Short-echo-time magnetic resonance phase-shift velocity mapping for assessment of heart valve and great vessel stenoses: three years' experience

Shortening the echo times of magnetic resonance (MR) sequences used for phase-shift velocity mapping to 3.6 ms has extended use of the technique to measurement of velocities in turbulent, poststenotic jet flows. We used a 0.5-T MR machine and field even-echo rephasing (FEER) sequences with 3.6 ms echo times for jet velocity mapping.In vitro trials used continuous flow through a phantom with a 6-mm stenosis. Fifteen patients with mitral and/or aortic valve stenosis and 20 patients with repaired aortic coarctation were studied prospectively, with Doppler ultrasonic measurement of peak jet velocity performed independently on the same day. The clinical contribution of MR jet velocity mapping, used during a 3-year period in 306 patients with congenital and acquired disease of heart valves, great vessels, and conduits, was assessed retrospectively. The 3.6-ms sequence allowed accurate measurement of jet velocities up to 6 m s^–1 in vitro (r=0.996). Prospective studies in patients showed good agreement between MR and Doppler measurements of peak velocity:n=38; range, 1.2–6.1 m s^–1; mean, 2.7 m s^–1; mean of differences (Doppler-MR), 0.22 ms^–1; standard deviation of differences, ±0.38 m s^–1 (±14%). MR jet velocity mapping proved particularly valuable for assessment and localization of stenoses at sites where ultrasonic access was limited. The technique represents a diagnostic advance which can obviate the need for catheterization in selected cases.     

Investigation of membrane disruption in the reaction catalyzed by cholesterol oxidase

Dye leakage experiments were undertaken to investigate the membrane disruption properties of cholesterol oxidase. Inspection of the X-ray crystal structures of cholesterol oxidase suggested that an active-site "lid" opens in order to bind substrate [Li, J., Vrielink, A., Brick, P., & Blow, D. M. (1993) Biochemistry 32, 11507-11515]. We tested whether the interaction of the putative active-site lid with the membrane was sufficiently disruptive of the membrane structure to cause leakage or lysis of the cell membrane. Vesicles (100 nm) composed of egg phosphatidylcholine, 2-palmitoyl-3-oleoyl-1-sn-phosphatidylethanolamine, and 2-palmitoyl-3-oleoyl-1-sn-phosphatidylcholine were used in this study to mimic biomembranes. To separate the effects of membrane binding from conversion of cholesterol to cholest-4-en-3-one, the active-site mutant E361Q was utilized. In the reaction catalyzed by E361Q, isomerization of the cholest-5-en-3-one intermediate is suppressed and cholest-5-en-3-one is the major product isolated. Furthermore, E361Q produces cholest-5-en-3-one 20-fold more slowly than wild type produces cholest-4-en-3-one from cholesterol. Wild-type and E361Q cholesterol oxidases bind to vesicles with an apparent K(D) of approximately 25 microM, as measured by quenching of intrinsic tryptophan fluorescence, irrespective of headgroup size and cholesterol content. Membrane disruption was measured by leakage of the encapsulated marker carboxyfluorescein. Leakage was observed with cholesterol-containing vesicles and wild-type enzyme only; the rate of leakage was dependent on the rate of cholest-4-en-3-one production. E361Q did not induce membrane disruption, regardless of vesicle type tested. Thus, binding of cholesterol oxidase to the membrane and partitioning of cholesterol into the active site does not sufficiently perturb the bilayer to cause leakage of vesicle contents. Formation of the product cholest-4-en-3-one, however, does increase membrane permeability. Expansion of the lipid bilayer upon conversion of cholesterol to cholest-4-en-3-one is the likely cause of this increased permeability.     

Slurry Utilization and Impact of Mixing Ratio in Biogas Production

The anaerobic conversion of waste to biogas in a biogas digester is influenced by a number of factors including mixing ratios, type of substrate, temperature, organic loading rate, pH, and carbon/nitrogen ratio. The appropriate mixing ratio of water and animal waste leads to effective biogas yield. Different mixing ratios proposed in literature for various animal waste for improving biogas yield are reviewed. Characterization and application of animal slurry, the choice of animal slurry as suitable feedstock for biogas production, the storage of animal slurry, the agitation process of slurry as well as mixing ratios of animal slurry are evaluated.     

A successive approximation vector quantizer for wavelet transformimage coding

A coding method for wavelet coefficients of images using vector quantization, called successive approximation vector quantization (SA-W-VQ) is proposed. In this method, each vector is coded by a series of vectors of decreasing magnitudes until a certain distortion level is reached. The successive approximation using vectors is analyzed, and conditions for convergence are derived. It is shown that lattice codebooks are an efficient tool for meeting these conditions without the need for very large codebooks. Regular lattices offer the extra advantage of fast encoding algorithms. In SA-W-VQ, distortion equalization of the wavelet coefficients can be achieved together with high compression ratio and precise bit-rate control. The performance of SA-W-VQ for still image coding is compared against some of the most successful image coding systems reported in the literature. The comparison shows that SA-W-VQ performs remarkably well at several bit rates and in various test images.     

Photophysics of Two-Dimensional Perovskites—Learning from Metal Halide Substitution

2D perovskites offers a rich playing field to explore exciton physics and they possess a great potential for a variety of opto-electronic applications. Whilst their photophysics shows intricate interactions of excitons with the lattice, most reports have so far relied on single compound studies. With the exception of variations of the organic spacer cations, the effect of constituent substitution on the photophysics and the nature of emitting species, in particular, have remained largely under-explored. Here PEA₂PbBr₄, PEA₂PbI₄, and PEA₂SnI₄ (where PEA stands for phenylethylammonoium) are studied through a variety of optical spectroscopy techniques to reveal a complex set of excitonic transitions at low temperature. Weak high-energy features are attributed to vibronic transitions breaking Kasha's, for which the responsible phonons cannot be accessed through simple Raman spectroscopy. Bright peaks at lower energy are due to two distinct electronic states, of which the upper is a convolution of the free exciton and a localized dark state and the lower is attributed to recombination involving shallow defects. This study offers deeper insights into the photophysics of 2D perovskites through compositional substitution and highlights critical limits to the communities’ current understanding of processes in these compounds.     

Comparative Study on the Performance of Aboveground and Underground Fixed-Dome Biogas Digesters

A comparative analysis of aboveground and underground biogas digesters is described in relation to their performance and effect of temperature, pH, and hydraulic retention time. The digestion chamber of the two biogas systems under study was fabricated using high-density polyethylene (HDPE) plastic, while the inlet and outlet chamber were constructed with bricks. The performance of small-scale digesters, particularly of the fixed-dome type, should be improved. Cow dung served as substrate in the performance test. Biogas production from the two systems was measured by a customized gas temperature pressure measurement sensor. Evidently, the insulation provided to the underground digester helped to achieve a more stable internal temperature for the digester.