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排序方式: 共有288条查询结果,搜索用时 15 毫秒
281.
Swati Rane John T. Spear Zhongliang Zu Manus J. Donahue John C. Gore 《Magnetic resonance imaging》2014
A novel approach for detecting blood oxygenation level-dependent (BOLD) signals in the brain is investigated using spin locking (SL) pulses to selectively edit the effects of extravascular diffusion in field gradients from different sized vascular structures. We show that BOLD effects from diffusion amongst susceptibility gradients will contribute significantly not only to transverse relaxation rates (R2* and R2) but also to R1ρ, the rate of longitudinal relaxation in the rotating frame. Similar to the ability of 180-degree pulses to refocus static dephasing effects in a spin echo, moderately strong SL pulses can also reduce contributions of diffusion in large-scale gradients and the choice of SL amplitude can be used to selectively emphasize smaller scale inhomogeneities (such as microvasculature) and to drastically reduce the influence of larger structures (such as veins). Moreover, measurements over a range of locking fields can be used to derive estimates of the spatial scales of intrinsic gradients. The method was used to detect BOLD activation in human visual cortex. Eight healthy young adults were imaged at 3 T using a single-slice, SL-prepped turbo spin echo (TSE) sequence with spin-lock amplitudes ω1 = 80 Hz and 400 Hz, along with conventional T2*-weighted and T2-prepped sequences. The BOLD signal varied from 1.1 ± 0.4 % (ω1 = 80 Hz) to 0.7 ± 0.2 % (at 400 Hz), whereas the T2-weighted sequence measured 1.3 ± 0.3 % and the T2* sequence measured 1.9 ± 0.3 %. This new R1ρ functional contrast can be made selectively sensitive to intrinsic gradients of different spatial scales, thereby increasing the spatial specificity of the evoked response. 相似文献
282.
S. R. Bhoyar R. S. Rane S. D. Katore A. G. Deshmukh 《International Journal of Theoretical Physics》2011,50(11):3467-3474
Taub (Ann. Math., 53:472?C490, 1951) has studied plane symmetry in Riemannian space-time by considering empty space-time admitting a three parameter group of motions. In this paper, we have deduced the line element of such a space-time for Z=Z(t/z)-type plane gravitational waves using suitable transformations following the concept of Takeno (Sci. Rep. Inst. Theor. Phys. Hiroshima Univ. 1, 1961), Lal and Ali (Tensor 20:281?C302, 1969). Furthermore it has been shown that the deduced space-time admit plane wave solutions of the field equations of general relativity containing electromagnetic terms. Also we have studied electromagnetic field except gauge transformation with particular cases with respect to ??. 相似文献
283.
Kale VS Sathe BR Kushwaha A Aslam M Shelke MV 《Chemical communications (Cambridge, England)》2011,47(27):7785-7787
A novel, catalyst-free strategy for the direct synthesis of vertically aligned silicon nanowire-carbon nanotube (SiNW-CNT) heterojunction arrays is presented. Such a heterojunction with the junction area in the nanoscale displays enhanced field emission characteristics at low turn-on field, with a nearly three times increase in the field enhancement factor. 相似文献
284.
Steam etched porous graphene oxide network for chemical sensing 总被引:1,自引:0,他引:1
Han TH Huang YK Tan AT Dravid VP Huang J 《Journal of the American Chemical Society》2011,133(39):15264-15267
Oxidative etching of graphene flakes was observed to initiate from edges and the occasional defect sites in the basal plane, leading to reduced lateral size and a small number of etch pits. In contrast, etching of highly defective graphene oxide and its reduced form resulted in rapid homogeneous fracturing of the sheets into smaller pieces. On the basis of these observations, a slow and more controllable etching route was designed to produce nanoporous reduced graphene oxide sheets by hydrothermal steaming at 200 °C. The degree of etching and the concomitant porosity can be conveniently tuned by etching time. In contrast to nonporous reduced graphene oxide annealed at the same temperature, the steamed nanoporous graphene oxide exhibited nearly 2 orders of magnitude increase in the sensitivity and improved recovery time when used as chemiresistor sensor platform for NO(2) detection. The results underscore the efficacy of the highly distributed nanoporous network in the low temperature steam etched GO. 相似文献
285.
Supriyo Paul Krishna Kumar Mahendra K. Verma Daniele Carati Arnab K. De Vinayak Eswaran 《Pramana》2010,74(1):75-82
In this paper we investigate two-dimensional (2D) Rayleigh-Bénard convection using direct numerical simulation in Boussinesq
fluids with Prandtl number P = 6.8 confined between thermally conducting plates. We show through the simulation that in a small range of reduced Rayleigh
number r (770 < r < 890) the 2D rolls move chaotically in a direction normal to the roll axis. The lateral shift of the rolls may lead to a
global flow reversal of the convective motion. The chaotic travelling rolls are observed in simulations with free-slip as
well as no-slip boundary conditions on the velocity field. We show that the travelling rolls and the flow reversal are due
to an interplay between the real and imaginary parts of the critical modes. 相似文献
286.
287.
Adimule Vinayak Bhat Vinay S. Joshi Rajeev Batakurki Sheetal Hegde Gurumurthy Yallur Basappa C. 《Journal of Solid State Electrochemistry》2023,27(2):511-529
Journal of Solid State Electrochemistry - In the present investigation, we have synthesized samarium (Sm) nanoparticles (NPs) and anchored them onto the surface of CuO:CoO nanostructure (NS) by... 相似文献
288.
Daniel Lin Matthew Coe Vinayak Krishnamurti Xanath Ispizua-Rodriguez G. K. Surya Prakash 《Chemical record (New York, N.Y.)》2023,23(9):e202300104
In the last few years, many reagents and protocols have been developed to allow for the efficient fluorofunctionalization of a diverse set of scaffolds ranging from alkanes, alkenes, alkynes, and (hetero)arenes. The concomitant rise of organofluorine chemistry and visible light-mediated synthesis have synergistically expanded the fields and have mutually benefitted from developments in both fields. In this context, visible light driven formations of radicals containing fluorine have been a major focus for the discovery of new bioactive compounds. This review details the recent advances and progress made in visible light-mediated fluoroalkylation and heteroatom centered radical generation. 相似文献