Measurement of cranial optical path length as a function of age using phase resolved near infrared spectroscopy

Noninvasive measurement of cerebral blood flow in adults using near-infrared spectroscopy and indocyanine green: a pilot study

This pilot study was designed to determine the feasibility of measuring cerebral blood flow noninvasively after an intravenous bolus of indocyanine green using near-infrared spectroscopy and pulse dye-densitometry. Feasibility aside, this study did not attempt to validate the measured values of cerebral blood flow against an established method of measurement. Twelve healthy volunteers were investigated after peripheral intravenous injection of indocyanine green. Arterial and cerebral changes in indocyanine green concentration were measured using pulse dye-densitometry and near-infrared spectroscopy, respectively. Two methods of calculating cerebral blood flow were used, and a blood flow index was also estimated. Absolute cerebral blood flow was calculated using a modification of the Fick principle and a deconvolution algorithm to derive the impulse residue function. Mean (range) estimated cerebral blood flow for the Fick method was 8.2 mL/100 g/min (4.2-16.2 mL/100 g/min) and 8.3 mL/100 g/min (4.7-15.3 mL/100 g/min) for the impulse residue function method. The impulse residue function method provided a more precise intrasubject estimation of cerebral blood flow compared with the modified Fick principle, with a coefficient of variation of 10.1% versus 25.5%. The blood flow index was 8.6 mg/sec (range: 5.6-17.3 mg/sec) with an intrasubject coefficient of variation of 12.0%. Estimation of cerebral blood flow using near-infrared spectroscopy and pulse dye-densitometry can be made at the bedside after intravenous injection of indocyanine green, and the precision can be improved using a deconvolution algorithm. Notwithstanding the low values obtained for absolute cerebral blood flow, further investigation and validation of this bedside technique is warranted.

Simultaneous measurements of cerebral oxygenation changes during brain activation by near-infrared spectroscopy and functional magnetic resonance imaging in healthy young and elderly subjects

Near infrared spectroscopy (NIRS) and functional magnetic resonance imaging (fMRI) both allow non-invasive monitoring of cerebral cortical oxygenation responses to various stimuli. To compare these methods in elderly subjects and to determine the effect of age on cortical oxygenation responses, we determined motor-task-related changes in deoxyhemoglobin concentration ([HHb]) over the left motor cortex in six healthy young subjects (age 35 +/- 9 years, mean +/- SD) and five healthy elderly subjects (age 73 +/- 3 years) by NIRS and blood-oxygen-level-dependent (BOLD) fMRI simultaneously. The motor-task consisted of seven cycles of 20-sec periods of contralateral finger-tapping at a rate as fast as possible alternated with 40-sec periods of rest. Time-locked averages over the seven cycles were used for further analysis. Task-related decreases in [HHb] over the motor cortex were measured by NIRS, with maximum changes of -0.83 +/- 0.38 mumol/L (P < 0.01) for the young and -0.32 +/- 0.17 mumol/L (P < 0.05) for the elderly subjects. The BOLD-fMRI signal increased over the cortex volume under investigation with NIRS, with maximum changes of 2.11 +/- 0.72% (P < 0.01) for the young and 1.75 +/- 0.71% (P < 0.01) for the elderly subjects. NIRS and BOLD-fMRI measurements showed good correlation in the young (r = -0.70, r(2) = 0.48, P < 0.001) and elderly subjects (r = -0.82, r(2) = 0.67, P < 0.001). Additionally, NIRS measurements demonstrated age-dependent decreases in task-related cerebral oxygenation responses (P < 0.05), whereas fMRI measurements demonstrated smaller areas of cortical activation in the elderly subjects (P < 0.05). These findings demonstrate that NIRS and fMRI similarly assess cortical oxygenation changes in young subjects and also in elderly subjects. In addition, cortical oxygenation responses to brain activation alter with aging.

Estimation of regional cerebral blood flow distribution in infants by near-infrared topography using indocyanine green

Near-infrared topography with indocyanine green was used to measure regional cerebral blood flow (rCBF) in the temporal lobes of infants. The mean rCBF in infants without neural abnormality was 14.5 +/- 3.1 ml/100 g/min, and the rCBFs in the fronto-temporal, temporal, and occipito-temporal regions were 15.1 +/- 3.9, 15.4 +/- 3.3, and 14.6 +/- 3.3 ml/100 g/min, respectively. Moreover, in one asphyxiated infant with infarction and one infant with subdural and intracerebellar hemorrhage, it was demonstrated that the area of defective blood flow could be detected as well as it can by SPECT. This technique makes it possible to estimate rCBF distribution in infants at the bedside. Thus, in the future, evaluation of various neonatal illnesses should be feasible.

Performance of near-infrared spectroscopy in measuring local O(2) consumption and blood flow in skeletal muscle

The aim of this study was to investigate local muscle O(2) consumption (muscV(O(2))) and forearm blood flow (FBF) in resting and exercising muscle by use of near-infrared spectroscopy (NIRS) and to compare the results with the global muscV(O(2)) and FBF derived from the well-established Fick method and plethysmography. muscV(O(2)) was derived from 1) NIRS using venous occlusion, 2) NIRS using arterial occlusion, and 3) the Fick method [muscV(O(2(Fick)))]. FBF was derived from 1) NIRS and 2) strain-gauge plethysmography. Twenty-six healthy subjects were tested at rest and during sustained isometric handgrip exercise. Local variations were investigated with two independent and simultaneously operating NIRS systems at two different muscles and two measurement depths. muscV(O(2)) increased more than fivefold in the active flexor digitorum superficialis muscle, and it increased 1.6 times in the brachioradialis muscle. The average increase in muscV(O(2(Fick))) was twofold. FBF increased 1.4 times independent of the muscle or the method. It is concluded that NIRS is an appropriate tool to provide information about local muscV(O(2)) and local FBF because both place and depth of the NIRS measurements reveal local differences that are not detectable by the more established, but also more global, Fick method.

Simultaneous near-infrared spectroscopy monitoring of left and right occipital areas reveals contra-lateral hemodynamic changes upon hemi-field paradigm

In this study we have shown that in humans it is possible to monitor non-invasively and simultaneously both hemispheres revealing cortical oxygenation changes in the occipital area in response to a contra-lateral hemi-field paradigm. A novel multi-channel near infrared spectroscopy approach with a high temporal resolution was used. The results confirm previous findings obtained by functional magnetic resonance imaging and positron emission tomography with the advantage to measure directly not only concentration changes in deoxyhemoglobin as measured by functional magnetic resonance imaging (MRI), but also in oxyhemoglobin with low cost instrumentation potentially useful to investigate the pathophysiology of vision.

Spontaneous low frequency oscillations of cerebral hemodynamics and metabolism in human adults

We investigated slow spontaneous oscillations in cerebral oxygenation in the human adult's visual cortex. The rationale was (1) to demonstrate their detectability by near infrared spectroscopy (NIRS); (2) to analyze the spectral power of as well as the phase relationship between the different NIRS parameters (oxygenated and deoxygenated hemoglobin and cytochrome-oxidase; oxy-Hb/deoxy-Hb/Cyt-ox). Also (3) influences of functional stimulation and hypercapnia on power and phase shifts were investigated. The results show that-in line with the literature-low frequency oscillations (LFO) centred around 0.1 s(-1) and even slower oscillations at about 0.04 s(-1) (very low frequency, VLFO) can be distinguished. Their respective power differs between oxy-Hb, deoxy-Hb, and Cyt-ox. Either frequency (LFO and VLFO) is altered in magnitude by functional stimulation of the cortical area examined. Also we find a change of the phase shift between the vascular parameters (oxy-Hb, tot-Hb) and the metabolic parameter (Cyt-ox) evoked by the stimulation. It is shown that hypercapnia attenuates the LFO in oxy-Hb and deoxy-Hb.

CONCLUSIONS

(1) spontaneous vascular and metabolic LFO and VLFO can be reproducibly detected by NIRS in the human adult. (2) Their spectral characteristics and their response to hypercapnia are in line with those described in exposed cortex (for review see (Hudetz et al., 1998)) and correspond to findings with transcranial doppler sonography (TCD) (Diehl et al., 1995) and fMRI (Biswal et al., 1997). (3) The magnitude of and phase relation between NIRS-parameters at the LFO may allow for a local noninvasive assessment of autoregulatory mechanisms in the adult brain.

Quantitative measurement of oxygen consumption and forearm blood flow in patients with mitochondrial myopathies

Five patients with chronic progressive external ophthalmoplegia (CPEO) and 27 healthy controls were examined by near-infrared spectroscopy (NIRS) for the non-invasive and direct quantitative measurement of muscle oxygen consumption during rest as well as during static isometric handgrip exercise at 10% of their maximum voluntary contraction. In patients with CPEO, we found a significantly decreased oxygen consumption during exercise, but more remarkably already during rest. Our results suggest that NIRS is able to discriminate between CPEO patients and healthy controls, which makes NIRS a promising tool in the diagnostic work-up of patients suspected of a mitochondrial myopathy.

Calibration of near-infrared frequency-domain tissue spectroscopy for absolute absorption coefficient quantitation in neonatal head-simulating phantoms

Frequency-domain tissue spectroscopy is a method to measure the absolute absorption coefficient of bulk tissues, assuming that a representative model can be found to recover the optical properties from measurements. While reliable methods exist to calculate absorption coefficients from source-detector measurements less than a few centimeters apart along a flat tissue volume, it is less obvious what methods can be used for transmittance through the larger tissue volumes typically associated with neonatal cerebral monitoring. In this study we compare the use of multiple distance frequency-domain measurements processed with (i) a modified Beer-Lambert law method, (ii) an analytic infinite-medium diffusion theory expression, and (iii) a numerical finite element solution of the diffusion equation, with the goal of recovering the absolute absorption coefficient of the medium. Based upon our observations, the modified Beer-Lambert method provides accurate absolute changes in the absorption coefficient, while analytic infinite-medium diffusion theory solutions or finite element-based numerical solutions can be used to calculate the absolute absorption coefficient, assuming that the data can be measured at multiple source-detector distances. We recommend that the infinite-medium multi-distance method or the finite element method be used across large tissue regions for calculation of the absolute absorption coefficient using frequency-domain near-infrared measurements at multiple positions along the head.

Noninvasive measurement of cerebral hemoglobin oxygen saturation using two near infrared spectroscopy approaches

Spatially resolved spectroscopy (SRS) is a new near infrared spectroscopy (NIRS) method that, using the multi-distance approach, measures local cerebral cortex hemoglobin oxygen saturation [J. Matcher, P. Kirkpatrick, K. Nahid, M. Cope, and D. T. Delpy, Proc. SPIE 2389, 486-495 (1995)]. Using a conventional continuous wave NIRS photometer, cerebral venous oxygen saturation (SvO2) can be calculated from oxyhemoglobin and total hemoglobin rise induced by partial occlusion of jugular vein [C. E. Elwell, S. J. Matcher, L. Tyszczuk, J. H. Meek, and D. T. Delpy, Adv. Exp. Med. Biol. 411, 453-460 (1997)]. The aim of this study was to compare direct measurements of forehead tissue oxygenation index (TOI) with the calculated SvO2 during venous occlusion in 16 adult volunteers using a clinical two-channel SRS oximeter (NIRO-300). Measured TOI and calculated SvO2 values of either right or left forehead did not significantly differ. A good agreement between the two NIRS methods was also demonstrated. On 16 other subjects, no significant differences were found between the right and left forehead TOI values measured simultaneously, and between the TOI values measured by channel 1 or 2 on the same side. The results confirm that cerebral cortex hemoglobin oxygen saturation, measured directly by the SRS method, reflects predominantly the saturation of the intracranial venous compartment of circulation.

Spontaneous oscillation of oxy- and deoxy- hemoglobin changes with a phase difference throughout the occipital cortex of newborn infants observed using non-invasive optical topography

We investigated spontaneous changes in the cerebral oxygenation state of infants during quiet sleeping by using a form of multi-channel near-infrared spectroscopy: non-invasive optical topography. Eight infants born at 32-39 weeks were studied at postconceptional term age (38-43 weeks). Spatially synchronized oscillations of changes in the concentration of oxy- and deoxy- hemoglobin ([oxy-Hb] and [deoxy-Hb]) were observed throughout the occipital cortex. Time series analysis based on the theory of non-linear oscillators showed that the mean periods of the oscillation for each infant ranged from 11 to 18 s. The phase lag of [oxy-Hb] relative to [deoxy-Hb] was stable at about 3pi/4. This phase difference may result from interplay between the vasomotion and the oxygen consumption in relation to brain activity.

Near-infrared spectroscopy: does it function in functional activation studies of the adult brain?

Changes in optical properties of biological tissue can be examined by near-infrared spectroscopy (NIRS). The relative transparency of tissues including the skull to near-infrared light is the prerequisite to apply the method to brain research. We describe the methodology with respect to its applicability in non-invasive functional research of the adult cortex. A summary of studies establishing the 'typical' response in NIRS vascular parameters, i.e. changes in the concentration of oxygenated and deoxygenated haemoglobin, over an activated area is followed by the validation of changes in the cytochrome-oxidase redox state in response to a visual stimulus. Proceeding from these findings a rough mapping of this metabolic response over the motion-sensitive extrastriate visual area is demonstrated. NIRS measures concentration changes in deoxygenated haemoglobin [deoxy-Hb] which are assumed to be the basis of fMRI BOLD contrast (blood oxygenation level-dependent). The method is therefore an excellent tool to validate assumptions on the physiological basis underlying the fMRI signal, due to its high specificity as to the parameters measured. Questions concerning the concept of 'activation'/'deactivation' and that of the linearity of the vascular response are discussed. To challenge the method we finally present results from a complex single-trial motor paradigm study testing the hypothesis, that premotor potentials (contingent negative variation) can be examined by functional techniques relying on the vascular response. Some of the work described here has been published elsewhere.

Measurement of cerebral blood volume using near-infrared spectroscopy and indocyanine green elimination

Methods for measuring cerebral blood volume (CBV) have traditionally used radioisotopes. More recently, near-infrared spectroscopy (NIRS) has been used to measure CBV by using a technique involving O(2) desaturation of cerebral tissue, where the observed change in the concentration of oxygenated hemoglobin is a marker of the volume of blood contained within the brain. A new integration method employing NIRS is described by using indocyanine green (ICG) as the intravascular marker. After bolus injection, concentration-time integrals of cerebral tissue ICG concentration ([ICG](tissue)) measured by NIRS are compared with corresponding integrals of the cerebral blood ICG concentrations ([ICG](blood)) estimated by high-performance liquid chromatography of peripheral blood samples with allowance for cerebral-to-large-vessel hematocrit ratio. It is shown that CBV = integral [ICG]tissue/[ICG]blood. Measurements in 10 adult volunteers gave a mean value of 1.1 +/- 0.39 (SD) ml/100 g illuminated tissue. This result, although lower than previous NIRS estimations, is consistent with the long extracerebral path of light in the adult head. Scaling of results is required to take into account this component of the optical pathlength.

Near-infrared oximetry of the brain

Near-infrared (IR) light easily penetrates biological tissue, and the information offered by in vivo spectroscopy of cerebral oxygenation is detailed and comes with a high temporal resolution. Near-IR light spectroscopy (NIRS) reflects cerebral oxygenation during arterial hypotension, hypoxic hypoxaemia and hypo- and hypercapnia. As determined by dual-wavelength NIRS, the cerebral O2 saturation integrates the arterial O2 content and the cerebral perfusion, and as established for skeletal muscle, NIRS obtains information on tissue oxygenation and metabolism beyond that obtained by venous blood sampling. Caveats of cerebral NIRS include insufficient light shielding, optode displacement and a sample volume including muscle or the frontal sinus mucous membrane. The relative influence from the extracranial tissue is minimized by optode separation and correction for an extracranial sample volume, or both. The natural pigment melatonin and also water are of little influence to spectroscopic analysis of cerebral oxygenation, whereas bilirubin systematically lowers ScO2 and attenuates the detection of changes in cerebral oxygenation. By NIRS, reduction of cytochrome oxidase is demonstrated during hypoxic hypoxaemia and head-up tilt-induced arterial hypotension, but the changes are small. In the clinical setting, NIRS offers useful information in patients with both systemic and local cerebral circulatory impairment, for example, during cranial trauma, surgery on the cerebral arteries, orthostasis and acute heart failure. Whereas mapping of the brain circulation is needed for jugular venous sampling to reflect either global or local oxygenation, the determination of cerebral oxygenation by NIRS has the advantage of localized monitoring of the cerebral cortex.

Noninvasive assessment of changes in cytochrome-c oxidase oxidation in human subjects during visual stimulation

In this study the authors used a whole-spectrum near-infrared spectroscopy approach to noninvasively assess changes in hemoglobin oxygenation and cytochrome-c oxidase redox state (Cyt-Ox) in the occipital cortex during visual stimulation. The system uses a white light source (halogen lamp). The light reflected from the subject's head is spectrally resolved by a spectrograph and dispersed on a cooled charge-coupled device camera. The authors showed the following using this approach: (1) Changes in cerebral hemoglobin oxygenation (increase in concentration of oxygenated hemoglobin, decrease in concentration of deoxygenated hemoglobin) in the human occipital cortex during visual stimulation can be assessed quantitatively. (2) The spectral changes during functional activation cannot be completely explained by changes in hemoglobin oxygenation solely; Cyt-Ox has to be included in the analysis. Only if Cyt-Ox is considered can the spectral changes in response to increased brain activity be explained. (3) Cytochrome-c oxidase in the occipital cortex of human subjects is transiently oxidized during visual stimulation. This allows us to measure vascular and intracellular energy status simultaneously.

New technologies in pediatric neurology. Near-infrared spectroscopy

Near-infrared spectroscopy (NIRS) is a relatively new technology that offers the enormous advantage of making measurements in vivo of changes in cerebral hemodynamics and oxygenation. Because NIRS is noninvasive and portable, it can provide real-time measurements of these changes at the bedside. Thus NIRS is ideally suited to the study of many physiological and pathological processes affecting the brain, particularly in the infant or young child in the intensive care unit or operating room. This review outlines the basic principles, advantages, and limitations of the current state of NIRS technology. An emphasis is placed on the animal and clinical studies that are relevant to the field of child neurology, with an eye to the future evolution and potential applications of this promising technique.

A multiwavelength frequency-domain near-infrared cerebral oximeter

This study tests a multiwavelength frequency-domain near-infrared oximeter (fdNIRS) in an in vitro model of the human brain. The model is a solid plastic structure containing a vascular network perfused with blood in which haemoglobin oxygen saturation (SO2) was measured by co-oximetry, providing a standard for comparison. Plastic shells of varying thickness (0.5-2 cm), with a vascular network of their own and encircling the brain model, were also added to simulate extracranial tissues of the infant, child and adult. The fdNIRS oximeter utilizes frequency-domain technology to monitor phaseshifts at 754 nm, 785 nm and 816 nm relative to a 780 nm reference to derive SO2 through photon transport and Beer-Lambert equations. We found a linear relationship between fdNIRS SO2 and co-oximetry SO2 with excellent correlation (r2 > or = 0.95) that fitted the line of identity in all experiments (n = 7). The bias of fdNIRS oximetry was -2% and the precision was 6%. Blood temperature and fdNIRS source-detector distance did not affect fdNIRS oximetry. Low haemoglobin concentration (6 g dl(-1)) altered the fdNIRS versus co-oximetry line slope and intercept, producing a 15% error at the extremes of SO2. The infant- and child-like shells overlying the brain model did not alter fdNIRS oximetry, whereas the adult-like shell yielded an error as high as 32%. In conclusion, fdNIRS accurately measures SO2 in an in vitro brain model, although low haemoglobin concentration and extracranial tissue of adult thickness influence accuracy.

Defining thresholds for critical ischemia by using near-infrared spectroscopy in the adult brain

OBJECT

Signal changes in adult extracranial tissues may have a profound effect on cerebral near-infrared spectroscopy (NIRS) measurements. During carotid surgery NIRS signals provide the opportunity to determine the relative contributions from the intra- and extracranial vascular territories, allowing for a more accurate quantification. In this study the authors applied multimodal monitoring methods to patients undergoing carotid endarterectomy and explored the hypothesis that NIRS can define thresholds for cerebral ischemia, provided extracranial NIRS signal changes are identified and removed. Relative criteria for intraoperative severe cerebral ischemia (SCI) were applied to 103 patients undergoing carotid endarterectomy.

METHODS

One hundred three patients underwent carotid endarterectomy. An intraoperative fall in transcranial Doppler-detected middle cerebral artery flow velocity (%deltaFV) of greater than 60% accompanied by a sustained fall in cortical electrical activity were adopted as criteria for SCI. Ipsilateral frontal NIRS recorded the total difference in concentrations of oxyhemoglobin and deoxyhemoglobin (Total deltaHb(diff)). Interrupted time series analysis following clamping of the external carotid artery (ECA) and the internal carotid artery (ICA) allowed the different vascular components of Total deltaHb(diff) (ECA deltaHb(diff) and ICA deltaHb(diff)) to be identified. Data obtained in 76 patients were deemed suitable. A good correlation between %deltaFV and ICA deltaHb(diff) (r = 0.73, p < 0.0001) was evident. Sixteen patients (21%) fulfilled the criteria for SCI. All patients who demonstrated an ICA deltaHb(diff) of greater than 6.8 micromol/L showed SCI, and in two patients within this group nondisabling watershed infarction developed, as seen on postoperative computerized tomography scans. No patient with an ICA deltaHb(diff) less than 5 micromol/L exhibited SCI or suffered a stroke. Within the resolution of the criteria used an ICA deltaHb(diff) threshold of 6.8 micromol/L provided 100% specificity for SCI, whereas an ICA deltaHb(diff) less than 5 micromol/L was 100% sensitive for excluding SCI. When Total deltaHb(diff) was used without removing the ECA component, no thresholds for SCI were apparent.

CONCLUSIONS

Carotid endarterectomy provides a stable environment for exploring NIRS-quantified thresholds for SCI in the adult head.

Cerebral blood flow is independent of mean arterial blood pressure in preterm infants undergoing intensive care

OBJECTIVE

Preterm infants are often presumed to have a pressure passive cerebral circulation implying that a low mean arterial blood pressure (MABP) results in reduced cerebral perfusion. The aim of this study was to determine whether cerebral blood flow (CBF) was compromised in preterm infants whose MABP fell below 30 mm Hg (4 kPa).

METHODS

Thirty preterm infants undergoing intensive care were studied within the first 24 hours of life. CBF was measured using near infrared spectroscopy. The infants were analyzed in two groups on the basis of their MABP at the time of study: group 1 had a MABP below 30 mm Hg and group 2 more than 30 mm Hg. CBF in the two groups was compared.

RESULTS

There was no significant difference in the mean CBF between the two groups. In group 1 the median MABP was 27.2 mm Hg (range, 23.7-29.9 mm Hg) and CBF was 13.9 (standard deviation, +/-6.9) mL . 100 g-1 . min-1. In group 2 the median MABP was 35.3 mm Hg (range, 30.1-39.3 mm Hg) and CBF was 12.3 (standard deviation, +/-6.4) mL . 100 g-1 . min-1. Mortality and incidence of cranial ultrasound scan abnormalities were also not significantly different.

CONCLUSION

These results indicate that preterm infants undergoing intensive care are able to maintain adequate cerebral perfusion at a MABP in the range of 23.7 to 39.3 mm Hg.

Measurement of cerebral blood flow during cardiopulmonary bypass with near-infrared spectroscopy

OBJECTIVE

A novel noninvasive method for repeatedly measuring cerebral blood flow during cardiopulmonary bypass by near-infrared spectroscopy is described. The reproducibility of the method is investigated and a comparison is made with an established technique.

METHODS AND RESULTS

The method is derived from the Fick principle and uses indocyanine green dye, injected into the bypass circuit, as an intravascular tracer. Cerebral blood flow was measured in nine children undergoing cardiopulmonary bypass on a total of 49 occasions. Results from this study suggest that an integrating period of 4 seconds provided a consistent measurement of global cerebral blood flow. The values obtained ranged from 3.2 to 32.4 (median 15.9) ml.100 gm-1.min-1. In an additional 10 children in whom repeated measurements were made, the coefficient of variation was 11% +/- 7% (mean +/- standard deviation). In a further study, the method was compared with microsphere injection in five piglets undergoing cardiopulmonary bypass. The comparison within each animal with the linear least squares method gave values for R2 in the range 0.91 to 0.99. The gradients of the fits ranged from 0.5 to 1.8 (median 1.0). The mean difference between the two techniques was 5.7 ml.100 gm-1.min-1 or 7%. The coefficient of variation for the piglets was 14% +/- 9% (mean +/- standard deviation).

CONCLUSIONS

Indocyanine green and near-infrared spectroscopy allow frequent and repeated measurements of cerebral blood flow during cardiopulmonary bypass. The measurements are reproducible and accurately reflect changes in cerebral blood flow. It may be widely applicable both in research and clinical practice.

Decrease in parietal cerebral hemoglobin oxygenation during performance of a verbal fluency task in patients with Alzheimer's disease monitored by means of near-infrared spectroscopy (NIRS)--correlation with simultaneous rCBF-PET measurements

We used near-infrared spectroscopy (NIRS) to study non-invasively changes in cerebral hemoglobin oxygenation in the frontal and parietal cortex during performance of a verbal fluency task in patients with Alzheimer's disease (AD). Whereas healthy elderly subjects (n = 19, age 67 +/- 10 years) showed increases in concentrations of oxygenated hemoglobin [HbO2] (mean (arbitrary units) +/- S.E.M., 1.44 +/- 0.59) and total hemoglobin [HbT] (0.92 +/- 0.81) over the left superior parietal cortex, patients with AD (n = 19, age 71 +/- 10 years) showed significant decreases in [HbO2] (-3.26 +/- 1.30, P < 0.01) as well as [HbT] (-4.45 +/- 1.57, P < 0.01). [HbR] decreased slightly in both groups (-0.62 +/- 0.29 and - 1.18 +/- 0.40, respectively). Using two pairs of NIRS optodes placed on the left superior partietal cortex and on the left prefrontal cortex simultaneous increases in [HbO2] as well as [HbT] in both cortical regions in the healthy elderly subjects (n = 8, age 60 +/- 15) were demonstrated during performance of the task. AD patients (n = 10, age 65 +/- 13 years) showed decreases in [HbO2] and [HbT] in the parietal cortex and, at the same time, increases in [HbO2] and [HbT] in the frontal cortex. Simultaneous NIRS-[HbT] and PET-rCBF measurements showed a significant correlation both when calculated in a 'banana' shaped volume approximated by using cortical thresholds as well as when calculated in a semisphere volume of brain tissue beneath the optodes placed on the head surface (patients with AD, n = 10). The correlation was dependent on the assumed penetration depth of the near-infrared light and was best for all three NIRS variables ([HbO2], [HbR] and [HbT]) when calculated using a semisphere radius of 0.45 cm to 1.35 cm. In conclusion, in Alzheimer's disease a marked reduction of regional cerebral blood flow and cerebral hemoglobin oxygenation may occur during activation of brain function, probably mainly in degenerating brain areas, such as the parietal cortex.

Theoretical and experimental investigation of near-infrared light propagation in a model of the adult head

Near-infrared light propagation in various models of the adult head is analyzed by both time-of-flight measurements and mathematical prediction. The models consist of three- or four-layered slabs, the latter incorporating a clear cerebrospinal fluid (CSF) layer. The most sophisticated model also incorporates slots that imitate sulci on the brain surface. For each model, the experimentally measured mean optical path length as a function of source-detector spacing agrees well with predictions from either a Monte Carlo model or a finite-element method based on diffusion theory or a hybrid radiosity-diffusion theory. Light propagation in the adult head is shown to be highly affected by the presence of the clear CSF layer, and both the optical path length and the spatial sensitivity profile of the models with a CSF layer are quite different from those without the CSF layer. However, the geometry of the sulci and the boundary between the gray and the white matter have little effect on the detected light distribution.

Near-infrared spectroscopy: theory and applications

In conclusion, NIRS appears to offer both a new monitoring modality and new information about cerebral oxygenation. Technical problems in the application of this technology persist, most notably determination of pathlength and the volume of tissue interrogated. Those familiar with the history of pulse oximetry will recall that although Millikan developed an ear oximeter in 1947, it was not until Aoyagi combined recognition of the pulse signal with spectroscopy in the 1970s that oximetry was transformed into a clinically applicable monitor. In much the same way, NIRS may find the same tremendous usefulness as a noninvasive monitor of cerebral oxygen utilization, pending resolution of the remaining technical problems.