Significance: Interstitial fiber-based spectroscopy is gaining interest for real-time in vivo optical biopsies, endoscopic interventions, and local monitoring of therapy. Different from other photonics approaches, time-domain diffuse optical spectroscopy (TD-DOS) can probe the tissue at a few cm distance from the fiber tip and disentangle absorption from the scattering properties. Nevertheless, the signal detected at a short distance from the source is strongly dominated by the photons arriving early at the detector, thus hampering the possibility of resolving late photons, which are rich in information about depth and absorption. Aim: To fully benefit from the null-distance approach, a detector with an extremely high dynamic range is required to effectively collect the late photons; the goal of our paper is to test its feasibility to perform TD-DOS measurements at null source-detector separations (NSDS). Approach: In particular, we demonstrate the use of a superconducting nanowire single photon detector (SNSPD) to perform TD-DOS at almost NSDS ð?150 ?mÞ by exploiting the high dynamic range and temporal resolution of the SNSPD to extract late arriving, deep-traveling photons from the burst of early photons. Results: This approach was demonstrated both on Monte Carlo simulations and on phantom measurements, achieving an accuracy in the retrieval of the water spectrum of better than 15%, spanning almost two decades of absorption change in the 700- to 1100-nm range. Additionally, we show that, for interstitial measurements at null source-detector distance, the scattering coefficient has a negligible effect on late photons, easing the retrieval of the absorption coefficient. Conclusions: Utilizing the SNSPD, broadband TD-DOS measurements were performed to successfully retrieve the absorption spectra of the liquid phantoms. Although the SNSPD has certain drawbacks for use in a clinical system, it is an emerging field with research progressing rapidly, and this makes the SNSPD a viable option and a good solution for future research in needle guided time-domain interstitial fiber spectroscopy.

This letter deals with the plane wave diffraction by a composite structure consisting of metallic and lossless dielectric 90 degrees blocks, which are adjacent and share the edge. Such a problem is first tackled in the frequency domain, and the related diffraction coefficients are determined by exploiting the uniform asymptotic physical optics approach. The inverse Laplace transform is applied to obtain the time-domain counterparts, which can be used to compute the transient diffracted field due to an arbitrary function plane wave.

A periodic monitoring of the adipose tissue functions due to interventions, such as calorie restriction and bariatric surgery, or pathophysiological processes, has an increasing relevance in clinical diagnostics. Diffuse Optical Spectroscopy (DOS) is a valuable non-invasive tool that can be used in that direction. In this work, we present a pilot study based on Time Domain Broadband Diffuse Optical Spectroscopy (TD DOS) to characterize in vivo the subcutaneous fat tissue in the abdominal region. A first of its kind, portable TD DOS instrumentation, already enrolled in clinical studies, was used. Three healthy male volunteers were considered. Three source-detector separation distances (1, 2, and 3 cm) were used over the broad wavelength range of 600-1100 nm. The analysis was performed using a method based on a heterogeneous model to account for the multi-layered nature of the subcutaneous adipose tissue, and to obtain the optical properties specific to this fat localization. Inter-subject variation of tissue composition data was observed.

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 task. (C) The Authors.

Closed form solutions are proposed for evaluating the time domain diffraction coefficients to be used when plane waves are orthogonally incident on obtuse-angled penetrable wedges. They are obtained by performing the inverse Laplace transform of the corresponding frequency domain solutions related to the internal region of the wedge and the surrounding space. The knowledge of the analytical expressions of the diffraction coefficients allows one to evaluate the response to an arbitrary incident transient field via a convolution integral. Numerical tests and comparisons with finite-difference time-domain results confirm the effectiveness of the proposed solutions. © 2013 Copyright Taylor and Francis Group, LLC.

The performance of a pontoon-type Very Large Floating Structure (VLFS) has been investigated with the aim of using the VLFS as a floating airport. The beam sea condition is assumed as being relevant for floating runways installed parallel to the coast line. Such type of platforms has a great horizontal extension and the sea floor topography can vary notably along the length and width of the structure. Therefore effects of variable bottom on the airport motion are investigated numerically in the present work. A Boundary Element Method (BEM) for the fluid domain and the thin plate theory for the structural motion are applied in the numerical simulation. The seakeeping problem of a pontoon-type VLFS model as a rigid body is simulated in time domain in a two dimensional numerical wave tank (NWT) with and without uneven bottom effects and by retaining the problem nonlinearities. The hydroelastic analysis of a pontoon-type VLFS is performed by assuming more general 3D conditions. The nonlinear effects are neglected and the problem is solved in frequency domain. Present results are compared with experimental data and other numerical solutions.

The authors present a new two-dimensional algorithm for interferometric synthetic aperture radar phase unwrapping based on the finite element method. The proposed technique allows an efficient least-squares solution carried out in the time domain. Weighting functions can be introduced without increasing the computational requirements. The possibility to speed up the computations via a multigrid technique is discussed. A noise-robust extension of the proposed technique is also investigated. The presented experiments on real ERS-I data, as well as on simulated patterns, demonstrate the precision and efficiency of the procedure