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T.Whit Athey Ph.D. Ronald J. Ross M.D. Paul S. Ruggera P.E. 《Magnetic resonance imaging》1982,1(3):149-154
Measurements were made of magnetic and electric field levels in and around a nuclear magnetic resonance imaging system undergoing a clinical trial. Magnetic field levels ranged from 0.04 tesla (T) in the imaging volume down to about 0.0006 T at the end of the patient table. The peak radio-frequency magnetic field level was 15 amperes per meter (A/m) in the imaging volume, while the rms value was 4.6 A/m. The specific absorption rate resulting from the radio-frequency magnetic field was calculated to be no more than 0.017 watts per kilogram (W/kg). The radio-frequency electric field was detectable only within a few centimeters of the coil assembly, and does not significantly contribute to the specific absorption rate. These exposure levels were much lower than existing guidelines for clinical NMR procedures. 相似文献
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Dean M. Philipp William A. Goddard Michael J. Mullins Phillip S. Athey 《Tetrahedron letters》2004,45(28):5441-5444
We have been engaged in a search for coordination catalysts for the copolymerization of polar monomers (such as vinyl chloride and vinyl acetate) with ethylene. We have been investigating complexes of late transition metals with heterocyclic ligands. In this report we describe the synthesis of a symmetrical bis-thiadiazole. We have characterized one of the intermediates using single crystal X-ray diffraction. Several unsuccessful approaches toward 1 are also described, which shed light on some of the unique chemistry of thiadiazoles. 相似文献
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Victor Krauthamer Howard J. Bryant Christopher C. Davis T. Whit Athey 《Journal of fluorescence》1991,1(4):207-213
With the use of a single, implantable, optical fiber, to excite fluorescence and detect changes from voltage-sensitive dyes, transmembrane potential changes were measured without the need for a clear line-of-sight path between the excitation light, the tissue, and the detector. In a previous study, we were required to use signal averaging and could detect only cardiac action potentials from frog. In the present study we improved this system so that unaveraged cardiac action potentials were resolved with high fidelity, and action potentials from single nerve axons were detected. Endeavors to optimize the signal-to-noise ratio resulted in the selection of a larger core fiber with a rounded tip, styryl dyes, and filters based upon fluorescence spectra of the dyes when bound to membrane (rather than in solution). The frog gave signals nearly comparable in magnitude and signal-to-noise ratio to those seen with systems that use a fluorescence microscope. Action potential-induced signals could be detected in single lobster axons with the intracellular injection of a dye. The improvement in the signal-to-noise ratio allowed the use of a reduced-intensity excitation illumination which produced less bleaching of the dye. 相似文献
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This report outlines a new and efficient synthesis of cyclen (1,4,7,10-tetraazacyclododecane, 1) utilizing bis-imidazoline, 6 (1,1'-ethylenedi-2-imidazoline), with 1,2-dibromoethane. General conditions were developed, allowing for the simple, three-step synthesis of 1 at the multigram scale with an isolated overall yield approaching 65%. The cyclization of 6 produced by the condensation of triethylene tetraamine (TETA) with N,N-dimethylformamide dimethyl acetal, gave the twelve-membered, imidazolinium, cyclized intermediate bromide salt, 7 (2,3,4,5,6,7,8,8c-octahydro-1H-4a,6a,8a-triaza-2a-azoniacyclopent[fg]acenaphthylene), which hydrolyzed to 1 with the use of hot, aqueous caustic. Hydrolysis of 7 under milder conditions formed the 1,4,7,10-tetraazabicyclo[8.2.1]tridecan-13-one (20). Mechanistically, the formation of 7 may be rationalized as involving a diaminocarbene that undergoes an intramolecular carbon-hydrogen insertion. 相似文献
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R. D. Athey W. A. Mosher N. W. Weston 《Journal of polymer science. Part A, Polymer chemistry》1977,15(6):1423-1430
The kinetic scheme describing chain transfer in free-radical polymerizations, the Mayo equation, has been rearranged to yield the probability of generating a functional end group by chain transfer (e.g., to a disulfide). The prediction is that a mixture of functional and nonfunctional ends occurs. Examination of literature data shows a number of systems produce more functionality than predicted. Hence, the challenge to devise a new kinetic scheme to account for the increased functionality is made (e.g., by selective reaction of initiator with chain-transfer agent). 相似文献
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Boone HW Athey PS Mullins MJ Philipp D Muller R Goddard WA 《Journal of the American Chemical Society》2002,124(30):8790-8791
Since the advent of Ziegler-Natta polymerization of ethylene, attempts have been made to extend coordination polymerization to commercially important monomers with polar functionality. In this study we examined the copolymerization of perdeuterated vinyl chloride (VC) and perdeuterated vinyl acetate (VA) with ethylene using a tridentate Fe(II) dichloride pyridine diimine metal catalyst. The resulting ethylene oligomers were examined by GC/MS and 2H NMR spectroscopy. It was shown that VC was inserted once for every approximately 180 ethylene monomers and VA was inserted once for every approximately 350 ethylene monomers. VC and VA behave as comonomers for coordination/insertion polymerizations with ethylene. However, we find that insertion with either monomer leads to termination of the growing chain via beta-elimination processes. The deuterium atoms are exclusively located at the olefin terminus for each of the monomers. 相似文献
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