Imaging in turbid media using quasi-ballistic photons

We study by means of experiments and Monte Carlo simulations, the scattering of light in random media, to determine the distance up to which photons travel along almost undeviated paths within a scattering medium, and are therefore capable of casting a shadow of an opaque inclusion embedded within the medium. Such photons are isolated by polarisation discrimination wherein the plane of linear polarisation of the input light is continuously rotated and the polarisation preserving component of the emerging light is extracted by means of a Fourier transform. This technique is a software implementation of lock-in detection. We find that images may be recovered to a depth far in excess of that predicted by the diffusion theory of photon propagation. To understand our experimental results, we perform Monte Carlo simulations to model the random walk behaviour of the multiply scattered photons. We present a new definition of a diffusing photon in terms of the memory of its initial direction of propagation, which we then quantify in terms of an angular correlation function. This redefinition yields the penetration depth of the polarisation preserving photons. Based on these results, we have formulated a model to understand shadow formation in a turbid medium, the predictions of which are in good agreement with our experimental results.     

Imaging through turbid media by polarized light

A theoretical approach to the calculation of the image of an inhomogeneity in transillumination of tissue-like media using polarized radiation is developed in the approximation of the basic polarization modes. The method is based on the solution of the nonstationary transfer equation in the Fokker-Planck approximation. An expression for the edge spread function of the image of a totally absorbing half-plane is derived. A generalization to the image profile of inhomogeneity that represents a finite-width stripe is presented. The spatial resolution (sharpness) and contrast of image are analyzed at various time intervals of detection. The image parameters are compared for the polarization-difference technique and imaging in unpolarized light. It is demonstrated that the imaging using the difference of linear polarizations is similar to the imaging using the pulsed unpolarized radiation with a detection interval of about z/2c (z is the thickness of the sample and c is the velocity of light). The results are in agreement with the experimental data on the differential polarization transillumination of tissue-like media.     

Influences of diffusive reflection intensity and pulse shaping of ultrashort lasers on turbid media

The influences of the absorption μ_a, the scattering μ_s, and the anisotropy coefficient g on the optical properties of ultrashort pulse in turbid media has been simulated based on the diffusive approximation theory. The laser pulse intensity will be attenuated and the diffusive scattering pulse shape will be widened in the turbid media. Various medium parameters have different influences on the reflection of the laser pulse. The intensity loss of the diffusive reflection light is obtained when μ_a and μ_s are increased in turbid media. The pulse width of the diffusive reflection pulse is rapidly increased far away from the incident point and at the same time the pulse times that are delayed have been numerical simulated in the boundary conditions of semi-infinite homogenous media.     

Light transfer problem in turbid media with surface reflectivity

Light transfer problems in turbid media with surface reflectivity satisfying Fresnel's law are formulated. The intensity of light is considered as a sum of collimated and diffuse radiances. The problem with a collimated source and boundary conditions with surface reflectivity is solved in terms of the corresponding source-free problem with simple boundary conditions. In order to solve the source-free problem, two-flux models in the differential and integral forms are obtained. For the differential form, weight functions are introduced in order to force the boundary conditions to be met, while in the integral form the boundary conditions are embedded. The integral form has the advantage over the differential form that it permits one to extend the validity of the model and to consider inhomogeneous media. Numerical results are given for albedos and partial fluxes depending upon the refractive index of the medium. The calculations are performed in both semi-infinite and finite media and are compared with the published calculations.     

CW laser transilluminance in turbid media with cylindrical inclusions

The present work analyzes the process of detection of small diffusive inclusions in turbid hosts. Experiments were carried out using a transillumination geometry and continuous wave laser radiation, considering cylindrical inclusions in different environments. A comparison between experimental data, theoretical approaches for the radiative transfer equation in the diffusion approximation, and numerical Monte Carlo simulations, is presented.     

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.     

Ultrasound detection through turbid media

Optical coherence-domain reflectometry and laser-based ultrasound detection have been combined with the use of adaptive optics to detect ultrasound through turbid media. The dynamic hologram in a photorefractive quantum-well device performs as a coherence gate that eliminates multiply scattered background. Quadrature homodyne detection conditions are selected by the choice of center wavelength of the pulse spectrum, requiring no active stabilization or feedback. A depth resolution of 30 microm was achieved, with a pulse duration of nominally 120 fs for ultrasound detection through turbid media up to optical thicknesses of 11 mean free scattering lengths.     

Transport-based image reconstruction in turbid media with small source-detector separations

We demonstrate a new method for imaging through several millimeters of a turbid sample with a resolution of approximately 100 mum by combining aspects of confocal reflectance microscopy and diffuse optical tomography. By laterally displacing the pinhole aperture of a confocal microscope we can achieve small source-detector separations and detect minimally scattered light. A reconstruction algorithm based on the first Born approximation to the radiative transport equation is then used to reconstruct an image of a 100-mum absorbing object located 2 mm beneath the surface.     

Zeeman laser-scanning confocal microscopy in turbid media

A novel Zeeman laser-scanning confocal microscope (ZLSCM) is proposed. It has the same configuration as the conventional laser-scanning confocal microscope (LSCM) in which a Zeeman laser in conjunction with a Glan-Thompson analyzer is used. In our system, the analyzer with the bandpass filter, which act simultaneously as a polarization gate and a coherence gate, enhance the collection efficiency of the weak-scattering photons and simultaneously suppress the multiple-scattering photons. The improvement in depth resolution of a ZLSCM in a scattering medium compared with that of a conventional LSCM is discussed and demonstrated experimentally.     

Laser beam widening mechanisms in turbid media

We examine theoretically and experimentally the transverse intensity profile of a laser beam as it traverses through a turbid medium. By increasing the concentrations of milk in an aqueous solution we examine the transition from the weakly scattering to the diffusive regime. The experimental data of the transverse beam profiles for various scattering strengths are obtained in a non-contact geometry from digital images of the exit surface of the medium. The intensity distributions are compared with theoretical data obtained from Monte Carlo simulations.     

Active spectral filtering through turbid media

We demonstrate controlled wavelength-dependent light focusing through turbid media using wavefront shaping. Due to the dispersion caused by multiple light scattering, light propagation through turbid media can be independently controlled between different wavelengths. Foci with various wavelengths can be generated by applying different optimized wavefronts to a highly scattering layer. Given the linearity of the transmission matrix, multiple foci with different wavelengths can also be simultaneously constructed.     

Quantitative spectroscopy of superficial turbid media

We report a novel diffuse optical spectroscopy probe design for determining optical properties of superficial volumes of turbid samples. The fiber-based probe employs a highly scattering layer placed in contact with the sample of interest. This layer diffuses photons from a collimated light source before they enter the sample and provides a basis for describing light transported in superficial media by the diffusion approximation. We compare the performance of this modified two-layer diffusion model with Monte Carlo simulations. A set of experiments that demonstrate the feasibility of this method in turbid tissue phantoms is also presented. Optical properties deduced by this approach are in good agreement with those derived by use of a benchmark method for determining optical properties. The average interrogation depth of the probe design investigated here is estimated to be less than 1 mm.     

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.     

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.     

Noninvasive detection of inhomogeneities in turbid media with time-resolved log-slope analysis

Detecting foreign objects embedded in turbid media using noninvasive optical tomography techniques is of great importance in many practical applications, such as in biomedical imaging and diagnosis, safety inspection on aircrafts and submarines, and LIDAR techniques. In this paper we develop a novel optical tomography approach based on slope analysis of time-resolved back-scattered signals collected at the medium boundaries where the light source is an ultrafast, short-pulse laser. As the optical field induced by the laser-pulse propagates, the detected temporal signals are influenced by the optical properties of the medium traversed. The detected temporal signatures therefore contain information that can indicate the presence of an inhomogeneity as well as its size and location relative to the laser source and detection systems. The log-slope analysis of the time-resolved back-scattered intensity is shown to be an effective method for extracting the information contained in the signal. The technique is validated by experimental results and by Monte Carlo simulations.     

Random walk of polarized light in turbid media

We study the propagation of polarized light in turbid media as a random walk of vector photons. Both propagation and polarization directions of light are found to isotropize, following a power law of the number of scattering events. The characteristic length scale governing light isotropization and linear depolarization, the isotropization length , is derived using the exact Mie scattering for spherical particles. A simple relation is obtained for Rayleigh-Gans scatterers where is the transport mean free path and is the mean cosine of scattering angles.     

Photon-transport forward model for imaging in turbid media

A photon-transport forward model for image reconstruction in turbid media is derived that treats weak inhomogeneities through a Born approximation of the Boltzmann radiative transfer equation. This model can conveniently replace the commonly used diffusion approximation in optical tomography. An analytical expression of the background Green's function is obtained from the cumulant solution of the Boltzmann equation. Our model provides the correct behavior of photon migration at early times and reduces at long times to the center-moved diffusion approximation. Numerical comparisons between this model and the standard and center-moved diffusion models are presented.     

Transmission of polarized light through turbid media

The depolarization of light in multiple-scattering media with large (larger than the light wavelength) inhomogeneities is considered. The polarization state of the scattered light is described in the principal-mode approximation. Using the Fokker-Planck model, the polarization and intensity distribution of light are calculated in the vicinity of an inhomogeneity in the shape of an absorbing half-plane. The results of the calculations agree with the experimental data on transmission of light through turbid media.     

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.