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111.
This paper considers the scattering of a plane, time-harmonic wave by an inclusion with heterogeneous nonlinear elastic properties embedded in an otherwise homogeneous linear elastic solid. When the inclusion and the surrounding matrix are both isotropic, the scattered second harmonic fields are obtained in terms of the Green's function of the surrounding medium. It is found that the second harmonic fields depend on two independent acoustic nonlinearity parameters related to the third order elastic constants. Solutions are also obtained when these two acoustic nonlinearity parameters are given as spatially random functions. An inverse procedure is developed to obtain the statistics of these two random functions from the measured forward and backscattered second harmonic fields. 相似文献
112.
Degnan AJ Young VE Tang TY Gill AB Graves MJ Gillard JH Patterson AJ 《Magnetic resonance imaging》2012,30(7):1017-1021
Purpose
Previous studies reporting relaxation times within atherosclerotic plaque have typically used dedicated small-bore high-field systems and small sample sizes. This study reports quantitative T1, T2 and T2? relaxation times within plaque tissue at 1.5 T using spatially co-matched histology to determine tissue constituents.Methods
Ten carotid endarterectomy specimens were removed from patients with advanced atherosclerosis. Imaging was performed on a 1.5-T whole-body scanner using a custom built 10-mm diameter receive-only solenoid coil. A protocol was defined to allow subsequent computation of T1, T2 and T2? relaxation times using multi-flip angle spoiled gradient echo, multi-echo fast spin echo and multi-echo gradient echo sequences, respectively. The specimens were subsequently processed for histology and individually sectioned into 2-mm blocks to allow subsequent co-registration. Each imaging sequence was imported into in-house software and displayed alongside the digitized histology sections. Regions of interest were defined to demarcate fibrous cap, connective tissue and lipid/necrotic core at matched slice-locations. Relaxation times were calculated using Levenberg-Marquardt's least squares curve fitting algorithm. A linear-mixed effect model was applied to account for multiple measurements from the same patient and establish if there was a statistically significant difference between the plaque tissue constituents.Results
T2 and T2? relaxation times were statistically different between all plaque tissues (P=.026 and P=.002 respectively) [T2: lipid/necrotic core was lower 47±13.7 ms than connective tissue (67±22.5 ms) and fibrous cap (60±13.2 ms); T2?: fibrous cap was higher (48±15.5ms) than connective tissue (19±10.6 ms) and lipid/necrotic core (24±8.2 ms)]. T1 relaxation times were not significantly different (P=.287) [T1: Fibrous cap: 933±271.9 ms; connective tissue (1002±272.9 ms) and lipid/necrotic core (1044±304.0 ms)]. We were unable to demarcate hemorrhage and calcium following histology processing.Conclusions
This study demonstrates that there is a significant difference between qT2 and qT2? in plaque tissues types. Derivation of quantitative relaxation times shows promise for determining plaque tissue constituents. 相似文献113.
提出了一种准相位型光子筛。该准相位型光子筛通过改变暗环上小孔直径与带宽的比值,对焦点处的相位分布进行调制,使通过亮环和暗环上小孔的光波在焦点处发生相长干涉,生成聚焦光斑。该准相位型光子筛无需对基底进行特殊处理,即可完成相位型光子筛的制作,降低了加工难度。和普通振幅型光子筛相比,在相同最小加工尺寸的条件下,准相位型光子筛具有较大的数值孔径,可降低聚焦光斑尺寸。并且,和多区光子筛相比,在同样的最小加工尺寸及数值孔径的条件下,准相位型光子筛光斑尺寸及质量均优于对方。为大数值孔径光子筛设计提供了一种新的设计方法。 相似文献
114.
115.
在放射性免疫治疗以及硼中子俘获治疗(BNCT)等放射性治疗过程中,粒子通过与人体组织材料相互作用产生次级电子将能量传递给人体组织,放射性核素在细胞尺度分布的不均匀性将严重影响靶区剂量分布。为深入了解放射性核素在细胞中不同位置分布对靶区剂量影响,采用基于历史凝聚算法的Monte Carlo 工具包Geant4编写了细胞S值计算程序。计算了2种细胞尺寸,12种粒子能量,3种源分布方式的细胞S值,与医学内照射剂量(MIRD)委员会解析算法的计算结果进行对比,发现两者差异在1%以内。证明了Geant4在m尺度下细胞剂量计算的可行性,并对BNCT治疗过程中产生的粒子(1.47 MeV与1.78 MeV)的细胞S值进行计算。 相似文献
116.
117.
118.
We report an experimental observation of dark pulse generation in a dispersion-managed erbium-doped fiber laser with net anomalous
cavity group-velocity dispersion. It is found that apart from the bright soliton pulses, dark pulses with spectral sidebands
could be obtained in the laser under appropriate operating conditions. The generation of dark pulses may be attributed to
soliton shaping in the cavity. 相似文献
119.
S. Y. Cheng Y. G. Wang P. Yu Y. J. Cheng J. Tang H. R. Chen W. F. Hsieh 《Laser Physics》2012,22(1):54-59
Using the vertical evaporation technique we fabricated saturable absorbers by transferring the water-soluble single wall carbon
nanotubes (SWCNT) onto a hydrophilic quartz substrate. The fast recovery times of the absorber were measured to be 136 and
790 fs. The modulation depth of the absorber was about 2%. Passive mode-locked Nd:GdVO4 laser using such an absorber was demonstrated. The continuous wave mode-locked pulses with the pulse duration of 12.4 ps
and the repetition of 120 MHz were achieved. The maximum average output power of the mode-locked laser is 2.4 W at the pump
power of 13 W. Such kind of absorber has potential to be put into practical use for high power solid-state laser mode locking. 相似文献
120.
锥形空间滤波孔是一种应用于高功率固体激光装置空间滤波的新型结构滤波小孔,其内部光滑的锥形结构可以有效滤除光束中的高频成分且避免等离子堵孔现象的发生。根据神光-Ⅲ主机装置结构特点,设计并加工了适用于主机装置的锥形空间滤波小孔,并开展了应用实验研究。通过与传统的平面结构滤波小孔对比可以看出,锥形小孔有效地避免了等离子体度堵孔现象,在与光束波前补偿技术配合使用条件下,保证光束顺利过孔,并成功实现了基频光7988 J的满能量输出。 相似文献