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At least 145 records · Page 8Linked to original sources
PubMed1996

A non-invasive protocol for the determination of lumbar spine mobility.

OBJECTIVE: This article presents a non-invasive protocol for the determination of lumbar spine mobility and compares intersegmental lumbar spinal mobility data from 45 subjects using both radiographic (invasive) and videographic (non-invasive) techniques. DESIGN: Non-linear mathematical models were developed to transfer skin marker coordinates into corresponding vertebral body positions. BACKGROUND: To relate skin surface motion with that of the underlying vertebrae and to enhance the reliability of using the non-invasive technique relied on a model of transformation. METHODS: This set of models was developed by taking the L(4) skinfold, the L(1)-S(1) skin distraction, and the non-linear difference between the lumbar spinal profile and the skin profile into consideration. Based on the digitized and model transformed data, the intersegmental joint angles (ISA) and intersegmental joint mobilities (ISM) were calculated and compared. RESULTS: In this study the non-invasive and invasive coordinate differences ranged from 0.485 to 1.099 cm; the mean ISA differences ranged from 0.29 to 2.08 degrees the mean ISM differences ranged from 0.62 to 1.88 degrees. CONCLUSIONS: With these small differences, the quality of the non-invasive protocol and the application of the transformation model were justified sufficient.

Journal Article↗
PubMed1988

A comparison of dose-response models for death from hematological depression in different species.

Many radiation-induced lethality experiments have been published for various mammalian species. From those studies a subset of studies reflecting useful biological and physical variables has been compiled into a database suitable to study interspecific variability of radiosensitivity, dose-rate dependence of sensitivity, dose-response behavior within each experiment, etc. The data compiled were restricted to continuous and nearly continuous exposures to photon radiations having source energies above 100 keV. Photon source energy, exposure geometry, and body weight considerations were used to select studies where the dose to hematopoietic tissue was approximately uniform. The database reflects 13 mammalian species ranging in size from mouse to cattle. Some 211 studies were compiled, but only 105 were documented in adequate detail to be useful in development and evaluation of dose-response models of interest to practical human exposures. Of the 105 studies, 70 were for various rodent species, and 35 were for non-rodent groups ranging from standard laboratory primates (body weight approximately 5 kg) to cattle (body weight approximately 375 kg). This paper considers seven different dose-response models which are tested for validity against those 105 studies. The dose-response models include: right-skewed extreme value, left-skewed extreme value, log-logistic, log-probit, logistic, probit, and Weibull models. In general, the log transformation models did not improve model performance and the extreme value models did not seem consistent with the preponderance of the data. Overall, the probit and the logistic models seemed preferable over the Weibull model.

Animals↗
PubMed2000

Visual detection of spatial contrast patterns: evaluation of five simple models.

The ModelFest Phase One dataset is a collection of luminance contrast thresholds for 43 two-dimensional monochromatic spatial patterns confined to an area of approximately two by two degrees. These data were collected by a collaboration among twelve laboratories, and were designed to provide a common database for calibration and testing of spatial vision models. Here I report fits of the ModelFest data with five models: Peak Contrast, Contrast Energy, Generalized Energy, a Gabor Channels model, and a Discrete Cosine Transform model. The Gabor Channels model provides the best fit, though the other, simpler models, with the exception of Peak Contrast, provide remarkably good fits as well. Though there are clear individual differences, regularities in the data suggest the possibility of constructing a standard observer for spatial vision.

Contrast Sensitivity↗
PubMed2004

Case analysis online: a strategic management case model for the health industry.

Despite the plethora of methods and tools available to support strategic management, the challenge for health executives in the next century will relate to their ability to access and interpret data from multiple and intricate communication networks. Integrated digital networks and satellite systems will expand the scope and ease of sharing information between business divisions, and networked systems will facilitate the use of virtual case discussions across universities. While the internet is frequently used to support clinical decisions in the healthcare industry, few executives rely upon the internetfor strategic analysis. Although electronic technologies can easily synthesize data from multiple information channels, research as well as technical issues may deter their application in strategic analysis. As digital models transform access to information, online models may become increasingly relevant in designing strategic solutions. While there are various pedagogical models available to support the strategic management process, this framework was designed to enhance strategic analysis through the application of technology and electronic research. A strategic analysis framework, which incorporated internet research and case analysis in a strategic managementcourse, is described alongwith design and application issues that emerged during the case analysis process.

Education, Graduate↗
PubMed1983

[Lightness decrease of the total area accompanying simultaneous lightness contrast].

Matches to the Munsell Lightness Scale were made in the two areas in a contrast-inducing pattern whose luminance difference was less than about 10%, and were compared with those of two uniform luminance field of corresponding luminances. The results showed (1) the simultaneous lightness contrast appeared in the contrast-inducing pattern, (2) the high luminance area in the contrast-inducing pattern appeared darker than the uniform luminance field of the same luminance. The lightness decrease in the high luminance area could be explained neither by Békésy's neural units model nor by Stevens' power transformation model, since Békésy's model predicts that high luminance area should appear lighter than the corresponding uniform luminance pattern, while Stevens' model expected the identical lightnesses between the two areas. In order to explain both the simultaneous lightness contrast and the lightness decrease, it was introduced a new model which included not only the antagonistic excitation and inhibition depending upon luminance-intensity, but also the non-antagonistic inhibition which is dependent upon the luminance-difference.

Adult↗
PubMed2005

Transforming kelp into a marine bioreactor.

The past decade has seen the genetic engineering of various types of seaweed. To date, genetic transformation studies have been carried out in several seaweeds, including the red seaweeds Porphyra, Gracilaria, Grateloupia, Kappaphycus and Ceramium and the green seaweed Ulva. A genetic transformation model system has been established in the most commonly cultivated seaweed, the brown seaweed Laminaria japonica (kelp), based on the transfer of technology used in land plant transformation and also by modulating the seaweed life cycle. This model showed the potential for application of transgenic kelp to the production of valuable products and an indoor cultivation system for transgenic kelp was proposed, taking into account necessary factors for bio-safety. In this review, the establishment at use of the kelp transformation model is introduced, highlighting the potential for transforming kelp into a marine bioreactor.

Bioreactors↗
PubMed1984

Quantitative comparisons of acute toxicity of organic chemicals to rat and fish.

Relationships between the acute toxicity of chemicals to fish (LC50) and rat (LD50) were analyzed using a Model II regression analysis after logarithmic transformation. (Model II regression assumes errors in both variables.) Significant correlations were found among bluegill and fathead minnow LC50S and rat LD50 values for the "priority pollutants." Fathead minnow and bluegill LC50S for 48 pesticides were highly correlated. No correlations were found between fish and rat for carbamate plus organophosphate pesticides. Correlations were obtained among all species for the combined priority pollutant plus pesticide data and for chlorinated pesticides.

Animals↗
PubMed1997

Object detection in natural backgrounds predicted by discrimination performance and models.

Many models of visual performance predict image discriminability, the visibility of the difference between a pair of images. We compared the ability of three image discrimination models to predict the detectability of objects embedded in natural backgrounds. The three models were: a multiple channel Cortex transform model with within-channel masking; a single channel contrast sensitivity filter model; and a digital image difference metric. Each model used a Minkowski distance metric (generalized vector magnitude) to summate absolute differences between the background and object plus background images. For each model, this summation was implemented with three different exponents: 2, 4 and infinity. In addition, each combination of model and summation exponent was implemented with and without a simple contrast gain factor. The model outputs were compared to measures of object detectability obtained from 19 observers. Among the models without the contrast gain factor, the multiple channel model with a summation exponent of 4 performed best, predicting the pattern of observer d's with an RMS error of 2.3 dB. The contrast gain factor improved the predictions of all three models for all three exponents. With the factor, the best exponent was 4 for all three models, and their prediction errors were near 1 dB. These results demonstrate that image discrimination models can predict the relative detectability of objects in natural scenes.

Adolescent↗
PubMed2003

Hierarchical active shape models, using the wavelet transform.

Active shape models (ASMs) are often limited by the inability of relatively few eigenvectors to capture the full range of biological shape variability. This paper presents a method that overcomes this limitation, by using a hierarchical formulation of active shape models, using the wavelet transform. The statistical properties of the wavelet transform of a deformable contour are analyzed via principal component analysis, and used as priors in the contour's deformation. Some of these priors reflect relatively global shape characteristics of the object boundaries, whereas, some of them capture local and high-frequency shape characteristics and, thus, serve as local smoothness constraints. This formulation achieves two objectives. First, it is robust when only a limited number of training samples is available. Second, by using local statistics as smoothness constraints, it eliminates the need for adopting ad hoc physical models, such as elasticity or other smoothness models, which do not necessarily reflect true biological variability. Examples on magnetic resonance images of the corpus callosum and hand contours demonstrate that good and fully automated segmentations can be achieved, even with as few as five training samples.

Algorithms↗
PubMed1998

Algorithms for radiological image registration and their clinical application.

This paper reviews recent work in radiological image registration and provides a classification of image registration by type of transformation and by methods employed to compute the transformation. The former includes transformation of 2D images to 2D images of the same individual, transformation of 3D images to 3D images of the same individual, transformation of images to an atlas or model, transformation of images acquired from a number of individuals, transformations for image guided interventions including 2D to 3D registration and finally tissue deformation in image guided interventions. Recent work on computing transformations for registration using corresponding landmark based registration, surface based registration and voxel similarity measures, including entropy based measures, are reviewed and compared. Recently fully automated algorithms based on voxel similarity measures and, in particular, mutual information have been shown to be accurate and robust at registering images of the head when the rigid body assumption is valid. Two approaches to modelling soft tissue deformation for applications in image guided interventions are described. Validation of complex processing tasks such as image registration is vital if these algorithms are to be used in clinical practice. Three alternative validation strategies are presented. These methods are finding application outside the original domain of radiological imaging.

Algorithms↗
PubMed2006

Discrete solitons and vortices in the two-dimensional Salerno model with competing nonlinearities.

An anisotropic lattice model in two spatial dimensions, with on-site and intersite cubic nonlinearities (the Salerno model), is introduced, with emphasis on the case in which the intersite nonlinearity is self-defocusing, competing with on-site self-focusing. The model applies, for example, to a dipolar Bose-Einstein condensate trapped in a deep two-dimensional (2D) optical lattice. Soliton families of two kinds are found in the model: ordinary ones and cuspons, with peakons at the border between them. Stability borders for the ordinary solitons are found, while all cuspons (and peakons) are stable. The Vakhitov-Kolokolov criterion does not apply to cuspons, but for the ordinary solitons it correctly identifies the stability limits. In direct simulations, unstable solitons evolve into localized pulsons. Varying the anisotropy parameter, we trace a transition between the solitons in 1D and 2D versions of the model. In the isotropic model, we also construct discrete vortices of two types, on-site and intersite centered (vortex crosses and squares, respectively), and identify their stability regions. In simulations, unstable vortices in the noncompeting model transform into regular solitons, while in the model with the competing nonlinearities they evolve into localized vortical pulsons, which maintain their topological character. Bound states of regular solitons and vortices are constructed too, and their stability is identified.

Journal Article↗
PubMed1995

Increased glycosylation of beta 1 integrins affects the interaction of transformed S115 mammary epithelial cells with laminin-1.

The effect of transformation on the expression and the functions of beta 1 integrins was studied using an in vitro cell transformation model. S115 mammary epithelial tumor cells undergo transformation into tumorigenic fibroblastoid cells in the presence of steroids. Transformation was found to reduce the attachment and the spreading of S115 cells on laminin-1 but not fibronectin. Adhesion of S115 cells to laminin-1 was inhibited in the presence of an antibody against the beta 1 integrin subunit. Both nontreated and transformed S115 cells expressed at least two putative laminin-1-binding beta 1 integrins at the same level. In transformed cells, however, the mature integrin subunits appeared to be structurally altered, showing a slower electrophoretic mobility. Treatment with N-glycosidase-F and tunicamycin abolished this mobility difference, suggesting that the presence of complex-type N-linked oligosaccharides was responsible. Detailed enzymatic analysis of the oligosaccharides present on the beta 1 subunits revealed that the difference in glycosylation is, at least partially, due to poly-N-lactosaminoglycan chains on beta 1 integrin from transformed cells. Removal of this difference in glycosylation by either cleavage of the polylactosaminoglycan chains with endo-beta-galactosidase or inhibiton of complex-type glycan formation with swainsonine repeatedly enhanced the spreading of transformed cells on laminin-1. Thus, the increased size of complex-type oligosaccharides on beta 1 integrin may affect cell-laminin-1 interactions. Similar changes may contribute to the altered adhesion of cancer cells during the invasion and metastasis.

Animals↗
PubMed1990

Numerical simulation of the crown of an incisiform tooth by conformal and polynomial regression mapping of a simple model.

We present a simple bowl-shaped model in the complex plane for the enamel and dentin structure of a tooth. Isochronous mode lines, and path lines representing the paths followed by individual ameloblasts and odontoblasts, form a simple regular mesh in the model. After the conformal map W = Z2, the transformed model is remarkably tooth like. To further improve the simulation of the crown of an incisiform tooth in particular, we compare our results to our own data from actual teeth and data from the literature. This leads us (a) to adjust the initial model so that the mesh of mode and path lines intersects the boundary of the tooth in a realistic way, and (b) to refine the mapping using least-squares regression to fit polynomial functions of Z and W to the available data.

Computer Simulation↗
PubMed2003

Signals for death and differentiation: a two-step mechanism for in vitro transformation of adult islets of Langerhans to duct epithelial structures.

Phenotypic change of adult pancreatic islets has been implicated in the development of certain pancreatic cancers and in islet transplant failure. The aim of this study was to characterize intracellular events that mediate changes in adult islet phenotype. Using an in vitro islet-to-duct transformation model, canine islets were induced to undergo phenotypic transformation to duct-like epithelial structures through a two-stage process. Stage one was characterized by widespread islet cell apoptosis associated with the formation of cavitary spaces within the islets. During this stage, c-Jun N-terminal regulated kinase (JNK) and caspase-3 activities were elevated, while extracellular signal-regulated kinase (ERK) and Akt activities were decreased. The second stage of the process was characterized by an inversion in the balance in activity between these signal transduction pathways and by a concomitant decrease in apoptosis. The transformed islets were no longer immunoreactive for islet cell hormones, but expressed the duct epithelial cell marker CK-AE1/AE3. In contrast to islet cells, these duct epithelial cells were highly proliferative. To clarify the role of the identified changes in signal transduction events, we performed additional studies using pharmacological inhibitors of enzyme activity and demonstrated that inhibition of JNK and caspase-3 activity prevented cystic transformation. Our results indicate that the balance in signaling activity between ERK/Akt and JNK/caspase-3 appears to be an important regulator of islet cell death and differentiation.

Animals↗
PubMed1994

A physician-based architecture for the construction and use of statistical models.

Physicians need specially tailored computer tools to take advantage of published research results. We present a knowledge-based computer framework--the physician-based (PB) architecture--for constructing such tools, and we use the problem of physicians' interpretation of two-arm parallel randomized clinical trials (TAPRCT) as a working example. Statistical models are represented by influence diagrams. The interpretation of influence-diagram elements are mapped into users' language in a domain-specific, physician-based user interface, called a patient-flow diagram. Statistical-model transformations that maintain the semantic relationships of the model and that embody clinical-epidemiological knowledge are encoded in a mediating structure called the cohort-state diagram. The algorithm that coordinates the interactions among the knowledge representations uses modular actions called construction steps. This architecture has been implemented in a Bayesian system, called THOMAS, that supports physician decision making in light of TAPRCT data. This support entails assessing clinical significance, prior beliefs, and methodological concerns. We suggest that the PB architecture applies to a wide range of statistical tools and users.

Algorithms↗
PubMed2004

Self modeling with flexible, random time transformations.

Methods for modeling sets of complex curves where the curves must be aligned in time (or in another continuous predictor) fall into the general class of functional data analysis and include self-modeling regression and time-warping procedures. Self-modeling regression (SEMOR), also known as a shape invariant model (SIM), assumes the curves have a common shape, modeled nonparametrically, and curve-specific differences in amplitude and timing, traditionally modeled by linear transformations. When curves contain multiple features that need to be aligned in time, SEMOR may be inadequate since a linear time transformation generally cannot align more than one feature. Time warping procedures focus on timing variability and on finding flexible time warps to align multiple data features. We draw on these methods to develop a SIM that models the time transformations as random, flexible, monotone functions. The model is motivated by speech movement data from the University of Wisconsin X-ray microbeam speech production project and is applied to these data to test the effect of different speaking conditions on the shape and relative timing of movement profiles.

Biomechanical Phenomena↗
PubMed2002

Model evaluation and calibration for prospective respiratory motion correction in coronary MR angiography based on 3-D image registration.

Image processing was used as a fundamental tool to derive motion information from magnetic resonance (MR) images, which was fed back into prospective respiratory motion correction during subsequent data acquisition to improve image quality in coronary MR angiography (CMRA) scans. This reduces motion artifacts in the images and, in addition, enables the usage of a broader gating window than commonly used today to increase the scan efficiency. The aim of the study reported in this paper was to find a suitable motion model to be used for respiratory motion correction in cardiac imaging and to develop a calibration procedure to adapt the motion model to the individual patient. At first, the performance of three motion models [one-dimensional translation in feet-head (FH) direction, three-dimensional (3-D) translation, and 3-D affine transformation] was tested in a small volunteer study. An elastic image registration algorithm was applied to 3-D MR images of the coronary vessels obtained at different respiratory levels. A strong intersubject variability was observed. The 3-D translation and affine transformation model were found to be superior over the conventional FH translation model used today. Furthermore, a new approach is presented, which utilizes a fast model-based image registration to extract motion information from time series of low-resolution 3-D MR images, which reflects the respiratory motion of the heart. The registration is based on a selectable global 3-D motion model (translation, rigid, or affine transformation). All 3-D MR images were registered with respect to end expiration. The resulting time series of model parameters were analyzed in combination with additionally acquired motion information from a diaphragmatic MR pencil-beam navigator to calibrate the respiratory motion model. To demonstrate the potential of a calibrated motion model for prospective motion correction in coronary imaging, the approach was tested in CMRA examinations in five volunteers.

Adult↗
PubMed2002

Proportional hazards models with frailties and random effects.

We discuss some of the fundamental concepts underlying the development of frailty and random effects models in survival. One of these fundamental concepts was the idea of a frailty model where each subject has his or her own disposition to failure, their so-called frailty, additional to any effects we wish to quantify via regression. Although the concept of individual frailty can be of value when thinking about how data arise or when interpreting parameter estimates in the context of a fitted model, we argue that the concept is of limited practical value. Individual random effects (frailties), whenever detected, can be made to disappear by elementary model transformation. In consequence, unless we are to take some model form as unassailable, beyond challenge and carved in stone, and if we are to understand the term 'frailty' as referring to individual random effects, then frailty models have no value. Random effects models on the other hand, in which groups of individuals share some common effect, can be used to advantage. Even in this case however, if we are prepared to sacrifice some efficiency, we can avoid complex modelling by using the considerable power already provided by the stratified proportional hazards model. Stratified models and random effects models can both be seen to be particular cases of partially proportional hazards models, a view that gives further insight. The added structure of a random effects model, viewed as a stratified proportional hazards model with some added distributional constraints, will, for group sizes of five or more, provide no more than modest efficiency gains, even when the additional assumptions are exactly true. On the other hand, for moderate to large numbers of very small groups, of sizes two or three, the study of twins being a well known example, the efficiency gains of the random effects model can be far from negligible. For such applications, the case for using random effects models rather than the stratified model is strong. This is especially so in view of the good robustness properties of random effects models. Nonetheless, the simpler analysis, based upon the stratified model, remains valid, albeit making a less efficient use of resources.

Humans↗