Archive for the 'NIRS' Category

IOP publish. highlights of 2008

http://www.iop.org/EJ/journal/-page=extra.highlights2008/0031-9155

initial dip

One page.

Related Articles, <!–
var Menu18479876 = [
[« UseLocalConfig », « jsmenu3Config », «  », «  »],
[« LinkOut », « window.top.location=’/sites/entrez?Cmd=ShowLinkOut&Db=pubmed&TermToSearch=18479876&ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »]
]
–>Links
Click here to read
Dynamics and nonlinearities of the BOLD response at very short stimulus durations.

Yeşilyurt B, Uğurbil K, Uludağ K.

Max-Planck-Institute for Biological Cybernetics, High-Field Magnetic Resonance Center, 72076 Tübingen, Germany.

In designing a functional imaging experiment or analyzing data, it is typically assumed that task duration and hemodynamic response are linearly related to each other. However, numerous human and animal studies have previously reported a deviation from linearity for short stimulus durations (<4 s). Here, we investigated nonlinearities of blood-oxygenation-level-dependent (BOLD) signals following visual stimulation of 5 to 1000 ms duration at two different luminance levels in human subjects. It was found that (a) a BOLD response to stimulus durations as short as 5 ms can be reliably detected; this stimulus duration is shorter than employed in any previous study investigating BOLD signal time courses; (b) the responses are more nonlinear than in any other previous study: the BOLD response to 1000 ms stimulation is only twice as large as the BOLD response to 5 ms stimulation although 200 times more photons were projected onto the retina; (c) the degree of nonlinearity depends on stimulus intensity; that is, nonlinearities have to be characterized not only by stimulus duration but also by stimulus features like luminance. These findings are especially of most practical importance in rapid event-related functional magnetic resonance imaging (fMRI) experimental designs. In addition, an ‘initial dip’ response – thought to be generated by a rapid increase in cerebral metabolic rate of oxygen metabolism (CMRO(2)) relative to cerebral blood flow – was observed and shown to colocalize well with the positive BOLD response. Highly intense stimulation, better tolerated by human subjects for short stimulus durations, causes early CMRO(2) increase, and thus, the experimental design utilized in this study is better for detecting the initial dip than standard fMRI designs. These results and those from other groups suggest that short stimulation combined with appropriate experimental designs allows neuronal events and interactions to be examined by BOLD signal analysis, despite its slow evolution.

PMID: 18479876 [PubMed – as supplied by publisher]


Related Articles, <!–
var Menu18479869 = [
[« UseLocalConfig », « jsmenu3Config », «  », «  »],
[« LinkOut », « window.top.location=’/sites/entrez?Cmd=ShowLinkOut&Db=pubmed&TermToSearch=18479869&ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »]
]
–>Links
Click here to read
Transient and sustained BOLD responses to sustained visual stimulation.

Uludağ K.

Max-Planck-Institute for Biological Cybernetics, High-Field Magnetic Resonance Center, 72076 Tübingen, Germany.

Examining the transients of the blood-oxygenation-level-dependent (BOLD) signal using functional magnetic resonance imaging is a tool to probe basic brain physiology. In addition to the so-called initial dip and poststimulus undershoot of the BOLD signal, occasionally, overshoot at the beginning and at the end of stimulation and stimulus onset and offset (‘phasic’) responses are observed. Hemifield visual stimulation was used in human subjects to study the latter transients. As expected, sustained (‘tonic’) stimulus-correlated contralateral activation in the visual cortex and LGN was observed. Interestingly, bilateral phasic responses were observed, which only partly overlapped with the tonic network and which would have been missed using a standard analysis. A biomechanical model of the BOLD signal (‘balloon model’) indicated that, in addition to phasic neuronal activity, vascular uncoupling can also give rise to phasic BOLD signals. Thus, additional physiological information (i.e., cerebral blood flow) and examination of spatial distribution of the activity might help to assess the BOLD signal transients correctly. In the current study, although vascular uncoupled responses cannot be ruled out as an explanation of the observed phasic BOLD network, the spatial distribution argues that sustained hemifield visual stimulation evokes both bilateral phasic and contralateral sustained neuronal responses. As a consequence, in rapid event-related experimental designs, both the phasic and tonic networks cannot be separated, possibly confounding the interpretation of BOLD signal data. Furthermore, a combination of phasic and tonic responses in the same region of interest might also mimic a BOLD response typically observed in adaptation experiments.

PMID: 18479869 [PubMed – as supplied by publisher]


Related Articles, <!–
var Menu17959391 = [
[« UseLocalConfig », « jsmenu3Config », «  », «  »],
[« LinkOut », « window.top.location=’/sites/entrez?Cmd=ShowLinkOut&Db=pubmed&TermToSearch=17959391&ordinalpos=3&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »]
]
–>Links
Click here to read
Independent components of the haemodynamic response in intrinsic optical imaging.

Schiessl I, Wang W, McLoughlin N.

Faculty of Life Sciences, University of Manchester, Manchester, M60 1QD, UK. i.schiessl@manchester.ac.uk

Functional brain imaging methods are prone to contamination from global vascular artefacts. A variety of methods have been proposed to help segment functional from non-specific changes. Here we quantify the improvement in the signal to noise ratio (SNR) of functional maps, derived from intrinsic optical imaging studies of macaque visual cortex, through the application of Extended Spatial Decorrelation (ESD). The resulting independent component maps and their corresponding time courses reveal for the first time a fast vascular component in the haemodynamic response. ESD is a blind source separation algorithm that utilises spatial statistical features in brain images to separate the recorded mixed sources into independent components. We have investigated differential and single condition experiments using a variety of visual stimuli. To calculate the improvement of the SNR in decibel (dB) we back project separated components onto the original single trial data and analyse the corresponding Fourier spectrum. The application of ESD improved SNR in the functional brain maps from 0.52 to 16.88 dB on differential imaging data and from 1.69 to 12.83 dB in the case of single condition experiments. Analysing the independent components further we found that they can separate different functional compartments of the cortical vasculature. Some of the components, classified as arterial through slit spectroscopy, revealed a strong fast response to the stimulus onset/offset starting approximately 0.2 s after the change of the stimulus and reaching a peak after approximately 0.4 s. This fast haemodynamic response raises new questions concerning the spatial specificity of the so-called « initial dip ».

Publication Types:

PMID: 17959391 [PubMed – indexed for MEDLINE]


Related Articles, <!–
var Menu17574868 = [
[« UseLocalConfig », « jsmenu3Config », «  », «  »],
[« Cited Articles » , « window.top.location=’/sites/entrez?Db=pubmed&DbFrom=pubmed&Cmd=Link&LinkName=pubmed_pubmed_refs&LinkReadableName=Cited%20Articles&IdsFromResult=17574868&ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »],
[« Substance (MeSH Keyword) » , « window.top.location=’/sites/entrez?Db=pcsubstance&DbFrom=pubmed&Cmd=Link&LinkName=pubmed_pcsubstance_mesh&LinkReadableName=Substance%20(MeSH%20Keyword)&IdsFromResult=17574868&ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »],
[« Free in PMC » , « window.top.location=’http://www.pubmedcentral.gov/articlerender.fcgi?tool=pubmed&pubmedid=17574868&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract&ordinalpos=4&prime; « , «  », «  »],
[« LinkOut », « window.top.location=’/sites/entrez?Cmd=ShowLinkOut&Db=pubmed&TermToSearch=17574868&ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »]
]
–>Links
Click here to read Click here to read
Temporal profiles and 2-dimensional oxy-, deoxy-, and total-hemoglobin somatosensory maps in rat versus mouse cortex.

Prakash N, Biag JD, Sheth SA, Mitsuyama S, Theriot J, Ramachandra C, Toga AW.

University of California, Los Angeles, David Geffen School of Medicine, Department of Neurology, Laboratory of Neuro Imaging, Los Angeles, CA 90095, USA. neal.prakash@gmail.com

BACKGROUND: Mechanisms of neurovascular coupling-the relationship between neuronal chemoelectrical activity and compensatory metabolic and hemodynamic changes-appear to be preserved across species from rats to humans despite differences in scale. However, previous work suggests that the highly cellular dense mouse somatosensory cortex has different functional hemodynamic changes compared to other species. METHODS: We developed novel hardware and software for 2-dimensional optical spectroscopy (2DOS). Optical changes at four simultaneously recorded wavelengths were measured in both rat and mouse primary somatosensory cortex (S1) evoked by forepaw stimulation to create four spectral maps. The spectral maps were converted to maps of deoxy-, oxy-, and total-hemoglobin (HbR, HbO, and HbT) concentration changes using the modified Beer-Lambert law and phantom HbR and HbO absorption spectra. RESULTS:: Functional hemodynamics were different in mouse versus rat neocortex. On average, hemodynamics were as expected in rat primary somatosensory cortex (S1): the fractional change in the log of HbT concentration increased monophasically 2 s after stimulus, whereas HbO changes mirrored HbR changes, with HbO showing a small initial dip at 0.5 s followed by a large increase 3.0 s post stimulus. In contrast, mouse S1 showed a novel type of stimulus-evoked hemodynamic response, with prolonged, concurrent, monophasic increases in HbR and HbT and a parallel decrease in HbO that all peaked 3.5-4.5 s post stimulus onset. For rats, at any given time point, the average size and shape of HbO and HbR forepaw maps were the same, whereas surface veins distorted the shape of the HbT map. For mice, HbO, HbR, and HbT forepaw maps were generally the same size and shape at any post-stimulus time point. CONCLUSIONS: 2DOS using image splitting optics is feasible across species for brain mapping and quantifying the map topography of cortical hemodynamics. These results suggest that during physiologic stimulation, different species and/or cortical architecture may give rise to different hemodynamic changes during neurovascular coupling.

Publication Types:

PMID: 17574868 [PubMed – indexed for MEDLINE]

PMCID: PMC2227950


Related Articles, <!–
var Menu17360498 = [
[« UseLocalConfig », « jsmenu3Config », «  », «  »],
[« Cited Articles » , « window.top.location=’/sites/entrez?Db=pubmed&DbFrom=pubmed&Cmd=Link&LinkName=pubmed_pubmed_refs&LinkReadableName=Cited%20Articles&IdsFromResult=17360498&ordinalpos=5&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »],
[« Compound (MeSH Keyword) » , « window.top.location=’/sites/entrez?Db=pccompound&DbFrom=pubmed&Cmd=Link&LinkName=pubmed_pccompound_mesh&LinkReadableName=Compound%20(MeSH%20Keyword)&IdsFromResult=17360498&ordinalpos=5&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »],
[« Substance (MeSH Keyword) » , « window.top.location=’/sites/entrez?Db=pcsubstance&DbFrom=pubmed&Cmd=Link&LinkName=pubmed_pcsubstance_mesh&LinkReadableName=Substance%20(MeSH%20Keyword)&IdsFromResult=17360498&ordinalpos=5&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »],
[« Free in PMC » , « window.top.location=’http://www.pubmedcentral.gov/articlerender.fcgi?tool=pubmed&pubmedid=17360498&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract&ordinalpos=5&prime; « , «  », «  »],
[« LinkOut », « window.top.location=’/sites/entrez?Cmd=ShowLinkOut&Db=pubmed&TermToSearch=17360498&ordinalpos=5&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »]
]
–>Links
Click here to read Click here to read
A coherent neurobiological framework for functional neuroimaging provided by a model integrating compartmentalized energy metabolism.

Aubert A, Pellerin L, Magistretti PJ, Costalat R.

Département de Physiologie, Université de Lausanne, 1005 Lausanne, Switzerland.

Functional neuroimaging has undergone spectacular developments in recent years. Paradoxically, its neurobiological bases have remained elusive, resulting in an intense debate around the cellular mechanisms taking place upon activation that could contribute to the signals measured. Taking advantage of a modeling approach, we propose here a coherent neurobiological framework that not only explains several in vitro and in vivo observations but also provides a physiological basis to interpret imaging signals. First, based on a model of compartmentalized energy metabolism, we show that complex kinetics of NADH changes observed in vitro can be accounted for by distinct metabolic responses in two cell populations reminiscent of neurons and astrocytes. Second, extended application of the model to an in vivo situation allowed us to reproduce the evolution of intraparenchymal oxygen levels upon activation as measured experimentally without substantially altering the initial parameter values. Finally, applying the same model to functional neuroimaging in humans, we were able to determine that the early negative component of the blood oxygenation level-dependent response recorded with functional MRI, known as the initial dip, critically depends on the oxidative response of neurons, whereas the late aspects of the signal correspond to a combination of responses from cell types with two distinct metabolic profiles that could be neurons and astrocytes. In summary, our results, obtained with such a modeling approach, support the concept that both neuronal and glial metabolic responses form essential components of neuroimaging signals.

Publication Types:

PMID: 17360498 [PubMed – indexed for MEDLINE]

PMCID: PMC1820730


Related Articles, <!–
var Menu16543814 = [
[« UseLocalConfig », « jsmenu3Config », «  », «  »],
[« Compound (MeSH Keyword) » , « window.top.location=’/sites/entrez?Db=pccompound&DbFrom=pubmed&Cmd=Link&LinkName=pubmed_pccompound_mesh&LinkReadableName=Compound%20(MeSH%20Keyword)&IdsFromResult=16543814&ordinalpos=6&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »],
[« Substance (MeSH Keyword) » , « window.top.location=’/sites/entrez?Db=pcsubstance&DbFrom=pubmed&Cmd=Link&LinkName=pubmed_pcsubstance_mesh&LinkReadableName=Substance%20(MeSH%20Keyword)&IdsFromResult=16543814&ordinalpos=6&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »],
[« Cited in PMC » , « window.top.location=’http://www.pubmedcentral.gov/tocrender.fcgi?action=cited&tool=pubmed&pubmedid=16543814&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract&ordinalpos=6&prime; « , «  », «  »],
[« LinkOut », « window.top.location=’/sites/entrez?Cmd=ShowLinkOut&Db=pubmed&TermToSearch=16543814&ordinalpos=6&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »]
]
–>Links
Click here to read
Intrinsic optical signal imaging of neocortical seizures: the ‘epileptic dip’.

Bahar S, Suh M, Zhao M, Schwartz TH.

Department of Neurological Surgery, Weill-Cornell Medical College, New York Presbyterian Hospital, New York, New York, USA. bahars@umsl.edu

Focal neocortical seizures, induced by injection of 4-aminopyridine, were imaged in the rat neocortex using the intrinsic optical signal, with incident light at various wavelengths. We observed focal, reproducible and prolonged reflectance drops following seizure onset, regardless of wavelength, in the ictal onset zone. A persistent drop in light reflectance with incident orange light, which corresponds to a decrease in oxygenated hemoglobin, was observed. We describe this phenomenon as an ‘epileptic dip’ as it is reminiscent of the ‘initial dip’ observed using the intrinsic optical signal, and also with blood oxygen level-dependent functional magnetic resonance imaging, after normal sensory processing, although with much longer duration. This persistent ictal ischemia was confirmed by direct measurement of tissue oxygenation using oxygen-sensitive electrodes.

Publication Types:

PMID: 16543814 [PubMed – indexed for MEDLINE]


Related Articles, <!–
var Menu16260743 = [
[« UseLocalConfig », « jsmenu3Config », «  », «  »],
[« Cited Articles » , « window.top.location=’/sites/entrez?Db=pubmed&DbFrom=pubmed&Cmd=Link&LinkName=pubmed_pubmed_refs&LinkReadableName=Cited%20Articles&IdsFromResult=16260743&ordinalpos=7&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »],
[« Compound (MeSH Keyword) » , « window.top.location=’/sites/entrez?Db=pccompound&DbFrom=pubmed&Cmd=Link&LinkName=pubmed_pccompound_mesh&LinkReadableName=Compound%20(MeSH%20Keyword)&IdsFromResult=16260743&ordinalpos=7&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »],
[« Substance (MeSH Keyword) » , « window.top.location=’/sites/entrez?Db=pcsubstance&DbFrom=pubmed&Cmd=Link&LinkName=pubmed_pcsubstance_mesh&LinkReadableName=Substance%20(MeSH%20Keyword)&IdsFromResult=16260743&ordinalpos=7&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »],
[« Free in PMC » , « window.top.location=’http://www.pubmedcentral.gov/articlerender.fcgi?tool=pubmed&pubmedid=16260743&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract&ordinalpos=7&prime; « , «  », «  »],
[« Cited in PMC » , « window.top.location=’http://www.pubmedcentral.gov/tocrender.fcgi?action=cited&tool=pubmed&pubmedid=16260743&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract&ordinalpos=7&prime; « , «  », «  »],
[« LinkOut », « window.top.location=’/sites/entrez?Cmd=ShowLinkOut&Db=pubmed&TermToSearch=16260743&ordinalpos=7&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »]
]
–>Links
Click here to read Click here to read
Brain lactate kinetics: Modeling evidence for neuronal lactate uptake upon activation.

Aubert A, Costalat R, Magistretti PJ, Pellerin L.

Département de Physiologie, Université de Lausanne, 1005 Lausanne, Switzerland.

A critical issue in brain energy metabolism is whether lactate produced within the brain by astrocytes is taken up and metabolized by neurons upon activation. Although there is ample evidence that neurons can efficiently use lactate as an energy substrate, at least in vitro, few experimental data exist to indicate that it is indeed the case in vivo. To address this question, we used a modeling approach to determine which mechanisms are necessary to explain typical brain lactate kinetics observed upon activation. On the basis of a previously validated model that takes into account the compartmentalization of energy metabolism, we developed a mathematical model of brain lactate kinetics, which was applied to published data describing the changes in extracellular lactate levels upon activation. Results show that the initial dip in the extracellular lactate concentration observed at the onset of stimulation can only be satisfactorily explained by a rapid uptake within an intraparenchymal cellular compartment. In contrast, neither blood flow increase, nor extracellular pH variation can be major causes of the lactate initial dip, whereas tissue lactate diffusion only tends to reduce its amplitude. The kinetic properties of monocarboxylate transporter isoforms strongly suggest that neurons represent the most likely compartment for activation-induced lactate uptake and that neuronal lactate utilization occurring early after activation onset is responsible for the initial dip in brain lactate levels observed in both animals and humans.

Publication Types:

PMID: 16260743 [PubMed – indexed for MEDLINE]

PMCID: PMC1297516


Related Articles, <!–
var Menu15837126 = [
[« UseLocalConfig », « jsmenu3Config », «  », «  »],
[« Compound (MeSH Keyword) » , « window.top.location=’/sites/entrez?Db=pccompound&DbFrom=pubmed&Cmd=Link&LinkName=pubmed_pccompound_mesh&LinkReadableName=Compound%20(MeSH%20Keyword)&IdsFromResult=15837126&ordinalpos=8&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »],
[« Substance (MeSH Keyword) » , « window.top.location=’/sites/entrez?Db=pcsubstance&DbFrom=pubmed&Cmd=Link&LinkName=pubmed_pcsubstance_mesh&LinkReadableName=Substance%20(MeSH%20Keyword)&IdsFromResult=15837126&ordinalpos=8&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »],
[« Cited in PMC » , « window.top.location=’http://www.pubmedcentral.gov/tocrender.fcgi?action=cited&tool=pubmed&pubmedid=15837126&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract&ordinalpos=8&prime; « , «  », «  »],
[« LinkOut », « window.top.location=’/sites/entrez?Cmd=ShowLinkOut&Db=pubmed&TermToSearch=15837126&ordinalpos=8&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »]
]
–>Links
Click here to read
Interaction between tissue oxygen tension and NADH imaging during synaptic stimulation and hypoxia in rat hippocampal slices.

Foster KA, Beaver CJ, Turner DA.

Research and Surgery Services, Durham Veterans Affairs Medical Center, Durham, NC 27710, USA. fosterka@duke.edu

Oxygen and NADH are essential components in the production of ATP in the CNS. This study examined the dynamic interaction between tissue oxygen tension (pO(2)) and NADH imaging changes within hippocampal tissue slices, during metabolic stresses including hypoxia and synaptic activation. The initiation of abrupt hypoxia (from 95% O(2) to 95% N(2)) caused a rapid decrease in pO(2), onset of hypoxic spreading depression (hsd; at 6.7+/-1.3 mm Hg; n=15), and a monophasic increase in NADH. Provided that reoxygenation was prompt, synaptic responses, pO(2) and NADH levels returned to baseline following hsd. Longer hypoxia caused irreversible neuronal dysfunction, an increase in pO(2) beyond baseline (due to decreased tissue demand), and hyperoxidation of NADH (10+/-2% decrease below baseline; n=7). Synaptic activation in ambient 95% O(2) caused a decrease or ‘initial dip’ in pO(2) and a biphasic NADH response (oxidation followed by reduction). The oxidizing phase of the NADH response was mitochondrial as it was synchronous with the ‘initial’ dip in pO(2). Following slow graded reductions in ambient oxygen levels to 8%, four of seven slices developed hsd following synaptic stimulation. The hypoxic threshold for graded oxygen reductions occurred at 7.9+/-5.8 mm Hg O(2) (n=7). Our hypoxic threshold range (6.7-7.9 mm Hg O(2) from abrupt and graded oxygen reduction, respectively) correlates well with reported in vivo values of <12 mm Hg O(2). The major findings of this study include: 1) determination of the critical physiological threshold of pO(2) (based upon hsd), which is a marker of imminent neuronal death if oxygen is not rapidly restored; 2) NADH hyperoxidation and an increase in pO(2) beyond baseline levels following longer periods of hypoxia; and 3) the occurrence of a pO(2) ‘dip’ during synaptic stimulation, which correlates with the early oxidizing phase of the biphasic NADH response.

Publication Types:

PMID: 15837126 [PubMed – indexed for MEDLINE]


Related Articles, <!–
var Menu15501093 = [
[« UseLocalConfig », « jsmenu3Config », «  », «  »],
[« Compound (MeSH Keyword) » , « window.top.location=’/sites/entrez?Db=pccompound&DbFrom=pubmed&Cmd=Link&LinkName=pubmed_pccompound_mesh&LinkReadableName=Compound%20(MeSH%20Keyword)&IdsFromResult=15501093&ordinalpos=9&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »],
[« Substance (MeSH Keyword) » , « window.top.location=’/sites/entrez?Db=pcsubstance&DbFrom=pubmed&Cmd=Link&LinkName=pubmed_pcsubstance_mesh&LinkReadableName=Substance%20(MeSH%20Keyword)&IdsFromResult=15501093&ordinalpos=9&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »],
[« Cited in PMC » , « window.top.location=’http://www.pubmedcentral.gov/tocrender.fcgi?action=cited&tool=pubmed&pubmedid=15501093&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract&ordinalpos=9&prime; « , «  », «  »],
[« LinkOut », « window.top.location=’/sites/entrez?Cmd=ShowLinkOut&Db=pubmed&TermToSearch=15501093&ordinalpos=9&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »]
]
–>Links
Click here to read
Modeling the hemodynamic response to brain activation.

Buxton RB, Uludağ K, Dubowitz DJ, Liu TT.

Department of Radiology, 0677, and Center for Functional MRI, University of California-San Diego, La Jolla, CA 92093-0677, USA. rbuxton@ecsd.edu

Neural activity in the brain is accompanied by changes in cerebral blood flow (CBF) and blood oxygenation that are detectable with functional magnetic resonance imaging (fMRI) techniques. In this paper, recent mathematical models of this hemodynamic response are reviewed and integrated. Models are described for: (1) the blood oxygenation level dependent (BOLD) signal as a function of changes in cerebral oxygen extraction fraction (E) and cerebral blood volume (CBV); (2) the balloon model, proposed to describe the transient dynamics of CBV and deoxy-hemoglobin (Hb) and how they affect the BOLD signal; (3) neurovascular coupling, relating the responses in CBF and cerebral metabolic rate of oxygen (CMRO(2)) to the neural activity response; and (4) a simple model for the temporal nonlinearity of the neural response itself. These models are integrated into a mathematical framework describing the steps linking a stimulus to the measured BOLD and CBF responses. Experimental results examining transient features of the BOLD response (post-stimulus undershoot and initial dip), nonlinearities of the hemodynamic response, and the role of the physiologic baseline state in altering the BOLD signal are discussed in the context of the proposed models. Quantitative modeling of the hemodynamic response, when combined with experimental data measuring both the BOLD and CBF responses, makes possible a more specific and quantitative assessment of brain physiology than is possible with standard BOLD imaging alone. This approach has the potential to enhance numerous studies of brain function in development, health, and disease.

Publication Types:

PMID: 15501093 [PubMed – indexed for MEDLINE]


Related Articles, <!–
var Menu14688611 = [
[« UseLocalConfig », « jsmenu3Config », «  », «  »],
[« Cited in PMC » , « window.top.location=’http://www.pubmedcentral.gov/tocrender.fcgi?action=cited&tool=pubmed&pubmedid=14688611&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract&ordinalpos=10&prime; « , «  », «  »],
[« LinkOut », « window.top.location=’/sites/entrez?Cmd=ShowLinkOut&Db=pubmed&TermToSearch=14688611&ordinalpos=10&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstract’ « , «  », «  »]
]
–>Links
Click here to read
Coupling of changes in cerebral blood flow with neural activity: what must initially dip must come back up.

Ances BM.

Department of Neurology, Hospital of the University of Pennsylvania, Philadelphia, PA 19104, USA. beau.ances@uphs.upenn.edu

Activation flow coupling, increases in neuronal activity leading to changes in cerebral blood flow (CBF), is the basis of many neuroimaging methods. An early rise in deoxygenation, the « initial dip, » occurs before changes in CBF and cerebral blood volume (CBV) and may provide a better spatial localizer of early neuronal activity compared with subsequent increases in CBF. Imaging modality, anesthetic, degree of oxygenation, and species can influence the magnitude of this initial dip. The observed initial dip may reflect a depletion of mitochondrial oxygen (O(2)) buffers caused by increased neuronal activity. Changes in CBF mediated by nitric oxide (NO) or other metabolites and not caused by a lack of O(2) or energy depletion most likely lead to an increased delivery of capillary O(2) in an attempt to maintain intracellular O(2) buffers.

Publication Types:

PMID: 14688611 [PubMed – indexed for MEDLINE]

DOT-NIRS-University College London-Delpy 1999-1988

Use of mitochondrial inhibitors to demonstrate that cytochrome oxidase near-infrared spectroscopy can measure mitochondrial dysfunction noninvasively in the brain.

Cooper CE, Cope M, Springett R, Amess PN, Penrice J, Tyszczuk L, Punwani S, Ordidge R, Wyatt J, Delpy DT.

Department of Biological Sciences, University of Essex, Colchester, UK.

The use of near-infrared spectroscopy to measure noninvasively changes in the redox state of cerebral cytochrome oxidase in vivo is controversial. We therefore tested these measurements using a multiwavelength detector in the neonatal pig brain. Exchange transfusion with perfluorocarbons revealed that the spectrum of cytochrome oxidase in the near-infrared was identical in the neonatal pig, the adult rat, and in the purified enzyme. Under normoxic conditions, the neonatal pig brain contained 15 micromol/L deoxyhemoglobin, 29 micromol/L oxyhemoglobin, and 1.2 micromol/L oxidized cytochrome oxidase. The mitochondrial inhibitor cyanide was used to determine whether redox changes in cytochrome oxidase could be detected in the presence of the larger cerebral hemoglobin concentration. Addition of cyanide induced full reduction of cytochrome oxidase in both blooded and bloodless animals. In the blooded animals, subsequent anoxia caused large changes in hemoglobin oxygenation and concentration but did not affect the cytochrome oxidase near-infrared signal. Simultaneous blood oxygenation level-dependent magnetic resonance imaging measurements showed a good correlation with near-infrared measurements of deoxyhemoglobin concentration. Possible interference in the near-infrared measurements from light scattering changes was discounted by simultaneous measurements of the optical pathlength using the cerebral water absorbance as a standard chromophore. We conclude that, under these conditions, near-infrared spectroscopy can accurately measure changes in the cerebral cytochrome oxidase redox state.

Publication Types:

PMID: 9886352 [PubMed – indexed for MEDLINE]


Experimental and theoretical comparison of NIR spectroscopy measurements of cerebral hemoglobin changes.

Firbank M, Elwell CE, Cooper CE, Delpy DT.

Department of Medical Physics and Bioengineering, University College London, London WC1E 6JA, United Kingdom CO4 3SQ.

Two near-infrared spectroscopy (NIRS) methods are available for measuring changes (Delta) in total cerebral hemoglobin concentration (CHC): 1) a continuous measurement of the changes in total hemoglobin concentration (Delta[Hb]tot) and 2) the difference between two absolute measurements of CHC, each derived from a small, controlled change in inspired O2 fraction. This paper investigates the internal consistency of these two methods by using an experimental and theoretical comparison. NIRS was used to measure [Hb]tot in five newborn piglets before and after a change in arterial PCO2. Delta[Hb]tot demonstrated a low coefficient of variation of 2.8 +/- 2.8 (SD) % which allowed changes in CO2-cerebral blood volume reactivity to be clearly discriminated. However, a high coefficient of variation of 22.8 +/- 3.5% on the DeltaCHC measurements obscured any CO2 reactivity changes. A theoretical analysis demonstrates the effects of optical pathlength, background absorption, scatter, and blood vessel diameter on both methods. For more accurate monitoring of CHC, individual measurements of optical pathlength and more accurate pulse oximetry are required.

Publication Types:

PMID: 9804599 [PubMed – indexed for MEDLINE]


A theoretical study of the signal contribution of regions of the adult head to near-infrared spectroscopy studies of visual evoked responses.

Firbank M, Okada E, Delpy DT.

University Department of Radiology, Royal Victoria Infirmary, Newcastle upon Tyne, United Kingdom.

Near-infrared (NIR) spectroscopy has been used in studies of the cerebral hemodynamic response to visual processing. In this paper, we present theoretical results from finite element and Monte Carlo modeling in order to help understand the contribution to the NIR signal from different parts of the head. The results from the models show that at the typical optode spacings used in these studies, an infrared spectroscopy measurement of intensity is sensitive to the outer 1-2 mm of the cortical gray matter and the partial optical path length in the gray matter is approximately 10 mm, compared with a total optical path length of 400 mm. When the NIR measurement is of change in mean photon arrival time (or phase shift), the signal comes from the upper 2-4 mm of the cortical surface and there is an increased lateral spread of the contributing tissue. We predict that for a 4-cm separation of input and detection optodes at 800 nm, a 1 microM change in hemoglobin concentration in the cortex corresponds to an attenuation change of approximately 0.001 OD (optical density) or 1 ps mean time change. Movement of the brain caused by this increase in volume will cause an absorption change of approximately half this magnitude, but does not affect the photon arrival time at 4-cm spacing. A discrepancy between the predicted and the experimentally measured intensities may support the supposition that the NIR signal is actually very sensitive to changes occurring in the pial cerebral vessels lying on the brain surface.

Publication Types:

PMID: 9698577 [PubMed – indexed for MEDLINE]


The relationship of oxygen delivery to absolute haemoglobin oxygenation and mitochondrial cytochrome oxidase redox state in the adult brain: a near-infrared spectroscopy study.

Cooper CE, Delpy DT, Nemoto EM.

Department of Biological Sciences, Central Campus, University of Essex, Wivenhoe Park, Colchester, CO4 3SQ, UK. ccooper@essex.ac.uk

Near-infrared spectroscopy was used to determine the effect of changes in the rate of oxygen delivery to the adult rat brain on the absolute concentrations of oxyhaemoglobin, deoxyhaemoglobin and the redox state of the CuA centre in mitochondrial cytochrome oxidase. The cytochrome oxidase detection algorithm was determined to be robust to large changes in haemoglobin oxygenation and concentration. By assuming complete haemoglobin deoxygenation and CuA reduction following mechanical ventilation on 100% N2O, the absolute concentration of oxyhaemoglobin (35 microM), deoxyhaemoglobin (27 microM) and the redox state of CuA (82% oxidized) were calculated in the normal adult brain. The mean arterial blood pressure was decreased by exsanguination. When the pressure reached 100 mmHg, haemoglobin oxygenation started to fall, but the total haemoglobin concentration and oxidized CuA levels only fell when cerebral blood volume autoregulation mechanisms failed at 50 mmHg. Haemoglobin oxygenation fell linearly with decreases in the rate of oxygen delivery to the brain, but the oxidized CuA concentration did not start to fall until this rate was 50% of normal. The results suggest that the brain maintains more than adequate oxygen delivery to mitochondria and that near-infrared spectroscopy may be a good measure of oxygen insufficiency in vivo.

Publication Types:

PMID: 9620863 [PubMed – indexed for MEDLINE]

PMCID: PMC1219521


In vivo measurements of the wavelength dependence of tissue-scattering coefficients between 760 and 900 nm measured with time-resolved spectroscopy.

Matcher SJ, Cope M, Delpy DT.

We present in vivo values for the optical transport coefficients (mu(a), mu(s)?) of the adult human forearm, calf, and head from 760 to 900 nm measured with time-resolved spectroscopy. The accuracy of the method is tested with tissue-simulating phantoms. We obtain mu(s)?(lambda) approximately 1.1 – (5.1 x 10(-4) lambda) mm(-1) (forearm), 1.6 – (8.9 x 10(-4) lambda) mm(-1) (calf), and 1.45 – (6.5 x 10(-4) lambda) mm(-1) (head), where lambda is measured in nanometers. At 800 nm we obtain mu(a) = 0.023 +/- 0.004 mm(-1) (forearm), 0.017 +/- 0.005 mm(-1) (calf), and 0.016 +/- 0.001 mm(-1) (head). Our values differ substantially from published in vitro data. In particular, our transport coefficients for the adult head are substantially lower than previously reported values for adult human cerebral matter and pig skull cortical bone measured in vitro.

PMID: 18250686 [PubMed – in process]


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

Okada E, Firbank M, Schweiger M, Arridge SR, Cope M, Delpy DT.

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.

PMID: 18250644 [PubMed – in process]


The effect of scalp ischaemia on measurement of cerebral blood volume by near-infrared spectroscopy.

Owen-Reece H, Elwell CE, Wyatt JS, Delpy DT.

Department of Anaesthesia, National Hospital for Neurology and Neurosurgery, London, UK.

Near-infrared spectroscopy (NIRS) is a noninvasive method of quantifying changes in cerebral haemodynamics from changes in the absorption of near-infrared light by oxyhaemoglobin and deoxyhaemoglobin. Measurement of neonatal cerebral blood volume (CBV) by NIRS was described in 1990 but it has been suggested that, in adults, scalp and skull blood content contribute a significant amount to the cerebral haemodynamic variables quantifiable by NIRS. To investigate this, CBV was measured in nine adult subjects, in the frontal region of the head, before and after inflating a pneumatic tourniquet proximal to the measurement site. Because a change in scalp blood content could potentially alter the pathlength of light passing through the head and hence affect the measured CBV, the optical pathlength factor was therefore also measured before and after tourniquet inflation. Blood flow occlusion was confirmed by laser Doppler velocimetry. The results showed that tourniquet inflation had no effect on the estimated value of CBV or the differential pathlength factor. We conclude that, provided the distance between light entry and exit on the surface of the scalp is sufficiently large, changes in scalp blood flow have no effect on NIRS measurement of cerebral haemodynamics.

Publication Types:

PMID: 8953626 [PubMed – indexed for MEDLINE]


Comment on:

Near-infrared spectroscopy and cerebral hemodynamics.

Owen-Reece H, Smith M, Elwell CE, Goldstone JC, Delpy DT.

Publication Types:

PMID: 8706504 [PubMed – indexed for MEDLINE]


Near-infrared spectroscopy: theory and applications.

Wahr JA, Tremper KK, Samra S, Delpy DT.

Department of Anesthesiology, University of Michigan, Ann Arbor, USA.

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.

Publication Types:

PMID: 8725427 [PubMed – indexed for MEDLINE]


The finite element method for the propagation of light in scattering media: boundary and source conditions.

Schweiger M, Arridge SR, Hiraoka M, Delpy DT.

Department of Medical Physics and Bioengineering, University College London, England.

This paper extends our work on applying the Finite Element Method (FEM) to the propagation of light in tissue. We address herein the topics of boundary conditions and source specification for this method. We demonstrate that a variety of boundary conditions stipulated on the Radiative Transfer Equation can be implemented in a FEM approach, as well as the specification of a light source by a Neumann condition rather than an isotropic point source. We compare results for a number of different combinations of boundary and source conditions under FEM, as well as the corresponding cases in a Monte Carlo model.

Publication Types:

PMID: 8587533 [PubMed – indexed for MEDLINE]


Performance comparison of several published tissue near-infrared spectroscopy algorithms.

Matcher SJ, Elwell CE, Cooper CE, Cope M, Delpy DT.

University College London Department of Medical Physics and Bioengineering, United Kingdom.

We have collected multiwavelength near-infrared (NIR) attenuation spectra on human forearm muscle, the adult rat head, and newborn piglet head to compare the changes in chromophore concentration derived from these data using published algorithms from four groups. We find differences between the results from the algorithms on each data set, particularly in their estimation of cytochrome oxidase (cyt-aa3) redox changes. We also find some differences when applying the same algorithm to the three data sets, suggesting possible difficulties in transferring algorithms between different physiological systems (e.g., Kurth, C. D., Steven, J. M., Benaron, D., and Chance, B. (1993) J. Clin. Monit. 9, 163-170). We have also compared the algorithms using simulated data generated using measured hemoglobin absorption spectra and a diffusion model for light transport in tissue. We find that while the algorithms from three groups are in broad agreement, that published by Piantadosi (Piantadosi, C. A. (1993) Methods Toxicol. 2, 107-126) produces significantly different results for cyt-aa3 and HbO2. Either the hemoglobin spectra used to produce the simulated data are inaccurate or the modeling is incorrect, or this algorithm is erroneous.

Publication Types:

PMID: 7668392 [PubMed – indexed for MEDLINE]


The spatial resolution performance of a time-resolved optical imaging system using temporal extrapolation.

Hebden JC, Hall DJ, Delpy DT.

Department of Medical Physics, University College London, England.

Optical imaging methods are being explored as a potential means of screening for breast cancer. Previous investigations of time-resolved imaging techniques have suggested that due to the lack of photons with sufficiently small pathlengths, the spatial resolution achievable through a human breast would be unlikely to be better than a centimeter. Experimental results presented here indicate, however, that higher resolution may be achieved by extrapolating the measured temporal distribution of transmitted photons. This is performed using a least-squares fit between data and an analytic model of photon transport. The spatial resolution of a time-resolved imaging system was evaluated by measuring the edge response produced by an opaque mask embedded in the center of a 51-mm-thick, very highly scattering medium. The limiting spatial resolution was improved from about 13 mm to about 5 mm.

Publication Types:

PMID: 7565351 [PubMed – indexed for MEDLINE]


Measurement of the optical properties of the skull in the wavelength range 650-950 nm.

Firbank M, Hiraoka M, Essenpreis M, Delpy DT.

Department of Medical Physics and Bioengineering, University College London, UK.

The optical properties of samples of bone from pig skull have been measured over the wavelength range 650-950 nm. The scattering phase function was measured on thin samples of the bone using a goniometer, and a value for the mean cosine g, of the scattering angle, was calculated. The scattering and absorption coefficients, mu s and mu a were then determined from measurements of diffuse reflectance and transmittance made with a pair of integrating spheres, by a step-wise search through a table of diffuse reflectance and transmittance versus mu a and mu s generated by a Monte Carlo model incorporating the measured scattering phase function. Values for g measured on six samples varied from 0.925 +/- 0.014 at 650 nm to 0.945 +/- 0.013 at 950 nm. Corresponding values for mu a and mu s measured on 18 samples were mu a = 0.04 +/- 0.002 mm-1, mu s = 35 +/- 0.7 mm-1 at 650 nm to mu a = 0.05 +/- 0.002 mm-1, mu s = 24 +/- 0.6 mm-1 at 950 nm.

Publication Types:

PMID: 8488176 [PubMed – indexed for MEDLINE]


Wavelength dependence of the differential pathlength factor and the log slope in time-resolved tissue spectroscopy.

Essenpreis M, Cope M, Elwell CE, Arridge SR, van der Zee P, Delpy DT.

Department of Medical Physics and Bioengineering, University College London, UK.

Publication Types:

PMID: 8362674 [PubMed – indexed for MEDLINE]

————–

Estimation of optical pathlength through tissue from direct time of flight measurement.

Delpy DT, Cope M, van der Zee P, Arridge S, Wray S, Wyatt J.

Department of Medical Physics, University College London, UK.

Quantitation of near infrared spectroscopic data in a scattering medium such as tissue requires knowledge of the optical pathlength in the medium. This can now be estimated directly from the time of flight of picosecond length light pulses. Monte Carlo modelling of light pulses in tissue has shown that the mean value of the time dispersed light pulse correlates with the pathlength used in quantitative spectroscopic calculations. This result has been verified in a phantom material. Time of flight measurements of pathlength across the rat head give a pathlength of 5.3 +/- 0.3 times the head diameter.

Publication Types:

DOT-NIRS-University College London Delpy 2004-2000

Mapping human skeletal muscle perforator vessels using a quantum well infrared photodetector (QWIP) might explain the variability of NIRS and LDF measurements.

Binzoni T, Leung T, Delpy DT, Fauci MA, Rüfenacht D.

Department of Radiology, Faculty of Medicine, University of Geneva, Switzerland. Tiziano.Binzoni@medecine.unige.ch

Near-infrared spectroscopy (NIRS) and laser Doppler flowmetry (LDF) have become the techniques of choice allowing the non-invasive study of local human skeletal muscle metabolism and blood perfusion on a small tissue volume (a few cm3). However, it has been shown that both NIRS and LDF measurements may show a large spatial variability depending on the position of the optodes over the investigated muscle. This variability may be due to local morphologic and/or metabolic characteristics of the muscle and makes the data interpretation and comparison difficult. In the present work, we use a third method to investigate this problem which permits fast, non-invasive mapping of the intramuscular vessel distribution in the human vastus latelralis muscle. This method uses an advanced, passive, infrared imaging sensor called a QWIP (quantum well infrared photodetector). We demonstrate, using a recovery-enhanced infrared imaging technique, that there is a significant presence of perforator vessels in the region of interest of approximately 30 x 18 cm (the number of vessels being: 14, 9, 8, 33, 17 and 18 for each subject, respectively). The presence of these vessels makes the skeletal muscle highly inhomogeneous, and may explain the observed NIRS and LDF spatial variability. We conclude that accurate comparison of the metabolic activity of two different muscle regions is not possible without reliable maps of vascular ‘singularities’ such as the perforator vessels, and that the QWIP-based imaging system is one method to obtain this information.

Publication Types:

PMID: 15272688 [PubMed – indexed for MEDLINE]


Spectral characteristics of spontaneous oscillations in cerebral haemodynamics are posture dependent.

Tachtsidis I, Elwell CE, Lee CW, Leung TS, Smith M, Delpy DT.

Department of Medical Physics & Bioengineering, University College London, London WC1E 6JA, UK. iliastac@medphys.ucl.ac.uk

Publication Types:

PMID: 15174599 [PubMed – indexed for MEDLINE]


Measurement of the optical properties of the adult human head with spatially resolved spectroscopy and changes of posture.

Leung TS, Elwell CE, Tachtsidis I, Henty JR, Delpy DT.

Department of Medical Physics & Bioengineering, University College London, London WC1E 6JA, UK. tsl@medphys.ucl.ac.uk

Publication Types:

PMID: 15174596 [PubMed – indexed for MEDLINE]


Investigation of cerebral haemodynamics by near-infrared spectroscopy in young healthy volunteers reveals posture-dependent spontaneous oscillations.

Tachtsidis I, Elwell CE, Leung TS, Lee CW, Smith M, Delpy DT.

Department of Medical Physics and Bioengineering, University College London, London WC1E 6JA, UK. iliastac@medphys.ucl.ac.uk

Autonomic reflexes enable the cardiovascular system to respond to gravitational displacement of blood during changes in posture. Spontaneous oscillations present in the cerebral and systemic circulation of healthy subjects have demonstrated a regulatory role. This study assessed the dynamic responses of the cerebral and systemic circulation upon standing up and the posture dependence of spontaneous oscillations. In ten young healthy volunteers, blood pressure and cerebral haemodynamics were continuously monitored non-invasively using the Portapres and near-infrared spectroscopy (NIRS), respectively. Oscillatory changes in the cerebral NIRS signals and the diastolic blood pressure (DBP) signal have been identified by the fast Fourier analysis. Blood pressure increased during standing and returned to basal level when volunteers sat on a chair. The mean value of cerebral tissue oxygen index (TOI) as measured by NIRS did not demonstrate any significant changes. Oscillatory changes in DBP, oxyhaemoglobin concentration [O2Hb] and TOI showed a significant increase when subjects were standing. Investigation of the low frequency component (approximately 0.1 Hz) of these fluctuations revealed posture dependence associated with activation of autonomic reflexes. Systemic and cerebral changes appeared to preserve adequate blood flow and cerebral perfusion during standing in healthy volunteers. Oscillatory changes in [O2Hb] and TOI, which may be related to the degree of cerebral sympathetic stimulation, are posture dependent in healthy subjects.

Publication Types:

PMID: 15132309 [PubMed – indexed for MEDLINE]


Monitoring cytochrome redox changes in the mitochondria of intact cells using multi-wavelength visible light spectroscopy.

Hollis VS, Palacios-Callender M, Springett RJ, Delpy DT, Moncada S.

Wolfson Institute for Biomedical Research, University College London, Cruciform Building, Gower Street, London, WC1E 6AE, UK.

We have developed an optical system based on visible light spectroscopy for the continuous study of changes in the redox states of mitochondrial cytochromes in intact mammalian cells. Cells are suspended in a closed incubation chamber in which oxygen and nitric oxide (NO) concentrations can be monitored during respiration. Simultaneously the cells are illuminated with a broad-band tungsten-halogen light source. Emergent light in the visible region (from 490-650 nm) is detected using a spectrophotometer and charge-coupled device camera system. Intensity spectra are then converted into changes in optical attenuation from a ‘steady-state’ baseline. The oxidised-minus-reduced absorption spectra of the mitochondrial cytochromes are fitted to the attenuation spectra using a multi-wavelength least-squares algorithm. Thus, the system can measure changes in the redox states of the cytochromes during cellular respiration. Here we describe this novel methodology and demonstrate its validity by monitoring the action of known respiratory chain inhibitors, including the endogenous signalling molecule NO, on cytochrome redox states in human leukocytes.

Publication Types:

PMID: 14670609 [PubMed – indexed for MEDLINE]


The oxygen dependency of cerebral oxidative metabolism in the newborn piglet studied with 31P NMRS and NIRS.

Springett RJ, Wylezinska M, Cady EB, Hollis V, Cope M, Delpy DT.

Department of Radiology, Dartmouth College, Hanover, New Hampshire, USA.

Mean cerebral saturation and changes in the oxidation state of the CuA centre of cytochrome oxidase were measured by near infra-red spectroscopy simultaneously with phosphorous metabolites and intracellular pH measured using 31P NMR spectroscopy during transient anoxia (inspired oxygen fraction = 0.0 for 105 seconds) in the newborn piglet brain. By collecting high quality 31P spectra every 10 seconds, it was possible to resolve the delay between the onset of anoxia and the fall in PCr and to show that the CuA centre of cytochrome oxidase reduced simultaneously with the fall in PCr. From these observations it is concluded that, at normoxia, oxygen tension at the mitochondrial level is substantially above a critical value at which oxidative metabolism becomes oxygen dependent.

Publication Types:

PMID: 14562751 [PubMed – indexed for MEDLINE]


Near-infrared light propagation in an adult head model. II. Effect of superficial tissue thickness on the sensitivity of the near-infrared spectroscopy signal.

Okada E, Delpy DT.

Department of Electronics and Electrical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan. okada@elec.keio.ac.jp

It is important for near-infrared spectroscopy (NIRS) and imaging to estimate the sensitivity of the detected signal to the change in hemoglobin that results from brain activation and the volume of tissue interrogated for a specific source-detector fiber spacing. In this study light propagation in adult head models is predicted by Monte Carlo simulation to investigate the effect of the superficial tissue thickness on the partial optical path length in the brain and on the spatial sensitivity profile. In the case of source-detector spacing of 30 mm, the partial optical path length depends mainly on the depth of the inner skull surface whereas the spatial sensitivity profile is significantly affected by the thickness of the cerebrospinal fluid layer. The mean optical path length that can be measured by time-resolved experiments increases when the skull thickness increases whereas the partial mean optical path length in the brain decreases when the skull thickness increases. These results indicate that it is not appropriate to use the mean optical path length as an alternative to the partial optical path length to compensate the NIRS signal for the difference in sensitivity caused by variation of the superficial tissue thickness.

Publication Types:

PMID: 12790440 [PubMed – indexed for MEDLINE]


Near-infrared light propagation in an adult head model. I. Modeling of low-level scattering in the cerebrospinal fluid layer.

Okada E, Delpy DT.

Department of Electronics and Electrical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan. okada@elec.keio.ac.jp

Adequate modeling of light propagation in a human head is important for quantitative near-infrared spectroscopy and optical imaging. The presence of a nonscattering cerebrospinal fluid (CSF) that surrounds the brain has been previously shown to have a strong effect on light propagation in the head. However, in reality, a small amount of scattering is caused by the arachnoid trabeculae in the CSF layer. In this study, light propagation in an adult head model with discrete scatterers distributed within the CSF layer has been predicted by Monte Carlo simulation to investigate the effect of the small amount of scattering caused by the arachnoid trabeculae in the CSF layer. This low scattering in the CSF layer is found to have little effect on the mean optical path length, a parameter that can be directly measured by a time-resolved experiment. However, the partial optical path length in brain tissue that relates the sensitivity of the detected signal to absorption changes in the brain is strongly affected by the presence of scattering within the CSF layer. The sensitivity of the near-infrared signal to hemoglobin changes induced by brain activation is improved by the effect of a low-scattering CSF layer.

Publication Types:

PMID: 12790439 [PubMed – indexed for MEDLINE]


A method for generating patient-specific finite element meshes for head modelling.

Gibson AP, Riley J, Schweiger M, Hebden JC, Arridge SR, Delpy DT.

Department of Medical Physics and Bioengineering, University College London, UK.

Finite element modelling of fields within the body, whether electrical or optical, requires knowledge of the geometry of the object being examined. It can be clinically impractical to obtain accurate surface information for individual patients, although a limited set of measurements such as the locations of sensors attached to the body, can be acquired more readily. In this paper, we describe how a generic surface taken from an adult head is warped to fit points measured on a neonatal head surface to provide a new, individual surface from which a finite element mesh was generated. Simulations show that data generated from this mesh and from the original neonatal head surface are similar to within experimental errors. However, data generated from a mesh of the best fit sphere were significantly different from data generated from the original neonatal head surface.

Publication Types:

PMID: 12630743 [PubMed – indexed for MEDLINE]


Simultaneous measurement of cerebral tissue oxygenation over the adult frontal and motor cortex during rest and functional activation.

Leung TS, Elwell CE, Henty JR, Delpy DT.

Department of Medical Physics & Bioengineering, University College London, London, WC1E 6JA, UK.

Publication Types:

PMID: 12580459 [PubMed – indexed for MEDLINE]


Three-dimensional optical tomography of the premature infant brain.

Hebden JC, Gibson A, Yusof RM, Everdell N, Hillman EM, Delpy DT, Arridge SR, Austin T, Meek JH, Wyatt JS.

Department of Medical Physics & Bioengineering, University College London, 11-20 Capper Street, London WC1E 6JA, UK.

For the first time, three-dimensional images of the newborn infant brain have been generated using measurements of transmitted light. A 32-channel time-resolved imaging system was employed, and data were acquired using custom-made helmets which couple source fibres and detector bundles to the infant head. Images have been reconstructed using measurements of mean flight time relative to those acquired on a homogeneous reference phantom, and using a head-shaped 3D finite-element-based forward model with an external boundary constrained to match the measured positions of the sources and detectors. Results are presented for a premature infant with a cerebral haemorrhage predominantly located within the left ventricle. Images representing the distribution of absorption at 780 nm and 815 nm reveal an asymmetry consistent with the haemorrhage, and corresponding maps of blood volume and fractional oxygen saturation are generally within expected physiological values.

Publication Types:

PMID: 12502040 [PubMed – indexed for MEDLINE]


Quantitative near infrared spectroscopy measurement of cerebral hemodynamics in newborn piglets.

Brown DW, Picot PA, Naeini JG, Springett R, Delpy DT, Lee TY.

Imaging Division, Lawson Health Research Institute, London, Ontario, Canada, N6A 4V2.

Severely premature infants are often at increased risk of cerebral hemorrhage and/or ischemic injury caused by immature autoregulatory control of blood flow to the brain. If blood flow is too high, the infant is at risk of hemorrhage, whereas too little blood flow can result in ischemic injury. The development of a noninvasive, bedside means of measuring cerebral hemodynamics would greatly facilitate both diagnosis and monitoring of afflicted individuals. It is to this end that we have developed a near infrared spectroscopy (NIRS) system that allows for quantitative, bedside measurement of cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT). The technique requires an i.v. injection of the near infrared chromophore indocyanine green. Six newborn piglets, median age of 18 h (range 6-54 h), median weight of 1.75 kg (range 1.5-2.1 kg), were studied. Measurements of CBF, CBV, and MTT were made at normocapnia, hypocapnia, and hypercapnia to test the technique over a range of hemodynamic conditions. The accuracy of our new approach has been determined by direct comparison with measurements made using a previously validated computed tomography technique. Paired t tests showed no significant difference between computed tomography and NIRS measurements of CBF, CBV, and MTT, and mean biases between the two methods were -2.05 mL x min(-1) x 100 g(-1), -0.18 mL x 100 g(-1), and 0.43 s, respectively. The precision of NIRS CBF, CBV, and MTT measurements, as determined by repeated-measures ANOVA, was 9.71%, 13.05%, and 7.57%, respectively.

Publication Types:

PMID: 11978878 [PubMed – indexed for MEDLINE]


Nitric oxide does not inhibit cerebral cytochrome oxidase in vivo or in the reactive hyperemic phase after brief anoxia in the adult rat.

De Visscher G, Springett R, Delpy DT, Van Reempts J, Borgers M, van Rossem K.

Department of Neuropathology, Discovery Research, Janssen Research Foundation, Turnhoutseweg 30, B-2340 Beerse, Belgium. gdvissch@janbe.jnj.com

In this study, near-infrared spectroscopy was applied to examine whether cytochrome oxidase in the rat brain is inhibited by nitric oxide in vivo. During normoxia, intravenous N(G)-nitro-L-arginine methyl ester (L-NAME) administration significantly decreased the cerebral saturation of hemoglobin with oxygen but did not alter the cytochrome oxidase redox state. Anoxia significantly reduced the cytochrome oxidase. The time course of the recovery of the redox state during reoxygenation was not altered by L-NAME. The results suggest that in adult rats, cytochrome oxidase is not inhibited by nitric oxide, either in physiologic conditions or during reoxygenation after a brief anoxic period.

PMID: 11973423 [PubMed – indexed for MEDLINE]


Local temperature changes and human skeletal muscle metabolism.

Binzoni T, Delpy D.

Departments of Radiology and Physiology, Faculty of Medicine, University of Geneva, Geneva, Switzerland.

The aim of this review is to describe the effects induced by local temperature changes on human skeletal muscle metabolism. More specifically, we will consider the influence of temperature on the mechanical properties of muscle contraction, on aerobic metabolism, anaerobic metabolism and on the Lohmann reaction. The text has been voluntarily organized on the basis of a simple bioenergetic model describing the different energy fluxes appearing in the muscle system. This approach should better highlight some of the points that still need to be investigated. Although it was not always possible to restrict the discussion to human muscle, the references report mainly data obtained directly on humans or on isolated human fibres. A short comment on skeletal muscle temperature measurement techniques, on humans, is also included.

Publication Types:

PMID: 11499164 [PubMed – indexed for MEDLINE]


Time resolved optical tomography of the human forearm.

Hillman EM, Hebden JC, Schweiger M, Dehghani H, Schmidt FE, Delpy DT, Arridge SR.

Department of Medical Physics and Bioengineering, University College London, UK.

A 32-channel time-resolved optical imaging instrument has been developed principally to study functional parameters of the new-born infant brain. As a prelude to studies on infants, the device and image reconstruction methodology have been evaluated on the adult human forearm. Cross-sectional images were generated using time-resolved measurements of transmitted light at two wavelengths. All data were acquired using a fully automated computer-controlled protocol. Images representing the internal scattering and absorbing properties of the arm are presented, as well as images that reveal physiological changes during a simple finger flexion exercise. The results presented in this paper represent the first simultaneous tomographic reconstruction of the internal scattering and absorbing properties of a clinical subject using purely temporal data, with additional co-registered difference images showing repeatable absorption changes at two wavelengths in response to exercise.

Publication Types:

PMID: 11324955 [PubMed – indexed for MEDLINE]


Oxygen dependency and precision of cytochrome oxidase signal from full spectral NIRS of the piglet brain.

Springett R, Newman J, Cope M, Delpy DT.

Department of Medical Physics and Bioengineering, University College London, London WC1E 6JA, United Kingdom. rspringett@medphys.ucl.ac.uk

Oxidation changes of the copper A (Cu(A)) center of cytochrome oxidase in the brain were measured during brief anoxic swings at both normocapnia and hypercapnia (arterial PCO(2) approximately 55 mmHg). Hypercapnia increased total hemoglobin from 37.5 +/- 9.1 to 50.8 +/- 12.9 micromol/l (means +/- SD; n = 7), increased mean cerebral saturation (Smc(O(2))) from 65 +/- 4 to 77 +/- 3%, and oxidized Cu(A) by 0.43 +/- 0.23 micromol/l. During the onset of anoxia, there were no significant changes in the Cu(A) oxidation state until Smc(O(2)) had fallen to 43 +/- 5 and 21 +/- 6% at normocapnia and hypercapnia, respectively, and the maximum reduction during anoxia was not significantly different at hypercapnia (1.49 +/- 0.40 micromol/l) compared with normocapnia (1.53 +/- 0.44 micromol/l). Residuals of the least squares fitting algorithm used to convert near-infrared spectra to concentrations are presented and shown to be small compared with the component of attenuation attributed to the Cu(A) signal. From these observations, we conclude that there is minimal interference between the hemoglobin and Cu(A) signals in this model, the Cu(A) oxidation state is independent of cerebral oxygenation at normoxia, and the oxidation after hypercapnia is not the result of increased cerebral oxygenation.

Publication Types:

PMID: 11045954 [PubMed – indexed for MEDLINE]


Optical tomography in the presence of void regions

Dehghani H, Arridge SR, Schweiger M, Delpy DT.

Department of Medical Physics and Bioengineering, University College London, United Kingdom.

There is a growing interest in the use of near-infrared spectroscopy for the noninvasive determination of the oxygenation level within biological tissue. Stemming from this application, there has been further research in the use of this technique for obtaining tomographic images of the neonatal head, with the view of determining the levels of oxygenated and deoxygenated blood within the brain. Owing to computational complexity, methods used for numerical modeling of photon transfer within tissue have usually been limited to the diffusion approximation of the Boltzmann transport equation. The diffusion approximation, however, is not valid in regions of low scatter, such as the cerebrospinal fluid. Methods have been proposed for dealing with nonscattering regions within diffusing materials through the use of a radiosity-diffusion model. Currently, this new model assumes prior knowledge of the void region location; therefore it is instructive to examine the errors introduced in applying a simple diffusion-based reconstruction scheme in cases in which there exists a nonscattering region. We present reconstructed images of objects that contain a nonscattering region within a diffusive material. Here the forward data is calculated with the radiosity-diffusion model, and the inverse problem is solved with either the radiosity-diffusion model or the diffusion-only model. The reconstructed images show that even in the presence of only a thin nonscattering layer, a diffusion-only reconstruction will fail. When a radiosity-diffusion model is used for image reconstruction, together with a priori information about the position of the nonscattering region, the quality of the reconstructed image is considerably improved. The accuracy of the reconstructed images depends largely on the position of the anomaly with respect to the nonscattering region as well as the thickness of the nonscattering region.

PMID: 10975376 [PubMed – as supplied by publisher]

DOT diffuse optical tomography

Diffuse optical tomography (DOT) uses near infrared light to generate images or signals of the body. The technique is sensitive to the optocal absorption of some components of the body, such as oxy-hemoglobin and deoxy-hemoglobin.

see also Near infrared spectroscopy (NIRS)


I MOVED THIS BLOG FROM WORDPRESS TO BLOGGER. Ce blog est à
ex-ample.blogspot.com

Blog Stats

  • 221 318 hits

localization

Flickr Photos

août 2019
L M M J V S D
« Oct    
 1234
567891011
12131415161718
19202122232425
262728293031