Comparison of statistical methods in MR imaging

The problem of recovering the true underlying scene from a noisy image is considered. Several methods are compared empirically by applying them to magnetic resonance (MR) images. It turns out that a simple method, the Gaussian window filter, gives good results. This method requires only “instantaneous” processing.     

Polarization properties of a nematic liquid-crystal spatial light modulator for phase modulation

The polarization properties of a nematic zero-twist liquid-crystal (NLC) spatial light modulator (SLM) were studied. A large ratio between the liquid-crystal (LC) layer thickness and the pixel pitch combined with spatial variations in the applied electric field causes fringing fields between pixels. Depending on the LC alignment, the electric field components within the LC layer can result in a twist deformation. The produced inhomogeneous optical anisotropy affects the polarization of light propagating through the device. We experimentally examined polarization effects in different diffraction orders for both binary and blazed phase gratings. Simulations of the LC deformation together with finite-difference time-domain simulations for the optical propagation were used to calculate the corresponding far-field intensities. It was demonstrated how rigorous simulations of the NLC SLM properties can be used to understand the polarization features of different diffraction orders.     

Feasibility of half-data image reconstruction in 3-D reflectivity tomography with a spherical aperture

Reflectivity tomography is an imaging technique that seeks to reconstruct certain acoustic properties of a weakly scattering object. Besides being applicable to pure ultrasound imaging techniques, the reconstruction theory of reflectivity tomography is also pertinent to hybrid imaging techniques such as thermoacoustic tomography. In this work, assuming spherical scanning apertures, redundancies in the three-dimensional (3-D) reflectivity tomography data function are identified and formulated mathematically. These data redundancies are used to demonstrate that knowledge of the measured data function over half of its domain uniquely specifies the 3-D object function. This indicates that, in principle, exact image reconstruction can be performed using a "half-scan" data function, which corresponds to temporally untruncated measurements acquired on a hemi-spherical aperture, or using a "half-time" data function, which corresponds to temporally truncated measurements acquired on the entire spherical aperture. Both of these minimal scanning configurations have important biological imaging applications. An iterative reconstruction method is utilized for reconstruction of a simulated 3-D object from noiseless and noisy half-scan and half-time data functions.     

Coherence and polarization of electromagnetic beams modulated by random phase screens and their changes on propagation in free space

The spectral degree of coherence and of polarization of some model electromagnetic beams modulated by a polarization-dependent phase-modulating device, such as a liquid-crystal spatial light modulator, acting as a random phase screen are examined on the basis of the recent theory formulated in terms of the 2 x 2 cross-spectral density matrix of the beam. The phase-modulating device is assumed to have strong polarization dependence that modulates only one of the orthogonal components of the electric vector, and the phase of the phase-modulating device is assumed to be a random function of position imitating a random phase screen and is assumed to obey Gaussian statistics with zero mean. The propagation of the modulated beam is also examined to show how the spectral degrees of coherence and of polarization of the beam change on propagation, even in free space. The results are illustrated by numerical examples.     

Nylons: A review of the literature on products of combustion and toxicity

The literature on polyamides was reviewed to determine the nature and extent of information available on these materials which are commonly used in consumer and industrial applications. This review was limited to aliphatic polyamides normally called nylon and excludes aromatic polyamides such as Nomex and bicomponent polymers consisting of nylon and other polymers. The review was further limited to those publications in English through June 1984. Typical pyrolysis products from a broad range of nylons do not appear to differ greatly. Many of the decomposition products detected in vacuum pyrolysis experiments appear as products of thermal degradation in inert and air atmospheres. In air, a general reduction in the quantities of heavier hydrocarbons is noted along with an increase in the production of CO, CO₂, H₂O, NH₃, HCN and NO_x. The toxicity of the thermal degradation products from various types of nylon has been evaluated by nine different protocols. Reported LC₅₀ values range from 10.8 m l^?1 to 61.9 mg l^?1. Dyes apparently do not affect the materials' combustion products toxicity but an increase in the amount of backcoating on a nylon fabric increases toxicity. Time to death measurements show that volatile products from nylons are less toxic than those from rayons or cotton, while the blending of wool with nylon greatly increases the toxicity of the thermal decomposition products. In general, however, the overall toxicity of the thermal degradation products from nylon do not appear to be greatly different than those from many other polymeric materials. Large-scale test results are ambiguous, and it is difficult to interpret the results in terms of a single component in a multicomponent system.     

Experimental study of cryogen spray properties for application in dermatologic laser surgery

Cryogenic sprays are used for cooling human skin during laser dermatologic surgery. In this paper, six straight-tube nozzles are characterized by photographs of cryogenic spray shapes, as well as measurements of average droplet diameter, velocity, and temperature. A single-droplet evaporation model to predict average spray droplet diameter and temperature is tested using the experimental data presented here. The results show two distinct spray patterns--sprays for 1.4-mm-diameter nozzles (wide nozzles) show significantly larger average droplet diameters and higher temperatures as a function of distance from the nozzle compared with those for 0.5-0.8-mm-diameter nozzles (narrow nozzles). These results complement and support previously reported studies, indicating that wide nozzles induce more efficient heat extraction than the narrow nozzles.     

Temperature alterations of infrared light absorption by cartilage and cornea under free-electron laser radiation

Like pure water, the water incorporated into cartilage and cornea tissue shows a pronounced dependence of the absorption coefficient on temperature. Alteration of the temperature by radiation with an IR free-electron laser was studied by use of a pulsed photothermal radiometric technique. A computation algorithm was modified to take into account the real IR absorption spectra of the tissue and the spectral sensitivity of the IR detector used. The absorption coefficients for several wavelengths within the 2.9- and 6.1-microm water absorption bands have been determined for various laser pulse energies. It is shown that the absorption coefficient for cartilage decreases at temperatures higher than 50 degrees C owing to thermal alterations of water-water and water-biopolymer interactions.     

Mythmaking and Knowledge Sharing: Living organizational myths and the broadening of opportunity structures for knowledge sharing in a Scandinavian engineering consultant company

With examples from Scandiaconsult, a Scandinavian engineering consultant company, this paper is an exploration of some of the constructive mythological aspects of living knowledge emerging in the symbolic interaction of organizational work life. Framed within the overall challenge of organizational knowledge sharing, and the approach of the communal resource as a (partial) solution to that, we more specifically look at ways of broadening the opportunity structures for knowledge sharing through the twin principles of organizational myths and rituals – as processes whereby people can identify and tap into the reservoir of living knowledge in the organization life, often in the realm of the informal or shadow organization. We explore in what ways the myths and rituals can leverage knowledge sharing especially in the face of distributed work environments inhabited by multiplex interested knowledge workers, and in our case with the additional challenge of mergers and acquisitions. We argue that active mythmaking is very far from manipulation or plain rhetoric, but rather a complex and interwoven processes of displaying cultural premises and transforming barriers and misbelieves into trustworthy relationships, and thus rendering possible conditions for lowering the costs of knowledge sharing. The mythmaking, of course, has to be deeply interwoven with the already existing mythic reservoir.     

Towards A Role-Based Framework for Distributed Systems Management

Roles have been widely used for modeling theauthority, responsibility, functions, and interactions,associated with manager positions within organizations.In this paper, we discuss the issues related tospecifying roles for both human and automated managersof distributed computer systems. The starting point isthat a role can be defined in terms of the authorizationand obligation policies, for a particular managerposition, which specify what actions the manager ispermitted or is obliged to do on a set of targetobjects. This permits individuals to be assigned orremoved from positions without respecifying the policiesfor the role. However these policies are insufficientfor fully specifying relationships between managers andthe targets they manage or between different managerroles. There is a need to specify the interactionprotocols and how managers coordinate and synchronizetheir activities. The role-based framework consists ofa set of tools enabling the creation of roles frompolicies, the specification of the concurrencyconstraints for role activities and the specification ofprotocols for role interaction. In addition, the issuesrelated to conflicts which can occur between policieswithin a role or between interacting roles are brieflydiscussed.