生物组织成像中用反相位脉冲技术提高二次谐波信噪比的研究

本文基于有限振幅声波在介质中的非线性传播理论,分析了反相位脉冲技术对生物组织中二次谐波增强的原理.实验中利用反相位脉冲激发超声换能器,对生物组织中传播的非线性信号相加分析.结果表明反相位脉冲技术可有效抑制基波及奇次谐波信号,而可增强偶次谐波信号6dB.与滤波器滤波法相比,反相位脉冲技术在抑制基波信号的同时,可有效地提高二次谐波的信噪比,因而在生物组织的二次谐波成像中具有广阔的应用前景.     

超谐波声场及其生物组织成像的理论及实验研究

理论及实验研究了平面活塞换能器激发的超谐波(三次、四次及五次谐波的线性幅度叠加)的轴向和径向声场特性,讨论了声源强度及传播距离对超谐波的强度及指向性的影响。结果表明在合适的声源强度及传播距离条件下,超谐波的幅度可大于二次谐波,同时-3dB波束宽度变窄,为基波和二次谐波的59%和77%。对生物组织进行了超谐波成像,并与基波、二次谐波图像对比。结果表明超谐波可提高超声非线性成像的灵敏度及分辨率。     

超短脉冲谐波截止频率与载波相位关系的研究

采用Lewenstein模型计算了两个不同脉宽的超短脉冲高次谐波截止频率与包络载波相位的关系, 结果显示,其截止频率随载波包络相位的增加先减小后增加。随着脉冲宽度的减小,高次谐波截止频率变化的幅度增加。我们按准经典的三步模型解释所得结果,并讨论了量子辐射效应。     

基于涡流脉冲相位热成像的金属材料裂纹检测

涡流热成像可以对金属材料表面、近表面缺陷进行快速准确的检测。采用涡流热成像与脉冲相位分析相结合的方法,针对裂纹附近相位谱的分布规律及温度场对相位谱的影响关系问题,分别对带有上表面裂纹和下表面裂纹的45#钢料进行感应加热仿真和实验分析。结果表明,相位谱分布与温度大小无关,只与温度变化趋势有关,相位谱图对不均匀加热有很好的抑制作用,可以扩大裂纹的检测范围,当频率低于12.5 Hz时能够清晰地识别裂纹。结合实验验证了仿真结论的正确性,为下一步利用相位信息实现缺陷的定量检测提供参考。     

载波相位对超短脉冲谐波谱的影响

数值计算了线偏振的超短激光脉冲(脉冲持续时间为两个光学周期)与一维模型原子相互作用产生的高次谐波发射功率谱. 研究表明,当载波相位发生变化时,超短脉冲谐波谱的截止频率也随之改变,而且在特定相位下,谐波谱出现了明显的双平台结构. 对此,采用半经典的“三步”模型给出了合理解释,并利用小波时频分析方法证实“三步”模型可以准确预言超短脉冲谐波谱的截止频率. 关键词： 高次谐波 超短脉冲 载波相位     

基于焦平面阵列的连续太赫兹波生物组织成像技术研究

太赫兹焦平面成像是一种实用成像技术,具有快速成像、结构简单的特点,适用于生物组织的太赫兹波段成像.为此,搭建了一套反射式连续太赫兹波焦平面成像系统,对新鲜猪肉组织、大鼠脑组织和人脑胶质瘤组织进行成像.实验发现,通过添加成像窗口可以减少目标不平整表面的散射;同时,采用平场校正可以消除非均匀光照带来的影响.结果表明,反射式...     

二次谐波中大相位失谐下入射啁啾脉冲对相位的影响

讨论了在大相位失谐下,基波为频率啁啾脉冲的二次谐波产生过程,结果表明选取合适的啁啾参量可以使二次谐波的光强提高,但同时会使其波形和相位发生变化.三波相位的畸变主要取决于入射的基波啁啾脉冲,而与相位匹配的失谐量关系不大.     

基于δ声波场的生物组织光学断层成像研究

利用δ声波场和近红外光漫射理论实现生物组织的断层成像是一种新颖医学 成像方法. 通过构建一个δ声波场并作用到生物组织中，仅改变组织内作用点上的光学特性参数，并影响近红外光在组织中的传播特性而改变组织表面的光强分布. 利用组织表面光强改变量与作用点上组织光学参数改变量之间的内在关系，可以通过对δ声波场作用点上断层的扫描实现组织内部的断层成像. 该方法具有广泛的临床医学应用前景，为乳腺癌的早期检测提供一种有效方法. 关键词： 光学断层成像 δ声波场 漫射方程 生物组织     

基于波面分割及多平面相位恢复的定量相位成像技术

提出了一种基于微透镜阵列分割波前及多平面相位恢复的定量相位成像技术。针对大动态范围的相位物体实现定量相位成像,该技术同时施加了横向波面分割、轴向多衍射平面和多波长照明三种约束。该技术记录了两种不同波长照明下,微透镜阵列焦面附近不同衍射距离的强度分布图,采用多平面相位恢复算法提取透过相位物体的数字复振幅光场,通过双波长数字复振幅光场相位提取算法,实现了大动态范围下的相位物体成像。数字仿真实验中,在640 nm和685 nm的照明下,对相位变化范围大、结构复杂的相位物体进行了模拟仿真,结果表明,该技术可以高效、便捷地实现高精度相位成像。     

基于时域剪切干涉的纳秒脉冲相位测量技术

提出一种时域剪切干涉技术测量纳秒激光脉冲的时间相位分布,该方法将待测脉冲分为彼此之间有数百个皮秒延迟量的两个脉冲;并在对其中一个脉冲加入适量的频移后和另一个脉冲合束,得到时域干涉条纹;最后采用相适应的算法,从记录的时域条纹计算得到纳秒激光脉冲的时间相位分布,并进一步计算得到激光脉冲的精细光谱结构.在对测量原理进行系统分析的基础上,利用数值模拟和实验对该相位测量技术的可行性进行了验证,并系统分析了其测量误差和非理想条件下的各种干扰因素的影响.由于该测量技术不采用任何非线光学方法,可对任何波长的激光脉冲进行测量,具有光路简单、测量精度高和适用范围广等优点,为需要对纳秒激光脉冲的时域相位分布进行测量的高功率激光等领域提供了一种简单便捷的测量新技术.     

Investigation of linear and nonlinear frequency modulated excitation effects on tissue harmonic imaging

The generation of tissue harmonics is due to nonlinear nature of ultrasound wave propagation in biological tissues.The tissue harmonics for imaging i.e.tissue harmonic imaging (THI)uses higher frequency components for imaging in which the resolution improves significantly but signal-to-noise ratio(SNR)and penetration depth remains low as compared to both fundamental and second harmonic imaging.The coded excitations have potential to improve the SNR which ultimately leads to improved penetration depth.In the present work,the linear frequency modulated(chirp/LFM)and nonlinear frequency modulated(NLFM)signals have been used to investigate the nonlinear ultrasound wave propagation and harmonic generation in biological tissues.The SNR has been found to be substantially improved for coded tissue harmonic imaging(CTHI)as well as for coded superharmonic imaging(CSHI).     

X-ray spatial harmonic imaging of phase objects

Refractive index gradients in materials or at material interfaces lead to x-ray diffraction. Interference of this radiation with adjacent x-ray waves causes phase contrast that can be used for imaging purposes if an x-ray source with sufficient spatial coherence is used. The imaging modality presented here uses hard x radiation diffracted at interfaces, but requires only little spatial coherence. We report experiments showing, first, that image contrast is not diminished by motional blurring, and second, that contrast can be increased by orders of magnitude relative to in-line x-ray phase-contrast imaging. These properties substantially broaden the applicability of phase-sensitive imaging to moving samples and very weak density gradients.     

Coded excitation for ultrasound tissue harmonic imaging

Coded excitation can improve the signal-to-noise ratio (SNR) in ultrasound tissue harmonic imaging (THI). However, it could suffer from the increased sidelobe artifact caused by incomplete pulse compression due to the spectral overlap between the fundamental and harmonic components of ultrasound signal after nonlinear propagation in tissues. In this paper, three coded tissue harmonic imaging (CTHI) techniques based on bandpass filtering, power modulation and pulse inversion (i.e., CTHI-BF, CTHI-PM, and CTHI-PI) were evaluated by measuring the peak range sidelobe level (PRSL) with varying frequency bandwidths. From simulation and in vitro studies, the CTHI-PI outperforms the CTHI-BF and CTHI-PM methods in terms of the PRSL, e.g., −43.5 dB vs. −24.8 dB and −23.0 dB, respectively.     

Nonlinear imaging of isoechogenic phantoms using phase conjugation of the second acoustic harmonic

Experiments on nonlinear C-scan acoustic imaging were performed using phase conjugation of the second harmonic of a focused incident beam. This method made it possible to obtain images of objects with close linear properties but with different nonlinear parameters. Nonlinear backward propagation of the conjugate wave was also considered and improvement of the image contrast was demonstrated for this case. The results obtained were generally in agreement with the theoretical predictions.     

Coded tissue harmonic imaging with nonlinear chirp signals

Coded tissue harmonic imaging with pulse inversion (CTHI-PI) based on a linear chirp signal can improve the signal-to-noise ratio with minimizing the peak range sidelobe level (PRSL), which is the main advantage over CTHI with bandpass filtering (CTHI-BF). However, the CTHI-PI technique could suffer from motion artifacts due to decreasing frame rate caused by two firings of opposite phase signals for each scanline. In this paper, a new CTHI method based on a nonlinear chirp signal (CTHI-NC) is presented, which can improve the separation of fundamental and harmonic components without sacrificing frame rate. The nonlinear chirp signal is designed to minimize the PRSL value by optimizing its frequency sweep rate and time duration. The performance of the CTHI-NC method was evaluated by measuring the PRSL and mainlobe width after compression. From the in vitro experiments, the CTHI-NC provided the PRSL of −40.6 dB and the mainlobe width of 2.1 μs for the transmit quadratic nonlinear chirp signal with the center frequency of 2.1 MHz, the fractional bandwidth at −6 dB of 0.6 and the time duration of 15 μs. These results indicate that the proposed method could be used for improving frame rates in CTHI while providing comparable image quality to CTHI-PI.     

The study of third harmonic demodulation in phase modulation optical heterodyne technique using optical resonator

With the technique of phase modulation optical heterodyne(PMOH), we detected the beat-frequency signal of the reflected wave from a F-P cavity. The frequency of the beat signal was three times of that of the modulation electric field, when the 3rd harmonic of the modulation electric field was used as the phase reference signal. The line shapes of the signal match those of the theoretical calculations, and the dispersion signal has good frequency-discrimination characteristic.     

Intravascular ultrasound tissue harmonic imaging: a simulation study

Recently, the in vivo feasibility of tissue harmonic imaging (THI) with a mechanically-rotated intravascular ultrasound (IVUS) catheter was experimentally demonstrated. To isolate the second harmonic signal content, both pulse inversion (PI) and analog filtering were used. In the present paper, we report the development of a simulation tool to investigate nonlinear IVUS beams and the influence of rotation on the efficiency of PI signal processing. Nonlinear 20 MHz beams were simulated in a homogeneous tissue-mimicking medium, resulting in second harmonic pressure fields at 40 MHz. The acoustic response from tissue was simulated by summing radio-frequency (RF) pulse-echo responses from many point-scatterers. When the transducer was rotated with respect to the point-scatterers, the fundamental frequency suppression using PI degraded rapidly with increasing inter-pulse angles. The results of this study will aid in the optimization of harmonic IVUS imaging systems.     

Statistical model of clutter suppression in tissue harmonic imaging

A statistical model is developed for the suppression of clutter in tissue harmonic imaging (THI). Tissue heterogeneity is modeled as a random phase screen that is characterized by its correlation length and variance. With the autocorrelation function taken to be Gaussian and for small variance, statistical solutions are derived for the mean intensities at the fundamental and second-harmonic frequencies in the field of a focused sound beam that propagates through the phase screen. The statistical solutions are verified by comparison with ensemble averaging of direct numerical simulations. The model demonstrates that THI reduces the aberration clutter appearing in the focal region regardless of the depth of the aberrating layer, with suppression of the clutter most effective when the layer is close to the source. The model is also applied to the reverberation clutter that is transmitted forward along the axis of the beam. As with aberration clutter, suppression of such reverberation clutter by THI is most pronounced when the tissue heterogeneity is located close to the source.     

Investigation on high efficiency temporal phase modulated broadband second harmonic laser

In this paper, it is the first time to experimentally demonstrate the second harmonic generation (SHG) of temporal phase modulated broadband laser. The SHG conversion efficiency and the characteristic of SHG spectrum are investigated in detail and the relation between SHG conversion efficiency and incident fundamental light intensity is obtained. The highest efficiency of nearly 70% is achieved at the fundamental pulse intensity of 1.6 GW/cm². The experimental results show that the SHG spectrum widths under high and low intensity are both half of the incident fundamental lights, and don’t result in the spectrum narrowing. The obtained results are very useful for operation at the second harmonic wavelength in inertial confinement fusion (ICF) laser facility.