Examination of the oxygenation state of hemoglobin in a phantom and in-vivo tissue applying absorption balancing with two and three laser wavelengths

Two-dimensional absorption measurements of hemoglobin in (i) a phantom, imitating the optical properties of tissue, and (ii) in-vivo breast tissue are reported. The method is based on quasi-simultaneous balanced absorption measurements of hemoglobin and oxygenated hemoglobin at selected wavelengths in order to minimize baseline fluctuations during spatial scanning of the sample. The method allows the detection of 0.8 cm-diameter inclusions of oxygenated hemoglobin differing by about 30% in its oxygenation state from the surrounding hemoglobin if two-wavelengths balancing is applied. However, small wavelength detunings from the optimum wavelength positions cause considerable fluctuations of the signal baseline and deteriorate the detection sensitivity, if there is an inhomogeneous distribution of another weak wavelength dependent absorber in the sample. Although the optical absorption of water is relatively small for the wavelengths applied, a considerable influence of the water on the signal baseline was observed in in-vivo measurements of the hemoglobin status in healthy breast tissue. For better compensation of interfering water absorption, a third laser wavelength was applied in order to balance the water absorption. The prospects of this new optical technique for screening of breast tumors are discussed. PACS 41.85.Ja; 42.30.Wb; 42.55.Px; 42.62.Be     

In vivo near-infrared spectral detection of pressure-induced changes in breast tissue

A diffuse near-infrared tomography system was used to measure dynamic changes in the absolute optical properties of the human breast that were induced through pressure applied to the tissue surface. Results from five subjects show that absorption and scattering coefficients changed measurably when pressure was increased and that these relative changes correlated with the subjects' body-mass index, indicating that the effect depends on tissue composition. Fitting the absolute absorption and scattering coefficients at six wavelengths to the molar absorption spectra of the three predominant chromophores revealed that both the average total hemoglobin and oxygen saturation increased by 10%, while water concentration decreased by more than 12%. These changes indicate that the pressure-induced variation is likely due to water displacement and vascular volume increase in the region being imaged, for mild application of pressure to the breast. These results suggest that the pressure applied during optical measurements of tissue may alter the tissue physiology, and care should be taken to factor this effect into the design of optical medical instrumentation. In addition, the technique provides a unique approach to measuring tissue elastic changes in vivo in the female breast and may offer a new method for dynamic contrast imaging based on elasto-optical measurements.     

基于代数迭代算法的燃烧火焰温度场和气体浓度场重建研究

基于TDLAS(tunable diode laser absorption spectroscopy)技术,以水汽作为目标气体,采用直接吸收的测量方式,探测了甲烷空气预混平焰炉燃烧区域水汽的吸收光谱信号,通过ART(algebraic reconstruction technique)代数迭代算法对燃烧场温度和水汽浓度分布进行了模拟重建和实验研究,模拟重建采取5×5共25个网格的正方形重建区域,假定25个网格的一个温度浓度二维分布,模拟28条激光束从不同的角度方位穿越重建区域,得到模拟射线下的投影值,经ART算法重建,结果显示温度场和水汽浓度场的重建偏差均在1%以内。实验采用分布反馈式激光器作为光源,选取H₂O的7 153.722,7 153.748和7 154.354 cm-1三条吸收线作为测温谱线,其中前两条线不区分作为一条吸收线来处理。使用平移台多方位平行扫描,共获取30路光谱吸收信号,经数据处理、算法重建和双线比值法测温原理得到了圆形平焰炉16个不同区域的温度浓度值,且炉面偏向中心区域温度浓度值较高,边缘较小,结果表明代数迭代算法能够很好地实现燃烧区域温度场和水汽浓度场的反演。     

针对多散斑图的差分压缩鬼成像方案研究

鬼成像方案实现所需设备、成像的质量以及成像所花的时间是决定鬼成像技术可实用化的重要因素. 本文提出一种针对多散斑图的差分压缩鬼成像方案. 该方案通过连续探测多个独立的散斑图, 降低了热光鬼成像方案对探测器高时间分辨力的要求; 通过采用差分方法, 抑制了背景噪声和其他噪声源的干扰; 通过使用压缩感知重建算法, 有效地降低了鬼成像所需时间并同时提升成像的质量. 数值仿真结果表明, 对于二灰度“N” 图, 本方案在8000次的采样情形下与多散斑图鬼成像方案35000次采样的结果相比, 均方误差降低了96.9%、峰值信噪比提升15.1 dB. 对于八灰度“Pepper”图, 本方案与多散斑图鬼成像方案相比, PSNR提升11.4 dB. 本方案可降低探测设备的要求、提高成像质量、降低重建时间, 具有广阔的应用前景.     

Characterization of small absorbers inside turbid media

We propose a novel noniterative near-infrared diffusive image reconstruction method that uses minimal a priori co-registered ultrasound information. Small absorbing targets embedded in a homogeneous background are described approximately in terms of their monopole, dipole, and quadrupole moments. With an approximate estimation of the center locations of these absorbers from ultrasound images, we show in simulations that the reconstruction accuracy of the absorption coefficient exceeds 80% if the noise level is less than 0.2%. We also demonstrate experimentally that the accuracy can be improved by use of additional ultrasound volume information even for a noise level as high as 1.5%.     

Modified adaptive algebraic tomographic reconstruction of gas distribution from incomplete projection by a two-wavelength absorption scheme

A modified adaptive algebraic reconstruction technique (MAART) with an auto-adjustment relaxation parameter and smoothness regularization is developed to reveal the tomographic reconstruction of H 2 O distribution in combustion from incomplete projections. Determinations of relaxation parameter and regularization factor are discussed with regard to the consideration of improvement in reconstruction and reduction of computational burden. A two-wavelength scheme from tunable diode laser absorption measurement, 7444.36 and 7185.59 cm 1 , is used to simplify the nonlinear solution problem for obtaining the tomographic distributions of concentration and temperature simultaneously. Results of calculations demonstrate that MAART can perform the reconstruction results more efficiently with little complex modification and much lower iterations as compared with the traditional algebraic reconstruction technique (ART) or simultaneous iterative reconstruction technique (SIRT) methods. The stability of the algorithm is studied by reconstruction from projections with random noise at different levels to demonstrate the dependence of reconstruction results on precise line-of-sight measurements.     

Comparison of the efficiencies of Fourier and wavelet expansions in passive acoustic thermal tomography

A comparison of the accuracy achieved in temperature-distribution reconstruction by the Fourier and wavelet expansions in passive acoustic tomography is carried out. Since the use of focused ultrasound in hyperthermia leads to local heating of the tissue, the wavelet representation of such temperature distributions with local fluctuations is more compact than their Fourier representation. It is demonstrated that the compactness of the wavelet representation provides an opportunity to reduce considerably the number of unknown quantities in solving the inverse problem of acoustic thermal tomography and to increase the accuracy of the temperature-distribution reconstruction in comparison with the case of using the Fourier representation. A method for choosing a compact wavelet basis for temperature distributions obtained in the hyperthermia process is proposed.     

Imager that combines near-infrared diffusive light and ultrasound

We introduce an imaging technique that combines complementary features of ultrasound and near-infrared diffusive light imaging. We achieve the combined technology experimentally by mounting an ultrasound array together with multiple laser source and optical detector fibers upon a hand-held probe. The technique is demonstrated with tissue phantoms wherein both acoustic and optical sensors image the volume underneath the probe. Coregistration of acoustic and optical images is achieved with an accuracy of 0.27+/-0.20cm, approximately half of the image pixel size of our prototype. Accurate determination of target optical absorption is also achieved by use of image segmentation on the ultrasound reconstruction. The combined technique may provide improved breast-cancer detection sensitivity and specificity.     

Optical tomography using the time-independent equation of radiative transfer—Part 2: inverse model

Optical tomography is a novel imaging modality that is employed to reconstruct cross-sectional images of the optical properties of highly scattering media given measurements performed on the surface of the medium. Recent advances in this field have mainly been driven by biomedical applications in which near-infrared light is used for transillumination and reflectance measurements of highly scattering biological tissues. Many of the reconstruction algorithms currently utilized for optical tomography make use of model-based iterative image reconstruction (MOBIIR) schemes. The imaging problem is formulated as an optimization problem, in which an objective function is minimized. In the simplest case the objective function is a normalized-squared error between measured and predicted data. The predicted data are obtained by using a forward model that describes light propagation in the scattering medium given a certain distribution of optical properties.In part I of this two-part study, we presented a forward model that is based on the time-independent equation of radiative transfer. Using experimental data we showed that this transport-theory-based forward model can accurately predict light propagation in highly scattering media that contain void-like inclusions. In part II we focus on the details of our image reconstruction scheme (inverse model). A crucial component of this scheme involves the efficient and accurate determination of the gradient of the objective function with respect to all optical properties. This calculation is performed using an adjoint differentiation algorithm that allows for fast calculation of this gradient. Having calculated this gradient, we minimize the objective function with a gradient-based optimization method, which results in the reconstruction of the spatial distribution of scattering and absorption coefficients inside the medium. In addition to presenting the mathematical and numerical background of our code, we present reconstruction results based on experimentally obtained data from highly scattering media that contain void-like regions. These types of media play an important role in optical tomographic imaging of the human brain and joints.     

一种基于多级小波域分解的时域扩散光学层析成像方法的研究

面向基于有限差分光扩散模型的平板乳腺扩散光学层析反演问题,提出了一种有效提高图像重建质量的方法.与基于全空间域离散像点的传统重建算法不同,本方法采用小波函数对吸收系数和散射系数在空间域进行多尺度分解,在减少低分辨率重建中光学参量的个数、有效改善重建过程病态性的同时,通过逐级细化分解尺度,最终提高了图像的量化度和空间分辨率.数值模拟表明,该方法可实现目标体边对边4 mm的空间分辨率,量化度明显高于传统算法.     

频域近红外光学成像法的苹果内部病变检测精度

苹果组织内部的病变会导致其光学参数发生变化。用频域近红外光学成像法（FD-DOT）对苹果组织进行吸收系数和约化散射系数的检测,并结合三维重构技术得到的重构图像可以直观地了解苹果内部的病变情况,从而实现对苹果内部病变的无损检测。选择可最大程度区分苹果正常组织与病变组织所对应的波长为740 nm的光作为激光光源。当FD-DOT的入射光调制频率不同、苹果内部病变的程度不同、病变位置和大小不同时,会导致成像精度的变化,设计了一系列模拟仿真实验研究以上因素对苹果内部病变检测精度的影响：设定不同的激光调制频率,研究调制频率对重构图像精度的影响;在苹果模型中某一位置添加不同大小的球形病变,研究病变区域大小对重构图像精度的影响;在苹果模型中不同位置添加一定大小的异质体,研究病变位置不同对重构图像精度的影响。首先用Abaqus建立苹果有限元网格模型,设计了12个740 nm的近红外激光光源和6个检测器均匀排布在苹果模型表面,根据实验需要,在组织体模型中添加代表病变的球形异质体,用经过高频调制的光源照射进苹果,检测出射光的交流幅度和相位延迟,然后借助开源软件NIRFAST计算并反推出待测苹果内部的吸收系数和约化散射系数分布并进行三维重构,重构结果可以用重构图像的吸收系数对比度噪声比（CNR值）和吸收系数分布图进行评价。实验结果表明,想要检测到尺寸较大苹果的深处病变,需要较高的入射光调制频率;该方法可以检测到大小适宜的苹果中大部分半径大于5 mm的球形病变区域,且随着病变区域在一定范围内扩大,重构图像的精度逐渐增加,但病变区域过大时,图像精度开始降低;病变区域距离检测器越来越近时,重构图像的精度逐渐增加,但当病变区域与检测器距离过小时,重构图像的精度有降低的趋势;病变区域距离检测器平面的垂直距离越近,重构图像的精度越高。以上实验结果将为应用频域近红外光学成像法对苹果进行无损检测奠定良好基础。     

A method for operative quantitative interpretation of multispectral images of biological tissues

A method for operative retrieval of spatial distributions of biophysical parameters of a biological tissue by using a multispectral image of it has been developed. The method is based on multiple regressions between linearly independent components of the diffuse reflection spectrum of the tissue and unknown parameters. Possibilities of the method are illustrated by an example of determining biophysical parameters of the skin (concentrations of melanin, hemoglobin and bilirubin, blood oxygenation, and scattering coefficient of the tissue). Examples of quantitative interpretation of the experimental data are presented.     

乳腺超声断层成像系统的有限元仿真分析

本文利用有限元仿真软件COMSOL进行乳腺超声断层成像仿真,仿真模型置于水中,重建区域包含乳腺软组织和一个圆形铁块障碍物。256个换能器等角度环形分布于乳腺外部,采用一个换能器发射、其余所有换能器接收的方式进行,依次进行256次仿真来实现环形扫描。仿真得到随时间变化的二维声压分布云图,提取接收电压获得信号矩阵。利用等角扇束滤波反投影重建算法进行图像重建,仿真实验结果证明了换能器呈360?进行环形扫描的方式可行。     

Effects of the approximations of light propagation on quantitative photoacoustic tomography using two-dimensional photon diffusion equation and linearization

Quantitative photoacoustic tomography (QPAT) employing a light propagation model will play an important role in medical diagnoses by quantifying the concentration of hemoglobin or a contrast agent. However, QPAT by the light propagation model with the three-dimensional (3D) radiative transfer equation (RTE) requires a huge computational load in the iterative forward calculations involved in the updating process to reconstruct the absorption coefficient. The approximations of the light propagation improve the efficiency of the image reconstruction for the QPAT. In this study, we compared the 3D/two-dimensional (2D) photon diffusion equation (PDE) approximating 3D RTE with the Monte Carlo simulation based on 3D RTE. Then, the errors in a 2D PDE-based linearized image reconstruction caused by the approximations were quantitatively demonstrated and discussed in the numerical simulations. It was clearly observed that the approximations affected the reconstructed absorption coefficient. The 2D PDE-based linearized algorithm succeeded in the image reconstruction of the region with a large absorption coefficient in the 3D phantom. The value reconstructed in the phantom experiment agreed with that in the numerical simulation, so that it was validated that the numerical simulation of the image reconstruction predicted the relationship between the true absorption coefficient of the target in the 3D medium and the reconstructed value with the 2D PDE-based linearized algorithm. Moreover, the the true absorption coefficient in 3D medium was estimated from the 2D reconstructed image on the basis of the prediction by the numerical simulation. The estimation was successful in the phantom experiment, although some limitations were revealed.

     

Intravascular photoacoustic imaging of lipid in atherosclerotic plaques in the presence of luminal blood

Intravascular photoacoustic (IVPA) imaging can characterize atherosclerotic plaque composition on the basis of the optical absorption contrast between different tissue types. Given the high optical absorption of lipid at 1720 nm wavelength, an atherosclerotic rabbit aorta was imaged at this wavelength ex vivo using an integrated intravascular ultrasound (IVUS) and IVPA imaging catheter in the presence of luminal blood. Strong optical absorption of lipid combined with low background signal from other tissues provides a high-contrast, depth-resolved IVPA image of lipid. The ability to image lipid at a single wavelength without removing luminal blood suggests that in vivo detection of lipid in atherosclerotic plaques using combined IVUS/IVPA imaging is possible.     

Initial studies of in vivo absorbing and scattering heterogeneity in near-infrared tomographic breast imaging

Simultaneously recovered absorption and scattering images that separate these optical property features within the female breast are demonstrated from frequency-domain measurements. A study of known absorbing and scattering objects is presented as a foundation for interpreting these in vivo images once the contrast space has been fully characterized. No measurable influence of absorbing-object contrast appears in the scattering images, whereas localized scattering contrast enhances the corresponding region within the absorption image by approximately 30% (e.g., a 2:1 scatterer also reconstructs as an approximately 1.3:1 absorber). Scattering and absorption images of a female volunteer with a 3.4-cm fibroadenoma show a clear 2:1 localized increase in absorption coefficient with little or no evidence of scattering enhancement in the lesion.     

Reconstruction of the distribution pattern for the bloodstream velocity vector in the acoustic tomography procedure

Results of the development of additive-correlation and multiplicative-correlation methods for a tomographic reconstruction of the distribution pattern of the total bloodstream velocity vector with the help of ultrasound are presented for relatively low frequencies (1–2) MHz, which provide a sufficient penetration depth. The reconstruction of the vector field uses a great body of spatial-correlation data and is closely related to the process of constructing the tomographic image of a given organ as a whole. A spatial cross-correlation of intermediate images is used, which makes it possible to estimate the bloodstream velocity vector independently of the value of the real blood transfer within the total measurement time. Results of numerical simulations for constant and varying distributions of the bloodstream velocity vector in the cross sections of blood vessels are presented.     

Analysis of algebraic reconstruction technique for accurate imaging of gas temperature and concentration based on tunable diode laser absorption spectroscopy

An improved algebraic reconstruction technique(ART) combined with tunable diode laser absorption spectroscopy(TDLAS) is presented in this paper for determining two-dimensional(2D) distribution of H_2O concentration and temperature in a simulated combustion flame.This work aims to simulate the reconstruction of spectroscopic measurements by a multi-view parallel-beam scanning geometry and analyze the effects of projection rays on reconstruction accuracy.It finally proves that reconstruction quality dramatically increases with the number of projection rays increasing until more than 180 for 20 × 20 grid,and after that point,the number of projection rays has little influence on reconstruction accuracy.It is clear that the temperature reconstruction results are more accurate than the water vapor concentration obtained by the traditional concentration calculation method.In the present study an innovative way to reduce the error of concentration reconstruction and improve the reconstruction quality greatly is also proposed,and the capability of this new method is evaluated by using appropriate assessment parameters.By using this new approach,not only the concentration reconstruction accuracy is greatly improved,but also a suitable parallel-beam arrangement is put forward for high reconstruction accuracy and simplicity of experimental validation.Finally,a bimodal structure of the combustion region is assumed to demonstrate the robustness and universality of the proposed method.Numerical investigation indicates that the proposed TDLAS tomographic algorithm is capable of detecting accurate temperature and concentration profiles.This feasible formula for reconstruction research is expected to resolve several key issues in practical combustion devices.     

Nonlinear pulsed ultrasound beams radiated by rectangular focused diagnostic transducers

A numerical model for simulating nonlinear pulsed beams radiated by rectangular focused transducers, which are typical of diagnostic ultrasound systems, is presented. The model is based on a KZK-type nonlinear evolution equation generalized to an arbitrary frequency-dependent absorption. The method of fractional steps with an operator-splitting procedure is employed in the combined frequency-time domain algorithm. The diffraction is described using the implicit backward finite-difference scheme and the alternate direction implicit method. An analytic solution in the time domain is employed for the nonlinearity operator. The absorption and dispersion of the sound speed are also described using an analytic solution but in the frequency domain. Numerical solutions are obtained for the nonlinear acoustic field in a homogeneous tissue-like medium obeying a linear frequency law of absorption and in a thermoviscous fluid with a quadratic frequency law of absorption. The model is applied to study the spatial distributions of the fundamental and second harmonics for a typical diagnostic ultrasound source. The nonlinear distortion of pulses and their spectra due to the propagation in tissues are presented. A better understanding of nonlinear propagation in tissue may lead to improvements in nonlinear imaging and in specific tissue harmonic imaging. Published in Russian in Akusticheskiĭ Zhurnal, 2006, Vol. 52, No. 4, pp. 560–570. This article was translated by the authors.