ABSTRACT We consider four different Monte Carlo methods, widely used in tissue optics, based on f... more ABSTRACT We consider four different Monte Carlo methods, widely used in tissue optics, based on four different ways to build photons' trajectories. By means of numerical results we compare the temporal point spread functions calculated by the four methods for a wide range of the optical properties in the slab and semi-infinite medium geometry. Therefore, we show the statistical equivalence of the four methods and some of their convergence characteristics.
Time-domain analytical solutions of the diffusion equation for photon migration through highly sc... more Time-domain analytical solutions of the diffusion equation for photon migration through highly scattering two- and three-layered slabs have been obtained. The effect of the refractive-index mismatch with the external medium is taken into account, and approximate boundary conditions at the interface between the diffusive layers have been considered. A Monte Carlo code for photon migration through a layered slab has also been developed. Comparisons with the results of Monte Carlo simulations showed that the analytical solutions correctly describe the mean path length followed by photons inside each diffusive layer and the shape of the temporal profile of received photons, while discrepancies are observed for the continuous-wave reflectance or transmittance.
ABSTRACT Using Monte Carlo simulations we demonstrate that a realistic absorption inhomogeneity e... more ABSTRACT Using Monte Carlo simulations we demonstrate that a realistic absorption inhomogeneity embedded in a diffusive medium can be effectively mimicked by a small black object of a proper volume (Equivalence Relation). Applying this concept we propose the construction of simple and well reproducible inhomogeneous phantoms.
A time-resolved optical imaging system using near-infrared light has been developed. The system h... more A time-resolved optical imaging system using near-infrared light has been developed. The system had three pulsed light sources and total 64 channels of detection, working simultaneously for acquisition of the time-resolved data of the pulsed light transmitted through scattering media like biological tissues. The light sources were provided by high power picosecond pulsed diode lasers, and optical switches directed one of the light sources to the object through an optical fiber. The light signals reemitted from the surface of the object were collected by optical fibers, and transmitted to a time-resolved detecting system. Each of the detecting channels consisted of an optical attenuator, a fast photomultiplier, and a time-correlated single photon counting circuit which contained a miniaturized constant fraction discriminator/time-to-amplitude converter module, and a signal acquisition unit with an A/D converter. The performance and potentiality of the imaging system have been examined by the image reconstruction from the measured data using solid phantoms.
Society of Photo Optical Instrumentation Engineers Conference Series, 2009
We report our studies on the optical signals measured non-invasively on electrically stimulated p... more We report our studies on the optical signals measured non-invasively on electrically stimulated peripheral nerves. The stimulation consists of the delivery of 0.1 ms current pulses, below the threshold for triggering any visible motion, to a peripheral nerve in human subjects (we have studied the sural nerve and the median nerve). In response to electrical stimulation, we observe an optical signal that peaks at about 100 ms post-stimulus, on a much longer time scale than the few milliseconds duration of the electrical response, or sensory nerve action potential (SNAP). While the 100 ms optical signal we measured is not a direct optical signature of neural activation, it is nevertheless indicative of a mediated response to neural activation. We argue that this may provide information useful for understanding the origin of the fast optical signal (also on a 100 ms time scale) that has been measured non-invasively in the brain in response to cerebral activation. Furthermore, the optical response to peripheral nerve activation may be developed into a diagnostic tool for peripheral neuropathies, as suggested by the delayed optical signals (average peak time: 230 ms) measured in patients with diabetic neuropathy with respect to normal subjects (average peak time: 160 ms).
We propose a new method for BOLD signal calculation that is more meaningful for comparison with N... more We propose a new method for BOLD signal calculation that is more meaningful for comparison with NIRS data. We provide evidence that BOLD signal can be highly localized within the region of activation not only in terms of the amplitude of activation but also for the nuances of the temporal trend. Therefore, in order to take into account of the
Proceedings of SPIE - The International Society for Optical Engineering
A method to measure the diffusion coefficient of homogeneous and layered media, based on multi-di... more A method to measure the diffusion coefficient of homogeneous and layered media, based on multi-distance measurements of time-resolved reflectance is proposed. The diffusion coefficient is retrieved by fitting procedures on the logarithm of the ratio of two reflectances measured at two different source-receiver distances. The inversion procedure uses analytical models derived from the diffusion equation and it is not affected by the boundary conditions used. For layered media the inversion procedure retrieves the diffusion coefficient of the first layer. The proposed procedure is simpler than others usually employed and also provides a reliable criterion for retrieval of information on the layered structure of the diffusive medium.
We show the potential of functional near-infrared spectroscopy for the discrimination of mental w... more We show the potential of functional near-infrared spectroscopy for the discrimination of mental workloads during a cognitive task with two different levels of difficulty. Standard data analysis based on filtering and folding average procedures were carried out to locate those source-detector pairs sensitive to the activated cortical regions. On these channels we applied two classification algorithms for the discrimination of
Optical Tomography and Spectroscopy of Tissue IX, 2011
ABSTRACT We report the development of an instrument for diffuse spectral imaging of the human bre... more ABSTRACT We report the development of an instrument for diffuse spectral imaging of the human breast operating over the wavelength range 650-900 nm. This instrument images the slightly compressed human breast in a planar geometry by performing a tandem scan, over the x-y plane, of a 3 mm illumination optical fiber and a 5 mm collection optical fiber that are collinear and located on opposite sides of the breast. An edge-correction algorithm accounts for breast thickness variability over the x-y plane, a second-derivative imaging algorithm enhances the display of optical inhomogeneities, and a paired-wavelength spectral method yields oxygenation maps. We report our results of oxygenation mapping in eighteen human subjects, two of which are breast cancer patients, one with a ductal carcinoma in situ, the other with an invasive ductal carcinoma.
ABSTRACT We consider four different Monte Carlo methods, widely used in tissue optics, based on f... more ABSTRACT We consider four different Monte Carlo methods, widely used in tissue optics, based on four different ways to build photons' trajectories. By means of numerical results we compare the temporal point spread functions calculated by the four methods for a wide range of the optical properties in the slab and semi-infinite medium geometry. Therefore, we show the statistical equivalence of the four methods and some of their convergence characteristics.
Time-domain analytical solutions of the diffusion equation for photon migration through highly sc... more Time-domain analytical solutions of the diffusion equation for photon migration through highly scattering two- and three-layered slabs have been obtained. The effect of the refractive-index mismatch with the external medium is taken into account, and approximate boundary conditions at the interface between the diffusive layers have been considered. A Monte Carlo code for photon migration through a layered slab has also been developed. Comparisons with the results of Monte Carlo simulations showed that the analytical solutions correctly describe the mean path length followed by photons inside each diffusive layer and the shape of the temporal profile of received photons, while discrepancies are observed for the continuous-wave reflectance or transmittance.
ABSTRACT Using Monte Carlo simulations we demonstrate that a realistic absorption inhomogeneity e... more ABSTRACT Using Monte Carlo simulations we demonstrate that a realistic absorption inhomogeneity embedded in a diffusive medium can be effectively mimicked by a small black object of a proper volume (Equivalence Relation). Applying this concept we propose the construction of simple and well reproducible inhomogeneous phantoms.
A time-resolved optical imaging system using near-infrared light has been developed. The system h... more A time-resolved optical imaging system using near-infrared light has been developed. The system had three pulsed light sources and total 64 channels of detection, working simultaneously for acquisition of the time-resolved data of the pulsed light transmitted through scattering media like biological tissues. The light sources were provided by high power picosecond pulsed diode lasers, and optical switches directed one of the light sources to the object through an optical fiber. The light signals reemitted from the surface of the object were collected by optical fibers, and transmitted to a time-resolved detecting system. Each of the detecting channels consisted of an optical attenuator, a fast photomultiplier, and a time-correlated single photon counting circuit which contained a miniaturized constant fraction discriminator/time-to-amplitude converter module, and a signal acquisition unit with an A/D converter. The performance and potentiality of the imaging system have been examined by the image reconstruction from the measured data using solid phantoms.
Society of Photo Optical Instrumentation Engineers Conference Series, 2009
We report our studies on the optical signals measured non-invasively on electrically stimulated p... more We report our studies on the optical signals measured non-invasively on electrically stimulated peripheral nerves. The stimulation consists of the delivery of 0.1 ms current pulses, below the threshold for triggering any visible motion, to a peripheral nerve in human subjects (we have studied the sural nerve and the median nerve). In response to electrical stimulation, we observe an optical signal that peaks at about 100 ms post-stimulus, on a much longer time scale than the few milliseconds duration of the electrical response, or sensory nerve action potential (SNAP). While the 100 ms optical signal we measured is not a direct optical signature of neural activation, it is nevertheless indicative of a mediated response to neural activation. We argue that this may provide information useful for understanding the origin of the fast optical signal (also on a 100 ms time scale) that has been measured non-invasively in the brain in response to cerebral activation. Furthermore, the optical response to peripheral nerve activation may be developed into a diagnostic tool for peripheral neuropathies, as suggested by the delayed optical signals (average peak time: 230 ms) measured in patients with diabetic neuropathy with respect to normal subjects (average peak time: 160 ms).
We propose a new method for BOLD signal calculation that is more meaningful for comparison with N... more We propose a new method for BOLD signal calculation that is more meaningful for comparison with NIRS data. We provide evidence that BOLD signal can be highly localized within the region of activation not only in terms of the amplitude of activation but also for the nuances of the temporal trend. Therefore, in order to take into account of the
Proceedings of SPIE - The International Society for Optical Engineering
A method to measure the diffusion coefficient of homogeneous and layered media, based on multi-di... more A method to measure the diffusion coefficient of homogeneous and layered media, based on multi-distance measurements of time-resolved reflectance is proposed. The diffusion coefficient is retrieved by fitting procedures on the logarithm of the ratio of two reflectances measured at two different source-receiver distances. The inversion procedure uses analytical models derived from the diffusion equation and it is not affected by the boundary conditions used. For layered media the inversion procedure retrieves the diffusion coefficient of the first layer. The proposed procedure is simpler than others usually employed and also provides a reliable criterion for retrieval of information on the layered structure of the diffusive medium.
We show the potential of functional near-infrared spectroscopy for the discrimination of mental w... more We show the potential of functional near-infrared spectroscopy for the discrimination of mental workloads during a cognitive task with two different levels of difficulty. Standard data analysis based on filtering and folding average procedures were carried out to locate those source-detector pairs sensitive to the activated cortical regions. On these channels we applied two classification algorithms for the discrimination of
Optical Tomography and Spectroscopy of Tissue IX, 2011
ABSTRACT We report the development of an instrument for diffuse spectral imaging of the human bre... more ABSTRACT We report the development of an instrument for diffuse spectral imaging of the human breast operating over the wavelength range 650-900 nm. This instrument images the slightly compressed human breast in a planar geometry by performing a tandem scan, over the x-y plane, of a 3 mm illumination optical fiber and a 5 mm collection optical fiber that are collinear and located on opposite sides of the breast. An edge-correction algorithm accounts for breast thickness variability over the x-y plane, a second-derivative imaging algorithm enhances the display of optical inhomogeneities, and a paired-wavelength spectral method yields oxygenation maps. We report our results of oxygenation mapping in eighteen human subjects, two of which are breast cancer patients, one with a ductal carcinoma in situ, the other with an invasive ductal carcinoma.
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Papers by Angelo Sassaroli