Interpreting Oxygenation-Based Neuroimaging Signals: The Importance and the Challenge of Understanding Brain Oxygen Metabolism

Functional magnetic resonance imaging is widely used to map patterns of brain activation based on blood oxygenation level dependent (BOLD) signal changes associated with changes in neural activity. However, because oxygenation changes depend on the relative changes in cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2), a quantitative interpretation of BOLD signals, and also other functional neuroimaging signals related to blood or tissue oxygenation, is fundamentally limited until we better understand brain oxygen metabolism and how it is related to blood flow. However, the positive side of the complexity of oxygenation signals is that when combined with dynamic CBF measurements they potentially provide the best tool currently available for investigating the dynamics of CMRO2. This review focuses on the problem of interpreting oxygenation-based signals, the challenges involved in measuring CMRO2 in general, and what is needed to put oxygenation-based estimates of CMRO2 on a firm foundation. The importance of developing a solid theoretical framework is emphasized, both as an essential tool for analyzing oxygenation-based multimodal measurements, and also potentially as a way to better understand the physiological phenomena themselves. The existing data, integrated within a simple theoretical framework of O2 transport, suggests the hypothesis that an important functional role of the mismatch of CBF and CMRO2 changes with neural activation is to prevent a fall of tissue pO2. Future directions for better understanding brain oxygen metabolism are discussed

Evolution of the dynamic changes in functional cerebral oxidative metabolism from tissue mitochondria to blood oxygen

The dynamic properties of the cerebral metabolic rate of oxygen consumption (CMR O2 ) during changes in brain activity remain unclear. Therefore, the spatial and temporal evolution of functional increases in CMR O2 was investigated in the rat somato-sensory cortex during forelimb stimulation under a suppressed blood flow response condition. Temporally, stimulation elicited a fast increase in tissue mitochondria CMR O2 described by a time constant of B1 second measured using flavoprotein autofluorescence imaging. CMR O2 -driven changes in the tissue oxygen tension measured using an oxygen electrode and blood oxygenation measured using optical imaging of intrinsic signal followed; however, these changes were slow with time constants of B5 and B10 seconds, respectively. This slow change in CMR O2 -driven blood oxygenation partly explains the commonly observed post-stimulus blood oxygen level-dependent (BOLD) undershoot. Spatially, the changes in mitochondria CMR O2 were similar to the changes in blood oxygenation. Finally, the increases in CMR O2 were well correlated with the evoked multi-unit spiking activity. These findings show that dynamic CMR O2 calculations made using only blood oxygenation data (e.g., BOLD functional magnetic resonance imaging (fMRI)) do not directly reflect the temporal changes in the tissue's mitochondria metabolic rate; however, the findings presented can bridge the gap between the changes in cellular oxidative rate and blood oxygenation

Dual-calibrated fMRI measurement of absolute cerebral metabolic rate of oxygen consumption and effective oxygen diffusivity

Dual-calibrated fMRI is a multi-parametric technique that allows for the quantification of the resting oxygen extraction fraction (OEF), the absolute rate of cerebral metabolic oxygen consumption (CMRO2), cerebral vascular reactivity (CVR) and baseline perfusion (CBF). It combines measurements of arterial spin labelling (ASL) and blood oxygenation level dependent (BOLD) signal changes during hypercapnic and hyperoxic gas challenges. Here we propose an extension to this methodology that permits the simultaneous quantification of the effective oxygen diffusivity of the capillary network (DC). The effective oxygen diffusivity has the scope to be an informative biomarker and useful adjunct to CMRO2, potentially providing a non-invasive metric of microvascular health, which is known to be disturbed in a range of neurological diseases. We demonstrate the new method in a cohort of healthy volunteers (n = 19) both at rest and during visual stimulation. The effective oxygen diffusivity was found to be highly correlated with CMRO2 during rest and activation, consistent with previous PET observations of a strong correlation between metabolic oxygen demand and effective diffusivity. The increase in effective diffusivity during functional activation was found to be consistent with previously reported increases in capillary blood volume, supporting the notion that measured oxygen diffusivity is sensitive to microvascular physiology

A dynamic model of neurovascular coupling: implications for blood vessel dilation and constriction

Neurovascular coupling in response to stimulation of the rat barrel cortex was investigated using concurrent multichannel electrophysiology and laser Doppler flowmetry. The data were used to build a linear dynamic model relating neural activity to blood flow. Local field potential time series were subject to current source density analysis, and the time series of a layer IV sink of the barrel cortex was used as the input to the model. The model output was the time series of the changes in regional cerebral blood flow (CBF). We show that this model can provide excellent fit of the CBF responses for stimulus durations of up to 16 s. The structure of the model consisted of two coupled components representing vascular dilation and constriction. The complex temporal characteristics of the CBF time series were reproduced by the relatively simple balance of these two components. We show that the impulse response obtained under the 16-s duration stimulation condition generalised to provide a good prediction to the data from the shorter duration stimulation conditions. Furthermore, by optimising three out of the total of nine model parameters, the variability in the data can be well accounted for over a wide range of stimulus conditions. By establishing linearity, classic system analysis methods can be used to generate and explore a range of equivalent model structures (e.g., feed-forward or feedback) to guide the experimental investigation of the control of vascular dilation and constriction following stimulation. (C) 2010 Elsevier Inc. All rights reserved

Quantification of the BOLD response via blood gas modulations

This thesis is intended to contribute to a quantitative understanding of the blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI) signal in order to increase its clinical potential. Here, the vascular, neuronal and physical processes which combine to give a resulting BOLD signal are investigated using respiratory challenges. The effect of isocapnic hyperoxia on vascular responses is investigated at 7 Tesla. No significant change was found in resting-state cerebral blood flow (CBF), resting-state cerebral blood volume (CBV) and task-evoked CBF. This challenges a previously held idea that hyperoxia is vasoconstrictive. The effect of isocapnic hyperoxia on neuronal oscillations was assessed with magnetoencephalography (MEG). Whilst a significant reduction in oscillatory power is reported in the occipital lobe, the change is significantly smaller than the global reduction previously measured with hypercapnia. These findings suggest that hyperoxia is an ideal tool for calibrated BOLD fMRI. The relationship between the change in blood oxygenation and change in transverse relaxation plays a key role in calibrated BOLD fMRI. However, previous measurements have been confounded by a change in CBV. Here, the relationship was found to be sub-linear across 1.5, 3 and 7 Tesla. Previous results which suggest a supralinear relationship at 1.5/3 Tesla and a linear relationship at 7 Tesla, are attributed to the relative contribution of intravascular/extravascular signals and their dependence on field strength, blood oxygenation and echo time. Finally, a comparison of single and multiphase ASL is made at 7 Tesla, with a modified Look-locker EPI sequence presented which allows simultaneous measurement of CBF and transit time, whilst increasing the available BOLD signal. This could have important implications for hypercapnia calibrated BOLD fMRI, where choice of ASL sequence may affect the estimated change in CMRO2. Furthermore, it provides a framework for future cerebral haemodynamic studies where simultaneous measurements are required

Quantification of the BOLD response via blood gas modulations

This thesis is intended to contribute to a quantitative understanding of the blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI) signal in order to increase its clinical potential. Here, the vascular, neuronal and physical processes which combine to give a resulting BOLD signal are investigated using respiratory challenges. The effect of isocapnic hyperoxia on vascular responses is investigated at 7 Tesla. No significant change was found in resting-state cerebral blood flow (CBF), resting-state cerebral blood volume (CBV) and task-evoked CBF. This challenges a previously held idea that hyperoxia is vasoconstrictive. The effect of isocapnic hyperoxia on neuronal oscillations was assessed with magnetoencephalography (MEG). Whilst a significant reduction in oscillatory power is reported in the occipital lobe, the change is significantly smaller than the global reduction previously measured with hypercapnia. These findings suggest that hyperoxia is an ideal tool for calibrated BOLD fMRI. The relationship between the change in blood oxygenation and change in transverse relaxation plays a key role in calibrated BOLD fMRI. However, previous measurements have been confounded by a change in CBV. Here, the relationship was found to be sub-linear across 1.5, 3 and 7 Tesla. Previous results which suggest a supralinear relationship at 1.5/3 Tesla and a linear relationship at 7 Tesla, are attributed to the relative contribution of intravascular/extravascular signals and their dependence on field strength, blood oxygenation and echo time. Finally, a comparison of single and multiphase ASL is made at 7 Tesla, with a modified Look-locker EPI sequence presented which allows simultaneous measurement of CBF and transit time, whilst increasing the available BOLD signal. This could have important implications for hypercapnia calibrated BOLD fMRI, where choice of ASL sequence may affect the estimated change in CMRO2. Furthermore, it provides a framework for future cerebral haemodynamic studies where simultaneous measurements are required

BOLD signal physiology: Models and applications

The BOLD contrast mechanism has a complex relationship with functional brain activity, oxygen metabolism, and neurovascular factors. Accurate interpretation of the BOLD signal for neuroscience and clinical applications necessitates a clear understanding of the sources of BOLD contrast and its relationship to underlying physiology. This review describes the physiological components that contribute to the BOLD signal and the steady-state calibrated BOLD models that enable quantification of functional changes with a separate challenge paradigm. The principles derived from these biophysical models are then used to interpret BOLD measurements in different neurological disorders in the presence of confounding vascular factors related to disease

Microscopic Studies of Neurovascular Coupling During Epilepsy in the Mouse Brain

Les mécanismes liant l’activité neuronale au changement local du flot sanguin sont regroupés dans un ensemble nommé couplage neurovasculaire. Ce lien neurovasculaire, qui est à la base de plusieurs principes d’imagerie fonctionnelle du cerveau, est altéré par l’épilepsie. Ces dernières années, des techniques d’imagerie tel l’IRMf, IOS et la NIRS ont été utilisées pour l’étude de cette maladie, montrant une forte corrélation entre l’activité épileptique et le signal mesuré. Par contre, la plupart de ces travaux se sont concentrés sur les changements d’hémoglobine, qui peuvent être liés à des phénomènes non-linéaires et qui ne renseignent pas directement sur la quantification de l’oxygène délivré localement. Le but de cette thèse est d’investiguer l’utilisation de la microscopie avec de nouvelles sondes moléculaires permettant l’imagerie de l’oxygénation des tissus durant les évènements épileptiques dans le cortex sensori-moteur de la souris. Dans un premier temps, une méthode de mesure de la pression partielle d’oxygène (PO2) en microscopie confocale du temps de vie de phosphorescence fut développée. Ce système permet une mesure minimalement invasive du PO2 dans les tissus corticaux à haute fréquences spatiale et temporelle lorsqu’il est utilisé conjointement avec la sonde phosphorescente OxyphorG4. Les mesures réalisées durant les crises épileptiques, induites avec l’agent 4-aminopyridine (4-AP), montrent des changements significatifs de l’oxygénation tissulaire. De plus, la distribution spatio-temporelle de la chute initiale de la réserve en oxygène, à proximité du point d’injection et le long des artérioles, a été caractérisé durant ces mêmes épisodes épileptiques. Une corrélation positive entre la variation du PO2 durant cette première phase et la durée de la crise épileptique a aussi été mesurée. Cette mesure pourrait s’avérer utile dans la localisation des foyers épileptique et dans la prédiction de la durée des crises. La deuxième étude présentée dans cette thèse se concentre sur le possible rôle joué par les astrocytes, qui sont un des acteurs importants dans le couplage neurovasculaire, dans la propagation des crises épileptiques. La concentration en ions calciques libres à la base axonale des astrocytes, conjointement avec le diamètre des artérioles adjacentes a été mesuré in-vivo en simultané sur des souris durant les épisodes épileptiques. Pour la mesure du calcium, la sonde fluorescente OregonGreen BAPTA-1 AM (OGB-1) a été utilisée en imagerie du temps de demie-vie de fluorescence avec un microscope 2-photons. Les résultats montrent que l’augmentation de calcium induirait une vasodilatation à chaque ictus dans la région du foyer épileptique. Dans les régions plus éloignées, cette même mesure corrèlerait plutôt avec une vasoconstriction dans les premiers moments de la crise, suivi par une vasodilatation selon la durée de l’épisode. De plus, une augmentation lente du niveau absolu de la concentration calcique a été observée lors de longues séquences d’évènements. Cette tendance à la hausse semble induire à son tour une constriction des artérioles dans les régions adjacentes. Ces observations confirment le rôle des astrocytes dans le contrôle local de la microcirculation et suggèrent un second rôle de modulation du niveau de la concentration calcique autour de leur base axonale. Puisqu’il n’a pas été possible de mesurer le PO2 en profondeur dans le cerveau ou de pouvoir imager adéquatement les réseaux de capillaires en microscopie confocale, et suivant le développement d’une sonde sensible aux ions d’oxygène en microscopie 2-photons, il a donc été possible, dans le cadre de la dernière étude de cette thèse, d’acquérir cette mesure en profondeur durant des épisodes épileptiques. Des changements significatifs du PO2 dans les tissus et les vaisseaux ont pu être observés. La distribution spatiale de la chute initiale de ce paramètre autour des artérioles, des capillaires, des veinules et du tissu près du foyer a pu être caractérisée. Les résultats obtenus pourraient avoir des implications profondes dans notre compréhension des mécanismes de livraison de l’oxygène dans les tissus en profondeur et leur capacité à supporter le cortex adéquatement dans les situations pathologiques. Le potentiel de la microscopie dans l’étude du couplage neurovasculaire et des changements liés à des pathologies a pu être pleinement démontré par les travaux de cette thèse.----------ABSTRACT Neurovascular coupling (NVC) is the mechanism that links a transient neural activity to the corresponding increase of cerebral blood flow (CBF). It underlies the local increase in blood flow during neural activity, forms the basis of functional brain imaging and is altered in epilepsy. For the last decades, functional imaging using BOLD fMRI, IOS and fNIRS and others have been applied to epilepsy, and yielded good correlation between epileptic activity and the measured signal. However, most previous work on epilepsy focused on the measurement of hemoglobin changes which sometimes leads to non-linear phenomena and does not quantify oxygen delivery in tissue. The aim of this thesis is to study oxygen delivery using microscopy with new oxygen sensitive molecular probes during epileptic events in the mouse somatosensory cortex. First, a confocal phosphorescence lifetime microscopy system for measuring brain oxygen partial pressure (PO2) was developed. This system enabled minimally invasive measurements of oxygen partial pressure in cerebral tissue with high spatial and temporal resolution using a dendritic phosphorescent probe, Oxyphor G4. Significant changes of PO2 in tissue were found at the epileptic focus and in remote areas during 4-aminopyridine (4-AP) induced epilepsy. The spatio-temporal distribution of the “initial dip” in PO2 near the injection site and along nearby arterioles was characterized by investigating epileptic events. A positive correlation between the percent change in the PO2 signal during the “initial dip” and the duration of seizure-like activity was revealed in this work, which may help localize the epileptic focus and predict the length of seizures. Because astrocytic calcium signalling is involved in neurovascular coupling, the second study investigated the role of this pathway in epilepsy. The free calcium concentration in astrocytic endfeet and diameter of adjacent arterioles were simultaneously monitored with the calcium-sensitive indicator OGB-1 by two-photon fluorescence lifetime measurements following 4-AP injection. Our results revealed that, increases in calcium concentration induced vasodilation for each ictal event in the focus. In the remote area, increases in calcium concentration correlated with vasoconstriction at the onset of seizure and vasodilation during the later part of the seizures. Furthermore, a slow increase in absolute calcium concentration following multiple seizures was observed, which in turn, caused a trend of arteriolar constriction both at the epileptic focus and remote areas. These observations confirmed the role of astrocytes in the control of local microcirculation and suggest a modulating role for baseline absolute calcium concentration in astrocytic endfeet. Since the confocal phosphorescence microscopy system was not able to measure PO2 deep in the cortex or resolve capillaries, two-photon phosphorescence microscopy was then used in the last project to study the PO2 delivery during epilepsy in deep tissue and vessels. Significant changes of PO2 in tissue and vasculature were observed during epileptic events. The spatial landscape of “initial dip” in PO2 signals around arterioles, veins and tissue near the injection site was characterized. These results may have profound implications for evaluating microvascular oxygen delivery capacity to support cerebral tissue in disease. The results of this thesis confirmed the potential of using microscopy to study neurovascular coupling during epilepsy