Near infrared spectroscopy (NIRS): a new tool to study hemodynamic changes during activation of brain function in human adults

Simultaneous assessment of cerebral oxygenation and hemodynamics during a motor task. A combined near infrared and transcranial Doppler sonography study

During performance of a sequential finger opposition task we measured changes in regional cerebral blood oxygenation (rCBO) over the motor cortex and blood flow velocity changes (CBFV) in the middle cerebral artery in a combined near-infrared spectroscopy (NIRS) and transcranial Doppler Sonography (TCD) study. Stimulus duration was 60 followed by a 90 s rest period. During performance of the motor task we observed an increase in [oxy-Hb] a decrease in [deoxy-Hb] and an increase in MCA flow velocity. These changes were significantly more pronounced contralaterally than ipsilaterally to the moving hand. The time course of changes in [oxy-Hb] and CBFV were strikingly similar, showing a pronounced initial over-shoot. This study proves the feasibility of a simultaneous assessment of microcirculatory hemodynamics and cerebral oxygenation at high temporal resolution.

Length of resting period between stimulation cycles modulates hemodynamic response to a motor stimulus

The influence of different lengths of the pre-stimulation resting period on the magnitude of a hemodynamic response evoked by motor stimulation was examined in 10 subjects by means of near-infrared spectroscopy (NIRS). A motor stimulus was used which has been previously established as a model for functional activation studies with NIRS. Subjects performed a 20 s finger opposition task in the hand contralateral to NIRS probe localization over left sensorimotor area (C3', according to the 10-20 system). The duration of the pre-stimulation resting period was varied from 10s to 50s and response magnitude was assessed for each of the interstimulus intervals (10 s, 20 s, 30 s, 40 s and 50 s). Data analysis showed that response magnitude in oxygenated and deoxygenated haemoglobin concentration changed with different interstimulation intervals. Interestingly the greatest NIRS response was obtained with resting period 30 s prior to stimulation; shorter and longer resting periods resulted in smaller responses. The time course and the dependence of response magnitude on interstimulus interval differed between [oxy-Hb] and [deoxy-Hb] changes. For [oxy-Hb] the previously described fast initial increase ('overshoot') and the post-stimulation undershoot was more clearly seen with long prestimulation resting periods. Cytochromeoxidase oxygenation changes did not change significantly with different interstimulus intervals. We conclude that comparisons between different functional activation studies with techniques relying on stimulus evoked changes in cerebral hemodynamics must take into account not only the quality of the experimental paradigm and the length of the stimulation period, but also that the resting period between repetitive stimulations is important for response amplitude and its time course.

Understanding functional neuroimaging methods based on neurovascular coupling

Functional neuroimaging techniques are usually grouped according to the employed apparatus into functional magnetic resonance imaging techniques (fMRI), nuclear medicine approaches such as single photon emission tomography (SPET) or positron emission tomography (PET), and optical approaches (measurement of intrinsic signals, near infrared spectroscopy (NIRS)). However, the physiological parameters that are measured with these methods do not necessarily follow this technical classification. On the one hand, using different imaging modalities the same physiological parameters are measured and on the other hand, using the same imaging devices completely different physiological parameters can be assessed. The present article covers those functional neuroimaging methods which measure the vascular response to functional brain activation (PET, SPET, fMRI and NIRS). First, starting with the traditional grouping of these methods, it is outlined how the specific methods assess vascular changes associated with brain activation in order to localize brain function. Based on the understanding of the underlying physiological events, subsequently, a new classification of functional neuroimaging methods is proposed.

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.

Cerebral oximetry and stroke distance: the future of emergency department monitoring?

OBJECTIVE

To illustrate the possible role of cerebral oximetry and stroke distance as measured by Doppler ultrasound in monitoring the critically ill patient non-invasively in the emergency department.

METHODS

Five critically ill patients were monitored with either cerebral oximetry or both cerebral oximetry and stroke distance (the distance travelled by blood in the aorta with each ventricular contraction), as measured by Doppler ultrasound of the aortic arch.

CONCLUSIONS

Stroke distance as measured by Doppler ultrasound was a good clinical indication of reduced stroke volume and hence of cardiac output. Cerebral oximetry appears to be a useful measure of tissue hypoxia in patients in whom pulse oximetry is either unrecordable or unreliable.

Simultaneous recording of cerebral blood oxygenation changes during human brain activation by magnetic resonance imaging and near-infrared spectroscopy

Changes in cerebral blood oxygenation due to functional activation of the primary sensorimotor cortex during a unilateral finger opposition task were simultaneously mapped by deoxyhemoglobin-sensitive magnetic resonance imaging (MRI) and monitored by near-infrared spectroscopy (NIRS). Activation foci along the contralateral central sulcus displayed task-associated increases in MRI signal intensity, indicating a concomitant decrease of the focal concentration of deoxyhemoglobin. This interpretation was confirmed by simultaneous reductions in deoxyhemoglobin measured optically. Since observation of the latter effect required exact spatial matching of the MRI-detected activation foci and position of the fiber optic bundles ("optodes") used for transmitting and receiving light, it may be concluded that optical recordings of changes in deoxyhemoglobin during functional challenge probe only a restricted brain tissue region. While deoxyhemoglobin responses seen by NIRS were smaller for ipsi- than for contralateral finger movements, task-related increases in oxyhemoglobin were rather similar between both conditions and, thus, seem to be less specific. Furthermore, no consistent changes were obtained for total hemoglobin during task performance, possibly due to the short timing of the repetitive protocol. In general, results underline, in humans, the hitherto assumed signal physiology for functional brain mapping by oxygenation-sensitive MRI and allow assessment of both constraints and practicability of functional studies by NIRS.

Cerebral Monitoring by Near-Infrared Spectroscopy

Near-infrared spectroscopy (NIRS) is a noninvasive optical monitoring technology that can provide information on relative cerebral oxyhemoglobin, deoxyhemoglobin, and oxidized cytochrome aa3 concentrations. Unlike other bedside clinical monitoring technologies, NIRS provides direct information on cerebral hemoglobin oxygenation and blood volume changes. NIRS recordings have documented changes in cerebral oxygenation in fetuses during labor, critically ill premature infants, cardiopulmonary bypass patients, and adult surgical patients. This information may improve understanding of the causes of brain injury and allow detection of inadequate oxygen delivery before brain injury occurs. Future developments in NIRS technology are likely to produce practical, quantitative bedside monitors of global and regional cerebral oxygenation.

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

Near infrared spectroscopy (NIRS) has been used to measure concentration changes of cerebral hemoglobin and cytochrome in neonates, children, and adults, to study cerebral oxygenation and hemodynamics. To derive quantitative concentration changes from measurements of light attenuation, the optical path length must be known. This is obtained by multiplying the source/ detector separation by a laboratory measured differential path length factor (DPF) which accounts for the increased distance traveled by light due to scattering. DPF has been measured by time of flight techniques on small populations of adults and postmortem infants. The values for adults are greater than those for newborns, and it is not clear how to interpolate the present data for studies on children. Recent developments in instrumentation using phase resolved spectroscopy techniques have produced a bedside unit which can measure optical path length on any subject. We have developed an intensity modulated optical spectrometer which measures path length at four wavelengths. Two hundred and eighty three subjects from 1 d of age to 50 y were studied. Measurements were made at a fixed frequency of 200 MHz and a source detector separation of 4.5 cm. Results suggest a slowly varying age dependence of DPF, following the relation DPF690 = 5.38 + 0.049A0.877, DPF744 = 5.11 + 0.106A0.723, DPF807 = 4.99 + 0.067A0.814, and DPF832 = 4.67 + 0.062A0.819, where DPF690 is the DPF measured at 690 nm and A is age is expressed in years from full term. There was a wide scatter of values, however, implying that ideally DPF should be measured at the time of each study.

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.

Influence of respiration and changes in expiratory pressure on cerebral haemoglobin concentration measured by near infrared spectroscopy

Near infrared spectroscopy (NIRS) was used to measure the changes in concentration of cerebral oxy- and deoxygenated haemoglobin ([HbO2] and [Hb]) in six healthy adult volunteers spontaneously breathing against increased expiratory pressures (IEPs) between 0 and 20 cm H2O. During expiration, an increase in [HbO2] was recorded, accompanied by a smaller decrease in [Hb], producing a small increase in total cerebral haemoglobin concentration ([Hbsum]). The mean plus/minus SD change in [Hbsum] at the maximum 1EP of 20 cm H2O was 1.2 +/- 0.7 micromol L-1 (equivalent to 1.4%). Changes in [Hbsum] correlated with IEP level (r = 0.95) and changes in MABP (r = 0.96). The results suggest that homeostatic mechanism do not maintain cerebral blood volume or flow constant over the period of a single breath in normal adults.

Non-invasive functional mapping with multi-channel near infra-red spectroscopic topography in humans

Near-infrared spectroscopy (NIRS) is a new technique for non-invasive monitoring of tissue oxygenation and its kinetics. Up to this date, it has been used solely in research for the global hemodynamic change of the brain and for rough regional activation after stimulating the brain physiologically. This paper describes functional brain mapping using multi channel (ten channel) NIRS by applying the motor stimulation in humans. Our results demonstrate that the regional hemodynamic change was detected in a small area around the motor cortex with a time resolution of 1-2 s. NIRS technique offers considerable potential for research and clinical applications with no invasion.

Near infrared spectroscopy in the diagnosis of Alzheimer's disease

Near infrared spectroscopy (NIRS) is a new technique that permits noninvasive monitoring of cerebral blood and tissue oxygenation. Recently, we and others have shown that NIRS measurements are sensitive enough to follow changes in cerebral hemoglobin oxygenation due to activation of brain function. Based on these findings we have assessed the influence of aging as well as the influence of neurodegeneration on cerebral hemoglobin oxygenation during mental work. The typical NIRS pattern in young healthy subjects while performing calculation tasks measured in the frontal cortex were increases in oxygenated hemoglobin [HbO2] and total hemoglobin [HbT] while reduced hemoglobin [HbR] decreased. Elderly healthy subjects showed a significant lower mean increase in [HbO2] and [HbT] levels. Regression analysis revealed an age-dependent decline in activation-induced local increase of [HbO2] as well as [HbT]. Furthermore, we monitored changes in cerebral hemoglobin oxygenation in the frontal cortex while patients with probable Alzheimer's disease (AD) performed cognitive tasks. Whereas elderly healthy subjects (as well as patients with major depression, age-associated memory impairments or vascular dementia) again showed clear increases in the local concentrations of [HbO2] and [HbT] during brain activation, AD patients showed significant decreases compared to the baseline levels in both variables that were most pronounced in the parietal cortex. To clarify whether the different patterns in cerebral hemoglobin oxygenation during cognitive activation were due to an altered functional brain organization in AD or to alterations in the cerebrovascular response to neuronal activation, we are currently performing simultaneous NIRS and (015-H20-)PET measurements during performance of a cognitive task (Stroop test). Our finding of a regional reduced oxygen supply during activation of brain function may be of relevance to the development and the time course of neurodegeneration.

Age dependency of changes in cerebral hemoglobin oxygenation during brain activation: a near-infrared spectroscopy study

We used near-infrared spectroscopy (NIRS) to study noninvasively the influence of aging on changes in the local concentration of oxygenated hemoglobin ([HbO2]), reduced hemoglobin ([HbR]), and total hemoglobin ([HbT] = [HbR] + [HbO2]) during activation of brain function. Young subjects (n = 12; age, 28 +/- 4 years) performing calculation tasks showed an increase in [HbO2] [mean (arbitrary units) +/- SD, 2.36 +/- 1.07] and [HbT] (2.24 +/- 1.13) in the frontal cortex, while [HbR] (-0.11 +/- 0.48) decreased. Elderly subjects (n = 17; age, 52 +/- 10 years) showed a significantly lower mean increase (p < 0.05) in [HbO2] and [HbT] levels (1.21 +/- 1.38 and 0.72 +/- 1.41, respectively). Regression analysis supports the hypothesis of an age-dependent decline in the activation-induced local increase in [HbO2] (y = -0.241x + 20.062; r = -0.431, p < 0.05) as well as [HbT] (y = -0.346x + 22.496; r = -0.568, p < 0.05). We conclude that NIRS is a promising approach for studying changes in Hb oxygenation during brain activation in physiological aging.

Successful Detection of Cerebral Hypoperfusion by near Infrared Spectroscopy in Two Patients with Dissecting Aortic Aneurysm

Near infrared spectroscopy is being used to monitor mixed cerebral oxygen saturation intraoperatively. In 2 patients receiving surgical treatment for acute aortic dissection, cerebral hypoperfusion was detected by near infrared spectroscopy (INVOS3100, Somanetics Corporation, Troy, MI, USA) and corrected accordingly. Near infrared spectroscopy may help to prevent serious neuropsychiatric complications in surgery for aortic arch dissection.

Spin-echo and gradient-echo EPI of human brain activation using BOLD contrast: a comparative study at 1.5 T

In this study, Blood Oxygenation Level Dependent (BOLD) contrast in the detection of human brain activation was compared between spin-echo and gradient-echo echo-planar sequences at 1.5 T. Time course series of spin-echo and gradient-echo images containing the primary motor cortex were collected during rest (no finger movement) and activation (finger movement). Each time course series was collected using a different TE. Resting and active state signal intensities at each TE were measured in identical regions in the motor cortex. From these data, resting and active state R2 (1/T2) and R2* (1/T2*) values were obtained. Across four subjects, brain activation produced an average R2 change of -0.16 +/- 0.02/s (+/- SE), and an average R2* change of -0.55 +/- 0.08/s. The average delta R2*/delta R2 ratio was 3.52 +/- 0.56. The average gradient-echo/spin-echo ratio of activation-induced signal changes at the TE for maximal BOLD contrast for each sequence (TE approximately T2* and T2) was calculated to be 1.87 +/- 0.40.

Applicability of laser-Doppler flowmetry for cerebral blood flow monitoring in neurological intensive care

Laser Doppler flowmetry (LDF) is a technique for real-time assessment of cerebral blood flow (CBF) changes with potential clinical applicability. Experimental studies have validated that LDF allows accurate measurement of changes in CBF due to physiological and pathophysiological stimuli. Absolute quantitation of flow in ml/100 g min by LDF is not possible. The technique may be used in patients during open brain surgery and postoperatively for bedside CBF monitoring. Disadvantages of the technique are that the flow measurement is highly localized (about 1 mm3) and artifacts may be produced by movement, light or probe placement over large surface vessels. The fibre optic probes for LDF are small enough to be introduced into routinely used intraventricular pressure catheters. We suggest that simultaneous monitoring of CBF and intracranial pressure by such a device holds promise for improved management of patients with critical brain injury.