Performance assessment of photon migration instruments: the MEDPHOT protocol

Effect of CPPU on optical properties of strawberry in near-infrared range during growth

To investigate the effect of CPPU (forchlorfenuron) on optical properties of strawberry during growth, the optical properties (absorption coefficient (μa) and reduced scattering coefficient (μs')) of strawberry treated with CPPU solutions at different concentrations (0, 2.5, 5.0 and 7.5 mg/L) were measured in white, color turning and red stages by using a single integrating sphere system over near-infrared wavelength range of 900-1700 nm. The physicochemical properties, i.e., single fruit weight, soluble solids content, firmness and moisture content, as well as microstructure of strawberry were also investigated. The results showed that in white stage, the μa of strawberry treated with 7.5 mg/L CPPU was significantly (p ≤ 0.05) lower than that of untreated strawberry at absorption peak of 1411 nm. In color turning stage, the μs' of strawberry treated with 5 mg/L CPPU was significantly lower than that of treated with 2.5 mg/L at absorption peaks of 975, 1197 and 1411 nm. In red stage, the μa of strawberry treated with 2.5 mg/L CPPU was significantly (p ≤ 0.05) different from that of treated with 7.5 mg/L at 1197 nm. The study indicates that the optical properties of strawberry were affected by CPPU, and it provides useful information for identifying CPPU treated strawberry.

Optical signatures of thermal damage on ex-vivo brain, lung and heart tissues using time-domain diffuse optical spectroscopy

Thermal therapies treat tumors by means of heat, greatly reducing pain, post-operation complications, and cost as compared to traditional methods. Yet, effective tools to avoid under- or over-treatment are mostly needed, to guide surgeons in laparoscopic interventions. In this work, we investigated the temperature-dependent optical signatures of ex-vivo calf brain, lung, and heart tissues based on the reduced scattering and absorption coefficients in the near-infrared spectral range (657 to 1107 nm). These spectra were measured by time domain diffuse optics, applying a step-like spatially homogeneous thermal treatment at 43 °C, 60 °C, and 80 °C. We found three main increases in scattering spectra, possibly due to the denaturation of collagen, myosin, and the proteins' secondary structure. After 75 °C, we found the rise of two new peaks at 770 and 830 nm in the absorption spectra due to the formation of a new chromophore, possibly related to hemoglobin or myoglobin. This research marks a significant step forward in controlling thermal therapies with diffuse optical techniques by identifying several key markers of thermal damage. This could enhance the ability to monitor and adjust treatment in real-time, promising improved outcomes in tumor therapy.

Development of a glass-based imaging phantom to model the optical properties of human tissue

The fabrication of a stable, reproducible optical imaging phantom is critical to the assessment and optimization of optical imaging systems. We demonstrate the use of an alternative material, glass, for the development of tissue-mimicking phantoms. The glass matrix was doped with nickel ions to approximate the absorption of hemoglobin. Scattering levels representative of human tissue were induced in the glass matrix through controlled crystallization at elevated temperatures. We show that this type of glass is a viable material for creating tissue-mimicking optical phantoms by providing controlled levels of scattering and absorption with excellent optical homogeneity, long-term stability and reproducibility.

In vivocharacterization of the optical and hemodynamic properties of the human sternocleidomastoid muscle through ultrasound-guided hybrid near-infrared spectroscopies

Objective.In this paper, we present a detailedin vivocharacterization of the optical and hemodynamic properties of the human sternocleidomastoid muscle (SCM), obtained through ultrasound-guided near-infrared time-domain and diffuse correlation spectroscopies.Approach.A total of sixty-five subjects (forty-nine females, sixteen males) among healthy volunteers and thyroid nodule patients have been recruited for the study. Their SCM hemodynamic (oxy-, deoxy- and total hemoglobin concentrations, blood flow, blood oxygen saturation and metabolic rate of oxygen extraction) and optical properties (wavelength dependent absorption and reduced scattering coefficients) have been measured by the use of a novel hybrid device combining in a single unit time-domain near-infrared spectroscopy, diffuse correlation spectroscopy and simultaneous ultrasound imaging.Main results.We provide detailed tables of the results related to SCM baseline (i.e. muscle at rest) properties, and reveal significant differences on the measured parameters due to variables such as side of the neck, sex, age, body mass index, depth and thickness of the muscle, allowing future clinical studies to take into account such dependencies.Significance.The non-invasive monitoring of the hemodynamics and metabolism of the sternocleidomastoid muscle during respiration became a topic of increased interest partially due to the increased use of mechanical ventilation during the COVID-19 pandemic. Near-infrared diffuse optical spectroscopies were proposed as potential practical monitors of increased recruitment of SCM during respiratory distress. They can provide clinically relevant information on the degree of the patient's respiratory effort that is needed to maintain an optimal minute ventilation, with potential clinical application ranging from evaluating chronic pulmonary diseases to more acute settings, such as acute respiratory failure, or to determine the readiness to wean from invasive mechanical ventilation.

Two-layered blood-lipid phantom and method to determine absorption and oxygenation employing changes in moments of DTOFs

Near-infrared spectroscopy (NIRS) is an established technique for measuring tissue oxygen saturation (StO₂), which is of high clinical value. For tissues that have layered structures, it is challenging but clinically relevant to obtain StO₂ of the different layers, e.g. brain and scalp. For this aim, we present a new method of data analysis for time-domain NIRS (TD-NIRS) and a new two-layered blood-lipid phantom. The new analysis method enables accurate determination of even large changes of the absorption coefficient (Δµ_a) in multiple layers. By adding Δµ_a to the baseline µ_a, this method provides absolute µ_a and hence StO₂ in multiple layers. The method utilizes (i) changes in statistical moments of the distributions of times of flight of photons (DTOFs), (ii) an analytical solution of the diffusion equation for an N-layered medium, (iii) and the Levenberg-Marquardt algorithm (LMA) to determine Δµ_a in multiple layers from the changes in moments. The method is suitable for NIRS tissue oximetry (relying on µ_a) as well as functional NIRS (fNIRS) applications (relying on Δµ_a). Experiments were conducted on a new phantom, which enabled us to simulate dynamic StO₂ changes in two layers for the first time. Two separate compartments, which mimic superficial and deep layers, hold blood-lipid mixtures that can be deoxygenated (using yeast) and oxygenated (by bubbling oxygen) independently. Simultaneous NIRS measurements can be performed on the two-layered medium (variable superficial layer thickness, L), the deep (homogeneous), and/or the superficial (homogeneous). In two experiments involving ink, we increased the nominal µ_a in one of two compartments from 0.05 to 0.25 cm^-1, L set to 14.5 mm. In three experiments involving blood (L set to 12, 15, or 17 mm), we used a protocol consisting of six deoxygenation cycles. A state-of-the-art multi-wavelength TD-NIRS system measured simultaneously on the two-layered medium, as well as on the deep compartment for a reference. The new method accurately determined µ_a (and hence StO₂) in both compartments. The method is a significant progress in overcoming the contamination from the superficial layer, which is beneficial for NIRS and fNIRS applications, and may improve the determination of StO₂ in the brain from measurements on the head. The advanced phantom may assist in the ongoing effort towards more realistic standardized performance tests in NIRS tissue oximetry. Data and MATLAB codes used in this study were made publicly available.

Solid optical tissue phantom tools based on upconverting nanoparticles for biomedical applications

SIGNIFICANCE

Phantoms play a critical role in the development of biophotonics techniques. There is a lack of novel phantom tools in the emerging field of upconverting nanoparticles (UCNPs) for biophotonics application. This work provides a range of UCNP-based phantom tools and a manufacturing recipe to bridge the gap and accelerate the development of UCNP-based biophotonics applications.

AIM

The study aims to provide a well-characterized UCNP-based solid phantom recipe and set of phantom tools to address a wide range of UCNP-based biophotonics applications.

APPROACH

A solid phantom recipe based on silicone matrix was developed to manufacture UCNP-based phantoms. A lab built UCNP imaging system was used to characterize upconverted fluorescence emission of phantoms for linearity, homogeneity, and long-term stability. A photon time-of-flight spectroscopy technique was used to characterize the optical properties of the phantoms.

RESULTS

In total, 24 phantoms classified into 4 types, namely homogeneous, multilayer, inclusion, and base phantoms, were manufactured. The phantoms exhibit linear behavior over the dosage range of UCNPs. The phantoms were found to be stable over a limited observed period of 4 months with a coefficient of variation of < 4 % . The deep tissue imaging case showed that increasing the thickness of tissue reduced the UCNP emission.

CONCLUSIONS

A first-of-its-kind UCNP-based solid phantom recipe was developed, and four types of UCNP phantom tools to explore biophotonics applications were presented. The UCNP phantoms exhibited a linear behavior with dosage and were stable over time. An example case showed the potential use of the phantom for deep tissue imaging applications. With recent advance in the use of UCNPs for biophotonics, we believe our recipe and tools will play a pivotal role in the growth of the UCNPs for biophotonics applications.

Reliable Fast (20 Hz) Acquisition Rate by a TD fNIRS Device: Brain Resting-State Oscillation Studies

A high power setup for multichannel time-domain (TD) functional near infrared spectroscopy (fNIRS) measurements with high efficiency detection system was developed. It was fully characterized based on international performance assessment protocols for diffuse optics instruments, showing an improvement of the signal-to-noise ratio (SNR) with respect to previous analogue devices, and allowing acquisition of signals with sampling rate up to 20 Hz and source-detector distance up to 5 cm. A resting-state measurement on the motor cortex of a healthy volunteer was performed with an acquisition rate of 20 Hz at a 4 cm source-detector distance. The power spectrum for the cortical oxy- and deoxyhemoglobin is also provided.

Label-Free Optical Spectroscopy for Early Detection of Oral Cancer

Oral cancer is the 16th most common cancer worldwide. It commonly arises from painless white or red plaques within the oral cavity. Clinical outcome is highly related to the stage when diagnosed. However, early diagnosis is complex owing to the impracticality of biopsying every potentially premalignant intraoral lesion. Therefore, there is a need to develop a non-invasive cost-effective diagnostic technique to differentiate non-malignant and early-stage malignant lesions. Optical spectroscopy may provide an appropriate solution to facilitate early detection of these lesions. It has many advantages over traditional approaches including cost, speed, objectivity, sensitivity, painlessness, and ease-of use in clinical setting for real-time diagnosis. This review consists of a comprehensive overview of optical spectroscopy for oral cancer diagnosis, epidemiology, and recent improvements in this field for diagnostic purposes. It summarizes major developments in label-free optical spectroscopy, including Raman, fluorescence, and diffuse reflectance spectroscopy during recent years. Among the wide range of optical techniques available, we chose these three for this review because they have the ability to provide biochemical information and show great potential for real-time deep-tissue point-based in vivo analysis. This review also highlights the importance of saliva-based potential biomarkers for non-invasive early-stage diagnosis. It concludes with the discussion on the scope of development and future demands from a clinical point of view.

Anthropomorphic Polydimethylsiloxane silicone-based phantom for Diffuse Optical Imaging

This work presents a method for constructing phantoms suitable for diffuse optical mammography. They are based on Polydimethylsiloxane silicones, with the characteristic of being anthropomorphic, and having similar mechanical and optical properties as a real breast. These phantoms are useful for testing the performance of diffuse optical imaging devices in the near infrared, both in transmittance and reflectance geometries, since they can be constructed containing inclusions, to simulate breast tumors. An alternative component to be used as scattering agent, that is easier to handle than traditional scattering agents, is also studied.

The optical properties of the phantoms were tested varying the concentration of scattering and absorbing agents, while their mechanical properties were modified by adding a silicone fluid to the basic mixture.

Finally, the phantoms were tested by Diffuse Optical Imaging experiments, and these images were compared to the ones obtained by conventional ultrasound techniques.

Results show that the constructed anthropomorphic phantoms properly reproduce the optical and mechanical characteristics of human breasts, and are suitable to be used in Diffuse Optical Imaging.

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We constructed anthropomorphic phantoms for Diffuse Optical Imaging.

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They simulate the optical and mechanical characteristics of a human breast.

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A new scattering agent was successfully introduced.

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Results of Diffuse Optical images are compared to Ultrasound images.

Lawrence K, Tachtsidis I, Tong Y, Torricelli A, Urner T, Wabnitz H, Wolf M, Wolf U, Xu S, Yang C, Yodh AG, Yücel MA, Zhou W. Optical imaging and spectroscopy for the study of the human brain: status report

This report is the second part of a comprehensive two-part series aimed at reviewing an extensive and diverse toolkit of novel methods to explore brain health and function. While the first report focused on neurophotonic tools mostly applicable to animal studies, here, we highlight optical spectroscopy and imaging methods relevant to noninvasive human brain studies. We outline current state-of-the-art technologies and software advances, explore the most recent impact of these technologies on neuroscience and clinical applications, identify the areas where innovation is needed, and provide an outlook for the future directions.

Multi-laboratory performance assessment of diffuse optics instruments: the BitMap exercise

SIGNIFICANCE

Multi-laboratory initiatives are essential in performance assessment and standardization-crucial for bringing biophotonics to mature clinical use-to establish protocols and develop reference tissue phantoms that all will allow universal instrument comparison.

AIM

The largest multi-laboratory comparison of performance assessment in near-infrared diffuse optics is presented, involving 28 instruments and 12 institutions on a total of eight experiments based on three consolidated protocols (BIP, MEDPHOT, and NEUROPT) as implemented on three kits of tissue phantoms. A total of 20 synthetic indicators were extracted from the dataset, some of them defined here anew.

APPROACH

The exercise stems from the Innovative Training Network BitMap funded by the European Commission and expanded to include other European laboratories. A large variety of diffuse optics instruments were considered, based on different approaches (time domain/frequency domain/continuous wave), at various stages of maturity and designed for different applications (e.g., oximetry, spectroscopy, and imaging).

RESULTS

This study highlights a substantial difference in hardware performances (e.g., nine decades in responsivity, four decades in dark count rate, and one decade in temporal resolution). Agreement in the estimates of homogeneous optical properties was within 12% of the median value for half of the systems, with a temporal stability of <5  %   over 1 h, and day-to-day reproducibility of <3  %  . Other tests encompassed linearity, crosstalk, uncertainty, and detection of optical inhomogeneities.

CONCLUSIONS

This extensive multi-laboratory exercise provides a detailed assessment of near-infrared Diffuse optical instruments and can be used for reference grading. The dataset-available soon in an open data repository-can be evaluated in multiple ways, for instance, to compare different analysis tools or study the impact of hardware implementations.

Special Section Guest Editorial: Tissue Phantoms to Advance Biomedical Optical Systems

The editorial introduces the JBO Special Section on Tissue Phantoms to Advance Biomedical Optical Systems.

Criteria for the design of tissue-mimicking phantoms for the standardization of biophotonic instrumentation

A lack of accepted standards and standardized phantoms suitable for the technical validation of biophotonic instrumentation hinders the reliability and reproducibility of its experimental outputs. In this Perspective, we discuss general criteria for the design of tissue-mimicking biophotonic phantoms, and use these criteria and state-of-the-art developments to critically review the literature on phantom materials and on the fabrication of phantoms. By focusing on representative examples of standardization in diffuse optical imaging and spectroscopy, fluorescence-guided surgery and photoacoustic imaging, we identify unmet needs in the development of phantoms and a set of criteria (leveraging characterization, collaboration, communication and commitment) for the standardization of biophotonic instrumentation.

Effects and correctability of pile-up distortion using established figures of merit in time-domain diffuse optics at extreme photon rates

Time-domain diffuse optics (TD-DO) allows one to probe diffusive media with recognized advantages over other working domains but suffers from a poor signal-to-noise ratio (SNR) resulting from the need to build-up the histogram of single-photon arrival times with maximum count rates (CR) of few percent of the laser pulse rate to avoid the so-called "pile-up" distortion. Here we explore the feasibility of TD-DO under severe pile-up conditions with a systematic in-silico/experimental study evaluating the effects and correctability of the distortion by means of shared figures of merit. In-silico, we demonstrate that pile-up correction allows one the retrieval of homogeneous optical properties with average error < 1% up to a CR > 99%, while the optimal CR needed to detect localized perturbation was found to be 83%. Experiments reported here confirm these findings despite exhibiting higher accuracy errors in the retrieval of homogeneous optical properties and higher noise in the detection of localized absorption perturbations, but in line with the state-of-the-art systems. This work validates a new working regime for TD-DO, demonstrating an increase of the SNR at constant acquisition time, but also potentially leading in the future to previously unrealizable measurements of dynamic phenomena or in spatial scanning applications.

Reproducibility of identical solid phantoms

SIGNIFICANCE

Tissue-like solid phantoms with identical optical properties, known within tolerant uncertainty, are of crucial importance in diffuse optics for instrumentation assessment, interlaboratory comparison studies, industrial standards, and multicentric clinical trials.

AIM

The reproducibility in fabrication of homogeneous solid phantoms is focused based on spectra measurements by instrument comparisons grounded on the time-resolved diffuse optics.

APPROACH

Epoxy-resin and silicone phantoms are considered as matrices and both employ three different instruments for time-resolved diffuse spectroscopy within the spectral range of 540 to 1100 nm. In particular, we fabricated two batches of five phantoms each in epoxy resin and silicone. Then, we evaluated the intra- and interbatch variability with respect to the instrument precision, by considering the coefficient of variation (CV) of absorption and reduced scattering coefficients.

RESULTS

We observed a similar precision for the three instruments, within 2% for repeated measurements on the same phantom. For epoxy-resin phantoms, the intra- and the interbatch variability reached the instrument precision limit, demonstrating a very good phantom reproducibility. For the silicone phantoms, we observed larger values for intra- and interbatch variability. In particular, at worst, for reduced scattering coefficient interbatch CV was about 5%.

CONCLUSIONS

Results suggest that the fabrication of solid phantoms, especially considering epoxy-resin matrix, is highly reproducible, even if they come from different batch fabrications and are measured using different instruments.

Development and Validation of Robust and Cost-Effective Liquid Heterogeneous Phantom for Time Domain Near-Infrared Optical Tomography

Diffused light imaging techniques, such as near-infrared optical tomography (NIROT), require a stable platform for testing and validation that imitates tissue optical properties. The aim of this work was to build a robust, but flexible liquid phantom for BORL time-domain NIROT system Pioneer. The phantom was designed to assess penetration depth and resolution of the system, and to provide a heterogeneous inner structure that can be changed in controllable manner with adjustable optical properties. We used only in-house produced 3D-printed elements and mechanics of a budget 3D-printer to build the phantom, and managed to keep the overall costs below $500. We achieved stable and repeatable movement of an arbitrary structure in 3+1 degree of freedom inside the phantom and demonstrated its performance in a series of tests. Thus, we presented a universal and cost-effective solution for testing NIROT, that can be easily customised to various systems or testing paradigms.

Kernel Flow: a high channel count scalable time-domain functional near-infrared spectroscopy system

SIGNIFICANCE

Time-domain functional near-infrared spectroscopy (TD-fNIRS) has been considered as the gold standard of noninvasive optical brain imaging devices. However, due to the high cost, complexity, and large form factor, it has not been as widely adopted as continuous wave NIRS systems.

AIM

Kernel Flow is a TD-fNIRS system that has been designed to break through these limitations by maintaining the performance of a research grade TD-fNIRS system while integrating all of the components into a small modular device.

APPROACH

The Kernel Flow modules are built around miniaturized laser drivers, custom integrated circuits, and specialized detectors. The modules can be assembled into a system with dense channel coverage over the entire head.

RESULTS

We show performance similar to benchtop systems with our miniaturized device as characterized by standardized tissue and optical phantom protocols for TD-fNIRS and human neuroscience results.

CONCLUSIONS

The miniaturized design of the Kernel Flow system allows for broader applications of TD-fNIRS.

Accurately calibrated frequency domain diffuse optical spectroscopy compared against chemical analysis of porcine adipose tissue

Frequency domain diffuse optical spectroscopy (fdDOS) is a noninvasive technique to estimate tissue composition and hemodynamics. While fdDOS has been established as a valuable modality for clinical research, comparison of fdDOS with direct chemical analysis (CA) methods has yet to be reported. To compare the two approaches, we propose a procedure to confirm accurate calibration by use of liquid emulsion and solid silicone phantoms. Tissue fat (FAT) and water (H₂ O) content of two ex vivo porcine tissue samples were optically measured by fdDOS and compared to CA values. We show an average H₂ O error (fdDOS minus CA) and SD of 1.9 ± 0.2% and -0.9 ± 0.2% for the two samples. For FAT, we report a mean error of -9.3 ± 1.3% and 0.8 ± 1.3%. We also measured various body sites of a healthy human subject using fdDOS with results suggesting that accurate calibration may improve device sensitivity.

Polydimethylsiloxane tissue-mimicking phantoms with tunable optical properties

SIGNIFICANCE

The polymer, polydimethylsiloxane (PDMS), has been increasingly used to make tissue simulating phantoms due to its excellent processability, durability, flexibility, and limited tunability of optical, mechanical, and thermal properties. We report on a robust technique to fabricate PDMS-based tissue-mimicking phantoms where the broad range of scattering and absorption properties are independently adjustable in the visible- to near-infrared wavelength range from 500 to 850 nm. We also report on an analysis method to concisely quantify the phantoms' broadband characteristics with four parameters.

AIM

We report on techniques to manufacture and characterize solid tissue-mimicking phantoms of PDMS polymers. Tunability of the absorption (μa  (  λ  )  ) and reduced scattering coefficient spectra (μs'(λ)) in the wavelength range of 500 to 850 nm is demonstrated by adjusting the concentrations of light absorbing carbon black powder (CBP) and light scattering titanium dioxide powder (TDP) added into the PDMS base material.

APPROACH

The μa  (  λ  )   and μs'(λ) of the phantoms were obtained through measurements with a broadband integrating sphere system and by applying an inverse adding doubling algorithm. Analyses of μa  (  λ  )   and μs'(λ) of the phantoms, by fitting them to linear and power law functions, respectively, demonstrate that independent control of μa  (  λ  )   and μs'(λ) is possible by systematically varying the concentrations of CBP and TDP.

RESULTS

Our technique quantifies the phantoms with four simple fitting parameters enabling a concise tabulation of their broadband optical properties as well as comparisons to the optical properties of biological tissues. We demonstrate that, to a limited extent, the scattering properties of our phantoms mimic those of human tissues of various types. A possible way to overcome this limitation is demonstrated with phantoms that incorporate polystyrene microbead scatterers.

CONCLUSIONS

Our manufacturing and analysis techniques may further promote the application of PDMS-based tissue-mimicking phantoms and may enable robust quality control and quality checks of the phantoms.

Time-domain NIRS system based on supercontinuum light source and multi-wavelength detection: validation for tissue oxygenation studies

We present and validate a multi-wavelength time-domain near-infrared spectroscopy (TD-NIRS) system that avoids switching wavelengths and instead exploits the full capability of a supercontinuum light source by emitting and acquiring signals for the whole chosen range of wavelengths. The system was designed for muscle and brain oxygenation monitoring in a clinical environment. A pulsed supercontinuum laser emits broadband light and each of two detection modules acquires the distributions of times of flight of photons (DTOFs) for 16 spectral channels (used width 12.5 nm / channel), providing a total of 32 DTOFs at up to 3 Hz. Two emitting fibers and two detection fiber bundles allow simultaneous measurements at two positions on the tissue or at two source-detector separations. Three established protocols (BIP, MEDPHOT, and nEUROPt) were used to quantitatively assess the system's performance, including linearity, coupling, accuracy, and depth sensitivity. Measurements were performed on 32 homogeneous phantoms and two inhomogeneous phantoms (solid and liquid). Furthermore, measurements on two blood-lipid phantoms with a varied amount of blood and Intralipid provide the strongest validation for accurate tissue oximetry. The retrieved hemoglobin concentrations and oxygen saturation match well with the reference values that were obtained using a commercially available NIRS system (OxiplexTS) and a blood gas analyzer (ABL90 FLEX), except a discrepancy occurs for the lowest amount of Intralipid. In-vivo measurements on the forearm of three healthy volunteers during arterial (250 mmHg) and venous (60 mmHg) cuff occlusions provide an example of tissue monitoring during the expected hemodynamic changes that follow previously well-described physiologies. All results, including quantitative parameters, can be compared to other systems that report similar tests. Overall, the presented TD-NIRS system has an exemplary performance evaluated with state-of-the-art performance assessment methods.

Near-Infrared Optical Spectroscopy In Vivo Distinguishes Subjects with Alzheimer's Disease from Age-Matched Controls

Background:

Medical imaging methods such as PET and MRI aid clinical assessment of Alzheimer’s disease (AD). Less expensive, less technically demanding, and more widely deployable technologies are needed to expand objective screening for diagnosis, treatment, and research. We previously reported brain tissue near-infrared optical spectroscopy (NIR) in vitro indicating the potential to meet this need.

Objective:

To determine whether completely non-invasive, clinical, NIR in vivo can distinguish AD patients from age-matched controls and to show the potential of NIR as a clinical screen and monitor of therapeutic efficacy.

Methods:

NIR spectra were acquired in vivo. Three groups were studied: autopsy-confirmed AD, control and mild cognitive impairment (MCI). A feature selection approach using the first derivative of the intensity normalized spectra was used to discover spectral regions that best distinguished “AD-alone” (i.e., without other significant neuropathology) from controls. The approach was then applied to other autopsy-confirmed AD cases and to clinically diagnosed MCI cases.

Results:

Two regions about 860 and 895 nm completely separate AD patients from controls and differentiate MCI subjects according to the degree of impairment. The 895 nm feature is more important in separating MCI subjects from controls (ratio-of-weights: 1.3); the 860 nm feature is more important for distinguishing MCI from AD (ratio-of-weights: 8.2).

Conclusion:

These results form a proof of the concept that near-infrared spectroscopy can detect and classify diseased and normal human brain in vivo. A clinical trial is needed to determine whether the two features can track disease progression and monitor potential therapeutic interventions.

Narrowband diffuse reflectance spectroscopy in the 900-1000 nm wavelength region to quantify water and lipid content of turbid media

We report a narrow wavelength band diffuse reflectance spectroscopy (nb-DRS) method to determine water and fat ratios of scattering media in the 900-1000 nm range. This method was shown to be linearly correlated with absolute water and fat concentrations as tested on a set of turbid emulsion phantoms with a range of water and lipid compositions. Robustness to scattering assumptions was demonstrated and compared against measured scattering by a frequency-domain photon migration system. nb-DRS was also tested on ex-vivo porcine samples and compared against direct tissue water extraction by analytical chemistry methods. We speculate nb-DRS has potential applications in portable devices such as clinical and digital health wearables.

The LUCA device: a multi-modal platform combining diffuse optics and ultrasound imaging for thyroid cancer screening

We present the LUCA device, a multi-modal platform combining eight-wavelength near infrared time resolved spectroscopy, sixteen-channel diffuse correlation spectroscopy and a clinical ultrasound in a single device. By simultaneously measuring the tissue hemodynamics and performing ultrasound imaging, this platform aims to tackle the low specificity and sensitivity of the current thyroid cancer diagnosis techniques, improving the screening of thyroid nodules. Here, we show a detailed description of the device, components and modules. Furthermore, we show the device tests performed through well established protocols for phantom validation, and the performance assessment for in vivo. The characterization tests demonstrate that LUCA device is capable of performing high quality measurements, with a precision in determining in vivo tissue optical and dynamic properties of better than 3%, and a reproducibility of better than 10% after ultrasound-guided probe repositioning, even with low photon count-rates, making it suitable for a wide variety of clinical applications.

The Use of Supercontinuum Laser Sources in Biomedical Diffuse Optics: Unlocking the Power of Multispectral Imaging

Optical techniques based on diffuse optics have been around for decades now and are making their way into the day-to-day medical applications. Even though the physics foundations of these techniques have been known for many years, practical implementation of these technique were hindered by technological limitations, mainly from the light sources and/or detection electronics. In the past 20 years, the developments of supercontinuum laser (SCL) enabled to unlock some of these limitations, enabling the development of system and methodologies relevant for medical use, notably in terms of spectral monitoring. In this review, we focus on the use of SCL in biomedical diffuse optics, from instrumentation and methods developments to their use for medical applications. A total of 95 publications were identified, from 1993 to 2021. We discuss the advantages of the SCL to cover a large spectral bandwidth with a high spectral power and fast switching against the disadvantages of cost, bulkiness, and long warm up times. Finally, we summarize the utility of using such light sources in the development and application of diffuse optics in biomedical sciences and clinical applications.

Optical signatures of radiofrequency ablation in biological tissues

Accurate monitoring of treatment is crucial in minimally-invasive radiofrequency ablation in oncology and cardiovascular disease. We investigated alterations in optical properties of ex-vivo bovine tissues of the liver, heart, muscle, and brain, undergoing the treatment. Time-domain diffuse optical spectroscopy was used, which enabled us to disentangle and quantify absorption and reduced scattering spectra. In addition to the well-known global (1) decrease in absorption, and (2) increase in reduced scattering, we uncovered new features based on sensitive detection of spectral changes. These absorption spectrum features are: (3) emergence of a peak around 840 nm, (4) redshift of the 760 nm deoxyhemoglobin peak, and (5) blueshift of the 970 nm water peak. Treatment temperatures above 100 °C led to (6) increased absorption at shorter wavelengths, and (7) further decrease in reduced scattering. This optical behavior provides new insights into tissue response to thermal treatment and sets the stage for optical monitoring of radiofrequency ablation.

In vivo test-driven upgrade of a time domain multi-wavelength optical mammograph

A recent upgrade of the time domain multi-wavelength optical mammograph developed by Politecnico di Milano achieved good performance in laboratory tests [Biomed. Opt. Express9, 755 (2018).10.1364/BOE.9.000755]. However, it proved unsatisfactory when in vivo measurements were finally performed. That led to a further upgrade, including the replacement of the time-to-digital converter with a new model, and the related set-up changes. The new instrument version offers improved laboratory performance (as assessed through established protocols: BIP and MEDPHOT) and good in vivo performance (extension of the scanned breast area, repeatability, consistency of estimated tissue composition with physiology). Besides introducing the new set-up and detailing its laboratory and in vivo performance, we highlight the importance of systematic in vivo testing before entering clinical trials.

Best practices for fNIRS publications

The application of functional near-infrared spectroscopy (fNIRS) in the neurosciences has been expanding over the last 40 years. Today, it is addressing a wide range of applications within different populations and utilizes a great variety of experimental paradigms. With the rapid growth and the diversification of research methods, some inconsistencies are appearing in the way in which methods are presented, which can make the interpretation and replication of studies unnecessarily challenging. The Society for Functional Near-Infrared Spectroscopy has thus been motivated to organize a representative (but not exhaustive) group of leaders in the field to build a consensus on the best practices for describing the methods utilized in fNIRS studies. Our paper has been designed to provide guidelines to help enhance the reliability, repeatability, and traceability of reported fNIRS studies and encourage best practices throughout the community. A checklist is provided to guide authors in the preparation of their manuscripts and to assist reviewers when evaluating fNIRS papers.

Probe-hosted large area silicon photomultiplier and high-throughput timing electronics for enhanced performance time-domain functional near-infrared spectroscopy

Two main bottlenecks prevent time-domain diffuse optics instruments to reach their maximum performances, namely the limited light harvesting capability of the detection chain and the bounded data throughput of the timing electronics. In this work, for the first time to our knowledge, we overcome both those limitations using a probe-hosted large area silicon photomultiplier detector coupled to high-throughput timing electronics. The system performances were assessed based on international protocols for diffuse optical imagers showing better figures with respect to a state-of-the-art device. As a first step towards applications, proof-of-principle in-vivo brain activation measurements demonstrated superior signal-to-noise ratio as compared to current technologies.

Wearable and wireless time-domain near-infrared spectroscopy system for brain and muscle hemodynamic monitoring

We present a wearable time-domain near infrared spectroscopy (TD-NIRS) system (two wavelengths, one detection channel), which fits in a backpack and performs real-time hemodynamic measurements on the brain and muscle tissues of freely moving subjects. It can provide concentration values of oxygenated hemoglobin (O2Hb), deoxygenated hemoglobin (HHb), total hemoglobin (tHb = O2Hb + HHb) and tissue oxygen saturation (StO2). The system is battery-operated and can be wirelessly controlled. By following established characterization protocols for performance assessment of diffuse optics instruments, we achieved results comparable with state-of-the-art research-grade TD-NIRS systems. We also performed in-vivo measurements such as finger tapping (motor cortex monitoring), breath holding (prefrontal cortex monitoring and forearm muscle monitoring), and outdoor bike riding (vastus lateralis muscle monitoring), in order to test the system capabilities in evaluating both muscle and brain hemodynamics.

Quantitative diffuse optical spectroscopy for noninvasive measurements of the malaria pigment hemozoin

Hemozoin (Hz) is a crystal by-product of hemoglobin consumption by malaria parasites. There are currently no in vivo deep tissue sensing methods that can quantify Hz presence noninvasively, which would be advantageous for malaria research and treatment. In this work, we describe the broadband near-infrared optical characterization of synthetic Hz in static and dynamic tissue-simulating phantoms. Using hybrid frequency domain and continuous-wave near-infrared spectroscopy, we quantified the broadband optical absorption and scattering spectra of Hz and identified the presence of Hz at a minimum tissue-equivalent concentration of 0.014 µg/mL in static lipid emulsion phantoms simulating human adipose. We then constructed a whole blood-containing tissue-simulating phantom and demonstrated the detection of Hz at physiologically-relevant tissue oxygen saturations ranging from 70-90%. Our results suggest that quantitative diffuse optical spectroscopy may be useful for detecting deep tissue Hz in vivo.

Direct estimation of the reduced scattering coefficient from experimentally measured time-resolved reflectance via Monte Carlo based lookup tables

A heuristic method for estimating the reduced scattering coefficient (µ_s') of turbid media using time-resolved reflectance is presented. The technique requires measurements of the distributions of times-of-flight (DTOF) of photons arriving at two identical detection channels placed at unique distances relative to a source. Measured temporal shifts in DTOF peak intensities at the two channels were used to estimate µ_s' of the medium using Monte Carlo (MC) simulation-based lookup tables. MC simulations were used to compute temporal shifts in modeled reflectance at experimentally employed source-detector separations (SDS) for media spanning a wide range of optical properties to construct look up tables. Experiments in Intralipid (IL) phantoms demonstrated that we could retrieve µ_s' with errors ranging between 6-25% of expected (literature) values, using reflectance measured across 650-800 nm and SDS of 5-15 mm. Advantages of the technique include direct processing of measured data without requiring iterative non-linear curve fitting. We also discuss applicability of this approach for media with low scattering coefficients where the commonly employed diffusion theory analysis could be inaccurate, with practical recommendations for use.

Non-invasive investigation of adipose tissue by time domain diffuse optical spectroscopy

The human abdominal region is very heterogeneous and stratified with subcutaneous adipose tissue (SAT) being one of the primary layers. Monitoring this tissue is crucial for diagnostic purposes and to estimate the effects of interventions like caloric restriction or bariatric surgery. However, the layered nature of the abdomen poses a major problem in monitoring the SAT in a non-invasive way by diffuse optics. In this work, we examine the possibility of using multi-distance broadband time domain diffuse optical spectroscopy to assess the human abdomen non-invasively. Broadband absorption and reduced scattering spectra from 600 to 1100 nm were acquired at 1, 2 and 3 cm source-detector distances on ten healthy adult male volunteers, and then analyzed using a homogeneous model as an initial step to understand the origin of the detected signal and how tissue should be modeled to derive quantitative information. The results exhibit a clear influence of the layered nature on the estimated optical properties. Clearly, the underlying muscle makes a relevant contribution in the spectra measured at the largest source-detector distance for thinner subjects related to blood and water absorption. More unexpectedly, also the thin superficial skin layer yields a direct contamination, leading to higher water content and steeper reduced scattering spectra at the shortest distance, as confirmed also by simulations. In conclusion, provided that data analysis properly accounts for the complex tissue structure, diffuse optics may offer great potential for the continuous non-invasive monitoring of abdominal fat.

Optical characterization of porcine tissues from various organs in the 650-1100 nm range using time-domain diffuse spectroscopy

We present a systematic characterization of the optical properties (µ_a and µ_s') of nine representative ex vivo porcine tissues over a broadband spectrum (650-1100 nm). We applied time-resolved diffuse optical spectroscopy measurements for recovering the optical properties of porcine tissues depicting a realistic representation of the tissue heterogeneity and morphology likely to be found in different ex vivo tissues. The results demonstrate a large spectral and inter-tissue variation of optical properties. The data can be exploited for planning or simulating ex vivo experiments with various biophotonics techniques, or even to construct artificial structures mimicking specific pathologies exploiting the wide assortment in optical properties.

Instrument response function acquisition in reflectance geometry for time-resolved diffuse optical measurements

In time-domain diffuse optical spectroscopy, the simultaneous acquisition of the time-of-flight distribution (DTOF) of photons traveling in a diffusive medium and of the instrument response function (IRF) is necessary to perform quantitative measurements of optical properties (absorption and reduced scattering coefficients) while taking into account the non-idealities of a real system (e.g. temporal resolution and time delays). The IRF acquisition can be a non-trivial and time-consuming operation that requires directly facing the injection and collection fibers. Since this operation is not always possible, a new IRF measurement scheme is here proposed where the IRF is acquired in reflectance geometry from a corrugate reflective surface. Validation measurements on a set of reference homogenous phantoms have been performed, resulting in an error in the optical properties estimation lower than 10% with respect to the typical IRF configuration. Thus, the proposed method proved to be a reliable approach that after a preliminary calibration can be exploited in a laboratory and clinical set-ups, leading to faster and more accurate measurements and reducing the operator-dependent performance.

Broadband Time Domain Diffuse Optical Reflectance Spectroscopy: A Review of Systems, Methods, and Applications

This review presents recent developments and a wide overview of broadband time domain diffuse optical spectroscopy (TD-DOS). Various topics including physics of photon migration, advanced instrumentation, methods of analysis, applications covering multiple domains (tissue chromophore, in vivo studies, food, wood, pharmaceutical industry) are elaborated. The key role of standardization and recent studies in that direction are discussed. Towards the end, a brief outlook is presented on the current status and future trends in broadband TD-DOS.

Systematic study of the effect of ultrasound gel on the performances of time-domain diffuse optics and diffuse correlation spectroscopy

Recently, multimodal imaging has gained an increasing interest in medical applications thanks to the inherent combination of strengths of the different techniques. For example, diffuse optics is used to probe both the composition and the microstructure of highly diffusive media down to a depth of few centimeters, but its spatial resolution is intrinsically low. On the other hand, ultrasound imaging exhibits the higher spatial resolution of morphological imaging, but without providing solid constitutional information. Thus, the combination of diffuse optical imaging and ultrasound may improve the effectiveness of medical examinations, e.g. for screening or diagnosis of tumors. However, the presence of an ultrasound coupling gel between probe and tissue can impair diffuse optical measurements like diffuse optical spectroscopy and diffuse correlation spectroscopy, since it may provide a direct path for photons between source and detector. A systematic study on the effect of different ultrasound coupling fluids was performed on tissue-mimicking phantoms, confirming that a water-clear gel can produce detrimental effects on optical measurements when recovering absorption/reduced scattering coefficients from time-domain spectroscopy acquisitions as well as particle Brownian diffusion coefficient from diffuse correlation spectroscopy ones. On the other hand, we show the suitability for optical measurements of other types of diffusive fluids, also compatible with ultrasound imaging.

Approaches for calibration and validation of near-infrared optical methods for oxygenation monitoring

Pulse oximetry for arterial oxygenation monitoring and tissue oximetry for monitoring of cerebral oxygenation or muscle oxygenation are based on quantitative in vivo diffuse optical spectroscopy. However, in both cases the information on absolute or relative concentration of human tissue constituents and especially on hemoglobin oxygenation can often not be retrieved by model-based analysis. An in vivo calibration against an accepted reference measurement can be a practical alternative. Pulse oximeters and most of commercial cerebral tissue oximeters rely on empirical calibration based on invasive controlled human desaturation studies. As invasive in vivo tests on healthy subjects are ethically disputable and should be limited to exceptional cases this calibration practice is unsatisfactory. We present the current status and problems of calibration and validation in pulse oximetry and cerebral tissue oximetry including the pros and cons of in vivo as well as in vitro methods. We emphasize various digital and physical phantom approaches and discuss the prospects of their application and possible further developments.

Accuracy and precision of tissue optical properties and hemodynamic parameters estimated by the BabyLux device: a hybrid time-resolved near-infrared and diffuse correlation spectroscopy neuro-monitor

We have investigated the accuracy and precision of "the BabyLux device", a hybrid time-resolved near-infrared (TRS) and diffuse correlation spectroscopy (DCS) neuro-monitor for the pre-term infant. Numerical data with realistic noise were simulated and analyzed using the BabyLux device as a reference system and different experimental and analysis parameters. The results describe the limits for the precision and the accuracy to be expected. The dependence of these limits on different experimental conditions and choices of the analysis method is also described. Experiments demonstrate comparable values for precision with respect to the simulation results.

Clinical Brain Monitoring with Time Domain NIRS: A Review and Future Perspectives

Near-infrared spectroscopy (NIRS) is an optical technique that can measure brain tissue oxygenation and haemodynamics in real-time and at the patient bedside allowing medical doctors to access important physiological information. However, despite this, the use of NIRS in a clinical environment is hindered due to limitations, such as poor reproducibility, lack of depth sensitivity and poor brain-specificity. Time domain NIRS (or TD-NIRS) can resolve these issues and offer detailed information of the optical properties of the tissue, allowing better physiological information to be retrieved. This is achieved at the cost of increased instrument complexity, operation complexity and price. In this review, we focus on brain monitoring clinical applications of TD-NIRS. A total of 52 publications were identified, spanning the fields of neonatal imaging, stroke assessment, traumatic brain injury (TBI) assessment, brain death assessment, psychiatry, peroperative care, neuronal disorders assessment and communication with patient with locked-in syndrome. In all the publications, the advantages of the TD-NIRS measurement to (1) extract absolute values of haemoglobin concentration and tissue oxygen saturation, (2) assess the reduced scattering coefficient, and (3) separate between extra-cerebral and cerebral tissues, are highlighted; and emphasize the utility of TD-NIRS in a clinical context. In the last sections of this review, we explore the recent developments of TD-NIRS, in terms of instrumentation and methodologies that might impact and broaden its use in the hospital.

Solid phantom recipe for diffuse optics in biophotonics applications: a step towards anatomically correct 3D tissue phantoms

We present a tissue mimicking optical phantom recipe to create robust well tested solid phantoms. The recipe consists of black silicone pigment (absorber), silica microspheres (scatterer) and silicone rubber (SiliGlass, bulk material). The phantom recipe was characterized over a broadband spectrum (600-1100 nm) for a wide range of optical properties (absorption 0.1-1 cm^-1, reduced scattering 5-25 cm^-1) that are relevant to human organs. The results of linearity show a proper scaling of optical properties as well as the absence of coupling between the absorber and scatterer at different concentrations. A reproducibility of 4% among different preparations was obtained, with a similar grade of spatial homogeneity. Finally, a 3D non-scattering mock-up phantom of an infant torso made with the same recipe bulk material (SiliGlass) was presented to project the futuristic aspect of our work that is 3D printing human organs of biomedical relevance.

BabyLux device: a diffuse optical system integrating diffuse correlation spectroscopy and time-resolved near-infrared spectroscopy for the neuromonitoring of the premature newborn brain

The BabyLux device is a hybrid diffuse optical neuromonitor that has been developed and built to be employed in neonatal intensive care unit for the noninvasive, cot-side monitoring of microvascular cerebral blood flow and blood oxygenation. It integrates time-resolved near-infrared and diffuse correlation spectroscopies in a user-friendly device as a prototype for a future medical grade device. We present a thorough characterization of the device performance using test measurements in laboratory settings. Tests on solid phantoms report an accuracy of optical property estimation of about 10%, which is expected when using the photon diffusion equation as the model. The measurement of the optical and dynamic properties is stable during several hours of measurements within 3% of the average value. In addition, these measurements are repeatable between different days of measurement, showing a maximal variation of 5% in the optical properties and 8% for the particle diffusion coefficient on a liquid phantom. The variability over test/retest evaluation is < 3 % . The integration of the two modalities is robust and without any cross talk between the two. We also perform in vivo measurements on the adult forearm during arterial cuff occlusion to show that the device can measure a wide range of tissue hemodynamic parameters. We suggest that this platform can form the basis of the next-generation neonatal neuromonitors to be developed for extensive, multicenter clinical testing.

MAESTROS: A Multiwavelength Time-Domain NIRS System to Monitor Changes in Oxygenation and Oxidation State of Cytochrome-C-Oxidase

We present a multiwavelength, multichannel, time-domain near-infrared spectroscopy system named MAESTROS. This instrument can measure absorption and scattering coefficients and can quantify the concentrations of oxy- and deoxy-haemoglobin ([HbO₂], [HHb]), and oxidation state of cytochrome-c-oxidase ([oxCCO]). This system is composed of a supercontinuum laser source coupled with two acousto-optic tuneable filters. The light is collected by four photomultipliers tubes, connected to a router to redirect the signal to a single time-correlated single-photon counting card. The interface between the system and the tissue is based on optical fibres. This arrangement allows us to resolve up to 16 wavelengths, within the range of 650-900 nm, at a sampling rate compatible with the physiology (from 0.5 to 2 Hz). In this paper, we describe the system and assess its performance based on two specifically designed protocols for photon migration instruments, the basic instrument protocol and nEUROPt protocols, and on a well characterized liquid phantom based on Intralipid and water. Then, the ability to resolve [HbO₂ ], [HHb], and [oxCCO] is demonstrated on a homogeneous liquid phantom, based on blood for [HbO₂], [HHb], and yeast for [oxCCO]. In the future, the system could be used to monitor brain tissue physiology.

Characterization of a Time-Resolved Diffuse Optical Spectroscopy Prototype Using Low-Cost, Compact Single Photon Avalanche Detectors for Tissue Optics Applications

Time-resolved diffuse optical spectroscopy (TR-DOS) is an increasingly used method to determine the optical properties of diffusive media, particularly for medical applications including functional brain, breast and muscle measurements. For medical imaging applications, important features of new generation TR-DOS systems are low-cost, small size and efficient inverse modeling. To address the issues of low-cost, compact size and high integration capabilities, we have developed free-running (FR) single-photon avalanche diodes (SPADs) using 130 nm silicon complementary metal-oxide-semiconductor (CMOS) technology and used it in a TR-DOS prototype. This prototype was validated using assessments from two known protocols for evaluating TR-DOS systems for tissue optics applications. Following the basic instrumental performance protocol, our prototype had sub-nanosecond total instrument response function and low differential non-linearity of a few percent. Also, using light with optical power lower than the maximum permissible exposure for human skin, this prototype can acquire raw data in reflectance geometry for phantoms with optical properties similar to human tissues. Following the MEDPHOT protocol, the absolute values of the optical properties for several homogeneous phantoms were retrieved with good accuracy and linearity using a best-fitting model based on the Levenberg-Marquardt method. Overall, the results of this study show that our silicon CMOS-based SPAD detectors can be used to build a multichannel TR-DOS prototype. Also, real-time functional monitoring of human tissue such as muscles, breasts and newborn heads will be possible by integrating this detector with a time-to-digital converter (TDC).

Agarose-based Tissue Mimicking Optical Phantoms for Diffuse Reflectance Spectroscopy

This protocol describes how to make agarose-based tissue-mimicking phantoms and demonstrates how to determine their optical properties using a conventional optical system with an integrating sphere. Measuring systems for the acquisition of the diffuse reflectance and total transmittance spectra are constructed with a broadband white light source, a light guide, an achromatic lens, an integrating sphere, a sample holder, an optical fiber probe, and a multi-channel spectrometer. An acrylic mold consisting of two rectangular acrylic pieces and a U-shaped acrylic piece is constructed to create an epidermal phantom and a dermal phantom with whole blood. The application of a sodium dithionite (Na2S2O4) solution to the dermal phantom enables the researcher to deoxygenate hemoglobin in red blood cells distributed in the dermal phantom. The inverse Monte Carlo simulation with the diffuse reflectance and total transmittance spectra measured by a spectrometer with an integrating sphere is performed to determine the absorption coefficient spectrum µa(λ) and the reduced scattering coefficient spectrum µs'(λ) of each layer phantom. A two-layered phantom mimicking the diffuse reflectance of human skin tissue is also demonstrated by piling up the epidermal phantom on the dermal phantom.

Broadband (550-1350 nm) diffuse optical characterization of thyroid chromophores

Thyroid plays an important role in the endocrine system of the human body. Its characterization by diffuse optics can open new path ways in the non-invasive diagnosis of thyroid pathologies. Yet, the absorption spectra of tyrosine and thyroglobulin–key tissue constituents specific to the thyroid organ–in the visible to near infrared range are not fully available. Here, we present the optical characterization of tyrosine (powder), thyroglobulin (granular form) and iodine (aqueous solution) using a time domain broadband diffuse optical spectrometer in the 550–1350 nm range. Various systematic errors caused by physics of photo migration and sample inherent properties were effectively suppressed by means of advanced time domain diffuse optical methods. A brief comparison with various other known tissue constituents is presented, which reveals key spectral regions for the quantification of the thyroid absorbers in an in vivo scenario.

High throughput detection chain for time domain optical mammography

A novel detection chain, based on 8 Silicon Photomultipliers (forming a wide-area custom-made detection probe) and on a time-to-digital converter, was developed to improve the signal level in multi-wavelength (635-1060 nm) time domain optical mammography. The performances of individual components and of the overall chain were assessed using established protocols (BIP and MEDPHOT). The photon detection efficiency was improved by up to 3 orders of magnitude, and the maximum count rate level was increased by a factor of 10 when compared to the previous system, based on photomultiplier tubes and conventional time-correlated single-photon counting boards. In the estimate of optical parameters, the novel detection chain provides performances comparable to the previous system, widely validated in clinics, but with higher signal level, higher robustness, and at a lower price per channel, thus targeting important requirements for clinical applications.

Time Domain Near Infrared Spectroscopy Device for Monitoring Muscle Oxidative Metabolism: Custom Probe and In Vivo Applications

Measurement of muscle oxidative metabolism is of interest for monitoring the training status in athletes and the rehabilitation process in patients. Time domain near infrared spectroscopy (TD NIRS) is an optical technique that allows the non-invasive measurement of the hemodynamic parameters in muscular tissue: concentrations of oxy- and deoxy-hemoglobin, total hemoglobin content, and tissue oxygen saturation. In this paper, we present a novel TD NIRS medical device for muscle oxidative metabolism. A custom-printed 3D probe, able to host optical elements for signal acquisition from muscle, was develop for TD NIRS in vivo measurements. The system was widely characterized on solid phantoms and during in vivo protocols on healthy subjects. In particular, we tested the in vivo repeatability of the measurements to quantify the error that we can have by repositioning the probe. Furthermore, we considered a series of acquisitions on different muscles that were not yet previously performed with this custom probe: a venous-arterial cuff occlusion of the arm muscle, a cycling exercise, and an isometric contraction of the vastus lateralis.

Towards the use of bioresorbable fibers in time-domain diffuse optics

In the last years bioresorbable materials are gaining increasing interest for building implantable optical components for medical devices. In this work we show the fabrication of bioresorbable optical fibers designed for diffuse optics applications, featuring large core diameter (up to 200 μm) and numerical aperture (0.17) to maximize the collection efficiency of diffused light. We demonstrate the suitability of bioresorbable fibers for time-domain diffuse optical spectroscopy firstly checking the intrinsic performances of the setup by acquiring the instrument response function. We then validate on phantoms the use of bioresorbable fibers by applying the MEDPHOT protocol to assess the performance of the system in measuring optical properties (namely, absorption and scattering coefficients) of homogeneous media. Further, we show an ex-vivo validation on a chicken breast by measuring the absorption and scattering spectra in the 500-1100 nm range using interstitially inserted bioresorbable fibers. This work represents a step toward a new way to look inside the body using optical fibers that can be implanted in patients. These fibers could be useful either for diagnostic (e. g. for monitoring the evolution after surgical interventions) or treatment (e. g. photodynamic therapy) purposes. Picture: Microscopy image of the 100 μm core bioresorbable fiber.