The interaction of light with biological materials, such as fruits and vegetables, is a complex process which involves both absorption, and scattering events at different scales. Measuring the optical properties of a fruit allows understanding the physical and chemical characteristics. In this paper, an optical bench based on the use of a continuous laser source and a CCD camera was developed to study the light diffusion inside apple tissue structures. The method refers to the well-known steady-state spatially resolved method. First, the optoelectronics system was tested with a tissue phantom in order to show the optimal sensing range required to obtain the best estimated optical properties. Second, experimental results were obtained using peeled and unpeeled apples as interrogated tissues. The data were confronted with a diffusion model in order to extract the optical properties at two wavelengths of 633, and 852 nm. To better understand the effect of the apple tissue structures, investigations into the propagation of light through a half cut apple were also performed.
The knowledge of the light fluence rate distribution inside a biological tissue irradiated by a Laser (or LED) is fundamental to achieve medical treatments. In this paper, we present a semi-analytical model based on the 2-D Fourier Transform of the diffusion equation. This method can be applied to any irradiation source (cylindrically symmetric or not) at the surface of the tissue. Two particular beam shapes are studied: planar irradiation and flat beam with finite radius. The total fluence rate along the depth in tissues was computed by adding the collimated and the diffuse components. The analytical solution was also used to study the effect of the beam radius on the light attenuation. Measurements were performed using a tank filled with a liquid-simulating medium (Milk), illuminated with a LED array (660 nm, 100mm×100mm). Several circular diaphragms were used to obtain uniform circular beams with well defined radii. An optical fibre (with an isotropic tip) was used to measure the fluence rate inside the medium. Preliminary experimental results are in agreement with theoretical predictions.
Optical imaging of turbid media is a challenging problem mainly due to the scattering process that reduces image
contrast and degrades spatial resolution. The development of fluorescent probes has recently improved the noninvasive
optical technique. In this paper, we are interested in the time gating fluorescence signals. The diffusion approximation is
used in order to describe the light propagation of a laser pulse in a turbid media that mimics breast like biological tissue.
A numerical model based on a finite element method is proposed. Fluorescence time dependent numerical simulations
are performed in order to compute time-gated intensities resulting from line scans across partially absorbing and
scattering slab configurations. Optical properties of embedded objects are chosen to be the same as optical properties of
breast tumor. Tacking into account two hidden objects, we investigate the lateral resolution aimed by fluorescence
modality, and we also compared the results to thus obtained by photon propagation. Different widths of the time gate are
computed and it is demonstrated that both lateral localization of one inclusion, and resolution of two inclusions, are
enhanced when the time-gate width (▵t) is decreased. The overall computations confirm that fluorescent time-gating
technique is very sensitive to local variations in optical properties that are due to breast-like tumors in turbid media.
Optical methods allow investigating biological tissue noninvasively without ionizing radiations. Moreover, considering
low absorption processes in the tissue in the near-infrared wavelengths range, biological tissue can be deeply
investigated. In this field, we studied the resolution limits of the detection of one and two tumour-like heterogeneities
embedded in the middle plane of a slab that mimics a breast enclosed between two transparent plates. We used the
diffusion equation in order to model the photons propagation in such slab. It is solved in the time-domain by means of a
finite element method. We computed time-resolved trans-illumination data based on lateral scan of the slab. The timedependent
transmitted light, received at the opposite of the source, was transformed in the frequency-domain and the
modulation and phase-shift of the signal are then obtained. The resulting phase-shift considering the embedded objects
was analyzed versus the distance between the objects. Then, the resolution limits were estimated considering different
modulation frequencies and a noise level. The overall combinations took into account a set of optical properties that
mimics realistic optical properties for healthy breast tissue and tumours.
In the framework of the depth detection of tumor using the diffusion equation, a finite element method is proposed in
order to solve the time-dependent light propagation in highly scattering media. A tumor-like object is positioned in the
media. The finite element method tacks into account Robin type air-tissue boundary conditions. This study is devoted to
the depth localization of a tumor enclosed into a breast tissue-like slab. Cartesian coordinates are used in order to solve
the time-dependent diffusion approximation. A short laser pulse of 1ps is considered. The transillumination technique is
able to laterally detect the objects when the source and detector are moved together on the same axis. In order to perform
the depth localization of the inclusion, we were interested in a non-coaxial transillumination technique conjugated to
interesting contrast functions based on the mean time of flight of photons. These functions allow to localize axially the
inclusion using the high scattering processes. Thus, we performed first results of a depth indicator of a tumor. We now
perform a parametric study. The optical properties of the slab are varying. Furthermore, different sizes of the objects are
tested. Thus, the influence and the variation of these parameters on the depth indicator are shown. Our study
demonstrates the possibility to deeply localize a tumor enclosed in a breast tissue using the high scattering processes
induced by a tumor. To enhance the scattering processes, an interesting way is then to use recent nanoparticles allowing
to modify the scattering coefficient.
KEYWORDS: Absorption, Reflectivity, Diffusion, Monte Carlo methods, Finite element methods, Data modeling, Skin, Tissues, Scattering, Near infrared spectroscopy
This work presents results on the modeling of the photon diffusion in a three-layered model, (skin, fat and muscle). The
Finite Element method was performed in order to calculate the temporal response of the above-mentioned structure. The
thickness of the fat layer was varied from 1 to 15 mm to investigate the effects of increasing fat thickness on the muscle
layer absorption coefficient measurements for a source-detector spacing of 30 mm.
The simulated time-resolved reflectance data, at different wavelengths, were fitted to the diffusion model to yield the
scattering and absorption coefficients of muscle. The errors in estimating muscle absorption coefficients &mgr;α depend on
the thickness of the fat layer and its optical properties. In addition, it was shown that it is possible to recover with a good
precision (~2.6 % of error) the absorption coefficient of muscle and this up to a thickness of the fat layer not exceeding
4mm. Beyond this limit a correction is proposed in order to make measurements coherent. The muscle-corrected
absorption coefficient can be then used to calculate hemoglobin oxygenation.
In the last few years, the propagation of diffuse photons in scattering media has become an important field of interest. This is mainly due to the possibility offered by the low absorption of light in the range 700 to 900nm. Indeed, this property leads to a potential deep penetration. But a non negligible limitation appears: the scattering processes strongly reduce both the contrast and the resolution. In this paper, the time-dependent light propagation in highly scattering media containing an inclusion is solved by means of a finite element method, tacking into account Robin type air-tissue boundary conditions. This study is devoted to the depth localization of a tumor enclosed into a breast-like slab. The tissue is modeled by a rectangular meshed domain that mimics a breast compressed between two transparent plates. Cartesian coordinates are used in order to solve the time-dependent diffusion approximation. A short laser pulse of 1ps is considered. The transillumination technique is able to laterally detect the object when the source and detector are moved together on the same axis. In order to perform the localization of the inclusion in this study, the optical properties of the object were varied. Knowing the lateral position of the inclusion, we derive interesting temporal contrast functions based on the mean time of flight of photons. These functions allow to localize in depth the inclusion under the assumption that the object is very diffusing. To conclude, our study demonstrates the possibility to detect laterally and axially a tumor-like inclusion enclosed in breast-like tissues.
The potentialities offered by time-gated transillumination of inhomogeneous tissue slab media are numerically investigated in this paper. A finite element model is firstly presented in order to solve the time-dependent light transport in mutiple-scattering optical media containing different embedded objects. The numerical procedure is based on the Galerkin formulation added to an implicite finite difference scheme (Backward Euler) to integrate the resulting matrix formulation with respect to time. The meshed domain refers to Cartesian-coordinates system (x,y) such that the computational grid can be adapted to scan along the longitudinal coordinate x. It takes into account the boundary conditions relative to air-tissue interfaces. The use of the method is demonstrated by the forward computations of time-gated intensities, resulting from line scans across either partially absorbing or scattering cylindrical objects. The overall computations confirm that time-gating technique is very sensitive to local variations in optical properties that are due to hidden objects in turbid media. It is also shown that the lateral localization of these inclusions is enhanced when the time-gate width (Δt) is decreased to about 30 ps.
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