Estimation of the scattering coefficients of turbid media using angle-resolved optical frequency-domain imaging

Noninvasive measurements of the scattering coefficients of optically turbid media using angle-resolved optical frequency-domain imaging (OFDI) are demonstrated. It is shown that, by incoherently averaging OFDI reflectance signals acquired at different backscattering angles, speckle noise is reduced, allowing scattering coefficients to be extracted from a single A-line with much higher accuracy than with measurements from conventional OFDI and optical coherence tomography systems. Modeling speckle as a random phasor sum, the relationship between the measurement accuracy and the number of compounded angles is derived. The sensitivity analysis is validated with measurements from a tissue phantom.     

Frequency-domain immersion technique for accurate optical property measurements of turbid media

We demonstrate that absorption coefficient micro(a) and reduced scattering coefficient micro(s)(?) of a small turbid object can be measured to high accuracy with a frequency-domain immersion technique. For this technique the sample is immersed in a calibrated scattering medium and the optical properties are obtained from a differential measurement. Compared with conventional approaches, the immersion technique improves accuracy, minimizes variations owing to probe coupling and motion, reduces the effects of boundary conditions, and offers simple and rapid measurement once the immersion medium is calibrated. Accuracy tests of immersion-based measurements of micro(a) and micro(s)(?) agree with reference values to within 3.6% and 2.6%, respectively. These tests are limited by the accuracy of the reference samples rather than by the accuracy of the immersion medium or the precision of the immersion approach. We demonstrate the in vivo capabilities of the technique through time-resolved measurements of micro(a) and micro(s)(?) for a human hand during cuff occlusion on the upper arm.     

Noncontact optical tomography of turbid media

Optical tomography of turbid media has so far been limited by systems that require fixed geometries or measurements employing fibers. We present a system that records noncontact optical measurements from diffuse media of arbitrary shapes and retrieves the three-dimensional surface information of the diffuse medium. We further present a novel method of combining this composite data set and obtain accurate fluorescence reconstructions. This approach offers significant experimental simplicity and yields high-information-content datasets. The performance of this novel tomographic approach is demonstrated with experimental reconstructions of phantoms.     

Simultaneous measurements of three-dimensional reflectivity distributions in scattering media based on optical frequency-domain reflectometry

A new type of tomography system based on optical frequency-domain reflectometry is presented. Using an area sensor, the system can simultaneously measure three-dimensional reflectivity distributions in scattering media without the need for mechanical scanning. In preliminary experiments we demonstrate that a target (resolution chart) placed behind biological tissue can be imaged with high depth resolution (47 microm) for a short measurement time.     

Bifocal optical coherenc refractometry of turbid media

We propose and demonstrate a novel technique, which we term bifocal optical coherence refractometry, for the rapid determination of the refractive index of a turbid medium. The technique is based on the simultaneous creation of two closely spaced confocal gates in a sample. The optical path-length difference between the gates is measured by means of low-coherence interferometry and used to determine the refractive index. We present experimental results for the refractive indices of milk solutions and of human skin in vivo. As the axial scan rate determines the acquisition time, which is potentially of the order of tens of milliseconds, the technique has potential for in vivo refractive-index measurements of turbid biological media under dynamic conditions.     

Reflectance properties of finite-size turbid media

Analytic expressions for the position-dependent reflectance of light were derived from a classical diffusion propagator. A modified transport propagator which describes the crossover from the ballistic to the diffusive propagation, and a cross-polarized component propagator. The theoretical reflectances are compared with the experimental ones and their potential for describing finite-size turbid media is discussed. It is found that the cross-polarized propagator provides the best agreement with the experimental data at the low volume fraction of scatterers, for a wide range of separation distances.     

Ring-resonator-based frequency-domain optical activity measurements of a chiral liquid

Chiral liquids rotate the plane of polarization of linearly polarized light and are therefore optically active. Here we show that optical rotation can be observed in the frequency domain. A chiral liquid introduced in a fiber-loop ring resonator that supports left and right circularly polarized modes gives rise to relative frequency shifts that are a direct measure of the liquid's circular birefringence and hence of its optical activity. The effect is in principle not diminished if the circumference of the ring is reduced. The technique is similarly applicable to refractive index and linear birefringence measurements.     

Tomographic imaging of absolute optical absorption coefficient in turbid media using combined photoacoustic and diffusing light measurements

We present a new method that can provide high resolution images of absolute optical absorption coefficient in heterogeneous turbid media. In this method, acoustic measurements in conventional photoacoustic tomography are combined with diffusing light measurements to separate the product of absorption coefficient and optical fluence or photon density. We validate this method using a series of tissuelike phantom experiments. The experimental results show that targets as small as 0.5 mm in diameter with optical absorption contrasts as low as 1.5 relative to a 50 mm diameter scattering background medium can be clearly detected.     

Determination of optical properties of slices of turbid media by diffuse CW laser light scattering profilometry

In this work we present a method for determining the optical parameters of turbid media, namely its absorption coefficient (μ_a) and its reduced scattering coefficient . It is based on the measurement of CW transmittance profiles and analysis of the experimental data by a theoretical model based on the diffusion approximation (DA) of the radiative transfer equation (RTE). The method developed has been investigated with solid polymer probes but it could be applied for liquid materials as well. Experimental results are presented and compared to those of other authors together with a discussion about the accuracy of measurements. In addition, measurements using integrating spheres as well as Monte Carlo simulations are also presented to validate these results.     

High-speed adaptive interferometer for optical coherence-domain reflectometry through turbid media

Two-wave mixing in a dynamic holographic film acts as the adaptive beam combiner in a short-coherence interferometer that performs optical coherence-domain reflectometry (OCDR) through turbid media. This approach combines the high spatial resolution and sensitivity of coherence-domain reflectometry with photorefractive quantum-well-based adaptive homodyne detection. A depth resolution of 28 microm and penetration through 16 mean free paths in a turbid medium have been obtained in this adaptive OCDR application.     

混沌介质光学常数测量

对基于漫射理论并应用CCD相机测量混沌介质的光学常数进行了实验研究.提出采用漫射中心环带约束法来提高逆向求解光学常数的精确性,以及采用在通过光的入射点和漫射中心的直线上截取一维原始数据的方法来提高算法的计算效率.以Intralipid脂肪乳剂作为混沌介质,对方法进行了实验验证.结果表明,本文提出的方法是可行的.     

Linear frequency modulation photoacoustic radar: optimal bandwidth and signal-to-noise ratio for frequency-domain imaging of turbid media

The development of the pulse compression photoacoustic (PA) radar using linear frequency modulation (LFM) demonstrated experimentally that spectral matching of the signal to the ultrasonic transducer bandwidth does not necessarily produce the best PA signal-to-noise ratio, and it was shown that the optical and acoustic properties of the absorber will modify the optimal bandwidth. The effects of these factors are investigated in frequency-domain (FD) PA imaging by employing one-dimensional and axisymmetric models of the PA effect, and a Krimholtz-Leedom-Matthaei model for the employed transducers. LFM chirps with various bandwidths were utilized and transducer sensitivity was measured to ensure the accuracy of the model. The theory was compared with experimental results and it was shown that the PA effect can act as a low-pass filter in the signal generation. Furthermore, with the PA radar, the low-frequency behavior of two-dimensional wave generation can appear as a false peak in the cross correlation signal trace. These effects are important in optimizing controllable features of the FD-PA method to improve image quality.     

Correlation characteristics of optical coherence tomography images of turbid media with statistically inhomogeneous optical parameters

Noisy structure of optical coherence tomography (OCT) images of turbid medium contains information about spatial variations of its optical parameters. We propose analytical model of statistical characteristics of OCT signal fluctuations from turbid medium with spatially inhomogeneous coefficients of absorption and backscattering. Analytically predicted correlation characteristics of OCT signal from spatially inhomogeneous medium are in good agreement with the results of correlation analysis of OCT images of different biological tissues. The proposed model can be efficiently applied for quantitative evaluation of statistical properties of absorption and backscattering fluctuations basing on correlation characteristics of OCT images.     

Computer simulation of time-resolved optical imaging of objects hidden in turbid media

We review research on time-resolved optical imaging of objects hidden in strongly scattering media, with emphasis on the application to breast cancer detection. A method is presented to simulate the propagation of light in turbid media. Based on a numerical algorithm to solve the time-dependent diffusion equation, the method takes into account spatial variations of the reduced scattering and absorption factors of the medium due to the presence of objects as well as random fluctuations of these factors. It is shown that the simulation method reproduces, without fitting, experimental results on tissue-like phantoms. Using experimental and simulation results, an assessment is made of the reliability for extracting the reduced scattering and absorption coefficients of the medium from time-resolved reflection and transillumination data. The simulation technique is employed to study the conditions for locating mm-sized objects immersed in a turbid medium, by direct, time-resolved imaging. We discuss a simple method to enhance the imaging power of the time-resolved technique. The mathematical justification of the method, as well as some applications to simple problems, is given. The simulation technique is employed to demonstrate the effectiveness of the data processing technique. Results of time-resolved reflection experiments and simulations are presented, showing that the use of the latter allow us to locate 1 mm diameter objects under conditions which would prevent detection otherwise. Our results demonstrate that the combination of simulation and the appropriate processing of the diffusive part of the time-resolved reflected or transmitted light intensity may substantially increase the potential of the time-resolved near-infrared diffusive light imaging technique as a diagnostic tool for breast cancer detection.     

Laminar optical tomography: demonstration of millimeter-scale depth-resolved imaging in turbid media

Laminar optical tomography (LOT) is a new technique that combines the advantages of diffuse optical tomography image reconstruction and a microscopy-based setup to allow noncontact imaging with 100-200-microm resolution effective over depths of 0-2.5 mm. LOT is being developed primarily for multispectral imaging of rat cortex, for which resolving functional dynamics in various layers of the brain's cortex (to depths of 1500 microm) is of increasing interest to neurophysiologists. System design and image reconstruction techniques are described, along with simulation and phantom results that demonstrate the characteristics and limitations of system accuracy and resolution.     

Inhomogeneity of turbid media and its effect on the MTF of an optical system

Summary A numerical procedure, partly based on the Monte Carlo method, is employed to examine the effect of inhomogeneity of a turbid medium on the Modulation Transfer Function of an optical system. The results, both for a particulate of small size and of size comparable with wavelength, show a general effect of the ?shower curtain? type.     

Polarized optical coherence imaging in turbid media by use of a Zeeman laser

A method that uses a Zeeman laser in conjunction with a Glan-Thompson analyzer to image an object in a turbid medium is proposed. A heterodyne signal is generated only when the scattering photons are partially polarized, and the spatial coherence is not seriously degraded after the signal propagates in the turbid medium. A system combining polarization discrimination with optical coherence detection to image the object in a scattering medium is successfully demonstrated. The medium is a solution of polystyrene microspheres measuring 1.072 mum in diameter suspended in distilled water contained in a 10-mm-thick quartz cuvette. The advantages of this optical system, including better selectivity of the weak partially polarized scattering photons and better imaging ability in higher-scattering media, are discussed.     

Effects of modulation phase of ultrasound-modulated light on the ultrasound-modulated optical image in turbid media

In this paper, our investigations suggest that the modulation phase of ultrasound-modulated light escaping from the different locations in the ultrasonic field is different. In turbid media, the modulation phase causes the ultrasound-modulated light intensity collected outside the media to fluctuate. However, the ultrasound-modulated optical technology uses the ultrasound-modulated light signals to image. Consequently, the modulation phase affects the quality of ultrasound-modulated optical imaging.     

Multiple passages of light through an absorption inhomogeneity in optical imaging of turbid media

Multiple passages of light through an absorption inhomogeneity of finite size deep within a turbid medium are analyzed for optical imaging by use of the self-energy diagram. The nonlinear correction becomes more important for an inhomogeneity of a larger size and with greater contrast in absorption with respect to the host background. The nonlinear correction factor agrees well with that from Monte Carlo simulations for cw light. The correction is approximately 50%-75% in the near infrared for an absorption inhomogeneity with the typical optical properties found in tissues and five times the size of the transport mean free path.     

Fluorescence-lifetime-based tomography for turbid media

We derive a novel algorithm to recover the in vivo distributions of fluorophores based on an asymptotic life-time analysis of time-domain fluorescence measurements with turbid tissue. We experimentally demonstrate the advantage offered by this method in localizing fluorophores with distinct lifetimes. This algorithm has wide applicability for diagnostic fluorescence imaging in the presence of several-centimeter-thick biological tissue, since fluorescence lifetime is a sensitive indicator of local tissue environment and interactions at the molecular level.