Rui Guerra

Tissue phantom for optical diagnostics based on a suspension of microspheres with a fractal size distribution

We demonstrate experimentally the possibility of reproducing the phase function, absorption, and scattering coefficients of a real biological tissue (adult brain white matter and liver) using a suspension of polystyrene microspheres with a fractal size distribution. The design of a light scattering goniometer with a cylindrical cell in air is discussed, and phase function measurements using the device are described. The scattering coefficient is measured using transmission spectrophotometry and the absorption and reduced scattering coefficients are measured using a time-resolved method. A good match between real tissue and phantom parameters is demonstrated.

Brownian motion of a dust particle in a plasma

A new version of the Brownian motion describing the motion of a dust particle in a turbulent plasma is considered. Here, the stochastic force acting on the dust particle is due to the fluctuations of the plasma potential and not due to the usual molecular collisions. Another significant difference is due to the fact that the dust electric charge is not constant but fluctuates with the potential. A four-dimensional formulation of the problem is also given.A new version of the Brownian motion describing the motion of a dust particle in a turbulent plasma is considered. Here, the stochastic force acting on the dust particle is due to the fluctuations of the plasma potential and not due to the usual molecular collisions. Another significant difference is due to the fact that the dust electric charge is not constant but fluctuates with the potential. A four-dimensional formulation of the problem is also given.

Flashing LEDs for Microalgal Production

Flashing lights are next-generation tools to mitigate light attenuation and increase the photosynthetic efficiency of microalgal cultivation systems illuminated by light-emitting diodes (LEDs). Optimal flashing light conditions depend on the reaction kinetics and properties of the linear electron transfer chain, energy dissipation, and storage mechanisms of a phototroph. In particular, extremely short and intense light flashes potentially mitigate light attenuation in photobioreactors without impairing photosynthesis. Intelligently controlling flashing light units and selecting electronic components can maximize light emission and energy efficiency. We discuss the biological, physical, and technical properties of flashing lights for algal production. We combine recent findings about photosynthetic pathways, self-shading in photobioreactors, and developments in solid-state technology towards the biotechnological application of LEDs to microalgal production.

Simultaneous determination of the mean and standard deviation of quasi-monodisperse size distributions of microspheres by static light scattering

The objective of this work is to quantify the accuracy and precision of particle sizing performed through light scattering goniometry. A rigorous characterization of this simple method depends on a complete description of the scattering geometry, and to our knowledge this has not yet been done. We have determined the central diameter (d0) and standard deviation (?) of samples of polystyrene spheres with a narrow normal distribution of diameters through a ?2 fit to the light scattering phase function. The fit is performed simultaneously in the variables d0 and ?. The model for the fit includes Mie scattering calculations, integration of the phase function over a normal distribution of diameters and a complete description of the scattering geometry. Experimental tests on spheres with diameters between 2 and 7 ?m show that this method provides excellent accuracy and precision for the determination of d0 and good accuracy for the determination of ?. The precision in ? is poor in relative terms but in absolute terms it is around 0.05 ?m within the range of sizes tested. We calculate the uncertainty limits for the determination of d0 and ? and show that they are consistent with all the supplier values except the value of ? for the 2 ?m spheres, where aggregation was detected by optical microscopy. Other topics included in this work are: (i) comparison between our method and the classical simple fit through a pure monodisperse system (? = 0); and (ii) dependence of the quality of sizing on the angular range of the measurement. Finally, comparison with published results shows that simple goniometry may outperform more complex methods.

Model selection for clustering of pharmacokinetic responses

BACKGROUND AND OBJECTIVE Pharmacokinetics comprises the study of drug absorption, distribution, metabolism and excretion over time. Clinical pharmacokinetics, focusing on therapeutic management, offers important insights towards personalised medicine through the study of efficacy and toxicity of drug therapies. This study is hampered by subjects high variability in drug blood concentration, when starting a therapy with the same drug dosage. Clustering of pharmacokinetics responses has been addressed recently as a way to stratify subjects and provide different drug doses for each stratum. This clustering method, however, is not able to automatically determine the correct number of clusters, using an user-defined parameter for collapsing clusters that are closer than a given heuristic threshold. We aim to use information-theoretical approaches to address parameter-free model selection. METHODS We propose two model selection criteria for clustering pharmacokinetics responses, founded on the Minimum Description Length and on the Normalised Maximum Likelihood. RESULTS Experimental results show the ability of model selection schemes to unveil the correct number of clusters underlying the mixture of pharmacokinetics responses. CONCLUSIONS In this work we were able to devise two model selection criteria to determine the number of clusters in a mixture of pharmacokinetics curves, advancing over previous works. A cost-efficient parallel implementation in Java of the proposed method is publicly available for the community.

A new path integral approximation to photon propagation in turbid media

The path integral approach is applied to describe the propagation of photons in bounded homogeneous turbid media. Introducing a new controlled perturbative expansion centred on the diffusion limit, new analytic solutions for continuous wave reflectance and transmittance are determined, as relevant for non-invasive measurements of tissue optical properties based on continuous wave photon migration instruments. A comparative analysis with the diffusion approximation of radiative transfer theory is made to show that non-diffusive effects are enhanced in turbid regions with increasing absorption coefficients and decreasing reduced scattering coefficients as well as when the distance between interface boundaries increases. The relation between the transport equation (which is the starting point for the path integral approach) and multiple scattering theory is also discussed.

Scattering of electromagnetic waves by counter‐rotating vortex streets in plasmas

The scattering of electromagnetic waves from counter‐rotating vortex streets associated with nonlinear convective cells in uniform plasmas has been considered. The vortex street solution of the Navier–Stokes or the Hasegawa–Mima (and of the ‘‘sinh‐Poisson’’) equation is adopted as a scatterer. Assuming arbitrary polarization and profile function for the incident electromagnetic field, a compact expression for the scattering cross section has been obtained. Specific results for the differential cross section are obtained for the case in which the incident beam has a Gaussian profile and propagates as an ordinary mode. The results show that when the characteristic wavelength of the vortex street (λv=2π/a) is larger than that of the incident electromagnetic wave (λi=2π/ki), the differential cross section dσ/dΩ has a very well‐defined angular periodicity; in fact, it is a collection of Gaussians varying as exp[−f(kiw)2], where w is the waist and f is a function expressing a kind of ‘‘Bragg condition.’’ On the...

Multiple scattering of electromagnetic waves by large amplitude drift turbulent vortex streets in a plasma

The scattering of electromagnetic waves from strong drift turbulent vortex streets in uniform plasmas has been considered. An integral equation describing the scattering is obtained in a Fourier representation. This integral equation is then solved approximately by the Galerkin method using two trial functions. Assuming arbitrary polarization and a plane wave for the incident electromagnetic field, a compact expression for the scattering cross-section is given. Specific results are derived for the differential cross-section for the case in which the incident wave propagates as an ordinary mode. It is shown that the the scattering cross-section thus obtained, which contains contributions from all orders in the scattering strength, is no more difficult to evaluate than the cross-section calculated to second order Born by the iterative technique. Keeping both trial functions and allowing the scattering strength tend to zero reduces in fact to the second Born solution, while keeping only one of the trial functions reduces to the usual first Born result in the same limit.