Accuracy of retrieving optical properties from liquid tissue phantoms using a single integrating sphere.

Using integrating spheres (ISs) in conjunction with the inverse adding-doubling algorithm (IAD) offers a well-established, rigorous protocol for determining optical absorption (µa) and reduced scattering (µs') coefficients of thin, optically homogeneous, turbid media. Here, we report the performance and use of a single IS system for experimentally retrieving optical properties in phantom media whose optical properties were well controlled. The IS system was used to measure the total reflectance and transmittance between 500 and 800 nm in liquid phantoms that were prepared to span a wide range of optical scattering and absorption coefficients. Measurements on phantoms were sequentially made using one of two broadband light sources-a halogen lamp or a supercontinuum laser. We report on the accuracy of IAD-derived optical coefficients using IS measurements made on phantoms-directly or by employing one of two previously reported correction methods. The first (sample-substitution error) correction was experimentally achieved while the second used Monte Carlo-based corrections with IAD. When experimentally calibrated reflectance and transmittance values were directly used as inputs to the IAD, mean absolute errors in recovered optical coefficients were larger than 0.4cm-1 for absorption and more than 6cm-1 for scattering across all phantoms and wavelengths measured. These errors reduced to 0.06-0.17cm-1 and 0.7-2cm-1 for µa and µs', respectively, with the use of corrections. Choice of light sources used, sample geometry (relative to optical coefficients), signal-to-noise of measurements, and the selection of correction methods are discussed.

In vitro Comparison of Adipogenic Differentiation in Human Adipose-Derived Stem Cells Cultured with Collagen Gel and Platelet-Rich Fibrin.

Background: Adipose-derived stem cells (ADSCs) are the most preferred cell type, based on their phenotypic characteristics, plasticity, and favorable immunological properties for applications in soft-tissue augmentation. Hence, the present in vitro study was aimed to evaluate the adipogenic differentiation potential of human ADSCs upon culturing individually with collagen gel and platelet-rich fibrin (PRF). Materials and methods: The collected lipoaspirate was used for establishing ADSCs using enzymatic digestion method. Then, the cells were analyzed for their morphology, viability, proliferation rate, population doubling time (PDT), colony-forming ability, cell surface markers expression, and osteogenic differentiation as biological properties. Further, ADSCs were evaluated for their adipogenicity using induction media alone, and by culturing with collagen gel and PRF individually for prospective tissue augmentation. Results: ADSCs were successfully established in vitro and exhibited a fibroblast-like morphology throughout the culture period. Cells had higher viability, proliferation potential and showed their ability to form colonies. The positive expression of cell surface markers and osteogenic ability confirmed the potency of ADSCs. The ADSCs cultured on collagen gel and PRF, individually, showed higher number of differentiated adipocytes than ADSCs grown with adipogenic induction medium alone. Conclusion: The extent of lipid accumulation by ADSCs was slightly higher when cultured on collagen gel than on PRF. Additional experiments are required to confirm better suitability of scaffold materials for soft-tissue regeneration.

Lock-in technique for extraction of pulse rates and associated confidence levels from video.

We investigate the practical applicability of video photoplethysmography (VPPG) to extract heart rates of subjects using noncontact color video recordings of human faces collected under typical indoor laboratory conditions using commercial video cameras. Videos were processed following three previously described simple VPPG algorithms to produce a time-varying plethysmographic signal. These time signals were then analyzed using, to the best of our knowledge, a novel, lock-in algorithm that was developed to extract the pulsatile frequency component. A protocol to associate confidence estimates for the extracted heart rates for each video stream is presented. Results indicate that the difference between heart rates extracted using the lock-in technique and gold-standard measurements, for videos with high-confidence metrics, was less than 4 beats per minute. Constraints on video acquisition and processing, including natural subject motion and the total duration of video recorded required for evaluating these confidence metrics, are discussed.

High Frequency Sampling of TTL Pulses on a Raspberry Pi for Diffuse Correlation Spectroscopy Applications.

Diffuse Correlation Spectroscopy (DCS) is a well-established optical technique that has been used for non-invasive measurement of blood flow in tissues. Instrumentation for DCS includes a correlation device that computes the temporal intensity autocorrelation of a coherent laser source after it has undergone diffuse scattering through a turbid medium. Typically, the signal acquisition and its autocorrelation are performed by a correlation board. These boards have dedicated hardware to acquire and compute intensity autocorrelations of rapidly varying input signal and usually are quite expensive. Here we show that a Raspberry Pi minicomputer can acquire and store a rapidly varying time-signal with high fidelity. We show that this signal collected by a Raspberry Pi device can be processed numerically to yield intensity autocorrelations well suited for DCS applications. DCS measurements made using the Raspberry Pi device were compared to those acquired using a commercial hardware autocorrelation board to investigate the stability, performance, and accuracy of the data acquired in controlled experiments. This paper represents a first step toward lowering the instrumentation cost of a DCS system and may offer the potential to make DCS become more widely used in biomedical applications.

Can diffuse reflectance spectroscopy identify shuntodynia in pediatric hydrocephalus patients?

Significance: Shuntodynia is patient reported pain at the site of the implanted ventriculoperitoneal (VP) shunt. Pediatric hydrocephalus requiring shunt placement is a chronic and prevalent standard of care treatment and requires lifetime management. Shuntodynia is a subjective measure of shunt dysfunction. Quantitative, white-light tissue spectroscopy could be used to objectively identify this condition in the clinic. Aim: Pediatric subjects were recruited for optical sensing during routine clinical follow-up visits, post-VP shunt implantations. Acquired optical signals were translated into skin-hemodynamic signatures and were compared between subjects that reported shuntodynia versus those that did not. Approach: Diffuse reflectance spectroscopy (DRS) measurements were collected between 450 and 700 nm using a single-channel fiber-optical probe from (N=35) patients. Multiple reflectance spectra were obtained by the attending physician from regions both proximal and distal to the VP shunt sites and from a matched contralateral site for each subject. Acquired reflectance spectra were processed quantitatively into functional tissue optical endpoints. A two-way, repeated measures analysis of variance was used to assess whether and which of the optical variables were statistically separable, across subjects with shuntodynia versus those without. Results: Analyses indicated that intrapatient differences in vascular oxygen saturation measured between shunt sites relative to that obtained at the scar or contralateral sites was significantly lower in the pain group. We also find that the total hemoglobin concentrations at the shunt site were lowest relative to the other sites for subjects reporting pain. These findings suggest that shuntodynia pain arises in the scalp tissue around the implanted shunts and may be caused due to hypoxia and inflammation. Conclusions: Optically derived hemodynamic variables were statistically significantly different in subjects presenting with shuntodynia relative to those without. DRS could provide a viable mode in routine bedside monitoring of subjects with VP shunts for clinical management and assessment of shuntodynia.

Neural network-based inverse model for diffuse reflectance spectroscopy.

In diffuse reflectance spectroscopy, the retrieval of the optical properties of a target requires the inversion of a measured reflectance spectrum. This is typically achieved through the use of forward models such as diffusion theory or Monte Carlo simulations, which are iteratively applied to optimize the solution for the optical parameters. In this paper, we propose a novel neural network-based approach for solving this inverse problem, and validate its performance using experimentally measured diffuse reflectance data from a previously reported phantom study. Our inverse model was developed from a neural network forward model that was pre-trained with data from Monte Carlo simulations. The neural network forward model then creates a lookup table to invert the diffuse reflectance to the optical coefficients. We describe the construction of the neural network-based inverse model and test its ability to accurately retrieve optical properties from experimentally acquired diffuse reflectance data in liquid optical phantoms. Our results indicate that the developed neural network-based model achieves comparable accuracy to traditional Monte Carlo-based inverse model while offering improved speed and flexibility, potentially providing an alternative for developing faster clinical diagnosis tools. This study highlights the potential of neural networks in solving inverse problems in diffuse reflectance spectroscopy.

Numerical approach to quantify depth-dependent blood flow changes in real-time using the diffusion equation with continuous-wave and time-domain diffuse correlation spectroscopy.

Diffuse correlation spectroscopy (DCS) is a non-invasive optical technique that can measure brain perfusion by quantifying temporal intensity fluctuations of multiply scattered light. A primary limitation for accurate quantitation of cerebral blood flow (CBF) is the fact that experimental measurements contain information about both extracerebral scalp blood flow (SBF) as well as CBF. Separating CBF from SBF is typically achieved using multiple source-detector channels when using continuous-wave (CW) light sources, or more recently with use of time-domain (TD) techniques. Analysis methods that account for these partial volume effects are often employed to increase CBF contrast. However, a robust, real-time analysis procedure that can separate and quantify SBF and CBF with both traditional CW and TD-DCS measurements is still needed. Here, we validate a data analysis procedure based on the diffusion equation in layered media capable of quantifying both extra- and cerebral blood flow in the CW and TD. We find that the model can quantify SBF and CBF coefficients with less than 5% error compared to Monte Carlo simulations using a 3-layered brain model in both the CW and TD. The model can accurately fit data at a rate of <10 ms for CW data and <250 ms for TD data when using a least-squares optimizer.

Efficient computation of the steady-state and time-domain solutions of the photon diffusion equation in layered turbid media.

Accurate and efficient forward models of photon migration in heterogeneous geometries are important for many applications of light in medicine because many biological tissues exhibit a layered structure of independent optical properties and thickness. However, closed form analytical solutions are not readily available for layered tissue-models, and often are modeled using computationally expensive numerical techniques or theoretical approximations that limit accuracy and real-time analysis. Here, we develop an open-source accurate, efficient, and stable numerical routine to solve the diffusion equation in the steady-state and time-domain for a layered cylinder tissue model with an arbitrary number of layers and specified thickness and optical coefficients. We show that the steady-state ([Formula: see text] ms) and time-domain ([Formula: see text] ms) fluence (for an 8-layer medium) can be calculated with absolute numerical errors approaching machine precision. The numerical implementation increased computation speed by 3 to 4 orders of magnitude compared to previously reported theoretical solutions in layered media. We verify our solutions asymptotically to homogeneous tissue geometries using closed form analytical solutions to assess convergence and numerical accuracy. Approximate solutions to compute the reflected intensity are presented which can decrease the computation time by an additional 2-3 orders of magnitude. We also compare our solutions for 2, 3, and 5 layered media to gold-standard Monte Carlo simulations in layered tissue models of high interest in biomedical optics (e.g. skin/fat/muscle and brain). The presented routine could enable more robust real-time data analysis tools in heterogeneous tissues that are important in many clinical applications such as functional brain imaging and diffuse optical spectroscopy.

Reconstruction of optical coefficients in turbid media using time-resolved reflectance and calibration-free instrument response functions.

Measurements of time-resolved reflectance from a homogenous turbid medium can be employed to retrieve the absolute values of its optical transport coefficients. However, the uncertainty in the temporal shift of the experimentally determined instrument response function (IRF) with respect to the real system response can lead to errors in optical property reconstructions. Instrument noise and measurement of the IRF in a reflectance geometry can exacerbate these errors. Here, we examine three reconstruction approaches that avoid requiring direct measurements of photon launch times. They work by (a) fitting relative shapes of the reflectance profile with a pre-determined constraint on the scattering coefficient, (b) calibrating launch-time differences via a reference sample, and (c) freely fitting for the launch-time difference within the inverse problem. Analysis methods that can place a tight bound on the scattering coefficient can produce errors within 5-15% for both absorption and scattering at source-detector separations of 10 and 15 mm. Including the time-shift in the fitting procedure also recovered optical coefficients to under 20% but showed large crosstalk between extracted scattering and absorption coefficients. We find that the uncertainty in the temporal shift greatly impacts the reconstructed reduced scattering coefficient compared to absorption.

Rapid ratiometric determination of hemoglobin concentration using UV-VIS diffuse reflectance at isosbestic wavelengths.

We developed a ratiometric method capable of estimating total hemoglobin concentration from optically measured diffuse reflectance spectra. The three isosbestic wavelength ratio pairs that best correlated to total hemoglobin concentration independent of saturation and scattering were 545/390, 452/390, and 529/390 nm. These wavelength pairs were selected using forward Monte Carlo simulations which were used to extract hemoglobin concentration from experimental phantom measurements. Linear regression coefficients from the simulated data were directly applied to the phantom data, by calibrating for instrument throughput using a single phantom. Phantoms with variable scattering and hemoglobin saturation were tested with two different instruments, and the average percent errors between the expected and ratiometrically-extracted hemoglobin concentration were as low as 6.3%. A correlation of r = 0.88 between hemoglobin concentration extracted using the 529/390 nm isosbestic ratio and a scalable inverse Monte Carlo model was achieved for in vivo dysplastic cervical measurements (hemoglobin concentrations have been shown to be diagnostic for the detection of cervical pre-cancer by our group). These results indicate that use of such a simple ratiometric method has the potential to be used in clinical applications where tissue hemoglobin concentrations need to be rapidly quantified 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 (µs') 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 µs' 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 µs' 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.

Light transport in biological tissue using three-dimensional frequency-domain simplified spherical harmonics equations.

The accuracy of the commonly used diffusion approximation as used in diffuse optical tomography is known to be limited in cases involving strong absorption and in these situations a higher ordered approximation is necessary. In this study, a light transport model has been developed based upon the three-dimensional frequency-domain simplified spherical harmonics (SP(N)) approximation for orders up to N = 7. The SP(N) data are tested against a semi-infinite multi-layered Monte Carlo model. It has been shown that the SP(N) approximation for higher orders (N >1) provides an increase in accuracy over the diffusion equation specifically near sources and at boundaries of regions with increased optical absorption. It is demonstrated that the error of fluence calculated near the sources between the diffusion approximation and the SP(N) model (N = 7) can be as large as 60%, therefore limiting the use of the diffusion approximation for small animal imaging and in situations where optical changes near sources are critical for tomographic reconstructions.

Diffuse optical monitoring of peripheral tissues during uncontrolled internal hemorrhage in a porcine model.

Reliable, continuous and noninvasive blood flow and hemoglobin monitoring in trauma patients remains a critical, but generally unachieved goal. Two optical sensing methods - diffuse correlation spectroscopy (DCS) and diffuse reflectance spectroscopy (DRS) - are used to monitor and detect internal hemorrhage. Specifically, we investigate if cutaneous perfusion measurements acquired using DCS and DRS in peripheral (thighs and ear-lobe) tissues could detect severe hemorrhagic shock in a porcine model. Four animals underwent high-grade hepato-portal injury in a closed abdomen, to induce uncontrolled hemorrhage and were subsequently allowed to bleed for 10 minutes before fluid resuscitation. DRS and DCS measurements of cutaneous blood flow were acquired using fiber optical probes placed on the thigh and earlobe of the animals and were obtained repeatedly starting from 1 to 5 minutes pre-injury, up to several minutes post shock. Clear changes were observed in measured optical spectra across all animals at both sites. DCS-derived cutaneous blood flow decreased sharply during hemorrhage, while DRS-derived vascular saturation and hemoglobin paralleled cardiac output. All derived optical parameters had the steepest changes during the rapid initial hemorrhage unambiguously. This suggests that a combined DCS and DRS based device might provide an easy-to-use, non-invasive, internal-hemorrhage detection system that can be used across a wide array of clinical settings.

Acute Stress Attenuates Cognitive Flexibility in Males Only: An fNIRS Examination.

Cognitive processes that afford us the ability to control thoughts and achieve goal-directed behavior are known as executive functions. Empirical evidence in the past few years has demonstrated that executive functions can be influenced by acute stress. The impact of acute stress on cognitive flexibility, a key aspect of executive functions, has received little attention in the literature. We present the results of two experiments conducted to examine the effect of acute stress on cognitive flexibility. Acute stress was induced using the cold pressor task. Cognitive flexibility was assessed using the Wisconsin Card Sorting Test (WCST). Across both experiments acute stress had an attenuating effect on task switching on the WCST. Our findings also indicate that this effect was moderated by the participant's gender. In Study 1, we observed that following stress exposure male participants in the stress condition made more perseverative errors than participants in the control group. In Study 2, we examined the bilateral hemodynamics in the prefrontal cortex (PFC) during acute stress induction using functional near infrared spectroscopy (fNIRS). Our analysis indicated that functional oxyHb signals fluctuated with greater amplitude than systemic components for participants in the stress group relative to those in the control group. In addition, oxyHb levels post stress induction were correlated with performance on the WCST for the male participants in the stress group only. Concordant with previous reports, our findings indicate that acute stress impacts cognitive flexibility in males and females differentially. Our work also demonstrates the feasibility of using fNIRS as a practical and objective technique for the examination of hemodynamics in the PFC during acute stress.

Neurovascular toxicity of N-methyl-d-aspartate is markedly enhanced in the developing mouse central nervous system.

Penumbral perfusion is critical to brain viability. Proximal arterial occlusion and deep brain stroke has variable effect on cortical dysfunction. Cortical microvessel collaterals may be recruited and at times sufficient for partial parenchymal perfusion. Postnatal neural and endothelial cells are markedly vulnerable to glutamate excitotoxicity. Early vascular cell stress may promote partial protective neural preconditioning though postnatally a developmental window of the cerebral microvasculature may be particularly vulnerable to injury. We tested the hypothesis that postnatal NMDA excitotoxic injury, when cerebral endothelial cells' central energy source is via glycolysis, is age specific. Neurovascular responses of cortical viability were directly identified with diffuse reflectance patterns of perfusion properties in a non-invasive manner, over time. Histological evaluation for neural and vascular cytoarchitectonic abnormalities were evaluated 4- 7days post injury. Optical diffuse reflectance recordings were obtained at the injection site prior to, immediately after and 48h post injury. Extent of neurovascular injury at the infarct zone was greatest at PND 5 and cortical perfusion responses identified with recordings of pattern change. These data further suggest excitotoxic injury to both neural and vascular cells, in vivo, can enhance CNS injury in the young and neuroprotective strategies may benefit from vascular directed therapies.

Novel diffuse optics system for continuous tissue viability monitoring - extended recovery in vivo testing in a porcine flap model.

In reconstructive surgery, tissue perfusion/vessel patency is critical to the success of microvascular free tissue flaps. Early detection of flap failure secondary to compromise of vascular perfusion would significantly increase the chances of flap salvage. We have developed a compact, clinically-compatible monitoring system to enable automated, minimally-invasive, continuous, and quantitative assessment of flap viability/perfusion. We tested the system's continuous monitoring capability during extended non-recovery surgery using an in vivo porcine free flap model. Initial results indicated that the system could assess flap viability/perfusion in a quantitative and continuous manner. With proven performance, the compact form constructed with cost-effective components would make this system suitable for clinical translation.

In vivo preclinical verification of a multimodal diffuse reflectance and correlation spectroscopy system for sensing tissue perfusion.

In reconstructive surgery, impeded blood flow in microvascular free flaps due to a compromise in arterial or venous patency secondary to blood clots or vessel spasms can rapidly result in flap failures. Thus, the ability to detect changes in microvascular free flaps is critical. In this paper, we report progress on in vivo pre-clinical testing of a compact, multimodal, fiber-based diffuse correlation and reflectance spectroscopy system designed to quantitatively monitor tissue perfusion in a porcine model's surgically-grafted free flap. We also describe the device's sensitivity to incremental blood flow changes and discuss the prospects for continuous perfusion monitoring in future clinical translational studies.

Compact dual-mode diffuse optical system for blood perfusion monitoring in a porcine model of microvascular tissue flaps.

In reconstructive surgery, the ability to detect blood flow interruptions to grafted tissue represents a critical step in preventing postsurgical complications. We have developed and pilot tested a compact, fiber-based device that combines two complimentary modalities-diffuse correlation spectroscopy (DCS) and diffuse reflectance spectroscopy-to quantitatively monitor blood perfusion. We present a proof-of-concept study on an in vivo porcine model (n=8). With a controllable arterial blood flow supply, occlusion studies (n=4) were performed on surgically isolated free flaps while the device simultaneously monitored blood flow through the supplying artery as well as flap perfusion from three orientations: the distal side of the flap and two transdermal channels. Further studies featuring long-term monitoring, arterial failure simulations, and venous failure simulations were performed on flaps that had undergone an anastomosis procedure (n=4). Additionally, benchtop verification of the DCS system was performed on liquid flow phantoms. Data revealed relationships between diffuse optical measures and state of occlusion as well as the ability to detect arterial and venous compromise. The compact construction of the device, along with its noninvasive and quantitative nature, would make this technology suitable for clinical translation.

Design verification of a compact system for detecting tissue perfusion using bimodal diffuse optical technologies.

It is essential to monitor tissue perfusion during and after reconstructive surgery, as restricted blood flow can result in graft failures. Current clinical procedures are insufficient to monitor tissue perfusion, as they are intermittent and often subjective. To address this unmet clinical need, a compact, low-cost, multimodal diffuse correlation spectroscopy and diffuse reflectance spectroscopy system was developed. We verified system performance via tissue phantoms and experimental protocols for rigorous bench testing. Quantitative data analysis methods were employed and tested to enable the extraction of tissue perfusion parameters. This design verification study assures data integrity in future in vivo studies.

Fluorescence quenching by polystyrene microspheres in UV-visible and NIR tissue-simulating phantoms.

Tissue-simulating phantoms are widely used for controlled studies of photon transport in turbid media. Here, we describe how polystyrene microspheres, which are often used to simulate optical scattering in such phantoms, can reduce fluorophore quantum yield via collisional quenching. We report studies on UV-visible (fluorescein-based) and NIR (IR125-based) phantoms with differing fluorophore and scatterer concentrations, as well as differing microsphere sizes. Results consistent with the Stern-Volmer relation suggest that the fluorophore intrinsic excited-state lifetime decreased due to collisional quenching from polystyrene microspheres and that the quenching efficiency was dependent on the concentration ratio of fluorophores to microspheres. Lifetime decreases ranging from 10-35% (20%) were measured for fluorescein-based (IR 125-based) phantoms. Since polystyrene microspheres are commonly used in tissue-simulating phantoms for quantitative studies of fluorescence light propagation, their quenching effects on fluorescence intensities may be difficult to separate from intensity losses attributed to optical absorption and scattering in the phantom unless fluorescence lifetime measurements are performed simultaneously.