Publications

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    ABSTRACT: The nEUROPt protocol is one of two new protocols developed within the European project nEUROPt to characterize the performances of time-domain systems for optical imaging of the brain. It was applied in joint measurement campaigns to compare the various instruments and to assess the impact of technical improvements. This protocol addresses the characteristic of optical brain imaging to detect, localize, and quantify absorption changes in the brain. It was implemented with two types of inhomogeneous liquid phantoms based on Intralipid and India ink with well-defined optical properties. First, small black inclusions were used to mimic localized changes of the absorption coefficient. The position of the inclusions was varied in depth and lateral direction to investigate contrast and spatial resolution. Second, two-layered liquid phantoms with variable absorption coefficients were employed to study the quantification of layer-wide changes and, in particular, to determine depth selectivity, i.e., the ratio of sensitivities for deep and superficial absorption changes. We introduce the tests of the nEUROPt protocol and present examples of results obtained with different instruments and methods of data analysis. This protocol could be a useful step toward performance tests for future standards in diffuse optical imaging.
    Journal of Biomedical Optics 08/2014; 19(8). · 2.75 Impact Factor
  • Journal of Biomedical Optics 07/2014; 19(7):76011. · 2.75 Impact Factor
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    ABSTRACT: In this paper, after a critical review of the literature, we present two forward solvers and a new methodology for description of photon migration in the presence of totally absorbing inclusions embedded in diffusive media in both time and CW domains. The first forward solver is a heuristic approach based on a higher order perturbation theory applied to the diffusion equation (DE) [denoted eighth-order perturbation theory (EOPT)]. The second forward solver [denoted eighth-order perturbation theory with the equivalence relation (EOPTER) ] is obtained by combining the EOPT solver with the adoption of the equivalence relation (ER) [J. Biomed. Opt.18, 066014 (2013)]. These forward solvers can possibly overcome some evident limitations of previous approaches like the theory behind the so-called banana-shape regions or exact analytical solutions of the DE in the presence of highly or totally absorbing inclusions. We also propose the ER to reformulate the problem of a totally absorbing inclusion in terms of another inclusion having a finite absorption contrast and a re-scaled volume. For instance, we have shown how this approach can indeed be used to simulate black inclusions with the Born approximation. By means of comparisons with the results of Monte Carlo simulations, we have shown that the EOPTER solver can model totally absorbing inclusions with an error smaller than about 10%, whereas the EOPT solver shows an error smaller than about 20%, showing a performance largely better than that observed with solvers proposed previously.
    Journal of the Optical Society of America A 03/2014; 31(3):460-9. · 1.67 Impact Factor
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    Rosario Esposito, Fabrizio Martelli, Sergio De Nicola
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    ABSTRACT: We have developed a theoretical model for photon migration through scattering media in the presence of an absorbing inhomogeneity. A closed-form solution for the average diffuse intensity has been obtained through an iterative approximation scheme of the steady-state diffusion equation. The model describes absorbing defects in a wide range of values. Comparisons with the results of Monte Carlo simulations show that the error of the model is lower than 3% for size inclusion lower than 4 mm and absorption contrast up to the threshold value of the "black defect." The proposed model provides a tractable mathematical basis for diffuse optical and photoacoustic tomographic reconstruction techniques.
    Optics Letters 02/2014; 39(4):826-9. · 3.39 Impact Factor
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    ABSTRACT: The design of inhomogeneous phantoms for diffuse optical imaging purposes using totally absorbing objects embedded in a diffusive medium is proposed and validated. From time-resolved and continuous-wave Monte Carlo simulations, it is shown that a given or desired perturbation strength caused by a realistic absorbing inhomogeneity of a certain absorption and volume can be approximately mimicked by a small totally absorbing object of a so-called equivalent black volume (equivalence relation). This concept can be useful in two ways. First, it can be exploited to design realistic inhomogeneous phantoms with different perturbation strengths simply using a set of black objects with different volumes. Further, it permits one to grade physiological or pathological changes on a reproducible scale of perturbation strengths given as equivalent black volumes, thus facilitating the performance assessment of clinical instruments. A set of plots and interpolating functions to derive the equivalent black volume corresponding to a given absorption change is provided. The application of the equivalent black volume concept for grading different optical perturbations is demonstrated for some examples.
    Journal of Biomedical Optics 06/2013; 18(6):66014. · 2.75 Impact Factor
  • Angelo Sassaroli, Fabrizio Martelli
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    ABSTRACT: Different strategies for Monte Carlo simulations are currently used in tissue optics. In this work we analyze and compare four Monte Carlo methods based on different ways to extract the photons’ trajectories. By using theoretical arguments we show that the four methods are statistically equivalent. Afterwards we study the convergence of the four methods both in time and continuous wave domains. Our results show that those Monte Carlo methods based on photons’ annihilation or survival converge faster for continuous wave calculations and at shorter source-detector distances. On the contrary Monte Carlo methods based on weight assignment provide a better representation of the temporal point spread function in time domain.
    03/2013
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    ABSTRACT: We propose and validate the design of inhomogeneous phantoms for diffuse optical imaging purposes using totally absorbing objects embedded in a diffusive medium. From Monte Carlo simulations, we show that a given or desired perturbation strength caused by an realistic absorbing inhomogeneity of a certain absorption and volume can be approximately mimicked by a small totally absorbing object of a so-called Equivalent Black Volume (Equivalence Relation). This concept can be useful to design realistic inhomogeneous phantoms using a set of black objects with different volumes. Further, it permits to grade physiological or pathological changes on a reproducible scale of equivalent black volumes, thus facilitating the performance assessment of clinical instruments. We have also provided a plot to derive the Equivalent Black Volume yielding the same effect of a realistic absorption object.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2013;
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    ABSTRACT: Novel protocols were developed and applied in the European project "nEUROPt" to assess and compare the performance of instruments for time-domain optical brain imaging and of related methods of data analysis. The objective of the first protocol, "Basic Instrumental Performance", was to record relevant basic instrumental characteristics in a direct way. The present paper focuses on the second novel protocol ("nEUROPt" protocol) that was devoted to the assessment of sensitivity, spatial resolution and quantification of absorption changes within inhomogeneous media. It was implemented with liquid phantoms based on Intralipid and ink, with black inclusions and, alternatively, in two-layered geometry. Small black cylinders of various sizes were used to mimic small localized changes of the absorption coefficient. Their position was varied in depth and lateral direction to address contrast and spatial resolution. Two-layered liquid phantoms were used, in particular, to determine depth selectivity, i.e. the ratio of contrasts due to a deep and a superficial absorption change of the same magnitude. We introduce the tests of the "nEUROPt" protocol and present exemplary results obtained with various instruments. The results are related to measurements with both types of phantoms and to the analysis of measured time-resolved reflectance based on time windows and moments. Results are compared for the different instruments or instrumental configurations as well as for the methods of data analysis. The nEUROPt protocol is also applicable to cw or frequency-domain instruments and could be useful for designing performance tests in future standards in diffuse optical imaging.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2013;
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    ABSTRACT: Intralipid 20% was recently suggested as a diffusive reference standard for tissue simulating phantoms. In this work, we extend previously obtained results to other fat emulsions, specifically Intralipid 10%, Intralipid 30%, Lipovenoes 10%, Lipovenoes 10% PhosphoLipid Reduced, Lipovenoes 20%, Lipofundin S 10%, and Lipofundin S 20%. Of particular importance for practical applications, our measurements carried out at a wavelength of 751 nm show the following features. First, these products show high stability and small batch-to-batch variations in their diffusive optical properties, similar to Intralipid 20%. Second, the absorption coefficient of Intralipid, Lipovenoes, and Lipofundin S are very similar and their measured values are within the experimental errors; moreover the reduced scattering coefficient of Intralipid 20%, Lipovenoes 20%, and Lipofundin S 20% are similar and their measured values are within 5%. Third, the reduced scattering coefficient of Intralipid 10% and Intralipid 30% can be scaled from that of Intralipid 20% with an error of 9% and 2%, respectively. A similar scaling property is valid for Lipovenoes and Lipofundin S. We have verified that this scaling property depends on the composition of the fat emulsions: If the ingredients exactly scale with the concentration then the reduced scattering coefficient almost exactly scale as well.
    Applied Optics 10/2012; 51(30):7176-82. · 1.69 Impact Factor
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    Angelo Sassaroli, Fabrizio Martelli
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    ABSTRACT: In the field of photon migration in turbid media, different Monte Carlo methods are usually employed to solve the radiative transfer equation. We consider four different Monte Carlo methods, widely used in the field of tissue optics, that are based on four different ways to build photons' trajectories. We provide both theoretical arguments and numerical results showing the statistical equivalence of the four methods. In the 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. The convergence of the methods is also briefly discussed.
    Journal of the Optical Society of America A 10/2012; 29(10):2110-7. · 1.67 Impact Factor
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    ABSTRACT: A novel protocol to determine sensitivity, spatial resolution and quantification of absorption changes in optical brain imaging was applied to assess time-domain instruments and methods of data analysis.
    Biomedical Optics; 04/2012
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    ABSTRACT: Intrinsic reduced scattering coefficient of Intralipid and intrinsic absorption coefficient of Indian ink at NIR wavelengths are accurately assessed in a multi-center study involving different techniques, instrumental set-ups, and analysis methods.
    04/2012;
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    ABSTRACT: We propose the use of a retrieval procedure for time-resolved near-infrared tissue spectroscopy based on the 'optimal estimation' method. The aim of this retrieval method is to obtain an improved estimate of the target parameters compared with standard nonlinear least-squares routines, since the inverse problem dedicated to retrieve the optical properties of tissue is ill posed. A priori information on target and forward model parameters is used, so that a larger number of target parameters can be retrieved, and/or a better accuracy and precision can be achieved on the retrieved target parameters. The procedure has been tested on time-resolved simulated experiments generated, using solutions of the diffusion equation and with solutions of the radiative transfer equation reconstructed with Monte Carlo simulations. The results obtained show that, by using a priori information on target parameters, we have a smaller difference between retrieved values and true values, and lower retrieved error bars. Similarly, a more correct estimate of the errors of the forward model parameters improves the retrieval of the target parameters.
    Physics in Medicine and Biology 04/2012; 57(10):2915-29. · 2.92 Impact Factor
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    ABSTRACT: The use of Intralipid 20% was recently suggested as a diusive reference standard for tissue simulating phantoms. We herein extend previously obtained results to other fat emulsions, specically Intralipid 10%, Intralipid 30%, Lipovenoes 10%, Lipovenoes 20%, Lipofundin S 10%, and Lipofundin S 20%. Of particular importance for practical applications, our measurements carried out at a wavelength of 751 nm show the following features. First, batch-to-batch variations of the optical properties of these products are less than 2 % similarly to Intralipid 20%. Second, the absorption coecient of Intralipid, Lipovenoes, and Lipofundin S are very similar and their measured values are within the experimental errors. Third, the reduced scattering of Intralipid 20%, Lipovenoes 20%, and Lipofundin S 20% are similar and their measured values are within 5%. Forth, the reduced scattering coecient of Intralipid 10% and Intralipid 30% can be scaled from that of Intralipid 20% with an error of 9% and 2%, respectively. A similar scaling property is valid for Lipovenoes and Lipofundin S.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2012;
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    ABSTRACT: The full characterization of the optical properties of nanofluids consisting of single-wall carbon nanohorns of different morphologies in aqueous suspensions is carried out using a novel spectrophotometric technique. Information on the nanofluid scattering and absorption spectral characteristics is obtained by analyzing the data within the single scattering theory and validating the method by comparison with previous monochromatic measurements performed with a different technique. The high absorption coefficient measured joint to the very low scattering albedo opens promising application perspectives for single-wall carbon nanohorn-based fluid or solid suspensions. The proposed approximate approach can be extended also to other low-scattering turbid media.PACS: 78.35.+c Brillouin and Rayleigh scattering, other light scattering; 78.40.Ri absorption and reflection spectra, fullerenes and related materials; 81.05.U- carbon/carbon-based materials; 78.67.Bf optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures, nanocrystals, nanoparticles, and nanoclusters.
    Nanoscale Research Letters 01/2012; 7(1):96. · 2.48 Impact Factor
  • Fabrizio Martelli
    Journal of Near Infrared Spectroscopy 01/2012; 20(1):29-. · 1.48 Impact Factor
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    Paola Di Ninni, Fabrizio Martelli, Giovanni Zaccanti
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    ABSTRACT: The calibration of optical tissue-simulating phantoms remains an open question in spite of the many techniques proposed for accurate measurements of optical properties. As a consequence, a reference phantom with well known optical properties is still missing. As a first step towards a reference phantom we have recently proposed to use dilutions of Intralipid 20%. In this paper we discuss a matter that is commonly ignored when dilutions are prepared, i.e., the possibility of deviations from the simple linear relationships between the optical properties of the dilution and the Intralipid concentration due to the effects of dependent scattering. The results of an experimental investigation showed that dependent scattering does not affect absorption. As for the reduced scattering coefficient the effect can be described adding a term proportional to the square of the concentration. However, for concentrations of interest for tissue optics deviations from linearity remain within about 2%. The experimental investigation also showed that the microphysical properties of Intralipid are not affected by dilution. These results show the possibility to easily obtain a liquid diffusive phantom whose optical properties are known with error smaller than about 1%. Due to the intrinsic limitations of the different techniques proposed for measuring the optical properties it seems difficult to obtain a similar accuracy for solid phantoms.
    Biomedical Optics Express 08/2011; 2(8):2265-78. · 3.50 Impact Factor
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    ABSTRACT: We present a multi-laboratory comparison of several independent forward solvers used for photon migration through layered media. Two main categories of forward solvers are presented: Monte Carlo procedures and solutions of the diffusion equation for the time domain. For Monte Carlo we have included four independent codes. For the solutions of the diffusion equation, we have presented: two semi-analytical approaches based on the Green's function method and one solution obtained with the finite element method. The comparisons between the different time-dependent solutions were performed for a two-layer medium.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2011;
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    ABSTRACT: We propose the use of a retrieval operator for biomedical applications in near-infrared spectroscopy. The proposed retrieval operator is based on the "Optimal Estimation" method. The main characteristic of this method relates to the possibility to include prior information both on target and on forward model parameters of the inversion procedure. The possibility of the retrieval operator to elaborate a-priori information can in principle be a benefit for the whole retrieval procedure. This means that a larger number of target parameters can be retrieved, or that a better accuracy can be achieved in retrieving the target parameters. The final goal of this inversion procedure is to have an improved estimate of the target parameters. The procedure has been tested on time-resolved simulated experiments obtained with a Monte Carlo code. The results obtained show that an improved performance of the inversion procedure is achieved when prior information on target and forward model parameters is available. With the use of a priori information on target parameters we have in average a lower difference between the retrieved values of the parameters and their true values, and the error bars determined by the inversion procedure on the retrieved parameters are significantly lower. At the same time a good estimate of the errors on the forward model parameters can significantly improve the retrieval of the target parameters.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2011;
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    Angelo Sassaroli, Fabrizio Martelli, Sergio Fantini
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    ABSTRACT: In this work, based on our previously proposed perturbation theory for the diffusion equation, we present new theoretical results in time and frequency domains. More specifically, we have developed a fourth order perturbation theory of the diffusion equation for absorbing defects. The method of Padé Approximants is used to extend the validity of the proposed theory to a wider range of absorbing contrasts between defects and background medium. The results of the theory are validated by comparisons with Monte Carlo simulations. In the frequency domain, the discrepancy between theoretical and Monte Carlo results for amplitude (AC) data are less than 10% up to an absorption contrast of Deltamua
    Proceedings of SPIE - The International Society for Optical Engineering 02/2011;

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