固体核磁共振中旋转回波激发的研究

本文提出的旋转回波脉冲序列具有激发带宽适中,实验参数调整简单的特点,可广泛应用于固体样品的核磁共振谱测量中.理论计算和实验结果表明它比传统的双90°回波序列的激发效果好. 关键词：     

自旋3/2粉末样品四极核中心跃迁的NMR回波序列

本文提出脉冲序列45_X~°—t_1—90_(±Y)~°—t_2,从理论和实验上证实了它能够有效地会聚粉末样品中I=3/2四极核中心跃迁的磁化矢量,并对该回波序列的激发谱宽作了数值计算。     

逆合成孔径成像激光雷达数据采样技术

逆合成孔径成像激光雷达能够实现对运动目标的高分辨实时成像,但激光信号的极大带宽和目标回波信号的微弱性给雷达回波数据的接收和处理带来了较大困难.针对这一问题,提出了基于光外差探测手段和压缩感知理论相结合的信号采样方法,首先通过光外差探测降低回波信号的有效带宽,再结合压缩感知理论实现对信号的稀疏化采样和重构.仿真结果证明了运用本文所提出的采样方法,在使用远低于奈奎斯特定理所规定的采样率时,仍然能够实现对目标的高质量成像.     

逆合成孔径成像激光雷达数据采样技术

逆合成孔径成像激光雷达能够实现对运动目标的高分辨实时成像,但激光信号的极大带宽和目标回波信号的微弱性给雷达回波数据的接收和处理带来了较大困难.针对这一问题,提出了基于光外差探测手段和压缩感知理论相结合的信号采样方法,首先通过光外差探测降低回波信号的有效带宽,再结合压缩感知理论实现对信号的稀疏化采样和重构.仿真结果证明了运用本文所提出的采样方法,在使用远低于奈奎斯特定理所规定的采样率时,仍然能够实现对目标的高质量成像.     

基于波长330nm激光激发多色激光导星回波光子数的数值计算与探讨

采用无模激光器发射波长为330 nm的激光激发多色激光导星,需要考虑脉冲激光重频率、激光带宽、激光初始光斑直径以及大气透过率对回波光子数的影响.通过数值模拟,计算了高斯光束的脉冲激光和连续激光激发多色激光导星在实际大气中后向辐射330 nm和2207 nm波长的回波光子数.数值计算结果表明,在垂直发射和接收的情况下,当到达大气中间层的激光能量为1 W时,连续激光能够获得更多的回波光子数,并且回波光子数几乎无起伏.对于脉冲激光,提高脉冲激光重频率达到50 kHz以上时,多色激光导星330 nm的回波光子数随脉冲重频率的增加趋于有限值;当大气能见度小于5 km且大气相干长度为12.8 cm时,大约需要34 W以上的激光发射能量,才能获得满足使用自然星全倾斜探测的330 nm回波光子数.对于连续激光,相同情况下,大约需要20 W以上的激光发射能量.     

结合脉冲梯度场的选择激发技术在溶剂峰抑制中的应用

对结合脉冲梯度场的选择激发技术在溶剂峰抑制中的四种可行序列进行比较,实验表明非对称的双梯度自旋回波序列在谱图的相位特性和溶剂峰抑制效果两方面均是最佳的方案,并将其推广应用到溶剂峰抑制的gCOSY实验中,获得较理想的溶剂峰抑制效果     

APPLICATION STUDY ON SOLVENT SIGNAL SUPPRESSION WITH SELECTIVE EXCITATION TECHNIQUE IN COMBINATION WITH PFG

对结合脉冲梯度场的选择激发技术在溶剂峰抑制中的四种可行序列进行比较,实验表明非对称的双梯度自旋回波序列在谱图的相位特性和溶剂峰抑制效果两方面均是最佳的方案,并将其推广应用到溶剂峰抑制的gCOSY实验中,获得较理想的溶剂峰抑制效果.     

固态NMR中自旋Ⅰ=1体系的魔术回波序列

虽然组合脉冲固体回波大大地减轻了有限脉冲宽度的影响,但是它们并没有完全消除有限脉冲宽度带来的失真,而且产生另一种失真,即所谓的位相失真。本文分析了组合脉冲序列,将它用来构成魔术回波序列,这样就能消除位相失真。理论和实验的结果都证明,自旋I=1的组合脉冲魔术回波在线型的测量上优于组合脉冲固体回波序列,基本上解决了²D核核磁共振中有限脉冲宽度带来的难题。     

InP基矩形马赫-曾德高速电光调制器行波电极设计与测试

提出了一种由新型沟槽耦合器和90°弯曲波导构成的InGaAsP/InP基矩形马赫-曾德电光调制器,对该调制器的L型波导相移臂设计了T型类微带行波电极.首先利用电极的等效电路估算带宽上限,进而在考虑阻抗匹配、回波损耗以及带宽等性能的基础上,使用有限元方法对电极的传输、输入/输出以及过渡区的结构参量进行优化.仿真结果表明由于受到电极输入及过渡区的性能限制,设计的整体行波电极匹配阻抗大于42Ω,回波损耗小于-15dB,带宽可达65GHz.测试制备的Ti/Au行波电极,得到回波损耗为-12dB和带宽为20GHz的最优性能.     

小动物高分辨扩散加权成像在临床 MRI 上的实现

在临床用MRI系统上对小动物扩散加权成像一般采用回波平面成像序列,但是回波平面成像易受偏共振效应的影响,得到的图像伪影大、几何变形严重、图像分辨率低,无法探究微小的生物组织结构. 该文报道了在临床用3 T MRI系统上采用自旋回波序列实现了高分辨扩散加权成像. 为减少运动伪影,序列中整合了导航回波矫正技术. 对脑缺血模型大鼠脑部的扫描结果显示,自旋回波扩散加权序列获得的图像基本没有发生形变,并且具有较高的分辨率和较好的信噪比.     

Conventional and collision photon echo in ytterbium vapor

The polarization properties of the photon echo generated by two linearly polarized pulses of resonant radiation at the (6s6p)³ P ₁ ? (6s ²)¹ S ₀ transition of ¹⁷⁴Yb are investigated. A complicated polarization behavior of the photon echo versus an angle between the polarization vectors of the excitation pulses is revealed in a mixture of ytterbium vapor with inert gas. For the angles ranging from 0° to 75°, a conventional echo with its linear polarization coinciding with the second excitation pulse dominates and the echo amplitude decreases with an increasing angle. For the angles ranging from 75° to 89°, the photon echo is elliptically polarized. Finally, for an angle of 90°, the conventional echo disappears and the collision echo becomes linearly polarized along the first excitation pulse.     

Comparative study of motions in dimethylsulfone by noise excitation and solid echo spectroscopy

2H NMR spectra of dimethylsulfone were measured with noise excitation and solid echo NMR spectroscopy in the temperature range from 125 to 355 K. Besides the known fact that broad NMR spectra can be measured with both methods, in comparable times it is shown that for noise excitation, the signal loss is negligible compared to echo spectroscopy in the regime when the correlation times of the motions are of the order of magnitude of the echo pulse spacing. For simulating the dynamic NMR spectra acquired with noise excitation, only the motional process must be taken into account and relaxation can be neglected. Furthermore, the problem of restricted acquisition bandwidth in noise NMR spectroscopy is discussed.     

Nonsecular contribution to the decay of spin-echo signals of quadrupolar nuclei in magnetically ordered materials

We analyze the influence of fluctuations of the nonsecular part of the spin Hamiltonian on the decay of ordinary and multiquantum signals of the two-pulse spin echo in a quadrupole spin system with an inhomogeneously broadened spectral line. Expressions are obtained for the rate of decay of an echo in the case of selective excitation of a signal from quadrupole nuclei with arbitrary spin. These expressions are then used to analyze the experimentally observed ordinary and multiquantum echo signals from quadrupole nuclei with spin I=3/2 (⁵³Cr, ⁶³Cu, and ⁶⁵Cu) in ferromagnetic chromium chalcogenide spinels. Zh. éksp. Teor. Fiz. 116, 204–216 (July 1999)     

Sodium 3-D MRI of the human torso using a volume coil

Sodium MR imaging is considered to provide clinically important information about the human body that is not achievable by hydrogen-based approaches. However, due to the low natural abundance in biological tissues, sodium signals usually lead to low spatial resolution, low SNR, and long acquisition times compared to conventional 1H imaging, even using well-adapted surface coils. For our study, a volume coil was designed with nearly homogeneous excitation/receive characteristics and a suitable geometry fitting the human torso. A sufficient penetration throughout the entire thorax, abdomen, or pelvis is provided allowing for sodium imaging of the kidneys, the liver with gall bladder, or the myocardium. All measurements were performed on a 1.5 T whole body scanner using a spoiled 3-D gradient echo sequence. Imaging parameters TE, TR, and readout bandwidth were optimized for sensitive recording of the sodium component with slow transverse relaxation. Nonselective RF excitation pulses with a duration of 2.5 ms and rectangular shape were applied to avoid SAR problems. Narrow receiver bandwidth and excitation near the Ernst angle provided clinically practicable examinations with measuring times of less than 15 min at a spatial resolution of 8 x 8 x 8 mm3. Under these conditions, SNR of 11 for the kidneys and vertebral disks, 9 for the spinal canal, and 6 for the liver was achieved. A special 3-D spin echo sequence was used to determine T2, times which resulted to 15.3 +/- 1.1 ms for liver, 27.7 +/- 7.2 ms for kidneys, and 24.0 +/- 4.7 ms for the content of the spinal canal.     

High spatial resolution BOLD fMRI using simultaneous multislice excitation with echo-shifting gradient echo at 7 Tesla

We introduce an accelerated gradient echo (GRE) sequence combining simultaneous multislice excitation (SMS) with echo-shifting technique for high spatial resolution blood oxygen level dependent (BOLD) functional MRI (fMRI). The simulation was conducted to optimize scan parameters. To validate the feasibility of the proposed technique, the visual and motor task experiments were performed at 7.0 Tesla (T). The single-shot EPI sequence was also applied in comparison with the proposed technique. The simulation results showed that an optimized flip angle of 9° provided maximal BOLD contrast for our scanning scheme, allowing low power deposition and SMS acceleration factor of 5. Additionally, parallel acquisition imaging with acceleration factor of 2 was utilized, which allowed a total acceleration factor of 10 in volunteer study. The experiment results showed that geometric distortion-free BOLD images with voxel size of 1.0 × 1.0 × 2.5 mm³ were obtained. Significant brain activation was identified in both visual and motor task experiments, which were in accordance with previous investigations. The proposed technique has potential for high spatial resolution fMRI at ultra-high field because of its sufficient BOLD sensitivity as well as improved acquisition speed over conventional GRE-based techniques.     

The quadrupole moment of176Lu

The intrinsic quadrupole moment of¹⁷⁶Lu has been determined by Coulomb excitation with α-particles to be 6.98±0.10 b. Evidence for a new level at approximately 578 keV with presumably collective structure is presented.     

Neutron binding and excitation energies of some Pb isotopes

The intrinsic quadrupole moment of¹⁷⁶Lu has been determined by Coulomb excitation with α-particles to be 6.98±0.10 b. Evidence for a new level at approximately 578 keV with presumably collective structure is presented.     

Efficiency of Spurious Signal Cancellation under the NQR Detection of Some Explosives by the SLIME-Type Pulse Sequences

Two versions of the spin-lock inversion mid-echo pulse sequence used for the prompt nuclear quadrupole resonance detection of explosives have been studied experimentally. These sequences are modifications of the conventional multiple pulse spin-locking sequence and differ from it by additional groups of pulses which invert echo signals. It was found that spurious signals produced by some magnetic objects are suppressed by one of these sequences much stronger than by the other. The attenuation of echo signals due to their inversion by the group of the inverting pulses is measured. The number of pulses is optimized in both the sequences to obtain maximum signal-to-noise ratio, taking into account some loss in the magnitude of inverted echoes.