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PubMed1982

Neural networks and physical systems with emergent collective computational abilities.

Computational properties of use of biological organisms or to the construction of computers can emerge as collective properties of systems having a large number of simple equivalent components (or neurons). The physical meaning of content-addressable memory is described by an appropriate phase space flow of the state of a system. A model of such a system is given, based on aspects of neurobiology but readily adapted to integrated circuits. The collective properties of this model produce a content-addressable memory which correctly yields an entire memory from any subpart of sufficient size. The algorithm for the time evolution of the state of the system is based on asynchronous parallel processing. Additional emergent collective properties include some capacity for generalization, familiarity recognition, categorization, error correction, and time sequence retention. The collective properties are only weakly sensitive to details of the modeling or the failure of individual devices.

Animals↗
PubMed1994

A comparison of three electron planning algorithms for a 16 MeV electron beam.

PURPOSE: We report results of a comparison of three electron planning algorithms, an Age-Diffusion Pencil beam algorithm and two (2-D) and three dimensional (3-D) Hogstrom pencil beam algorithms, using simple 2 x 2 cm air and hard bone inhomogeneities and a complex anthropomorphic head and neck phantom. METHODS AND MATERIALS: The simple inhomogeneities have variable dimensions outside the plane of calculation to test the effects of out of plane scattering on 2-D algorithms, compared with dose measured by film below the inhomogeneity in the dose fall-off range. Comparisons are also made of a parotid treatment field for 16 MeV electrons, and the dose measured by high sensitivity thermoluminescent dosimeters in the head and neck phantom. RESULTS: Behind the simple inhomogeneities, the electron algorithms are found to underestimate the dose behind the air cavity by up to 40% and overestimated the dose behind bone by up to 30%. In the head phantom, the presence of inhomogeneities also presents problems for the algorithms, with overestimations of dose of up to 20% found behind bone-tissue interfaces, apparently due to shielding by high density bone. Overestimations of up to 17% are also found beside interfaces parallel to the beam. Underestimations of dose of up to 10% are found on the beam-side of interfaces, due to under-prediction of backscattered electrons. All three investigated algorithms underestimate the dose by up to 20% behind extreme surface curvature. One algorithm is found to underestimate the dose in the falloff region while another overestimates the dose around the 90% isodose. CONCLUSION: Clinicians should be aware of the limitations of their planning systems.

Algorithms↗
PubMed1985

Design and fitting of neural network transfer functions.

An algorithm is presented which (a) allows construction of mathematical models involving arbitrary combinations of linear cascades, parallel pathways, and feedback loops, (b) computes a total transfer function of the system, (c) performs a least-squares optimization of model parameters to best fit the model to experimental data, and (d) provides a measure of goodness-of-fit to the data. The technique has been employed to construct and test models of neural networks which mimic a class of responses observed in the cat vestibular nuclei in response to tilt, namely responses which show both a gain increase and progressive phase lag as the stimulation frequency goes from 0.01 to 2 Hz. A network consisting of a simple gain element in parallel with an inhibitory high-pass filtered version of the input provided a satisfactory fit to these data.

Animals↗
PubMed1984

Evoked potential techniques in the evaluation of visual function.

Visual evoked potentials (VEPs) can be used in a multitude of ways to assess the various levels of visual processing. The human visual system consists of multiple, parallel channels which process different information, and each channel constitutes a set of sequential processes. An algorithm of sequential steps that can be used to assess visual function is reviewed. The pathophysiology of retinal, anterior visual pathways and retrochiasmal pathways can be objectively evaluated by VEPs.

Adolescent↗
PubMed1991

Sample size estimation for comparing two or more treatment groups in clinical trials.

Methods for estimating required sample size for comparing two population means have been published. Most involve the use of complicated formulae and tables. These methods are limited to comparing two groups. Although techniques exist to determine sample sizes for comparing more than two groups, they are intrinsically far more complicated. A simple linear nomogram is proposed as a solution to these problems, and its use is illustrated with examples of parallel group, ordered parallel group and factorial designs.

Algorithms↗
PubMed1991

Asymmetric Boltzmann machines.

We study asymmetric stochastic networks from two points of view: combinatorial optimization and learning algorithms based on relative entropy minimization. We show that there are non trivial classes of asymmetric networks which admit a Lyapunov function L under deterministic parallel evolution and prove that the stochastic augmentation of such networks amounts to a stochastic search for global minima of L. The problem of minimizing L for a totally antisymmetric parallel network is shown to be associated to an NP-complete decision problem. The study of entropic learning for general asymmetric networks, performed in the non equilibrium, time dependent formalism, leads to a Hebbian rule based on time averages over the past history of the system. The general algorithm for asymmetric networks is tested on a feed-forward architecture.

Algorithms↗
PubMed1989

A two-dimensional pencil-beam algorithm for calculation of arc electron dose distributions.

A two-dimensional pencil-beam algorithm is presented for the calculation of arc electron dose distributions in any plane that is perpendicular to the axis of rotation. The dose distributions are calculated by modelling the arced beam as a single broad beam defined by the irradiated surface of the patient. The algorithm is two-dimensional in that the anatomical cross section of the patient and the skin collimators are assumed identical in parallel planes outside the plane of calculation. The broad beam is modelled as a collection of strip beams, each strip beam being characterised by its planar fluence, mean projected angular direction and a root-mean-square spread about the mean direction. Using these parameters, the dose distribution is calculated using pencil-beam theory. Examples of strip-beam parameters and resulting dose distributions for patient geometries are presented. Features of the algorithm, which include (1) incorporation of pencil-beam theory for the calculation of dose in heterogeneous tissue, (2) run times of only about twice that of comparable-sized fixed electron fields and (3) the input requirement of only a single depth dose and four off-axis dose profiles of measured data, make the algorithm practical for clinical use.

Algorithms↗
PubMed1994

Graded changes in the response of individual human basophils to stimulation: distributional behavior of early activation events.

These studies examine the distribution of single-cell responses in basophil preparations in the context of four events that may be associated with early activation by anti-immunoglobulin E (IgE) antibody and the bacterial peptide fMet-Leu-Phe (fMLP). In general, we measured the single-cell response distributions after challenge with a concentration of stimulus that resulted in an optimal response and compared this with the distribution that occurred after challenge with suboptimal concentrations of the same stimulus. The elevation in cytosolic calcium, as detected in Fura-2-labeled basophils, after challenge with anti-IgE or fMLP showed graded characteristics in that the distributions were unimodal under conditions of optimal or suboptimal challenge with little skewing from a normal distribution. Similarly, the up-regulation of the cell surface adhesion molecule CD11b, as determined by flow cytometry, showed graded unimodal increases after challenge with anti-IgE antibody at optimal and suboptimal concentrations. In addition, stimulation of basophils led to increased F-actin polymerization. After challenge with an optimal concentration of anti-IgE antibody, the F-actin content of basophils increased to a maximum between 10 and 15 min and returned to near prechallenge levels by 60 min. There was a close correlation between the maximum increase in F-actin content and histamine release regardless of the stimulus; anti-IgE antibody, fMLP, and phorbol ester (PMA) responses lay on the same regression line. The single-cell F-actin polymerization distributions were also unimodal and graded according to the magnitude of the histamine release response. During measurements of the calcium response under the microscope we noted that basophils underwent significant changes in morphology after challenge with any stimulus. These changes were related to both degranulation and nondegranulation events and could be quantitated by a series of image-processing algorithms, which are presented. The kinetics of the morphological change, measured as a change in cell perimeter, paralleled degranulation. Single-cell distributions of the morphologic changes were also unimodal under conditions of both optimal and suboptimal stimulation. Therefore, no evidence of all-or-nothing responses could be observed in the context of these four early activation events. In general, the response distributions resembled normal distributions at both optimal and suboptimal levels of stimulation, which indicated that single basophils responded in a graded manner.

Basophils↗
PubMed1987

High-resolution CT of the wrist: initial experience with scaphoid disorders and surgical fusions.

We performed high-resolution CT scans on 30 wrists in 27 patients with either previous surgical intercarpal fusion or known or suspected scaphoid abnormalities. Most examinations used the same protocol: contiguous 1.5-mm axial sections parallel to the long axis of the body of the scaphoid, small reconstruction circle, and bone reconstruction algorithm. Fifteen patients were evaluated for union of previous surgical carpal fusions; eight patients were evaluated for healing of scaphoid fractures; and the other four patients had miscellaneous disorders, including a midcarpal dislocation with a scaphoid fracture. High-resolution CT clearly showed whether the carpal fusions were united. In addition, metallic fixation devices were easy to locate and did not significantly degrade image quality. Osseous union of healing scaphoid fractures was reliably assessed. We conclude that high-resolution CT of the wrist is a useful method for evaluating surgical carpal fusions and various disorders of the scaphoid.

Arthrodesis↗
PubMed1995

Multimedic system for telepathology and interdisciplinary councils between doctors and various hospitals.

At present, the exchange of information between clinicians and pathologists are limited to mail, telefax and telephone. The bi-directional video communication using at least two digitized phone lines (ISDN) is an additional medium for video-conference and for image exchange. The MECOM system (Medical Communication) has been set up in the department of pathology of Aixla-Chapelle Hospital (Germany) and has routinely used for two years between 4 local hospitals. Now it is mainly used for staff discussions between pathologists and practitioners involved in different oncological fields. Mecom can transmit any kind of high resolution images (x rays films, scanner images, histologic slides...) which are useful or necessary during medical staff discussions. This system requires only two phone lines to be operative. However it can turn on up to twelve parallel ISDN lines which allow to transmit 768 kbytes/sec. It manages the SG3, HVQ and H320 compression algorithms and allows the transmission of still images at a resolution varying from 256/240 to 512/480 pixels. Mecom is a modulable system which adapts to the needs. It has the following advantages. Emergency inter-disciplinary conferences. Real time images, discussion between a pathologist and an expert to confirm histo-pathological diagnosis. Abolition of distances. That is specially useful or necessary in countries with low medical population and poor transportation network.

Computer Communication Networks↗
PubMed1992

Deblurring of 3-dimensional patterns of evoked rat cerebellar cortical activity: a study using voltage-sensitive dyes and optical sectioning.

One of the benefits of imaging neuronal activity is the capability of resolving spatial patterns in the x-y plane. With optical sectioning microscopy, the 3-dimensional (3-D) structure may also be studied without physical deformation by serially moving the focal plane of the microscope through the volume of interest along the focal axis. However, each image is blurred by contributions from neighboring planes. This degradation is most severe for low numerical aperture lenses and large amounts of defocus. In this study, an image restoration method using the optical properties of an aberration-free, defocused optical system has been developed for improving optical signals from voltage-sensitive dyes. Deblurring based on the optical transfer function (OTF) of the system was applied on two test sets of serially sectioned images: (1) fluorescent beads and (2) in vivo rat cerebellar cortex stained with the voltage-sensitive dye RH795. This method was shown to reduce significantly the out-of-focus contribution to the images, improving the spatial resolution not only in the x-y plane, but also the z axis. The algorithms were then applied to optical signals obtained by stimulation of the cerebellar surface. Optical signals having a distinct beam-like pattern were evoked and recorded over depths ranging from 0 to 300 microns prior to deblurring. Application of the deblurring algorithm reduced the depth of cerebellar cortex over which the optical signals were observed. In agreement, field potential recordings of the evoked parallel fiber volley and post-synaptic components were restricted to a narrow range of depths similar to the deblurred optical images. Removal of out-of-focus information is an essential step in the serial sectioning of central nervous system structures for neuronal imaging and 3-D reconstruction.

Algorithms↗
PubMed1994

Protein design by optimization of a sequence-structure quality function.

An automated procedure for protein design by optimization of a sequence-structure quality has been developed. The method selects a statistically optimal sequence for a particular structure, on the assumption that such a protein will adopt the desired structure. We present two optimization algorithms: one provides an exact optimization while the other uses a combinatorial technique for comparatively rapid results. Both are suitable for massively parallel computers. A prototype system was used to design sequences which should adopt the four-helix bundle conformation of myohemerythrin. These appear satisfactory to secondary structure and profile analysis. Detailed inspection reveals that the sequences are generally plausible but, as expected, lack some specific structural features. The design parameters provide some insight into the general determinants of protein structure.

Algorithms↗
PubMed1990

PR interval behavior during exercise: implications for physiological pacemakers.

The relationship between heart rate response and the dynamic changes in the PR interval was assessed in 631 patients undergoing routine cardiac exercise tests for a variety of clinical indications. Patients were stratified into four subsets: nonmedicated normals (n = 437), patients on beta-antagonist agents (n = 118), those on antiarrhythmic agents alone (n = 61) and those with a clinical diagnosis of advanced (New York Heart Association [NYHA] Class III or IV) congestive heart failure. All patients were in stable sinus rhythm throughout the test. PR intervals were measured at rest, at mid-exercise and at peak exercise. Mean PR intervals shortened to a statistically significant degree in most subgroups. This effect was predominantly observed in the earlier stages of exercise. In patients with advanced heart failure, there was no statistically significant shortening of exercise PR intervals later in exercise, demonstrating a parallel with their relatively blunted heart rate response. These changes in exercise PR intervals suggest that implanted pacemaker algorithms may be constructed to maximize hemodynamic benefit in patients requiring physiological pacemakers.

Adolescent↗
PubMedDate not supplied

Cone-beam SPECT with a tilted detector.

In this article we report on the TOMOCONIC project for cerebral SPECT using cone-beam collimators. First, we describe our experimental set-up. The cone-beam collimator improves both spatial resolution and sensitivity. We tilt the detector to center the focal point rotation plane on the region to reconstruct. Then, to minimize cone-beam artefacts, we use the Grangeat algorithm for image reconstruction. We describe here how it can be generated to this tilted acquisition geometry. Finally, we present our first experimental results and a comparison with parallel-beam SPECT. The improvement ratio on transverse spatial resolution is 1.5. We conclude with our first clinical images.

Algorithms↗
PubMed1987

Rapid three-dimensional treatment planning: I. Ray-tracing approach to primary component dose calculations.

Algorithms for fully three-dimensional divergent-beam radiotherapy treatment planning have been developed to achieve very high sampling of dose in heterogeneous (inhomogeneous density) tissue throughout an arbitrarily oriented patient volume, in clinically acceptable times of calculation. Dose is calculated at points along numerous rays which sample each beam. To display the dose distribution, the calculated dose values for each beam are interpolated onto rectilinear grids of (arbitrary) parallel planes, scaled for beam weight and finally merged with the weighted dose contributions of other beams. In this paper we describe and demonstrate the algorithm for the primary component of the three-dimensional photon dose distribution delivered to a patient.

Clinical Trials as Topic↗
PubMed1994

Conductance catheter measurements of left ventricular volume in the intact dog: parallel conductance is independent of left ventricular size.

OBJECTIVE: It has recently been suggested that conductance catheter parallel conductance (alpha Vc) is a function of left ventricular volume. To confirm this, alpha Vc was measured in this study over a wide range of steady state volumes. In addition, conductance derived volumes were compared to those obtained by radionuclide angiography to determine if the conductance catheter can be used to measure absolute left ventricular volume accurately in the intact dog heart. METHODS: Seven dogs were anaesthetised and instrumented with left ventricular conductance and pressure tip catheters, a flow through rho cuvette to continually measure blood resistance, a thermodilution catheter, and a venous catheter for volume infusion/withdrawal. Conductance and angiographic data were acquired at 8(SD 1) variably loaded states. Parallel conductance was measured twice at each state using a saline dilution technique and a new non-linear algorithm that allows variability in the observations of both maximum and minimum conductance volumes. RESULTS: The mean value of alpha Vc was 89.1(18.0) ml (71.8 to 111.3 ml) with a mean within-animal coefficient of variation of 7.3(3.4)%. Multiple linear regression using dummy variables to account for the large interanimal variability did not reveal any relationship between alpha Vc and either maximum or minimum left ventricular volume. Furthermore, no difference was found when alpha Vc values measured at the lowest and highest loading levels in each dog were compared. Linear regression showed good agreement between conductance and radionuclide derived end diastolic volumes (slope = 0.94, R = 0.9, p < 0.001). CONCLUSIONS: While alpha Vc varies between animals, it remains constant within any given animal over a broad range of left ventricular volumes. Thus the conductance catheter can provide reliable absolute left ventricular volume measurements under steady state conditions.

Animals↗