Arianna Giusto

Time-resolved spectrally constrained method for the quantification of chromophore concentrations and scattering parameters in diffusing media

We have devised and experimentally validated, on tissue-simulating phantoms and in vivo, a time-resolved spectral fitting analysis for direct assessment of chromophore concentrations and scattering parameters. Experimental data have been acquired with a time-resolved broadband system based on supercontinuum light generated in a photonic crystal fiber and a 32 channel Time Correlated Single Photon Counting system. The novel method is more robust than conventional techniques, especially at low signal-to-noise ratio.

OPTICAL PROPERTIES OF COMPOSITE INTERSTELLAR GRAINS: A MORPHOLOGICAL ANALYSIS

In the framework of the transition matrix approach, we calculate the relevant optical properties of cosmic dust grains of amorphous carbon and astronomical silicates, modeled as aggregates of spherical monomers. Two mechanisms of aggregation were considered, producing clusters with different structure and degree of fluffiness: ballistic particle-cluster aggregation (BPCA) and ballistic cluster-cluster aggregation (BCCA). Our results are very different from those obtained through computational approaches based on effective medium theories and might have major implications both on the modeling procedure and on the dust-mass balance in the interstellar medium.

Efficient light-scattering calculations for aggregates of large spheres

Calculation of the scattering pattern from aggregates of spheres through the T-matrix approach yields high-precision results but at a high-computational cost, especially when the aggregate concerned is large or is composed of large-size spheres. With reference to a specific but representative aggregate, we discuss how and to what extent the computational effort can be reduced but still preserve the qualitative features of the signature of the aggregate concerned.

Feasibility of white-light time-resolved optical mammography

We demonstrate the feasibility of white-light time-resolved optical mammography. The instrumentation is based on supercontinuum light generated in photonic crystal fiber and 32-channel parallel time-correlated single-photon-counting detection. Total measurement time is of the order of 10 min for typical clinical applications. Preliminary measurements performed on volunteers show the ability of the system to determine tissue constituent concentrations and structure over the entire breast area. Furthermore, measurements on a tissue-like sample demonstrate detection and characterization of inclusions.

ULTRAVIOLET RADIATION INSIDE INTERSTELLAR GRAIN AGGREGATES. I. THE DENSITY OF RADIATION

We study the distribution of energy density inside dust grain aggregates through an approach based on the multipole expansion of the electromagnetic fields. A significant fraction of the energy of the impinging wave is found throughout the interiors of grains. Implications for extraterrestrial prebiotic chemistry are discussed.

On the formation and survival of complex prebiotic molecules in interstellar grain aggregates

The aggregation of interstellar grains as a result of ballistic collisions produces loosely packed structures with much of their internal volume composed by vacuum (cavities). The molecular material present on the surfaces of the cavities gives rise to a series of reactions induced by cosmic rays, UV radiation, thermal shocks, etc., in high reducing conditions. Thus, a terrestrial type chemistry is given the possibility to evolve inside these cavities. The resulting products are different and of a wider range than those from gas-phase or surface chemistry in molecular clouds. Under conditions similar to those in the aggregate cavities, laboratory experiments have produced amino acids, sugars and other organic compounds from simple precursors. In dense star-forming regions, the molecular species inside aggregates are efficiently shielded against the local UV field. The same molecules were incorporated in the material which formed the Earth, as well as other planets, during the process of its formation and afterwards fell on the surface via comets, meteorites, interstellar dust, etc. This was the source material that can produce, under favorable circumstances, the biopolymers needed for life. The astronomical observations of organic molecules in star-forming regions and the results of analyses of meteorites and cometary dust seem to support the present hypothesis that complex prebiotic molecules form inside dust aggregates and therein survive the journey to planetary systems. The Miller experiment is revisited through innumerable repetitions inside dust grain aggregates.

Monitoring Absorption Changes in a Layered Diffusive Medium by White-Light Time-Resolved Reflectance Spectroscopy

Diffuse spectroscopy of turbid media has assumed a crucial role in the characterization of biological tissues. In particular, broadband time-resolved optical spectroscopy allows the direct determination in a single measurement of both the optical parameters of the tissue and the concentration of its main constituents. Moreover, the possibility of performing parallel wavelength measurements allows the recording of data in real time, providing a system that is able to perform dynamic measurements. We used a white-light time-resolved spectroscopy system to monitor absorption changes in a layered diffusive medium. Measurements were performed in reflectance geometry, with a 2-cm source-detector distance, on a two-layer liquid phantom with optical properties similar to those of human tissues. By varying the concentrations of three inks with different spectral features, we changed the absorption coefficient of the layers to mimic functional brain activation and the systemic response in the scalp. Data were analyzed by a time-resolved spectrally constrained fitting method based on a homogeneous model of photon diffusion. Although this approach is based on a homogeneous model and employs a single source-detector distance, the technique is able to monitor changes in the lower layer, while it is scarcely affected by variations in the upper layer. These results were confirmed by numerical simulations based on a perturbation approach to diffusion theory. Preliminary in vivo measurements have been performed on healthy volunteers to monitor oxy- and deoxy-hemoglobin changes in the brain during a motor task. Although the overall sensitivity of the technique is reduced, in vivo results are in general agreement with the findings of the dedicated system for functional brain activity.

Ultraviolet Radiation inside Interstellar Grain Aggregates. II. Field Depolarization

We study the polarization of the UV light within the cavities of interstellar grain aggregates modeled as homogeneous spheres containing several spherical voids. The incident field is a linearly polarized plane wave. We found that field depolarization occurs in all examined cases so that the field within the cavities has the features of an elliptically polarized wave. The depolarization of the field does not depend on the material of the grains but on the geometry of the problem only. The implications of this result for the interstellar photochemistry are briefly discussed.

Expulsion of dust aggregates from galaxies

In this paper we analyse the possibility of dust being repelled away from a galactic disk by radiation pressure exerted by starlight, studying the effect of a grain aggregate model for the dust. It was found that the aggregated structure enhances the effect of radiation pressure.

Assessment of collagen absorption and related potential diagnostic applications

The sensitivity to collagen may be useful for diagnostic purposes in mammography, as collagen seems to be involved in the development of breast cancer. Moreover, collagen content is expected to be related to breast density (i.e. breast parenchymal pattern) and its quantification could allow the classification of breast type. Thus we have measured the absorption properties of collagen from 610 to 1040 nm. Absorption spectra of breast from healthy volunteers were then interpreted adding collagen to the other absorbers previously considered (i.e. oxy- and deoxyhemoglobin, water, and lipids). A significant amount of collagen, depending on breast type, is estimated to be present and seems to correlate with breast type. Moreover, adding collagen to the fitting procedure affects remarkably the estimated values of blood content and oxygenation. We have also upgraded our time-resolved multi-wavelength optical mammograph, adding a long wavelength (1060 nm) to improve the spectral information and, in particular, the sensitivity to collagen. Breast measurements on volunteers have recently started.