An optimized solenoidal head radiofrequency coil for low-field magnetic resonance imaging

Applications of low-field magnetic resonance imaging (MRI) systems (<0.3 T) are limited due to the signal-to-noise ratio (SNR) being lower than that provided by systems based on superconductive magnets (≥1.5 T). Therefore, the design of radiofrequency (RF) coils for low-field MRI requires careful consideration as significant gains in SNR can be achieved with the proper design of the RF coil. This article describes an analytical method for the optimization of solenoidal coils. Coil and sample losses are analyzed to provide maximum SNR and optimum B₁ field homogeneity. The calculations are performed for solenoidal coils optimized for the human head at 0.2 T, but the method could also be applied to any solenoidal coil for imaging other anatomical regions at low field. Several coils were constructed to compare experimental and theoretical results. A head magnetic resonance image obtained at 0.2 T with the optimum design is presented.     

Spin glasses with long-range interaction at high temperatures

We study the Ising andN-vector spin glasses with exchange couplings J=(J _ij ;i, jZ ^d ), which are independent random variables with EJ_ij=0 andEJ ⁿ _ij ⁿ n!¦i–j¦ ^–nd , forn, some finite constant >0, and >1/2. For sufficiently small, we show that forE-a.a.J there is a weakly unique, extremal, infinite-volume Gibbs measure _J for which the expectation of a single (component of) spin vanishes and which has the cluster property inL ₂(E) with the same decay as interaction. This work is based on results and methods of Fröhlich and Zegarlinski.     

Hydroxylamine Oxidation on Polycrystalline Gold Electrodes in Aqueous Electrolytes: Quantitative On-Line Mass Spectrometry under Forced Convection

Herein, a method is presented that allows quantitative determination of faradaic efficiencies for dinitrogen (N₂) generation during the electrochemical oxidation of hydroxylamine (NH₂OH), , on a polycrystalline gold Au(poly) disk electrode in aqueous electrolytes over a wide pH range. This tactic involves the use of an impinging jet electrolyte configuration incorporating a gas porous ring connected in turn to a mass spectrometer. The actual amount of N₂ generated at the Au(poly) disk was assayed using the oxidation of hydrazine (N₂H₄) in aqueous phosphate buffer (pH 7). This redox process yields N₂ as the only product, allowing a direct correlation to be established between the changes in the partial pressures of N₂ and the current flowing through the disk electrode. An analysis of the data collected revealed a strong dependence of both on pH and the applied potential. Although values of as high as 20 to 30 % were found in acid and neutral media over a narrow potential region, those in alkaline solution were far smaller in the entire potential range examined.     

Rigorous results for the free energy in the Hopfield model

We prove that the free energy of the Hopfield model with a finite number of patterns can be represented in terms of an asymptotic series expansion in inverse powers of the neurons number. The series is Borel summable for large temperatures. We also establish mathematically some other interesting properties, partly used before in a seminal paper by Amit, Gutfreund and Sompolinsky.     

Sharp Weak-Type Inequalities for Analytic Functions on the unit Disc

If Q is a Steiner symmetric domain, symmetric about the y-axis,then it can be proved that for an analytic function F:D C (D= {zC: |z| < 1}), such that Im F(0) = 0, and for t> 0,the inequality holds, where denotes the Lebesgue measure of a set A T and C_Q is some constant dependent onlyon Q. The best constantC_Q is found. Some weak-type inequalitiesfollow, which generalize results of Baernstein.     

New alkyl-phosphate bonded stationary phases for liquid chromatographic separation of biologically active compounds

A new type of bonded stationary phase for liquid chromatography, with the properties of immobilized artificial membranes, has been synthesized. Alkyl-phosphate adsorbents were obtained by modification of aminopropyl silica gel. The structures of the synthesized materials were confirmed by use of instrumental techniques--elemental analysis, infrared spectroscopy (FTIR), and (13)C and (29)Si CP/MAS NMR. Analysis revealed that the adsorbents mimic the phospholipids present in natural cell membranes. The new synthesized alkyl-phosphate stationary phases may be used for liquid chromatographic separation of biologically active compounds of different polarity.     

Palladium(0)-Mediated Synthesis of Acetylated Unsaturated 1,4-Disaccharides

Abstract

Alkylation of ethyl 6-O-tert-butyldiphenylsilyl-4-O-methoxycarbonyl-2,3-dideoxy-α-D-erythro-hex-2-enopyranoside (1) with various peracetylated 1-hydroxy sugars in the presence of a catalytic amount of palladium(O) gave the corresponding unsaturated 1,4-disaccharides and trisaccharides. In all cases the reaction is regio- and stereospecific according to the unsaturated moiety, alkylation occuring only at C-4 of the unsaturated carbohydrate, with overall retention of configuration.