Experimental Study on Localization of Absorbers within Tissue Phantom Using a Simple Phased Array System

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Experimental Study on Localization of Absorbers within Tissue Phantom Using a Simple Phased-Array System

A simple phased-array system with two anti-phase near infrared (NIR) light sources whose intensities are modulated sinusoidally at 200 MHz is established, and experiments for exploring the light distribution in homogeneous tissue phantom (0.25% intralipid) and the perturbations of intensity null and phase transition by absorbers are carried out. Results conform closely with the prediction by diffusion theory and the results of other groups[1,2]. Our measurements clearly demonstrate the distortion of the light distribution by the existence of absorbers and the effects on the perturbations caused by different sizes and locations of the objects. Particularly, we have studied the possibility of distinguishing between two absorbers and obtained initial results.     

Multiple passive element enriched photoacoustic computed tomography

Recently, we presented a method using laser-induced ultrasound from an external absorber (passive element) to image the ultrasound transmission parameters of an object under photoacoustic tomographic investigation. The method suffers from long measurement times due to the requirement for a large number of views and consequently physical projections around the object. Here we propose and validate an approach that permits a multitude of views to be obtained within a limited projection scenario. The approach uses a plurality of spatially distributed external absorbers in the path of the light, that results in multiple laser-induced ultrasound sources to interrogate the object from a number of angles. This reduces the required number of rotation angles or physical projections around the object, permitting a considerable reduction in imaging time without significant degradation in image quality. The approach brings the concept of hybrid imaging of ultrasound transmission parameters together with photoacoustic imaging, into the realm of practical application.     

Autofluorescence endoscopy for the gastrointestinal tract

This review focuses on our basic study results and clinical experience of fluorescence endoscopy for the gastrointestinal (GI) tract. Collagen, which fluoresces in the green wavelength range, is one of the major sources of tissue autofluorescence (AF) and AF imaging systems are now available. With their use, however, it is important to take into account tissue changes other than, or in addition to, changes in gross tissue morphology. These may include alterations in the local blood volume, tissue metabolic activity, and relative fluorophore concentrations. New AF imaging systems are very easy to use, because white light endoscopy can be changed to AF at the push of a button, and hold great promise for diagnosis of early carcinomas and premalignant lesions in the GI tract. In particular, AF endoscopy has potential for identification of small or flat tumors, tumor margins and premalignant lesions in Barrett’s esophagus, as well as for assessing tumor grade and response to therapy. However, large-scale studies are needed to clarify the clinical impact of this new diagnostic approach.     

Human uses of ultrasound: ancient and modern

For untold millennia certain animals have used ultrasound to probe places where light is unavailable, echo-locating bats being among the most adept. With ultrasonics, bats can quickly and safely 'see' at night in pursuing insects or flying in dark caves. Unable to hear ultrasound, humans have nevertheless made use of it. They did this anciently by taming wolves, with their keen ultrasonic hearing, for aiding in the hunt. Currently, they are doing this by developing technology to detect, generate and process ultrasound for searching in air or other gases, in water or other liquids, and in solids. The story of these technological developments is a large and fascinating mirror of human history involving the advent of such discoveries and inventions as magnetostriction, piezoelectricity, sonar, ultrasonic microscopy, etc.--the list is long. By now we are skilled in probing for underwater objects, the internal structure in materials, organs inside the human body, etc.--again the list is long. A number of different ultrasonic systems can be categorized into one of three key generic approaches: pulse-echo exploration, intensity mapping, and phase-amplitude measurement. In addition, each of these categories can be combined with the others to produce hybrid systems for which an unambiguous categorization is difficult or impossible. Challenging problems remain but solutions are being found. New principles and techniques are being discovered that will improve the use of ultrasound. Employing tomo-holographic techniques to reduce ambiguity in probing three-dimensional objects, near-field techniques to boost resolution and using limited-diffraction beams to provide image construction with ultra high frame rates are cases in point.     

Frequency-swept ultrasound-modulated optical tomography in biological tissue by use of parallel detection

A frequency-swept ultrasonic beam was focused into a biological tissue sample to modulate the laser light passing through the ultrasonic beam inside the tissue. Parallel detection of the speckle field formed by the transmitted laser light was implemented with the source-synchronous-illumination lock-in technique to improve the signal-to-noise ratio. The ultrasound-modulated laser light reflects the local optical and mechanical properties in the ultrasonic beam and can be used for tomographic imaging of the tissue. Sweeping the ultrasonic frequency provides spatial resolution along the ultrasonic axis, which is scalable with the frequency span of the sweep. Two-dimensional images of biological tissue with buried objects were successfully obtained experimentally.     

Analysis of saturable absorbers,interacting with gaussian pulses

Mode-locked lasers, Q-switch systems and discrimination amplifiers make use of saturable absorbers. The interaction of those absorbers with light pulses is well known, either for very long or very short pulses, but not, if the pulse width is comparable with the absorber life time, which occurs very often in experiments. Moreover, the spatial structure of the pulse is mostly not considered. In this paper, we investigate in detail the interaction of gaussian pulses (spatial and temporal) with the two-level-absorber. Energy transmission, pulse shortening and pulse asymmetry are calculated as functions of pulse width and small signal transmission of the absorber.     

Fibre-optic systems for dynamic light scattering — a review

‘Photon correlation spectroscopy’ has become a standard method in applied science for analysing the properties of submicron particles in a fluid environment. The fast, local fluctuations in the intensity of the light scattered from such particles are recorded and analysed in terms of the local motions. This has led to the use of the method both to characterize particle geometry and to monitor particle translation as a marker for fluid flow. The advent of fibre-optics has enabled portable, small, remote and more flexible systems to be established in recent years. This paper reviews those instrument designs which have represented advances in the development of fibre-based systems. Emphasis is laid on instruments (a) for monitoring blood flow and (b) for measuring Brownian-induced translatory diffusion in colloidal suspensions.     

Diffuse Optical Tomography using Time-resolved Photon Path Distribution

Our research goal is to develop diffuse optical tomography (DOT) capable of quantitative measurement. Information on optical pathlength is essential for reconstructing images with quantitative properties, and we have performed image reconstruction with a simulation model using a time-resolved photon path distribution (time-resolved PPD). The results showed that a DOT image reconstruction algorithm using this PPD is effective in quantifying the absorbers in a scattering medium such as human tissue. This algorithm uses a photon distribution independent of absorption by simply assuming that the measurement object is homogeneous, which means that PPD needs to be calculated only once. Our technique is therefore applicable to short-time imaging of measurement objects for which absorption changes flatness such as that in human tissue.     

Modeling of thermal effects in antivascular ultrasound therapy

Antivascular ultrasound consisting of low-intensity sonication in the presence of circulating microbubbles of an ultrasound contrast agent has been demonstrated to disrupt blood flow in solid cancers. In this study a mathematical framework is described for the microbubble-induced heating that occurs during antivascular ultrasound. Biological tissues are modeled as a continuum of microbubble-filled vasculature, cells, and interstitial fluids with compressibility equal to the sum of the compressibility of each component. The mathematical simulations show that the absorption of ultrasound waves by viscous damping of the microbubble oscillations induced significant local heating of the tissue vasculature. The extent and the rate of temperature increase not only depends on the properties of the microbubbles and the sonication parameters but is also influenced markedly by the blood flow. Slow flow conditions lead to higher tissue temperatures due to a stronger interaction between microbubbles and ultrasound and reduced heat dissipation. Because tumors have slower blood flow than healthy tissue, the microbubble-induced ultrasound antivascular therapy is likely to affect cancerous tissue more extensively than healthy tissue, providing a way to selectively target the vasculature of cancers.     

超声调制介质中漫散射光自相关性质研究

研究了超声调制随机介质中漫散射光自相关性质,推导出均匀无限介质中有超声场调制时漫散射光时间自相关函数的表达式,讨论了调制参数、调制幅度及其衰减与超声参数、光散射参数等的关系.用Monte Carlo方法研究了随机介质中隐含异物成像的可行性.结果表明,正常生物组织和病变生物组织的超声调制自相关函数有明显的差别,该方法为光学医学诊断提供了一种新参考 关键词： 自相关 漫散射光 超声调制 光学成像     

High-resolution ultrasound-modulated optical tomography in biological tissues

We present a novel implementation of high-resolution ultrasound-modulated optical tomography that, based on optical contrast, can image several millimeters deep into soft biological tissues. A long-cavity confocal Fabry-Perot interferometer, which provides a large etendue and a short response time, was used to detect the ultrasound-modulated coherent light that traversed the scattering biological tissue. Using 15-MHz ultrasound, we imaged with high-contrast light-absorbing structures placed >3 mm below the surface of chicken breast tissue. The resolution along the axial and the lateral directions with respect to the ultrasound propagation direction was better than 70 and 120 microm, respectively. The resolution can be scaled down further by use of higher ultrasound frequencies. This technology is complementary to other imaging technologies, such as confocal microscopy and optical-coherence tomography, and has the potential for broad biomedical applications.     

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%.     

Use of local fuzzy variance to extract the scattered regions of spatial stray light influence in hyperspectral images

Spatial stray light is a phenomenon caused by scattering of photons from target objects, and it is generated within and among adjacent objects. The intensity and radiometric composition of spatial stray light depends on the light source properties and chemical and physical properties of the objects being imaged. A special kind of region exists in hyperspectral images, referred to as the “scattered region” of spatial stray light, which can be regarded as a boundary between different objects or between an object and background. In this study, local fuzzy variance was used to extract the regions, in which spatial stray light may have significant influence. The proposed method was applied to three data sets: synthetic array data and two hyperspectral imaging datasets (1: plastic card and 2: maize kernels from three inbred lines). Using local fuzzy variance method, scattered regions of spatial stray light could be extracted and visual inspection of data sets demonstrated the effectiveness and sensitivity of the proposed method.     

The effect of dissolve gas concentration in the initial growth stage of multi cavitation bubbles. Differences between vacuum degassing and ultrasound degassing

The sonochemical luminescence intensity from luminol was measured at a sampling rate of several kilohertz. This was noted at three different periods: first, the latent period in which no light emission occurs at all; second, the increased emission period from the start of light emission to the time when a steady state is reached; and third, the steady state period in which light emission occurs at the steady state value. When irradiated with ultrasound of different intensities, the times of the latent period and increased emission period are shorter for higher ultrasound intensities. To know how the dissolved oxygen content is involved in early-stage cavitation growth, an experiment was conducted using solutions with varying dissolved oxygen contents from 100% to 37%. For dissolved air content of 50% or less, it was found that the latent period was 30 times longer in a saturated condition. It was also found that the increased emission period was 10 times longer. However, the emission intensity in the steady state did not change at all even when the initial dissolved gas concentration of the sample was changed. From this, it was found that the reuse of collapsed bubbles takes place efficiently in the steady state. Dissolved oxygen was reduced by the use of a vacuum pump and by the degassing action of ultrasound, and it was discovered that the behavior of transient emission differed for the two ways of degassing.     

The impact of point thermal absorbers in ablation of poly(methyl methacrylate)

Molecular dynamics (MD) simulations are used to study the interactions of photons with a poly(methyl methacrylate), or PMMA, substrate and point thermal absorbers. The point thermal absorbers are embedded within the polymer matrix which can be excited and transfer energy to the surrounding particles through an internal vibrational mode. Using a fluence above the ablation threshold, two excitation channels are studied—one includes a direct heating of the polymer and the other includes the excitation of the thermal absorbers. Although the yield of ejected particles is similar for both simulations, the plume composition differs. For the simulation of the excitation of the point thermal absorbers, the plume consists of a greater number of smaller substrate fragments due to local high temperature regions.     

Doppler ultrasound technique for measuring capillary-speed flow velocities with strong stationary echoes.

Ultrasound Doppler is widely used for low measurement in both medicine and industry, having the advantages of being non-invasive and comparatively simple and therefore inexpensive. The technique has not however been used for capillary blood flow measurement, because of the relatively low velocities encountered and because of the presence of strong interfering signals from the encompassing tissue. An ultrasound Doppler system capable of measuring flow velocities of one millimetre per second in the presence of one thousand times stronger interfering signals is described, as well as test results using both thread and flow phantoms.     

Correlation transfer and diffusion of ultrasound-modulated multiply scattered light

We develop a temporal correlation transfer equation (CTE) and a temporal correlation diffusion equation (CDE) for ultrasound-modulated multiply scattered light. These equations can be applied to an optically scattering medium with embedded optically scattering and absorbing objects to calculate the power spectrum of light modulated by a nonuniform ultrasound field. We present an analytical solution based on the CDE and Monte Carlo simulation results for light modulated by a cylinder of ultrasound in an optically scattering slab. We further validate with experimental measurements the numerical calculations for an actual ultrasound field. The CTE and CDE are valid for moderate ultrasound pressures and on a length scale comparable with the optical transport mean-free path. These equations should be applicable to a wide spectrum of conditions for ultrasound-modulated optical tomography of soft biological tissues.     

Photoacoustic Doppler effect from flowing small light-absorbing particles

From the flow of a suspension of micrometer-scale carbon particles, the photoacoustic Doppler shift is observed. As predicted theoretically, the observed Doppler shift equals half of that in Doppler ultrasound and does not depend on the direction of laser illumination. This new physical phenomenon provides a basis for developing photoacoustic Doppler flowmetry, which can potentially be used for detecting fluid flow in optically scattering media and especially low-speed blood flow of relatively deep microcirculation in biological tissue.