Pietro Luigi Indovina

EXPERIMENTAL TESTS OF DIFFERENT SOLUTIONS TO THE DIFFUSION EQUATION FOR OPTICAL CHARACTERIZATION OF SCATTERING MEDIA BY TIME-RESOLVED TRANSMITTANCE

A detailed investigation of the use of time-resolved trasmittance for the optical characterization of scattering media by use of different analytical solutions to the diffusion equation has been performed. A femtosecond Ti:sapphire laser working at 800 nm and a streak camera with a time resolution of a few picoseconds were employed. Different latex and Intralipid solutions as well as biological samples were investigated. Reduced scattering coefficients were evaluated, and good agreement with the Mie predictions was found. An estimation of the order of magnitude of the absorption coefficient was obtained for the low-absorbance samples examined. These studies confirm experimentally that time-resolved trasmittance can be employed usefully for evaluating s values of thick scattering samples when a proper theoretical description that takes into account realistic boundary conditions is used.

A perturbation approach to characterize absorptive inclusions in diffusing media by time-resolved contrast measurements

Time-resolved measurements of ultrashort light pulses transmitted through turbid scattering media have been performed to determine the possibilities and the limitations of a first-order perturbation approach to recover the absorption coefficient of different tissue-like cylindrical absorptive phantoms. The model accounts for the effect of a spatially varying absorption coefficient of an inclusion on the time-resolved transmittance. We have determined the absorption coefficient of the inhomogeneities located at the midplane of a scattering cell by fitting the diffusion approximation-based perturbation model to the experimental time-resolved contrast measurements. The accuracy of the model predictions is discussed. An error analysis of the fitting procedure is also presented.

Data analysis in Raman measurements of biological tissues using wavelet techniques.

Raman spectroscopy of oral tissues is a promising tool for in vivo diagnosis of oral pathologies, due to the high chemical and structural information content of Raman spectra. However, measurements on biological tissues are usually hindered by low level signals and by the presence of interfering noise and background components due to light diffusion or fluorescence processes. Numerical methods can be used in data analysis, in order to overcome these problems. In this work the wavelet multicomponent decomposition approach has been tested in a series of micro-Raman measurements performed on “in vitro” animal tissue samples. The experimental set-up was mainly composed by a He-Ne laser and a monochromator equipped with a liquid nitrogen cooled CCD equipped with a grating of 1800 grooves/mm. The laser light was focused on the sample surface by means of a 50 X optical objective. The resulting spectra were analysed using a wavelet software package and the contribution of different vibration modes have been singled out. In particular, the C=C stretching mode, and the CH2 bending mode of amide I and amide III and tyrosine contributions were present. The validity of wavelet approach in the data treatment has been also successfully tested on aspirin.

Perturbation approach to the time-resolved transmittance for a spatially varying scattering inclusion in a diffusive slab

In the framework of the perturbation approach to the diffusion equation, an analytical expression is derived to describe the effects on the time-resolved transmittance due to the presence of a spatially varying scattering inclusion hidden inside a diffusive slab. This formula assumes that the reduced scattering coefficient of the inclusion is spatially Gaussian distributed and complements that obtained for the absorptive case. The accuracy and the application range of the perturbed transmittance are investigated through comparisons with the numerical solutions of the time-dependent diffusion equation given by using the finite-element method. The proposed perturbation model is validated through a fitting procedure that determines the relative error in retrieving the scattering perturbation parameter of the inclusion located at the midplane of the slab.

Investigation of optical properties of scattering solutions by time-resolved transmittance

The optical properties of controlled size latex particles suspended in water have been investigated by using two different time-resolved transmittance set-ups. Least-mean square fitting between experimental data and analytical solution to diffusion approximation equation has given values of optical parameters in good agreement with the predictions of the Mie theory. In this way, the validity of these predictions was checked in the investigated experimental conditions and the data analysis confirms that time-resolved transmittance can be a reliable technique to measure the optical properties of scattering solutions.

Random walk analysis of time-resolved transmittance measurements

Many biomedical applications of time-resolved transmittance require precise measurements of optical properties of scattering samples. Recently, random walk theory has been proposed to accurately model the time-resolved transmittance and reflectance across uniform slabs of scattering media as well as slabs containing isolated targets of different scattering or absorbing properties. In this paper random walk description of light propagation in turbid media has been used to analyse transmittance measurements for optical characterization of scattering and absorbing media. A femtosecond Ti:Sa laser working at 800 nm and a streak camera with a time resolution of a few picoseconds have been employed. Various Intralipid suspensions with and without absorbing ink have been investigated. These studies have experimentally confirmed that random walk theory can be usefully employed to discriminate small changes in absorbing and scattering properties. An appropriate fitting procedure, without arbitrary constrains, have been checked and used. In addition, the detailed investigation here reported can help in defining the applicability range of random walk description. This analysis can be important since random walk is a powerful and versatile theory as fast and efficient solution of the inverse problem in optical tomography also for sophisticated geometry and for a wide range of optical properties.

Optical property measurements in scattering media by time-correlated single-photon counting system (TCSPCS)

Time-Correlated Single-Photon-Counting (TCSPC) systems have been often adopted to prepare more compact and less expensive instruments for medical imaging. Then it is important to investigate in detail the performances of each imaging system in measuring the optical properties of turbid media. Our experimental apparatus is composed by an Hamamatsu PLP02 pulsed diode laser at 824 nm with 1 Mhz repetition rate and a pulse duration of 40 ps. The signal has been collected from the investigated sample by means of fiber bundles and has been analyzed by an Edinburgh Instrument TCSPCS equipped with an 8 channels Hamamatsu multichannel plate R411OU-F008 and an SPC300 acquisition module. Solutions of distilled water and commercial Intralipid 10% at different concentrations have been investigated. To obtain optical parameters, the experimental data have been fitted with an analytic solution to the diffusion equation. Also the convolution effect of the measured Temporal Point Spread Functions (TPSF) by the Impulse Response Function (IRF) of the system has been investigated. A linear trend has been obtained for the reduced scattering coefficient (mu) s with concentration in solution of the scattering agent (Intralipid 10%) and an agreement within few percent has been reported with Mie theory predictions. The results here obtained confirm that all the details in fitting and convolution procedures become particularly important when slight difference in absorption and scattering coefficients have to be examined.

Differences in optical properties of normal and tumoral tissues: a comparison to accuracy limits in laser techniques for optical imaging

The use of optical techniques for diagnostic purpose relies on the capability to measure the optical properties of healthy and pathological tissues and in appreciating their relative differences. In fact, a degree of contrast must exist between absorption and scattering coefficients for effective detection of a tissue alteration using optical imaging. In this contest, it is important to study the accuracy limits of different optical measurement techniques in order to establish their performances in recovering the optical parameters. The accuracy limit of laser techniques in the determination of optical methods have been recovered partly from literature and, as far as concerns time-resolved techniques, from experimental work carried on in our laboratory using a conventional time-resolved system. The results from this analysis allow us to better identify the role of different experimental techniques, which are generally proposed in optical imaging for diagnostic purpose.

Fluorescence spectroscopy of scattering media in visible and infrared range

Fluorescence spectra of scattering and non scattering solutions of rhodamine 6G and IR125 have been measured. The experimental data have been analyzed with a Gaussian parametric model to give some parameters that are related to the changes of scattering properties of fluorescent solutions. In addition, complementary measurements of diffuse reflectance have allowed us to test an analytical model generally used in order to recover intrinsic fluorescence spectra.

EDGE RESPONSE FUNCTION IN OPTICAL IMAGING BY TIME-RESOLVED LASER TRANSILLUMINATION

The image quality in optical imaging is actually under intense investigation in order to ultimately define the role of optical techniques for medical diagnosis purposes. In this work we have investigated the spatial resolution, noise and contrast parameters by using edge response function measurements with masks having well-controlled optical properties. The experimental set-up mainly consisted of an Argon pumped Titanium-Sapphire laser working in femtosecond regime and a streak camera with a few picosecond of temporal resolution. The investigated samples were solutions of Intralipid 10% with distilled water in order to approximate optical properties of biological tissues. The experimental data were analyzed with different imaging algorithms and compared with theoretical predictions about the above- mentioned image quality parameters. The use of spatial resolution, contrast and signal-to-noise ratio allowed us in identifying the best working conditions for optical imaging systems and in evaluating the efficiency of different image reconstruction algorithms.