Determining changes in NIR absorption using a layered model of the human head

Dynamics of cortical neurovascular coupling analyzed by simultaneous DC-magnetoencephalography and time-resolved near-infrared spectroscopy

Functional magnetic resonance imaging (fMRI) visualizes activated brain areas with a high spatial resolution. The activation signal is determined by the local change of cerebral blood oxygenation, blood volume and blood flow which serve as surrogate marker for the neuronal signal itself. Here, the complex coupling between these parameters and the electrophysiologic activity is characterized non-invasively in humans during a simple motor task using simultaneously DC-magnetoencephalography (DC-MEG), for the detection of neuronal signals, and time-resolved near-infrared spectroscopy (trNIRS), for cortical metabolic/vascular responses: over the left primary motor cortex hand area of healthy subjects DC-fields and trNIRS parameters followed closely the 30 s motor task cycles, i.e., finger movements of the right hand alternating with rest. In subjects showing a sufficient signal-to-noise ratio the analysis of variance of photon time of flight proved that the task-related trNIRS changes originated from the cortex. While onset and relaxation started simultaneously, trNIRS signals reached 50% of the maximum level 1-4 s later than the DC-MEG-signals. The non-invasive 'dual' setup helps to characterize simultaneously the two complementary aspects of the 'hemodynamic inverse problem', i.e., the coupling of neuronal and vascular/metabolic signals, in healthy subjects and provides a new analysis perspective for pathophysiological coupling concepts in diverse diseases, e.g., in stroke, hypertension and Alzheimer's disease.

Optical brain imaging in vivo: techniques and applications from animal to man

Optical brain imaging has seen 30 years of intense development, and has grown into a rich and diverse field. In-vivo imaging using light provides unprecedented sensitivity to functional changes through intrinsic contrast, and is rapidly exploiting the growing availability of exogenous optical contrast agents. Light can be used to image microscopic structure and function in vivo in exposed animal brain, while also allowing noninvasive imaging of hemodynamics and metabolism in a clinical setting. This work presents an overview of the wide range of approaches currently being applied to in-vivo optical brain imaging, from animal to man. Techniques include multispectral optical imaging, voltage sensitive dye imaging and speckle-flow imaging of exposed cortex, in-vivo two-photon microscopy of the living brain, and the broad range of noninvasive topography and tomography approaches to near-infrared imaging of the human brain. The basic principles of each technique are described, followed by examples of current applications to cutting-edge neuroscience research. In summary, it is shown that optical brain imaging continues to grow and evolve, embracing new technologies and advancing to address ever more complex and important neuroscience questions.

Retinotopic mapping of adult human visual cortex with high-density diffuse optical tomography

Functional neuroimaging is a vital element of neuroscience and cognitive research and, increasingly, is an important clinical tool. Diffuse optical imaging is an emerging, noninvasive technique with unique portability and hemodynamic contrast capabilities for mapping brain function in young subjects and subjects in enriched or clinical environments. We have developed a high-performance, high-density diffuse optical tomography (DOT) system that overcomes previous limitations and enables superior image quality. We show herein the utility of the DOT system by presenting functional hemodynamic maps of the adult human visual cortex. The functional brain images have a high contrast-to-noise ratio, allowing visualization of individual activations and highly repeatable mapping within and across subjects. With the improved spatial resolution and localization, we were able to image functional responses of 1.7 cm in extent and shifts of <1 cm. Cortical maps of angle and eccentricity in the visual field are consistent with retinotopic studies using functional MRI and positron-emission tomography. These results demonstrate that high-density DOT is a practical and powerful tool for mapping function in the human cortex.

DC-magnetoencephalography and time-resolved near-infrared spectroscopy combined to study neuronal and vascular brain responses

The temporal relation between vascular and neuronal responses of the brain to external stimuli is not precisely known. For a better understanding of the neuro-vascular coupling changes in cerebral blood volume and oxygenation have to be measured simultaneously with neuronal currents. With this motivation modulation dc-magnetoencephalography was combined with multi-channel time-resolved near-infrared spectroscopy to simultaneously monitor neuronal and vascular parameters on a scale of seconds. Here, the technique is described, how magnetic and optical signals can be measured simultaneously. In a simple motor activation paradigm (alternating 30 s of finger movement with 30 s of rest for 40 min) both signals were recorded non-invasively over the motor cortex of eight subjects. The off-line averaged signals from both modalities showed distinct stimulation related changes. By plotting changes in oxy- or deoxyhaemoglobin as a function of magnetic field a characteristic trajectory was created, which was similar to a hysteresis loop. A parametric analysis allowed quantitative results regarding the timing of coupling: the vascular signal increased significantly slower than the neuronal signal.

Time-resolved optical imager for assessment of cerebral oxygenation

A time-resolved optical instrument allowing for noninvasive assessment of cerebral oxygenation is presented. The instrument is equipped with picosecond diode lasers, fast photodetectors, and time-correlated single photon counting electronics. This technology enables depth-resolved estimation of changes in absorption and, in consequence, assessment of changes in hemoglobin concentrations in the brain cortex. Changes in oxyhemoglobin (HbO(2)) and deoxyhemoglobin (Hb) can be evaluated selectively in extra- and intracerebral tissue compartments using the moments of distributions of times of flight of photons measured at two wavelengths in the near-infrared region. The combination of the data acquired from multiple sources and detectors located on the surface of the head with the depth-resolved analysis, based on the moments, enables imaging of cortex oxygenation. Results of the tests on physical phantoms as well as in vivo validation of the instrument during the motor stimulation experiment are presented.

The oxygenation response to functional stimulation: is there a physiological meaning to the lag between parameters?

To investigate the regulation of the hemodynamic response to functional stimulation, functional near-infrared spectroscopy (fNIRS) has been used, due to its ability to assess the dynamics of oxygenated, deoxygenated and total hemoglobin concentration ([oxy-Hb], [deoxy-Hb] and [tot-Hb]). Concerning the latency of these parameters, recent studies have returned a consistent picture when comparing the oxygenation response in the sensorimotor to the visual system: changes in [oxy-Hb] lead those in [deoxy-Hb] by 1.6+/-0.2 s (mean+/-SD) for the sensorimotor system but not for the visual system (0.1+/-0.3 s). A number of physiological differences between these cortical areas may account for such a discrepancy, however, the methodological properties of transcranial NIRS also have a relevant influence. Here we show that for the motor system the latency between changes in oxy- compared to deoxy-Hb vanishes once efforts are made to reduce the effects of a systemic response accompanying sensorimotor activity. We apply two independent approaches to reduce the systemic response and find a simultaneous change in [oxy-Hb] and [deoxy-Hb] even in response to a motor paradigm. The two approaches are: (i) an experimental paradigm with alternating contralateral and ipsilateral motor performance without interspersed rest periods designed to minimize systemic changes and (ii) a global correction scheme in an experiment, comparing a unilateral motor performance to rest. These data shed some doubt on the alleged fundamental physiological difference between cortical hemodynamic regulation in motor and visual cortex and highlight the relevance to respect contributions of the systemic hemodynamics.

Relevance of depth resolution for cerebral blood flow monitoring by near-infrared spectroscopic bolus tracking during cardiopulmonary bypass

OBJECTIVE

Noninvasive near-infrared spectroscopy (NIRS) is increasingly used to monitor cerebral oxygenation and blood flow status, which is also of high relevance during cardiovascular surgical interventions with cardiopulmonary bypass. Contamination of the cerebral signal by contamination from overlaying extracerebral tissue, however, has been proposed to reduce sensitivity and cerebral selectivity of this promising technique.

METHODS

We evaluated a novel depth-resolved approach for the determination of cerebral hemodynamics by near-infrared spectroscopic tracking of intravenously administered indocyanine green boluses. A frequency domain technique was applied, allowing simultaneous determination of light absorption changes and time of flight of single photons and enabling the differentiation between extracerebral and intracerebral tracer kinetics. Depth-resolved near-infrared spectroscopy was tested in 4 patients undergoing cardiopulmonary bypass and compared with data derived by conventional continuous-wave near-infrared spectroscopy.

RESULTS

Depth resolution extracted the differential responses of extracerebral and intracerebral blood vessels from near-infrared bolus tracking signals. Postoperative blood flow indices derived from the intracerebral time course exceeded preoperative values by 1.5 +/- 0.2 times, indicating a significant increase of cerebral blood flow not detectable by conventional near-infrared spectroscopy.

CONCLUSION

The depth-resolved approach provides additional and relevant data for the interpretation of intraoperative cerebral perfusion during cardiothoracic surgery. The validity of this approach for patients with preexisting risk factors for cerebral hypoperfusion remains to be determined in larger clinical trials.

Comparison between a time-domain and a frequency-domain system for optical tomography

The quality of phase and amplitude data from two medical optical tomography systems were compared. The two systems are a 32-channel time-domain system developed at University College London (UCL) and a 16-channel frequency-domain system developed at Helsinki University of Technology (HUT). Difference data measured from an inhomogeneous and a homogeneous phantom were compared with a finite-element method (diffusion equation) and images of scattering and absorption were reconstructed based on it. The measurements were performed at measurement times between 1 and 30 s per source. The mean rms errors in the data measured by the HUT system were 3.4% for amplitude and 0.51 deg for phase, while the corresponding values for the UCL data were 6.0% and 0.46 deg, respectively. The reproducibility of the data measured with the two systems was tested with a measurement time of 5 s per source. It was 0.4% in amplitude for the HUT system and 4% for the UCL system, and 0.08 deg in phase for both systems. The image quality of the reconstructions from the data measured with the two systems were compared with several quantitative criteria. In general a higher contrast was observed in the images calculated from the HUT data.

Decomposition of a laser-Doppler spectrum for estimation of speed distribution of particles moving in an optically turbid medium: Monte Carlo validation study

A method for measurement of distribution of speed of particles moving in an optically turbid medium is presented. The technique is based on decomposition of the laser-Doppler spectrum. The theoretical background is shown together with the results of Monte Carlo simulations, which were performed to validate the proposed method. The laser-Doppler spectra were obtained by Monte Carlo simulations for assumed uniform and Gaussian speed distributions of particles moving in the turbid medium. The Doppler shift probability distributions were calculated by Monte Carlo simulations for several anisotropy factors of the medium, assuming the Hanyey-Greenstein phase function. The results of the spectra decomposition show that the calculated speed distribution of moving particles match well the distribution assumed for Monte Carlo simulations. This result was obtained for the spectra simulated in optical conditions, in which the photon is scattered with the Doppler shift not more than once during its travel between the source and detector. Influence of multiple scattering of the photon is analysed and a perspective of spectrum decomposition under such conditions is considered. Potential applications and limitations of the method are discussed.

Optimization of the Monte Carlo code for modeling of photon migration in tissue

The Monte Carlo method is frequently used to simulate light transport in turbid media because of its simplicity and flexibility, allowing to analyze complicated geometrical structures. Monte Carlo simulations are, however, time consuming because of the necessity to track the paths of individual photons. The time consuming computation is mainly associated with the calculation of the logarithmic and trigonometric functions as well as the generation of pseudo-random numbers. In this paper, the Monte Carlo algorithm was developed and optimized, by approximation of the logarithmic and trigonometric functions. The approximations were based on polynomial and rational functions, and the errors of these approximations are less than 1% of the values of the original functions. The proposed algorithm was verified by simulations of the time-resolved reflectance at several source-detector separations. The results of the calculation using the approximated algorithm were compared with those of the Monte Carlo simulations obtained with an exact computation of the logarithm and trigonometric functions as well as with the solution of the diffusion equation. The errors of the moments of the simulated distributions of times of flight of photons (total number of photons, mean time of flight and variance) are less than 2% for a range of optical properties, typical of living tissues. The proposed approximated algorithm allows to speed up the Monte Carlo simulations by a factor of 4. The developed code can be used on parallel machines, allowing for further acceleration.

Perturbation theory for the diffusion equation by use of the moments of the generalized temporal point-spread function. I. Theory

We approach the perturbative solution to the diffusion equation for the case of absorbing inclusions embedded in a heterogeneous scattering medium by using general properties of the radiative transfer equation and the solution of the Fredholm equation of the second kind given by the Neumann series. The terms of the Neumann series are used to obtain the expression of the moments of the generalized temporal point-spread function derived in transport theory. The moments are calculated independently by using Monte Carlo simulations for validation of the theory. While the mixed moments are correctly derived from the theory by using the solution of the diffusion equation in the geometry of interest, in order to obtain the self moments we should reframe the problem in transport theory and use a suitable solution of the radiative transfer equation for the calculation of the multiple integrals of the corresponding Neumann series. Since the rigorous theory leads to impractical formulas, in order to simplify and speed up the calculation of the self moments, we propose a heuristic method based on the calculation of only a single integral and some scaling parameters. We also propose simple quadrature rules for the calculation of the mixed moments for speeding up the computation of perturbations due to multiple defects. The theory can be developed in the continuous-wave domain, the time domain, and the frequency domain. In a companion paper [J. Opt. Soc. Am. A23, 2119 (2006)] we discuss the conditions of applicability of the theory in practical cases found in diffuse optical imaging of biological tissues.

Absorption and scattering perturbations in homogeneous and layered diffusive media probed by time-resolved reflectance at null source-detector separation

We characterize the capability of time-resolved reflectance measurements at small source-detector separation (less than 5 mm) to localize small inhomogeneities embedded in an otherwise homogeneous or layered diffusive medium. By considering both absorption and scattering inhomogeneities, we demonstrate the improvement of this approach in terms of contrast and spatial resolution, as compared to more typical set-ups involving larger source-detection separations (few centimeters). Simulations are performed exploiting an analytical perturbation approach to diffusion theory and a four-layer heterogeneous time-resolved Monte Carlo code, considering realistic tissue geometries. Exhaustive investigation in the parameters space is reported.

Effective scattering coefficient of the cerebral spinal fluid in adult head models for diffuse optical imaging

An efficient computation of the time-dependent forward solution for photon transport in a head model is a key capability for performing accurate inversion for functional diffuse optical imaging of the brain. The diffusion approximation to photon transport is much faster to simulate than the physically correct radiative transport equation (RTE); however, it is commonly assumed that scattering lengths must be much smaller than all system dimensions and all absorption lengths for the approximation to be accurate. Neither of these conditions is satisfied in the cerebrospinal fluid (CSF). Since line-of-sight distances in the CSF are small, of the order of a few millimeters, we explore the idea that the CSF scattering coefficient may be modeled by any value from zero up to the order of the typical inverse line-of-sight distance, or approximately 0.3 mm(-1), without significantly altering the calculated detector signals or the partial path lengths relevant for functional measurements. We demonstrate this in detail by using a Monte Carlo simulation of the RTE in a three-dimensional head model based on clinical magnetic resonance imaging data, with realistic optode geometries. Our findings lead us to expect that the diffusion approximation will be valid even in the presence of the CSF, with consequences for faster solution of the inverse problem.

Time-gated optical system for depth-resolved functional brain imaging

We present a time-domain optical system for functional imaging of the adult head. We first describe the instrument, which is based on a Ti:Sapphire pulsed laser (wavelength 750-850 nm) and an intensified CCD camera enabling parallel detection of multiple fibers. We characterize the system in terms of sensitivity and signal-to-noise ratio, instrument response function, cross-talk, stability, and reproducibility. We then describe two applications of the instrument: the characterization of baseline optical properties of homogeneous scattering media, and functional brain imaging. For the second application, we developed a two-part probe consisting in two squares of 4 x 4 sources and 3 x 3 detectors. The laser source is time-multiplexed to define 4 states of 8 sources that can be turned on during the same camera frame while minimizing cross-talk. On the detection side, we use for each detector 7 fibers of different lengths creating an optical delay, and enabling simultaneous detection in 7 windows (by steps of 500 ps) for each detector. This multiple window detection allows depth sensitivity. The imaging probe was tested on dynamic phantoms and a preliminary result on an adult performing a motor task shows discrimination between superficial and cortical responses to the stimulus on both hemispheres.

Non-invasive detection of fluorescence from exogenous chromophores in the adult human brain

This is the first report on results proving that fluorescence of exogenous dyes inside the human brain can be excited and detected non-invasively at the surface of the adult head. Boli of indocyanine green (ICG) were intravenously applied to healthy volunteers, and the passage of the contrast agent in the brain was monitored by detecting the corresponding fluorescence signal following pulsed laser excitation at 780 nm. Our hypothesis that the observed fluorescence signal contains a considerable cortical fraction was corroborated by performing measurements with picosecond temporal resolution and analyzing distributions of times of arrival of photons, hence taking advantage of the well-known depth selectivity of that method. Our experimental findings are explained by Monte Carlo simulations modeling the head as a layered medium and taking into account realistic bolus kinetics within the extra- and intracerebral compartment. Although a particular non-specific dye (ICG) was used, the results clearly demonstrate that fluorescence-mediated imaging of the adult human brain is generally feasible. In particular, we will discuss how these results serve as proof of concept for non-invasive fluorescence brain imaging and may thus open the door towards optical molecular imaging of the human brain.

Illuminating the BOLD signal: combined fMRI–fNIRS studies

Functional magnetic resonance imaging (fMRI) is currently combined with electrophysiological methods to identify the relationship between neuronal activity and the blood oxygenation level-dependent (BOLD) signal. Several processes like neuronal activity, synaptic activity, vascular dilation, blood volume and oxygenation changes underlie both response modalities, that is, the electrophysiological signal and the vascular response. However, accessing single process relationships is absolutely mandatory when aiming at a deeper understanding of neurovascular coupling and necessitates studies on the individual building blocks of the vascular response. Combined fMRI and functional near-infrared spectroscopy studies have been performed to validate the correlation of the BOLD signal to the hemodynamic changes in the brain. Here we review the current status of the integration of both technologies and judge these studies in the light of recent findings on neurovascular coupling.

Measurement of cerebral oxidative metabolism with near-infrared spectroscopy: a validation study

Predicting the onset of secondary energy failure after a hypoxic-ischemic insult in newborns is critical for providing effective treatment. Measuring reductions in the cerebral metabolic rate of oxygen (CMRO(2)) may be one method for early detection, as hypoxia-ischemia is believed to impair oxidative metabolism. We have developed a near-infrared spectroscopy (NIRS) technique based on the Fick Principle for measuring CMRO(2). This technique combines cerebral blood flow (CBF) measurements obtained using the tracer indocyanine green with measurements of the cerebral deoxy-hemoglobin (Hb) concentration. In this study, NIRS measurements of CMRO(2) were compared with CMRO(2) determined from the product of CBF and the cerebral arteriovenous difference in oxygen measured from blood samples. The blood samples were collected from a peripheral artery and the sagittal sinus. Eight piglets were subjected to five cerebral metabolic states created by varying the plane of anesthesia. No significant difference was found between CMRO(2) measurements obtained with the two techniques at any anesthetic level (P>0.5). Furthermore, there was a strong correlation when concomitant CMRO(2) values from the two techniques were compared (R(2)=0.88, P<0.001). This work showed that CMRO(2) can be determined accurately by combining NIRS measurements of CBF and Hb. Since NIRS is safe and measurements can be obtained at the bedside, it is believed that this technique could assist in the early diagnosis of cerebral energy dysfunction after hypoxia-ischemia.

Estimation of cerebral oxy- and deoxy-haemoglobin concentration changes in a layered adult head model using near-infrared spectroscopy and multivariate statistical analysis

The non-invasive measurement of cerebral oxy- (DeltaHbO(br)2) and deoxy-haemoglobin (DeltaHHb(br)) changes using near-infrared spectroscopy instruments is often affected by the absorption in the extracerebral layer. We have exploited the multivariate calibration (partial least squares, PLS) method to minimize the errors for a range of blood volume, oxygen saturation and extracerebral layer thicknesses. The changes in the mean time of flight of photons (Delta tau) and attenuation (DeltaA) on the surface of a 3D adult head model were simulated using a finite-element method based on the diffusion equation. The PLS was then performed to identify the optimal number of detectors, their positions and weightings, to optimize the estimation of DeltaHbO(br)2 and DeltaHHb(br). We define the 'nominal accuracy' as the accuracy of estimating DeltaHbO(br)2 and DeltaHHb(br) over a nominal range of extracerebral layer thicknesses and 'robustness' as the accuracy beyond the nominal range. The results showed that for one or two detectors, Delta tau performed better than DeltaA while using them together gave the best performance. When more detectors were used, the performances of using Delta tau, DeltaA or both together became comparable, showing that a larger number of detectors can compensate for the performance of a simple DeltaA measurement despite this measurement having a relatively lower sensitivity to intracerebral absorption changes.

Extraction of depth-dependent signals from time-resolved reflectance in layered turbid media

We try a new approach with near-IR time-resolved spectroscopy, to separate optical signals originated in the upper layer from those in the lower layer and to selectively determine the absorption coefficient (mu(a)) of each layer in a two-layered turbid medium. The difference curve in the temporal profiles of light attenuation between a target and a reference medium is divided into segments along the time axis, and a slope of each segment is calculated to determine the depth-dependent mu(a). The depth-dependent mu(a) values are estimated under various conditions in which mu(a) and the reduced scattering coefficient (mu(s)') of each layer are changed with a Monte Carlo simulation and in phantom experiments. Temporal variation of them represents the difference in mu(a) between two layers when mu(s)' of a reference is the same as that of the upper layer of the target. The discrepancies between calculated mu(a) and the real mu(a) depend on the ratio of the real mu(a) of the upper layer to that of the lower layer, and our approach enables us to estimate the ratio of mu(a) between the two layers. These results suggest the potential that mu(a) of the lower layer can be determined by our procedure.

Measuring microlymphatic flow using fast video microscopy

Despite advances in the measurement of lymphatic function, little is known about the actual velocities of flow in microlymphatic ( approximately 100 mum diam) vessels. In this work, video microscopy and particle tracking methods are adapted and integrated with an ultra-high-speed imaging camera to obtain measurements of lymph velocities throughout the entire lymphatic contraction cycle in the ratmesentery, something that previous systems were incapable of measuring. To determine the system's accuracy, calibration experiments are conducted across the hypothesized physiologically significant range of velocities for microlymphatic flow (up to 15 mmsec). The system shows high accuracy, less than 2% error, when comparing actual with measured velocities. Microspheres flowing through 140-mum-diam tubing are imaged to demonstrate the system's ability to determine flow rates in these small vessels by measuring particle velocities. To demonstrate biological applicability, mesenteric microlymphatics in loops of the small intestine of three male Sprague-Dawley rats are exteriorized and imaged with the high-speed system at a rate of 500 framessec for several contraction sequences. Lymph velocity fluctuates cyclically with the vessel wall contractions, ranging from -1 to 7 mmsec. These rates are higher than would be possible with standard video microscopy (3.75 mmsec maximum).

Reevaluation of near-infrared light propagation in the adult human head: implications for functional near-infrared spectroscopy

Using both experimental and theoretical methods, we examine the contribution of different parts of the head to near-IR (NIR) signal. Time-resolved spectroscopy is employed to measure the mean optical path length (PL), and the absorption (mu(a)) and reduced scattering (mu(s)') coefficients in multiple positions of the human head. Monte Carlo simulations are performed on four-layered head models based on an individual magnetic resonance imaging (MRI) scan to determine mu(a) and mu(s)' in each layer of the head by solving inverse problems, and to estimate the partial path length in the brain (p-PL) and the spatial sensitivity to regions in the brain at the source-detector separation of 30 mm. The PL is closely related to the thickness of the scalp, but not to that of other layers of the head. The p-PL is negatively related to the PL and its contribution ratio to the PL is 5 to 22% when the differential path length factor is 6. Most of the signal attributed to the brain comes from the upper 1 to 2 mm of the cortical surface. These results indicate that the NIR signal is very sensitive to hemodynamic changes associated with functional brain activation in the case that changes in the extracerebral tissue are ignorable.

Direct characterization and removal of interfering absorption trends in two-layer turbid media

We propose a method to isolate absorption trends confined to the lower layer of a two-layer turbid medium, as is desired in near-infrared spectroscopy (NIRS) of cerebral hemodynamics. Several two-layer Monte Carlo simulations of NIRS time series were generated using a physiologically relevant range of optical properties and varying the absorption coefficients due to bottom-layer, top-layer, and/or global fluctuations. Initial results showed that by measuring absorption trends at two source-detector separations and performing a least-squares fit of one to the other, processed signals strongly resemble the simulated bottom-layer absorption properties. Through this approach, it was demonstrated that fitting coefficients can be estimated within less than +/- 2% of the ideal value without any a priori knowledge of the optical properties present in the model. An analytical approximation for the least-squares coefficient provides physical insight into the nature of errors and suggests ways to reduce them.

Time-resolved reflectance at null source-detector separation: improving contrast and resolution in diffuse optical imaging

We propose a novel approach to imaging in diffusive media based on time-resolved reflectance measurements at null source-detector separation. This approach yields better spatial resolution and contrast as compared to the classical approach, which typically employs a separation of 20-40 mm. Results are obtained by an analytical perturbation approach to diffusion theory and on Monte Carlo simulations. Practical implementation with state-of-the-art technology and performance of a complementary approach based on the use of small but not null source-detector separation are also discussed.

The fast optical signal--robust or elusive when non-invasively measured in the human adult?

Near infrared spectroscopy (NIRS) can detect vascular changes in cerebral cortical tissue elicited by functional stimulation. For some 10 years, another optical signal has been reported to be accessible by NIRS. This signal has been reported to correlate to the electrophysiological response rendering NIRS an exquisite non-invasive approach to investigate neurovascular coupling in the human adult. Due to their typical latency of up to hundreds of milliseconds, these signals have been termed "fast" optical signals and have been postulated to stem from scatter changes in neuronal tissue, as a fingerprint of the electrophysiological response. Here, we utter a less optimistic view on the non-invasive detectability of these changes in the human, motivated by an upper limit signal size estimation, predicting a signal size by orders of magnitude smaller than those previously reported. Also, we discuss the influence of small stimulus correlated movement artifacts potentially mimicking a fast optical signal. Based on invasive studies, we perform an upper limit estimation for changes in intensity and mean time of flight, which can be expected assuming a scatter change in the cerebral cortex while measuring on the surface of the head of an adult subject. Since the resulting numbers are far below those previously reported, we constructed a simple system, which minimizes technical noise. The system allows us to detect rather small intensity changes (2 x 10(-3)%) when averaging over approximately 3000 stimuli. Despite this outstandingly low noise level of the system, we find a reliable change in response to a sub-motor-threshold steady state median nerve stimulation in just one single subject (8 subjects examined, 4 subjects twice). Exceeding the motor threshold leads to large stimulus related artifacts, on a similar time scale and with comparable amplitude as previously reported signals. To check for potential modality specific problems, we next performed a visual stimulation study, avoiding potential motor artifacts. For the steady state visually evoked response, no subject yielded a reliable result (11 subjects examined, 4 subjects twice). The paper discusses these findings by a review of the literature on fast optical signals and their being accessible in the adult human.

Simulation study of magnetic resonance imaging-guided cortically constrained diffuse optical tomography of human brain function

Diffuse optical imaging can measure brain activity noninvasively in humans through the scalp and skull by measuring the light intensity modulation arising from localized-activity-induced absorption changes within the cortex. Spatial resolution and localization accuracy are currently limited by measurement geometry to approximately 3 cm in the plane parallel to the scalp. Depth resolution is a more significant challenge owing to the limited angle tomography permitted by reflectance-only measurements. We combine previously established concepts for improving image quality and demonstrate, through simulation studies, their application for improving the image quality of adult human brain function. We show in a three-dimensional human head model that localization accuracy is significantly improved by the addition of measurements that provide overlapping samples of brain tissue. However, the reconstructed absorption contrast is significantly underestimated because its depth is underestimated. We show that the absorption contrast amplitude accuracy can be significantly improved by providing a cortical spatial constraint in the image reconstruction to obtain a better depth localization. The cortical constraint makes physiological sense since the brain-activity-induced absorption changes are occurring in the cortex and not in the scalp, skull, and cerebral spinal fluid. This spatial constraint is provided by segmentation of coregistered structural magnetic resonance imaging (MRI). However, the absorption contrast deep within the cortex is reconstructed superficially, resulting in an underestimation of the absorption contrast. The synthesis of techniques described here indicates that multimodality imaging of brain function with diffuse optical imaging and MRI has the potential to provide more quantitative estimates of the total and deoxyhemoglobin response to brain activation, which is currently not provided by either method independently. However, issues of depth resolution within the cortex remain to be resolved.

Improved sensitivity to cerebral hemodynamics during brain activation with a time-gated optical system: analytical model and experimental validation

Time domain (TD) diffuse optical measurement systems are being applied to neuroimaging, where they can detect hemodynamics changes associated with cerebral activity. We show that TD systems can provide better depth sensitivity than the more traditional continuous wave (CW) systems by gating late photons, which carry information about deep layers of the brain, and rejecting early light, which is sensitive to the superficial physiological signal clutter. We use an analytical model to estimate the contrast due to an activated region of the brain, the instrumental noise of the systems, and the background signal resulting from superficial physiological signal clutter. We study the contrast-to-noise ratio and the contrast-to-background ratio as a function of the activation depth and of the source-detector separation. We then present experimental results obtained with a time-gated instrument on the motor cortex during finger-tapping exercises. Both the model and the experimental results show a similar contrast-to-noise ratio for CW and TD, but that estimation of the contrast is experimentally limited by background fluctuations and that a better contrast-to-background ratio is obtained in the TD case. Finally, we use the time-gated measurements to resolve in depth the brain activation during the motor stimulus.

Bilateral prefrontal cortex oxygenation responses to a verbal fluency task: a multichannel time-resolved near-infrared topography study

The letter-fluency task-induced response over the prefrontal cortex is investigated bilaterally on eight subjects using a recently developed compact, eight-channel, time-resolved, near-IR system. The cross-subject mean values of prefrontal cortex oxygen saturation (SO2) were 68.8+/-3.2% (right) and 71.0+/-3.6% (left), and of total hemoglobin concentration (tHb) were 69.6+/-9.6 microM (right) and 69.5+/-9.9 microM (left). The typical cortical activation response to the cognitive task [characterized by an increase in oxyhemoglobin (O2Hb) with a concurrent decrease in deoxyhemoglobin (HHb)] at each measurement point is observed in only four subjects. In this subset, the amplitude of the O2Hb increase and HHb decrease is uniform over each prefrontal cortex area and comparable between the two hemispheres. These findings agree with previous studies using continuous wave functional near-IR spectroscopy and functional magnetic resonance imaging, therefore demonstrating the potential of a time-resolved spectroscopy approach. In addition, a significant increase in SO2 levels was observed in the right (1.1+/-0.5%) compared to left side of the prefrontal cortex (0.9+/-0.5%) (P=0.005). A different pattern of cortical activation (characterized by the lack of HHb decrease or even increased HHb) was observed in the remaining subjects.

Bed-side assessment of cerebral perfusion in stroke patients based on optical monitoring of a dye bolus by time-resolved diffuse reflectance

We present a minimally invasive optical method, that is, multi-channel time-domain diffuse near-infrared reflectometry of the head to assess cerebral blood perfusion that is applicable at the bed-side and repetitively at short intervals. Following intravenous injection of an ICG bolus, its transit through intra- and extracerebral tissue is monitored based on changes in moments of distributions of times of flight of photons, recorded with a 4-channel instrument simultaneously on both hemispheres. In healthy volunteers, we found that variance of distributions of times of flight of photons is well suited to assess latency and initial slope of the increase in absorption of intracerebral tissue due to the bolus. We successfully applied our method in two patients demonstrating a reversible cerebral perfusion deficit in an ischemic stroke patient who was treated by thrombolysis and in another patient with a permanent impaired unilateral perfusion due to ipsilateral internal carotid artery occlusion. In either case, we observed a difference in bolus transit time between the hemispheres. In the stroke patient, this difference resolved when re-evaluated 1 day after thrombolysis. The study demonstrates the necessity of a technique with sub-nanosecond time resolution to allow for depth discrimination if clinical perfusion monitoring of cerebrovascular diseases is addressed by optical methods.

Diffuse optical measurement of blood flow, blood oxygenation, and metabolism in a human brain during sensorimotor cortex activation

We combine diffuse optical and correlation spectroscopies to simultaneously measure the oxyhemoglobin and deoxyhemoglobin concentration and blood flow in an adult human brain during sensorimotor stimulation. The observations permit calculation of the relative cerebral metabolic rate of oxygen in the human brain, for the first time to our knowledge, by use of all-optical methods. The feasibility for noninvasive optical measurement of blood flow through the skull of an adult brain is thus demonstrated, and the clinical potential of this hybrid, all-optical noninvasive, methodology can now be explored.

Separability and cross talk: optimizing dual wavelength combinations for near-infrared spectroscopy of the adult head

By means of noninvasive near-infrared spectroscopy (NIRS), cerebral concentration changes in oxygenated and deoxygenated hemoglobin ([oxy-Hb] and [deoxy-Hb]) can be determined. The quality of the concentration changes' assessment critically depends on the wavelength combination used. Trying to optimize this combination, two spectroscopic effects must be taken into account: cross talk and separability. Cross talk between [oxy-Hb] and [deoxy-Hb] occurs because the assumption made in the analysis-that there is a homogeneous concentration change-does not hold true for the adult human head. Separability-to be introduced in this paper-is a measure for the degree of physical noise of the measurement that will influence the noise of the concentration changes' assessment. In other words, high separability corresponds to a low noise with respect to the concentration changes assessed. Here, we present analytical expressions for both measures and provide model-based estimates of cross talk and separability for any combination of two wavelengths between 610 and 920 nm. These theoretical considerations allow for two predictions: (a) if both wavelengths used are greater than approximately 780 nm, cross talk is high and separability is low resulting in erroneous and noisy concentration data. (b) If one wavelength is chosen below 720 nm while the other is greater than 730 nm, cross talk is low and separability is high resulting in accurate concentration changes. We show the relevance of these theoretical results for noninvasive NIRS by testing the predictions on experimental data obtained in adults undergoing visual stimulation.

Time-resolved multidistance near-infrared spectroscopy of the adult head: intracerebral and extracerebral absorption changes from moments of distribution of times of flight of photons

We report on multidistance time-resolved diffuse reflectance spectroscopy of the head of a healthy adult after intravenous administration of a bolus of indocyanine green. Intracerebral and extracerebral changes in absorption are deduced from moments (integral, mean time of flight, and variance) of the distributions of times of flight of photons (DTOFs), recorded simultaneously at four different source-detector separations. We calculate the sensitivity factors converting depth-dependent changes in absorption into changes of moments of DTOFs by Monte Carlo simulations by using a layered model of the head. We validate our method by analyzing moments of DTOFs simulated for the assumed changes in absorption in different layers of the head model.

Cytochrome-c-oxidase redox changes during visual stimulation measured by near-infrared spectroscopy cannot be explained by a mere cross talk artefact

The detection of redox changes in cytochrome-c-oxidase ([Cyt-ox]) in response to cerebral activation by non-invasive NIRS is hampered by methodological spectroscopic issues related to the modification of the Beer-Lambert law. Also, the question whether a change in the enzyme's redox-state is elicited by functional stimulation is unresolved. In a previous study, we found physiological evidence in favour of an activation-induced increase in oxidation of the enzyme [J. Cereb. Blood Flow Metab. 19 (1999) 592], while in a second study on spectroscopic cross talk, we found that the [Cyt-ox] changes to potentially be an artefact of the spectroscopic approach [J. Biomed. Opt. 7 (2002) 51]. Here, we use two different stimuli which differentially activate areas either rich or poor in [Cyt-ox] content (blob/interblob in visual cortex V1 and pale/thin stripes in V2) to further clarify this apparent discrepancy. In a first experiment, two stimuli were presented in an alternating fashion for 20 s and all stimulation periods were separated by resting periods of 40 s. We observed similar changes in [Cyt-ox] for both stimuli. To become more sensitive to the potentially very small optical changes related to changes in [Cyt-ox], we tried to minimise global haemodynamic and metabolic effects in a second experiment by omitting the resting periods. Our hypothesis was that [Cyt-ox] changes could be fully explained by cross talk as it is predicted from our last study [J. Biomed. Opt. 7 (2002) 51]. However, in more than half of the experiments, we were not able to model the changes in Cyt-ox calculated from measured attenuation spectra as a cross talk artefact. We interpret this finding as an argument in favour of the existence of [Cyt-ox] changes in response to functional stimulation. This finding, however, does not lessen the liability of the [Cyt-ox] changes to cross talk and calls for great caution when [Cyt-ox] changes are derived from NIRS measurements based on the modified Beer-Lambert approach. Further (invasive) validation studies are required.

Mapping of calf muscle oxygenation and haemoglobin content during dynamic plantar flexion exercise by multi-channel time-resolved near-infrared spectroscopy

A compact and fast multi-channel time-resolved near-infrared spectroscopy system for tissue oximetry was developed. It employs semiconductor laser and fibre optics for delivery of optical signals. Photons are collected by eight 1 mm fibres and detected by a multianode photomultiplier. A time-correlated single photon counting board is used for the parallel acquisition of time-resolved reflectance curves. Estimate of the reduced scattering coefficient is achieved by fitting with a standard model of diffusion theory, while the modified Lambert-Beer law is used to assess the absorption coefficient. In vivo measurements were performed on five healthy volunteers to monitor spatial changes in calf muscle (medial and lateral gastrocnemius; MG, LG) oxygen saturation (SmO2) and total haemoglobin concentration (tHb) during dynamic plantar flexion exercise performed at 50% of the maximal voluntary contraction. At rest SmO2 was 73.0 +/- 0.9 and 70.5 +/- 1.7% in MG and LG, respectively (P = 0.045). At the end of the exercise, SmO2 decreased (69.1 +/- 1.8 and 63.8 +/- 2.1% in MG and LG, respectively; P < 0.01). The LG desaturation was greater than the MG desaturation (P < 0.02). These results strengthen the role of time-resolved near-infrared spectroscopy as a powerful tool for investigating the spatial and temporal features of muscle SmO2 and tHb.

Diffuse optical imaging of brain activation: approaches to optimizing image sensitivity, resolution, and accuracy

Near-infrared spectroscopy (NIRS) and diffuse optical imaging (DOI) are finding widespread application in the study of human brain activation, motivating further application-specific development of the technology. NIRS and DOI offer the potential to quantify changes in deoxyhemoglobin (HbR) and total hemoglobin (HbT) concentration, thus enabling distinction of oxygen consumption and blood flow changes during brain activation. While the techniques implemented presently provide important results for cognition and the neurosciences through their relative measures of HbR and HbT concentrations, there is much to be done to improve sensitivity, accuracy, and resolution. In this paper, we review the advances currently being made and issues to consider for improving optical image quality. These include the optimal selection of wavelengths to minimize random and systematic error propagation in the calculation of the hemoglobin concentrations, the filtering of systemic physiological signal clutter to improve sensitivity to the hemodynamic response to brain activation, the implementation of overlapping measurements to improve image spatial resolution and uniformity, and the utilization of spatial prior information from structural and functional MRI to reduce DOI partial volume error and improve image quantitative accuracy.

Factors affecting the accuracy of near-infrared spectroscopy concentration calculations for focal changes in oxygenation parameters

Near-infrared spectroscopy (NIRS) can be used to noninvasively measure changes in the concentrations of oxy- and deoxyhemoglobin in tissue. We have previously shown that while global changes can be reliably measured, focal changes can produce erroneous estimates of concentration changes (NeuroImage 13 (2001), 76). Here, we describe four separate sources for systematic error in the calculation of focal hemoglobin changes from NIRS data and use experimental methods and Monte Carlo simulations to examine the importance and mitigation methods of each. The sources of error are: (1). the absolute magnitudes and relative differences in pathlength factors as a function of wavelength, (2). the location and spatial extent of the absorption change with respect to the optical probe, (3). possible differences in the spatial distribution of hemoglobin species, and (4). the potential for simultaneous monitoring of multiple regions of activation. We found wavelength selection and optode placement to be important variables in minimizing such errors, and our findings indicate that appropriate experimental procedures could reduce each of these errors to a small fraction (<10%) of the observed concentration changes.

Beyond the visible--imaging the human brain with light

Optical approaches to investigate cerebral function and metabolism have long been applied in invasive studies. From the neuron cultured to the exposed cortex in the human during neurosurgical procedures, high spatial resolution can be reached and several processes such as membrane potential, cell swelling, metabolism of mitochondrial chromophores, and vascular response can be monitored, depending on the respective preparation. The authors focus on an extension of optical methods to the noninvasive application in the human. Starting with the pioneering work of Jöbsis 25 years ago, near-infrared spectroscopy (NIRS) has been used to investigate functional activation of the human cerebral cortex. Recently, several groups have started to use imaging systems that allow the generation of images of a larger area of the subject's head and, thereby, the production of maps of cortical oxygenation changes. Such images have a much lower spatial resolution compared with the invasively obtained optical images. The noninvasive NIRS images, however, can be obtained in undemanding set-ups that can be easily combined with other functional methods, in particular EEG. Moreover, NIRS is applicable to bedside use. The authors briefly review some of the abundant literature on intrinsic optical signals and the NIRS imaging studies of the past few years. The weaknesses and strengths of the approach are critically discussed. The authors conclude that NIRS imaging has two major advantages: it can address issues concerning neurovascular coupling in the human adult and can extend functional imaging approaches to the investigation of the diseased brain.

Lateral frontal cortex oxygenation changes during translation and language switching revealed by non-invasive near-infrared multi-point measurements

The organisation of language in the brain of multilingual people remains controversial. Using a high temporal resolution 12-channel near-infrared continuous wave spectroscopy system, we have demonstrated that it is possible to monitor non-invasively, comfortably and, without the interferences due to intrinsic limitations of positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), cortical oxygenation changes in the Broca's area in response to translation of short sentences and language switching. Eight Dutch students proficient in English translated aloud from their native language into English or vice versa or alternating (switching) short visually presented sentences. These tasks provoked, in the left inferior frontal cortex which includes the Broca's area, a consistent and incremental rise in oxyhaemoglobin accompanied by a smaller decrease in deoxyhaemoglobin. The investigated cortical areas surrounding the Broca's area showed no uniform and consistent oxygenation changes upon the three different translation tasks. These results confirm that Broca's area is involved in the translation process and its so called activation is unaffected by the direction of the translation. In addition, these results strengthen the role of near-infrared multi-point measurements as a powerful tool for investigating the spatial and temporal features of the cortical oxygenation changes during language processing.

Educational neuroimaging: a proposed neuropsychological application of near-infrared spectroscopy (nIRS)

OBJECTIVE

To provide a description of an emerging neuroimaging methodology, near-infrared spectroscopy (nIRS), and a potential educational application of the unique aspects of this technology.

SUMMARY

nIRS is documented for its potential as a personal, portable, brain imaging system that may prove useful for cerebral monitoring in applied settings such as home, school, and work. The basis of nIRS brain imaging is reviewed, with summary descriptions of optical and neurovascular issues as well as a brief comparison to other brain imaging methodologies. Recent developments in nIRS technology are discussed, including ongoing validation efforts and potential applications for neuropsychologists. We describe one potential application of nIRS (i.e., educational neuroimaging) as an illustration of the use of nIRS technology and the potential expansion of the neuropsychologist's role in the educational setting.

CONCLUSION

nIRS holds the potential of opening new clinical questions and opportunities for neuropsychologists, and may provide a low-cost means of repeatable, neurovascular monitoring in nonmedical settings.

Noninvasive monitoring of cerebral blood flow by a dye bolus method: separation of brain from skin and skull signals

Tracking a bolus of contrast agent traveling through the cerebral vasculature provides a measure of the blood flow velocity in the respective cerebral tissue. This principle has been the basis for the first approaches in functional magnetic resonance (MR) imaging and is of great value for investigating patients with vascular disease, especially stroke. While bolus measurements are a standard procedure in MR imaging, optical bolus tracking is as yet not established. Here we study optical absorption changes induced by a bolus of the dye indocyanine-green with near infrared spectroscopy in healthy volunteers. The aim is to assess the latency and shape of the change in absorption. Since application in the adult human critically depends on differentiation between extra- and intracerebral vascular compartments we focus on an approach for such a separation. To do this frequency-domain and multidistance measurements are analyzed by a Monte Carlo based model for photon transport in tissue. Based on measurements of both the photon's mean time of flight (phase) and the intensity, our results allow differentiation between an upper (skin and skull) and a lower layer (brain). The bolus in the deeper tissue layers has a peak of about 10 s width, while the change in absorption in the upper layers shows a much longer recovery time. This is in qualitative agreement with MR imaging results using a gadolinium bolus. This result is promising with respect to the potential of bedside monitoring of mean transit time (MTT) changes in patients with stroke or related vascular disease.

Influence of hypoxia on wavelength dependence of differential pathlength and near-infrared quantification

Continuous wave near-infrared spectroscopy (NIRS) measurements of cardiac correlated changes in attenuation in the adult human head were computed using a Fourier analysis technique that eliminates the positive error bias associated with the magnitude of the Fourier coefficient. These attenuation changes were used to determine wavelength dependence of differential pathlength, DP(lambda), at four stages during progressive hypoxia (21, 17, 13 and 9% FIO2) in normal volunteers. The effects of incorporating DP(lambda) into NIRS algorithms to compute relative concentration changes and absolute concentration of oxyhaemoglobin and deoxyhaemoglobin are discussed. Because variations in DP(lambda) are restricted to wavelengths below 780 nm, absolute concentration calculations are influenced by hypoxia-induced changes while relative concentrations are unaffected. However, even accounting for changes in DP(lambda) did not allow computation of physiologically reasonable absolute concentrations of the haemoglobin species.

Cross talk in the Lambert-Beer calculation for near-infrared wavelengths estimated by Monte Carlo simulations

Using the modified Lambert-Beer law to analyze attenuation changes measured noninvasively during functional activation of the brain might result in an insufficient separation of chromophore changes ("cross talk") due to the wavelength dependence of the partial path length of photons in the activated volume of the head. The partial path length was estimated by performing Monte Carlo simulations on layered head models. When assuming cortical activation (e.g., in the depth of 8-12 mm), we determine negligible cross talk when considering changes in oxygenated and deoxygenated hemoglobin. But additionally taking changes in the redox state of cytochrome-c-oxidase into account, this analysis results in significant artifacts. An analysis developed for changes in mean time of flight--instead of changes in attenuation--reduces the cross talk for the layers of cortical activation. These results were validated for different oxygen saturations, wavelength combinations and scattering coefficients. For the analysis of changes in oxygenated and deoxygenated hemoglobin only, low cross talk was also found when the activated volume was assumed to be a 4-mm-diam sphere.

Reconstruction of absorber concentrations in a two-layer structure by use of multidistance time-resolved reflectance spectroscopy

The characterization of a two-layer structure was investigated by use of time-resolved reflectance over a wide spectral range. We exploited the nonlinear dependence of the measured spectra on the upper-and lower-layer properties to formulate an algorithm for the recovery of absorber concentrations in both layers. The method assumes that the spectral features of the key absorbers are known, but it does not rely on a priori knowledge of the layer thickness. Phantom tests confirmed the accuracy of the estimate of the absorber concentrations to within 10% for thickness values ranging from 0.3 to 1.2 cm. Multidistance absorption spectra from 610 to 1000 nm were obtained in vivo from the forearms of human subjects, allowing us to estimate the concentration of key tissue constituents in a two-layer approximation. Good agreement between the reconstructed spectra and the experimental data taken from two volunteers with opposite predominance of adipose and muscular tissues demonstrated the validity of this approach.

Three-dimensional optical tomography of hemodynamics in the human head

We report on the first three-dimensional, volumetric, tomographic localization of vascular reactivity in the brain. To this end we developed a model-based iterative image reconstruction scheme that employs adjoint differentiation methods to minimize the difference between measured and predicted data. The necessary human-head geometry and optode locations were determined with a photogrammetric method. To illustrate the performance of the technique, the three-dimensional distribution of changes in the concentration of oxyhemoglobin, deoxyhemoglobin, and total hemoglobin during a Valsalva maneuver were visualized. The observed results are consistent with previously reported effects concerning optical responses to hemodynamic perturbations.