k-Space tutorial: an MRI educational tool for a better understanding of k-space

A main difference between Magnetic Resonance (MR) imaging and other medical imaging modalities is the control over the data acquisition and how it can be managed to finally show the adequate reconstructed image. With some basic programming adjustments, the user can modify the spatial resolution, field of view (FOV), image contrast, acquisition velocity, artifacts and so many other parameters that will contribute to form the final image. The main character and agent of all this control is called k-space, which represents the matrix where the MR data will be stored previously to a Fourier transformation to obtain the desired image

Properties of the Broad-Range Nematic Phase of a Laterally Linked H-Shaped Liquid Crystal Dimer

In search for novel nematic materials, a laterally linked H-shaped liquid crystal dimer have been synthesized and characterized. The distinct feature of the material is a very broad temperature range (about 50 oC) of the nematic phase, which is in contrast with other reported H-dimers that show predominantly smectic phases. The material exhibits interesting textural features at the scale of nanometers (presence of smectic clusters) and at the macroscopic scales. Namely, at a certain temperature, the flat samples of the material show occurrence of domain walls. These domain walls are caused by the surface anchoring transition and separate regions with differently tilted director. Both above and below this transition temperature the material represents a uniaxial nematic, as confirmed by the studies of defects in flat samples and samples with colloidal inclusions, freely suspended drops, X-ray diffraction and transmission electron microscopy.Comment: 30 pages (including Supplementary Information), 7 Figure

Tuning charge carrier transport and optical birefringence in liquid-crystalline thin films : a new design space for organic light-emitting diodes

B.L. acknowledges financial support from the Binational Science Foundation under grant No 2014396 and from the National Science Foundation under grant No 1639073. C.K. acknowledges funding from the Kent State University Internal Post-Doctoral Competition. M.C.K. acknowledges funding from the Belgian Agentschap voor innovatie door wetenschap en techniek under grant No IWT 131498.Liquid-crystalline organic semiconductors exhibit unique properties that make them highly interesting for organic optoelectronic applications. Their optical and electrical anisotropies and the possibility to control the alignment of the liquid-crystalline semiconductor allow not only to optimize charge carrier transport, but to tune the optical property of organic thin-film devices as well. In this study, the molecular orientation in a liquid-crystalline semiconductor film is tuned by a novel blading process as well as by different annealing protocols. The altered alignment is verified by cross-polarized optical microscopy and spectroscopic ellipsometry. It is shown that a change in alignment of the liquid-crystalline semiconductor improves charge transport in single charge carrier devices profoundly. Comparing the current-voltage characteristics of single charge carrier devices with simulations shows an excellent agreement and from this an in-depth understanding of single charge carrier transport in two-terminal devices is obtained. Finally, p-i-n type organic light-emitting diodes (OLEDs) compatible with vacuum processing techniques used in state-of-the-art OLEDs are demonstrated employing liquid-crystalline host matrix in the emission layer.Publisher PDFPeer reviewe

Formation of a Silicate L 3 Phase with Continuously Adjustable Pore Sizes

the magnitude of the gain. Thus, the delay time of ϳ0.5 s observed in REFERENCES AND NOTES ___________________________ Since the demonstration that surfactants could be used in the fabrication of silica mesophases (1), amphiphiles have been used to produce inorganic materials with a variety of mesomorphic structures, including lamellar, hexagonally packed tubular, and cubic forms (2-12). Surfactant-induced assembly of inorganic structures is now recognized as a way to make novel nanoporous materials with larger pore sizes than was previously possible. However, techniques developed thus far have limited capability to produce very large pores of a predetermined size. Here we describe the synthesis and characterization of a new, random, bicontinuous silicate mesomorph for which predetermined pore sizes, over a very large size range, may be obtained. Most procedures for forming mesoporous silicates rely on the micelle-forming properties of a surfactant, typically at a low surfactant concentration. The addition of an inorganic precursor, such as an alkoxysilane, leads to association and coassembly into a mesophase precipitant whose structural dimensions are controlled by the surfactant length. Polymerization of the inorganic precursor and removal of the surfactant results in a rigid silica shell conforming to the structural shape of the mesophase. However, the use of dilute surfactant solutions limits the ability to predict the topology of the mesophase. Also, the typical product of the process is a powder of micrometer-sized particles, thereby limiting uses in filtration, optical, or electronic applications, where large-area thin films or large uniform monoliths of material are required. Finally, the pore volume is filled with surfactant; that is, the surfactant must be removed before the pores can be accessed. These difficulties may be partially avoided by the use of high-concentration surfactant systems in which either the inorganic precursors minimally perturb a preexisting surfactant-water liquid crystalline (LC) structure or the LC nature of the system may be recovered under appropriate experimental conditions, as shown by Attard et al. (6). Also, because the inorganic precursor does not precipitate out of solution, the resultant material conforms to the shape of the container in which it forms, thereby allowing fabrication of large monoliths of a desired size and shape. However, even in these cases, the pore size is limited by the surfactant and the limited range of compositions on the phase diagram for a given mesomorphic structure. Applications of silicate mesophases as filtration media, optical materials, and nanocomposites would be facilitated if th

Host preferences and differential contributions of deciduous tree species shape mycorrhizal species richness in a mixed Central European forest

Mycorrhizal species richness and host ranges were investigated in mixed deciduous stands composed of Fagus sylvatica, Tilia spp., Carpinus betulus, Acer spp., and Fraxinus excelsior. Acer and Fraxinus were colonized by arbuscular mycorrhizas and contributed 5% to total stand mycorrhizal fungal species richness. Tilia hosted similar and Carpinus half the number of ectomycorrhizal (EM) fungal taxa compared with Fagus (75 putative taxa). The relative abundance of the host tree the EM fungal richness decreased in the order Fagus > Tilia >> Carpinus. After correction for similar sampling intensities, EM fungal species richness of Carpinus was still about 30–40% lower than that of Fagus and Tilia. About 10% of the mycorrhizal species were shared among the EM forming trees; 29% were associated with two host tree species and 61% with only one of the hosts. The latter group consisted mainly of rare EM fungal species colonizing about 20% of the root tips and included known specialists but also putative non-host associations such as conifer or shrub mycorrhizas. Our data indicate that EM fungal species richness was associated with tree identity and suggest that Fagus secures EM fungal diversity in an ecosystem since it shared more common EM fungi with Tilia and Carpinus than the latter two among each other