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261.
N-acetylaspartic acid (NAA), a prominent peak in the proton spectrum, is an amino acid thought to be present almost exclusively in neurons and their dendritic and axonal extensions. While 1H MRS studies are showing promise in identifying NAA deficits in different patient groups, unwanted lipid signal from subcutaneous fat surrounding the skull, and necessarily large voxels have limited investigators' ability to assess NAA in cortical gray matter. Here we report a technique developed to derive estimates of NAA signal from cortical gray matter. This approach uses an inversion recovery imaging pulse sequence with a long TE to suppress lipid signal from the scalp and information from concurrently obtained structural MR images to determine the CSF, white and gray matter composition of each spectroscopic voxel. A regression analysis is then used to estimate what NAA levels would be in "pure" white and gray matter voxels. This technique has been applied to demonstrate reduced NAA gray/white levels in the brains of five healthy older compared with five healthy younger women. 相似文献
262.
KO Donner 《Canadian Metallurgical Quarterly》1977,3(2):171-173
Rod dark adaptation in the amphibian retina appears to be due to three processes: 1. background adaptation, occurring immediately after the extinction of an adapting or bleaching light, 2. intermediate adaptation, that frequently lasts 30 min or more and 3. opsin adaptation, which in the isolated retina where regeneration of rhodopsin is insignificant, is observed a a permanent loss of sensitivity after the completion of intermediate adaptation. Intermediate adaptation is characterized by a linear relation between log threshold and the amount of "retinal" present, a similar relation is obtained between log threshold and the amount of rhodopsin bleached in opsin adaptation. These adaptation processes are discussed in terms of a model of the rod outer segment. 相似文献
263.
The stable crack growth in thin steel sheets is the topic of this paper. The crack opening was observed using a videoextensometry system, allowing the crack extension determination. JR-curve and δR-curve were established from obtained data. The ductile tearing properties of different thin sheets of steel were determined, including the impact of the specimen orientation, from test performed on compact tension specimens loaded under two conditions. The effect of the material, the rolling direction, and loading rate on the crack growth resistance of thin steel sheets was analyzed. In addition to the crack growth resistance, J-integral values for crack initiation were also estimated. The relation between J i and J0.2 was assessed using the basic mathematical and statistical methods. This relation was described by a linear regression model. 相似文献
264.
265.
磁流变阻尼器对斜拉索振动控制研究 总被引:10,自引:3,他引:10
大跨度斜拉桥拉索的风雨振现象越来越引起人们的重视,应用磁流变阻尼器进行拉索振动控制是一种新的探索。本文采用模态坐标表示的拉索运动方程和Bingham模型,对磁流变阻尼器-拉索系统的阻尼特性进行了全面仿真模拟,得到了阻尼器安装位置、外部激励大小及阻尼器输入电压等参数对系统前三阶模态阻尼比的影响规律,介绍了在洞庭湖大桥应用磁流变阻尼器进行拉索振动控制的试验研究情况,试验与仿真结果具有很好的一致性,表明仿真计算模型和方法是正确的和可行的。对磁流变阻尼器减振效果的仿真计算表明,该阻尼器具有很好的减振性能。在实际风雨振发生时实测的加速度响应进一步印证了仿真结果,表明应用磁流变阻尼器能很好地抑制拉索风雨振现象,磁流变阻尼器将是抑制拉索风雨振一种非常有效的手段。这些对于应用磁流变阻尼器进行拉索振动控制具有重要的指导意义。 相似文献
266.
In recent years, robots have come to be required to operate in various environments and situations. Therefore, robots have to recognize the environment they are in and adapt to any situations that present themselves. The purpose of this research is to increase the tracking speed and robustness of a visual servoing system. A 3‐link planar manipulator provided with a CCD camera is used to track the target. In order to track the target, an image‐based tracking method, suitable for high‐speed tracking, is used. This paper proposes an image‐space observer (IOB), which compensates disturbances in the image space. A zero‐order hold is introduced to the IOB to enhance the performance of the visual servoing system. The validity of the proposed method is confirmed through experiments. 相似文献
267.
KO Pui Hang DU Ruxu 《机械工程学报(英文版)》2007,20(5):12-15
A new type of air bearing with flexure structure is introduced. The new bearing is designed for precision mechanical engineering devices such as mechanical watch movement. The new design uses the flexure structure to provide 3D damping to absorb shocks from all directions. Two designs are presented: one has 12 T-shape slots in the radian direction while the other has 8 spiral slots in the radian direction. Both designs have flexure mountings on the axial directions. Based on the finite element analysis (FEA), the new bearing can reduce the vibration (displacement) by as much as 8.37% and hence, can better protect the shafts. 相似文献
268.
The aim of this study was to assess the repeatability of the program-usage in everyday listening situations. A blind field test was used. The Widex Quattro (WQ) system served as a model for multiple memory hearing aids (linear amplification) Eleven experienced WQ wearers (41-73 years) with mild to moderate, recruiting, cochlear hearing losses participated. Eight of them regularly used all four available programs (all used at least three programs). The participants stated in duplicate the best hearing aid program in 15 real-world listening situations. The percentage of subjects who selected identical programs (repeatability) surpassed the level corresponding to pure guess under almost all listening conditions (14). Maximum repeatability (100%) was achieved by the five subjects who visited an industrial environment twice. Interestingly, the repeatability exceeded 70% in demanding listening situations such as: party, conversation in group, etc. Support for this high repeatability was given by a distinct improvement in the mean aided speech-to-noise threshold (3.7 dB, p < 0.005). 相似文献
269.
CHEON-BAE SOHN SANG-MI LEE MYUNG-HEE KIM JEONG HEON KO KYOUNG-SOOK KIM JI-EUN CHANG YONG-KEUN AHN CHEORL-HO KIM 《Journal of food science》1996,61(1):230-234
An extracellular β-amylase, which was easily adsorbable onto raw corn starch, was purified 22.5-fold from a new isolate of Bacillus polymyxa No 26–1 with a Mr of 53 kDa and pI of 9.1. The optimum temperature was 45°C and pH 5.5 for raw corn starch. Thermal stability at 40°C and pH stability at 5.0–8.5 were shown. The degradation ofraw starch by β-amylase was greatly stimulated by pullulanase addition. Scanning electron micrographs revealed that starch granule degradation by the enzyme alone occured at the equatorial grooves of lecticular granules. Corn starch granules hydrolyzed by β-amylase had large holes on granule surfaces. 相似文献
270.