Echo spectroscopy and atom optics billiards

We discuss a recently demonstrated type of microwave spectroscopy of trapped ultra-cold atoms known as “echo spectroscopy” [Phys. Rev. Lett., 2003;90:023001[1]–[4]]. Echo spectroscopy can serve as an extremely sensitive experimental tool for investigating quantum dynamics of trapped atoms even when a large number of states are thermally populated. We show numerical results for the stability of eigenstates of an atom-optics billiard of the Bunimovich type, and discuss its behavior under different types of perturbations. Finally, we propose to use special geometrical constructions to make a dephasing free dipole trap.     

基于AFI的快速发射场B1+成像方法

该文提出了一种基于实际翻转角成像(Actual Flip-angle Imaging,AFI)的快速发射场测量方法(Fast AFI,FAFI),将多次激发平面回波成像(Multi-shot Echo Planar Imaging,Multi-shot EPI)的采集方式运用于AFI发射场(B_1~+)测量中,充分利用AFI序列中采集的等待时间,高倍数加速了水模和人体头部、腹部及盆腔的发射场测量.该文在水模和人体(n=16)实验中,验证了采用FAFI序列得到的B_1~+测量结果与AFI结果的一致性.FAFI序列大幅加速了发射场测量,为实现动态B_1~+匀场(B_1~+shimming)和快速局部激发提供了高效的发射场测量方法.     

野战有线通信系统二线全双工接口测试电路设计

野战有线通信系统全双工接口在单对线上同时收发信号,难以直接进行数据采集和测试。针对上述问题,研究了一种基于收发分离和自适应回波抵消的二线全双工接口测试方法。首先,设计电子收发分离器,实现线路上收发信号的隔离;然后,使用自适应回波抵消器,完成了近端回波干扰信号的抵消;最后,借鉴软件无线电技术和存储测试技术,构建测试系统,实现激励信号生成与数据采集。仿真和实验表明,该测试方法能够实现有效完成收发信号隔离以及回波干扰信号抵消,适用于野战有线通信系统二线全双工接口测试。     

A modified PGSE for measuring diffusion in the presence of static magnetic field gradients

With a proper timing of pi pulses, it is possible to reduce the effect of the static internal magnetic field gradient on the measurement of diffusion with the pulsed gradient spin echo (PGSE). A pulse sequence that in the first order eliminates the effect of weak internal static gradients in a standard PGSE experiment is introduced. The method should be applied in the cases, where strong and short magnetic gradient pulses are used to investigate the motion of liquid in heterogeneous samples with large susceptibility differences such as porous media.     

Studies on optical dephasing of amorphous semiconductor with incoherent light

Dephasing in a-Si_(0.65)C_(0.35): H is observed by time-delayed four-wave mixing incoherent light. Results are in correspondance with consequences of disorder and localization on nonlinear optical response of the solid.     

基于二能级光量子态存储效率的研究

本文研究利用沉默再生效回声(ROSE)技术实现光量子态存储的效率。利用自然非均匀展宽的二能级原子系综,不需应用任何外部的电场或磁场进行调控。在均匀光谱分布下,我们对吸收过程和读取过程进行分析,得出量子的存储效率。发现对于光子向前读取过程,存储效率的最大值能达到54%;而当光深度达到无穷大时,向后的读取效率可达到100%。我们给出了详细的理论推导过程。     

Critical behavior of the XY spin chain with three-site interaction studied in terms of the Loschmidt echo

The effect of the three-site interaction (α) on the critical behaviors of the XY spin chain is studied in terms of the Loschmidt echo (LE). The critical lines can be shifted by α, and the anisotropy parameter of the XY chain has no effect on the critical lines. The scaling behaviors of the LE reveal that the dynamical behaviors of the LE are reliable for characterizing quantum phase transition (QPT).     

Diffusion-weighted MRI in prostate cancer -- comparison between single-shot fast spin echo and echo planar imaging sequences

Diffusion-weighted (DW) MRI at 1.5 T was carried out in two groups of patients. MRI data were correlated with the biopsy and histopathology (where available). The performance of two sequences -- a single-shot FSE (14 patients) and a single-shot EPI (15 patients) -- was compared. Average ADC values from the normal peripheral zone (PZ), central gland (CG) and the tumour [prostate carcinoma (PCa)] were calculated from b values of 0 and 600. Tukey-Kramer test was used for statistical analysis. EPI produced higher values of ADC (10(-3) mm(2)/s) than FSE sequence: 1.992+/-0.208 vs. 1.573+/-0.270 in PZ (P<.001), 1.518+/-0.126 vs. 1.373+/-0.179 in CG and 1.214+/-0.254 vs. 0.993+/-0.158 in PCa (P<.01). In conclusion, both EPI and FSE sequences showed differences in ADC between normal PZ, CG and PCa; however, EPI produced significantly higher ADC values than FSE.     

核磁共振实验的计算机辅助教学

基于核磁共振的经典理论——Bloch方程,运用Matlab程序对核磁共振实验的各个环节(观察核磁共振信号、自由衰减信号、自旋回波信号等)进行了可视化模拟,帮助学生认识和理解核磁共振,辅助实验教学.     

Registering spherical navigators with spherical harmonic expansions to measure three-dimensional rotations in magnetic resonance imaging

Subject motion remains a challenging problem to overcome in clinical and research applications of magnetic resonance imaging (MRI). Subject motion degrades the quality of MR images and the integrity of experimental data. A promising method to correct for subject motion in MRI is the spherical navigator (SNAV) echo. Spherical navigators acquire k-space data on the surface of a sphere in order to measure three-dimensional (3D) rigid-body motion. Analysis begins by registering the magnitude of two SNAVs to determine the 3D rotation between them. Several different methods to register SNAV data exist, each with specific capabilities and limitations. In this study, we assessed the accuracy, precision and computational requirements of measuring rotations about all three coordinate axes by correlating the spherical harmonic expansions of SNAV data. We compare the results of this technique to previous SNAV studies and show that, although computationally expensive, the spherical harmonic technique is a highly accurate, precise and robust method to register SNAVs and detect 3D rotations in MRI. A key advantage to the spherical harmonic technique is the ability to optimize the accuracy, precision, processing time and memory requirements by adjusting parameters used in the registration. While present developments are aimed at improving the programming efficiency and memory handling of the algorithm, this registration technique is currently well suited for retrospective motion correction applications, such as removing motion-related image artifacts and aligning slices within a high-resolution 3D volume.