T1Effects in Sequential Dynamic Susceptibility Contrast Experiments

Residual effects of an initial bolus of gadolinium contrast agent have been previously demonstrated in sequential dynamic susceptibility contrast MR experiments. While these residual effects quickly reach a saturation steady state, their etiology is uncertain, and they can lead to spurious estimates of hemodynamic parameters in activation experiments. The possible influence ofT₁effects is now investigated with experiments in whichT₁weighting is varied as well as with serial regionalT₁measurements. Little evidence for significant residualT₁effects is found, suggesting instead that susceptibility effects underlie these observations. An initial saturation dose of contrast agent minimizes this effect.     

The Laplacian and Dirac operators on critical planar graphs

On a periodic planar graph whose edge weights satisfy a certain simple geometric condition, the discrete Laplacian and Dirac operators have the property that their determinants and inverses only depend on the local geometry of the graph. We obtain explicit expressions for the logarithms of the (normalized) determinants, as well as the inverses of these operators. We relate the logarithm of the determinants to the volume plus mean curvature of an associated hyperbolic ideal polyhedron. In the associated dimer and spanning tree models, for which the determinants of the Dirac operator and the Laplacian respectively play the role of the partition function, this allows us to compute the entropy and correlations in terms of the local geometry. In addition, we define a continuous family of special discrete holomorphic functions which, via convolutions, gives a general process for constructing discrete holomorphic functions and discrete harmonic functions on critical planar graphs. Oblatum 6-III-2002 & 12-VI-2002?Published online: 6 August 2002     

Operator stable laws: Series representations and domains of normal attraction

LetX,X ₁,X ₂,... be i.i.d. random vectors in ^d. The limit laws that can arise by suitable affine normalizations of the partial sums,S _n=X ₁+...+X _n, are calledoperator-stable laws. These laws are a natural extension to ^d of the stable laws on. Thegeneralized domain of attraction of [GDOA()] is comprised of all random vectorsX whose partial sums can be affinely normalized to converge to . If the linear part of the affine transformation is restricted to take the formn ^–B for some exponent operatorB naturally associated to thenX is in thegeneralized domain of normal attraction of [GDONA()]. This paper extends the theory of operator-stable laws and their domains of attraction and normal attraction.     

A new route to 2-styrylbenzimidazoles

An attempt was made to prepare 2-benzylquinoxalin-3-one by hydrolyzing the azlactone, 2-phenyl-4-benzylidene-5-oxazolone to β-phenylpyruvic acid and then treating this in situ with o-phenylenediamine (OPDA). The initial hydrolysis apparantly proceeded only as far as opening the azlactone ring forming 2-benzamidocinnamic acid which condensed with OPDA to form a substituted styrylbenzimidazole.     

Enhancement of phosphoprotein analysis using a fluorescent affinity tag and mass spectrometry

A fluorescent affinity tag (FAT) was synthesized and was utilized to selectively modify phosphorylated serine and threonine residues via beta-elimination and Michael addition chemistries in a 'one-step' reaction. This labeling technique was used for covalent modification of both phosphoproteins and phosphopeptides, allowing identification of these molecular species by fluorescence imaging after solution- or gel-based separation methods. In addition to the strong fluorescence of the rhodamine tag, a commercially available antibody can be used to enrich low-abundance post-labeled phosphopeptides present in complex mixtures. Application of this methodology to phosphorylation-site mapping has been evaluated for a phosphoprotein standard, bovine beta-casein. Initial results demonstrated low femtomole detection limits after fluorescence image analysis of FAT-labeled proteins or peptides.     

Oxygen, carbon, and sulfur segregation in annealed and unannealed zerovalent iron substrates

Finely ground and pretreated iron substrates known as "zerovalent iron" or "Fe0" are used as reductants in the environmental remediation of halogenated hydrocarbons, and the composition of their surfaces significantly affects their reactivity. Samples of unannealed and annealed (heat-treated under H2/N2) zerovalent iron were analyzed using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Surface concentration of the iron and of the impurities observed by XPS and AES, carbon, chlorine, sulfur, and oxygen, were measured before and after soaking in trichloroethylene (TCE) and in water saturated with TCE (H2O/TCE) to simulate chlorocarbon remediation conditions. Samples pretreated by annealing at high temperature under H2 contained less iron carbide. The carbide contaminant was evident in both iron and carbon XPS spectra, with binding energies of 709.0 and 283.3 eV for the Fe 2p3/2 and C 1s, respectively. The annealed Fe0 surface also contained more sulfur. The carbide concentration was essentially unchanged by TCE and H2O/TCE exposure, whereas the sulfur decreased in proportion to chlorine adsorption following the dechlorination reaction. While oxygen concentration is initially lower on the annealed substrate surface, it rapidly increased during the model TCE remediative treatment process and thus does not represent a significant effect of the annealing process on surface reactivity.     

Alternating structures in copolymers as elucidated by microozonolysis

The technique of microozonolysis followed by gas chromatography has been used to determine the amount of 1,2-structures occurring as 1,4–1,2–1,4 sequences in n-butyl-lithium-initiated polybutadienes. The product obtained from the ozonolysis of these sequences, 3-formly-1,6-hexanedial, was directly proportional to the 1,2 (vinyl) content of the polymers as measured by infrared or NMR spectroscopy. An unusual ozonolysis product, 4-octene-1,8-dial, has been found in the ozonolysis products of high-1,4 poly-butadienes. Ozonolysis has also been used to determine alternating structures in propylene-butadiene copolymers. The product obtained from ozonolysis of such structures is 3-methyl-1,6-hexanedial. Ozonolysis of a polyisoprene containing equal amounts of 1,4 and 3,4 structures indicated it to have a nonalternating structure with long blocks of 1,4-isoprene units. Gas chromatographic separation was accomplished by using sucrose acetate isobutyrate with Carbowax 20M as the liquid phase. Retention times were established by using model compounds where possible.     

Fibrin proliferation at model surfaces: influence of surface properties

Fibrin proliferation from both human fibrinogen solutions and platelet-poor plasma was studied quantitatively as a function of substrate surface properties. A quartz crystal microbalance was used to monitor both protein adsorption and fibrin proliferation in real time at hydrophobic, hydrophilic, positively charged, and negatively charged surfaces. Scanning electron microscopy was used to characterize the morphology of the polymerized fibrin layers. The observed changes in mass indicate that fibrinogen adsorption occurs rapidly and mediates subsequent fibrin proliferation. Notably, substrate surface properties significantly affect the ability of adsorbed fibrinogen to promote fibrin proliferation.     

Surface phase behavior and microstructure of lipid/PEG-emulsifier monolayer-coated microbubbles

Langmuir trough methods and fluorescence microscopy were combined to investigate the phase behavior and microstructure of monolayer shells coating micron-scale bubbles (microbubbles) typically used in biomedical applications. The monolayer shell consisted of a homologous series of saturated acyl chain phospholipids and an emulsifier containing a single hydrophobic stearate chain and polyethylene glycol (PEG) head group. PEG-emulsifier was fully miscible with expanded phase lipids and phase separated from condensed phase lipids. Phase coexistence was observed in the form of dark condensed phase lipid domains surrounded by a sea of bright, emulsifier-rich expanded phase. A rich assortment of condensed phase area fractions and domain morphologies, including networks and other novel structures, were observed in each batch of microbubbles. Network domains were reproduced in Langmuir monolayers under conditions of heating–cooling followed by compression–expansion, as well as in microbubble shells that underwent surface flow with slight compression. Domain size decreased with increased cooling rate through the phase transition temperature, and domain branching increased with lipid acyl chain length at high cooling rates. Squeeze-out of the emulsifier at a surface pressure near 35 mN/m was indicated by a plateau in Langmuir isotherms and directly visualized with fluorescence microscopy, although collapse of the solid lipid domains occurred at much higher surface pressures. Compression of the monolayer past the PEG-emulsifier squeeze-out surface pressure resulted in a dark shell composed entirely of lipid. Under certain conditions, the PEG-emulsifier was reincorporated upon subsequent expansion. Factors that affect shell formation and evolution, as well as implications for the rational design of microbubbles in medical applications, are discussed.