Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Models, Anatomic”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 703 records · Page 39Linked to original sources
PubMed1995

Potential distributions and neural excitation patterns in a rotationally symmetric model of the electrically stimulated cochlea.

In spite of many satisfactory results, the clinical outcome of cochlear implantation is poorly predictable and further insight into the fundamentals of electrical nerve stimulation in this complex geometry is necessary. For this purpose we developed a rotationally symmetric volume conductor model of the implanted cochlea, using the Boundary Element Method (BEM). This configuration mimics the cochlear anatomy more closely than previous, unrolled models. The calculated potential distribution in the cochlea due to stimulating electrodes is combined with a multiple non-linear node model of auditory nerve fibres, which we recently developed. The combined model is used to compute excitation profiles of the auditory nerve for a variety of stimulus levels and electrode positions. The model predicts that the excitation threshold, the spatial selectivity and the dynamic range depend on the exact position of the electrode in the scala tympani. These results are in good agreement with recently published electrical ABR data. It is shown that the use of actively modelled nerve fibres is essential to obtain correct predictions for the biphasic stimuli typically used in cochlear implants and that unrolling the cochlear duct as done in previous models leads to erroneous predictions regarding modiolar stimulation.

Animals↗
PubMed1995

Neuronal correlates of perceptual amplitude-modulation detection.

The goal of the present paper is to relate the coding of amplitude modulation (AM) in the auditory pathway to the behavioral detection performance. To address this issue, the detectability of AM was estimated by modelling a single neuron located in the central nucleus of the inferior colliculus (IC). The computational model is based on cochlear nucleus responses and a coincidence detection mechanism. The model replicated the main feature of the neuronal AM transfer function, namely a bandpass function. The IC-unit model was initially tuned to a 200-Hz modulation frequency. A single neurometric function for AM detection at this modulation frequency was generated using a 2-interval, 2-alternative forced-choice paradigm. On each trial of the experiments, AM was taken to be correctly detected by the model if the number of spikes in response to the modulated signal exceeded the number of spikes in an otherwise identical interval that contained an unmodulated signal. Psychometric functions for 4 human subjects were also measured under the same stimulus conditions. Comparison of the simulated neurometric and psychometric functions suggested that there was sufficient information in the rate response of an IC neuron well-tuned in the modulation-frequency domain to support behavioral detection performance.

Acoustic Stimulation↗
PubMedDate not supplied

Matching pulmonary structure and perfusion via combined dynamic multislice CT and thin-slice high-resolution CT.

Taking advantage of two scan modes of an electron beam CT scanner (Imatron), we have developed a method utilizing x-ray CT for relating pulmonary perfusion to global and regional anatomy. A high temporal resolution mode, used to follow bolus contrast agent, is combined with a high spatial resolution mode to obtain the structure-function fusion. A software module has been developed for our image analysis package (VIDA) to automatically calculate physiologic parameters of flow and integrate these color coded functional measurements into a corresponding high spatial resolution data set. We present the scanning methodology details and give examples from our physiologic based research to demonstrate strengths of combining dynamic and high resolution CT to uniquely characterize pulmonary normal and pathophysiology.

Algorithms↗
PubMedDate not supplied

CT volumetric data-based left ventricle motion estimation: an integrated approach.

This paper describes a novel approach to left ventricle motion analysis via the integration of image segmentation with shape deformation analysis using computerized tomography (CT) volumetric image data. This approach is different from traditional image analysis scenario in which the image segmentation and shape analysis were considered separately. The advantage of integrating the image segmentation with the shape analysis lies in the fact that the shape characteristics of the object can be used as effective constraints in the process of segmentation while original image data can be made useful along with the segmentation results in the process of shape analysis. In the case of left ventricle motion estimation, such an integration can be applied to obtain the estimation results that are consistent with both given image data and a priori shape knowledge. The initial segmentation of the images is obtained through adaptive K-mean classification and the region-of-interest is then identified based on the initial segmentation. The shape analysis is accomplished through fitting the boundary points of the region-of-interest to the surface modeling primitives. These two processes are integrated through the feedforward and feedback channels so that the surface fitting is constrained by the confidence measures of the boundary points and segmentation refinement is guided by the result of surface modeling. Global motion parameters are obtained by comparing the parameters of the fitted surface model at consecutive time instances. The segmentation and shape analysis results obtained show that the integrated approach is capable of providing promising improvement over traditional approaches.

Algorithms↗
PubMedDate not supplied

3D reconstructions of neurofunctional structures for neuroimaging.

An improved method for computer-aided 3D reconstruction of neuroanatomical structures derived from intracranially fixed human brains is demonstrated. The embedded brains are cut into slices. The sliced surfaces are photographed. The outlines of the neuroanatomical structures are drawn onto transparencies which are scanned. The surfaces of the structures are 3D reconstructed with the triangulation method. The cortical structures of the human brain are complex, resulting in possible ambiguities for triangulation which must be solved interactively. These surface reconstructions allow for 3D visualization of the brain and its components comparable with intravital conditions found in clinical neuroimaging.

Algorithms↗
PubMed1993

Bayesian inference for model-based segmentation of computed radiographs of the hand.

We present a method for medical image understanding by computer that uses model-based, hierarchical Bayesian inference to accurately segment imaged anatomy. A first application is a prototype system that automatically segments and measures symptoms of arthridities in hand radiographs. This is potentially useful in radiological diagnosis and tracking of arthridities. Key steps of the model-based, Bayesian inference approach are: (1) prediction of imagery features from 3D models of anatomy, parameterized by population statistics, (2) local image feature extraction in predicted sub-regions, and (3) the use of a probabilistic calculus to accrue results of image processing and image feature matching procedures in support or denial of hypotheses about the imaged anatomy. The prototype system for hand radiograph analysis accurately segments normal and somewhat degenerated hand anatomy. Results are shown of the ability of the automated system to 'fail soft', recognizing when segmentation is inadequate for accurate measurement. This self evaluation capability improves reliability of measurements for potential clinical use.

Arthritis↗
PubMed1993

Early events in auditory processing.

During the last year, further evidence has appeared concerning the basis of frequency selectivity in the cochlea, which may ultimately depend on a motile mechanism residing within the walls of the outer hair cells. Evidence has also appeared on hair-cell mechanotransduction, and on the way that the stimulus is coupled to the mechanotransducer channels.

Animals↗
PubMed1996

Discussion on the design of a hip joint simulator.

Hip joint simulators were developed for predicting, by attempting to duplicate in vitro physiological loads and motion, the wear rate that total hip replacements are likely to show in vivo. From a theoretical point of view, loading and motion cycles of hip joints could be closely reproduced by three rotation actuators and three force actuators. However existing devices have been designed assuming that some of these degrees of freedom are negligible, in order to reduce the complexity of the equipment. The present study singles out some preliminary indications on the design choices regarding the spatial configuration of loading and motion actuators. The aim is to define theoretically a simplified simulator but still able to apply the most physiologically realistic loading cycle to the specimen.

Biomechanical Phenomena↗
PubMed1996

Sensitivity of femoral strain pattern analyses to resultant and muscle forces at the hip joint.

An automated geometrical preprocessor was developed with the aim of creating three-dimensional finite element models (FEM) of the human femur. On the basis of postprocessed computed tomography data, this preprocessor makes possible rapid, flexible and regular meshing with 'brick' elements. Three different material properties were modelled at the present stage of development. Sensitivity analyses demonstrated that the strain energy density (SED) patterns of the different femoral parts were most sensitive to the implementation of an iliotibial tract force. The variation of the resultant hip force and abductor force direction within the sagittal plane demonstrated a SED minimum at an anterior inclination of 13 degrees; the variation of the resultant force direction within the frontal plane demonstrated a minimum SED at a medial inclination of 21 degrees relative to the mechanical axis of the lower limb. The orientation of the connecting line between the surface-SED-peaks in the horizontal view was found to be most sensitive to the variation of the resultant force within the sagittal plane.

Biomechanical Phenomena↗
PubMed1996

Computation of vascular flow dynamics from intravascular ultrasound images.

Analysis of three-dimensional velocity profiles and wall shear stress distribution in a segment of an artery reconstructed from in vivo imaging data are presented in this study. Cross-sectional images of a segment of the abdominal aorta in dogs were obtained using intravascular ultrasound (IVUS) imaging employing a constant pull back technique. Simultaneous measurement of pressures distal and proximal to the vessel segment along with gated pulsed Doppler velocity measurements were also obtained. The three-dimensional geometry of the vascular segment was reconstructed from the IVUS images during peak forward flow phase, and a computational mesh was constructed from the data. A quasi-steady analysis of incompressible Newtonian fluid was performed with a finite difference general purpose computational analysis program FLOW3D. The velocity at the inlet and pressure at the outlet measured at the corresponding time (time referenced to ECG) were used to specify the boundary conditions for the computational flow model. The computed results compared favorably with previously reported results. The purpose of the present study was to analyze the hemodynamics in vascular segments from morphologically realistic three-dimensional reconstructions. The method can be potentially employed in analyzing the hemodynamics in the region of atherosclerotic plaques at various stages of development and the reactivity of the vessel in response to pharmacological and mechanical interventions.

Animals↗
PubMed1996

The integrated design of mechanical bi-leaflet prosthetic heart valves.

A flow model utilizing an irrotational, inviscid algorithm of vortex-ring elements simulating the leaflets and source/sink elements simulating the aortic root coupled with a boundary layer model has been developed to model the internal flow phenomena of bi-leaflet mechanical heart valves implanted in the aortic root. The inviscid representation evaluates the aerodynamic lift, the induced drag, the pitching moment and flow velocity along the leaflet surface thus providing data for evaluating the boundary-layer thickness, the shear stress and flow separation point by the boundary layer theory. Full integration with the geometry enables immediate updates of the flow solution when changes in geometry have been made. It is shown that the effects of the internal flow domain model are necessary in the correct evaluation of lift and drag for subsequent dynamic analysis. The environment presented provides for the ability to produce significant and immediate design changes so that crucial decisions may be made whilst still within the software design loop. New designs are shown along with data for the improved flow model.

Algorithms↗
PubMed1996

A minimal parametric model of the femur to describe axial elastic strain in response to loads.

Evaluating the state of stress/strain for a given geometry and load in femurs can be done both experimentally, measuring strain at a limited number of locations, and theoretically with finite element models. Another approach is to describe the state of strain with a few synthetic indices. For this purpose the reverse elastic problem (i.e. bone parameters are estimated given the strain distribution and loads) needs to be solved as opposed to the finite element direct problem. Such reverse models can be then used: (1) to describe simply the strain distribution by means of few synthetic indices; (2) to explain the state of strain; and (3) to predict the strain distribution under different loading conditions. Various linear models, characterized by two to five bone related parameters, were tested on (1) 12 femurs, (2) a finite element model, and (3) data taken from the literature, for a total of 43 loading cases. Three and four-parameter models were able to fit the experimental strain distributions with mean squared residuals smaller than 5% of the strain range. The consistency of the model was proved by the repeatability of the parameters estimate for identical femurs. Furthermore, the bone-related coefficients were able to detect the stiffening effect of the implantation of an uncemented stem. Finally, the model can be used for predictive purposes if the parameter estimates are used with different loading conditions.

Biomechanical Phenomena↗
PubMed1996

Dynamic analysis and geometry models for the design of bi-leaflet prosthetic mechanical heart valves.

This paper provides new geometry definitions for the axi-symmetric stiffening (or sewing) ring, as part of a bi-leaflet prosthetic heart valve, and presents a dynamic behaviour analysis of the leaflet. An optimal stiffening ring geometry may be constructed by considering the point of flow separation on the stiffening ring (measured in the downstream coordinate), the effective orifice area of the stiffening ring and the associated dynamic behaviour of the leaflet. The dynamic model is accomplished by utilizing a second-order rotating system to simulate the opening and closing characteristics of the leaflet. The moments due to the aerodynamic loads are evaluated from an irrotational inviscid flow model, coupled with boundary layer theory, modelling the internal flow phenomena of the bi-leaflet heart valve implanted in the aortic root. It has been shown that this internal flow model provides the correct evaluation of lift and induced drag and the subsequent dynamic characteristics.

Algorithms↗
PubMedDate not supplied

Evaluation of theories of complex movement planning in different levels of gravity.

Due to high redundancy of degrees of freedom in the human body, we can perform any movement, from the simplest to the most complex, in many different ways. Several studies are still trying to identify the motor strategies that master this redundancy and generate the movements whose characteristics are highly stereotyped. The aim of this work is to build a simulator that is able to evaluate different motor planning hypotheses. The most interesting applications of this tool occur in studies of the motor strategy in microgravity conditions. The comparison between simulated movements and kinematics data recorded both on Earth, and during a 5-month mission on board the Mir station shows that for a complex whole-body movement (such as trunk bending) a single planning criterion cannot explain all movement aspects. However, the simulator allows an understanding of the motor planning adaptation of astronauts. In space, the lack of equilibrium constraint (which on Earth brings about the center of mass control) leads to a new motor strategy that minimizes dynamic interactions with the floor.

Adaptation, Physiological↗
PubMed2004

Postretention relapse of mandibular anterior crowding in patients treated without mandibular premolar extraction.

Treatment stability is one of the most important objectives in orthodontics, but, despite decades of research, it is still agreed that the stability of aligned teeth is variable and largely unpredictable. This study aimed to evaluate the relapse of mandibular anterior crowding in patients treated without mandibular premolar extraction. The sample comprised 40 patients of both sexes with Class I or II malocclusions who received nonextraction treatment in the mandibular arch with edgewise mechanics. Lateral cephalograms and dental casts of each patient were obtained at pretreament, posttreatment, and 5 years postretention. Relapse of mandibular anterior crowding was assessed, and associations between this relapse and other clinical factors were also investigated. Mandibular anterior crowding was measured by the Little irregularity index, and the data were evaluated by the Mann-Whitney test. The mean relapse of mandibular anterior crowding was 1.95 mm (26.54%) over the long term. No clinical factor studied was predictive of crowding relapse in the long term.

Bicuspid↗
PubMed2004

Class II treatment success rate in 2- and 4-premolar extraction protocols.

This study compared the occlusal success rate of Class II orthodontic treatment results with either 2- or 4-premolar extraction protocols. Group 1 comprised dental study models of 81 patients treated with 2 premolar extractions, and group 2 comprised dental study models of 50 patients treated with 4 premolar extractions. The initial mean ages of the groups were 13.9 and 12.9 years, respectively. Grainger's treatment priority index (TPI) was used to assess the initial and final occlusal status of each patient. Individual variables, such as maxillary canine anteroposterior positioning, overjet, and overbite improvements were also evaluated. Independent t tests were used to compare the variables at the pretreatment and posttreatment stages and their improvement between the groups. The results showed a statistically significant difference in most of the variables and in their improvement at the end of treatment between the groups. The variables showed a better dental relationship in group 1, and the improvements in group 1 were larger than in group 2. Treatment of Class II malocclusion with 2 premolar extractions gives a better occlusal success rate than treatment with 4 premolar extractions.

Adolescent↗
PubMed2004

Role of shielding in modulating the effects of solar particle events: Monte Carlo calculation of absorbed dose and DNA complex lesions in different organs.

Distributions of absorbed dose and DNA clustered damage yields in various organs and tissues following the October 1989 solar particle event (SPE) were calculated by coupling the FLUKA Monte Carlo transport code with two anthropomorphic phantoms (a mathematical model and a voxel model), with the main aim of quantifying the role of the shielding features in modulating organ doses. The phantoms, which were assumed to be in deep space, were inserted into a shielding box of variable thickness and material and were irradiated with the proton spectra of the October 1989 event. Average numbers of DNA lesions per cell in different organs were calculated by adopting a technique already tested in previous works, consisting of integrating into "condensed-history" Monte Carlo transport codes--such as FLUKA--yields of radiobiological damage, either calculated with "event-by-event" track structure simulations, or taken from experimental works available in the literature. More specifically, the yields of "Complex Lesions" (or "CL", defined and calculated as a clustered DNA damage in a previous work) per unit dose and DNA mass (CL Gy-1 Da-1) due to the various beam components, including those derived from nuclear interactions with the shielding and the human body, were integrated in FLUKA. This provided spatial distributions of CL/cell yields in different organs, as well as distributions of absorbed doses. The contributions of primary protons and secondary hadrons were calculated separately, and the simulations were repeated for values of Al shielding thickness ranging between 1 and 20 g/cm2. Slight differences were found between the two phantom types. Skin and eye lenses were found to receive larger doses with respect to internal organs; however, shielding was more effective for skin and lenses. Secondary particles arising from nuclear interactions were found to have a minor role, although their relative contribution was found to be larger for the Complex Lesions than for the absorbed dose, due to their higher LET and thus higher biological effectiveness.

Astronauts↗