Biomedical subjects

Fabrizio Martelli

Publications and source records attributed to Fabrizio Martelli.

10 recordsLinked to original sources

PubMed2004

Phantom validation and in vivo application of an inversion procedure for retrieving the optical properties of diffusive layered media from time-resolved reflectance measurements.

An experimental validation of an inversion procedure for retrieving the optical properties of layered media from multidistance time-resolved reflectance measurements is presented. The results cover a wide range of optical properties, showing excellent effectiveness and reliability of the procedure in reconstructing the optical properties of a two-layered medium. The optical properties of the first layer and the absorption of the second layer could be retrieved with excellent precision, whereas the reduced scattering coefficient of the second layer was reconstructed with a large error. The first layer thickness could be retrieved with an error less than 50%. An example of retrieval of the optical properties of muscle from in vivo measurements during an arterial occlusion is shown.

Algorithms↗

PubMed2004

Effect of the refractive index mismatch on light propagation through diffusive layered media.

The effect of the refractive index mismatch on light propagation through diffusive layers has been investigated. The refractive index mismatch changes the balance of energy inside the medium determining a temporal and spatial redistribution of light. Light penetration through the medium is obstructed (facilitated) by a negative (positive) refractive index step variation. An analytical solution of the time-dependent diffusion equation that accounts for this effect has been obtained. The solution has been validated by comparisons with the results of Monte Carlo simulations. An excellent description of light propagation is given even for a high refractive index mismatch.

Animals↗

PubMed2003

Procedure for retrieving the optical properties of a two-layered medium from time-resolved reflectance measurements.

A procedure for retrieving the optical properties of a two-layered diffusive medium based on an exact analytical solution of the diffusion equation and on relative multidistance time-resolved reflectance measurements is presented. The method overcomes some limitations of previously developed procedures. Five parameters of the medium have been fitted: the absorption and the reduced scattering coefficients of both layers and the thickness of the first layer. The actual values of the parameters are correctly retrieved by the procedure. The inversion procedure does not require an initial guess for the unknown optical properties, but the starting value for the thickness of the first layer needs to be estimated with an error smaller than 50%.

Journal Article↗

PubMed2003

Measurements of optical properties of high-density media.

Measurements of optical properties (scattering coefficient mu(s), absorption coefficient mu(a), reduced scattering coefficient mu(s)', and asymmetry factor g) have been carried out up to a volume particle concentration of rho = 0.227. The results for mu(s) and mu(s)' show significant deviations from the linear dependence on rho as expected when the independent scattering assumption is fulfilled. The asymmetry factor also changed significantly. In contrast, the dependence of mu(a) remained linear even at the largest concentration investigated. The simple linear dependence of absorption on the chromophore concentration expected from the independent scattering assumption is thus applicable also to spectroscopic measurements of dense media. A comparison with an approximate theoretical model based on the Foldy-Twersky equation is also reported. The model provides a good description of the dependence of mu(s) on particle concentration.

Computer Simulation↗

PubMed2003

Solution of the time-dependent diffusion equation for layered diffusive media by the eigenfunction method.

An exact solution of the time-dependent diffusion equation for the case of a two- and a three-layered finite diffusive medium is proposed. The method is based on the decomposition of the fluence rate in a series of eigenfunctions and upon the solution of the consequent transcendental equation for the eigenvalues obtained from the boundary conditions. Comparisons among the solution of the diffusion equation and the results of Monte Carlo simulations show the correctness of the proposed model.

Journal Article↗

PubMed2003

Assessment of an in situ temporal calibration method for time-resolved optical tomography.

A 32-channel time-resolved optical imaging device is developed at University College London to produce functional images of the neonatal brain and the female breast. Reconstruction of images using time-resolved measurements of transmitted light requires careful calibration of the temporal characteristics of the measurement system. Since they can often vary over a period of time, it is desirable to evaluate these characteristics immediately after, or prior to, the acquisition of image data. A calibration technique is investigated that is based on the measurement of light back-reflected from the surface of the object being imaged. This is facilitated by coupling each detector channel with an individual source fiber. A Monte Carlo model is employed to investigate the influence of the optical properties of the object on the back-reflected signal. The results of simulations indicate that their influence may be small enough to be ignored in some cases, or could be largely accounted for by a small adjustment to the calibrated data. The effectiveness of the method is briefly demonstrated by imaging a solid object with tissue-equivalent optical properties.

Brain↗

PubMed2002

Penetration depth of light re-emitted by a diffusive medium: theoretical and experimental investigation.

The depth at which photons penetrate into a diffusive medium before being re-emitted has been investigated with reference to a semi-infinite homogeneous medium illuminated by a pencil beam. By using the diffusion equation analytical expressions have been obtained for the probability that photons penetrate at a certain depth before being detected, and for the mean path length they travel inside each layer of the medium. Expressions have been obtained both for the cw and the time domain, and simple approximate scaling relationships describing the dependence on the scattering properties of the medium have been found. For time-resolved measurements both the probability and the mean path length are expected to be independent of the distance from the light beam at which the detector is placed and of the absorption coefficient of the medium. The penetration depth increases as the time of flight increases. In contrast, for cw measurements both the probability and the mean path length strongly depend on the distance and absorption. The penetration depth increases as the distance increases or absorption decreases. The accuracy of the analytical expressions has been demonstrated by comparisons with cw experimental results. The penetration depth and the mean path length provide useful information, for instance, for measurements of tissue oxygenation and for functional imaging of muscle and brain. In particular, the depth reached by received photons provides overall information on the volume of the tissue actually investigated, while the mean path is strictly related to the sensitivity to local variations of absorption.

Absorption↗

PubMed2002

Method to measure the optical properties of small volumes of diffusive media.

The method consists of measuring the perturbation provoked by a small volume of the diffusive medium on light propagating through a medium of known optical properties. The absorption and the reduced scattering coefficients of the medium are retrieved from multidistance continuous-wave measurements of transmittance. The inversion procedure is based on the solution of the diffusion equation obtained with a perturbative approach. The method has been validated with Monte Carlo results. Examples of experimental results are reported.

Diffusion↗

PubMed2002

Analytical approximate solutions of the time-domain diffusion equation in layered slabs.

Time-domain analytical solutions of the diffusion equation for photon migration through highly scattering two- and three-layered slabs have been obtained. The effect of the refractive-index mismatch with the external medium is taken into account, and approximate boundary conditions at the interface between the diffusive layers have been considered. A Monte Carlo code for photon migration through a layered slab has also been developed. Comparisons with the results of Monte Carlo simulations showed that the analytical solutions correctly describe the mean path length followed by photons inside each diffusive layer and the shape of the temporal profile of received photons, while discrepancies are observed for the continuous-wave reflectance or transmittance.

Diagnostic Imaging↗

PubMedDate not supplied

ARS Component B: structural characterization, tissue expression and regulation of the gene and protein (SLURP-1) associated with Mal de Meleda.

The ARS Component B gene (EMBL ID: HSARS81S, AC: X99977) encodes a 9 kD non-glycosylated polypeptide (also known as SLURP-1, SwissProt/TrEMBL: P55000), a soluble member of the human Ly6/uPAR superfamily. ARS Component B gene mutations have been implicated in Mal de Meleda. In this study we show by immunohistochemistry that SLURP-1 (secreted Ly-6/uPAR related protein, the protein product of the ARS Component B gene) is localized to human skin, exocervix, gums, stomach and esophagus. In the epidermis, keratinocytes underlying the stratum corneum are highly positive for SLURP1 immunostaining and cultured keratinocytes secrete the expected 9 kD protein. Circulating SLURP1 is detected in human plasma and urine. In the mouse, expression is evident in skin, eye, whole lung, trachea, esophagus and stomach. Human ARS Component B mRNA expression is regulated by retinoic acid, epidermal growth factor and interferon-gamma. The tissue localization and the association with Mal de Meleda suggest that ARS Component B and its protein product SLURP1 are implicated in maintaining the physiological and structural integrity of the keratinocyte layers of the skin.

Amino Acid Sequence↗