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At least 811 records · Page 45Linked to original sources
PubMedDate not supplied

The role of intra-abdominal pressure in spinal unloading.

Previous studies on how an increase in intra-abdominal pressure (IAP) effects the loading of the lumbar spine during back extension show diverging results. From a critical review of the literature we deduce a simplified, but consistent, model of the mechanisms involved in IAP-induced unloading of the lumbar spine. The model is then expanded by explicitly incorporating equilibrium equations for the pressurised abdomen and the abdominal wall. It is shown that the unloading effect of IAP can be viewed as that of a pressurised column of fixed cross-sectional area, between the rib cage and pelvis. Different abdominal forms are examined and a form with zero longitudinal curvature is found to have some important mechanical benefits for the generation of IAP-induced alleviation of compressive loading of the lumbar spine.

Abdomen↗
PubMed2006

Measuring the acoustic effects of compression amplification on speech in noise.

This letter contains a description of an inversion technique that allows for separation of speech and noise and its application to quantifying the acoustic effects of wide-dynamic-range compression (WDRC) on speech in background noise. Three main findings are reported: that fast-acting WDRC further degrades signal-to-noise ratio; that the effective compression ratio is lower for speech in noise than speech in quiet; and that in contrast to speech in quiet, the amplitude envelope of speech is mostly unaffected when compressed in background noise.

Algorithms↗
PubMed2001

Mechanical compression influences intracellular Ca2+ signaling in chondrocytes seeded in agarose constructs.

Ca2+ signaling forms part of a possible mechanotransduction pathway by which chondrocytes may alter their metabolism in response to mechanical loading. In this study, a well-characterized model system utilizing bovine articular chondrocytes embedded in 4% agarose constructs was used to investigate the effect of physiological mechanical compressive strain applied after 1 and 3 days in culture. The intracellular Ca2+ concentration was measured by use of the ratiometric Ca2+ indicator indo 1-AM and confocal microscopy. A positive Ca2+ response was defined as a percent increase in Ca2+ ratio above a preset threshold. A significantly greater percentage of cells exhibited a positive Ca2+ response in strained constructs compared with unstrained controls at both time points. In strained constructs, treatment with either Ga3+ or EGTA significantly reduced the number of positive Ca2+ responders compared with untreated controls. These results represent an important step in understanding the physiological role of intracellular Ca2+ in chondrocytes under mechanical compression.

Algorithms↗
PubMedDate not supplied

Genetic algorithm applied to the selection of factors in principal component-artificial neural networks: application to QSAR study of calcium channel antagonist activity of 1,4-dihydropyridines (nifedipine analogous).

A QSAR algorithm, principal component-genetic algorithm-artificial neural network (PC-GA-ANN), has been applied to a set of newly synthesized calcium channel blockers, which are of special interest because of their role in cardiac diseases. A data set of 124 1,4-dihydropyridines bearing different ester substituents at the C-3 and C-5 positions of the dihydropyridine ring and nitroimidazolyl, phenylimidazolyl, and methylsulfonylimidazolyl groups at the C-4 position with known Ca(2+) channel binding affinities was employed in this study. Ten different sets of descriptors (837 descriptors) were calculated for each molecule. The principal component analysis was used to compress the descriptor groups into principal components. The most significant descriptors of each set were selected and used as input for the ANN. The genetic algorithm (GA) was used for the selection of the best set of extracted principal components. A feed forward artificial neural network with a back-propagation of error algorithm was used to process the nonlinear relationship between the selected principal components and biological activity of the dihydropyridines. A comparison between PC-GA-ANN and routine PC-ANN shows that the first model yields better prediction ability.

Algorithms↗
PubMed1994

MOUSE-III: learning rules of conformational analysis from X-ray data.

MOUSE-III is learning program that finds rules of conformational analysis from raw crystallographic data. The program perceives molecular features, finds conformational classes in the data and then learns rules that link features to classes. The rules that MOUSE learns are capable of correctly assigning conformations to ring systems that were not used for training with greater than 95% accuracy, when MOUSE was presented with sufficient data. The rules also show a compression of as much as 99% when compared to the raw data. This is accomplished through abstraction and generalization. The algorithm is presented along with a carefully worked example. An example of a learned rule is also presented and analyzed. Some conclusions about the scope and limitations of the learning process are presented.

Algorithms↗
PubMed2003

Mechanical consequences of different scenarios for simulated bone atrophy and recovery in the distal radius.

Metabolic bone diseases such as osteoporosis usually cause a decrease in bone mass and a deterioration of bone microarchitecture leading to a decline in bone strength. Methods to predict bone strength in patients are currently based on bone mass only. It has been suggested that an improved prediction of bone strength might be possible if structural changes are taken into account as well. In this study we evaluated which structural parameters (other than bone mass) are the best predictors for changes in bone mechanical properties of the human radius after different bone atrophy scenarios and whether the original strength of the affected bone can be recovered if bone loss is restored by thickening of the remaining structures. To answer these questions, a human radius was measured with a microcomputer tomography scanner to extract the full three-dimensional architecture of the distal radius at an isotropic resolution of 80 microm. Eight models with modified bone architecture were created and the mechanical variations due to these modifications were studied using microfinite element (micro-FE) simulations. In four models mass was lowered by 20%, either by reducing cortical thickness, trabecular thickness, or number of trabeculae or by overall thinning of structures. In the other four models bone mass was restored to the original value using a trabecular bone thickening procedure. The micro-FE analyses revealed that most load was carried by the cortical bone. For this reason, bone strength was affected most in the reduced cortical thickness model. For the same reason, the trabecular bone atrophy scenarios, all of which affected bone strength in a very similar way, resulted in less dramatic bone strength reduction. The restoration of bone mass did not recover the original bone strength. These findings demonstrate that the importance of different parameters for the prediction of bone strength also depends on the mechanical loading. This could explain why results of earlier studies on the importance of structural parameters can be inconsistent and site-dependent.

Adult↗
PubMed2006

Adaptive MAP error concealment for dispersively packetized wavelet-coded images.

In this paper, we present an adaptive maximum a posteriori (MAP) error concealment algorithm for dispersively packetized wavelet-coded images. We model the subbands of a wavelet-coded image as Markov random fields, and use the edge characteristics in a particular subband, and regularity properties of subband/wavelet samples across scales, to adapt the potential functions locally. The resulting adaptive MAP estimation gives PSNR advantages of up to 0.7 dB compared to the competing algorithms. The advantage is most evident near the edges, which helps improve the visual quality of the reconstructed images.

Algorithms↗
PubMed2005

A practical approach for the differential diagnosis of chronic leg pain in the athlete.

Chronic lower leg pain results from various conditions, most commonly, medial tibial stress syndrome, stress fracture, chronic exertional compartment syndrome, nerve entrapment, and popliteal artery entrapment syndrome. Symptoms associated with these conditions often overlap, making a definitive diagnosis difficult. As a result, an algorithmic approach was created to aid in the evaluation of patients with complaints of lower leg pain and to assist in defining a diagnosis by providing recommended diagnostic studies for each condition. A comprehensive physical examination is imperative to confirm a diagnosis and should begin with an inquiry regarding the location and onset of the patient's pain and tenderness. Confirmation of the diagnosis requires performing the appropriate diagnostic studies, including radiographs, bone scans, magnetic resonance imaging, magnetic resonance angiography, compartmental pressure measurements, and arteriograms. Although most conditions causing lower leg pain are treated successfully with nonsurgical management, some syndromes, such as popliteal artery entrapment syndrome, may require surgical intervention. Regardless of the form of treatment, return to activity must be gradual and individualized for each patient to prevent future athletic injury.

Algorithms↗
PubMed2001

Three-dimensional time-resolved optical tomography of a conical breast phantom.

A 32-channel time-resolved imaging device for medical optical tomography has been employed to evaluate a scheme for imaging the human female breast. The fully automated instrument and the reconstruction procedure have been tested on a conical phantom with tissue-equivalent optical properties. The imaging protocol has been designed to obviate compression of the breast and the need for coupling fluids. Images are generated from experimental data with an iterative reconstruction algorithm that employs a three-dimensional (3D) finite-element diffusion-based forward model. Embedded regions with twice the background optical properties are revealed in separate 3D absorption and scattering images of the phantom. The implications for 3D time-resolved optical tomography of the breast are discussed.

Journal Article↗
PubMed2000

Sequence complexity for biological sequence analysis.

A new statistical model for DNA considers a sequence to be a mixture of regions with little structure and regions that are approximate repeats of other subsequences, i.e. instances of repeats do not need to match each other exactly. Both forward- and reverse-complementary repeats are allowed. The model has a small number of parameters which are fitted to the data. In general there are many explanations for a given sequence and how to compute the total probability of the data given the model is shown. Computer algorithms are described for these tasks. The model can be used to compute the information content of a sequence, either in total or base by base. This amounts to looking at sequences from a data-compression point of view and it is argued that this is a good way to tackle intelligent sequence analysis in general.

Algorithms↗
PubMed2004

Data preprocessing by wavelets and genetic algorithms for enhanced multivariate analysis of LC peptide mapping.

Peptide mapping by means of liquid chromatography is a powerful technique used for the characterisation and analysis of the primary structure of proteins. Subtle changes in the covalent structure of the protein can be detected by means of the chromatographic profile (fingerprint). Chromatographic methods, however, display variations in the chromatographic profile even at identical instrumental settings and sample conditions. These variations may be due to changes of the chromatographic conditions, e.g. slight shifts in column temperature, and degradation or alterations of the stationary phase or small changes in the trifluoroacetic acid (TFA) concentration. Such variations may result in varying retention times and peak shapes of the analytes and differences in the chromatographic baseline, thereby having a detrimental impact on the results obtained on multivariate analysis of peptide maps. In order to reduce the non-sample-related variations and to be able to more fully extract the information in peptide mapping, approaches for achieving this objective are outlined in the present study. These methods are denoising and data compression of the chromatograms by wavelets, baseline corrections by linear interpolation, and peak shift alignments towards a target chromatogram by means of a genetic algorithm. Visual inspections of preprocessed chromatograms and principal component analysis (PCA) score plots demonstrate the efficiency of the methodology used. Furthermore, deliberately added changes, e.g. insertions of small Gaussian peaks (outliers), are more easily detected by the proposed methods than from the original chromatograms by multivariate analysis.

Algorithms↗
PubMed2006

Varying complexity in tree-structured image distribution models.

Probabilistic models of image statistics underlie many approaches in image analysis and processing. An important class of such models have variables whose dependency graph is a tree. If the hidden variables take values on a finite set, most computations with the model can be performed exactly, including the likelihood calculation, training with the EM algorithm, etc. Crouse et al. developed one such model, the hidden Markov tree (HMT). They took particular care to limit the complexity of their model. We argue that it is beneficial to allow more complex tree-structured models, describe the use of information theoretic penalties to choose the model complexity, and present experimental results to support these proposals. For these experiments, we use what we call the hierarchical image probability (HIP) model. The differences between the HIP and the HMT models include the use of multivariate Gaussians to model the distributions of local vectors of wavelet coefficients and the use of different numbers of hidden states at each resolution. We demonstrate the broad utility of image distributions by applying the HIP model to classification, synthesis, and compression, across a variety of image types, namely, electrooptical, synthetic aperture radar, and mammograms (digitized X-rays). In all cases, we compare with the HMT.

Algorithms↗
PubMed1999

Loss of resolution in a corneal topography system.

BACKGROUND: Videokeratoscopes employing a spherically biased reconstruction algorithm are known to inaccurately model radially aspheric surfaces. The purpose of this study was to quantify the effect of a spherically biased reconstruction on the accuracy and resolution of corneal shape measurements. METHODS: Corneal asphericity and apical radius of curvature values were calculated from the local radius of curvature data produced by the EyeSys Corneal Analysis System running version 2.1.08 of the software on 40 normal eyes. A theoretical model was constructed to simulate the effect of the spherically biased reconstruction. RESULTS: A significant difference was found between the asphericity values from this study and previous studies of corneal shape (P < 0.05 all cases). A large compression of the range of asphericity values was also found. Theoretical simulation demonstrated that these changes could be explained by the inherent error in the reconstruction algorithm alone. CONCLUSIONS: The measured asphericity value was lower by a factor of 3 than accepted values for normal corneas. There was a similar reduction in the range of asphericities recorded. The consequence is a loss of resolution by a factor of 3 when assessing corneal shape change as measured by corneal asphericity. This may compromise longitudinal clinical studies or be important when one is making measurements on different corneal topography systems.

Adult↗
PubMed1993

Unjustified use of the Quality of Well-Being Scale in priority setting in Oregon.

The Quality of Well-Being Scale (QWB) is an instrument for valuing health states on a continuum from unity (healthy) to zero (dead). While it seems safe to assume that the values have ordinal properties, there is neither theoretical nor empirical basis for claiming that they have the cardinal properties that are required in calculations of social benefit in cost-utility analysis. Failure to recognize this led the Oregon Health Services Commission to produce a QWB-based priority list with a number of counterintuitive rankings. A set of health state values based on upper end compression would have produced a list more in accordance with public preferences.

Algorithms↗
PubMedDate not supplied

Information preserving image compression for archiving NMR images.

This paper presents a result on information preserving compression of NMR images for the archiving purpose. Both Lynch-Davisson coding and linear predictive coding have been studied. For NMR images of 256 x 256 x 12 resolution, the Lynch-Davisson coding with a block size of 64 as applied to prediction error sequences in the Gray code bit planes of each image gave an average compression ratio of 2.3:1 for 14 testing images. The predictive coding with a third order linear predictor and the Huffman encoding of the prediction error gave an average compression ratio of 3.1:1 for 54 images under test, while the maximum compression ratio achieved was 3.8:1. This result is one step further toward the improvement, albeit small, of the information preserving image compression for medical applications.

Algorithms↗
PubMed2004

Integrin alpha2beta1 affects mechano-transduction in slowly and rapidly adapting cutaneous mechanoreceptors in rat hairy skin.

The role of a transmembrane protein, integrin alpha2beta1, to modulate the neural responses of cutaneous mechanoreceptors to mechanical indentation was examined using an isolated skin-nerve preparation in a rat model. Skin and its intact innervation were harvested from the medial thigh of the hindlimb and placed in a dish containing synthetic interstitial fluid. Using a standard teased nerve preparation, the neural responses of single slowly or rapidly adapting mechanoreceptors (SA or RA, respectively) were identified and the afferents categorized according to standard protocols (i.e. response to constant stimuli). The most sensitive spot of a mechanoreceptor's receptive field was identified and then stimulated using controlled compressive stress (constant or dynamic loads between threshold and saturation load for SAs and RAs, respectively). Loads were applied before, during, and after passive diffusion into the skin of a function-blocking anti-integrin alpha2 monoclonal antibody (FBmAb) or one of two types of control antibodies (immunoglobulin G or a FBmAb conjugated with a secondary antibody). The sensitivities of both SA and RA mechanoreceptors were profoundly reduced in the presence of the FBmAb, while not changing the waveforms of their action potentials or their adaptation properties. Both control antibodies had no significant effect on mechanoreceptors' sensitivities. Following removal of the FBmAb, the effects in some neurons were partially reversible. Taken together, the data from this study support the hypothesis that integrin alpha2beta1 plays a significant role in modulating mechanoreceptive response to compressive indentation.

Adaptation, Physiological↗
PubMed2006

Turbulent mixing with physical mass diffusion.

Simulated mixing rates of the Rayleigh-Taylor instability for miscible fluids with physical mass diffusion are shown to agree with experiment; for immiscible fluids with physical values of surface tension the numerical data lie in the center of the range of experimental values. The simulations are based on an improved front tracking algorithm to control numerical surface tension and on improved physical modeling to allow physical values of mass diffusion or surface tension. Compressibility, after correction for variable density effects, has also been shown to have a strong influence on mixing rates. In summary, we find significant dependence of the mixing rates on scale breaking phenomena. We introduce tools to analyze the bubble merger process and confirm that bubble interactions, as in a bubble merger model, drive the mixing growth rate.

Journal Article↗