Carotid endarterectomy is not a treatment for stroke or TIA but is performed only to reduce the risk of stroke. When a patient has a TIA or nondisabling stroke and is found to have carotid stenosis of 50% or greater o...
详细信息
Carotid endarterectomy is not a treatment for stroke or TIA but is performed only to reduce the risk of stroke. When a patient has a TIA or nondisabling stroke and is found to have carotid stenosis of 50% or greater on the side corresponding to the ocular or cerebral event, carotid endarterectomy affords an appropriate significant opportunity to prevent a subsequent stroke. Carotid endarterectomy for patients with severe but asymptomatic disease can also provide an opportunity to reduce the risk of stroke. However, since the risk-benefit ratio is much smaller in these cases, the comorbid conditions and patient preferences need to be factored into the decision making. For both symptomatic and asymptomatic carotid artery stenosis, the surgeon should be experienced in performing carotid endarterectomy and should have surgical outcomes similar to those of the surgeons who participated in the carotid endarterectomy trials. Carotid angioplasty with or without stenting is still being evaluated in clinical trials to determine its role in reducing the risk of stroke in patients with carotid artery stenosis.
Neurite growth and guidance depends on the transduction of extracellular guidance cues into motile responses by the sensory apparatus at the tip of the neurite, the growth cone. Contact of the growth cone with extrace...
详细信息
Neurite growth and guidance depends on the transduction of extracellular guidance cues into motile responses by the sensory apparatus at the tip of the neurite, the growth cone. Contact of the growth cone with extracellular ligands leads to the cytoskeletal reorganisation required for changes in rate of motility and direction of outgrowth. Differential adhesion mediated by cell adhesion molecules and signal transduction pathways mediated by growth cone receptors were once seen as separate but cooperative events in controlling growth cone motility. However, recent findings suggest that cell adhesion molecules can activate novel signalling pathways in the growth cone by the recruitment of fibroblast growth factor receptors leading to neurite outgrowth. This Review focuses on work by various laboratories centering on the intracellular consequences of the cell adhesion molecule-mediated activation of the fibroblast growth factor receptor. These include activation of a lipase cascade including phospholipase C and diacylglycerol lipase and culminating in the release of arachidonic acid. This release of arachidonic acid is proposed to activate the transient opening of voltage dependent ion-channels leading to localised rises in growth Ca2+. Recent findings demonstrating this previously undetectable rise in Ca2+ in the growth cone are discussed in light of the proposed roles and mechanisms of Ca2+ in controlling neurite outgrowth. The Ca2+ rises are thought to induce the activation of GAP43 and Ca2+/calmodulin-dependent kinase II, molecules implicated in the modulation of cytoskeletal remodelling. The evidence that this pathway may be involved in the guidance of retinal ganglion cells is evaluated. (C) 2000 Academic Press.
The literature reflects that numerous factors impact a young child's ability to have playmates and develop friendships. This in turn can influence health, happiness, and stability throughout life. Social relations...
详细信息
We fit a functional relationship between aptitude and achievement test scores and show how to use it to allocate educational resources. As an example we use the PSAT-Mathematics test to predict performance on the Coll...
详细信息
We fit a functional relationship between aptitude and achievement test scores and show how to use it to allocate educational resources. As an example we use the PSAT-Mathematics test to predict performance on the College Board's Advanced Placement Test in calculus, as a guide to student and school participation, for school or system assessment, and to project future nationwide expansion. In addition to the PSAT-AP test score relations, we consider the distribution of student ability, school policies of student selection and recruitment, and teacher skill in presenting the material and in motivating students. This overall result provides an indication of just how remarkable was Jaime Escalante's accomplishment in Los Angeles's Garfield High School. We find little evidence for differences in educational quality between such diverse schools as in the inner city of Detroit and the affluent suburb of La Canada, California. We comment briefly on the role of the AP in international assessments.
The National Aeronautics and Space Administration's (NASA's) Marshall Space Plight Center (MSFC) has been developing and testing video-based sensors for automated spacecraft guidance for several years. The vid...
详细信息
ISBN:
(纸本)0819438235
The National Aeronautics and Space Administration's (NASA's) Marshall Space Plight Center (MSFC) has been developing and testing video-based sensors for automated spacecraft guidance for several years. The video sensor currently under development is to have a tracking rate of 50 Hz while delivering full 3-dimensional relative information (X, Y, Z, Pitch, Yaw, and Roll.) Prior systems have been developed and tested in both open-loop and closed-loop simulations. The prototype Video guidance Sensor (VGS) was built for a flight experiment and performed well on two separate Space Shuttle flights. The VGS uses two wavelengths of light to illuminate a target that has a pattern of filtered retro-reflectors. The filters pass only one wavelength of light and absorb the other. Two fast, successive pictures are taken of the target, each picture illuminated by a different wavelength. When the background picture is subtracted from the foreground, a high signal to noise ratio is achieved, and the target spots are easy to track. The next generation VGS will be using a CMOS imaging chip for higher-speed target tracking and a Texas Instruments DSP for higher-speed image processing. The system is being designed to have lower weight and power requirements than the previous generation, and it will be suitable for other applications.
Preliminary results of a new computational approach to generate aggressive trajectories in real-time for constrained mechanical systems are presented. The algorithm is based on a combination of nonlinear control theor...
详细信息
ISBN:
(纸本)0780366387
Preliminary results of a new computational approach to generate aggressive trajectories in real-time for constrained mechanical systems are presented. The algorithm is based on a combination of nonlinear control theory, spline theory, and sequential quadratic programming. It is demonstrated that read-time trajectory generation for constrained mechanical systems is possible by mapping the problem to one of finding trajectory curves in a lower dimensional space. Performance of the algorithm is compared with existing optimal trajectory generation techniques. Numerical results are reported using the NTG software package.
An incremental-learning-by-navigation approach to vision-based autonomous land vehicle (ALV) guidance in indoor environments is proposed. The approach consists of three stages: initial learning, navigation, and model ...
详细信息
An incremental-learning-by-navigation approach to vision-based autonomous land vehicle (ALV) guidance in indoor environments is proposed. The approach consists of three stages: initial learning, navigation, and model updating. In the initial learning stage, the ALV is driven manually, and environment images and other status data are recorded automatically, Then, an off-line procedure is performed to build an initial environment model. In the navigation stage, the ALV moves along the learned environment automatically, locates itself by model matching, and records necessary information for model updating. In the model updating stage, an off-line procedure is performed to refine the learned model. A more precise model is obtained after each navigation-and-update iteration. Used environment features are vertical straight lines in camera views. A multiweighted generalized Hough transform is proposed for model matching. A real ALV was used as the testbed, and successful navigation experiments show the feasibility of the proposed approach.
作者：
Lu, PHanson, JMIowa State Univ
Dept Aerosp Engn & Engn Mech Ames IA 50011 USA NASA
George C Marshall Space Flight Ctr Flight Mech Guidance Navigat & Control Syst Branc Huntsville AL 35812 USA
The entry Eight of the X-33 Advanced Technology Demonstrated is first examined by using a trajectory optimization approach. For closed-loop guidance the nominal entry trajectory is defined by a piecewise linear drag v...
详细信息
The entry Eight of the X-33 Advanced Technology Demonstrated is first examined by using a trajectory optimization approach. For closed-loop guidance the nominal entry trajectory is defined by a piecewise linear drag vs an energy profile. Under a nonlinear tracking control law and an onboard drag-profile updating scheme, the vehicle reaches the designated target point accurately. To further meet the stringent heading requirement at this point, a ground-track design method and the corresponding trajectory control law, which can be applied at a distance before the vehicle reaches the target point, are developed. Simulations with the X-33 vehicle and trajectory data demonstrate good performance of the guidance algorithms.
High quality and sustainable agricultural production in the open field will be supported by centimetre-precision guidance of agricultural implements. In this paper, a complete guidance system is proposed and simulated...
详细信息
High quality and sustainable agricultural production in the open field will be supported by centimetre-precision guidance of agricultural implements. In this paper, a complete guidance system is proposed and simulated using real sensor data obtained from a real time kinematic (RTK) differential global positioning system (DGPS). This type of the satellite navigation system became available commercially in 1997 and is claimed to reach cm accuracy. It uses real time kinematics to improve the accuracy df a position fix by phase comparison of the carrier signal. The simulation primarily aims to investigate the accuracy of the guidance system that can be obtained with RTK DGPS. Using real DGPS measurement errors the simulated system showed a tracking error less than 0.005 m when the PLD-controller had settled. (C) 1998 Elsevier Science B.V. All rights reserved.
The system error of guidance instruments together with that of orbit tracking is considered and a mathematical model for estimating both of them is established. A thorough investigation of the accurate calculation of ...
详细信息
The system error of guidance instruments together with that of orbit tracking is considered and a mathematical model for estimating both of them is established. A thorough investigation of the accurate calculation of circumstance function and the effects of the system error model is made. Finally, the estimator of the two kinds of errors is presented based on nonlinear regression model and the accuracy of the presented estimator is given. Theoretical analysis and simulation results confirm the validity and effectiveness of this method.
暂无评论