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Most magnetic resonance imaging sequences employ field gradients and amplitude modulated RF pulses to excite only those spins lying in a specific plane. The fidelity of the resulting magnetization distribution is crucial to overall image resolution. Conventional RF-pulse design techniques rely on the small tip-angle approximation to Bloch's equation, which is inadequate for the design of 90 degrees and 180 degrees pulses. This paper demonstrates the existence of a selective pulse, and provides a sound mathematical and computational basis for pulse design. It is shown that the pulses are optimal in the class of piecewise continuous functions of duration T. An optimal pulse is defined as the pulse on the interval that achieves a magnetization profile "closest" to the desired distribution. Optimal control theory provides the mathematical basis for the new pulse design technique. Computer simulations have verified the efficacy of the 90 degrees and the 180 degrees inversion and "pancake-flip" optimal pulses. 相似文献
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人体红外辐射及温度特性与呼吸等生理活动密切相关。针对远程监测需要,提出了一种基于面部红外图像分析的在体呼吸监测方法。为了提高红外热成像中鼻孔区域识别及跟踪精度,研究了基于Harris角点法的提取方法,及基于光流场方法的特征跟踪识别方法;分析对比了不同环境中在体呼吸信号时频特征及统计参数。实验结果表明,本文提出的方法可以准确监测在体呼吸信号,分析呼吸模式变化,为红外热成像方法在临床医学的应用提供了参考。 相似文献
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Smith DG Potter SR Lee BR Ko HW Drummond WR Telford JK Partin AW 《IEEE transactions on bio-medical engineering》2000,47(10):1403-1405
A noninvasive electromagnetic method has been developed that can effectively measure the in-vivo conductivity difference between rat tumor lines having a low and high metastatic potential. These tumor lines are used in the study of human prostate tumor. 相似文献
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Caprihan A 《IEEE transactions on medical imaging》1983,2(4):169-175
If NMR imaging uses selective excitation, then it has to do it correctly as it affects the image resolution. We have related the properties of the selected slice, such as its width, its rate of attenuation, and its side lobes to the amplitude modulation of the radio frequency pulse. An interesting observation was that multiplying sinc(t) function by a triangle can give better results than multiplying it with a Gauss function. We have used computer simulation and tried to emphasize the qualitative relationship between the properties. We conclude with some practical requirements and show the need of a numerical design procedure if the slice properties have to be optimized. 相似文献
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提出了一种新的基于椭圆曲线密码体制的公平盲签名方案.该方案结合了已有的公平盲签名和基于双线性配对函数的短签名方案.方案中裁判者可在特殊情况下揭示签名文件的原文,可有效阻止利用盲签名的匿名性进行犯罪的行为.最后对方案作了安全性分析. 相似文献
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Electronic transaction through e-payment protocol will grow tremendous in the coming years. In this article, by conversely using blind signature, the authors propose a fair electronic payment scheme for electronic commerce, which can ensure two participants' right simultaneously in electronic transaction process. This scheme is different from other existing schemes and it does not require strong trust relation between customer and merchant. In the scheme, a semi-trusted third party (S-TTP) is involved to provide a fair commerce environment. The S-TTP takes part in the protocol on-line and no secure information about the business is leaked. Moreover, the participants do not need to register to S-TTP. This scheme does not require intervention of a third party in case of dispute, if one user cheats or simply crashes. 相似文献
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With the development of blockchain,huge storage space is needed to store all of blockchain data.In addition,data can’t be changed once it is packaged into the chain,and it may be possible for overdue data to take up large storage space.Firstly,a threshold ring signature scheme was improved,and then a deletable blockchain scheme based on the mechanism of proof of space (PoSpace) consensus was constructed.Most of nodes could sign and delete a block together when block data was overdue,and the overall structure of the blockchain was unchanged.Several experiments in a simulated environment were executed,and the results show that the proposed blockchain scheme has high efficiency in generating and delegating a block.Meanwhile,the deletion of a block doesn’t influence the storage and use of other blocks. 相似文献
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Efficient blind signature scheme based on quadratic residues 总被引:4,自引:0,他引:4
The authors propose an efficient blind signature scheme based on quadratic residues. It takes only a few computations to obtain and verify a legal signature in their scheme. The security of the scheme depends on the difficulty of solving the square roots of quadratic residues without trapdoors 相似文献
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通过分析基于大整数分解、离散对数和双线性对等数学问题的特殊可传递签名方案,抽象出了可传递签名实现方法的共性。以此为基础,提出了一个基于同态加密体制的通用可传递签名方案,该方案利用同态加密体制能支持密文运算的特性实现了可传递签名及验证的一般模型,为基于同态密码体制构造安全可靠的可传递签名方案提供了一种通用框架。其次,通过适当定义安全目标和设计安全性实验,完成了该通用可传递签名方案的可证明安全性,指出若使用的同态加密方案是CPA安全而标准签名是CMA安全的,则所提出的方案就达到CMA安全。最后,给出了该通用可传递签名方案并进行了性能分析与比较。 相似文献
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特征提取是目标自动识别的关键。本文给出了改进的回波预处理算法、基于图像处理理论的目标特征提取算法以及基于统计和变换特征的目标回波特征提取算法。第一种算法能有效提高识别效率,后两种算法相互补充,在实际工程应用中有较强的适应性。 相似文献
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新颖的基于并发签名的公平交易协议 总被引:4,自引:1,他引:3
基于完美的并发签名,提出了一个新颖的公平交易协议,在双方起初交换2个模糊签名及相关交易数据的时候,任何人都无法辨认是谁签了哪一个签名,直到发起方公布一个秘密信息,2个签名才同时绑定到各自的签署方,之后另一方发送解密商品的密钥,从而让双方都能公平的得到对方的物品,而且本协议无需可信第三方(TTP)的参与即可让交易双方公平地交易数据条目,避免了两方勾结的可能性以及可信第三方参与协议的瓶颈.通过分析,协议很好地保证了公平性、不可否认性以及不可滥用性等性质,而且协议只需两方发送4条消息,简洁高效. 相似文献
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基于Java2的数字签名技术 总被引:1,自引:0,他引:1
数字签名是现代互联网中应用十分广泛的一种信息安全技术,用于保证在不安全的公共网络中,信息传输过程中的完整性以及信息发送的不可抵赖性。结合Java2特有的安全体系结构,详细讨论了数字签名的定义、作用和基本原理,并给出了一个Java2中数字签名的具体实例。 相似文献