幅值和相位配准技术及其在光学相干层析血流成像中的应用

频域光学相干层析系统中扫描机构定位精度、 机械抖动及样品移动会造成A扫描信号幅值和相位发生波动, 影响生物组织成像质量. 利用最小灰度差匹配、Lorentzian曲线极值拟合和谱域光程差补偿等方法对A 扫描信号进行幅值配准. 通过对A扫描信号相位分布特征的匹配实现相位差检测与配准. 通过求已配准的A 扫描复信号之差, 消除静态组织对血流成像的影响. 进行了人眼扫描实验, 有效提取了视网膜三维血流图像. 实验结果表明, 提出的幅值及相位配准方法大大减小了系统扫描精度、人眼跳动等因素对生物组织在体成像质量的影响. 快速、精确的相位配准方法也可广泛应用在多普勒OCT、相位显微等与相位分辨有关的光学成像领域. 关键词： 频域光学相干层析 配准 血流成像 相位分布特征     

Real-time in vivo blood-flow imaging by moving-scatterer-sensitive spectral-domain optical Doppler tomography

We present a moving-scatterer-sensitive optical Doppler tomography (MSS-ODT) technique for in vivo blood flow imaging in real time by using a spectral-domain optical coherence tomography system. In MSS-ODT the influence of stationary scatterers is suppressed by subtracting adjacent complex axial scans before calculating the Doppler frequency shift. We demonstrate that MSS-ODT is a useful technique for accurate determination of blood vessel size by imaging flow in a small capillary tube with a 75 microm inner diameter. The flow profile obtained with MSS-ODT yields a substantially more accurate tube diameter than that obtained with the conventional phase-resolved method, which underestimates the diameter by about 23%. We also demonstrate that MSS-ODT provides improved sensitivity over the conventional phase-resolved method for imaging in vivo blood flow in small vessels in a mouse ear.     

一维均匀Morse晶格体系的热流棘齿效应

本文系统研究了系统两端无平均温差时一维均匀Morse晶格中的热流棘齿效应. Morse晶格的两端分别与两个热浴相接触, 其中一端热浴温度周期调制,另一端热浴温度保持不变, 两端热浴温度长时平均相等. 数值结果表明, 当对一端热浴温度进行周期调制时, 系统中便会有稳定的定向热流产生. 通过改变调制频率和强度, 可以控制热流的大小及方向. 在合适的频率范围内, 可观察到一种非常有趣的现象——非定态负热导现象, 即系统中产生的定向热流逆着系统温度梯度方向由低温端流向高温端. 通过热波动力学分析(分析热流及温度分     

基于波长调制技术的激光器调制特性研究

在流场诊断技术中,可调谐半导体吸收光谱技术（TDLAS）成为主要的诊断技术之一,其可实现非接触、原位检测。波长调制（WMS）和直接吸收(DA)是两种最常用的TDLAS气体传感方法,在目标含量很低或者极端流场环境下,波长调制技术呈现出更多的优势,检测灵敏度与直接吸收相比可以提高1～2个数量级。在近红外波长调制技术应用领域,分布反馈式（DFB）半导体激光器成为流场诊断技术的光源选择之一,无论利用谐波信号（或者归一化谐波信号）的线型拟合,还是选择谐波信号的峰值来反演流场参数,吸收模型的准确建立均十分重要。在模型建立时,激光器频率-时间响应以及光强-时间响应的准确表示尤为重要。为解决吸收模型准确建立问题,提出了一种准确测量激光器调制参数的完整方法,通过实验测量了用于探测水汽吸收的1 392和1 469 nm激光器的调制特性,研究了分布反馈式激光器的调制参数随调制幅度,调制频率以及工作温度的变化。根据该方法得到的调制参数,建立吸收模型,测得常温下空气中水汽浓度为1.97%,直接吸收方法测得浓度为1.99%,验证了该测量方法的准确性。研究表明,调制深度随调制幅度的增加线性增加,随调制频率的增加非线性单调减小,随工作温度的升高线性增加;激光器的出光强度和频率同时被调制,强度变化超前频率变化的相位,随调制幅度的变化不明显,随调制频率的增加单调增加,随工作温度的升高单调减小;归一化一次谐波振幅和二次振幅均随调制幅度的增加而增加,随调制频率的增加而减小,随工作温度的变化不明显。在吸收光谱应用领域,波长调制技术发挥的作用愈加重要,调制系数与谐波信号的峰值息息相关,在波长调制技术应用时,选取适当的调制参数,有利于得到合适的谐波信号,可通过改变调制幅度、调制频率、工作温度得到最优调制系数。研究了近红外分布反馈式半导体激光器的调制特性,该方法同样适用于不同封装和不同波段激光器调制特性的研究,利于推广吸收光谱技术在各领域的应用。     

Single-detector polarization-sensitive optical frequency domain imaging using high-speed intra A-line polarization modulation

We demonstrate a novel high-speed polarization-sensitive optical frequency domain imaging system employing high-speed polarization modulation. Rapid and continuous polarization modulation of light prior to illumination of the sample is accomplished by shifting the frequency of one polarization eigenstate by an amount equal to one quarter of the digitization sampling frequency. This approach enables polarization-sensitive imaging with a single detection channel and overcomes artifacts that may arise from temporal variations of the birefringence in fiber-optic imaging probes and spatial variation of birefringence in the sample.     

Recognizing cat-eye targets with dual criterions of shape and modulation frequency

We present an image recognition method to distinguish targets with cat-eye effect from the dynamic background based on target shape and modulation frequency.Original image sequences to be processed are acquired through an imaging mechanism that utilizes a pulsed laser as active illuminator and an industrial camera as detection device.There are two criterions to recognize a target: one exploits shape priors and the other is the active illuminator’s modulation frequency.The feasibility of the proposed method and its superiority over the single criterion method have been demonstrated by practical experiments.     

Cerebral blood flow imaged with ultrahigh-resolution optical coherence angiography and Doppler tomography

Speckle contrast based optical coherence angiography (OCA) and optical coherence Doppler tomography (ODT) have been applied to image cerebral blood flow previously. However, the contrast mechanisms of these two methods are not fully studied. Here, we present both flow phantom and in vivo animal experiments using ultrahigh-resolution OCA (μOCA) and ODT (μODT) to investigate the flow sensitivity differences between these two methods. Our results show that the high sensitivity of μOCA for visualizing minute vasculature (e.g., slow capillary beds) is due to the enhancement by random Brownian motion of scatterers (e.g., red and white blood cells) within the vessels; whereas, μODT permits detection of directional flow below the Brownian motion regime (e.g., laser-induced microischemia) and is, therefore, more suitable for brain functional imaging.     

Fundamental study on subharmonic imaging by irradiation of amplitude-modulated ultrasound waves

The second harmonic and subharmonic components, the frequencies of which are twice and one half the fundamental frequency, are included in echoes from contrast agents. An imaging method, which employs a second harmonic (second harmonic imaging), is widely used in medical diagnoses. On the other hand, subharmonic is expected to provide a higher contrast between biological tissues and blood flow because echo signals are generated only from blood containing the contrast agents. However, the subharmonic component echo signal power from contrast agents is relatively low. This has resulted in little progress in the field of subharmonic imaging. In this study, a new imaging method is proposed using amplitude-modulated waves as transmitted waves combined with the pulse inversion method to enhance subharmonic echo signals. Two optimal frequencies are set, including the modulated waves, F(1) and F(2), so that the subharmonic frequency of F(1) and the second harmonic frequency of F(2) may result in the same value. This allows a more powerful signal at the frequency band because the second harmonic and subharmonic components are integrated. Furthermore, a B-mode ultrasound image of an agar phantom that imitated biological tissue and showed the effectiveness of our method was reconstructed. As a result, the echo power of the subharmonic component was enhanced by approximately 11.8 dB more than the conventional method and the signal to noise ratio showed an improvement of 7.6 dB.     

模板法测量图像传感器调制传递函数方法研究

在干涉条纹投射法测量的基础上，提出了利用正弦模板法测量图像传感器调制传递函数的新方法.通过将一定空间频率的正弦模板直接投射在图像传感器上，获得了同频率光强呈正弦分布的图像，通过计算求出图像传感器的调制传递函数.讨论了其具体实现方法，并用空间频率为50l/mm的计量光栅做了近似实验，采用了相应的数据处理方法，获得了较为满意的调制传递系数.     

Full range complex ultrahigh sensitive optical microangiography

This Letter presents a useful method that combines the full range complex Fourier domain optical coherence tomography (OCT) with the ultrahigh sensitive optical microangiography (OMAG) to achieve full range complex imaging of blood flow within microcirculatory tissue beds in vivo. We propose to use the fast scanning axis to realize the full range complex imaging, while using the slow axis to achieve OMAG imaging of blood flow. We demonstrate the proposed method by using a high speed 1310?nm OCT/OMAG system running at 92?kHz line scan rate to image the flow phantoms in vitro, and the blood flows in tissue beds in vivo.     

接触缺陷的振动调制超声导波检测技术研究

针对常规线性超声检测方法无法实现板结构接触类缺陷(如微裂纹、脱粘等)检测问题,将超声导波技术与振动声调制技术相结合,利用稀疏分布传感器发展了一种板结构中接触缺陷非线性超声检测方法。通过低频振动改变缺陷的接触状况,使得通过接触面的高频导波信号的相位和幅值受到调制。对受低频振动调制的超声导波二维时间序列进行时频分析,由于接触类缺陷的存在,在振动调制超声导波序列的时频分布上出现明显的低频振动频率分量。利用提取出的低频振动频率下的超声导波信号,进行了结构接触缺陷成像处理。检测试验表明,基于振动声调制的超声导波缺陷成像方法可以实现结构中的接触类缺陷检测。      

Spectral- and frequency-encoded fluorescence imaging

A method for obtaining fluorescence images with a high number of resolvable points by using spectral and frequency encoding is presented. Broadband excitation light is encoded with a wavelength-dependent frequency modulation and dispersed onto the sample with a grating to simultaneously illuminate an entire image line. The Fourier transform of the frequency-encoded fluorescence emission provides one line of the image. Mechanical scanning along a direction orthogonal to the wavelength-encoded axis allows creation of the two-dimensional fluorescent image. This method is applicable for developing submillimeter diameter endoscopes. The principles of the technique are validated by imaging indocyanine green fluorescence in microfluidic channels.     

A directional quantifying Doppler system for measurement of transport velocity of blood.

A transcutaneous Doppler device has been developed that measures primarily the directional transport velocity of blood, averaged over the vessel diameter, irrespective of its flow in adjacent vessels. Directional information is obtained by high or low-pass filtering of frequency converted versions of the received Doppler signals, applying low-cost, sharp filters in a superheterodyne system. Upper and lower channel signals are quantified separately to average directional velocity. Linear results from in vitro measurements are obtained.     

电离层调制加热产生极低频/甚低频波定向辐射的理论分析

通过低电离层调制加热能够产生极低频/甚低频(ELF/VLF)波. 基于调制加热理论, 并引入相控阵天线思想, 建立了通过双波束幅度调制模式(DAM)和圆形几何调制模式(CGM)产生ELF/VLF波的定向辐射模型, 并通过与实验数据的对比验证了模型的正确性. 据此模型, 采用HARRP加热阵参数, 对比分析了上述两种定向辐射模式与常规幅度调制模式(AM)之间的特性差异, 并研究了调制频率(f_ELF/VLF)和加热波束与垂直方向倾角(ψ)对各模式的影响. 结果表明: 相对AM模式, 通过合理设置初始相位、f_ELF/VLF和ψ, DAM 模式和CGM模式在实现ELF/VLF信号定向辐射的同时还可以提高其辐射强度, 相对AM模式, CGM模式信号强度最大提高约11.3 dB. 关键词： 定向辐射 双波束幅度调制 圆形几何调制     

Temperature field formed inside a blood vessel under the action of pulsed laser radiation

A model that simulates the dynamics of the temperature field formed by pulsed laser radiation inside a biological structure containing blood vessels is developed. The threshold conditions of denaturation of vessel walls and subsequent blocking the blood flow are determined based on the thermochemical concept. The possibility of application of a pulsed modulation of the radiation for increasing the homogeneity of coagulation of vessel walls and reducing the risk of damage of tissues caused by the phase transition is considered. The modulation frequency range of radiation that ensures the realization of this effect is determined.     

A sensitivity function-based conjugate gradient method for optical tomography with the frequency-domain equation of radiative transfer

The Sensitivity Function-based Conjugate Gradient Method (SFCGM) is described. This method is used to solve the inverse problems of function estimation, such as the local maps of absorption and scattering coefficients, as applied to optical tomography for biomedical imaging. A highly scattering, absorbing, non-reflecting, non-emitting medium is considered here and simultaneous reconstructions of absorption and scattering coefficients inside the test medium are achieved with the proposed optimization technique, by using the exit intensity measured at boundary surfaces. The forward problem is solved with a discrete-ordinates finite-difference method on the framework of the frequency-domain full equation of radiative transfer. The modulation frequency is set to 600 MHz and the frequency data, obtained with the source modulation, is used as the input data. The inversion results demonstrate that the SFCGM can retrieve simultaneously the spatial distributions of optical properties inside the medium within a reasonable accuracy, by significantly reducing a cross-talk between inter-parameters. It is also observed that the closer-to-detector objects are better retrieved.     

Hemoglobin contrast in magnetomotive optical Doppler tomography

We introduce a novel contrast mechanism for imaging blood flow by use of magnetomotive optical Doppler tomography (MM-ODT), which combines an externally applied temporally oscillating high-strength magnetic field with ODT to detect erythrocytes moving according to the field gradient. Hemoglobin contrast was demonstrated in a capillary tube filled with moving blood by imaging the Doppler frequency shift, which was observed independently of blood flow rate and direction. Results suggest that MM-ODT may be a promising technique with which to image blood flow.     

Diffusion anisotropy indexes are sensitive to selecting the EPI readout-encoding bandwidth at high-field MRI

Diffusion tensor magnetic resonance imaging (DT-MRI) is generally performed using an echo planar imaging (EPI) acquisition to map directional water diffusion. However, the oscillating magnetic field gradients of the EPI acquisition can result in considerable mechanical vibrations, which lead, in turn, to magnetic field fluctuations causing Nyquist ghosting in the EPI data. The objective of this study was to investigate effects of EPI readout gradient modulation frequency, which is directly associated with the EPI readout bandwidth (BW), on the accuracy of DT-MRI measurements in a high magnetic field system. A spherical water phantom was used to study the relationship between the EPI BW and the Nyquist ghost for a spin-echo EPI acquisition with a matrix size of 128x128, complemented by diffusion sensitization gradients of up to b=800 s/mm(2) along six directions for DT-MRI. Nine volunteers (four males and five females) were studied using EPI at different BW acquisitions. Analysis of variance was used to investigate the EPI BW effects. The phantom studies demonstrated a systematic relationship between BWs and the intensities of Nyquist ghosts. In the human brain studies, EPI BW variations substantially corrupted diffusion anisotropy indexes (i.e., fractional anisotropy and relative anisotropy) (F=10.5, P=.0001) but were unrelated to diffusion-encoding directions (F=0.14, P=.98). It was possible to minimize BW dependence (F=1.48, P=.25) by tuning the modulation frequency of the EPI readout gradient. In conclusion, diffusion anisotropic indexes are sensitive to the readout BW of EPI due to associated Nyquist ghosting. However, the effect can be minimized by tuning the modulation frequency of the EPI readout gradient, that is, the EPI BW, to a range outside the harmonics of mechanical gradient vibrations.     

Effect of deviation from plane wave conditions on the Doppler spectrum from an ultrasonic blood flow detector.

Deviation from plane wave conditions within the ultrasound beam of a Doppler blood flow detector leads to a non-linear relationship between the phase angle of the back-scattered signal and the scatterer position. This in turn leads to frequency modulation of the Doppler signal and an increase in the Doppler spectrum width. The relationship between the ultrasound beam and the observed signal spectrum has been investigated by employing a computer-based model of the ultrasound field which enabled the calculation of: 1, pressure (amplitude and phase angle) field distributions from plane disc and focused transducers with unapodized and apodized aperture field distributions; 2, the Doppler signal from a scatterer moving through the field; and 3, the spectrum of this signal. The increase in spectral width resulting from deviations from plane wave conditions was calculated by comparing this spectrum with that of the signal from which frequency modulation had been removed.     

Hierarchical clustering to measure connectivity in fMRI resting-state data

Low frequency oscillations, which are temporally correlated in functionally related brain regions, characterize the mammalian brain, even when no explicit cognitive tasks are performed. Functional connectivity MR imaging is used to map regions of the resting brain showing synchronous, regional and slow fluctuations in cerebral blood flow and oxygenation. In this study, we use a hierarchical clustering method to detect similarities of low-frequency fluctuations. We describe one measure of correlations in the low frequency range for classification of resting-state fMRI data. Furthermore, we investigate the contribution of motion and hardware instabilities to resting-state correlations and provide a method to reduce artifacts. For all cortical regions studied and clusters obtained, we quantify the degree of contamination of functional connectivity maps by the respiratory and cardiac cycle. Results indicate that patterns of functional connectivity can be obtained with hierarchical clustering that resemble known neuronal connections. The corresponding voxel time series do not show significant correlations in the respiratory or cardiac frequency band.