Effect of respiratory CO(2) changes on the temporal dynamics of the hemodynamic response in functional MR imaging

Multi-Echo Investigations of Positive and Negative CBF and Concomitant BOLD Changes: Positive and negative CBF and BOLD changes

Unlike the positive blood oxygenation level-dependent (BOLD) response (PBR), commonly taken as an indication of an 'activated' brain region, the physiological origin of negative BOLD signal changes (i.e. a negative BOLD response, NBR), also referred to as 'deactivation' is still being debated. In this work, an attempt was made to gain a better understanding of the underlying mechanism by obtaining a comprehensive measure of the contributing cerebral blood flow (CBF) and its relationship to the NBR in the human visual cortex, in comparison to a simultaneously induced PBR in surrounding visual regions. To overcome the low signal-to-noise ratio (SNR) of CBF measurements, a newly developed multi-echo version of a center-out echo planar-imaging (EPI) readout was employed with pseudo-continuous arterial spin labeling (pCASL). It achieved very short echo and inter-echo times and facilitated a simultaneous detection of functional CBF and BOLD changes at 3 T with improved sensitivity. Evaluations of the absolute and relative changes of CBF and the effective transverse relaxation rate,R₂^* the coupling ratios, and their dependence on CBF at rest, CBF_rest indicated differences between activated and deactivated regions. Analysis of the shape of the respective functional responses also revealed faster negative responses with more pronounced post-stimulus transients. Resulting differences in the flow-metabolism coupling ratios were further examined for potential distinctions in the underlying neuronal contributions.

Characterization of Hemodynamic Alterations in Schizophrenia and Bipolar Disorder and Their Effect on Resting-State fMRI Functional Connectivity

Common and distinct neural bases of Schizophrenia (SZ) and bipolar disorder (BP) have been explored using resting-state fMRI (rs-fMRI) functional connectivity (FC). However, fMRI is an indirect measure of neural activity, which is a convolution of the hemodynamic response function (HRF) and latent neural activity. The HRF, which models neurovascular coupling, varies across the brain within and across individuals, and is altered in many psychiatric disorders. Given this background, this study had three aims: quantifying HRF aberrations in SZ and BP, measuring the impact of such HRF aberrations on FC group differences, and exploring the genetic basis of HRF aberrations. We estimated voxel-level HRFs by deconvolving rs-fMRI data obtained from SZ (N = 38), BP (N = 19), and matched healthy controls (N = 35). We identified HRF group differences (P < .05, FDR corrected) in many regions previously implicated in SZ/BP, with mediodorsal, habenular, and central lateral nuclei of the thalamus exhibiting HRF differences in all pairwise group comparisons. Thalamus seed-based FC analysis revealed that ignoring HRF variability results in false-positive and false-negative FC group differences, especially in insula, superior frontal, and lingual gyri. HRF was associated with DRD2 gene expression (P < .05, 1.62 < |Z| < 2.0), as well as with medication dose (P < .05, 1.75 < |Z| < 3.25). In this first study to report HRF aberrations in SZ and BP, we report the possible modulatory effect of dopaminergic signalling on HRF, and the impact that HRF variability can have on FC studies in clinical samples. To mitigate the impact of HRF variability on FC group differences, we suggest deconvolution during data preprocessing.

Imaging faster neural dynamics with fast fMRI: A need for updated models of the hemodynamic response

Fast fMRI enables the detection of neural dynamics over timescales of hundreds of milliseconds, suggesting it may provide a new avenue for studying subsecond neural processes in the human brain. The magnitudes of these fast fMRI dynamics are far greater than predicted by canonical models of the hemodynamic response. Several studies have established nonlinear properties of the hemodynamic response that have significant implications for fast fMRI. We first review nonlinear properties of the hemodynamic response function that may underlie fast fMRI signals. We then illustrate the breakdown of canonical hemodynamic response models in the context of fast neural dynamics. We will then argue that the canonical hemodynamic response function is not likely to reflect the BOLD response to neuronal activity driven by sparse or naturalistic stimuli or perhaps to spontaneous neuronal fluctuations in the resting state. These properties suggest that fast fMRI is capable of tracking surprisingly fast neuronal dynamics, and we discuss the neuroscientific questions that could be addressed using this approach.

BOLD fMRI and hemodynamic responses to somatosensory stimulation in anesthetized mice: spontaneous breathing vs. mechanical ventilation

Mouse functional MRI (fMRI) has been of great interest due to the abundance of transgenic models. Due to a mouse's small size, spontaneous breathing has often been used. Because the vascular physiology affecting fMRI might not be controlled normally, its effects on functional responses were investigated with optical intrinsic signal (OIS) imaging and 9.4 T BOLD fMRI. Three conditions were tested in C57BL/6 mice: spontaneous breathing under ketamine and xylazine anesthesia (KX), mechanical ventilation under KX, and mechanical ventilation under isoflurane. Spontaneous breathing under KX induced an average pCO₂ of 83 mmHg, whereas a mechanical ventilation condition achieved a pCO₂ of 37-41 mmHg within a physiological range. The baseline diameter of arterial and venous vessels was only 7%-9% larger with spontaneous breathing than with mechanical ventilation under KX, but it was much smaller than that in normocapnic isoflurane-anesthetized mice. Three major functional studies were performed. First, CBV-weighted OIS and arterial dilations to 4-second forepaw stimulation were rapid and larger at normocapnia than hypercapnia under KX, but very small under isoflurane. Second, CBV-weighted OIS and arterial dilations by vasodilator acetazolamide were measured for investigating vascular reactivity and were larger in the normocapnic condition than in the hypercapnic condition under KX. Third, evoked OIS and BOLD fMRI responses in the contralateral mouse somatosensory cortex to 20-second forepaw stimulation were faster and larger in the mechanical ventilation than spontaneous breathing. BOLD fMRI peaked at the end of the 20-second stimulation under hypercapnic spontaneous breathing, and at ~9 seconds under mechanical ventilation. The peak amplitude of BOLD fMRI was 2.2% at hypercapnia and ~3.4% at normocapnia. Overall, spontaneous breathing induces sluggish reduced hemodynamic and fMRI responses, but it is still viable for KX anesthesia due to its simplicity, noninvasiveness, and well-localized BOLD activity in the somatosensory cortex.

Differential effects of aquaporin-4 channel inhibition on BOLD fMRI and diffusion fMRI responses in mouse visual cortex

The contribution of astrocytes to the BOLD fMRI and DfMRI responses in visual cortex of mice following visual stimulation was investigated using TGN-020, an aquaporin 4 (AQP4) channel blocker, acting as an astrocyte function perturbator. Under TGN-020 injection the amplitude of the BOLD fMRI response became significantly higher. In contrast no significant changes in the DfMRI responses and the electrophysiological responses were observed. Those results further confirm the implications of astrocytes in the neurovascular coupling mechanism underlying BOLD fMRI, but not in the DfMRI responses which remained unsensitive to astrocyte function perturbation.

The Contribution of Functional Magnetic Resonance Imaging to the Understanding of the Effects of Acute Physical Exercise on Cognition

The fact that a single bout of acute physical exercise has a positive impact on cognition is well-established in the literature, but the neural correlates that underlie these cognitive improvements are not well understood. Here, the use of neuroimaging techniques, such as functional magnetic resonance imaging (fMRI), offers great potential, which is just starting to be recognized. This review aims at providing an overview of those studies that used fMRI to investigate the effects of acute physical exercises on cerebral hemodynamics and cognition. To this end, a systematic literature survey was conducted by two independent reviewers across five electronic databases. The search returned 668 studies, of which 14 studies met the inclusion criteria and were analyzed in this systematic review. Although the findings of the reviewed studies suggest that acute physical exercise (e.g., cycling) leads to profound changes in functional brain activation, the small number of available studies and the great variability in the study protocols limits the conclusions that can be drawn with certainty. In order to overcome these limitations, new, more well-designed trials are needed that (i) use a more rigorous study design, (ii) apply more sophisticated filter methods in fMRI data analysis, (iii) describe the applied processing steps of fMRI data analysis in more detail, and (iv) provide a more precise exercise prescription.

Cardiorespiratory noise correction improves the ASL signal

Cardiorespiratory fluctuations such as changes in heart rate or respiration volume influence the temporal dynamics of cerebral blood flow (CBF) measurements during arterial spin labeling (ASL) fMRI. This "physiological noise" can confound estimates of resting state network activity, and it may lower the signal-to-noise ratio of ASL during task-related experiments. In this study we examined several methods for minimizing the contributions of both synchronized and non-synchronized physiological noise in ASL measures of CBF, by combining the RETROICOR approach with different linear deconvolution models. We evaluated the amount of variance in CBF that could be explained by each method during physiological rest, in both resting state and task performance conditions. To further demonstrate the feasibility of this approach, we induced low-frequency cardiorespiratory deviations via peripheral adrenergic stimulation with isoproterenol, and determined how these fluctuations influenced CBF, before and after applying noise correction. By suppressing physiological noise, we observed substantial improvements in the signal-to-noise ratio at the individual and group activation levels. Our results suggest that variations in cardiac and respiratory parameters can account for a large proportion of the variance in resting and task-based CBF, and indicate that regressing out these non-neuronal signal variations improves the intrinsically low signal-to-noise ratio of ASL. This approach may help to better identify and control physiologically driven activations in ASL resting state and task-based analyses.

Impact of aberrant cerebral perfusion on resting-state functional MRI: A preliminary investigation of Moyamoya disease

The impact of chronic cerebral hypoperfusion on resting-state blood oxygen level-dependent signal fluctuations remains unknown. We aimed to determine whether chronic ischemia induces changes in amplitude of low-frequency fluctuations (ALFF) and to investigate the correlation between ALFF and perfusion-weighted magnetic resonance imaging (PWI) parameters in patients with moyamoya disease (MMD). Thirty patients with pre- and postoperative resting-state functional magnetic resonance imaging and PWI were included, and thirty normal controls underwent resting-state functional magnetic resonance imaging. A decrease in preoperative frontal lobe ALFF was observed in patients with MMD. Postoperative frontal lobe ALFF showed moderate improvement but still remained lower than those in normal controls. The values of mean transit time and time-to-peak, but not cerebral blood volume and cerebral blood flow, correlated significantly with frontal lobe ALFF. Moreover, there were significant negative correlations between changes in frontal lobe PWI parameters and changes in frontal lobe ALFF on both operated side and contralateral side after the unilateral revascularization surgery. Our results demonstrate that reduced ALFF are closely related to the abnormal PWI parameters and vary with the alteration of cerebral perfusion in patients with MMD.

The absolute CBF response to activation is preserved during elevated perfusion: Implications for neurovascular coupling measures

Functional magnetic resonance imaging (fMRI) techniques in which the blood oxygenation level dependent (BOLD) and cerebral blood flow (CBF) response to a neural stimulus are measured, can be used to estimate the fractional increase in the cerebral metabolic rate of oxygen consumption (CMRO2) that accompanies evoked neural activity. A measure of neurovascular coupling is obtained from the ratio of fractional CBF and CMRO2 responses, defined as n, with the implicit assumption that relative rather than absolute changes in CBF and CMRO2 adequately characterise the flow-metabolism response to neural activity. The coupling parameter n is important in terms of its effect on the BOLD response, and as potential insight into the flow-metabolism relationship in both normal and pathological brain function. In 10 healthy human subjects, BOLD and CBF responses were measured to test the effect of baseline perfusion (modulated by a hypercapnia challenge) on the coupling parameter n during graded visual stimulation. A dual-echo pulsed arterial spin labelling (PASL) sequence provided absolute quantification of CBF in baseline and active states as well as relative BOLD signal changes, which were used to estimate CMRO2 responses to the graded visual stimulus. The absolute CBF response to the visual stimuli were constant across different baseline CBF levels, meaning the fractional CBF responses were reduced at the hyperperfused baseline state. For the graded visual stimuli, values of n were significantly reduced during hypercapnia induced hyperperfusion. Assuming the evoked neural responses to the visual stimuli are the same for both baseline CBF states, this result has implications for fMRI studies that aim to measure neurovascular coupling using relative changes in CBF. The coupling parameter n is sensitive to baseline CBF, which would confound its interpretation in fMRI studies where there may be significant differences in baseline perfusion between groups. The absolute change in CBF, as opposed to the change relative to baseline, may more closely match the underlying increase in neural activity in response to a stimulus.

Investigating Human Neurovascular Coupling Using Functional Neuroimaging: A Critical Review of Dynamic Models

The mechanisms that link a transient neural activity to the corresponding increase of cerebral blood flow (CBF) are termed neurovascular coupling (NVC). They are possibly impaired at early stages of small vessel or neurodegenerative diseases. Investigation of NVC in humans has been made possible with the development of various neuroimaging techniques based on variations of local hemodynamics during neural activity. Specific dynamic models are currently used for interpreting these data that can include biophysical parameters related to NVC. After a brief review of the current knowledge about possible mechanisms acting in NVC we selected seven models with explicit integration of NVC found in the literature. All these models were described using the same procedure. We compared their physiological assumptions, mathematical formalism, and validation. In particular, we pointed out their strong differences in terms of complexity. Finally, we discussed their validity and their potential applications. These models may provide key information to investigate various aspects of NVC in human pathology.

Dependence of BOLD signal fluctuation on arterial blood CO2 and O2: Implication for resting-state functional connectivity

Blood oxygenation level dependent (BOLD) functional MRI signal is known to be modulated by the CO2 level. Typically only end-tidal CO2, rather than the arterial partial pressure of CO2 (paCO2), was measured while the arterial partial pressure of O2 (paO2) level was not controlled due to free breathing, making their contribution not separable. Especially, the influences of paO2 and paCO2 on resting-state functional connectivity are not well studied. In this study, we investigated the relationship between paCO2 and resting as well as stimulus-evoked BOLD signals under hyperoxic and hypercapnic manipulation with tight control of arterial paO2. Rats under isoflurane anesthesia were subjected to six inspired gas conditions: 47% O2 in air (Normal), adding 1%, 2% or 5% CO2, carbogen (95% O2/5% CO2), and 100% O2. Somatosensory BOLD activation was significantly increased under 100% O2, while reduced with increased paCO2 levels. However, while resting BOLD connectivity pattern expanded and bilateral correlation increased under 100% O2, the correlation coefficient between the left and right somatosensory cortex was generally not dependent on paCO2 or paO2. Interestingly, the correlation in 0.04-0.07Hz range significantly increased with CO2 levels. Intracortical electrophysiological recordings showed a similar trend as the BOLD but the neurovascular coupling varied. The results suggest that paO2 and paCO2 together rather than paCO2 alone alter the BOLD signal. The response is not purely vascular in nature but has strong neuronal origins. This should be taken into consideration when designing calibrated BOLD experiment and interpreting functional connectivity data especially in aging, under drug, or neurological disorders.

Functional imaging of cerebral perfusion

The functional imaging of perfusion enables the study of its properties such as the vasoreactivity to circulating gases, the autoregulation and the neurovascular coupling. Downstream from arterial stenosis, this imaging can estimate the vascular reserve and the risk of ischemia in order to adapt the therapeutic strategy. This method reveals the hemodynamic disorders in patients suffering from Alzheimer's disease or with arteriovenous malformations revealed by epilepsy. Functional MRI of the vasoreactivity also helps to better interpret the functional MRI activation in practice and in clinical research.

Steroid treatment in Ataxia-Telangiectasia induces alterations of functional magnetic resonance imaging during prono-supination task

BACKGROUND

Ataxia-Teleangiectasia (A-T) is a rare neurodegenerative disorder characterized by progressive cerebellar degeneration. Till few years ago only supportive care was available to improve the neurological function in A-T patients. Even though A-T remains an incurable disease, we recently demonstrated a drug dependent amelioration of neurological signs in A-T patients during a short-term treatment with oral betamethasone.

AIMS

The aim of this study is to evaluate whether the steroid induced motor performance changes in A-T are associated with functional magnetic resonance imaging (fMRI) modifications. This represents a preliminary pilot study, which requires a validation on a larger cohort of patients.

METHODS

Six A-T patients received a 10-days cycle of oral betamethasone at 0.03 mg/kg/day. fMRI studies were carried out at T0 and at the end of the cycle. The neurological evaluation was performed through the Scale for the Assessment and Rating of Ataxia (SARA) quantification. The fMRI protocol was a block design with alternating epochs of rest and prono-supination of the dominant (right) hand.

RESULTS

The voxel-based comparison showed a remarkable increase in the number of activated voxels within the motor cortex under the on-therapy condition as compared with the cortical activity under baseline condition in the 2 patients who completed the study protocol.

CONCLUSIONS

Changes in motor performance in A-T patients treated with betamethasone are coupled with an increase in the activation in relevant cortical areas, thus suggesting that in A-T patients steroid treatment could improve motor performance facilitating cortical compensatory mechanisms.

The hypnotic zolpidem increases the synchrony of BOLD signal fluctuations in widespread brain networks during a resting paradigm

Networks of brain regions having synchronized fluctuations of the blood oxygen level-dependent functional magnetic resonance imaging (BOLD fMRI) time-series at rest, or "resting state networks" (RSNs), are emerging as a basis for understanding intrinsic brain activity. RSNs are topographically consistent with activity-related networks subserving sensory, motor, and cognitive processes, and studying their spontaneous fluctuations following acute drug challenge may provide a way to understand better the neuroanatomical substrates of drug action. The present within-subject double-blind study used BOLD fMRI at 3T to investigate the functional networks influenced by the non-benzodiazepine hypnotic zolpidem (Ambien). Zolpidem is a positive modulator of γ-aminobutyric acid(A) (GABA(A)) receptors, and engenders sedative effects that may be explained in part by how it modulates intrinsic brain activity. Healthy participants (n=12) underwent fMRI scanning 45 min after acute oral administration of zolpidem (0, 5, 10, or 20mg), and changes in BOLD signal were measured while participants gazed at a static fixation point (i.e., at rest). Data were analyzed using group independent component analysis (ICA) with dual regression and results indicated that compared to placebo, the highest dose of zolpidem increased functional connectivity within a number of sensory, motor, and limbic networks. These results are consistent with previous studies showing an increase in functional connectivity at rest following administration of the positive GABA(A) receptor modulators midazolam and alcohol, and suggest that investigating how zolpidem modulates intrinsic brain activity may have implications for understanding the etiology of its powerful sedative effects.

End-tidal CO2: an important parameter for a correct interpretation in functional brain studies using speech tasks

The aim was to investigate the effect of different speech tasks, i.e. recitation of prose (PR), alliteration (AR) and hexameter (HR) verses and a control task (mental arithmetic (MA) with voicing of the result on end-tidal CO2 (PETCO2), cerebral hemodynamics and oxygenation. CO2 levels in the blood are known to strongly affect cerebral blood flow. Speech changes breathing pattern and may affect CO2 levels. Measurements were performed on 24 healthy adult volunteers during the performance of the 4 tasks. Tissue oxygen saturation (StO2) and absolute concentrations of oxyhemoglobin ([O2Hb]), deoxyhemoglobin ([HHb]) and total hemoglobin ([tHb]) were measured by functional near-infrared spectroscopy (fNIRS) and PETCO2 by a gas analyzer. Statistical analysis was applied to the difference between baseline before the task, 2 recitation and 5 baseline periods after the task. The 2 brain hemispheres and 4 tasks were tested separately. A significant decrease in PETCO2 was found during all 4 tasks with the smallest decrease during the MA task. During the recitation tasks (PR, AR and HR) a statistically significant (p<0.05) decrease occurred for StO2 during PR and AR in the right prefrontal cortex (PFC) and during AR and HR in the left PFC. [O2Hb] decreased significantly during PR, AR and HR in both hemispheres. [HHb] increased significantly during the AR task in the right PFC. [tHb] decreased significantly during HR in the right PFC and during PR, AR and HR in the left PFC. During the MA task, StO2 increased and [HHb] decreased significantly during the MA task. We conclude that changes in breathing (hyperventilation) during the tasks led to lower CO2 pressure in the blood (hypocapnia), predominantly responsible for the measured changes in cerebral hemodynamics and oxygenation. In conclusion, our findings demonstrate that PETCO2 should be monitored during functional brain studies investigating speech using neuroimaging modalities, such as fNIRS, fMRI to ensure a correct interpretation of changes in hemodynamics and oxygenation.

Biophysical and physiological origins of blood oxygenation level-dependent fMRI signals

After its discovery in 1990, blood oxygenation level-dependent (BOLD) contrast in functional magnetic resonance imaging (fMRI) has been widely used to map brain activation in humans and animals. Since fMRI relies on signal changes induced by neural activity, its signal source can be complex and is also dependent on imaging parameters and techniques. In this review, we identify and describe the origins of BOLD fMRI signals, including the topics of (1) effects of spin density, volume fraction, inflow, perfusion, and susceptibility as potential contributors to BOLD fMRI, (2) intravascular and extravascular contributions to conventional gradient-echo and spin-echo BOLD fMRI, (3) spatial specificity of hemodynamic-based fMRI related to vascular architecture and intrinsic hemodynamic responses, (4) BOLD signal contributions from functional changes in cerebral blood flow (CBF), cerebral blood volume (CBV), and cerebral metabolic rate of O(2) utilization (CMRO(2)), (5) dynamic responses of BOLD, CBF, CMRO(2), and arterial and venous CBV, (6) potential sources of initial BOLD dips, poststimulus BOLD undershoots, and prolonged negative BOLD fMRI signals, (7) dependence of stimulus-evoked BOLD signals on baseline physiology, and (8) basis of resting-state BOLD fluctuations. These discussions are highly relevant to interpreting BOLD fMRI signals as physiological means.

Multi-echo acquisition

The rapid development of fMRI was paralleled early on by the adaptation of MR spectroscopic imaging (MRSI) methods to quantify water relaxation changes during brain activation. This review describes the evolution of multi-echo acquisition from high-speed MRSI to multi-echo EPI and beyond. It highlights milestones in the development of multi-echo acquisition methods, such as the discovery of considerable gains in fMRI sensitivity when combining echo images, advances in quantification of the BOLD effect using analytical biophysical modeling and interleaved multi-region shimming. The review conveys the insight gained from combining fMRI and MRSI methods and concludes with recent trends in ultra-fast fMRI, which will significantly increase temporal resolution of multi-echo acquisition.

Individual variability in the shape and amplitude of the BOLD-HRF correlates with endogenous GABAergic inhibition

It has previously been demonstrated that there is a negative correlation between the amplitude of the BOLD response and resting γ amino-butyric acid (GABA) concentration in visual cortex. The work here is the first to empirically characterize individual variability in the haemodynamic response functions (HRFs) in response to a simple visual stimulus and baseline GABA concentration in a population of young adult males (n = 15, aged 20-28 years). The results demonstrate that GABA concentration is negatively correlated with BOLD response amplitude (r = -0.64, P < 0.02) and positively correlated with HRF width (r = 0.67, P < 0.002), that is, individuals with higher resting GABA concentration tend to exhibit smaller and wider HRFs. No correlations were observed with resting cerebral blood flow and GABA concentration and similarly, no correlations were observed between GABA and the proportional tissue content of the MRS voxel. We argue that correlation of the height of the HRF is supportive of the view that the previously observed correlations between BOLD amplitudes and GABA are reflective of differences in neuronal activity. However, the changes in HRF shape in individuals with higher baseline GABA levels are suggestive that differing vascular response characteristics may also make a significant contribution. Our results reinforce the view that variability in endogenous factors, such as neurotransmitter concentration, can have a profound effect on the vascular haemodynamic response. This has important implications for between-cohort fMRI studies in which variation in parameters such as GABA concentration may lead to group differences in the BOLD signal.

Assessment of brain responses to innocuous and noxious electrical forepaw stimulation in mice using BOLD fMRI

Functional magnetic resonance imaging (fMRI) using the blood oxygen level-dependent (BOLD) contrast was used to study sensory processing in the brain of isoflurane-anesthetized mice. The use of a cryogenic surface coil in a small animal 9.4T system provided the sensitivity required for detection and quantitative analysis of hemodynamic changes caused by neural activity in the mouse brain in response to electrical forepaw stimulation at different amplitudes. A gradient echo-echo planar imaging (GE-EPI) sequence was used to acquire five coronal brain slices of 0.5mm thickness. BOLD signal changes were observed in primary and secondary somatosensory cortices, the thalamus and the insular cortex, important regions involved in sensory and nociceptive processing. Activation was observed consistently bilateral despite unilateral stimulation of the forepaw. The temporal BOLD profile was segregated into two signal components with different temporal characteristics. The maximum BOLD amplitude of both signal components correlated strongly with the stimulation amplitude. Analysis of the dynamic behavior of the somatosensory 'fast' BOLD component revealed a decreasing signal decay rate constant k(off) with increasing maximum BOLD amplitude (and stimulation amplitude). This study demonstrates the feasibility of a robust BOLD fMRI protocol to study nociceptive processing in isoflurane-anesthetized mice. The reliability of the method allows for detailed analysis of the temporal BOLD profile and for investigation of somatosensory and noxious signal processing in the brain, which is attractive for characterizing genetically engineered mouse models.

Caffeine increases the linearity of the visual BOLD response

Although the blood oxygenation level dependent (BOLD) signal used in most functional magnetic resonance imaging (fMRI) studies has been shown to exhibit nonlinear characteristics, most analyses assume that the BOLD signal responds in a linear fashion to stimulus. This assumption of linearity can lead to errors in the estimation of the BOLD response, especially for rapid event-related fMRI studies. In this study, we used a rapid event-related design and Volterra kernel analysis to assess the effect of a 200 mg oral dose of caffeine on the linearity of the visual BOLD response. The caffeine dose significantly (p<0.02) increased the linearity of the BOLD response in a sample of 11 healthy volunteers studied on a 3 T MRI system. In addition, the agreement between nonlinear and linear estimates of the hemodynamic response function was significantly increased (p=0.013) with the caffeine dose. These findings indicate that differences in caffeine usage should be considered as a potential source of bias in the analysis of rapid event-related fMRI studies.

Caffeine reduces resting-state BOLD functional connectivity in the motor cortex

In resting-state functional magnetic resonance imaging (fMRI), correlations between spontaneous low-frequency fluctuations in the blood oxygenation level dependent (BOLD) signal are used to assess functional connectivity between different brain regions. Changes in resting-state BOLD connectivity measures are typically interpreted as changes in coherent neural activity across spatially distinct brain regions. However, this interpretation can be complicated by the complex dependence of the BOLD signal on both neural and vascular factors. For example, prior studies have shown that vasoactive agents that alter baseline cerebral blood flow, such as caffeine and carbon dioxide, can significantly alter the amplitude and dynamics of the task-related BOLD response. In this study, we examined the effect of caffeine (200 mg dose) on resting-state BOLD connectivity in the motor cortex across a sample of healthy young subjects (N=9). We found that caffeine significantly (p<0.05) reduced measures of resting-state BOLD connectivity in the motor cortex. Baseline cerebral blood flow and spectral energy in the low-frequency BOLD fluctuations were also significantly decreased by caffeine. These results suggest that caffeine usage should be carefully considered in the design and interpretation of resting-state BOLD fMRI studies.

Quantitative evaluation of hemodynamic response after hypercapnia among different brain territories by fMRI

The brain vascular system has an autoregulatory mechanism that maintains blood perfusion within normal limits at the capillary level. Partially due to its clinical importance, it is of interest to better understand the mechanisms involved in vascular regulation. Therefore, using functional magnetic resonance imaging (fMRI), we quantitatively investigated hemodynamic response characteristics of regions supplied by the main cerebral arteries, during two breath holding tests (BHT): after inspiration and after expiration. We used an auto-regressive method capable of estimating four signal parameters: onset delay, full width at half maximum (FWHM), time-to-peak and amplitude. The onset delay was significantly longer for the posterior cerebral artery (PCA) than for middle cerebral artery (MCA) and anterior arteries (ACA). FWHM and time-to-peak were larger in the ACA territory, indicating a slower blood flow in this region. Differences were also observed in the amplitude among the three areas, where MCA and PCA territories showed the smallest and the highest amplitudes, respectively. Moreover, differences were found in amplitude and onset when BHT was performed after inspiration as compared to BHT after expiration. Time-to-peak and FWHM showed no statistical differences between these two challenges. Such results are related to regional anatomical specificities and biochemical mechanisms responsible for vasodilation, such as those related to vascularity and vessel sizes.

BOLD functional MRI in disease and pharmacological studies: room for improvement?

In the past decade the use of blood oxygen level-dependent (BOLD) fMRI to investigate the effect of diseases and pharmacological agents on brain activity has increased greatly. BOLD fMRI does not measure neural activity directly, but relies on a cascade of physiological events linking neural activity to the generation of MRI signal. However, most of the disease and pharmacological studies performed so far have interpreted changes in BOLD fMRI as "brain activation," ignoring the potential confounds that can arise through drug- or disease-induced modulation of events downstream of the neural activity. This issue is especially serious in diseases (like multiple sclerosis, brain tumours and stroke) and drugs (like anaesthetics or those with a vascular action) that are known to influence these physiological events. Here we provide evidence that, to extract meaningful information on brain activity in patient and pharmacological BOLD fMRI studies, it is important to identify, characterise and possibly correct these influences that potentially confound the results. We suggest a series of experimental measures to improve the interpretability of BOLD fMRI studies. We have ranked these according to their potential information and current practical feasibility. First-line, necessary improvements consist of (1) the inclusion of one or more control tasks, and (2) the recording of physiological parameters during scanning and subsequent correction of possible between-group differences. Second-line, highly recommended important aim to make the results of a patient or drug BOLD study more interpretable and include the assessment of (1) baseline brain perfusion, (2) vascular reactivity, (3) the inclusion of stimulus-related perfusion fMRI and (4) the recording of electrophysiological responses to the stimulus of interest. Finally, third-line, desirable improvements consist of the inclusion of (1) simultaneous EEG-fMRI, (2) cerebral blood volume and (3) rate of metabolic oxygen consumption measurements and, when relevant, (4) animal studies investigating signalling between neural cells and blood vessels.

Quantitative regional cerebral blood flow MRI of animal model of attention-deficit/hyperactivity disorder

The spontaneously hypertensive rat (SHR) has been widely used as an animal model for attention-deficit/hyperactivity disorder (AD/HD), a developmental disorder that affects 3-5% of school-age children. Quantitative high-resolution (180 x 180 x 1500 microm) perfusion magnetic resonance imaging was performed to evaluate regional CBF in AD/HD rats (SHR, n=7) and control Wistar Kyoto rats (WKY, n=9) in the frontal cortex, motor cortex, sensory cortex, corpus callosum, hippocampus, thalamus, globus pallidus, caudoputamen and whole brain. The accuracy of repeated cerebral blood flow (CBF) measurements within animals in these brain regions ranged from 3% to 10% (7 repeated measures) and across animals ranged from 15% to 18% (n=7 rats), respectively, indicating highly accurate and reproducible CBF measurements. Regional CBF of the SHR were statistically different from those of the WKY rats in all structures analyzed (P<0.05) except for the caudate putamen (P=0.09) and the globus pallidus (P=0.12). Whole brain CBF of the SHR (1.5+/-0.2 ml/g/min, mean+/-S.D.) was approximately 25% higher than that of the WKY rats (1.2+/-0.2 ml/g/min), likely due to the hypertensive nature of the AD/HD rat model. Following normalization to eliminate global CBF differences, CBF in the medial prefrontal cortex, a structure thought to be the equivalent of the human dorsolateral prefrontal cortex and widely implicated in AD/HD, was found to be higher in SHR compared to WKY rats (P<0.05). The only other structure that was found to be statistically different after normalization is the corpus callosum (P<0.05). Since resting cerebral blood flow is intricately coupled to resting neural activity, these results suggest that there was abnormal resting neural activity in the medial prefrontal cortex and the corpus callosum between the control and AD/HD animals, consistent with the hyperactivity, impulsivity, inattention, and other AD/HD-like behaviors in this animal model.

Haemodynamic and neural responses to hypercapnia in the awake rat

The relationship between localized changes in brain activity and metabolism, and the blood oxygenation level-dependent (BOLD) signal used in functional magnetic resonance imaging studies is not fully understood. One source of complexity is that stimulus-elicited changes in the BOLD signal arise both from changes in oxygen consumption due to increases in activity and purely 'haemodynamic' changes such as increases in cerebral blood flow. It is well established that robust cortical haemodynamic changes can be elicited by increasing the concentration of inspired CO(2) (inducing hypercapnia) and it is widely believed that these haemodynamic changes occur without significant effects upon neural activity or cortical metabolism. Hypercapnia is therefore commonly used as a calibration condition in functional magnetic resonance imaging studies to enable estimation of oxidative metabolism from subsequent stimulus-evoked functional magnetic resonance imaging BOLD signal changes. However, there is little research that has investigated in detail the effects of hypercapnia upon all components of the haemodynamic response (changes in cerebral blood flow, volume and oxygenation) in addition to recording neural activity. In awake animals, we used optical and electrophysiological techniques to measure cortical haemodynamic and field potential responses to hypercapnia (60 s, 5% CO(2)). The main findings are that firstly, in the awake rat, the temporal structure of the haemodynamic response to hypercapnia differs from that reported previously in anaesthetized preparations in that the response is more rapid. Secondly, there is evidence that hypercapnia alters ongoing neural activity in awake rats by inducing periods of cortical desynchronization and this may be associated with changes in oxidative metabolism.

Anxiety, respiration, and cerebral blood flow: implications for functional brain imaging

Brain functional imaging methods, such as fMRI, are sensitive to changes in cerebral blood flow (CBF) that are normally associated with changes in regional neural activation. However, other endogenous and exogenous factors can alter CBF independently of brain neural activity, thus complicating the interpretation of functional imaging data. The presence of an anxiety disorder, as well as change in state anxiety, is often accompanied by respiratory alterations that affect arterial CO(2) tensions and produce significant changes in CBF that are independent of task-related neural activation. Therefore, the effects of trait and state anxiety need to be given close consideration in interpreting functional imaging findings. In this paper, we review the dependence of most brain functional imaging methods on localized changes in CBF and the potentially confounding effects of anxiety-related alterations of respiration on interpreting patterns of functional activation. Approaches for addressing these effects are discussed.

Sources of systematic bias in hypercapnia-calibrated functional MRI estimation of oxygen metabolism

The change in cerebral rate of oxidative metabolism (CMR(O(2))) during neural activation may be estimated from blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) and arterial spin-labeling (ASL) fMRI measurements. The established method relies on an epoch of iso-metabolic blood flow increase, typically induced by CO2 breathing, to calibrate the BOLD-CBF relationship at resting-state CMR(O2). Here, we discuss the systematic bias in CMR(O2)-CBF data that can be introduced depending on the value derived for the calibration constant (M) from the CO2 breathing epoch. We demonstrate that the fidelity of BOLD-CBF data acquired during the neural activation task have low impact on the tightness of CMR(O2)-CBF coupling, as well as the coupling slope, when the derived calibration value is of a relatively moderate amplitude (M in the range of, or greater than, 10-15 at 1.5 T). Via the standard reformulation of a grid in BOLD-CBF space into the CMR(O2)-CBF plane, we demonstrate the non-linear transformation that takes place and the sources of systematic bias that result. We find that the outcome of a neurovascular coupling study may be predicted to a large extent purely from the value of the calibration constant, M, that is used. Our results suggest that the accurate determination of M is of greater importance than thought previously and indicate that BOLD-CBF data must always be supplied when considering CMR(O2)-CBF behavior in a particular brain region.

EEG–fMRI of idiopathic and secondarily generalized epilepsies

We used simultaneous EEG and functional MRI (EEG-fMRI) to study generalized spike wave activity (GSW) in idiopathic and secondary generalized epilepsy (SGE). Recent studies have demonstrated thalamic and cortical fMRI signal changes in association with GSW in idiopathic generalized epilepsy (IGE). We report on a large cohort of patients that included both IGE and SGE, and give a functional interpretation of our findings. Forty-six patients with GSW were studied with EEG-fMRI; 30 with IGE and 16 with SGE. GSW-related BOLD signal changes were seen in 25 of 36 individual patients who had GSW during EEG-fMRI. This was seen in thalamus (60%) and symmetrically in frontal cortex (92%), parietal cortex (76%), and posterior cingulate cortex/precuneus (80%). Thalamic BOLD changes were predominantly positive and cortical changes predominantly negative. Group analysis showed a negative BOLD response in the cortex in the IGE group and to a lesser extent a positive response in thalamus. Thalamic activation was consistent with its known role in GSW, and its detection in individual cases with EEG-fMRI may in part be related to the number and duration of GSW epochs recorded. The spatial distribution of the cortical fMRI response to GSW in both IGE and SGE involved areas of association cortex that are most active during conscious rest. Reduction of activity in these regions during GSW is consistent with the clinical manifestation of absence seizures.

Caffeine reduces the initial dip in the visual BOLD response at 3 T

Localized changes in oxygen consumption related to increased neural activity can result in a small and transient "initial dip" of the blood oxygenation level-dependent (BOLD) signal used in functional magnetic resonance imaging (fMRI). The initial dip has been of great interest to the fMRI community because it may provide a more accurate and localized measure of neural activity than the conventional BOLD signal increase. Although potentially useful as a technique for human brain mapping, the initial dip is not always detected and has been a source of some controversy. In this study, the BOLD response to a 4-s long visual stimulus was measured with a 3-T MRI system in 5 healthy volunteers both before and immediately after a 200-mg oral caffeine dose. The caffeine dose significantly (P < 0.001) reduced or eliminated the initial dip in all subjects. These findings suggest that caffeine usage may be a key factor in the detection of the initial dip in human fMRI studies.

Hypocapnia related changes in pain-induced brain activation as measured by functional MRI

Stress, acute pain and chronic pain may often result in hyperventilation (HV) which produces hypocapnia. The aim of this fMRI-study was to investigate the influence of hypocapnia on cortical activation during noxious stimulation in 14 healthy volunteers. The intensity of voluntary HV was controlled by capnometry Three tasks were performed in the fMRI sessions: (I) three 3-min HV periods with 7-min periods of recovery in between; (II) mechanically induced phasic pain stimulation--pain task (PT); (III) tapping--motor task (MT). The last two of these protocols were performed under normocapnic and hypocapnic conditions. HV decreased the fMRI signal by 3-7% in all regions of the cortex and subcortical nuclei. This decrease was most prominent in the opercular, frontal and temporal areas. When the PT was performed during hypocapnia a strong reduction in cluster sizes and lower t-values in S1 and insular cortex were found. In contrast MT was accompanied by an increase in cluster sizes and higher t-values. From this we conclude that hypocapnia significantly influences the BOLD signal in nociceptive and motor systems, indicating that either the coupling between the BOLD effect and neuronal processing changed or that the activity in the cortical network which represents the pain processing is decreased. These effects should be considered for functional brain imaging studies on the nociceptive system.

The role of fMRI in drug discovery

Pharmacological functional (phMRI) studies are making a significant contribution to our understanding of drug-effects on brain systems. Pharmacological fMRI has an additional contribution to make in the translation of disease models and candidate compounds from preclinical to clinical investigation and in the early clinical stages of drug development. Here it can demonstrate a proof-of-concept of drug action in a small human cohort and thus contribute substantially to decision-making in drug development. We review the methods underlying pharmacological fMRI studies and the links that can be made between animal and human investigations. We discuss the potential fMRI markers of drug effect, experimental designs and caveats in interpreting hemodynamic fMRI data as reflective of changes in neuronal activity. Although there are no current published examples of fMRI applied to novel compounds, we illustrate the potential of fMRI across a range of applications and with specific reference to processing of pain in the human brain and pharmacological analgesia. Pharmacological fMRI is developing to meet the neuroscientific challenges. Electrophysiological methods can be used to corroborate the drug effects measured hemodynamically with fMRI. In future, pharmacological fMRI is likely to extend to examinations of the spinal cord and into pharmacogenetics to relate genetic polymorphisms to differential responses of the brain to drugs.

An arteriolar compliance model of the cerebral blood flow response to neural stimulus

Although functional magnetic resonance imaging (fMRI) is a widely used and powerful tool for studying brain function, the quantitative interpretation of fMRI measurements for basic neuroscience and clinical studies can be complicated by inter-subject and inter-session variability arising from modulation of the baseline vascular state by disease, aging, diet, and pharmacological agents. In particular, recent studies have shown that the temporal dynamics of the cerebral blood flow (CBF) and the blood oxygenation level dependent (BOLD) responses to stimulus are modulated by changes in baseline CBF induced by various vasoactive agents and by decreases in vascular compliance associated with aging. These effects are not readily explained using current models of the CBF and BOLD responses. We present here a second-order nonlinear feedback model of the evoked CBF response in which neural activity modulates the compliance of arteriolar smooth muscle. Within this model framework, the baseline vascular state affects the dynamic response by changing the relative contributions of an active smooth muscle component and a passive connective tissue component to the overall vessel compliance. Baseline dependencies of the BOLD signal are studied by coupling the arteriolar compliance model with a previously described balloon model of the venous compartment. Numerical simulations show that the proposed model describes to first order the observed dependence of CBF and BOLD responses on the baseline vascular state.

Effects of hypoxia, hyperoxia, and hypercapnia on baseline and stimulus-evoked BOLD, CBF, and CMRO2 in spontaneously breathing animals

Functional magnetic resonance imaging (fMRI) was used to investigate the effects of inspired hypoxic, hyperoxic, and hypercapnic gases on baseline and stimulus-evoked changes in blood oxygenation level-dependent (BOLD) signals, cerebral blood flow (CBF), and the cerebral metabolic rate of oxygen (CMRO2) in spontaneously breathing rats under isoflurane anesthesia. Each animal was subjected to a baseline period of six inspired gas conditions (9% O2, 12% O2, 21% O2, 100% O2, 5% CO2, and 10% CO2) followed by a superimposed period of forepaw stimulation. Significant stimulus-evoked fMRI responses were found in the primary somatosensory cortices. Relative fMRI responses to forepaw stimulation varied across gas conditions and were dependent on baseline physiology, whereas absolute fMRI responses were similar across moderate gas conditions (12% O2, 21% O2 100% O2, and 5% CO2) and were relatively independent of baseline physiology. Consistent with data obtained using well-established techniques, baseline and stimulus-evoked CMRO2 were invariant across moderate physiological perturbations thereby supporting a CMRO2-fMRI technique for non-invasive CMRO2 measurement. However, under 9% O2 and 10% CO2, stimulus-evoked CBF and BOLD were substantially reduced and the CMRO2 formalism appeared invalid, likely due to attenuated neurovascular coupling and/or a failure of the model under extreme physiological perturbations. These findings demonstrate that absolute fMRI measurements help distinguish neural from non-neural contributions to the fMRI signals and may lend a more accurate measure of brain activity during states of altered basal physiology. Moreover, since numerous pharmacologic agents, pathophysiological states, and psychiatric conditions alter baseline physiology independent of neural activity, these results have implications for neuroimaging studies using relative fMRI changes to map brain activity.

Caffeine alters the temporal dynamics of the visual BOLD response

The blood oxygenation level-dependent (BOLD) responses to visual stimuli, using both a 1-s long single trial stimulus and a 20-s long block stimulus, were measured in a 4-T magnetic field both before and immediately after a 200-mg caffeine dose. In addition, resting levels of cerebral blood flow (CBF) were measured using arterial spin labeling. For the single trial stimulus, the caffeine dose significantly (p<0.05) reduced the time to peak (TTP), the time after the peak at which the response returned to 50% of the peak amplitude (TA50), and the amplitude of the poststimulus undershoot in all subjects (N=5). Other parameters, such as the full-width half-maximum (FWHM) and the peak amplitude, also showed significant changes in the majority of subjects. For the block stimulus, the TTP, TA50, and the time for the response to reach 50% of the peak amplitude (T50) were significantly reduced. In some subjects, oscillations were observed in the poststimulus portion of the response with median peak periods of 9.1 and 9.5 s for the single trial and block responses, respectively. Resting CBF was reduced by an average of 24%. The reproducibility of the results was verified in one subject who was scanned on 3 different days. The dynamic changes are similar to those previously reported for baseline CBF reductions induced by hypocapnia and hyperoxia.

Echo-planar BOLD fMRI of mice on a narrow-bore 9.4 T magnet

The feasibility of BOLD fMRI in association with electrical somatosensory stimulation on spontaneously breathing, isoflurane-anesthetized mice was investigated using spin-echo, echo-planar imaging (EPI) on a vertical narrow-bore 9.4 T magnet. Three experiments were performed to derive an optimal fMRI protocol. In Experiment 1 (n = 9), spin-echo BOLD responses to 10% CO2 challenge under graded isoflurane (0.25-1.25%) ranged from 10 +/- 2% to 3.5 +/- 0.9%; the optimal BOLD contrast-to-noise ratio peaked at 0.75% isoflurane. In Experiment 2 (n = 6), hindpaw somatosensory stimulations using 1-7 mA under 0.75% isoflurane revealed the optimal BOLD response was at 6 mA. In Experiment 3 (n = 5), BOLD responses to 4 and 6 mA stimulation under 0.75% and 1% isoflurane were evaluated in detail, confirming the optimal conditions in Experiment 2. These results demonstrated that BOLD fMRI using single-shot, spin-echo EPI in a mouse somatosensory stimulation model could be routinely performed on high-field, vertical, narrow-bore magnets. This protocol might prove useful for fMRI studies of transgenic mice.

Imaging oxygen consumption in forepaw somatosensory stimulation in rats under isoflurane anesthesia

The cerebral metabolic rate of oxygen (CMRO2) was dynamically evaluated on a pixel-by-pixel basis in isoflurane-anesthetized and spontaneously breathing rats following graded electrical somatosensory forepaw stimulations (4, 6, and 8 mA). In contrast to alpha-chloralose, which is the most widely used anesthetic in forepaw-stimulation fMRI studies of rats under mechanical ventilation, isoflurane (1.1-1.2%) provided a stable anesthesia level over a prolonged period, without the need to adjust the ventilation volume/rate or sample blood gases. Combined cerebral blood flow signals (CBF) and blood oxygenation level-dependent (BOLD) fMRI signals were simultaneously measured with the use of a multislice continuous arterial spin labeling (CASL) technique (two-coil setup). CMRO2 was calculated using the biophysical BOLD model of Ogawa et al. (Proc Natl Acad Sci USA 1992;89:5951-5955). The stimulus-evoked BOLD percent changes at 4, 6, and 8 A were, respectively, 0.5% +/- 0.2%, 1.4% +/- 0.3%, and 2.0% +/- 0.3% (mean +/- SD, N = 6). The CBF percent changes were 23% +/- 6%, 58% +/- 9%, and 87% +/- 14%. The CMRO2 percent changes were 14% +/- 4%, 24% +/- 6%, and 43% +/- 11%. BOLD, CBF, and CMRO2 activations were localized to the forepaw somatosensory cortices without evidence of plateau for oxygen consumption, indicative of partial coupling of CBF and CMRO2. This study describes a useful forepaw-stimulation model for fMRI, and demonstrate that CMRO2 changes can be dynamically imaged on a pixel-by-pixel basis in a single setting with high spatiotemporal resolution.

Haemodynamic responses to sensory stimulation are enhanced following acute cocaine administration

Cocaine enhances neural activity in response to sensory stimulation, an effect that may play a role in the development of drug craving. However, cocaine-induced sensory enhancement may be difficult to study in humans using neuroimaging if the global increases in baseline haemodynamic parameters, which cocaine produces, interfere with the ability of enhanced sensory-related neural activity to lead to enhanced haemodynamic responses. To investigate the effect of cocaine-induced baseline haemodynamic changes on sensory-related haemodynamic (and electrophysiological) responses, field potential (FP) and haemodynamic responses (obtained using optical imaging spectroscopy and laser-Doppler flowmetry) in the barrel cortex of the anaesthetised rat were measured during mechanical whisker stimulation following cocaine (0.5 mg/kg) or saline administration. During cocaine infusion, the relationship between blood flow and volume transiently decoupled. Following this, cocaine caused large baseline increases in blood flow (133%) and volume (33%), which peaked after approximately 6 min and approached normal levels again after 25 min. During the peak baseline increases, FP responses to whisker stimulation were similar to saline whereas several haemodynamic response parameters were slightly reduced. After the peak, significant increases in FP responses were observed, accompanied by significantly enhanced haemodynamic responses, even though the haemodynamic baselines remained elevated. Hence, the haemodynamic response to sensory stimulation is transiently reduced in the presence of large increases in baseline but, after the baseline peak, enhanced neural responses are faithfully accompanied by enhanced haemodynamic responses. The findings suggest that any cocaine-induced enhancement of sensory-related neural activity in humans is likely to be detectable by neuroimaging.

Resting fluctuations in arterial carbon dioxide induce significant low frequency variations in BOLD signal

Carbon dioxide is a potent cerebral vasodilator. We have identified a significant source of low-frequency variation in blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) signal at 3 T arising from spontaneous fluctuations in arterial carbon dioxide level in volunteers at rest. Fluctuations in the partial pressure of end-tidal carbon dioxide (Pet(CO(2))) of +/-1.1 mm Hg in the frequency range 0-0.05 Hz were observed in a cohort of nine volunteers. Correlating with these fluctuations were significant generalized grey and white matter BOLD signal fluctuations. We observed a mean (+/-standard error) regression coefficient across the group of 0.110 +/- 0.033% BOLD signal change per mm Hg CO(2) for grey matter and 0.049 +/- 0.022% per mm Hg in white matter. Pet(CO(2))-related BOLD signal fluctuations showed regional differences across the grey matter, suggesting variability of the responsiveness to carbon dioxide at rest. Functional magnetic resonance imaging (fMRI) results were corroborated by transcranial Doppler (TCD) ultrasound measurements of the middle cerebral artery (MCA) blood velocity in a cohort of four volunteers. Significant Pet(CO(2))-correlated fluctuations in MCA blood velocity were observed with a lag of 6.3 +/- 1.2 s (mean +/- standard error) with respect to Pet(CO(2)) changes. This haemodynamic lag was adopted in the analysis of the BOLD signal. Doppler ultrasound suggests that a component of low-frequency BOLD signal fluctuations is mediated by CO(2)-induced changes in cerebral blood flow (CBF). These fluctuations are a source of physiological noise and a potentially important confounding factor in fMRI paradigms that modify breathing. However, they can also be used for mapping regional vascular responsiveness to CO(2).

BOLD-contrast functional MRI signal changes related to intermittent rhythmic delta activity in EEG during voluntary hyperventilation-simultaneous EEG and fMRI study

Differences in the blood oxygen level dependent (BOLD) signal changes were studied during voluntary hyperventilation (HV) between young healthy volunteer groups, (1) with intermittent rhythmic delta activity (IRDA) (N = 4) and (2) controls (N = 4) with only diffuse arrhythmic slowing in EEG (normal response). Subjects hyperventilated (3 min) during an 8-min functional MRI in a 1.5-T scanner, with simultaneous recording of EEG (successful with N = 3 in both groups) and physiological parameters. IRDA power and average BOLD signal intensities (of selected brain regions) were calculated. Hypocapnia showed a tendency to be slightly lighter in the controls than in the IRDA group. IRDA power increased during the last minute of HV and ended 10-15 s after HV. The BOLD signal decreased in white and gray matter after the onset of HV and returned to the baseline within 2 min after HV. The BOLD signal in gray matter decreased approximately 30% more in subjects with IRDA than in controls, during the first 2 min of HV. This difference disappeared (in three subjects out of four) during IRDA in EEG. BOLD signal changes seem to depict changes, which precede IRDA. IRDA due to HV in healthy volunteers represent a model with a clearly defined EEG pattern and an observable BOLD signal change.

Coupling of changes in cerebral blood flow with neural activity: what must initially dip must come back up

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.

Regional cerebral blood flow and BOLD responses in conscious and anesthetized rats under basal and hypercapnic conditions: implications for functional MRI studies

Anesthetics, widely used in magnetic resonance imaging (MRI) studies to avoid movement artifacts, could have profound effects on cerebral blood flow (CBF) and cerebrovascular coupling relative to the awake condition. Quantitative CBF and tissue oxygenation (blood oxygen level-dependent [BOLD]) were measured, using the continuous arterial-spin-labeling technique with echo-planar-imaging acquisition, in awake and anesthetized (2% isoflurane) rats under basal and hypercapnic conditions. All basal blood gases were within physiologic ranges. Blood pressure, respiration, and heart rates were within physiologic ranges in the awake condition but were depressed under anesthesia (P < 0.05). Regional CBF was heterogeneous with whole-brain CBF values of 0.86 +/- 0.25 and 1.27 +/- 0.29 mL. g-1. min-1 under awake and anesthetized conditions, respectively. Surprisingly, CBF was markedly higher (20% to 70% across different brain conditions) under isoflurane-anesthetized condition compared with the awake state (P < 0.01). Hypercapnia decreased pH, and increased Pco(2) and Po(2). During 5% CO(2) challenge, under awake and anesthetized conditions, respectively, CBF increased 51 +/- 11% and 25 +/- 4%, and BOLD increased 7.3 +/- 0.7% and 5.4 +/- 0.4%. During 10% CO(2) challenge, CBF increased 158 +/- 28% and 47 +/- 11%, and BOLD increased 12.5 +/- 0.9% and 7.2 +/- 0.5%. Since CBF and BOLD responses were substantially higher under awake condition whereas blood gases were not statistically different, it was concluded that cerebrovascular reactivity was suppressed by anesthetics. This study also shows that perfusion and perfusion-based functional MRI can be performed in awake animals.

Neural responses during Valsalva maneuvers in obstructive sleep apnea syndrome

The repetitive upper airway muscle atonic episodes and cardiovascular sequelae of obstructive sleep apnea (OSA) suggest dysfunction of specific neural sites that integrate afferent airway signals with autonomic and somatic outflow. We determined neural responses to the Valsalva maneuver by using functional magnetic resonance imaging. Images were collected during a baseline and three Valsalva maneuvers in 8 drug-free OSA patients and 15 controls. Multiple cortical, midbrain, pontine, and medullary regions in both groups showed intensity changes correlated to airway pressure. In OSA subjects, the left inferior parietal cortex, superior temporal gyrus, posterior insular cortex, cerebellar cortex, fastigial nucleus, and hippocampus showed attenuated signal changes compared with controls. Enhanced responses emerged in the left lateral precentral gyrus, left anterior cingulate, and superior frontal cortex of OSA patients. The anterior cingulate, cerebellar cortex, and posterior insula exhibited altered response timing patterns between control and OSA subjects. The response patterns in OSA subjects suggest deficits in particular neural pathways that normally mediate the Valsalva maneuver and compensatory actions in other structures.

Brain responses associated with the Valsalva maneuver revealed by functional magnetic resonance imaging

The Valsalva maneuver, a test frequently used to evaluate autonomic function, recruits discrete neural sites. The time courses of neural recruitment relative to accompanying cardiovascular and breathing patterns are unknown. We examined functional magnetic resonance imaging signal changes within the brain to repeated Valsalva maneuvers and correlated these changes with physiological trends. In 12 healthy subjects (age, 30-58 yr), a series of 25 volumes (20 gradient echo echo-planar image slices per volume) was collected using a 1.5-Tesla scanner during a 60-s baseline and 90-s challenge period consisting of three Valsalva maneuvers. Regions of interest were examined for signal intensity changes over baseline and challenge conditions in cardiorespiratory-related regions. In addition, whole brain correlations between signal intensity and heart rate and airway load pressure were performed on a voxel-by-voxel basis. Significant signal changes, correlated with the time course of load pressure and heart rate, emerged within multiple areas, including the amygdala and hippocampus, insular and lateral frontal cortices, dorsal pons, dorsal medulla, lentiform nucleus, and fastigial and dentate nuclei of the cerebellum. Signal intensities peaked early in the Valsalva maneuver within the hippocampus and amygdala, later within the dorsal medulla, pons and midbrain, and deep cerebellar nuclei, and last within the lentiform nuclei and the lateral prefrontal cortex. The ventral pontine signals increased during the challenge, but not in a fashion correlated to load pressure or heart rate. Sites showing little or no correlation included the vermis and medial prefrontal cortex. These data suggest an initiating component arising in rostral brain areas, a later contribution from cerebellar nuclei, basal ganglia, and lateral prefrontal cortex, and a role for the ventral pons in mediating longer term processes.

Ethical issues in the clinical application of fMRI: factors affecting the validity and interpretation of activations

The ability of functional magnetic resonance imaging (fMRI) to localize activations in a single patient, along with the safety and widespread availability of this methodology, has lead to an increasing use of fMRI for clinical purposes such as pre-surgical planning. As methodology continues to improve and more experience with fMRI in the clinical setting is acquired, clinical functional neuroimaging will likely have an increasing influence over patient care. Therefore, ethical use of fMRI, as with other medical techniques, requires understanding the factors impacting the interpretation of the methodology. Issues affecting the validity and interpretation of clinical functional neuroimaging, including effects of altered hemodynamic response function, head motion, and structural changes in the brain, are reviewed. The distinction between correlated and necessary activation in a clinical context is discussed. Different types of statistical errors in fMRI analysis are described, along with their consequences to the patient. Finally, for the future of clinical fMRI development, the need for normative patient data, as well as standardized tasks, scan protocols, and data analyses, is discussed.