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Gradient Optimized Gradient-Echo Gradient Moment Nulling Sequences for Flow Compensation of Brain Images

  • Jahng, Geon-Ho;Stephen Pickup
    • Investigative Magnetic Resonance Imaging
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    • v.4 no.1
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    • pp.20-26
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    • 2000
  • Gradient moment nulling techniques require the introduction of an additional gradient on each axis for each order of motion correction to be applied. The additional gradients introduce new constraints on the sequence design and increase the demands on the gradient system. The purpose of this paper is to demonstrate techniques for optimization of gradient echo gradient moment nulling sequences within the constraints of the gradient hardware. Flow compensated pulse sequences were designed and implemented on a clinical magnetic resonance imaging system. The design of the gradient moment nulling sequences requires the solution of a linear system of equations. A Mathematica package was developed that interactively solves the gradient moment nulling problem. The package allows the physicist to specify the desired order of motion compensation and the duration of the gradients in the sequence with different gradient envelopes. The gradient echo sequences with first, second, and third order motion compensation were implemented with minimum echo time. The sequences were optimized to take full advantage of the capabilities of the gradient hardware. The sequences were used to generate images of phantoms and human brains. The optimized sequences were found to have better motion compensation than comparable standard sequences.

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Experimental study of rockburst under true-triaxial gradient loading conditions

  • Liu, Xiqi;Xia, Yuanyou;Lin, Manqing;Benzerzour, Mahfoud
    • Geomechanics and Engineering
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    • v.18 no.5
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    • pp.481-492
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    • 2019
  • Due to the underground openings, the tangentially concentrated stress of the tunnel remains larger at excavation boundary and decreases toward the interior of the surrounding rock with a certain gradient. In order to study the effect of different gradient stress on rockburst, the true-triaxial gradient and hydraulic-pneumatic combined test apparatus were carried out to simulate the rockburst processes. Under the different gradient stress conditions, the rock-like specimen (gypsum) was tested independently through three principal stress directions loading--fast unloading of single surface--top gradient and hydraulic-pneumatic combined loading, which systematically analyzed the macro-mesoscopic damage phenomena, force characteristics and acoustic emission (AE) signals of the specimen during rockburst. The experimental results indicated that the rockburst test under the gradient and hydraulic-pneumatic combined loading conditions could perfectly reflect the rockburst processes and their stress characteristics; Relatively high stress loading could cause specimen failure, but could not determine its mode. The rockburst under the action of gradient stress suggested that the failure mode of specimen mainly depended on the stress gradient. When the stress gradient was lower, progressive and static spalling failure occured and the rockburst grades were relatively slight. On the other hand, shear fractures occurred in rockbursts accounted for increasingly large proportion as the stress gradient increased and the rockburst occurred more intensely and suddenly, the progressive failure process became unconspicuous, and the rockburst grades were moderate or even stronger.

ON GRADIENT RICCI SOLITONS AND YAMABE SOLITONS

  • Choi, Jin Hyuk;Kim, Byung Hak;Lee, Sang Deok
    • Journal of the Chungcheong Mathematical Society
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    • v.33 no.2
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    • pp.219-226
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    • 2020
  • In this paper, we consider gradient Ricci solitons and gradient Yamabe solitons in the warped product spaces. Also we study warped product space with harmonic curvature related to gradient Ricci solitons and gradient Yamabe solitons. Consequently some theorems are generalized and we derive differential equations for a warped product space to be a gradient Ricci soliton.

A new gradient coil design technique for open magnetic resonance imaging systems (개방형 자기공명영상시스템용 경사자계코일의 새로운 설계기법)

  • Lee, Soo-Yeol;Park, Bu-Sik;Yi, Jeong-Han;Yi, Wan
    • Journal of the Korean Institute of Telematics and Electronics S
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    • v.34S no.1
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    • pp.72-79
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    • 1997
  • Most open magnetic resonance imaging systems have used the planar gradient coils whose inductances were minimized through the magnetic energy minimization procedure in the spatial frequency domain. Though the planar gradient coils have smaller inductance than conventional gradient coils, the planar gradient coils often suffer from their poor magnetic field linearity. Scaling the spatial frequencies of the current density function designed by the magnetic energy minimization, magnetic field linearity of the planar gradient coils can be greatly improved with small sacrifice of gradient coil inductance. We have found that the figure of merit of the planar gradient coils, defined by the gradient strength divided by the linearity error and the inductance, can be improved by proposed technique.

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Study on Proximal Convergence/Accommodation(PC/A) Ratio by Comparison of Gradient AC/A Ratio and Calculated AC/A Ratio (Gradient AC/A비와 Calculated AC/A비의 비교에 의한 근접성 폭주비(PC/A)에 관한 연구)

  • Han, Gyeong-Ae;Sung, A-Young
    • Journal of Korean Ophthalmic Optics Society
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    • v.9 no.2
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    • pp.223-231
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    • 2004
  • In most previous studies, the assessment of accommodative convergence to accommodative stimulus (AC/A) ratio was commonly made by measuring gradient AC/A ratio. This study deals with the proximal convergence/accommodation(PC/A)ratio measured by comparing values of the gradient AC/A ratio and the calculated AC/A ratio to prevail the clinical use of the AC/A ratio. Visual acuities of All 124 subjects had been corrected to at least 1.0 with either eye through their habitual refractive correction and the MEM dynamic retinoscopy was performed to estimate their accommodative response. And then the PC/A ratio was calculated by making use of the calculated AC/A ratio and the gradient AC/A ratio. This study showed that the difference between the mean calculated AC/A ratio and the mean gradient AC/A ratio in subgroups may be attributable to proximal convergence. Consequently, further studies on proximity cues including the PC/A ratio could be helpful to prevail the clinical use of the AC/A ratio.

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Combined strain gradient and concrete strength effects on flexural strength and ductility design of RC columns

  • Chen, M.T.;Ho, J.C.M.
    • Computers and Concrete
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    • v.15 no.4
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    • pp.607-642
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    • 2015
  • The stress-strain relationship of concrete in flexure is one of the essential parameters in assessing the flexural strength and ductility of reinforced concrete (RC) columns. An overview of previous research studies revealed that the presence of strain gradient would affect the maximum concrete stress developed in flexure. However, no quantitative model was available to evaluate the strain gradient effect on concrete under flexure. Previously, the authors have conducted experimental studies to investigate the strain gradient effect on maximum concrete stress and respective strain and developed two strain-gradient-dependent factors k3 and ko for modifying the flexural concrete stress-strain curve. As a continued study, the authors herein will extend the investigation of strain gradient effects on flexural strength and ductility of RC columns to concrete strength up to 100 MPa by employing the strain-gradient-dependent concrete stress-strain curve using nonlinear moment-curvature analysis. It was evident from the results that both the flexural strength and ductility of RC columns are improved under strain gradient effect. Lastly, for practical engineering design purpose, a new equivalent rectangular concrete stress block incorporating the combined effects of strain gradient and concrete strength was proposed and validated. Design formulas and charts have also been presented for flexural strength and ductility of RC columns.

Wind characteristics observed in the vicinity of tropical cyclones: An investigation of the gradient balance and super-gradient flow

  • Tse, K.T.;Li, S.W.;Lin, C.Q.;Chan, P.W.
    • Wind and Structures
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    • v.19 no.3
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    • pp.249-270
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    • 2014
  • Through comparing the mean wind profiles observed overland during the passages of four typhoons, and the gradient wind speeds calculated based on the sea level pressure data provided by a numerical model, the present paper discusses, (a) whether the gradient balance is a valid assumption to estimate the wind speed in the height range of 1250 m ~ 1750 m, which is defined as the upper-level mean wind speed, in a tropical cyclone over land, and (b) if the super-gradient feature is systematically observed below the height of 1500 m in the tropical cyclone wind field over land. It has been found that, (i) the gradient balance is a valid assumption to estimate the mean upper-level wind speed in tropical cyclones in the radial range from the radius to the maximum wind (RMW) to three times the RMW, (ii) the super-gradient flow dominates the wind field in the tropical cyclone boundary layer inside the RMW and is frequently observed in the radial range from the RMW to twice the RMW, (iii) the gradient wind speed calculated based on the post-landfall sea level pressure data underestimates the overall wind strength at an island site inside the RMW, and (iv) the unsynchronized decay of the pressure and wind fields in the tropical cyclone might be the reason for the underestimation.

GRADIENT ESTIMATES AND HARNACK INEQUALITES OF NONLINEAR HEAT EQUATIONS FOR THE V -LAPLACIAN

  • Dung, Ha Tuan
    • Journal of the Korean Mathematical Society
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    • v.55 no.6
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    • pp.1285-1303
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    • 2018
  • This note is motivated by gradient estimates of Li-Yau, Hamilton, and Souplet-Zhang for heat equations. In this paper, our aim is to investigate Yamabe equations and a non linear heat equation arising from gradient Ricci soliton. We will apply Bochner technique and maximal principle to derive gradient estimates of the general non-linear heat equation on Riemannian manifolds. As their consequence, we give several applications to study heat equation and Yamabe equation such as Harnack type inequalities, gradient estimates, Liouville type results.

GRADIENT ALMOST RICCI SOLITONS WITH VANISHING CONDITIONS ON WEYL TENSOR AND BACH TENSOR

  • Co, Jinseok;Hwang, Seungsu
    • Journal of the Korean Mathematical Society
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    • v.57 no.2
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    • pp.539-552
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    • 2020
  • In this paper we consider gradient almost Ricci solitons with weak conditions on Weyl and Bach tensors. We show that a gradient almost Ricci soliton has harmonic Weyl curvature if it has fourth order divergence-free Weyl tensor, or it has divergence-free Bach tensor. Furthermore, if its Weyl tensor is radially flat, we prove such a gradient almost Ricci soliton is locally a warped product with Einstein fibers. Finally, we prove a rigidity result on compact gradient almost Ricci solitons satisfying an integral condition.

The Development of Gradient Response CVT for a Small Size Electric Vehicle (소형 전기차량용 구배반응 무단변속기 개발)

  • Kim, Gyu-Sung;Kwon, Young-Woong
    • Journal of Power System Engineering
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    • v.19 no.6
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    • pp.33-38
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    • 2015
  • In this study new CVT(Continuously Variable Transmission) system which is adaptable to a small size electric vehicle is proposed available to gradient response CVT. New pulleys consist of springs adapted driving pulley and driven pulley. At the moment a small electric vehicle drive a slope, new system respond to a gradient as overcoming tensional force of springs. We made prototype of gradient response CVT to test parts performance and travelling performance test. As a result of test, belt pitch diameter varied for high torque direction at the gradient. In the flat travelling, acceleration travelling and gradient travelling performance test, the small electric vehicle with gradient response CVT get improved perfomance than the small electric vehicle with reduction gear.