High-resolution venography of the brain using magnetic resonance imaging

The purpose of this study was to evaluate a non-flow related magnetic resonance imaging method to visualize small veins independent of arteries in the human brain. A long TE, high-resolution 3D gradient echo MR acquisition was used to highlight venous information. The method is based on the paramagnetic property of deoxyhemoglobin and the resulting phase difference between veins and brain parenchyma at long echo times. The MR magnitude images were masked with a phase mask filter to enhance small structure visibility.. Venous information down to sub-pixel vessel diameters of several hundred microns is visible. Venous data are displayed in an angiographic manner using a minimum intensity projection algorithm. Both superficial veins and deep white matter veins are visible. The method has been successfully applied in volunteers. Preliminary results in patients with cerebral arteriovenous malformations indicate its potential in clinical applications. The proposed method is easy to implement and does not require administration of a contrast agent or application of specially designed rf pulses to highlight the veins. Rather it exploits the intrinsic magnetic properties (BOLD-effect) and the prolonged T ^_2* of venous blood. The method may be of diagnostic potential in the assessment of arteriovenous malformations or other vascular venous lesions. © 1998 Elsevier Science B.V. All rights reserved.     

Electrooptic tunable filters for infrared wavelengths

This paper describes the performance characteristics of an electronically tunable infrared filter based on the electrooptic effect. Like the acoustooptic tunable filter, this device operates by selectively coupling principal polarizations in a birefringent crystal at a Phase-matched wavelength by means of a spatially periodic refractive index perturbation. Instead of a traveling acoustic wave, however, the electrooptic tunable filter employs a temporally static electric field. The main advantages of this filter are its very low power consumption and its versatility of passband programming by virtue of separately addressable voltages under microprocessor control. Two experimental embodiments of the filter are described, one using longitudinal fields, collinear with the light beam, the other using transverse fields in a "thick waveguide" configuration. Experimental results of filter characteristics in the wavelength range2-10 mum are presented. We also present examples of passband synthesis that are achieved by apodizing the amplitude of the applied electric field perturbations. These include passband broadening and sidelobe suppression. The experimental transfer characteristics exhibit good agreement with the theoretical computer plots. Typical driver voltages are ∼ 500 V.     

A novel technique for preparing organophilic silica by water-in-oil microemulsions

Summary A novel technique to prepare ultramicro spherical silica particles containing vinyl groups on their surfaces is presented. This process is a combination of the sol-gel technique and the water-in-oil microemulsion technique in which hydrolysis and condensation of TEOS [Si(OEt)₄] and MPS (trimethoxysilylpropylmethacrylate) take place. Spherical silica particles with a size range from 20 to 70 nm were obtained and the surface concentrations of the double bonds per nm² were from 4 to 7.     

The hydrogenation of toluene over nickel loaded Y zeolites

The catalytic activities of a range of hydrogen reduced nickel Y zeolites for the hydrogenation of toluene were measured and correlated with the following catalytic parameters: reaction temperature; reaction time; coke deposition. The role of the alkali metal co-cation (Li⁺, Na⁺, K⁺, Rb⁺ or Cs⁺) in influencing the overall hydrogenation activity of the supported nickel metal was probed. The effect of poisoning the surface Bronsted acidity by the adsorption of ammonia is discussed. For comparative purposes, data on the hydrogenation of benzene over the same catalysts are included.     

Hot Corrosion of Silica

The high-temperature corrosion of bulk silica glass was studied in pure oxygen and in SO₃-containing oxygen atmospheres in the presence of liquid sulfate deposits at temperatures of 700° and 1000°C. No reaction and devitrification were observed without Na₂SO₄ on the surface. The wetting of the silica by the sulfate, the tendency toward basic fluxing, and the crystallization of the silica incrased with the activity of Na₂O. The most extensive degradation of vitreous silica occurred by crystallization, and the resulting spalling under basic conditions and thermal cycling at basic conditions were parabolic. This behavior is explained by a model in which the crystallization is controlled by sodium at the glass-crystal interface and its diffusion into the glass. This sodium diffuses into the glass before crystallization and is swept ahead of the crystallization front.     

Enantioselective two-phase hydrogenation of α-amino acid precursors with water soluble rhodium complexes of the cationic ligand (S,S)-2,4-bis[bis-(p-N,N,N-trimethylammoniumphenyl)phosphino]pentane, [CH3CH(P(p-C6H4NMe3)2CH2CH(P(p-C6H4NMe3)2CH3]4+

Rhodium complexes of the chiral cationic phosphine, (S,S)-2,4-bis[bis-(p-N,N.N-trimethyl-ammoniumphenyl)phosphino]pentane show excellent enantioselectivity for the hydrogenation of the DOPA precursor. (Z)-3-OMe,4-OAcC₆H₃CHC(COOH)(NHCOCH₃), and dehydro-phenylalanineacetamide as slurries in water. The catalytic reactions may be done in either a two-phase system with the catalyst in the aqueous phase or as a slurry in water provided the substrates have some water solubility. Enantioselectivities of as high as 95% can be obtained.     

Challenges and opportunities for next-generation intracortically based neural prostheses

Neural prosthetic systems aim to help disabled patients by translating neural signals from the brain into control signals for guiding computer cursors, prosthetic arms, and other assistive devices. Intracortical electrode arrays measure action potentials and local field potentials from individual neurons, or small populations of neurons, in the motor cortices and can provide considerable information for controlling prostheses. Despite several compelling proof-of-concept laboratory animal experiments and an initial human clinical trial, at least three key challenges remain which, if left unaddressed, may hamper the translation of these systems into widespread clinical use. We review these challenges: achieving able-bodied levels of performance across tasks and across environments, achieving robustness across multiple decades, and restoring able-bodied quality proprioception and somatosensation. We also describe some emerging opportunities for meeting these challenges. If these challenges can be largely or fully met, intracortically based neural prostheses may achieve true clinical viability and help increasing numbers of disabled patients.     

An autocorrelation-based time domain analysis technique for monitoring perfusion and oxygenation in transplanted organs

In designing an implantable sensor for perfusion monitoring of transplant organs the ability of the sensor to gather perfusion information with limited power consumption and in near real time is paramount. The following work was performed to provide a processing method that is able to predict perfusion and oxygenation change within the blood flowing through a transplanted organ. For this application, an autocorrelation-based algorithm was used to reduce the acquisition time required for fast Fourier transform (FFT) analysis while retaining the accuracy inherent to FFT analysis. In order to provide data proving that the developed method is able to predict perfusion as accurately as FFT two experiments were developed isolating both periodic and quasi-periodic cardiac frequencies. It was shown that the autocorrelation-based method was able to perform comparably with FFT (limited to a sampling frequency of 300 Hz) and maintain accuracy down to acquisition times as low as 4 s in length.     

Correspondence between the location of evoked potential generators and sites of maximal sensitivity to stimulation

The potential recorded by a set of electrodes as an action potential traverses a small axonal segment is proportional to the transmembrane potential produced during stimulation of that axon segment by the same set of recording electrodes, under certain circumstances. First, the membrane must have a constant thickness which is so small that the difference between the surface area of the inner and outer surfaces is minimal. Second, all media must be linear. Third, there must be a monotonically increasing relation between the mean transmembrane potential induced by a stimulus and the maximum transmembrane potential. Fourth, as each axon segment depolarizes, the transmembrane current and change in membrane potential during this time are same. This principle remains true for magnetic stimulation and recording as long as currents generated at the boundaries between regions of differing conductivity outside the axon contribute minimally to the field at the axon. This allows the identification of the point at which an action potential generates a maximal extracellular potential as the point that is stimulated with the lowest threshold.     

平板投影光楔的图象质量

我们报道了一种厚度为20mm的14英寸平板投影显示屏,其亮度可达5000尼特、分辨率为600×800。这种显示器由现成的有机玻璃制成,使用只带梯形预畸变的无需调节的视频投影仪。在厚度为20mm对角线为50英寸屏幕上光学模拟预示XGA分辨率,亮度为500尼特、对比度与传统投影仪相当。