Non-invasive optical monitoring of the newborn piglet brain using continuous-wave and frequency-domain spectroscopy.

We have used continuous-wave (CW) and frequency-domain spectroscopy to investigate the optical properties of the newborn piglet brain in vivo and non-invasively. Three anaesthetized, intubated, ventilated and instrumented newborn piglets were placed into a stereotaxic instrument for optimal experimental stability, reproducible probe-to-scalp optical contact and 3D adjustment of the optical probe. By measuring the absolute values of the brain absorption and reduced scattering coefficients at two wavelengths (758 and 830 nm), frequency-domain spectroscopy provided absolute readings (in contrast to the relative readings of CW spectroscopy) of cerebral haemoglobin concentration and saturation during experimentally induced perturbations in cerebral haemodynamics and oxygenation. Such perturbations included a modulation of the inspired oxygen concentration, transient brain asphyxia, carotid artery occlusion and terminal brain asphyxia. The baseline cerebral haemoglobin saturation and concentration, measured with frequency-domain spectroscopy, were about 60% and 42 microM respectively. The cerebral saturation values ranged from a minimum of 17% (during transient brain asphyxia) to a maximum of 80% (during recovery from transient brain asphyxia). To analyse the CW optical data, we have (a) derived a mathematical relationship between the cerebral optical properties and the differential pathlength factor and (b) introduced a method based on the spatial dependence of the detected intensity (dc slope method). The analysis of the cerebral optical signals associated with the arterial pulse and with respiration demonstrates that motion artefacts can significantly affect the intensity recorded from a single optode pair. Motion artefacts can be strongly reduced by combining data from multiple optodes to provide relative readings in the dc slope method. We also report significant biphasic changes (initial decrease and successive increase) in the reduced scattering coefficient measured in the brain after the piglet had been sacrificed.     

Measuring brain hemodynamic changes in a songbird: responses to hypercapnia measured with functional MRI and near-infrared spectroscopy

Songbirds have been evolved into models of choice for the study of the cerebral underpinnings of vocal communication. Nevertheless, there is still a need for in vivo methods allowing the real-time monitoring of brain activity. Functional Magnetic Resonance Imaging (fMRI) has been applied in anesthetized intact songbirds. It relies on blood oxygen level-dependent (BOLD) contrast revealing hemodynamic changes. Non-invasive near-infrared spectroscopy (NIRS) is based on the weak absorption of near-infrared light by biological tissues. Time-resolved femtosecond white laser NIRS is a new probing method using real-time spectral measurements which give access to the local variation of absorbing chromophores such as hemoglobins. In this study, we test the efficiency of our time-resolved NIRS device in monitoring physiological hemodynamic brain responses in a songbird, the zebra finch (Taeniopygia guttata), using a hypercapnia event (7% inhaled CO(2)). The results are compared to those obtained using BOLD fMRI. The NIRS measurements clearly demonstrate that during hypercapnia the blood oxygen saturation level increases (increase in local concentration of oxyhemoglobin, decrease in deoxyhemoglobin concentration and total hemoglobin concentration). Our results provide the first correlation in songbirds of the variations in total hemoglobin and oxygen saturation level obtained from NIRS with local BOLD signal variations.     

The effects of maternal hypercapnia on foetal oxygenation and uterine blood flow in the pig.

1. The purpose of this investigation was to determine the effects of maternal hyperoxaemia and hypercapnia on the uterine vascular bed and foetal oxygenation in the large white sow at 80-90 days gestation. 2. When maternal hyperoxaemia was induced with 100% oxygen, there was a highly significant rise in the maternal arterial oxygen tension, but no other significant blood gas or vascular changes were observed. 3. When mild maternal hypercapnia was superimposed on maternal hyperoxaemia (oxygen plus 6% carbon dioxide), the oxygen tension and saturation of both the maternal uterine venous and foetal umbilical venous bloods were found when severe hypercapnia was induced (oxygen plus 50% carbon dioxide) but in this case all blood samples showed dramatic changes in PCO2 and pH. These changes were accompanied by an increase in the systemic blood pressure and uterine blood flow, and a decrease in uterine vascular resistance. 4. When mild hypercapnia was induced without hyperoxaemia (air plus 5% carbon dioxide) significant increases were recorded in the oxygen tension and saturation of uterine venous and foetal umbilical venous bloods. Systemic and uterine vascular resistance fell. 5. It was concluded that the increased foetal oxygen tension during maternal hypercapnia was the result of the increased uterine blood flow and greater mass delivery of oxygen to the placenta, so that once the oxygen requirements of the placental tissues themselves were exceeded there would be an increased oxygen gradient at the site of gas exchange. 6. Carbon dioxide concentration in arterial blood plays an important role in determining blood flow through the pregnant uterus in the sow.     

Correlation between near-infrared spectroscopy and magnetic resonance imaging of rat brain oxygenation modulation

We measure the tissue oxygen and haemoglobin concentrations in the rat brain during modulation of inhaled oxygen concentration (FiO2), using non-invasive frequency domain near-infrared oximetry. The rise in oxygenated haemoglobin concentration and the decline in deoxygenated haemoglobin concentration are demonstrated in correspondence with the modulation of FiO2, which is changed from 20% to 100% in increments of 20%. Furthermore, the tissue oxygenation saturation also shows the corresponding trend and changes ranging from approximately 70% to 90%. The relative changes in deoxygenated haemoglobin concentration are compared to the blood-oxygenation-level-dependent (BOLD) MRI signal recorded during a similar FiO2 protocol. A linear relationship with high correlation coefficient between the relative changes in the BOLD MRI signal and the NIRS signal is observed.     

Noninvasive imaging of hemoglobin concentration and oxygenation in the rat brain using high-resolution photoacoustic tomography

Simultaneous transcranial imaging of two functional parameters, the total concentration of hemoglobin and the hemoglobin oxygen saturation, in the rat brain in vivo is realized noninvasively using laser-based photoacoustic tomography (PAT). As in optical diffusion spectroscopy, PAT can assess the optical absorption of endogenous chromophores, e.g., oxygenated and deoxygenated hemoglobins, at multiple optical wavelengths. However, PAT can provide high spatial resolution because its resolution is diffraction-limited by photoacoustic signals rather than by optical diffusion. Laser pulses at two wavelengths are used sequentially to acquire photoacoustic images of the vasculature in the cerebral cortex of a rat brain through the intact skin and skull. The distributions of blood volume and blood oxygenation in the cerebral cortical venous vessels, altered by systemic physiological modulations including hyperoxia, normoxia, and hypoxia, are visualized successfully with satisfactory spatial resolution. This technique, with its prominent sensitivity to endogenous contrast, can potentially contribute to the understanding of the interrelationship between neural, hemodynamic, and metabolic activities in the brain.     

Time-domain scanning optical mammography: II. Optical properties and tissue parameters of 87 carcinomas

Within a clinical trial on scanning time-domain optical mammography reported on in a companion publication (part I), craniocaudal and mediolateral projection optical mammograms were recorded from 154 patients, suspected of having breast cancer. Here we report on in vivo optical properties of the subset of 87 histologically validated carcinomas which were visible in optical mammograms recorded at two or three near-infrared wavelengths. Tumour absorption and reduced scattering coefficients were derived from distributions of times of flight of photons recorded at the tumour site employing the model of diffraction of photon density waves by a spherical inhomogeneity, located in an otherwise homogeneous tissue slab. Effective tumour radii, taken from pathology, and tumour location along the compression direction, deduced from off-axis optical scans of the tumour region, were included in the analysis as prior knowledge, if available. On average, tumour absorption coefficients exceeded those of surrounding healthy breast tissue by a factor of about 2.5 (670 nm), whereas tumour reduced scattering coefficients were larger by about 20% (670 nm). From absorption coefficients at 670 nm and 785 nm total haemoglobin concentration and blood oxygen saturation were deduced for tumours and surrounding healthy breast tissue. Apart from a few outliers total haemoglobin concentration was observed to be systematically larger in tumours compared to healthy breast tissue. In contrast, blood oxygen saturation was found to be a poor discriminator for tumours and healthy breast tissue; both median values of blood oxygen saturation are the same within their statistical uncertainties. However, the ratio of total haemoglobin concentration over blood oxygen saturation further improves discrimination between tumours and healthy breast tissue. For 29 tumours detected in optical mammograms recorded at three wavelengths (670 nm, 785 nm, 843 nm or 884 nm), scatter power was derived from transport scattering coefficients. Scatter power of tumours tends to be larger than that of surrounding healthy breast tissue, yet the 95% confidence intervals of both medians overlap.     

Mapping of the cerebral vascular response to hypoxia and hypercapnia using quantitative perfusion MRI at 3 T

Changes in breathing change the concentration of oxygen and carbon dioxide in arterial blood resulting in changes in cerebral blood flow (CBF). This mechanism can be described by the cerebral vascular response (CVR), which has been shown to be altered in different physiological and pathophysiological states. CBF maps of grey matter (GM) were determined with a pulsed arterial spin labelling technique at 3 T in a group of 19 subjects under baseline conditions, hypoxia, and hypercapnia. Experimental conditions allowed a change in either arterial oxygen (hypoxia) or carbon dioxide (hypercapnia) concentration compared with the baseline, leaving the other variable constant, in order to separate the effects of these two variables. From these results, maps were calculated showing the regional distribution of the CVR to hypoxia and hypercapnia in GM. Maps of CVR to hypoxia showed very high intra-subject variations, with some GM regions exhibiting a positive response and others a negative response. Per 10% decrease in arterial oxygen saturation, there was a statistically significant 7.0 +/- 2.9% (mean +/- SEM) increase in GM-CBF for the group. However, 70% of subjects showed an overall positive CVR (positive responders), and the remaining 30% an overall negative CVR (negative responders). Maps of CVR to hypercapnia showed less intra-subject variation. Per 1 mm Hg increase in partial pressure of end-tidal carbon dioxide, there was a statistically significant 5.8 +/- 0.9% increase in GM-CBF, all subjects showing an overall positive CVR. As the brain is particularly vulnerable to hypoxia, a condition associated with cardiorespiratory diseases, CVR maps may help in the clinic to identify the areas most prone to damage because of a reduced CVR.     

A method for determining blood flow and oxygen consumption in the rat brain.

Cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRo2) were measured in rats under nitrous oxide anaesthesia, using a 133Xenon modification of the Kety and Schmidt inert gas technique with sampling of cerebral venous blood from the retroglenoid vein. Extracerebral contamination of the venous blood sampled was studied by comparing the rates at which the activity of 133Xenon decreased in blood and tissues. Contamination was avoided by gentle compression of the contralateral retroglenoid vein during sampling. CBF and CMRo2 of the rat brain were 80+/-2 and 7.6+/-0.2 ml-(100g)-1-min-1, respectively. These values are about 25% lower than those previously obtained for cerebral cortical tissue under similar conditions. Induced hypercapnia (Paco2 about 70 mm Hg) or hypocapnia (Paco2 15-20 mm Hg) gave rise to expected changes in CBF but did not alter CMRo2. The CMRo2 of the rat brain is at least twice that of the human brain. This species difference, which is similar to that previously reported for the oxygen uptake of cerebral tissue in vitro, probably reflects on inverse relationship between brain weight and neuronal packing density.     

Non-invasive and quantitative near-infrared haemoglobin spectrometry in the piglet brain during hypoxic stress, using a frequency-domain multidistance instrument

The frequency-domain multiple-distance (FDMD) method is capable of measuring the absolute absorption and reduced scattering coefficients of optically turbid media. Absolute measurement of absorption at two near-infrared (NIR) wavelengths makes possible the quantitation of tissue haemoglobin concentration and tissue haemoglobin oxygen-saturation (StO2). However, errors are introduced by the uncertainties of background absorption and the dissimilarities between real tissues and the simplified mathematical model on which these measurements are based. An FDMD-based tissue instrument has been used for the monitoring of tissue haemoglobin concentration and oxygenation in the brain of newborn piglets during periods of hypoxia and hyperoxia. These tissue haemoglobin saturation values were compared with arterial saturation (SaO2) and venous saturation (SvO2) measured by blood gas analyses. A linear correlation was observed between StO2 and the average of SaO2 and SvO2. However, StO2 is not equal to any fixed weighted average of SaO2 and SvO2 unless we introduce an effective background tissue absorption. The magnitude of the background absorption was about 0.08 cm(-1) at 758 nm and 0.06 cm(-1) at 830 nm, and it was nearly consistent between piglets. The origin of this 'effective' background absorption may be real, an artefact caused by the application of a simplified model to a complex sample, or a combination of factors.     

Two-photon microscopy of cortical NADH fluorescence intensity changes: correcting contamination from the hemodynamic response

Quantification of nicotinamide adenine dinucleotide (NADH) changes during functional brain activation and pathological conditions provides critical insight into brain metabolism. Of the different imaging modalities, two-photon laser scanning microscopy (TPLSM) is becoming an important tool for cellular-resolution measurements of NADH changes associated with cellular metabolic changes. However, NADH fluorescence emission is strongly absorbed by hemoglobin. As a result, in vivo measurements are significantly affected by the hemodynamics associated with physiological and pathophysiological manipulations. We model NADH fluorescence excitation and emission in TPLSM imaging based on precise maps of cerebral microvasculature. The effects of hemoglobin optical absorption and optical scattering from red blood cells, changes in blood volume and hemoglobin oxygen saturation, vessel size, and location with respect to imaging location are explored. A simple technique for correcting the measured NADH fluorescence intensity changes is provided, with the utilization of a parallel measurement of a physiologically inert fluorophore. The model is applied to TPLSM measurements of NADH fluorescence intensity changes in rat somatosensory cortex during mild hypoxia and hyperoxia. The general approach of the correction algorithm can be extended to other TPLSM measurements, where changes in the optical properties of the tissue confound physiological measurements, such as the detection of calcium dynamics.     

Continuous-wave near-infrared spectroscopy using pathlength-independent hypoxia normalization

A general physiological model for the hemodynamic response during altered blood flow, oxygenation, and metabolism is presented. Calculations of oxy-, deoxy-, and total hemoglobin changes during stimulation are given. It is shown that by using a global hyperoxic or mild hypoxic challenge it is possible to normalize the activation response in terms of the fractional changes in the cerebral blood volume, tissue oxygenation index, and oxygen extraction ratio, which are independent of the optical pathlength. Using a dual wavelength spectrometer, the method is validated by measuring pathlength-independent hemodynamic responses during mild hypercarbia in a rat model. Phantom experiments showed that the changes in optical pathlength were small as the hemoglobin concentration was varied over a wide range. The determination of quantitative parameters facilitates the use of continuous-wave transcranial methods by providing a means by which to characterize activation response across subjects.     

Cerebral oxygen consumption and blood flow in Fischer-344 rats of different ages

Regional cerebral oxygen consumption and blood flow were determined and compared in conscious male Fischer-344 rats at 3, 12, 24 and 33 months of age to correlate the reported regional neurochemical and morphological changes which occur in these parameters during development, maturation, aging and senescence. Cerebral blood flow was determined with 14[C]-labelled iodoantipyrine and regional cerebral arterial and venous oxygen saturation was measured microspectrophotometrically. Oxygen consumption was obtained by multiplying cerebral blood flow and oxygen extraction. Systolic and diastolic blood pressure and heart rate decreased significantly with age. Average cerebral blood flow, oxygen extraction and oxygen consumption/100 g did not differ significantly between the four age groups examined. Oxygen consumption averaged 2.9 +/- 0.1 ml O2/min/100 g (mean +/- S.E.M.) in the 3-month-old group and 3.6 +/- 0.1 ml O2/min/100 g in the 33-month-old group. Differences in flow among the examined brain regions, which were present in the mature, 12-month-old brain, were not present in the developing, aging or senescent rat brain. Compensatory alterations in the efficiency or organization of neurochemical activity which occur during development, aging and senescence may modify the inter-regional differences in cerebral blood flow and oxygen consumption noted during maturation. There was no correlation found between the neurochemical and morphological changes which have been reported during maturation, aging or senescence and regional cerebral oxygen consumption.     

Validation of diffuse correlation spectroscopic measurement of cerebral blood flow using phase-encoded velocity mapping magnetic resonance imaging

Diffuse correlation spectroscopy (DCS) is a novel optical technique that appears to be an excellent tool for assessing cerebral blood flow in a continuous and non-invasive manner at the bedside. We present new clinical validation of the DCS methodology by demonstrating strong agreement between DCS indices of relative cerebral blood flow and indices based on phase-encoded velocity mapping magnetic resonance imaging (VENC MRI) of relative blood flow in the jugular veins and superior vena cava. Data were acquired from 46 children with single ventricle cardiac lesions during a hypercapnia intervention. Significant increases in cerebral blood flow, measured both by DCS and by VENC MRI, as well as significant increases in oxyhemoglobin concentration, and total hemoglobin concentration, were observed during hypercapnia. Comparison of blood flow changes measured by VENC MRI in the jugular veins and by DCS revealed a strong linear relationship, R=0.88, p<0.001, slope=0.91±0.07. Similar correlations were observed between DCS and VENC MRI in the superior vena cava, R=0.77, slope=0.99±0.12, p<0.001. The relationship between VENC MRI in the aorta and DCS, a negative control, was weakly correlated, R=0.46, slope=1.77±0.45, p<0.001.     

Measurement of intravascular Na(+) during increased CBF using (23)Na NMR with a shift reagent.

Sodium ions are intimately involved with neural activity. Thus, it is highly desirable to devise a way of mapping brain activity via sodium imaging. Sodium ions exist in the extravascular and intravascular spaces. To separate the two components, the shift reagent Tm(DOTP)(5-) was intravenously introduced into rats. Intravascular sodium changes in the rat brain were measured during increased blood flow induced by hypercapnia using volume-localized (23)Na-NMR. The intravascular sodium changes, equivalent to cerebral blood volume changes, are significant during hypercapnia conditions and correlate well with the increase in arterial pCO(2). This suggests that the intravascular sodium change is dominant in total (23)Na spectroscopy or imaging of the brain during blood flow increase induced by external perturbation.     

In vivo determination of the optical properties of infant brain using frequency-domain near-infrared spectroscopy

We investigate the optical properties of the brain in 23 neonates in vivo using a frequency domain near-infrared spectroscopy (NIRS). In this study, a calibration procedure is employed to determine the absorption and reduced scattering coefficients with single source-detector separation. The absorption coefficients of the infant foreheads are lower than the values reported in adults. A large intersubject variation in the reduced scattering coefficients is also demonstrated. Furthermore, physiological parameters are derived from the absorption coefficients at two wavelengths (788 and 832 nm). The mean total hemoglobin concentration (THC) is 39.7+/-9.8 microM and the mean cerebral blood oxygen saturation (StO2) is 58.7+/-11.2%. Our preliminary results show that this bedside frequent domain NIRS could provide quantitative optical measurement of the infant brain.     

A Method for Determining Blood Flow and Oxygen Consumption in the Rat Brain

Cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRo₂) were measured in rats under nitrous oxide anaesthesia, using a ¹³³Xenon modification of the Kety and Schmidt inert gas technique with sampling of cerebral venous blood from the retroglenoid vein. Extracerebral contamination of the venous blood sampled was studied by comparing the rates at which the activity of ¹³³Xenon decreased in blood and tissues. Contamination was avoided by gentle compression of the contralateral retroglenoid vein during sampling. CBF and CMRo₂ of the rat brain were 80±2 and 7.6 ± 0.2 ml·(100 g)^-1, min^-1, respectively. These values are about 25% lower than those previously obtained for cerebral cortical tissue under similar conditions. Induced hypercapnia (Paco₂ about 70 mm Hg) or hypocapnia (Paco² 15–20 mm Hg) gave rise to expected changes in CBF but did not alter CMRo² The CMRo² of the rat brain is at least twice that of the human brain. This species difference, which is similar to that previously reported for the oxygen uptake of cerebral tissue in vitro, probably reflects on inverse relationship between brain weight and neuronal packing density.     

Can the cerebral metabolic rate of oxygen be estimated with near-infrared spectroscopy?

We have measured the changes in oxy-haemoglobin and deoxy-haemoglobin in the adult human brain during a brief finger tapping exercise using near-infrared spectroscopy (NIRS). The cerebral metabolic rate of oxygen (CMRO2) can be estimated from these NIRS data provided certain model assumptions. The change in CMRO2 is related to changes in the total haemoglobin concentration, deoxy-haemoglobin concentration and blood flow. As NIRS does not provide a measure of dynamic changes in blood flow during brain activation, we relied on a Windkessel model that relates dynamic blood volume and flow changes, which has been used previously for estimating CMRO2 from functional magnetic resonance imaging (fMRI) data. Because of the partial volume effect we are unable to quantify the absolute changes in the local brain haemoglobin concentrations with NIRS and thus are unable to obtain an estimate of the absolute CMRO2 change. An absolute estimate is also confounded by uncertainty in the flow-volume relationship. However, the ratio of the flow change to the CMRO2 change is relatively insensitive to these uncertainties. For the linger tapping task, we estimate a most probable flow-consumption ratio ranging from 1.5 to 3 in agreement with previous findings presented in the literature, although we cannot exclude the possibility that there is no CMRO2 change. The large range in the ratio arises from the large number of model parameters that must be estimated from the data. A more precise estimate of the flow-consumption ratio will require better estimates of the model parameters or flow information, as can be provided by combining NIRS with fMRI.     

Relationship between baseline cerebral blood flow and vascular responses to changes in PaCO2 measured by positron emission tomography in humans: implication of inter-individual variations of cerebral vascular tone

Aim: Inter‐individual variations in normal human cerebral blood flow (CBF) at rest condition have been reported. Inter‐individual variation of cerebral vascular tone is considered to contribute to this, and several determinants of cerebral vascular tone have been proposed. In the present study, the relationship between CBF and cerebral vascular tone to inter‐individual variation at rest condition was investigated using positron emission tomography (PET). Methods: CBF was measured using PET with H₂¹⁵O in each of 20 healthy subjects (20–28 years) under three conditions: at rest (baseline), during hypercapnia and during hypocapnia. The vascular response to change in P_aCO₂ was calculated as the percentage changes in CBF per absolute change in P_aCO₂ in response to hypercapnia and hypocapnia. Results: A significant negative correlation between baseline CBF and the vascular response to hypocapnia was observed in the thalamus, temporal cortex, parietal cortex, occipital cortex and cerebral cortex (P < 0.05). A trend towards negative correlation between baseline CBF and the vascular response to hypocapnia was observed in the cerebellum and putamen (P < 0.1). A significant negative correlation between baseline CBF and the vascular response to hypercapnia was observed in the occipital cortex (P < 0.05). No significant correlation was observed between baseline CBF and haemoglobin concentration, and P_aCO₂. Conclusion: These findings support the assumption that cerebral vascular tone might incline towards vasoconstriction and vasodilatation when baseline CBF is low and high between individuals respectively. Although several determinants of cerebral vascular tone have been proposed, the mechanism of such inter‐individual differences in cerebral vascular tone is unknown.