Fabrizio Martelli

Medical Physics, Optics, Computational Physics

PhD
34.49

Publications

  • Journal of Biomedical Optics 12/2015; 20(12):121304. DOI:10.1117/1.JBO.20.12.121304 · 2.75 Impact Factor
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    ABSTRACT: The in-vivo optical properties of the human head are investigated in the 600–1100 nm range on different subjects using continuous wave and time domain diffuse optical spectroscopy. The work was performed in collaboration with different research groups and the different techniques were applied to the same subject. Data analysis was carried out using homogeneous and layered models and final results were also confirmed by Monte Carlo simulations. The depth sensitivity of each technique was investigated and related to the probed region of the cerebral tissue. This work, based on different validated instruments, is a contribution to fill the existing gap between the present knowledge and the actual in-vivo values of the head optical properties.
    Biomedical Optics Express 06/2015; 6(7):2609-2623. DOI:10.1364/BOE.6.002609 · 3.50 Impact Factor
  • Tiziano Binzoni · Fabrizio Martelli
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    ABSTRACT: It is shown that an analytical noise-free implementation of Monte Carlo simulations [Appl. Opt.54, 2400 (2015).10.1364/AO.54.002400APOPAI1559-128X] for diffuse correlation spectroscopy (DCS) may be successfully used to check the ability of a given DCS model to generate a reliable estimator of tissue blood flow. As an example, four different DCS models often found in the scientific literature are tested on a simulated tissue (semi-infinite geometry) with a Maxwell-Boltzmann probability distribution function for red blood cell speed. It is shown that the random model is the best model for the chosen speed distribution but that (1) some inaccuracies in the DCS model in taking into account red blood cell concentration and (2) some inaccuracies, probably due to a low-order approximation of the DCS model, are still observed. The method can be easily generalized for other speed/flow probability distribution functions of the red blood cells.
    Applied Optics 06/2015; 54(17):5320. DOI:10.1364/AO.54.005320 · 1.78 Impact Factor
  • Optics Express 06/2015; 23(11):13937. DOI:10.1364/OE.23.013937 · 3.49 Impact Factor
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    ABSTRACT: Light is a powerful tool to non-invasively probe highly scattering media for clinical applications ranging from oncology to neurology, but also for molecular imaging, and quality assessment of food, wood and pharmaceuticals. Here we show that, for a paradigmatic case of diffuse optical imaging, ideal yet realistic time-domain systems yield more than 2-fold higher depth penetration and many decades higher contrast as compared to ideal continuous-wave systems, by adopting a dense source-detector distribution with picosecond time-gating. Towards this aim, we demonstrate the first building block made of a source-detector pair directly embedded into the probe based on a pulsed Vertical-Cavity Surface-Emitting Laser (VCSEL) to allow parallelization for dense coverage, a Silicon Photomultiplier (SiPM) to maximize light harvesting, and a Single-Photon Avalanche Diode (SPAD) to demonstrate the time-gating capability on the basic SiPM element. This paves the way to a dramatic advancement in terms of increased performances, new high impact applications, and availability of devices with orders of magnitude reduction in size and cost for widespread use, including quantitative wearable imaging.
    Biomedical Optics Express 05/2015; 6(5). DOI:10.1364/BOE.6.001749 · 3.50 Impact Factor
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    ABSTRACT: This article is about resettled Afghan Hazaras in Australia, many of whom are currently undergoing a complex process of transition (from transience into a more stable position) for the first time in their lives. Despite their permanent residency status, we show how resettlement can be a challenging transitional experience. For these new migrants, we argue that developing a sense of belonging during the transition period is a critical rite of passage in the context of their political and cultural identity. A study of forced migrants such as these, moving out of one transient experience into another transitional period (albeit one that holds greater promise and permanence) poses a unique intellectual challenge. New understandings about the ongoing, unpredictable consequences of ‘transience’ for refugee communities is crucial as we discover what might be necessary, as social support structures, to facilitate the process of transition into a distinctly new environment. The article is based on a doctoral ethnographic study of 30 resettled Afghan Hazara living in the region of Dandenong in Melbourne, Australia. Here, we include four of these participants’ reflections of transition during different phases of their resettlement. These reflections were particularly revealing of the ways in which some migrants deal with change and acquire a sense of belonging to the community. Taking a historical view, and drawing on Bourdieu’s notion of symbolic social capital to highlight themes in individual experiences of belonging, we show how some new migrants adjust and learn to ‘embody’ their place in the new country. Symbolic social capital illuminates how people access and use resources such as social networks as tools of empowerment, reflecting how Hazara post-arrival experiences are tied to complex power relations in their everyday social interactions and in their life trajectories as people in transition. We learned that such tools can facilitate the formation of Hazara migrant identities and are closely tied to their civic community participation, English language development, and orientation in, as well as comprehension of local cultural knowledge and place. This kind of theorization allows refugee, post-refugee and recent migrant narratives to be viewed not merely as static expressions of loss, trauma or damage, but rather as individual experiences of survival, adaptation and upward mobility.
    Journal of the Optical Society of America A 04/2015; 32(4). DOI:10.1364/JOSAA.32.000586 · 1.45 Impact Factor
  • Tiziano Binzoni · Fabrizio Martelli
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    ABSTRACT: Classical Monte Carlo (MC) simulations for laser-Doppler flowmetry (LDF) often necessitate too long computation times and specialized hardware. This is particularly true for LDF at large interoptode spacing with low absorption coefficients and large anisotropic factors representing real biological tissues. For this reason, a random numbers free "analytical" implementation of the classical MC (MC<sub>an</sub>) is proposed. The MC<sub>an</sub> approach allows to obtain noise exempt LDF spectra in a short time and with a simple personal laptop. The proposed MC<sub>an</sub> holds for a diffusive regime of light propagation and it is practically implemented for a semi-infinite geometry. Its validity is demonstrated by comparisons with the classical MC.
    Applied Optics 03/2015; 54(9):2400-6. DOI:10.1364/AO.54.002400 · 1.78 Impact Factor
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    ABSTRACT: The adoption of a short source-detector distance, combined with a time-resolved acquisition, can be advantageous in diffuse optical imaging due to the stricter spatial localization of the probing photons, provided that the strong burst of early photons is suppressed using a time-gated detection scheme. We propose a model for predicting the effect of the time-gated measurement system using a time-variant operator built on the system response acquired at different gate delays. The discrete representation of the system operator, termed Spread Matrix, can be analyzed to identify the bottlenecks of the detection system with respect to the physical problem under study. Measurements performed on tissue phantoms, using a time-gated single-photon avalanche diode and an interfiber distance of 2 mm, demonstrate that inhomogeneities down to 3 cm can be detected only if the decay constant of the detector is lower than 100 ps, while the transient opening of the gate has a less critical impact.
    Journal of Physics D Applied Physics 02/2015; 48(4):045401. DOI:10.1088/0022-3727/48/4/045401 · 2.72 Impact Factor
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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). DOI:10.1117/1.JBO.19.8.086012 · 2.75 Impact Factor
  • Tiziano Binzoni · Fabrizio Martelli
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    ABSTRACT: Analytical models, describing laser Doppler flowmetry and its derived applications, are based on fundamental assumptions of photon scattering angles. It is shown by means of Monte Carlo simulations that, even in the case these assumptions are correct, the presence of a specific source–detector configuration may bias the shape of the probability density functions describing scattering angle behavior. It is found that these biased shapes are generated by selective filtering of photons induced by a particular source–detector configuration. In some specific cases, this phenomenon might invalidate laser Doppler analytical models.
    Applied Optics 07/2014; 53(20). DOI:10.1364/AO.53.004580 · 1.78 Impact Factor
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    ABSTRACT: We present the experimental implementation and validation of a phantom for diffuse optical imaging based on totally absorbing objects for which, in the previous paper [J. Biomed. Opt. 18(6), 066014, (2013)], we have provided the basic theory. Totally absorbing objects have been manufactured as black polyvinyl chloride (PVC) cylinders and the phantom is a water dilution of intralipid-20% as the diffusive medium and India ink as the absorber, filled into a black scattering cell made of PVC. By means of time-domain measurements and of Monte Carlo simulations, we have shown the reliability, the accuracy, and the robustness of such a phantom in mimicking typical absorbing perturbations of diffuse optical imaging. In particular, we show that such a phantom can be used to generate any absorption perturbation by changing the volume and position of the totally absorbing inclusion. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License.
    Journal of Biomedical Optics 07/2014; 19(7):76011. DOI:10.1117/1.JBO.19.7.076011 · 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. DOI:10.1364/JOSAA.31.000460 · 1.45 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. DOI:10.1364/OL.39.000826 · 3.18 Impact Factor
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    ABSTRACT: The optical properties of the human head in the range from 600 nm to 1100 nm have been non-invasively investigated by various research groups using different diffuse optics techniques and data analysis methods.
    Biomedical Optics; 01/2014
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    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.
    European Conferences on Biomedical Optics; 08/2013
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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. DOI:10.1117/1.JBO.18.6.066014 · 2.75 Impact Factor
  • F. Martelli · A. Sassaroli
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    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.
    European Conference on Biomedical Optics; 05/2013
  • 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.
    Conference on Optical Tomography and Spectroscopy of Tissue X; 03/2013
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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; DOI:10.1117/12.2002438 · 0.20 Impact Factor
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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; DOI:10.1117/12.2003665 · 0.20 Impact Factor

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