Diffuse optical tomography to investigate the newborn brain

Review of measurements and imaging of cytochrome-c-oxidase in humans using near-infrared spectroscopy: an update

This review examines advancements in the measurement and imaging of oxidized cytochrome-c-oxidase (oxCCO) using near-infrared spectroscopy (NIRS) in humans since 2016. A total of 34 published papers were identified, with a focus on both adult and neonate populations. The NIRS-derived oxCCO signal has been demonstrated to correlate with physiological parameters and hemodynamics. New instrumentation, such as systems that allow the imaging of changes of oxCCO with diffuse optical tomography or combine the oxCCO measurement with diffuse correlation spectroscopy measures of blood flow, have advanced the field in the past decade. However, variability in its response across different populations and paradigms and lack of standardization limit its potential as a reliable and valuable indicator of brain health. Future studies should address these issues to fulfill the vision of oxCCO as a clinical biomarker.

High-Speed Time-Domain Diffuse Optical Tomography with a Sensitivity Equation-based Neural Network

Steady progress in time-domain diffuse optical tomography (TD-DOT) technology is allowing for the first time the design of low-cost, compact, and high-performance systems, thus promising more widespread clinical TD-DOT use, such as for recording brain tissue hemodynamics. TD-DOT is known to provide more accurate values of optical properties and physiological parameters compared to its frequency-domain or steady-state counterparts. However, achieving high temporal resolution is still difficult, as solving the inverse problem is computationally demanding, leading to relatively long reconstruction times. The runtime is further compromised by processes that involve 'nontrivial' empirical tuning of reconstruction parameters, which increases complexity and inefficiency. To address these challenges, we present a new reconstruction algorithm that combines a deep-learning approach with our previously introduced sensitivity-equation-based, non-iterative sparse optical reconstruction (SENSOR) code. The new algorithm (called SENSOR-NET) unfolds the iterations of SENSOR into a deep neural network. In this way, we achieve high-resolution sparse reconstruction using only learned parameters, thus eliminating the need to tune parameters prior to reconstruction empirically. Furthermore, once trained, the reconstruction time is not dependent on the number of sources or wavelengths used. We validate our method with numerical and experimental data and show that accurate reconstructions with 1 mm spatial resolution can be obtained in under 20 milliseconds regardless of the number of sources used in the setup. This opens the door for real-time brain monitoring and other high-speed DOT applications.

Skeletal muscle optoacoustics reveals patterns of circulatory function and oxygen metabolism during exercise

Imaging skeletal muscle function and metabolism, as reported by local hemodynamics and oxygen kinetics, can elucidate muscle performance, severity of an underlying disease or outcome of a treatment. Herein, we used multispectral optoacoustic tomography (MSOT) to image hemodynamics and oxygen kinetics within muscle during exercise. Four healthy volunteers underwent three different hand-grip exercise challenges (60s isometric, 120s intermittent isometric and 60s isotonic). During isometric contraction, MSOT showed a decrease of HbO₂, Hb and total blood volume (TBV), followed by a prominent increase after the end of contraction. Corresponding hemodynamic behaviors were recorded during the intermittent isometric and isotonic exercises. A more detailed analysis of MSOT readouts revealed insights into arteriovenous oxygen differences and muscle oxygen consumption during all exercise schemes. These results demonstrate an excellent capability of visualizing both circulatory function and oxygen metabolism within skeletal muscle under exercise, with great potential implications for muscle research, including relevant disease diagnostics.

Optical Property Measurement and Temperature Monitoring in High-Intensity Focused Ultrasound Therapy by Diffuse Optical Tomography: A Correlation Study

In this article, we propose a new approach utilizing diffuse optical tomography (DOT) to monitoring the changes in tissues’ optical properties and temperature in high-intensity focused ultrasound (HIFU) therapy. By correlating the tissue reduced scattering coefficient (μs’) reconstructed by DOT and the temperature measured by a thermocouple, the quantitative relationship between μs’ and temperature in HIFU treatment was explored. The experiments were conducted using porcine and chicken breast muscle tissues during HIFU; the temperature of each tissue sample was recorded using a thermocouple. To incorporate the temperature dependency of tissue optical properties, both polynomial and exponential models were utilized to fit the experimental data. The results show that the change of μs’ during HIFU treatment could be detected in real-time using DOT and that this change of μs’ is quantitatively correlated with tissue temperature. Furthermore, while the tissue-type-dependent relationship between μs’ and temperature is non-linear in nature, it is stable and repeatable. Therefore, our approach has the potential to be used to predict temperature of tissue during HIFU treatment.

Transcranial photoacoustic characterization of neurovascular physiology during early-stage photothrombotic stroke in neonatal pigletsin vivo

OBJECTIVE

Perinatal ischemic stroke is estimated to occur in 1/2300-1/5000 live births, but early differential diagnosis from global hypoxia-ischemia is often difficult. In this study, we tested the ability of a hand-held transcranial photoacoustic (PA) imaging probe to non-invasively detect a focal photothrombotic stroke (PTS) within 2 h of stroke onset in a gyrencephalic piglet brain.

APPROACH

About 17 stroke lesions of approximately 1 cm²area were introduced randomly in anterior or posterior cortex via the light/dye PTS technique in anesthetized neonatal piglets (n= 11). The contralateral non-ischemic region served as control tissue for discrimination contrast for the PA hemoglobin metrics: oxygen saturation, total hemoglobin (tHb), and individual quantities of oxygenated and deoxygenated hemoglobin (HbO₂and HbR).

MAIN RESULTS

The PA-derived tissue oxygen saturation at 2 h yielded a significant separation between control and affected regions-of-interest (p< 0.0001), which were well matched with 24 h post-stroke cerebral infarction confirmed in the triphenyltetrazolium chloride-stained image. The quantity of HbO₂also displayed a significant contrast (p= 0.021), whereas tHb and HbR did not. The analysis on receiver operating characteristic curves and multivariate data analysis also agreed with the results above.

SIGNIFICANCE

This study shows that a hand-held transcranial PA neuroimaging device can detect a regional thrombotic stroke in the cerebral cortex of a neonatal piglet. In particular, we conclude that the oxygen saturation metric can be used alone to identify regional stroke lesions. The lack of change in tHb may be related to arbitrary hand-held imaging configuration and/or entrapment of red blood cells within the thrombotic stroke.

Photoacoustic assessment of the fetal brain and placenta as a method of non-invasive antepartum and intrapartum monitoring

A noninvasive monitor for concurrent evaluation of placental and fetal sagittal sinus sO 2 for both antepartum surveillance at the late 2nd and 3rd trimesters and intrapartum monitoring would be a great advantage over current methods. A PA fetal brain and placental monitor has potential value to rapidly identify the fetus at risk for developing hypoxia and ischemia of a sufficient degree that brain injury or death may develop, which may be prevented by intervention with delivery and other follow-up treatments.

2.5 Hz sample rate time-domain near-infrared optical tomography based on SPAD-camera image tissue hemodynamics

Time-domain near-infrared optical tomography (TD NIROT) techniques based on diffuse light were gaining performance over the last years. They are capable of imaging tissue at several centimeters depth and reveal clinically relevant information, such as tissue oxygen saturation. In this work, we present the very first in vivo results of our SPAD camera-based TD NIROT reflectance system with a temporal resolution of ∼116 ps. It provides 2800 time of flight source-detector pairs in a compact probe of only 6 cm in diameter. Additionally, we describe a 3-step reconstruction procedure that enables accurate recovery of structural information and of the optical properties. We demonstrate the system's performance firstly in reconstructing the 3D-structure of a heterogeneous tissue phantom with tissue-like scattering and absorption properties within a volume of 9 cm diameter and 5 cm thickness. Furthermore, we performed in vivo tomography of an index finger located within a homogeneous scattering medium. We employed a fast sampling rate of 2.5 Hz to detect changes in tissue oxygenation. Tomographic reconstructions were performed in true 3D, and without prior structural information, demonstrating the powerful capabilities of the system. This shows its potential for clinical applications.

Extraction of tissue optical property and blood flow from speckle contrast diffuse correlation tomography (scDCT) measurements

Measurement of blood flow in tissue provides vital information for the diagnosis and therapeutic monitoring of various vascular diseases. A noncontact, camera-based, near-infrared speckle contrast diffuse correlation tomography (scDCT) technique has been recently developed for 3D imaging of blood flow index (αD_B) distributions in deep tissues up to a centimeter. A limitation with the continuous-wave scDCT measurement of blood flow is the assumption of constant and homogenous tissue absorption coefficient (μ _a ). The present study took the advantage of rapid, high-density, noncontact scDCT measurements of both light intensities and diffuse speckle contrast at multiple source-detector distances and developed two-step fitting algorithms for extracting both μ _a and αD_B. The new algorithms were tested in tissue-simulating phantoms with known optical properties and human forearms. Measurement results were compared against established near-infrared spectroscopy (NIRS) and diffuse correlation spectroscopy (DCS) techniques. The accuracies of our new fitting algorithms with scDCT measurements in phantoms (up to 16% errors) and forearms (up to 23% errors) are comparable to relevant study results (up to 25% errors). Knowledge of μ _a not only improved the accuracy in calculating αD_B but also provided the potential for quantifying tissue blood oxygenation via spectral measurements. A multiple-wavelength scDCT system with new algorithms is currently developing to fit multi-wavelength and multi-distance data for 3D imaging of both blood flow and oxygenation distributions in deep tissues.

Depth-recognizable time-domain fluorescence molecular tomography in reflective geometry

Conventional fluorescence molecular tomography (FMT) reconstruction requires photons penetrating the whole object, which limits its applications to small animals. However, by utilizing reflective photons, fluorescence distribution near the surface could be reconstructed regardless of the object size, which may extend the applications of FMT to surgical navigation and so on. Therefore, time-domain reflective fluorescence molecular tomography (TD-rFMT) is proposed in this paper. The system excites and detects the emission light from the same angle within a field of view of 5 cm. Because the detected intensities of targets depend strongly on the depth, the reconstruction of targets in deep regions would be evidently affected. Therefore, a fluorescence yield reconstruction method with depth regularization and a weighted separation reconstruction strategy for lifetime are developed to enhance the performance for deep targets. Through simulations and phantom experiments, TD-rFMT is proved capable of reconstructing fluorescence distribution within a 2.5-cm depth with accurate reconstructed yield, lifetime, and target position(s).

Longitudinal infant fNIRS channel-space analyses are robust to variability parameters at the group-level: An image reconstruction investigation

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First investigation of validity of longitudinal infant channel-space fNIRS analysis.

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Novel image reconstruction analysis conducted.

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Variability in array position is dominant factor driving different inferences.

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Channel-space fNIRS analyses robust to implicit assumptions at group-level.

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Hope to encourage more widespread use of image reconstruction in infant analyses.

The first 1000 days from conception to two-years of age are a critical period in brain development, and there is an increasing drive for developing technologies to help advance our understanding of neurodevelopmental processes during this time. Functional near-infrared spectroscopy (fNIRS) has enabled longitudinal infant brain function to be studied in a multitude of settings. Conventional fNIRS analyses tend to occur in the channel-space, where data from equivalent channels across individuals are combined, which implicitly assumes that head size and source-detector positions (i.e. array position) on the scalp are constant across individuals. The validity of such assumptions in longitudinal infant fNIRS analyses, where head growth is most rapid, has not previously been investigated. We employed an image reconstruction approach to analyse fNIRS data collected from a longitudinal cohort of infants in The Gambia aged 5- to 12-months. This enabled us to investigate the effect of variability in both head size and array position on the anatomical and statistical inferences drawn from the data at both the group- and the individual-level. We also sought to investigate the impact of group size on inferences drawn from the data. We found that variability in array position was the driving factor between differing inferences drawn from the data at both the individual- and group-level, but its effect was weakened as group size increased towards the full cohort size (N = 53 at 5-months, N = 40 at 8-months and N = 45 at 12-months). We conclude that, at the group sizes in our dataset, group-level channel-space analysis of longitudinal infant fNIRS data is robust to assumptions about head size and array position given the variability in these parameters in our dataset. These findings support a more widespread use of image reconstruction techniques in longitudinal infant fNIRS studies.

Neuroimaging of depression with diffuse optical tomography during repetitive transcranial magnetic stimulation

Repetitive transcranial magnetic stimulation (rTMS) is an effective and safe treatment for depression; however, its potential has likely been hindered due to non-optimized targeting, unclear ideal stimulation parameters, and lack of information regarding how the brain is physiologically responding during and after stimulation. While neuroimaging is ideal for obtaining such critical information, existing modalities have been limited due to poor resolutions, along with significant noise interference from the electromagnetic spectrum. In this study, we used a novel diffuse optical tomography (DOT) device in order to advance our understanding of the neurophysiological effects of rTMS in depression. Healthy and depressed subjects aged 18–70 were recruited. Treatment parameters were standardized with targeting of the left dorsolateral prefrontal cortex with a magnetic field intensity of 100% of motor threshold, pulse frequency of 10 per second, a 4 s stimulation time and a 26 s rest time. DOT imaging was simultaneously acquired from the contralateral dorsolateral prefrontal cortex. Six healthy and seven depressed subjects were included for final analysis. Hemoglobin changes and volumetric three-dimensional activation patterns were successfully captured. Depressed subjects were observed to have a delayed and less robust response to rTMS with a decreased volume of activation compared to healthy subjects. In this first-in-human study, we demonstrated the ability of DOT to safely and reliably capture and compare cortical response patterns to rTMS in depressed and healthy subjects. We introduced this emerging optical functional imaging modality as a novel approach to investigating targeting, new treatment parameters, and physiological effects of rTMS in depression.

Quo Vadis, Nanoparticle-Enabled In Vivo Fluorescence Imaging?

The exciting advancements that we are currently witnessing in terms of novel materials and synthesis approaches are leading to the development of colloidal nanoparticles (NPs) with increasingly greater tunable properties. We have now reached a point where it is possible to synthesize colloidal NPs with functionalities tailored to specific societal demands. The impact of this new wave of colloidal NPs has been especially important in the field of biomedicine. In that vein, luminescent NPs with improved brightness and near-infrared working capabilities have turned out to be optimal optical probes that are capable of fast and high-resolution in vivo imaging. However, luminescent NPs have thus far only reached a limited portion of their potential. Although we believe that the best is yet to come, the future might not be as bright as some of us think (and have hoped!). In particular, translation of NP-based fluorescence imaging from preclinical studies to clinics is not straightforward. In this Perspective, we provide a critical assessment and highlight promising research avenues based on the latest advances in the fields of luminescent NPs and imaging technologies. The disillusioned outlook we proffer herein might sound pessimistic at first, but we consider it necessary to avoid pursuing "pipe dreams" and redirect the efforts toward achievable-yet ambitious-goals.

Real-time interleaved spectroscopic photoacoustic and ultrasound (PAUS) scanning with simultaneous fluence compensation and motion correction

For over two decades photoacoustic imaging has been tested clinically, but successful human trials have been limited. To enable quantitative clinical spectroscopy, the fundamental issues of wavelength-dependent fluence variations and inter-wavelength motion must be overcome. Here we propose a real-time, spectroscopic photoacoustic/ultrasound (PAUS) imaging approach using a compact, 1-kHz rate wavelength-tunable laser. Instead of illuminating tissue over a large area, the fiber-optic delivery system surrounding an US array sequentially scans a narrow laser beam, with partial PA image reconstruction for each laser pulse. The final image is then formed by coherently summing partial images. This scheme enables (i) automatic compensation for wavelength-dependent fluence variations in spectroscopic PA imaging and (ii) motion correction of spectroscopic PA frames using US speckle tracking in real-time systems. The 50-Hz video rate PAUS system is demonstrated in vivo using a murine model of labelled drug delivery.

Neonatal NIRS monitoring: recommendations for data capture and review of analytics

Brain injury is one of the most consequential problems facing neonates, with many preterm and term infants at risk for cerebral hypoxia and ischemia. To develop effective neuroprotective strategies, the mechanistic basis for brain injury must be understood. The fragile state of neonates presents unique research challenges; invasive measures of cerebral blood flow and oxygenation assessment exceed tolerable risk profiles. Near-infrared spectroscopy (NIRS) can safely and non-invasively estimate cerebral oxygenation, a correlate of cerebral perfusion, offering insight into brain injury-related mechanisms. Unfortunately, lack of standardization in device application, recording methods, and error/artifact correction have left the field fractured. In this article, we provide a framework for neonatal NIRS research. Our goal is to provide a rational basis for NIRS data capture and processing that may result in better comparability between studies. It is also intended to serve as a primer for new NIRS researchers and assist with investigation initiation.

Role of Optical Neuromonitoring in Neonatal Encephalopathy-Current State and Recent Advances

Neonatal encephalopathy (NE) in term and near-term infants is a significant global health problem; the worldwide burden of disease remains high despite the introduction of therapeutic hypothermia. Assessment of injury severity and effective management in the neonatal intensive care unit (NICU) relies on multiple monitoring modalities from systemic to brain-specific. Current neuromonitoring tools provide information utilized for seizure management, injury stratification, and prognostication, whilst systemic monitoring ensures multi-organ dysfunction is recognized early and supported wherever needed. The neuromonitoring technologies currently used in NE however, have limitations in either their availability during the active treatment window or their reliability to prognosticate and stratify injury confidently in the early period following insult. There is therefore a real need for a neuromonitoring tool that provides cot side, early and continuous monitoring of brain health which can reliably stratify injury severity, monitor response to current and emerging treatments, and prognosticate outcome. The clinical use of near-infrared spectroscopy (NIRS) technology has increased in recent years. Research studies within this population have also increased, alongside the development of both instrumentation and signal processing techniques. Increasing use of commercially available cerebral oximeters in the NICU, and the introduction of advanced optical measurements using broadband NIRS (BNIRS), frequency domain NIRS (FDNIRS), and diffuse correlation spectroscopy (DCS) have widened the scope by allowing the direct monitoring of oxygen metabolism and cerebral blood flow, both key to understanding pathophysiological changes and predicting outcome in NE. This review discusses the role of optical neuromonitoring in NE and why this modality may provide the next significant piece of the puzzle toward understanding the real time state of the injured newborn brain.

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.

Design and validation of a mechanically flexible and ultra-lightweight high-density diffuse optical tomography system for functional neuroimaging of newborns

Significance: Neonates are a highly vulnerable population. The risk of brain injury is greater during the first days and weeks after birth than at any other time of life. Functional neuroimaging that can be performed longitudinally and at the cot-side has the potential to improve our understanding of the evolution of multiple forms of neurological injury over the perinatal period. However, existing technologies make it very difficult to perform repeated and/or long-duration functional neuroimaging experiments at the cot-side. Aim: We aimed to create a modular, high-density diffuse optical tomography (HD-DOT) technology specifically for neonatal applications that is ultra-lightweight, low profile and provides high mechanical flexibility. We then sought to validate this technology using an anatomically accurate dynamic phantom. Approach: An advanced 10-layer rigid-flexible printed circuit board technology was adopted as the basis for the DOT modules, which allows for a compact module design that also provides the flexibility needed to conform to the curved infant scalp. Two module layouts were implemented: dual-hexagon and triple-hexagon. Using in-built board-to-board connectors, the system can be configured to provide a vast range of possible layouts. Using epoxy resin, thermochromic dyes, and MRI-derived 3D-printed moulds, we constructed an electrically switchable, anatomically accurate dynamic phantom. This phantom was used to quantify the imaging performance of our flexible, modular HD-DOT system. Results: Using one particular module configuration designed to cover the infant sensorimotor system, the device provided 36 source and 48 detector positions, and over 700 viable DOT channels per wavelength, ranging from 10 to ∼ 45    mm over an area of approximately 60    cm 2 . The total weight of this system is only 70 g. The signal changes from the dynamic phantom, while slow, closely simulated real hemodynamic response functions. Using difference images obtained from the phantom, the measured 3D localization error provided by the system at the depth of the cortex was in the of range 3 to 6 mm, and the lateral image resolution at the depth of the neonatal cortex is estimated to be as good as 10 to 12 mm. Conclusions: The HD-DOT system described is ultra-low weight, low profile, can conform to the infant scalp, and provides excellent imaging performance. It is expected that this device will make functional neuroimaging of the neonatal brain at the cot-side significantly more practical and effective.

Construction and validation of a database of head models for functional imaging of the neonatal brain

The neonatal brain undergoes dramatic structural and functional changes over the last trimester of gestation. The accuracy of source localisation of brain activity recorded from the scalp therefore relies on accurate age-specific head models. Although an age-appropriate population-level atlas could be used, detail is lost in the construction of such atlases, in particular with regard to the smoothing of the cortical surface, and so such a model is not representative of anatomy at an individual level. In this work, we describe the construction of a database of individual structural priors of the neonatal head using 215 individual-level datasets at ages 29-44 weeks postmenstrual age from the Developing Human Connectome Project. We have validated a method to segment the extra-cerebral tissue against manual segmentation. We have also conducted a leave-one-out analysis to quantify the expected spatial error incurred with regard to localising functional activation when using a best-matching individual from the database in place of a subject-specific model; the median error was calculated to be 8.3 mm (median absolute deviation 3.8 mm). The database can be applied for any functional neuroimaging modality which requires structural data whereby the physical parameters associated with that modality vary with tissue type and is freely available at www.ucl.ac.uk/dot-hub.

The Various Oximetric Techniques Used for the Evaluation of Blood Oxygenation

Adequate oxygen delivery to a tissue depends on sufficient oxygen content in arterial blood and blood flow to the tissue. Oximetry is a technique for the assessment of blood oxygenation by measurements of light transmission through the blood, which is based on the different absorption spectra of oxygenated and deoxygenated hemoglobin. Oxygen saturation in arterial blood provides information on the adequacy of respiration and is routinely measured in clinical settings, utilizing pulse oximetry. Oxygen saturation, in venous blood (SvO₂) and in the entire blood in a tissue (StO₂), is related to the blood supply to the tissue, and several oximetric techniques have been developed for their assessment. SvO₂ can be measured non-invasively in the fingers, making use of modified pulse oximetry, and in the retina, using the modified Beer–Lambert Law. StO₂ is measured in peripheral muscle and cerebral tissue by means of various modes of near infrared spectroscopy (NIRS), utilizing the relative transparency of infrared light in muscle and cerebral tissue. The primary problem of oximetry is the discrimination between absorption by hemoglobin and scattering by tissue elements in the attenuation measurement, and the various techniques developed for isolating the absorption effect are presented in the current review, with their limitations.

A bench-top model of middle ear effusion diagnosed with optical tympanometry

OBJECTIVES

To assess the validity of a bench-top model of an optical tympanometry device to diagnose in vitro model of middle ear effusion (MEE).

METHODS AND MATERIALS

We illuminated an in vitro model of ear canal and tympanic membrane with broadband light and relayed remitted light to a spectrometer system. We then used our proprietary algorithm to extract spectral features that, together with our logistic regression classifiers, led us to calculate a set of simplified indices related to different middle ear states. Our model included a glass vial covered with a porcine submucosa (representing the tympanic membrane) and filled with air, water, or milk solution (representing different MEE), and a set of cover-glass slips filled with either blood (representing erythema) or cerumen. By interchanging fluid types and cover-glass slips, we made measurements on combinations corresponding to normal healthy ear and purulent or serous MEE.

RESULTS

Each simulated condition had a distinct spectral profile, which was then employed by our algorithm to discriminate clean and cerumen-covered purulent and serous MEE. Two logistic purulent and serous MEE classifiers correctly classified all in vitro middle ear states with 100% accuracy assessed by leave-one-out and k-fold cross validation.

CONCLUSIONS

This proof-of-concept in vitro study addressed an unmet need by introducing a device that easily and accurately can assess middle ear effusion. Future in vivo studies aimed at collecting data from clinical settings are warranted to further elucidate the validity of the technology in diagnosing pediatric acute otitis media.

Optics Based Label-Free Techniques and Applications in Brain Monitoring

Functional near-infrared spectroscopy (fNIRS) has been utilized already around three decades for monitoring the brain, in particular, oxygenation changes in the cerebral cortex. In addition, other optical techniques are currently developed for in vivo imaging and in the near future can be potentially used more in human brain research. This paper reviews the most common label-free optical technologies exploited in brain monitoring and their current and potential clinical applications. Label-free tissue monitoring techniques do not require the addition of dyes or molecular contrast agents. The following optical techniques are considered: fNIRS, diffuse correlations spectroscopy (DCS), photoacoustic imaging (PAI) and optical coherence tomography (OCT). Furthermore, wearable optical brain monitoring with the most common applications is discussed.

Relationship between maternal pregnancy-related anxiety and infant brain responses to emotional speech - a pilot study

BACKGROUND

Maternal pregnancy-related anxiety (PRA) is reportedly related to neurodevelopmental outcomes of infants. However, the relationship between maternal PRA and the processing of emotions in the infant brain has not been extensively studied with neuroimaging. The objective of the present pilot study is to investigate the relationship between maternal PRA and infant hemodynamic responses to emotional speech at two months of age.

METHODS

The study sample included 19 mother-infant dyads from a general sample of a population of Caucasian mothers. Self-reported Pregnancy-Related Anxiety Questionnaire (PRAQ-R2) data was collected from mothers during pregnancy at gestational weeks (gwks) 24 (N = 19) and 34 (N = 18). When their infants were two months old, the infants' brains functional responses to emotional speech in the left fronto-temporoparietal cortex were recorded using diffuse optical tomography (DOT).

RESULTS

Maternal PRAQ-R2 scores at gwk 24 correlated negatively with the total hemoglobin (HbT) responses to sad speech on both sides of the temporoparietal junction (Spearman's rank correlation coefficient ρ = -0.87). The correlation was significantly greater at gwk 24 than gwk 34 (ρ = -0.42).

LIMITATIONS

The field of view of the measurement did not include the right hemisphere or parts of the frontal cortex. The sample size is moderate and the mothers were relatively highly educated, thus there may be some differences between the study sample and the general population.

CONCLUSIONS

Maternal pregnancy-related anxiety may affect child brain emotion processing development. Further research is needed to understand the functional and developmental significance of the findings.

A Mini-Review on Functional Near-Infrared Spectroscopy (fNIRS): Where Do We Stand, and Where Should We Go?

This mini-review is aimed at briefly summarizing the present status of functional near-infrared spectroscopy (fNIRS) and predicting where the technique should go in the next decade. This mini-review quotes 33 articles on the different fNIRS basics and technical developments and 44 reviews on the fNIRS applications published in the last eight years. The huge number of review articles about a wide spectrum of topics in the field of cognitive and social sciences, functional neuroimaging research, and medicine testifies to the maturity achieved by this non-invasive optical vascular-based functional neuroimaging technique. Today, fNIRS has started to be utilized on healthy subjects while moving freely in different naturalistic settings. Further instrumental developments are expected to be done in the near future to fully satisfy this latter important aspect. In addition, fNIRS procedures, including correction methods for the strong extracranial interferences, need to be standardized before using fNIRS as a clinical tool in individual patients. New research avenues such as interactive neurosciences, cortical activation modulated by different type of sport performance, and cortical activation during neurofeedback training are highlighted.

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.

Diffuse optical tomography for the detection of perinatal stroke at the cot side: a pilot study

BACKGROUND

Perinatal stroke is a potentially debilitating injury, often under-diagnosed in the neonatal period. We conducted a pilot study investigating the role of the portable, non-invasive brain monitoring technique, diffuse optical tomography (DOT), as an early detection tool for infants with perinatal stroke.

METHODS

Four stroke-affected infants were scanned with a DOT system within the first 3 days of life and compared to four healthy control subjects. Spectral power, correlation, and phase lag between interhemispheric low frequency (0.0055-0.3 Hz) hemoglobin signals were assessed. Optical data analyses were conducted with and without magnetic resonance imaging (MRI)-guided stroke localization to assess the efficacy of DOT when used without stroke anatomical information.

RESULTS

Interhemispheric correlations of both oxyhemoglobin and deoxyhemoglobin concentration were significantly reduced in the stroke-affected group within the very low (0.0055-0.0095 Hz) and resting state (0.01-0.08 Hz) frequencies (p < 0.003). There were no interhemispheric differences for spectral power. These results were observed even without MRI stroke localization.

CONCLUSION

This suggests that DOT and correlation-based analyses in the low-frequency range can potentially aid the early detection of perinatal stroke, prior to MRI acquisition. Additional methodological advances are required to increase the sensitivity and specificity of this technique.

Spectral correction for handheld optoacoustic imaging by means of near-infrared optical tomography in reflection mode

In vivo imaging of tissue/vasculature oxygen saturation levels is of prime interest in many clinical applications. To this end, the feasibility of combining two distinct and complementary imaging modalities is investigated: optoacoustics (OA) and near-infrared optical tomography (NIROT), both operating noninvasively in reflection mode. Experiments were conducted on two optically heterogeneous phantoms mimicking tissue before and after the occurrence of a perturbation. OA imaging was used to resolve submillimetric vessel-like optical absorbers at depths up to 25 mm, but with a spectral distortion in the OA signals. NIROT measurements were utilized to image perturbations in the background and to estimate the light fluence inside the phantoms at the wavelength pair (760 nm, 830 nm). This enabled the spectral correction of the vessel-like absorbers' OA signals: the error in the ratio of the absorption coefficient at 830 nm to that at 760 nm was reduced from 60%-150% to 10%-20%. The results suggest that oxygen saturation (SO ₂ ) levels in arteries can be determined with <10% error and furthermore, that relative changes in vessels' SO ₂ can be monitored with even better accuracy. The outcome relies on a proper identification of the OA signals emanating from the studied vessels.

Back-propagation neural network-based reconstruction algorithm for diffuse optical tomography

Diffuse optical tomography (DOT) is a promising noninvasive imaging modality and is capable of providing functional characteristics of biological tissue by quantifying optical parameters. The DOT image reconstruction is ill-posed and ill-conditioned, due to the highly diffusive nature of light propagation in biological tissues and limited boundary measurements. The widely used regularization technique for DOT image reconstruction is Tikhonov regularization, which tends to yield oversmoothed and low-quality images containing severe artifacts. It is necessary to accurately choose a regularization parameter for Tikhonov regularization. To overcome these limitations, we develop a noniterative reconstruction method, whereby optical properties are recovered based on a back-propagation neural network (BPNN). We train the parameters of BPNN before DOT image reconstruction based on a set of training data. DOT image reconstruction is achieved by implementing a single evaluation of the trained network. To demonstrate the performance of the proposed algorithm, we compare with the conventional Tikhonov regularization-based reconstruction method. The experimental results demonstrate that image quality and quantitative accuracy of reconstructed optical properties are significantly improved with the proposed algorithm.

Non-contact dynamic diffuse optical tomography imaging system for evaluating lower extremity vasculature

A novel multi-view non-contact dynamic diffuse optical tomographic imaging system for the clinical evaluation of vasculature in the lower extremities is presented. The system design and implementation are described in detail, including methods for simultaneously obtaining and reconstructing diffusely reflected and transmitted light using a system of mirrors and a single CCD camera. The system and its performance using numeric simulations and optical phantoms. Measurements of a healthy foot in vivo demonstrates the potential of the system in assessing perfusion within the foot.

Weighting function effects in a direct regularization method for image-guided near-infrared spectral tomography of breast cancer

Structural image-guided near-infrared spectral tomography (NIRST) has been developed as a way to use diffuse NIR spectroscopy within the context of image-guided quantification of tissue spectral features. A direct regularization imaging (DRI) method for NIRST has the value of not requiring any image segmentation. Here, we present a comprehensive investigational study to analyze the impact of the weighting function implied when weighting the recovery of optical coefficients in DRI based NIRST. This was done using simulations, phantom and clinical patient exam data. Simulations where the true object is known indicate that changes to this weighting function can vary the contrast by 10%, the contrast to noise ratio by 20% and the full width half maximum (FWHM) by 30%. The results from phantoms and human images show that a linear inverse distance weighting function appears optimal, and that incorporation of this function can generally improve the recovered total hemoglobin contrast of the tumor to the normal surrounding tissue by more than 15% in human cases.

Validation of noninvasive photoacoustic measurements of sagittal sinus oxyhemoglobin saturation in hypoxic neonatal piglets

We hypothesize that noninvasive photoacoustic imaging can accurately measure cerebral venous oxyhemoglobin saturation (So₂) in a neonatal model of hypoxia-ischemia. In neonatal piglets, which have a skull thickness comparable to that of human neonates, we compared the photoacoustic measurement of sagittal sinus So₂ against that measured directly by blood sampling over a wide range of conditions. Systemic hypoxia was produced by decreasing inspired oxygen stepwise (i.e., 100, 21, 19, 17, 15, 14, 13, 12, 11, and 10%) with and without unilateral or bilateral ligation of the common carotid arteries to enhance hypoxia-ischemia. Transcranial photoacoustic sensing enabled us to detect changes in sagittal sinus O₂ saturation throughout the tested range of 5-80% without physiologically relevant bias. Despite lower cortical perfusion and higher oxygen extraction in groups with carotid occlusion at equivalent inspired oxygen, photoacoustic measurements successfully provided a robust linear correlation that approached the line of identity with direct blood sample measurements. Receiver-operating characteristic analysis for discriminating So₂ <30% showed an area under the curve of 0.84 for the pooled group data, and 0.87, 0.91, and 0.92 for hypoxia alone, hypoxia plus unilateral occlusion, and hypoxia plus bilateral occlusion subgroups, respectively. The detection precision in this critical range was confirmed with sensitivity (87.0%), specificity (86.5%), accuracy (86.8%), positive predictive value (90.5%), and negative predictive value (81.8%) in the combined dataset. These results validate the capability of photoacoustic sensing technology to accurately monitor sagittal sinus So₂ noninvasively over a wide range and support its use for early detection of neonatal hypoxia-ischemia. NEW & NOTEWORTHY We present data to validate the noninvasive photoacoustic measurement of sagittal sinus oxyhemoglobin saturation. In particular, this paper demonstrates the robustness of this methodology during a wide range of hemodynamic and physiological changes induced by the stepwise decrease of fractional inspired oxygen to produce hypoxia and by unilateral and bilateral ligation of the common carotid arteries preceding hypoxia to produce hypoxia-ischemia. This technique may be useful for diagnosing risk of neonatal hypoxic-ischemic encephalopathy.

Neuromonitoring in the neonatal ECMO patient

Utilization of extraocorporeal membrane oxygenation (ECMO) has become increasingly widespread as a bridging therapy for neonates with severe, reversible respiratory or cardiac diseases. While significant risks remain, due to advances in medical and surgical management, overall mortality has decreased. However, short and long-term neurological morbidity has remained high. Therefore, increasing attention has been focused on multimodal neuromonitoring to track and optimally, minimize or prevent intracranial injury. This review will explore the the indications, advantages, disadvantages, timing, frequency, duration, and any known correlation with neurodevelopmental outcomes of common types of neuromonitoring in the neonatal ECMO population. Investigational monitoring techniques such as NIRS will be briefly reviewed.

Functional Near Infrared Spectroscopy: Enabling Routine Functional Brain Imaging

Functional Near-Infrared Spectroscopy (fNIRS) maps human brain function by measuring and imaging local changes in hemoglobin concentrations in the brain that arise from the modulation of cerebral blood flow and oxygen metabolism by neural activity. Since its advent over 20 years ago, researchers have exploited and continuously advanced the ability of near infrared light to penetrate through the scalp and skull in order to non-invasively monitor changes in cerebral hemoglobin concentrations that reflect brain activity. We review recent advances in signal processing and hardware that significantly improve the capabilities of fNIRS by reducing the impact of confounding signals to improve statistical robustness of the brain signals and by enhancing the density, spatial coverage, and wearability of measuring devices respectively. We then summarize the application areas that are experiencing rapid growth as fNIRS begins to enable routine functional brain imaging.