There's plenty of light at the bottom: Statistics of photon penetration depth in random media

We propose a comprehensive statistical approach describing the penetration depth of light in random media. The presented theory exploits the concept of probability density function f(z|ρ, t) for the maximum depth reached by the photons that are eventually re-emitted from the surface of the medium at distance ρ and time t. Analytical formulas for f, for the mean maximum depth 〈zmax〉 and for the mean average depth 〈z〉 reached by the detected photons at the surface of a diffusive slab are derived within the framework of the diffusion approximation to the radiative transfer equation, both in the time domain and the continuous wave domain. Validation of the theory by means of comparisons with Monte Carlo simulations is also presented. The results are of interest for many research fields such as biomedical optics, advanced microscopy and disordered photonics

Study of optimal measurement conditions for time-domain diffuse optics systems

Light is a powerful non-invasive tool that can be exploited to probe highly scattering media like biological tissues for different purposes, from the detection of brain activity to the characterization of cancer lesions. In the last decade, timedomain diffuse optics (TDDO) systems demonstrated improved sensitivity when using time-gated acquisition chains and short source-detector separations (ρ), both theoretically and experimentally. However, the sensitivity to localized absorption changes buried inside a diffusive medium strongly depends on many parameters such as: SDS, laser power, delay and width of the gating window, absorption and scattering properties of the medium, instrument response function (IRF) shape, etc. In particular, relevant effects due to slow tails in the IRF were noticed, with detrimental effects on performances. We present simulated experimental results based on the diffusion approximation of the Radiative Transfer Equation and the perturbation theory subjected to the Born approximation. To quantify the system sensitivity to deep (few cm) and localized absorption perturbations, we exploited contrast and contrast-to-noise ratio (CNR), which are internationally agreed on standardized figures of merit. The purpose of this study is to determine which parameters have the greatest impact on these figures of merit, thus also providing a range of best operative conditions. The study is composed by two main stages: the former is a comparison between simulations and measurements on tissue-mimicking phantom, while the latter is a broad simulation study in which all relevant parameters are tuned to determine optimal measurement conditions. This study essentially demonstrates that under the influence of the slow tails in the IRF, the use of a small SDS no longer corresponds to optimal contrast and CNR. This work sets the ground for future studies with next-generation of TDDO components, presently under development, providing useful hints on relevant features to which one should take care when designing TDDO components

Instrumental, optical and geometrical parameters affecting time-gated diffuse optical measurements: a systematic study

In time-domain diffuse optics the sensitivity to localized absorption changes buried inside a diffusive medium depends strongly on the interplay between instrumental, optical and geometrical parameters, which can hinder the theoretical advantages of novel measurement strategies like the short source-detector distance approach. Here, we present a study based on experimental measurements and simulations to comprehensively evaluate the effect of all different parameters. Results are evaluated exploiting standardized figures of merit, like contrast and contrast-to-noise ratio, to quantify the system sensitivity to deep localized absorption perturbations. Key findings show that the most critical hardware parameter is the memory effect which ultimately limits the dynamic range. Further, a choice of the source-detector distance around 10 mm seems to be a good compromise to compensate non-idealities in practical systems still preserving the advantages of short distances. This work provides both indications for users about the best measurement conditions and strategies, and for technology developers to identify the most crucial hardware features in view of next generation diffuse optics systems

Probe-hosted silicon photomultipliers for time-domain functional near-infrared spectrscopy: phantom and in vivo tests

We report the development of a compact probe for time-domain (TD) functional near-infrared spectroscopy (fNIRS) based on a fast silicon photomultiplier (SiPM) that can be put directly in contact with the sample without the need of optical fibers for light collection. We directly integrated an avalanche signal amplification stage close to the SiPM, thus reducing the size of the detection channel and optimizing the signal immunity to electromagnetic interferences. The whole detection electronics was placed in a plastic screw holder compatible with the electroencephalography standard cap for measurement on brain or with custom probe holders. The SiPM is inserted into a transparent and insulating resin to avoid the direct contact of the scalp with the 100-V bias voltage. The probe was integrated in an instrument for TD fNIRS spectroscopy. The system was characterized on tissue phantoms in terms of temporal resolution, responsivity, linearity, and capability to detect deep absorption changes. Preliminary in vivo tests on adult volunteers were performed to monitor hemodynamic changes in the arm during a cuff occlusion and in the brain cortex during a motor tas

An innovative 8 channels system for time-resolved diffuse optical tomography based on SiPMs

We present the design of a novel 8 channels system for time resolved optical tomography based on Silicon Photomultipliers (SiPMs), therefore knocking down cost and complexity of this technique and paving the way to a widespread diffusion. We validated the system performances on phantoms

Non-invasive optical estimate of tissue composition to differentiate malignant from benign breast lesions: A pilot study

Several techniques are being investigated as a complement to screening mammography, to reduce its false-positive rate, but results are still insufficient to draw conclusions. This initial study explores time domain diffuse optical imaging as an adjunct method to classify non-invasively malignant vs benign breast lesions. We estimated differences in tissue composition (oxy-and deoxyhemoglobin, lipid, water, collagen) and absorption properties between lesion and average healthy tissue in the same breast applying a perturbative approach to optical images collected at 7 red-near infrared wavelengths (635-1060 nm) from subjects bearing breast lesions. The Discrete AdaBoost procedure, a machine-learning algorithm, was then exploited to classify lesions based on optically derived information (either tissue composition or absorption) and risk factors obtained from patient's anamnesis (age, body mass index, familiarity, parity, use of oral contraceptives, and use of Tamoxifen). Collagen content, in particular, turned out to be the most important parameter for discrimination. Based on the initial results of this study the proposed method deserves further investigation

Characterization of homogeneous tissue phantoms for performance tests in diffuse optics

Solid homogeneous turbid phantoms can be employed to mimic the attenuation and angular distribution of light emerging from tissue, e.g., to assess the responsivity of the detection system of diffuse optics instrumentation and to support standardized performance tests of functional near-infrared spectroscopy devices. We present three methods to quantify the wavelength-dependent diffuse transmittance, relying on (1) measurement of radiance exiting the phantom by a detector far from the exit aperture, (2) simple recording of radiance by a power meter close to the exit aperture and correction for the finite distance between phantom surface and detector, (3) determination of the reduced scattering and absorption coefficients by time-resolved diffuse transmittance measurements and forward calculation of the time-integrated diffuse transmittance based on the diffusion model. The implications of the different approximations related to these approaches are discussed. The various methods were applied to characterize solid slab phantoms, and the results were compared. Specifically, for an epoxy-resin based phantom having a thickness of 2 cm, a reduced scattering coefficient of about 0.5/mm and an absorption coefficient of about 0.01/mm, the diffuse transmittance values obtained by the three different methods were found to agree within about 10%

Diffuse optical characterization of collagen absorption from 500 to 1700 nm

Reduction in scattering, high absorption, and spectral features of tissue constituents above 1000 nm could help in gaining higher spatial resolution, penetration depth, and specificity for in vivo studies, opening possibilities of near-infrared diffuse optics in tissue diagnosis. We present the characterization of collagen absorption over a broadband range (500 to 1700 nm) and compare it with spectra presented in the literature. Measurements were performed using a time-domain diffuse optical technique. The spectrum was extracted by carefully accounting for various spectral distortion effects, due to sample and system properties. The contribution of several tissue constituents (water, lipid, collagen, oxy, and deoxy-hemoglobin) to the absorption properties of a collagen-rich in vivo bone location, such as radius distal in the 500-to 1700-nm wavelength region, is also discussed, suggesting bone diagnostics as a potential area of interest

Frequency offset Raman spectroscopy (FORS) for depth probing of diffusive media

We present a new technique, frequency offset Raman spectroscopy (FORS), to probe Raman spectra of diffusive media in depth. The proposed methodology obtains depth sensitivity exploiting changes in optical properties (absorption and scattering) with excitation wavelengths. The approach was demonstrated experimentally on a two-layer tissue phantom and compared with the already consolidated spatially offset Raman spectroscopy (SORS) technique. FORS attains a similar enhancement of signal from deep layers as SORS, namely 2.81 against 2.62, while the combined hybrid FORS-SORS approach leads to a markedly higher 6.0 enhancement. Differences and analogies between FORS and SORS are discussed, suggesting FORS as an additional or complementary approach for probing heterogeneous media such as biological tissues in depth

Forward solvers for photon migration in the presence of highly and totally absorbing objects embedded inside diffusive media

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