Jens Steinbrink

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

UNLABELLED We investigated slow spontaneous oscillations in cerebral oxygenation in the human adults 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.

Somatosensory evoked fast optical intensity changes detected non-invasively in the adult human head

This work is the first to report optical intensity changes (DeltaI/I approximately 0.05%) with a latency between 60 and 160ms after electrical median nerve stimulation at 5Hz detected non-invasively through the intact adult human skull in volunteers. The signal is localised and reproducible when measuring at the same position on successive examinations. Compared to previous reports of fast optical changes in the human adult by a single group (Psychophysiology, 32 (1995) 505) the here reported changes are much smaller. They are in line with results from a photon transport calculation on a head model employing data from exposed cortical tissue. The origin of the signal found here is still unclear, however, they might be the non-invasive equivalent to the scattering changes found in exposed cortical tissue studies (J. Neurophysiol., 78 (1997) 1707).

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 enzymes 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.

Editorial comment: Cerebral near-infrared spectroscopy: how far away from a routine diagnostic tool?

Pathophysiological events in cerebrovascular disorders are heterogeneous, eg, the consequences of a carotid stenosis on cerebral circulation can differ considerably between patients and also the cerebral consequences of acute stroke are extremely heterogeneous, not only between different patients but also within one patient over time. Thus, there may not be the one optimal therapy for all patients, but rather the ideal therapeutic option should be tailored to the individual pathophysiological situation. Based on these considerations, a variety of methods have been developed to identify pathophysiological situations relevant for therapeutic decisions. One key issue when describing cerebral pathophysiology in the context of vascular disorders is the interplay between blood flow and oxygen consumption, and the most authoritative findings are still based on positron-emission tomography (PET) measurements. Cost, lack of general availability, poor temporal resolution, which prohibits any monitoring approach, and the exposure to radiation have motivated the search for alternatives. Beside MRI-based methods, near-infrared spectroscopy (NIRS) is a potential alternative allowing for an assessment of cerebral blood flow (CBF) and hemoglobin (Hb) oxygenation parameters of the human cerebral cortex noninvasively through skin and skull. NIRS has been termed a “promising method” for more than a quarter of a century now.1 The principle of a NIRS tool is based on a modified Beer-Lambert Law and is …

The Blushing Brain: Changes in Brain Colour Indicate the Functional State of the Cerebral Cortex

Summary Near infrared spectroscopy of the adult human head has been increasingly applied in functional activation studies. The methodological approach mostly used is based on the Beer-Lambert law and determines changes in oxygenated and deoxygenated haemoglobin from changes in attenuation at multiple wavelengths of the NIR spectrum (650–950 nm). Here we give an overview over our work to validate the method and its application in functional activation studies to better understand mechanisms of neuro-vascular coupling. Such coupling is the basis of modern imaging techniques such as PET and BOLD contrast fMRI, and is thereby of utmost interest when linking the broad knowledge on functional anatomy obtained by invasive studies in animals to the results of the ever increasing body of studies in the adult human. Besides this application in basic physiology of imaging techniques we briefly summarise our view on potential application in neurological disease especially stroke and epilepsy.