多重指数势的phantom字宙学

利用相空间方法研究具有Ⅳ个标量场的多重指数势的phantom动力学模型．特别地，作者详细研究了N=2的phantom宇宙学，指出在给定势的条件下允许存在bigrip吸引子．     

有质量phantom场的Casimir效应

近年来,phantom场受到许多宇宙学学者的关注.研究了狄利克雷边界条件下有质量phantom场的Casimir效应,利用Abel-Plana公式,得出了狄利克雷边界条件下有质量phantom场的Casimir能量和Casimir力,表明phantom场的Ca-simir能量和Casimir力是分段均匀的,当质量趋近于零时,其结果回到无质量phantom场的情形.     

无未来奇异的超加速膨胀宇宙

讨论由phantom驱动的超加速膨胀宇宙.对于态方程w=-1-Wt-n, phantom能量密度被导出.当n》1,它光滑增加并渐近地趋于常数,这指宇宙不存在未来奇异性.给出了该情形下phantom宇宙的标度因子.     

双重指数势的O(N)phantom宇宙动力学

利用相空间方法研究了双重指数势下具有O（N）对称性的phantom宇宙学模型，指出在给定势的条件下允许存在bignp吸引子，但并不存在deSiaer吸引子．此外，通过数值分析，研究了这个模型的动力学演化．     

避免小撕裂的一种机制

Phantom宇宙的晚期演化可能会导致包括大小撕裂在内的各种未来奇性.例如,在广义相对论中,小撕裂产生于具有平方势的phantom场的演化.本文作者在1/R引力框架下,提出了一种新的机制以避免小撕裂命运.在带有R2修正的1/R引力中,通过数值计算研究了具有各种势的phantom场,包括平方势、立方势和四次方势,在这些势下,奇性都可避免.     

具有双重指数势的phantom宇宙学

利用相空间分析方法,系统研究了具有双重指数势的phantom动力学．主要目的在于研究由M理论引起的势,但同时还分析了指数势中系数比为正的所有可能情况．根据宇宙的晚期行为,给出了解的完整分类．与quintessence情形相反,该体系存在异宿轨线解.     

Development of fully 3D image reconstruction techniques for pinhole SPECT imaging

The purpose of this study was to develop fully 3D image reconstruction techniques for pinhole SPECT imaging with our Micro-SPECT system.Our studies involve in the derivation of projection operators,analysis of the sampling characteristics of pinhole SPECT imaging in Radon space,development of effective geometric calibration method for system misalignment,and 3D image reconstruction development and implementation with quantitative degrading compensation for pinhole SPECT with both circular and helical scan.The performances of pinhole SPECT imaging were evaluated using computer simulations and experiments with the Ultra-Micro Hot-Spot phantom,Ultra-Micro Defrise phantom and small-animal imaging.The results from the computer simulations and phantom imaging experiments indicate that the statistically-based iterative algorithms with quantitative compensation provide overall image quality improvement,and the system resolution is significantly recovered for quantitative imaging.The helical pinhole SPECT improves the axial field-of-view (FOV) as compared with the standard pinhole SPECT with circular-orbit scan.The mouse bone imaging experiment shows that the helical pinhole SPECT imaging also provides decent high-resolution whole-body small animal imaging.In conclusion,we have successfully developed a set of valid fully 3D image reconstruction techniques for single-pinhole SPECT imaging.These techniques can be easily extended to multi-pinhole SPECT imaging.     

Experiment on building a real-time temperature field distribution model of the prostate using special data encryption multi-pole radiofrequency ablation and a visualization phantom

The aim of this study was to explore the utility of a visualization phantom for building a real-time temperature field distribution model of the prostate created by the application of special data encryption(SDE) multi-pole radiofrequency(RF) ablation.We prepared phantoms(phantom group) using acrylamide as the raw material and egg-white as a color developing reagent.The prostate specimens of male adult dogs were fixed in this phantom to create a prostate phantom(prostate group).The SDE multi-pole RF electrode was inserted into the two groups for RF ablation,and the temperature changes at relevant points in space were detected.We set the x-axis as the left and right direction,the y-axis as the fore and aft direction,and the z-axis as the insertion direction of the RF electrode.After this process,the effect of RF ablation on the appearance of each of the two groups was observed.Using temperature measurements taken during the operation,a three-dimensional(3D) surface model of temperature was constructed by drawing the boundary areas of different temperatures on the maximum x-z plane of the two groups of ablation lesions,with y equivalent to 0.Further comparison of the temperature-time change curves of four space points were made by setting y equivalent to 0,z equivalent to 0.5,and x equivalent to 0,0.25,0.5 and 0.75.White solid lesions with a diameter of approximately 12 mm could be found in both groups after RF ablation.During the ablation of the phantom group,the temperature ascended faster in a zone 1 cm from the needlepoint to the needle body,and 0.5 cm about the radiofrequency needle.The time-property result of the x-z plane with y equivalent to 0 showed that the boundary areas of the two groups were closer at 50 and 60°C,but the difference between the two groups at 70°C in the high temperature zone was relatively large.The 3D temperature surface model further proved that the 3D temperature outline at 60°C was in better concordance with the actual effect of RF ablation in the prostate tissue.In the comparison of temperature-time changing curves of the two groups at the four space points,despite there being a significant difference in the changing process(P=0.0001),there was no significant statistical difference in the final temperature of the two groups(P>0.05).The results of this work indicated that ablation appearance was in good agreement with in-vitro biologic tissues and that the RF temperature field distribution can be obtained by using a visualization phantom.In particular,the shape of the isothermal line at 60°C was close to the actual ablation lesion.These results may serve as a valuable reference to future clinical trials.     

Radiation theory comparison for magnetoacoustic tomography with magnetic induction （MAT-MI）

Based on the principle of Lorentz force induced acoustic vibration, radiation theory comparison between acoustic point and dipole sources was conducted for magnetoacoustic tomography with magnetic induction （MAT-MI）. It is proved that each acoustic source of MAT- MI is produced by the divergence of the magnetically induced Lorentz force, and the detected acoustic pressure is the integral of all diffraction sources inside the object. Wave clusters are produced by abrupt pressure changes at conductivity boundaries, and only the configurations in terms of shape and size of phantom models can be recon- structed. However, different from point source, positive and negative pressures are generated by the radiation pat- tern of dipole sources. Reverse vibration phases of wave clusters in collected waveforms and opposite polarities of borderline stripes in reconstructed images are produced at conductivity boundaries, representing the direction of conductivity changes. The experimentally collected wave- forms and reconstructed images of the aluminum foil cylinder and cylindrical saline gel phantom model agree well with simulated results. The favorable results prove the validity of the radiation theory of acoustic dipole source and provide basis for further investigation of conductivity reconstruction for MAT-MI.     

平面2N体问题的正N边形中心构型

在对N体问题中心构型的研究中,正多边形解是研究对象之一.其特点是,N个等质量的质点,位于一个正N边形的顶点上.有没有2 N个质点,构成一个正N边形解呢?即N个等质量的质点,位于一个正N边形的顶点上,另外N个等质量的质点,位于该正N边形的边的中点.本文研究了这种解的存在条件,并给出了具体的解.