees may be obtained for strong low-resolution reflections by this procedure. An extension of this approach to handle crystal structures containing non-crystallographic symmetry is described.

Chemical Phenomena↗

PubMed2005

Demosaicing by successive approximation.

In this paper, we present a fast and high-performance algorithm for color filter array (CFA) demosaicing. CFA demosaicing is formulated as a problem of reconstructing correlated signals from their downsampled versions with an opposite phase. The major contributions of this work include (1) a new iterative demosaicing algorithm in the color difference domain and (2) a spatially adaptive stopping criterion for suppressing color misregistration and zipper artifacts in the demosaiced images. We have compared the proposed demosaicing algorithm with two current state-of-the-art techniques reported in the literature. Ours outperforms both of them on demosaicing performance and computational cost.

Algorithms↗

PubMed2005

Statistical bias in 3-D reconstruction from a monocular video.

The present state-of-the-art in computing the error statistics in three-dimensional (3-D) reconstruction from video concentrates on estimating the error covariance. A different source of error which has not received much attention is the fact that the reconstruction estimates are often significantly statistically biased. In this paper, we derive a precise expression for the bias in the depth estimate, based on the continuous (differentiable) version of structure from motion (SfM). Many SfM algorithms, or certain portions of them, can be posed in a linear least-squares (LS) framework Ax = b. Examples include initialization procedures for bundle adjustment or algorithms that alternately estimate depth and camera motion. It is a well-known fact that the LS estimate is biased if the system matrix A is noisy. In SfM, the matrix A contains point correspondences, which are always difficult to obtain precisely; thus, it is expected that the structure and motion estimates in such a formulation of the problem would be biased. Existing results on the minimum achievable variance of the SfM estimator are extended by deriving a generalized Cramer-Rao lower bound. A detailed analysis of the effect of various camera motion parameters on the bias is presented. We conclude by presenting the effect of bias compensation on reconstructing 3-D face models from rendered images.

Algorithms↗

PubMed2005

Figure-ground segmentation from occlusion.

Layered video representations are increasingly popular; see [2] for a recent review. Segmentation of moving objects is a key step for automating such representations. Current motion segmentation methods either fail to segment moving objects in low-textured regions or are computationally very expensive. This paper presents a computationally simple algorithm that segments moving objects, even in low-texture/low-contrast scenes. Our method infers the moving object templates directly from the image intensity values, rather than computing the motion field as an intermediate step. Our model takes into account the rigidity of the moving object and the occlusion of the background by the moving object. We formulate the segmentation problem as the minimization of a penalized likelihood cost function and present an algorithm to estimate all the unknown parameters: the motions, the template of the moving object, and the intensity levels of the object and of the background pixels. The cost function combines a maximum likelihood estimation term with a term that penalizes large templates. The minimization algorithm performs two alternate steps for which we derive closed-form solutions. Relaxation improves the convergence even when low texture makes it very challenging to segment the moving object from the background. Experiments demonstrate the good performance of our method.

Algorithms↗

PubMed2006

Relevance feedback for CBIR: a new approach based on probabilistic feature weighting with positive and negative examples.

In content-based image retrieval, understanding the user's needs is a challenging task that requires integrating him in the process of retrieval. Relevance feedback (RF) has proven to be an effective tool for taking the user's judgement into account. In this paper, we present a new RF framework based on a feature selection algorithm that nicely combines the advantages of a probabilistic formulation with those of using both the positive example (PE) and the negative example (NE). Through interaction with the user, our algorithm learns the importance he assigns to image features, and then applies the results obtained to define similarity measures that correspond better to his judgement. The use of the NE allows images undesired by the user to be discarded, thereby improving retrieval accuracy. As for the probabilistic formulation of the problem, it presents a multitude of advantages and opens the door to more modeling possibilities that achieve a good feature selection. It makes it possible to cluster the query data into classes, choose the probability law that best models each class, model missing data, and support queries with multiple PE and/or NE classes. The basic principle of our algorithm is to assign more importance to features with a high likelihood and those which distinguish well between PE classes and NE classes. The proposed algorithm was validated separately and in image retrieval context, and the experiments show that it performs a good feature selection and contributes to improving retrieval effectiveness.

Algorithms↗

PubMed2006

Pattern generation using likelihood inference for cellular automata.

Cellular automata are discrete dynamical systems which evolve on a discrete grid. Recent studies have shown that cellular automata with relatively simple rules can produce highly complex patterns. We develop likelihood-based methods for estimating rules of cellular automata aimed at the re-generation of observed regular patterns. Under noisy data, our approach is equivalent to estimating the local map of a stochastic cellular automaton. Direct computations of the maximum likelihood estimates are possible for regular binary patterns. The likelihood formulation of the problem is congenial with the use of the minimum description length principle as a model selection tool. We illustrate our method with a series of examples using binary images.

Algorithms↗

PubMed2003

Entropy-based dual-portal-to-3-DCT registration incorporating pixel correlation.

For patient setup verification in external beam radiotherapy (EBRT) of prostate cancer, we developed an information theoretic registration framework, called the minimax entropy registration framework, to simultaneously and iteratively segment portal images and register them to three-dimensional (3-D) computed tomography (CT) image data. The registration framework has two steps, the max step and the min step, and evaluates appropriate entropies to estimate segmentations of the portal images and to find the transformation parameters. In the initial version of the algorithm (Bansal et al. 1999), we assumed image pixels to be independently distributed, an assumption not true in general. Thus, to better segment the portal images and to improve the accuracy of the estimated registration parameters, in this initial formulation of the problem, the correlation among pixel intensities is modeled using a one-dimensional Markov random process. Line processes are incorporated into the model to improve the estimation of segmentation of the portal images. In the max step, the principle of maximum entropy is invoked to estimate the probability distribution on the segmentations. The estimated distribution is then incorporated into the min step to estimate the registration parameters. Performance of the proposed framework is evaluated and compared to that of a mutual information-based registration algorithm using both simulated and real patient data. In the proposed registration framework, registration of the 3-D CT image and the portal images is guided by an estimated segmentation of the pelvic bone. However, as the prostate can move with respect to the pelvic structure, further localization of the prostate using ultrasound image data is required, an issue to be further explored in future.

Algorithms↗

PubMed2004

Identification of complex systems based on neural and Takagi-Sugeno fuzzy model.

The paper describes a neuro-fuzzy identification approach, which uses numerical data as a starting point. The proposed method generates a Takagi-Sugeno fuzzy model, characterized with transparency, high accuracy and a small number of rules. The process of self-organizing of the identification model consists of three phases: clustering of the input-output space using a self-organized neural network; determination of the parameters of the consequent part of a rule from over-determined batch least-squares formulation of the problem, using singular value decomposition algorithm; and on-line adaptation of these parameters using recursive least-squares method. The verification of the proposed identification approach is provided using four different problems: two benchmark identification problems, speed estimation for a dc motor drive, and estimation of the temperature in a tunnel furnace for clay baking.

Journal Article↗

PubMed2000

Does evolution of iteroparous and semelparous reproduction call for spatially structured systems?

A persistent question in the evolution of life histories is the fitness trade-off between reproducing only once (semelparity) in a lifetime or reproducing repeated times in different seasons (iteroparity). The problem can be formulated into a research agenda by assuming that one reproductive strategy is resident (has already evolved) and by asking whether invasion (evolution) of an alternative reproductive strategy is possible. For a spatially nonstructured system, Bulmer (1994) derived the relationship v + PA < 1 (PA is adult survival; vbS and bS are offspring numbers for iteroparous and semelparous breeding strategies, respectively) at which semelparous population cannot be invaded by an iteroparous mutant. When the inequality is changed to v + PA > 1, invasion of a semelparous mutant is not possible. From the inequalities, it is easy to see that possibilities for evolutionary establishment of a novel reproductive strategy are rather narrow. We extended the evolutionary scenario into a spatially structured system with dispersal linkage among the subunits. In this domain, a rare reproductive strategy can easily invade a population dominated by a resident reproductive strategy. The parameter space enabling invasion is far more generous with spatially structured evolutionary scenarios than in a spatially nonstructured system.

Animals↗

PubMed1995

Quality assurance in palliative care: some of the problems.

This review considers some of the difficulties associated with quality assurance in the context of palliative care. In particular, it will focus on the key question of who should be responsible for assessing quality and the problems of over-reliance on one particular perspective. Problems encountered in formulating criteria, setting standards and developing outcome measures, many of which are exacerbated in the palliative care setting, are discussed. A review of some of the methods currently available for assessing quality and their limitations are delineated. Finally, attention is drawn to some of the practical and methodological difficulties currently confronting palliative care with regard to quality assurance.

Attitude of Health Personnel↗

PubMed1992

Evaluation of antimicrobial interactions between chlorhexidine, quaternary ammonium compounds, preservatives and excipients.

The antimicrobial interactions of 49 combinations of chlorhexidine, quaternary ammonium compounds, preservatives and excipients were evaluated by the method of Berenbaum and the checkerboard titration method, with Staphylococcus aureus CIP 53154 and Escherichia coli CIP 54127 as test strains. MIC determinations were carried out as a preliminary step, and relative growth intensity was used to describe the bacteriostatic activity of surface-active agents (Amonyl 380 BA, Amonyl 671 SB). In the study of combinations, results were interpreted with Fractional Inhibitory Concentration indexes and represented by isobolograms. A fair correlation was shown between the method of Berenbaum and the checkerboard titration method. Combinations between chlorhexidine, cetrimonium bromide and benzalkonium chloride were synergistic or additive; combinations of antiseptics and preservatives were generally not antagonistic. The methods were also well adapted to the study of interactions involving surface-active agents, a critical problem in the formulation of topical antimicrobial agents.

Anti-Infective Agents, Local↗

PubMed1986

Improving risk communication.

This paper explores reasons for difficulties in communicating risks among analysts, the laypublic, media, and regulators. Formulating risk communication problems as decisions involving objectives and alternatives helps to identify strategies for overcoming these difficulties. Several strategies are suggested to achieve risk communication objectives like improving public knowledge about risks and risk management, encouraging risk reduction behavior, understanding public values and concerns, and increasing trust and credibility.

Communication↗

PubMed1991

Planning replacement of natural gas distribution systems under constraints on acceptable risk from explosions.

Natural gas distribution systems in the United States were developed primarily in the first half of this century, utilizing materials such as cast iron and then steel. Over time, cast iron and steel pipe sections became weak from corrosion and are subject to failure which in turn can lead to explosions and possible injury and loss of life. Gas utilities maintain system integrity through repair-replacement programs where pipe sections are prioritized for replacement in any given year through cost-benefit analysis; however, the total annual amount to be budgeted for replacement is left to engineering judgment. This approach has left some utilities vulnerable to criticism that their current replacement rate on cast iron pipe is not great enough and that public safety is being compromised. This paper addresses the problem situation by formulating a linear programming replacement decision model which augments cost-benefit analysis with explicit constraints on acceptable risk to human life from fire/explosion. The model is illustrated for a hypothetical utility.

Cost-Benefit Analysis↗

PubMed2003

Nonlinear parameter estimation for solution-diffusion models of membrane pervaporation.

An optimization-based procedure for estimating unknown parameters in solution-diffusion models of membrane pervaporation is presented. Permeation of two components through a polymer membrane is described by distinct solution and diffusion models. The solution model is based on a modified form of Flory-Huggins theory that accounts for interactions between the two penetrants. The diffusion model is derived from Fick's law, where the diffusion coefficients are allowed to depend on the local concentration of each component in the membrane. A phenomenologic relation is used to account for the effect of temperature on the component fluxes. The solution and diffusion models, as well as the temperature-flux relation, contain parameters that are not directly measurable. It is shown that these parameters can be estimated effectively from sorption and flux data by the solution of suitably formulated nonlinear optimization problems. The separation of styrene and ethylbenzene with a polyurethane membrane is used to illustrate the parameter estimation procedure.

Chemistry↗

PubMed1980

The analysis of interpersonal perceptions as a repertory grid.

In order to set goals and evaluate progress in therapy with couples it is important to begin with a clear formulation of the problem. The technique described provides quantitative data on three aspects of a relationship. Firstly, the extent to which a couple are in fact using a common language. Second, the extent to which they agree, understand each other's point of view, and realize how they are understood. Thirdly, the differences between their perception of the present and their goals for themselves and one another in the future. The investigation uses personal constructs elicited from the couple and the results are analysed as a repertory grid. Summary measures provided by the grid analysis organize the large amount of data and provide a general context for understanding the relationship within which particular aspects can be identified for more detailed analysis.

Adult↗

PubMed1996

An episode of syncope attacks in adolescent schoolgirls: investigations, intervention and outcome.

An increasing number of students in a secondary convent girls school developed syncope attacks over a time course of about two months. Fourteen students who suffered from syncope and 12 other students from the same class with no symptoms were assessed by a team of psychologists and paediatricians with the aim of identifying the cause of the problem and to formulate possible remedial action. Psychological assessments included a mental state examination, developmental, personal and psychological history, state-trait anxiety, self-esteem, hypnotic suggestibility, and students' beliefs about the cause and nature of the syncope attacks. Physical investigations included physical examination, blood pressure and electrocardiogram. The results indicated that most participants and controls had no physical or psychological pathologies. The two groups were not different on the physical and psychological measures. Analyses of the interview data, however, indicated that all the syncope sufferers belonged to a cohesive and exclusive social network. Social psychological circumstances rather than individual psychopathology were noted to be primarily responsible for the spread and maintenance of the mass hysterical influence. Intervention consisted of health education, authoritative reassurance and back-up support. Follow-up assessment after three and 12 months indicated no further syncope episodes.

Adolescent↗