https://sam.ensam.eu:443 The DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material. Tue, 21 Apr 2026 14:17:52 GMT 2026-04-21T14:17:52Z http://hdl.handle.net/10985/10053 Influence of the size and skin thickness of apple varieties on the retrieval of internal optical properties using Vis/NIR spectroscopy: A Monte Carlo-based study VAUDELLE, Fabrice; L'HUILLIER, Jean-Pierre Visible/near-infrared spectroscopy is a well-established method to measure optical properties of tissues, assuming that a light propagation model can be used to recover absorption and reduced scattering coefficients from non-invasive probing. Spectroscopic measurements have achieved success in non-destructive assessment of apple optical properties and quality attributes. However, the spectroscopy of apples must consider the size of the fruit and the presence of the thin skin layer that surrounds the flesh, to correctly read the signals acquired on the boundary. In this research, the fruit was modelled as a two layer spherical structure with various radii and finite thickness of the upper skin layer. Monte Carlo computations were performed to generate time-resolved reflectance and spatially-resolved reflectance measurements. Simulated data were then fitted using a procedure based on Levenberg–Marquardt algorithm with specific semi-infinite models. The errors in the retrieved optical properties of the flesh (absorption coefficient μa, and reduced scattering coefficient μ′s) were studied as functions of apple radius, skin thickness, and source–detector distance, for given optical parameter sets assigned to the flesh and the skin. The results suggest that the time-resolved reflectance spectroscopy may probe optical properties of the flesh regardless of the skin layer, when a sufficient source–detector distance (15 mm) is used for the measurements. Similar results were found in case of using the spatially resolved spectroscopy, because measurements extend up to 15–29 mm by steps of 1 mm or 2 mm. The computations also show that the curvature of the boundary has noticeable effect on the errors in the retrieved optical coefficients of the flesh. However, results from time-resolved spectroscopy are more influenced by the size of apples, compared with the spatially-resolved spectroscopy. Thu, 01 Jan 2015 00:00:00 GMT http://hdl.handle.net/10985/10053 2015-01-01T00:00:00Z VAUDELLE, Fabrice L'HUILLIER, Jean-Pierre Visible/near-infrared spectroscopy is a well-established method to measure optical properties of tissues, assuming that a light propagation model can be used to recover absorption and reduced scattering coefficients from non-invasive probing. Spectroscopic measurements have achieved success in non-destructive assessment of apple optical properties and quality attributes. However, the spectroscopy of apples must consider the size of the fruit and the presence of the thin skin layer that surrounds the flesh, to correctly read the signals acquired on the boundary. In this research, the fruit was modelled as a two layer spherical structure with various radii and finite thickness of the upper skin layer. Monte Carlo computations were performed to generate time-resolved reflectance and spatially-resolved reflectance measurements. Simulated data were then fitted using a procedure based on Levenberg–Marquardt algorithm with specific semi-infinite models. The errors in the retrieved optical properties of the flesh (absorption coefficient μa, and reduced scattering coefficient μ′s) were studied as functions of apple radius, skin thickness, and source–detector distance, for given optical parameter sets assigned to the flesh and the skin. The results suggest that the time-resolved reflectance spectroscopy may probe optical properties of the flesh regardless of the skin layer, when a sufficient source–detector distance (15 mm) is used for the measurements. Similar results were found in case of using the spatially resolved spectroscopy, because measurements extend up to 15–29 mm by steps of 1 mm or 2 mm. The computations also show that the curvature of the boundary has noticeable effect on the errors in the retrieved optical coefficients of the flesh. However, results from time-resolved spectroscopy are more influenced by the size of apples, compared with the spatially-resolved spectroscopy. http://hdl.handle.net/10985/17256 Use of steady-state imaging setup for assessing the internal optical properties of non-spherical apple samples ASKOURA, Mohamed Lamine; VAUDELLE, Fabrice; L'HUILLIER, Jean-Pierre The aim of this paper was to retrieve the absorption (µa) and reduced scattering (µ's) coefficients of whole apples which exhibit a complex shape. The effect of the local boundary curvature on the retrieved optical properties was investigated by means of numerical simulations and measurements carried out at the wavelength of 633 nm. A first attempt was made by performing Monte Carlo simulations on an apple-like spheroid model covered with a thin skin layer of thickness 80 µm. Monte Carlo data were then analyzed to depict the changes of photon densities, diffusively reflected images and optical properties as a function of the light source location over the surface of such target. Second, spatially-resolved backscattered images were acquired from 207 ‘Royal Gala’, and the values of µa and µ's were retrieved using an inverse algorithm to fit the scattering profiles with a diffusion theory model, in a selected fitting range of 2.8–10 mm. The results confirm the theoretical prediction and show that the absorption coefficient µa may be overestimated, while the reduced scattering coefficient µ's is slowly changed when the measurements are performed on these apple species. Finally, experiments carried out on 200 apples still show that µ's is negatively correlated to the fruit firmness with a correlation coefficient (r) of 0.63. The spatially-resolved technique provides an efficient means for measuring the optical properties of fruits, and may be also useful for assessing the apple firmness. Tue, 01 Jan 2019 00:00:00 GMT http://hdl.handle.net/10985/17256 2019-01-01T00:00:00Z ASKOURA, Mohamed Lamine VAUDELLE, Fabrice L'HUILLIER, Jean-Pierre The aim of this paper was to retrieve the absorption (µa) and reduced scattering (µ's) coefficients of whole apples which exhibit a complex shape. The effect of the local boundary curvature on the retrieved optical properties was investigated by means of numerical simulations and measurements carried out at the wavelength of 633 nm. A first attempt was made by performing Monte Carlo simulations on an apple-like spheroid model covered with a thin skin layer of thickness 80 µm. Monte Carlo data were then analyzed to depict the changes of photon densities, diffusively reflected images and optical properties as a function of the light source location over the surface of such target. Second, spatially-resolved backscattered images were acquired from 207 ‘Royal Gala’, and the values of µa and µ's were retrieved using an inverse algorithm to fit the scattering profiles with a diffusion theory model, in a selected fitting range of 2.8–10 mm. The results confirm the theoretical prediction and show that the absorption coefficient µa may be overestimated, while the reduced scattering coefficient µ's is slowly changed when the measurements are performed on these apple species. Finally, experiments carried out on 200 apples still show that µ's is negatively correlated to the fruit firmness with a correlation coefficient (r) of 0.63. The spatially-resolved technique provides an efficient means for measuring the optical properties of fruits, and may be also useful for assessing the apple firmness. http://hdl.handle.net/10985/17255 Approximate analytical effective phase function obtained for a thin slab geometry VAUDELLE, Fabrice The reflection and transmission of light from a slab containing a turbid medium provide a scattering effective phase function from which the true optical anisotropy factor may not be always easily retrieved. From the statistical Poissonian theory and thanks to approximations about the optical path related to the first scattering events, a simplified relationship is established between angular phase function and effective phase function. Therefore, with a modified Gegenbauer or a Two-Terms Henyey-Greenstein phase function, some adjustable analytic functions are proposed in order to fit the measurements linked to the true effective phase function. An efficiency of the approximate analytical function is proved, thanks to the light modelling by Monte Carlo method, for optical thickness lower or equal to 2. This is confirmed by comparisons of the anisotropy retrieval with other methods. Concerning applications, several fits were made on experimental effective phase functions corresponding to goniometric measurements from usual diffusing materials and biological tissues. Sun, 01 Jan 2017 00:00:00 GMT http://hdl.handle.net/10985/17255 2017-01-01T00:00:00Z VAUDELLE, Fabrice The reflection and transmission of light from a slab containing a turbid medium provide a scattering effective phase function from which the true optical anisotropy factor may not be always easily retrieved. From the statistical Poissonian theory and thanks to approximations about the optical path related to the first scattering events, a simplified relationship is established between angular phase function and effective phase function. Therefore, with a modified Gegenbauer or a Two-Terms Henyey-Greenstein phase function, some adjustable analytic functions are proposed in order to fit the measurements linked to the true effective phase function. An efficiency of the approximate analytical function is proved, thanks to the light modelling by Monte Carlo method, for optical thickness lower or equal to 2. This is confirmed by comparisons of the anisotropy retrieval with other methods. Concerning applications, several fits were made on experimental effective phase functions corresponding to goniometric measurements from usual diffusing materials and biological tissues. http://hdl.handle.net/10985/17260 Multispectral measurement of scattering-angular light distribution in apple skin and flesh samples ASKOURA, Mohamed Lamine; VAUDELLE, Fabrice; L'HUILLIER, Jean-Pierre Knowledge of the optical properties of apple tissues such as skin and flesh is essential to better understand the light–tissue interaction process and to apply optical methods for apple quality inspection. This work aimed at estimating the anisotropy factor of thin skin and flesh samples extracted from three apple cultivars. The scatter-ing-angular light distribution in each tissue sample was measured at four wavelengths (λ 633, 763, 784, and 852 nm), by means of a goniometer setup. Based on the recorded angular intensity I θ;λ , the effective anisotropy factor geff of each tissue type was first estimated using the mean statistics applied to the random variable cos θ. Next, the measured data were fitted with three predefined and modified phase functions—Henyey-Greenstein (pMHG), Gegenbauer kernel (pMGK), and Mie (pMie)—in order to retrieve the corresponding anisotropy factors gMHG, gMGK, and gM Mie. Typically, the anisotropy factors of skin and flesh amount to 0.6–0.8 in the abovementioned wavelength range. Fri, 01 Jan 2016 00:00:00 GMT http://hdl.handle.net/10985/17260 2016-01-01T00:00:00Z ASKOURA, Mohamed Lamine VAUDELLE, Fabrice L'HUILLIER, Jean-Pierre Knowledge of the optical properties of apple tissues such as skin and flesh is essential to better understand the light–tissue interaction process and to apply optical methods for apple quality inspection. This work aimed at estimating the anisotropy factor of thin skin and flesh samples extracted from three apple cultivars. The scatter-ing-angular light distribution in each tissue sample was measured at four wavelengths (λ 633, 763, 784, and 852 nm), by means of a goniometer setup. Based on the recorded angular intensity I θ;λ , the effective anisotropy factor geff of each tissue type was first estimated using the mean statistics applied to the random variable cos θ. Next, the measured data were fitted with three predefined and modified phase functions—Henyey-Greenstein (pMHG), Gegenbauer kernel (pMGK), and Mie (pMie)—in order to retrieve the corresponding anisotropy factors gMHG, gMGK, and gM Mie. Typically, the anisotropy factors of skin and flesh amount to 0.6–0.8 in the abovementioned wavelength range. http://hdl.handle.net/10985/17262 Experimental Study of Light Propagation in Apple Tissues Using a Multispectral Imaging System ASKOURA, Mohamed Lamine; VAUDELLE, Fabrice; L'HUILLIER, Jean-Pierre This work aimed at high lighting the role played by the skin in the light propagation through the apple flesh. A multispectral Visible-Near Infrared (Vis-NIR) steady-state imaging setup based on the use of four continuous laser sources (633, 763, 784, and 852 nm) and a charge–coupled–device (CCD) camera was developed to record light diffusion inside apple tissues. Backscattering images and light reflectance profiles were studied to reveal optical features of three whole and half-cut apple varieties with and without skin. The optical absorption and scattering properties (µa, µ’s) of intact apples and peeled apples were also retrieved in reflectance mode, using an optimal sensing range of 2.8–10 mm. A relative difference for ∆µa ranging from 3.4% to 24.7% was observed for intact apples with respect to peeled apples. Under the same conditions, no significant changes were noted for ∆µ’s, which ranged from 0.1% to 1.7%. These findings show that the apple skin cannot be ignored when using Vis-NIR optical imaging as a non-destructive sensing means to reveal major quality attributes of fruits. Fri, 01 Jan 2016 00:00:00 GMT http://hdl.handle.net/10985/17262 2016-01-01T00:00:00Z ASKOURA, Mohamed Lamine VAUDELLE, Fabrice L'HUILLIER, Jean-Pierre This work aimed at high lighting the role played by the skin in the light propagation through the apple flesh. A multispectral Visible-Near Infrared (Vis-NIR) steady-state imaging setup based on the use of four continuous laser sources (633, 763, 784, and 852 nm) and a charge–coupled–device (CCD) camera was developed to record light diffusion inside apple tissues. Backscattering images and light reflectance profiles were studied to reveal optical features of three whole and half-cut apple varieties with and without skin. The optical absorption and scattering properties (µa, µ’s) of intact apples and peeled apples were also retrieved in reflectance mode, using an optimal sensing range of 2.8–10 mm. A relative difference for ∆µa ranging from 3.4% to 24.7% was observed for intact apples with respect to peeled apples. Under the same conditions, no significant changes were noted for ∆µ’s, which ranged from 0.1% to 1.7%. These findings show that the apple skin cannot be ignored when using Vis-NIR optical imaging as a non-destructive sensing means to reveal major quality attributes of fruits. http://hdl.handle.net/10985/17264 Light source distribution and scattering phase function influence light transport in diffuse multi-layered media VAUDELLE, Fabrice; L'HUILLIER, Jean-Pierre; ASKOURA, Mohamed Lamine Red and near-Infrared light is often used as a useful diagnostic and imaging probe for highly scattering media such as biological tissues, fruits and vegetables. Part of diffusively reflected light gives interesting information related to the tissue subsurface, whereas light recorded at further distances may probe deeper into the interrogated turbid tissues. However, modelling diffusive events occurring at short source-detector distances requires to consider both the distribution of the light sources and the scattering phase functions. In this report, a modified Monte Carlo model is used to compute light transport in curved and multi-layered tissue samples which are covered with a thin and highly diffusing tissue layer. Different light source distributions (ballistic, diffuse or Lambertian) are tested with specific scattering phase functions (modified or not modified Henyey-Greenstein, Gegenbauer and Mie) to compute the amount of backscattered and transmitted light in apple and human skin structures. Comparisons between simulation results and experiments carried out with a multi-spectral imaging setup confirm the soundness of the theoretical strategy and may explain the role of the skin on light transport in whole and half-cut apples. Other computational results show that a Lambertian source distribution combined with a Henyey-Greenstein phase function provides a higher photon density in the stratum corneum than in the upper dermis layer. Furthermore, it is also shown that the scattering phase function may affect the shape and the magnitude of the Bidirectional Reflectance Distribution (BRDF) exhibited at the skin surface. Sun, 01 Jan 2017 00:00:00 GMT http://hdl.handle.net/10985/17264 2017-01-01T00:00:00Z VAUDELLE, Fabrice L'HUILLIER, Jean-Pierre ASKOURA, Mohamed Lamine Red and near-Infrared light is often used as a useful diagnostic and imaging probe for highly scattering media such as biological tissues, fruits and vegetables. Part of diffusively reflected light gives interesting information related to the tissue subsurface, whereas light recorded at further distances may probe deeper into the interrogated turbid tissues. However, modelling diffusive events occurring at short source-detector distances requires to consider both the distribution of the light sources and the scattering phase functions. In this report, a modified Monte Carlo model is used to compute light transport in curved and multi-layered tissue samples which are covered with a thin and highly diffusing tissue layer. Different light source distributions (ballistic, diffuse or Lambertian) are tested with specific scattering phase functions (modified or not modified Henyey-Greenstein, Gegenbauer and Mie) to compute the amount of backscattered and transmitted light in apple and human skin structures. Comparisons between simulation results and experiments carried out with a multi-spectral imaging setup confirm the soundness of the theoretical strategy and may explain the role of the skin on light transport in whole and half-cut apples. Other computational results show that a Lambertian source distribution combined with a Henyey-Greenstein phase function provides a higher photon density in the stratum corneum than in the upper dermis layer. Furthermore, it is also shown that the scattering phase function may affect the shape and the magnitude of the Bidirectional Reflectance Distribution (BRDF) exhibited at the skin surface. http://hdl.handle.net/10985/17261 Numerical Study of Light Transport in Apple Models Based on Monte Carlo Simulations ASKOURA, Mohamed Lamine; VAUDELLE, Fabrice; L'HUILLIER, Jean-Pierre This paper reports on the quantification of light transport in apple models using Monte Carlo simulations. To this end, apple was modeled as a two-layer spherical model including skin and flesh bulk tissues. The optical properties of both tissue types used to generate Monte Carlo data were collected from the literature, and selected to cover a range of values related to three apple varieties. Two different imaging-tissue setups were simulated in order to show the role of the skin on steady-state backscattering images, spatially-resolved reflectance profiles, and assessment of flesh optical properties using an inverse nonlinear least squares fitting algorithm. Simulation results suggest that apple skin cannot be ignored when a Visible/Near-Infrared (Vis/NIR) steady-state imagingsetupisusedforinvestigatingqualityattributesofapples. Theyalsohelptoimproveoptical inspection techniques in the horticultural products. Thu, 01 Jan 2015 00:00:00 GMT http://hdl.handle.net/10985/17261 2015-01-01T00:00:00Z ASKOURA, Mohamed Lamine VAUDELLE, Fabrice L'HUILLIER, Jean-Pierre This paper reports on the quantification of light transport in apple models using Monte Carlo simulations. To this end, apple was modeled as a two-layer spherical model including skin and flesh bulk tissues. The optical properties of both tissue types used to generate Monte Carlo data were collected from the literature, and selected to cover a range of values related to three apple varieties. Two different imaging-tissue setups were simulated in order to show the role of the skin on steady-state backscattering images, spatially-resolved reflectance profiles, and assessment of flesh optical properties using an inverse nonlinear least squares fitting algorithm. Simulation results suggest that apple skin cannot be ignored when a Visible/Near-Infrared (Vis/NIR) steady-state imagingsetupisusedforinvestigatingqualityattributesofapples. Theyalsohelptoimproveoptical inspection techniques in the horticultural products. http://hdl.handle.net/10985/17263 Light scattering in thin turbid tissue including macroscopic porosities: A study based on a Monte Carlo model VAUDELLE, Fabrice A Monte Carlo code is built taking into account macroscopic spheroid cavities inside a turbid medium, i.e. in mixing Multi-Layer Monte Carlo (MLMC) and Monte Carlo Ray Tracing (MCRT). That simulates a tissue with a strong and heterogeneous porosity, such as flesh tissues of fruit or bone tissues. This kind of tissue, which has two scales of porosity (microscopic and macroscopic), differs notably of the homogeneous and continuous model used in the usual radiative transfer equation. The influence of the presence of spheroids can be observed on the shape of the effective phase function, on the effect related to the time-resolved diffusion solution or also on the scattering coefficient retrieved by means of the Beer–Lambert relationship. For instance, the reduced scattering coefficients retrieved thanks to time-resolved transmittance from MLMC-MCRT models having a lot of intertwined large cavities show variations coherent with those retrieved from bone tissue. Furthermore, the effect of porosity on optical transmission seems to have a real impact when relative refractive index is close to 1. In this case, the equivalence problem between such porous MLMC-MCRT model and a homogeneous turbid medium, can be discussed at the level of the angular intensity distribution over the plane boundaries. This requires to fit this angular distribution by an Adding-Doubling model using optimized optical depth and scattering phase function. Experimental scattering phase functions obtained from apple tissues are considered in order to test this idea, and then compared with those computed with a MLMC-MCRT model. Mon, 01 Jan 2018 00:00:00 GMT http://hdl.handle.net/10985/17263 2018-01-01T00:00:00Z VAUDELLE, Fabrice A Monte Carlo code is built taking into account macroscopic spheroid cavities inside a turbid medium, i.e. in mixing Multi-Layer Monte Carlo (MLMC) and Monte Carlo Ray Tracing (MCRT). That simulates a tissue with a strong and heterogeneous porosity, such as flesh tissues of fruit or bone tissues. This kind of tissue, which has two scales of porosity (microscopic and macroscopic), differs notably of the homogeneous and continuous model used in the usual radiative transfer equation. The influence of the presence of spheroids can be observed on the shape of the effective phase function, on the effect related to the time-resolved diffusion solution or also on the scattering coefficient retrieved by means of the Beer–Lambert relationship. For instance, the reduced scattering coefficients retrieved thanks to time-resolved transmittance from MLMC-MCRT models having a lot of intertwined large cavities show variations coherent with those retrieved from bone tissue. Furthermore, the effect of porosity on optical transmission seems to have a real impact when relative refractive index is close to 1. In this case, the equivalence problem between such porous MLMC-MCRT model and a homogeneous turbid medium, can be discussed at the level of the angular intensity distribution over the plane boundaries. This requires to fit this angular distribution by an Adding-Doubling model using optimized optical depth and scattering phase function. Experimental scattering phase functions obtained from apple tissues are considered in order to test this idea, and then compared with those computed with a MLMC-MCRT model. http://hdl.handle.net/10985/10055 Assessment of tissue optical parameters in a spherical geometry using three different optical spectroscopy methods: comparison based on a theoretical approach VAUDELLE, Fabrice; ASKOURA, Mohamed Lamine; L'HUILLIER, Jean-Pierre The non-invasive research of information inside the biological tissues can be made by means of continuous, time dependent or frequency modulated light source, emitting in the visible or infrared range. Moreover, the biological structures such as brain, breast or fruits, can be seen as closer to a spherical shape than a slab. This paper focus on the retrieval of tissue optical parameters in a spherical geometry using fittings with an analytical solution adapted for semi infinite geometry. The data were generated using three different optical spetroscopy methods: frequency-resolved, spatially-resolved, and time-resolved. Simulations based on a Monte Carlo code were performed on a homogeneous sphere, with 18 spaced detectors located at the periphery. First, data are examinated in the frequency domain, then, they are treated with optimization algorithms to assess the optical coefficients. The computations show that the spatially resolved measurements are often more robust than those obtained by the frequency-resolved mode. In the temporal domain, errors on the estimates are also exhibited with the fitting by the Fourier transform of a solution based on the semi-infinite geometry. Furthermore, when the analytical solution is modified to take into account the sphere geometry, the retrieval of the coefficients is improved. Thu, 01 Jan 2015 00:00:00 GMT http://hdl.handle.net/10985/10055 2015-01-01T00:00:00Z VAUDELLE, Fabrice ASKOURA, Mohamed Lamine L'HUILLIER, Jean-Pierre The non-invasive research of information inside the biological tissues can be made by means of continuous, time dependent or frequency modulated light source, emitting in the visible or infrared range. Moreover, the biological structures such as brain, breast or fruits, can be seen as closer to a spherical shape than a slab. This paper focus on the retrieval of tissue optical parameters in a spherical geometry using fittings with an analytical solution adapted for semi infinite geometry. The data were generated using three different optical spetroscopy methods: frequency-resolved, spatially-resolved, and time-resolved. Simulations based on a Monte Carlo code were performed on a homogeneous sphere, with 18 spaced detectors located at the periphery. First, data are examinated in the frequency domain, then, they are treated with optimization algorithms to assess the optical coefficients. The computations show that the spatially resolved measurements are often more robust than those obtained by the frequency-resolved mode. In the temporal domain, errors on the estimates are also exhibited with the fitting by the Fourier transform of a solution based on the semi-infinite geometry. Furthermore, when the analytical solution is modified to take into account the sphere geometry, the retrieval of the coefficients is improved. http://hdl.handle.net/10985/8511 Time-resolved optical fluorescence spectroscopy of heterogeneous turbid media with special emphasis on brain tissue structures including diseased regions: A sensitivity analysis VAUDELLE, Fabrice; L'HUILLIER, Jean-Pierre Fluorescence-enhanced optical imaging based on near-infrared light provides a promising tool to differentiate diseased lesions from normal tissue. However, the measurement sensitivity of the fluorescence signals acquired at the output surface of the tissue is greatly influenced by the tissue structure, the optical properties, the location and the size of the target. In this paper, we present a numerical model based on the Monte Carlo method that allows to simulate time-resolved reflectance signals acquired on the surface of the scalp of a human head model bearing a fluorescent diseased region (tumor, glioma). The influence of tumor depth, tumor size and tumor shape evolution on the computed signals are analyzed by taking into account the multi-layered tissue structure. The simulations show that the mean-time-of-flight and the difference between two mean-times acquired at two source–detector distances are both relevant to this problem type. Furthermore, the simulations suggest that the use of the difference between mean-flight-times may be interesting to probe scattering changes that occur in the cerebrospinal fluid (CSF). Tue, 01 Jan 2013 00:00:00 GMT http://hdl.handle.net/10985/8511 2013-01-01T00:00:00Z VAUDELLE, Fabrice L'HUILLIER, Jean-Pierre Fluorescence-enhanced optical imaging based on near-infrared light provides a promising tool to differentiate diseased lesions from normal tissue. However, the measurement sensitivity of the fluorescence signals acquired at the output surface of the tissue is greatly influenced by the tissue structure, the optical properties, the location and the size of the target. In this paper, we present a numerical model based on the Monte Carlo method that allows to simulate time-resolved reflectance signals acquired on the surface of the scalp of a human head model bearing a fluorescent diseased region (tumor, glioma). The influence of tumor depth, tumor size and tumor shape evolution on the computed signals are analyzed by taking into account the multi-layered tissue structure. The simulations show that the mean-time-of-flight and the difference between two mean-times acquired at two source–detector distances are both relevant to this problem type. Furthermore, the simulations suggest that the use of the difference between mean-flight-times may be interesting to probe scattering changes that occur in the cerebrospinal fluid (CSF).