An optimal design method for magnetic resonance imaging gradient waveforms

A method of using nonlinear constrained optimization to design gradient waveforms for magnetic resonance imaging is described. Formulation and solution of the waveform optimization problem are described and example waveforms are presented for a variety of design objectives and constraint sets. Most design objectives can be expressed as linear or quadratic functions of the discrete parameter set, and most constraint functions are linear. Thus, linear and quadratic programming techniques can be utilized to solve the optimization problem. Among the objectives considered are: minimize RMS current; minimize waveform slewing; minimize waveform moments to reduce motion induced dephasing; minimize echo time (TE) for given imaging and motion refocusing conditions; maximize the gradient amplitude during RF application and sampling and the area of the phase encoding waveform to maximize resolution; and minimize or maximize the gradient b factor or diffusion sensitivity. This optimal design procedure produces physically realizable waveforms which optimally achieve specific imaging and motion artifact reduction goals, and it is likely to reduce waveform design time by making it more scientifically (rather than heuristically) based.     

Prediction of the Durability of Cyclically Loaded Structural Elements

A procedure of determination of the durability of cyclically loaded notched specimens is proposed and experimentally verified. The procedure is based on the concepts of the unified model of fatigue fracture treating the processes of initiation and propagation of a fatigue macrocrack from the common point of view. For specimens with structural stress concentrators of two types, we compute the periods of initiation and growth of a fatigue macrocrack and the number of loading cycles to failure on the basis of the diagrams of fatigue crack growth rates. The numerical results agree with the experimental data with an error of at most 38% depending on the method of calculations and durability. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 4, pp. 39–44, July–August, 2005.     

Optimization of nuclear reactor strategies under uncertainty

Operations research is applied to analysing uncertainties in the selection of nuclear reactor types. Dynamic programming methods have been extended to account for uncertain decision factors by including in the objective function a variance-related uncertainty factor as well as expected cost values. A technique is also described for obtaining total cost probability distributions. A case study is presented which uses the method to analyze a long-term planning problem for a given electric utility system. This problem is also solved by using fuzzy programming methods.     

The role of alcohol in Thailand motorcycle crashes

In order to identify motorcycle accident cause factors and countermeasures in Thailand, a large prospective study was undertaken. Researchers conducted on-scene, in-depth investigation and reconstruction of 969 collisions involving 1082 motorcycle riders. Accidents were randomly sampled and included all levels of injury severity. Alcohol proved to be the most outstanding cause factor, with 393 drinking riders in crashes. Alcohol accidents were distinctly different from non-alcohol crashes. Alcohol accidents were more frequent on weekends and particularly at night, usually when the rider was on his way home. Drinking riders were more likely to lose control of the motorcycle, usually by running off the road. They were more likely to be in a single vehicle accident, to violate traffic control signals, and to be in non-intersection collisions. Males were far more likely to drink and ride than females. Drinking riders were far more likely to be inattentive to the driving task just before they crashed, and to be the primary or sole cause of the accident. One-fourth of all riders did not go to the hospital, and another 42% needed only treatment in the emergency room. Drinking riders were more likely to be hospitalized and far more likely to be killed. The higher hospitalization and fatality rates of drinking riders resulted from the kinds of accidents in which they were involved, not from the minimal differences in speeds and helmet use. Problems with balance and coordination were about equally rare among drinking and non-drinking riders. Inattention was a far greater contributing factor.     

Influence of the Elevated Contents of Silicon and Manganese on the Operating Characteristics of High-Strength Wheel Steel

Materials Science - We study the temperature dependences of strength and plasticity, the cyclic crack-growth resistance after the thermal and force loading in the course of braking, and the wear...     

60 GHz indoor WLANs: insights into performance and power consumption

This paper presents a feasibility study of 60 GHz indoor WLANs. We evaluate 60 GHz performance in a typical academic office building under the primary assumption that 60 GHz WLAN APs and clients will be equipped with relatively wide-beam antennas to cope with client mobility. In contrast to previous works which measured performance at a single layer using custom, non-standard compliant hardware, we investigate performance across multiple layers using primarily 802.11ad-compliant wide-beam COTS devices. Our study shows that the large number of reflective surfaces in typical indoor WLAN environments combined with wider beams makes performance highly unpredictable and invalidates several assumptions that hold true in static, narrow-beam, Line-Of-Sight scenarios. Additionally, we present the first measurements, to our best knowledge, of power consumption of an 802.11ad NIC and examine the impact of a number of factors on power consumption.     

Use of a contact-type measurement device to detect robots'' hand positions

The article presents procedures for using a contact-type measurement device to detect and calibrate robots' hand positions. The device used has recently been designed and fabricated at the National Institute of Standards and Technology (NIST) and is suitable for testing industrial robots utilized in precision manufacturing operations. The device measures accuracy and repeatability of robot hand positions in a three-dimensional work space. The need for such a measurement device is well documented in the literature; however, the prime reason can be summarized in two steps: first, there exists a need to provide an inexpensive, industrial-type measurement system applicable to test a wide variety of robots; second, standardization of measurement methodologies and procedures is highly desired, thus common ground rules can be established between manufacturers and robot users. The device used here for robot testing has an accuracy of 34 μm and is considered a contact-type apparatus because it directly connects to a fixture affixed to the robot hand. To compare the device usage and describe its usefulness, the article highlights four other measurement systems that are in the process of development at NIST.     

Economics of nuclear versus coal Big is beautiful?

The economics of nuclear power versus coal-fired power generation is discussed with special account for scale effects. It is assumed that new plant capacity will be needed at the beginning of 1990s. The comparison of levelized power production costs indicates the apparent cost advantage of large 1000 MW nuclear units over coal-fire power plants, but the advantage is lost as the unit size is decreased to 500 MW. Further analyses show that the position of small nuclear units is not improved in the case of demand uncertainty. For nuclear alternatives the effect of scale economies largely outweighs the penalty of possible overcapacity.     

A Heuristic Algorithm for the Mixed Chinese Postman Problem

The Chinese Postman Problem (CPP) is to find a minimum-cost Eulerian tour in a given graph. CPP is efficiently solvable when the original graph is either undirected or directed. For a mixed graph consisting of both edges and arcs, the Mixed Chinese Postman Problem (MCPP) is known to be NP-complete. Many heuristics and optimal algorithms have been devised for solving MCPPs. A new heuristic is proposed. The heuristic finds the initial solution by using a Minimum Cost Flow algorithm; then it repeatedly uses the shortest path algorithm with slightly modified costs of the edges/arcs. The heuristic improves the solution by trying to find the correct direction of unoriented edges and simultaneously it deletes some of the additional edges/arcs. A number of previous heuristics are tested, analyzed, and compared with the proposed approach. Based upon computational results, the proposed heuristic on average outperforms all previous heuristics.