Changes in global cerebral blood flow in humans: effect on regional cerebral blood flow during a neural activation task

Clinical targeting of the cerebral oxygen cascade to improve brain oxygenation in patients with hypoxic-ischaemic brain injury after cardiac arrest

The cerebral oxygen cascade includes three key stages: (a) convective oxygen delivery representing the bulk flow of oxygen to the cerebral vascular bed; (b) diffusion of oxygen from the blood into brain tissue; and (c) cellular utilisation of oxygen for aerobic metabolism. All three stages may become dysfunctional after resuscitation from cardiac arrest and contribute to hypoxic-ischaemic brain injury (HIBI). Improving convective cerebral oxygen delivery by optimising cerebral blood flow has been widely investigated as a strategy to mitigate HIBI. However, clinical trials aimed at optimising convective oxygen delivery have yielded neutral results. Advances in the understanding of HIBI pathophysiology suggest that impairments in the stages of the oxygen cascade pertaining to oxygen diffusion and cellular utilisation of oxygen should also be considered in identifying therapeutic strategies for the clinical management of HIBI patients. Culprit mechanisms for these impairments may include a widening of the diffusion barrier due to peri-vascular oedema and mitochondrial dysfunction. An integrated approach encompassing both intra-parenchymal and non-invasive neuromonitoring techniques may aid in detecting pathophysiologic changes in the oxygen cascade and enable patient-specific management aimed at reducing the severity of HIBI.

Amygdalar functional connectivity during resting and evoked pain in youth with functional abdominal pain disorders

ABSTRACT

Pediatric functional abdominal pain disorders (FAPD) are highly prevalent, difficult to diagnose, and challenging to treat. The brain systems supporting FAPD remain poorly understood. This investigation examined the neuromechanisms of FAPD during a well-tolerated visceral pain induction task, the water load symptom provocation task (WL-SPT). Youth between the ages of 11 and 17 years participated. Functional connectivity (FC) was examined through the blood oxygenation level-dependent effect using the left and right amygdala (AMY) as seed regions. Relationships of the time courses within these seeds with voxels across the whole brain were evaluated. Arterial spin labeling was used to assess regional brain activation by examining cerebral blood flow. Increased FC between the left AMY with regions associated with nociceptive processing (eg, thalamus) and right AMY FC changes with areas associated with cognitive functioning (dorsolateral prefrontal cortex) and the default mode network (DMN; parietal lobe) were observed in youth with FAPD after the WL-SPT. These changes were related to changes in pain unpleasantness. Amygdala FC changes post-WL-SPT were also related to changes in pain intensity. Amygdala FC with the DMN in youth with FAPD also differed from healthy controls. Global cerebral blood flow changes were also noted between FAPD and healthy controls, but no significant differences in grey matter were detected either between groups or during the WL-SPT in youth with FAPD. Findings confirm youth with FAPD undergo changes in brain systems that could support the development of biomarkers to enhance understanding of the mechanisms of pain and treatment response.

Regulation of cerebral blood flow by arterial PCO2 independent of metabolic acidosis at 5050 m

KEY POINTS

We investigated the influence of arterial PCO₂ (PaCO₂ ) with and without experimentally altered pH on cerebral blood flow (CBF) regulation at sea level and with acclimatization to 5050 m. At sea level and high altitude, we assessed stepwise alterations in PaCO₂ following metabolic acidosis (via 2 days of oral acetazolamide; ACZ) with and without acute restoration of pH (via intravenous sodium bicarbonate; ACZ+HCO₃ ^- ). Total resting CBF was unchanged between trials at each altitude even though arterial pH and [HCO₃ ^- ] (i.e. buffering capacity) were effectively altered. The cerebrovascular responses to changes in arterial [H⁺ ]/pH were consistent with the altered relationship between PaCO₂ and [H⁺ ]/pH following ACZ at high altitude (i.e. leftward x-intercept shifts). Absolute cerebral blood velocity (CBV) and the sensitivity of CBV to PaCO₂ was unchanged between trials at high altitude, indicating that CBF is acutely regulated by PaCO₂ rather than arterial pH.

ABSTRACT

Alterations in acid-base balance with progressive acclimatization to high altitude have been well-established. However, how respiratory alkalosis and the resultant metabolic compensation interact to regulate cerebral blood flow (CBF) is uncertain. We addressed this via three separate experimental trials at sea level and following partial acclimatization (14 to 20 days) at 5050 m; involving: (1) resting acid-base balance (control); (2) following metabolic acidosis via 2 days of oral acetazolamide at 250 mg every 8 h (ACZ; pH: Δ -0.07 ± 0.04 and base excess: Δ -5.7 ± 1.9 mEq⋅l^-1 , trial effects: P < 0.001 and P < 0.001, respectively); and (3) after acute normalization of arterial acidosis via intravenous sodium bicarbonate (ACZ + HCO₃ ^- ; pH: Δ -0.01 ± 0.04 and base excess: Δ -1.5 ± 2.1 mEq⋅l^-1 , trial effects: P = 1.000 and P = 0.052, respectively). Within each trial, we utilized transcranial Doppler ultrasound to assess the cerebral blood velocity (CBV) response to stepwise alterations in arterial PCO₂ (PaCO₂ ), i.e. cerebrovascular CO₂ reactivity. Resting CBF (via Duplex ultrasound) was unaltered between trials within each altitude, indicating that respiratory compensation (i.e. Δ -3.4 ± 2.3 mmHg PaCO₂ , trial effect: P < 0.001) was sufficient to offset any elevations in CBF induced via the ACZ-mediated metabolic acidosis. Between trials at high altitude, we observed consistent leftward shifts in both the PaCO₂ -pH and CBV-pH responses across the CO₂ reactivity tests with experimentally reduced arterial pH via ACZ. When indexed against PaCO₂ - rather than pH - the absolute CBV and sensitivity of CBV-PaCO₂ was unchanged between trials at high altitude. Taken together, following acclimatization, CO₂ -mediated changes in cerebrovascular tone rather than arterial [H⁺ ]/pH is integral to CBF regulation at high altitude.

Cerebral and tumoral blood flow in adult gliomas: a systematic review of results from magnetic resonance imaging

Objective:

Blood flow is the rate of blood movement and relevant to numerous processes, though understudied in gliomas. The aim of this review was to pool blood flow metrics obtained from MRI modalities in adult supratentorial gliomas.

Methods:

MEDLINE, EMBASE and the Cochrane database were queried 01/01/2000–31/12/2019. Studies measuring blood flow in adult Grade II–IV supratentorial gliomas using dynamic susceptibility contrast (DSC) MRI, dynamic contrast enhanced MRI (DCE-MRI) or arterial spin labelling (ASL) were included. Absolute and relative cerebral blood flow (CBF), peritumoral blood flow and tumoral blood flow (TBF) were reported.

Results:

34 studies were included with 1415 patients and 1460 scans. The mean age was 52.4 ± 7.3 years. Most patients had glioblastoma (n = 880, 64.6%). The most common imaging modality was ASL (n = 765, 52.4%) followed by DSC (n = 538, 36.8%). Most studies were performed pre-operatively (n = 1268, 86.8%). With increasing glioma grade (II vs IV), TBF increased (70.8 vs 145.5 ml/100 g/min, p < 0.001) and CBF decreased (85.3 vs 49.6 ml/100 g/min, p < 0.001). In Grade IV gliomas, following treatment, CBF increased in ipsilateral (24.9 ± 1.2 vs 26.1 ± 0.0 ml/100 g/min, p < 0.001) and contralateral white matter (25.6 ± 0.2 vs 26.0± 0.0 ml/100 g/min, p < 0.001).

Conclusion:

Our findings demonstrate that increased mass effect from high-grade gliomas impairs blood flow within the surrounding brain that can improve with surgery.

Advances in knowledge:

This systematic review demonstrates how mass effect from brain tumours impairs blood flow in the surrounding brain parenchyma that can improve with treatment.

Anterior insular cortex plays a critical role in interoceptive attention

Accumulating evidence indicates that the anterior insular cortex (AIC) mediates interoceptive attention which refers to attention towards physiological signals arising from the body. However, the necessity of the AIC in this process has not been demonstrated. Using a novel task that directs attention toward breathing rhythm, we assessed the involvement of the AIC in interoceptive attention in healthy participants using functional magnetic resonance imaging and examined the necessity of the AIC in interoceptive attention in patients with AIC lesions. Results showed that interoceptive attention was associated with increased AIC activation, as well as enhanced coupling between the AIC and somatosensory areas along with reduced coupling between the AIC and visual sensory areas. In addition, AIC activation was predictive of individual differences in interoceptive accuracy. Importantly, AIC lesion patients showed disrupted interoceptive discrimination accuracy and sensitivity. These results provide compelling evidence that the AIC plays a critical role in interoceptive attention.

Modeling of dynamic cerebrovascular reactivity to spontaneous and externally induced CO2 fluctuations in the human brain using BOLD-fMRI

In this work, we investigate the regional characteristics of the dynamic interactions between arterial CO₂ and BOLD (dynamic cerebrovascular reactivity - dCVR) during normal breathing and hypercapnic, externally induced step CO₂ challenges. To obtain dCVR curves at each voxel, we use a custom set of basis functions based on the Laguerre and gamma basis sets. This allows us to obtain robust dCVR estimates both in larger regions of interest (ROIs), as well as in individual voxels. We also implement classification schemes to identify brain regions with similar dCVR characteristics. Our results reveal considerable variability of dCVR across different brain regions, as well as during different experimental conditions (normal breathing and hypercapnic challenges), suggesting a differential response of cerebral vasculature to spontaneous CO₂ fluctuations and larger, externally induced CO₂ changes that are possibly associated with the underlying differences in mean arterial CO₂ levels. The clustering results suggest that anatomically distinct brain regions are characterized by different dCVR curves that in some cases do not exhibit the standard, positive valued curves that have been previously reported. They also reveal a consistent set of dCVR cluster shapes for resting and forcing conditions, which exhibit different distribution patterns across brain voxels.

Application of machine learning algorithms for multiparametric MRI-based evaluation of murine colitis

Magnetic resonance imaging (MRI) allows non-invasive evaluation of inflammatory bowel disease (IBD) by assessing pathologically altered gut. Besides morphological changes, relaxation times and diffusion capacity of involved bowel segments can be obtained by MRI. The aim of this study was to assess the use of multiparametric MRI in the diagnosis of experimentally induced colitis in mice, and evaluate the diagnostic benefit of parameter combinations using machine learning. This study relied on colitis induction by Dextran Sodium Sulfate (DSS) and investigated the colon of mice in vivo as well as ex vivo. Receiver Operating Characteristics were used to calculate sensitivity, specificity, positive- and negative-predictive values (PPV and NPV) of these single values in detecting DSS-treatment as a reference condition. A Model Averaged Neural Network (avNNet) was trained on the multiparametric combination of the measured values, and its predictive capacity was compared to those of the single parameters using exact binomial tests. Within the in vivo subgroup (n = 19), the avNNet featured a sensitivity of 91.3% (95% CI: 86.6–96.0%), specificity of 92.3% (95% CI: 85.1–99.6%), PPV of 96.9% (94.0–99.9%) and NPV of 80.0% (95% CI: 69.9–90.1%), significantly outperforming all single parameters in at least 2 accuracy measures (p < 0.003) and performing significantly worse compared to none of the single values. Within the ex vivo subgroup (n = 30), the avNNet featured a sensitivity of 87.4% (95% CI: 82.6–92.2%), specificity of 82.9% (95% CI: 76.1–89.7%), PPV of 88.9% (84.3–93.5%) and NPV of 80.8% (95% CI: 73.8–87.9%), significantly outperforming all single parameters in at least 2 accuracy measures (p < 0.015), exceeded by none of the single parameters. In experimental mouse colitis, multiparametric MRI and the combination of several single measured values to an avNNet can significantly increase diagnostic accuracy compared to the single parameters alone. This pilot study will provide new avenues for the development of an MR-derived colitis score for optimized diagnosis and surveillance of inflammatory bowel disease.

Using a simulation approach to optimize time-domain diffuse correlation spectroscopy measurement on human head

Time-domain diffuse correlation spectroscopy (TD-DCS) has been recently proposed to improve detection of deep blood flow dynamics in a biological tissue, such as human brain. To obtain a high sensitive measurement, several experimental parameters such as the source-detector (SD) distance, gate opening time, and width need to be considered and optimized. We use a simulation approach to optimize these parameters based on Monte Carlo computations using a realistic human head model. Two cortical regions are investigated including the frontal and temporal lobes. SD distance ranging from 0 to 45 mm, gate opening time from 400 to 1000 ps, and gate width from 50 to 3000 ps are considered. The goal is to find out the optimal combinations of these parameters by which the higher contrast measurement on the cortical dynamics can be achieved. The simulations show that with an acceptable input power of light, the combinations of SD distance ranging from 0 to 15 mm, gate opening time at 700 to 800 ps, and gate width of 800 ps are optimal for achieving higher contrast measurement on the cortical dynamics. The simulation approach and results are helpful for the optimization of TD-DCS experimental design focused on brain functional detection.

Sex differences of human cortical blood flow and energy metabolism

Brain energy metabolism is held to reflect energy demanding processes in neuropil related to the density and activity of synapses. There is recent evidence that men have higher density of synapses in temporal cortex than women. One consequence of these differences would be different rates of cortical energy turnover and blood flow in men and women. To test the hypotheses that rates of oxygen consumption (CMRO2) and cerebral blood flow are higher in men than in women in regions of cerebral cortex, and that the differences persist with aging, we used positron emission tomography to determine cerebral blood flow and cerebral metabolic rate of oxygen as functions of age in healthy volunteers of both sexes. Cerebral metabolic rate of oxygen did not change with age for either sex and there were no differences of mean values of cerebral metabolic rate of oxygen between men and women in cerebral cortex. Women had significant decreases of cerebral blood flow as function of age in frontal and parietal lobes. Young women had significantly higher cerebral blood flow than men in frontal and temporal lobes, but these differences had disappeared at age 65. The absent sex difference of cerebral energy turnover suggests that the known differences of synaptic density between the sexes are counteracted by opposite differences of individual synaptic activity.

Lawrence K. Mapping Long-Term Functional Changes in Cerebral Blood Flow by Arterial Spin Labeling

Although arterial spin labeling (ASL) is appealing for mapping long-term changes in functional activity, inter-sessional variations in basal blood flow, arterial transit times (ATTs), and alignment errors, can result in significant false activation when comparing images from separate sessions. By taking steps to reduce these sources of noise, this study assessed the ability of ASL to detect functional CBF changes between sessions. ASL data were collected in three sessions to image ATT, resting CBF and CBF changes associated with motor activation (7 participants). Activation maps were generated using rest and task images acquired in the same session and from sessions separated by up to a month. Good agreement was found when comparing between-session activation maps to within-session activation maps with only a 16% decrease in precision (within-session: 90 ± 7%) and a 13% decrease in the Dice similarity (within-session: 0.75 ± 0.07) coefficient after a month. In addition, voxel-wise reproducibility (within-session: 4.7 ± 4.5%) and reliability (within-session: 0.89 ± 0.20) of resting grey-matter CBF decreased by less than 18% for the between-session analysis relative to within-session values. ATT variability between sessions (5.0 ± 2.7%) was roughly half the between-subject variability, indicating that its effects on longitudinal CBF were minimal. These results demonstrate that conducting voxel-wise analysis on CBF images acquired on different days is feasible with only modest loss in precision, highlighting the potential of ASL for longitudinal studies.

Effects of short-term mechanical hyperventilation on cerebral blood flow and dynamic cerebral autoregulation in critically ill patients with sepsis

In sepsis, higher PaCO2 levels are associated with impaired dynamic cerebral autoregulation (dCA), which may expose the brain to hypo- and hyperperfusion during acute fluctuations in blood pressure. We hypothesised that short-term mechanical hyperventilation would dCA in critically ill patients with sepsis. Seven mechanically ventilated septic patients were included. We assessed dCA before and after 30 min of mechanical hyperventilation. Transfer function analysis of spontaneous oscillations in transcranial Doppler-based middle cerebral artery blood flow velocity (MCAv) and invasive mean arterial blood pressure was used to assess dCA. Mechanical enhance hyperventilation reduced the median PaCO2 from 5.3 (IQR, 5.0-6.5) to 4.7 (IQR, 4.2-5.1) kPa (p < 0.05). This was associated with a reduction in the median MCAv from 57 (IQR, 33-68) to 32 (IQR, 21-40) cm sec(-1) (p < 0.05). Apart from a small increase in gain in the low frequency range (2.32 [IQR 1.80-2.41] vs. 2.59 (2.40-4.64) cm mmHg(-1) sec(-1); p < 0.05), this was not associated with any enhancement in dCA. In conclusion, cerebral CO2 vasoreactivity was found to be preserved in septic patients; nevertheless, and in contrast to our working hypothesis, short-term mechanical hyperventilation did not enhance dCA.

Brain Areas Involved in Anticipation of Clinically Relevant Pain in Low Back Pain Populations With High Levels of Pain Behavior

The purpose of this study was to identify neural correlates of pain anticipation in people with nonspecific low back pain (NSLBP) that correlated with pain-related distress and disability, thus providing evidence for mechanisms underlying pain behavior in this population. Thirty NSLBP sufferers, with either high levels of pain behavior or low levels on the basis of Waddell signs, were scanned with functional magnetic resonance imaging while a straight-leg raise (of the side deemed to cause moderate pain in the lower back) was performed. On each trial colored stimuli were presented and used to indicate when the leg definitely would be raised (green; 100% certainty), might be raised (yellow; 50% certainty), or would definitely not be raised (red; 100% certainty). In response to expected versus unexpected pain the group difference in activation between patients with high levels of pain behavior and low levels of pain behavior covaried as a function of anxiety scores in the right insula and pregenual anterior cingulate cortex and as a function of catastrophizing in prefrontal and parietal cortex and hippocampus. The results suggest NSLBP populations with the highest levels of pain-related distress are more likely to attend to and infer threat from innocuous cues, which may contribute to the maintenance of pain behavior associated with some chronic pain states.

PERSPECTIVE

This article shows a likely neural network for exacerbating pain anticipation in NSLBP contributing to high levels of pain behavior in some people. This information could potentially help clinicians and patients to understand how anticipation of pain may contribute to patient pain and disability.

Regional Cerebrovascular Responses to Hypercapnia and Hypoxia

A limited number of studies using differing imaging approaches suggest that there are regional variation in the cerebrovascular response to hypercapnia and hypoxia. However there are limitations to these studies. In particular, it is not clear if existing studies of hypoxia have fully accounted for the confounding effects of the changes in arterial PCO2 on cerebral perfusion that, if uncontrolled, will accompany the hypoxic stimulus. We determined quantitative maps of grey matter cerebral blood flow using a multi-slice pulsed arterial spin labelling MRI method at 3 T at rest, during conditions of isocapnic euoxia, hypercapnia, and mild isocapnic hypoxia. From these data, we determined grey matter cerebrovascular reactivity maps which show the spatial distribution of the responses to these interventions. Whilst, overall, cerebral perfusion increased with hypercapnia and hypoxia, hypoxia cerebrovascular reactivity maps showed very high variation both within and between individuals: most grey matter regions exhibiting a positive cerebrovascular reactivity, but some exhibiting a negative reactivity. The physiological explanation for this variation remains unclear and it is not known if these local differences will vary with state or with regional brain activity. The potential interaction between hypoxic or hypercapnic cerebrovascular changes and neurally related changes in brain perfusion is of particular interest for functional imaging studies of brain activation in which arterial blood gases are altered. We have determined the interaction between global hypoxia and hypercapnia-induced blood oxygen level-dependent (BOLD) MRI signal and local neurally related BOLD signal. Although statistically significant interactions were present, physiologically the effects were weak and, in practice, they did not change the statistical outcome related to the analysis of the neurally related signals. These data suggest that such respiratory-related confounds can be successfully accounted for in functional imaging studies.

Additive global cerebral blood flow normalization in arterial spin labeling perfusion imaging

To determine how different methods of normalizing for global cerebral blood flow (gCBF) affect image quality and sensitivity to cortical activation, pulsed arterial spin labeling (pASL) scans obtained during a visual task were normalized by either additive or multiplicative normalization of modal gCBF. Normalization by either method increased the statistical significance of cortical activation by a visual stimulus. However, image quality was superior with additive normalization, whether judged by intensity histograms or by reduced variability within gray and white matter.

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.

Effects of amisulpride on human resting cerebral perfusion

RATIONALE

Quantitative neuroimaging studies show that different neuroleptics have similar effects on resting metabolism/perfusion in the basal ganglia, but vary in their effect on the cortex, especially in the prefrontal and temporal lobes. These differences may represent signatures of the action of medication on distinctive receptor combinations.

OBJECTIVES

This study seeks to determine the effect on cerebral perfusion at rest of low-dose amisulpride, a neuroleptic with a receptor profile relatively selective to dopaminergic D2-receptors and both antidepressant and antipsychotic efficacy.

METHODS

Continuous arterial spin labelling in a placebo-controlled, double blind, crossover study at steady state of N = 20 healthy male adults.

RESULTS

Relative to placebo, amisulpride was associated with extensive and significant cortical decrements in resting perfusion levels, particularly in the prefrontal lobes (p = 0.01, corrected). Decrements spared the basal ganglia, where perfusion was slightly increased.

CONCLUSIONS

In contrast to earlier reports on other neuroleptics, amisulpride was associated with intense cortical perfusion decrements at rest. These results are consistent with an existing model in which dopaminergic blockade is associated not only with metabolism/perfusion increases in the basal ganglia, but also with decreases in the cerebral cortex that in most neuroleptics are compensated by action on other receptor systems. The selective receptor profile of amisulpride may explain the extensive cortical decrements.

Low Residual CBF Variability in Alzheimer's Disease after Correction for CO(2) Effect

We tested the claim that inter-individual CBF variability in Alzheimer's disease (AD) is substantially reduced after correction for arterial carbon dioxide tension (PaCO(2)). Specifically, we tested whether the variability of CBF in brain of patients with AD differed significantly from brain of age-matched healthy control subjects (HC). To eliminate the CO(2)-induced variability, we developed a novel and generally applicable approach to the correction of CBF for changes of PaCO(2) and applied the method to positron emission tomographic (PET) measures of CBF in AD and HC groups of subjects. After correction for the differences of CO(2) tension, the patients with AD lost the inter-individual CBF variability that continued to characterize the HC subjects. The difference (ΔK(1)) between the blood-brain clearances (K(1)) of water (the current measure of CBF) and oxygen (the current measure of oxygen clearance) was reduced globally in AD and particularly in the parietal, occipital, and temporal lobes. We then showed that oxygen gradients calculated for brain tissue were similar in AD and HC, indicating that the low residual variability of CBF in AD may be due to low functional demands for oxidative metabolism of brain tissue rather than impaired delivery of oxygen.

Influence of 100% and 40% oxygen on penumbral blood flow, oxygen level, and T2*-weighted MRI in a rat stroke model

Accurate imaging of the ischemic penumbra is a prerequisite for acute clinical stroke research. T(2)(*) magnetic resonance imaging (MRI) combined with an oxygen challenge (OC) is being developed to detect penumbra based on changes in blood deoxyhemoglobin. However, inducing OC with 100% O(2) induces sinus artefacts on human scans and influences cerebral blood flow (CBF), which can affect T(2)(*) signal. Therefore, we investigated replacing 100% O(2) OC with 40% O(2) OC (5 minutes 40% O(2) versus 100% O(2)) and determined the effects on blood pressure (BP), CBF, tissue pO(2), and T(2)(*) signal change in presumed penumbra in a rat stroke model. Probes implanted into penumbra and contralateral cortex simultaneously recorded pO(2) and CBF during 40% O(2) (n=6) or 100% O(2) (n=8) OC. In a separate MRI study, T(2)(*) signal change to 40% O(2) (n=6) and 100% O(2) (n=5) OC was compared. Oxygen challenge (40% and 100% O(2)) increased BP by 8.2% and 18.1%, penumbra CBF by 5% and 15%, and penumbra pO(2) levels by 80% and 144%, respectively. T(2)(*) signal significantly increased by 4.56% ± 1.61% and 8.65% ± 3.66% in penumbra compared with 2.98% ± 1.56% and 2.79% ± 0.66% in contralateral cortex and 1.09% ± 0.82% and -0.32% ± 0.67% in ischemic core, respectively. For diagnostic imaging, 40% O(2) OC could provide sufficient T(2)(*) signal change to detect penumbra with limited influence in BP and CBF.

Correcting for the echo-time effect after measuring the cerebral blood flow by arterial spin labeling

PURPOSE

To take into account the echo time (TE) influence on arterial spin labeling (ASL) signal when converting it in regional cerebral blood flow (rCBF). Gray matter ASL signal decrease with increasing TE as a consequence of the difference in the apparent transverse relaxation rates between labeled water in capillaries and nonlabeled water in the tissue (δR 2*). We aimed to measure ASL/rCBF changes in different parts of the brain and correct them.

MATERIALS AND METHODS

Fifteen participants underwent ASL measurements at TEs of 9.7-30 ms. Decreases in ASL values were localized by statistical parametric mapping. The corrections assessed were a subject-per-subject adjustment, an average δR 2* value adjustment, and a two-compartment model adjustment.

RESULTS

rCBF decreases associated with increasing TEs were found for gray matter and were corrected using an average δR 2* value of 20 s(-1) . Conversely, for white matter, rCBF values increased with increasing TEs (δR 2* = -23 s(-1)).

CONCLUSION

Our correction was as good as using a two-compartment model. However, it must be done separately for the gray and white matter rCBF values because the capillary R 2* values are, respectively, larger and smaller than those of surrounding tissues.

Robustly measuring vascular reactivity differences with breath-hold: normalising stimulus-evoked and resting state BOLD fMRI data

Inter-subject differences in local cerebral blood flow (CBF) and cerebral blood volume (CBV) contribute to differences in BOLD signal reactivity and, therefore, unmodelled variance in group level fMRI analyses. A simple way of elevating blood CO(2) concentrations to characterise subject differences in vascular reactivity is through breath-holds but two aspects of this measure are often neglected: (1) breath-holds are usually modelled as blocks even though CO(2) accumulates over time and (2) increases in CO(2) differ between subjects. This study demonstrates that the BOLD breath-hold response is best modelled by convolving the end-tidal CO(2) trace with a standard haemodynamic response function and including its temporal derivative. Inclusion of the BOLD breath-hold response as a voxel-dependent covariate in a group level analysis increases the spatial extent of activation in stimulus evoked and resting state datasets. By expressing the BOLD breath-hold response as a percentage signal increase with respect to an absolute change in the partial pressure of CO(2) (expressed in mmHg), the spatial extent of stimulus-evoked activation is further improved. This demonstrates that individual end-tidal CO(2) increases to breath-hold should be accounted for to provide an accurate measure of vascular reactivity resulting in more statistically active voxels in group level analyses.

Cortico-limbic circuitry and the airways: insights from functional neuroimaging of respiratory afferents and efferents

After nearly two decades of active research, functional neuroimaging has demonstrated utility in the identification of cortical, limbic, and paralimbic (cortico-limbic) brain regions involved in respiratory control and respiratory perception. Before the recent boon of human neuroimaging studies, the location of the principal components of respiratory-related cortico-limbic circuitry had been unknown and their function had been poorly understood. Emerging neuroimaging evidence in both healthy and patient populations suggests that cognitive and emotional/affective processing within cortico-limbic circuitry modulates respiratory control and respiratory perception. This paper will review functional neuroimaging studies of respiration with a focus on whole brain investigations of sensorimotor pathways that have identified respiratory-related neural circuitry known to overlap emotional/affective cortico-limbic circuitry. To aid the interpretation of present and future findings, the complexities and challenges underlying neuroimaging methodologies will also be reviewed as applied to the study of respiration physiology.

Model-derived assessment of cerebrovascular resistance and cerebral blood flow following traumatic brain injury

The published guidelines point out the need for the development of methods that individualize patient cerebral perfusion management and minimize secondary ischemic complications associated with traumatic brain injury. A laboratory method has been developed to determine model-derived assessments of cerebrovascular resistance (mCVR) and cerebral blood flow (mCBF) from cerebrovascular pressure transmission, and the dynamic relationship between arterial blood pressure (ABP) and intracranial pressure (ICP). The aim of this two-fold study is to (1) evaluate relative changes in the model-derived parameters of mCVR and mCBF with the corresponding changes in the pial arteriolar vascular parameters of pial arteriolar resistance (PAR) and relative pial arteriolar blood flow (rPABF); and (2) examine the efficacy of the proposed modeling methodology for continuous assessment of the state of cerebrovascular regulation by evaluating relative changes in the model-derived parameters of CBF and cerebrovascular resistance in relation to changes of cerebral perfusion pressure prior to and following fluid percussion brain injury. Changes of ABP, ICP, PAR, relative arteriolar blood flow (rPABF) and the corresponding model-derived parameters of mCBF and mCVR induced by acute hypertensive challenge were evaluated before and following fluid percussion injury in piglets equipped with cranial windows. Before fluid percussion, hypertensive challenge resulted in a significant increase of PAR and mCVR, whereas both rPABF and mCBF remained constant. Following fluid percussion, hypertensive challenge resulted in a significant decrease of PAR and mCVR and consistent with impaired cerebrovascular regulation. Hypertensive challenge significantly increased both rPABF and mCBF, which approximately doubled with increased CPP with correlation values of r = 0.96 (P < 0.01) and r = 0.97 (P Collapse

Key Words

• cerebrovascular resistance
• cerebral blood flow
• traumatic brain-injury

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MESH Headings

• Animals
• Arterioles/physiology
• Arterioles/physiopathology
• Blood Pressure/physiology
• Brain Injuries/physiopathology
• Cerebrovascular Circulation/physiology
• Humans
• Intracranial Pressure/physiology
• Male
• Models, Cardiovascular
• Pia Mater/blood supply
• Regional Blood Flow
• Swine
• Vascular Resistance/physiology

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Grants

• R01 HL034059-25 NHLBI NIH HHS
• R01HL34059 NHLBI NIH HHS
• R01 HL034059 NHLBI NIH HHS
• R01 HL034059-24 NHLBI NIH HHS
• R37 HL042851-18 NHLBI NIH HHS
• R15NS051331 NINDS NIH HHS
• R37 HL042851 NHLBI NIH HHS
• R37HL042851 NHLBI NIH HHS
• R37 HL042851-17 NHLBI NIH HHS

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Affiliation(s)

Michael L. Daley Department of Electrical and Computer Engineering, The University of Memphis, Engineering Science Building, Rm. 208B, Memphis, TN 38152-3180, Phone: 901-678-3254, Fax 901-678-5469,
Nithya Narayanan Department of Electrical and Computer Engineering, The University of Memphis, Engineering Science Building, Rm. 208B, Memphis, TN 38152-3180, Phone: 901-678-4332, Fax 901-678-5469,
Charles W. Leffler Department of Physiology, The University of Tennessee Health Science Center, Memphis, TN 38163, Phone: 901-448-7122, Fax: 901-448-7126,

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Components of acquisition-to-acquisition variance in continuous arterial spin labelling (CASL) imaging

BACKGROUND

Images of perfusion estimates obtained with the continuous arterial spin labelling technique are characterized by variation between single acquisitions. Little is known about the spatial determinants of this variation during the acquisition process and their impact on voxel-by-voxel estimates of effects.

RESULTS

We show here that the spatial patterns of covariance between voxels arising during the acquisition of these images uncover distinct mechanisms through which this variance arises: through variation in global perfusion levels; through the action of large vessels and other, less well characterized, large anatomical structures; and through the effect of noisy areas such as the edges of the brain.

CONCLUSIONS

Knowledge of these covariance patterns is important to experimenters for a correct interpretation of findings, especially for studies where relatively few acquisitions are made.

Opioids depress cortical centers responsible for the volitional control of respiration

Respiratory depression limits provision of safe opioid analgesia and is the main cause of death in drug addicts. Although opioids are known to inhibit brainstem respiratory activity, their effects on cortical areas that mediate respiration are less well understood. Here, functional magnetic resonance imaging was used to examine how brainstem and cortical activity related to a short breath hold is modulated by the opioid remifentanil. We hypothesized that remifentanil would differentially depress brain areas that mediate sensory-affective components of respiration over those that mediate volitional motor control. Quantitative measures of cerebral blood flow were used to control for hypercapnia-induced changes in blood oxygen level-dependent (BOLD) signal. Awareness of respiration, reflected by an urge-to-breathe score, was profoundly reduced with remifentanil. Urge to breathe was associated with activity in the bilateral insula, frontal operculum, and secondary somatosensory cortex. Localized remifentanil-induced decreases in breath hold-related activity were observed in the left anterior insula and operculum. We also observed remifentanil-induced decreases in the BOLD response to breath holding in the left dorsolateral prefrontal cortex, anterior cingulate, the cerebellum, and periaqueductal gray, brain areas that mediate task performance. Activity in areas mediating motor control (putamen, motor cortex) and sensory-motor integration (supramarginal gyrus) were unaffected by remifentanil. Breath hold-related activity was observed in the medulla. These findings highlight the importance of higher cortical centers in providing contextual awareness of respiration that leads to appropriate modulation of respiratory control. Opioids have profound effects on the cortical centers that control breathing, which potentiates their actions in the brainstem.

Relationship between middle cerebral artery blood velocity and end-tidal PCO2 in the hypocapnic-hypercapnic range in humans

This study examined the relationship between cerebral blood flow (CBF) and end-tidal PCO2 (PETCO2) in humans. We used transcranial Doppler ultrasound to determine middle cerebral artery peak blood velocity responses to 14 levels of PETCO2 in a range of 22 to 50 Torr with a constant end-tidal PO2 (100 Torr) in eight subjects. PETCO2 and end-tidal PO2 were controlled by using the technique of dynamic end-tidal forcing combined with controlled hyperventilation. Two protocols were conducted in which PETCO2 was changed by 2 Torr every 2 min from hypocapnia to hypercapnia (protocol I) and vice-versa (protocol D). Over the range of PETCO2 studied, the sensitivity of peak blood velocity to changes in PETCO2 (CBF-PETCO2 sensitivity) was nonlinear with a greater sensitivity in hypercapnia (4.7 and 4.0%/Torr, protocols I and D, respectively) compared with hypocapnia (2.5 and 2.2%/Torr). Furthermore, there was evidence of hysteresis in the CBF-PETCO2 sensitivity; for a given PETCO2, there was greater sensitivity during protocol I compared with protocol D. In conclusion, CBF-PETCO2 sensitivity varies depending on the level of PETCO2 and the protocol that is used. The mechanisms underlying these responses require further investigation.

The impact of global signal regression on resting state correlations: are anti-correlated networks introduced?

Low-frequency fluctuations in fMRI signal have been used to map several consistent resting state networks in the brain. Using the posterior cingulate cortex as a seed region, functional connectivity analyses have found not only positive correlations in the default mode network but negative correlations in another resting state network related to attentional processes. The interpretation is that the human brain is intrinsically organized into dynamic, anti-correlated functional networks. Global variations of the BOLD signal are often considered nuisance effects and are commonly removed using a general linear model (GLM) technique. This global signal regression method has been shown to introduce negative activation measures in standard fMRI analyses. The topic of this paper is whether such a correction technique could be the cause of anti-correlated resting state networks in functional connectivity analyses. Here we show that, after global signal regression, correlation values to a seed voxel must sum to a negative value. Simulations also show that small phase differences between regions can lead to spurious negative correlation values. A combination breath holding and visual task demonstrates that the relative phase of global and local signals can affect connectivity measures and that, experimentally, global signal regression leads to bell-shaped correlation value distributions, centred on zero. Finally, analyses of negatively correlated networks in resting state data show that global signal regression is most likely the cause of anti-correlations. These results call into question the interpretation of negatively correlated regions in the brain when using global signal regression as an initial processing step.

Potential use of oxygen as a metabolic biosensor in combination with T2*-weighted MRI to define the ischemic penumbra

We describe a novel magnetic resonance imaging technique for detecting metabolism indirectly through changes in oxyhemoglobin:deoxyhemoglobin ratios and T2(*) signal change during 'oxygen challenge' (OC, 5 mins 100% O(2)). During OC, T2(*) increase reflects O(2) binding to deoxyhemoglobin, which is formed when metabolizing tissues take up oxygen. Here OC has been applied to identify tissue metabolism within the ischemic brain. Permanent middle cerebral artery occlusion was induced in rats. In series 1 scanning (n=5), diffusion-weighted imaging (DWI) was performed, followed by echo-planar T2(*) acquired during OC and perfusion-weighted imaging (PWI, arterial spin labeling). Oxygen challenge induced a T2(*) signal increase of 1.8%, 3.7%, and 0.24% in the contralateral cortex, ipsilateral cortex within the PWI/DWI mismatch zone, and ischemic core, respectively. T2(*) and apparent diffusion coefficient (ADC) map coregistration revealed that the T2(*) signal increase extended into the ADC lesion (3.4%). In series 2 (n=5), FLASH T2(*) and ADC maps coregistered with histology revealed a T2(*) signal increase of 4.9% in the histologically defined border zone (55% normal neuronal morphology, located within the ADC lesion boundary) compared with a 0.7% increase in the cortical ischemic core (92% neuronal ischemic cell change, core ADC lesion). Oxygen challenge has potential clinical utility and, by distinguishing metabolically active and inactive tissues within hypoperfused regions, could provide a more precise assessment of penumbra.

Mapping of the cerebral vascular response to hypoxia and hypercapnia using quantitative perfusion MRI at 3 T

Changes in breathing change the concentration of oxygen and carbon dioxide in arterial blood resulting in changes in cerebral blood flow (CBF). This mechanism can be described by the cerebral vascular response (CVR), which has been shown to be altered in different physiological and pathophysiological states. CBF maps of grey matter (GM) were determined with a pulsed arterial spin labelling technique at 3 T in a group of 19 subjects under baseline conditions, hypoxia, and hypercapnia. Experimental conditions allowed a change in either arterial oxygen (hypoxia) or carbon dioxide (hypercapnia) concentration compared with the baseline, leaving the other variable constant, in order to separate the effects of these two variables. From these results, maps were calculated showing the regional distribution of the CVR to hypoxia and hypercapnia in GM. Maps of CVR to hypoxia showed very high intra-subject variations, with some GM regions exhibiting a positive response and others a negative response. Per 10% decrease in arterial oxygen saturation, there was a statistically significant 7.0 +/- 2.9% (mean +/- SEM) increase in GM-CBF for the group. However, 70% of subjects showed an overall positive CVR (positive responders), and the remaining 30% an overall negative CVR (negative responders). Maps of CVR to hypercapnia showed less intra-subject variation. Per 1 mm Hg increase in partial pressure of end-tidal carbon dioxide, there was a statistically significant 5.8 +/- 0.9% increase in GM-CBF, all subjects showing an overall positive CVR. As the brain is particularly vulnerable to hypoxia, a condition associated with cardiorespiratory diseases, CVR maps may help in the clinic to identify the areas most prone to damage because of a reduced CVR.

Influence of hypercapnic vasodilation on cerebrovascular autoregulation and pial arteriolar bed resistance in piglets

Changes in both pial arteriolar resistance (PAR) and simulated arterial-arteriolar bed resistance (SimR) of a physiologically based biomechanical model of cerebrovascular pressure transmission, the dynamic relationship between arterial blood pressure and intracranial pressure, are used to test the hypothesis that hypercapnia disrupts autoregulatory reactivity. To evaluate pressure reactivity, vasopressin-induced acute hypertension was administered to normocapnic and hypercapnic (N = 12) piglets equipped with closed cranial windows. Pial arteriolar diameters were used to compute arteriolar resistance. Percent change of PAR (%DeltaPAR) and percent change of SimR (%DeltaSimR) in response to vasopressin-induced acute hypertension were computed and compared. Hypercapnia decreased cerebrovascular resistance. Indicative of active autoregulatory reactivity, vasopressin-induced hypertensive challenge resulted in an increase of both %DeltaPAR and %DeltaSimR for all normocapnic piglets. The hypercapnic piglets formed two statistically distinct populations. One-half of the hypercapnic piglets demonstrated a measured decrease of both %DeltaPAR and %DeltaSimR to pressure challenge, indicative of being pressure passive, whereas the other one-half demonstrated an increase in these percentages, indicative of active autoregulation. No other differences in measured variables were detectable between regulating and pressure-passive piglets. Changes in resistance calculated from using the model mirrored those calculated from arteriolar diameter measurements. In conclusion, vasodilation induced by hypercapnia has the potential to disrupt autoregulatory reactivity. Our physiologically based biomechanical model of cerebrovascular pressure transmission accurately estimates the changes in arteriolar resistance during conditions of active and passive cerebrovascular reactivity.

Normalization in PET group comparison studies--the importance of a valid reference region

INTRODUCTION

In positron emission tomography (PET) studies of cerebral blood flow (CBF) and metabolism, the large interindividual variation commonly is minimized by normalization to the global mean prior to statistical analysis. This approach requires that no between-group or between-state differences exist in the normalization region. Given the variability typical of global CBF and the practical limit on sample size, small group differences in global mean easily elude detection, but still bias the comparison, with profound consequences for the physiological interpretation of the results.

MATERIALS AND METHODS

Quantitative [15O]H2O PET recordings of CBF were obtained in 45 healthy subjects (21-81 years) and 14 patients with hepatic encephalopathy (HE). With volume-of-interest (VOI) and voxel-based statistics, we conducted regression analyses of CBF as function of age in the healthy group, and compared the HE group to a subset of the controls. We compared absolute CBF values, and CBF normalized to the gray matter (GM) and white matter (WM) means. In additional simulation experiments, we manipulated the cortical values of 12 healthy subjects and compared these to unaltered control data.

RESULTS

In healthy aging, CBF was shown to be unchanged in WM and central regions. In contrast, with normalization to the GM mean, CBF displayed positive correlation with age in the central regions. Very similar artifactual increases were seen in the HE comparison and also in the simulation experiment.

CONCLUSION

Ratio normalization to the global mean readily elevates CBF in unchanged regions when a systematic between-group difference exists in gCBF, also when this difference is below the detection threshold. We suggest that the routine normalization to the global mean in earlier studies resulted in spurious interpretations of perturbed CBF. Normalization to central WM yields less biased results in aging and HE and could potentially serve as a normalization reference region in other disorders as well.

Stroke with subarachnoid hemorrhage: assessment of cerebrovascular pressure regulation and simulated cerebrovascular resistance

BACKGROUND

Monitoring methods designed to assess cerebrovascular regulation and increased cerebrovascular resistance (CVR) of patients with subarachnoid hemorrhage (SAH) would facilitate therapeutic intervention and potentially reduce secondary complications. The aim of this study was to assess changes of cerebrovascular regulation and CVR by evaluating changes of cerebrovascular pressure transmission in patients with SAH.

METHODS

Admission Hunt-Hess grades, Fisher scores, Glasgow Outcome Scores (GOS) at 6 months, and pressure recordings were obtained from 20 patients. Biomechanical models of cerebrovascular pressure transmission were constructed over one-minute intervals for the initial and final two hours of post-hemorrhage monitoring.

FINDINGS

Classified according to the GOS score at 6 months, eight patients died (GOS 1), five were severely disabled (GOS 3), and seven patients were moderately disabled (GOS 4). During the initial monitoring period 100%, 80%, and 28.6% of groups with GOS 1, 3, and 4 demonstrated impairment of cerebrovascular regulation; whereas, in the final monitoring period 100%, 100%, and 14.3% respectively demonstrated impairment. Between monitoring periods, simulated CVR (sCVR) significantly increased (p < 0.001) for patients with GOS 1 and 3 and decreased for those with GOS 4 with mean resistance for the latter group significantly lower (p < 0.001) than other means.

CONCLUSIONS

Loss of cerebrovascular regulation and increased sCVR were observed in SAH patients with poor outcome.

Gray matter blood flow change is unevenly distributed during moderate isocapnic hypoxia in humans

Hypoxia increases cerebral blood flow (CBF), but it is unknown whether this increase is uniform across all brain regions. We used H(2)(15)O positron emission tomography imaging to measure absolute blood flow in 50 regions of interest across the human brain (n = 5) during normoxia and moderate hypoxia. Pco(2) was kept constant ( approximately 44 Torr) throughout the study to avoid decreases in CBF associated with the hypocapnia that normally occurs with hypoxia. Breathing was controlled by mechanical ventilation. During hypoxia (inspired Po(2) = 70 Torr), mean end-tidal Po(2) fell to 45 +/- 6.3 Torr (means +/- SD). Mean global CBF increased from normoxic levels of 0.39 +/- 0.13 to 0.45 +/- 0.13 ml/g during hypoxia. Increases in regional CBF were not uniform and ranged from 9.9 +/- 8.6% in the occipital lobe to 28.9 +/- 10.3% in the nucleus accumbens. Regions of interest that were better perfused during normoxia generally showed a greater regional CBF response. Phylogenetically older regions of the brain tended to show larger vascular responses to hypoxia than evolutionary younger regions, e.g., the putamen, brain stem, thalamus, caudate nucleus, nucleus accumbens, and pallidum received greater than average increases in blood flow, while cortical regions generally received below average increases. The heterogeneous blood flow distribution during hypoxia may serve to protect regions of the brain with essential homeostatic roles. This may be relevant to conditions such as altitude, breath-hold diving, and obstructive sleep apnea, and may have implications for functional brain imaging studies that involve hypoxia.

Human cerebrovascular and ventilatory CO2 reactivity to end-tidal, arterial and internal jugular vein PCO2

This study examined cerebrovascular reactivity and ventilation during step changes in CO(2) in humans. We hypothesized that: (1) end-tidal P(CO(2)) (P(ET,CO(2))) would overestimate arterial P(CO(2)) (P(a,CO(2))) during step variations in P(ET,CO(2)) and thus underestimate cerebrovascular CO(2) reactivity; and (2) since P(CO(2)) from the internal jugular vein (P(jv,CO(2))) better represents brain tissue P(CO(2)), cerebrovascular CO(2) reactivity would be higher when expressed against P(jv,CO(2)) than with P(a,CO(2)), and would be related to the degree of ventilatory change during hypercapnia. Incremental hypercapnia was achieved through 4 min administrations of 4% and 8% CO(2). Incremental hypocapnia involved two 4 min steps of hyperventilation to change P(ET,CO(2)), in an equal and opposite direction, to that incurred during hypercapnia. Arterial and internal jugular venous blood was sampled simultaneously at baseline and during each CO(2) step. Cerebrovascular reactivity to CO(2) was expressed as the percentage change in blood flow velocity in the middle cerebral artery (MCAv) per mmHg change in P(a,CO(2)) and P(jv,CO(2)). During hypercapnia, but not hypocapnia, P(ET,CO(2)) overestimated P(a,CO(2)) by +2.4 +/- 3.4 mmHg and underestimated MCAv-CO(2) reactivity (P < 0.05). The hypercapnic and hypocapnic MCAv-CO(2) reactivity was higher ( approximately 97% and approximately 24%, respectively) when expressed with P(jv,CO(2)) than P(a,CO(2)) (P < 0.05). The hypercapnic MCAv-P(jv,CO(2)) reactivity was inversely related to the increase in ventilatory change (R(2) = 0.43; P < 0.05), indicating that a reduced reactivity results in less central CO(2) washout and greater ventilatory stimulus. Differences in the P(ET,CO(2)), P(a,CO(2)) and P(jv,CO(2))-MCAv relationships have implications for the true representation and physiological interpretation of cerebrovascular CO(2) reactivity.

Remifentanil-induced cerebral blood flow effects in normal humans: dose and ApoE genotype

BACKGROUND

Opioids have been linked to limbic system activation and, in animals, to neurotoxicity. Limbic system nonpharmacologic activation patterns have been linked to the Apolipoprotein E (ApoE) allelic distribution. We tested the hypothesis that, in the absence of surgery, small doses of remifentanil produce limbic system activation in humans which varies with dose and ApoE genotype.

METHODS

Twenty-seven ASA I-II volunteers received a remifentanil (Ultiva) infusion at four sequentially increasing doses: 0, 0.05, 0.1, and 0.2 microg x kg(-1) x min(-1) while receiving 100% oxygen. Cerebral blood flow (CBF) was measured at each dose globally and in the amygdala, cingulate, hippocampus, insula, and thalamus regions by pulsed arterial spin labeling magnetic resonance imaging. ApoE single nucleotide polymorphisms were determined in each subject.

RESULTS

Significant dose-related CBF increases, without correction for Paco(2), were detected in all areas. After normalizing for global CBF to correct for Paco(2) effects, the remifentanil-mediated increased CBF in the cingulate persisted, with decreased flow occurring in the hippocampus and amygdala. All these Paco(2)-corrected effects were reversed in the presence of the ApoE4 polymorphism.

CONCLUSION

Remifentanil at sedative doses produces both activating and depressing effects in various limbic system structures. The cingulate cortex seems to have the most susceptibility to remifentanil activation, and ApoE4 seems to produce relative activation of the hippocampus and amygdala.

Cerebral blood flow and BOLD responses to a memory encoding task: a comparison between healthy young and elderly adults

Functional magnetic resonance imaging (fMRI) studies of the medial temporal lobe have primarily made use of the blood oxygenation level dependent (BOLD) response to neural activity. The interpretation of the BOLD signal as a measure of medial temporal lobe function can be complicated, however, by changes in the cerebrovascular system that can occur with both normal aging and age-related diseases, such as Alzheimer's disease. Quantitative measures of the functional cerebral blood flow (CBF) response offer a useful complement to BOLD measures and have been shown to aid in the interpretation of fMRI studies. Despite these potential advantages, the application of ASL to fMRI studies of cognitive tasks and at-risk populations has been limited. In this study, we demonstrate the application of ASL fMRI to obtain measures of the CBF and BOLD responses to the encoding of natural scenes in healthy young (mean 25 years) and elderly (mean 74 years) adults. The percent CBF increase in the medial temporal lobe was significantly higher in the older adults, whereas the CBF levels during baseline and task conditions and during a separate resting-state scan were significantly lower in the older group. The older adults also showed slightly higher values for the BOLD response amplitude and the absolute change in CBF, but the age group differences were not significant. The percent CBF and BOLD responses are consistent with an age-related increase in the cerebral metabolic rate of oxygen metabolism (CMRO(2)) response to memory encoding.

Experimental comparison of dyspnea and pain

Dyspnea and pain are similarly unpleasant, alarming physical sensations, but studies examining both sensations in combination are lacking. In the present study, dyspnea was induced in 7 healthy volunteers by breathing through inspiratory resistive loads and the effects were compared with those of a heat pain stimulus. End-tidal partial pressures of carbon dioxide (PET(CO2)), inspiratory time (Ti), breathing frequency (f), experienced unpleasantness, and intensity were measured. No difference was observed between dyspnea and pain in experienced intensity and unpleasantness (p > .05). During dyspneic stimulation, slightly higher Ti was found than for pain (p < .08). PET(CO2) showed slight increases during the dyspneic versus the baseline and painful conditions (deltaPET(CO2) = 1.5 and 1.3 mmHg, respectively; p < .01 andp < .05). This study shows that the effects of dyspnea and heat pain can be compared within one experiment; both stimuli can be presented with similar intensity and unpleasantness, which is a prerequisite for comparing responses to them. The changes in PET(CO2) between our conditions were minimal, allowing an application of the present design to future fMRI studies.

Neuroimaging methods in affective neuroscience: selected methodological issues

A current goal of affective neuroscience is to reveal the relationship between emotion and dynamic brain activity in specific neural circuits. In humans, noninvasive neuroimaging measures are of primary interest in this endeavor. However, methodological issues, unique to each neuroimaging method, have important implications for the design of studies, interpretation of findings, and comparison across studies. With regard to event-related brain potentials, we discuss the need for dense sensor arrays to achieve reference-independent characterization of field potentials and improved estimate of cortical brain sources. Furthermore, limitations and caveats regarding sparse sensor sampling are discussed. With regard to event-related magnetic field (ERF) recordings, we outline a method to achieve magnetoencephalography (MEG) sensor standardization, which improves effects' sizes in typical neuroscientific investigations, avoids the finding of ghost effects, and facilitates comparison of MEG waveforms across studies. Focusing on functional magnetic resonance imaging (fMRI), we question the unjustified application of proportional global signal scaling in emotion research, which can greatly distort statistical findings in key structures implicated in emotional processing and possibly contributing to conflicting results in affective neuroscience fMRI studies, in particular with respect to limbic and paralimbic structures. Finally, a distributed EEG/MEG source analysis with statistical parametric mapping is outlined providing a common software platform for hemodynamic and electromagnetic neuroimaging measures. Taken together, to achieve consistent and replicable patterns of the relationship between emotion and neuroimaging measures, methodological aspects associated with the various neuroimaging techniques may be of similar importance as the definition of emotional cues and task context used to study emotion.

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.

Does the brain have a baseline? Why we should be resisting a rest

In the last few years, the notion that the brain has a default or intrinsic mode of functioning has received increasing attention. The idea derives from observations that a consistent network of brain regions shows high levels of activity when no explicit task is performed and participants are asked simply to rest. The importance of this putative "default mode" is asserted on the basis of the substantial energy demand associated with such a resting state and of the suggestion that rest entails a finely tuned balance between metabolic demand and regionally regulated blood supply. These observations, together with the fact that the default network is more active at rest than it is in a range of explicit tasks, have led some to suggest that it reflects an absolute baseline, one that must be understood and used if we are to develop a comprehensive picture of brain functioning. Here, we examine the assumptions that are generally made in accepting the importance of the "default mode". We question the value, and indeed the interpretability, of the study of the resting state and suggest that observations made under resting conditions have no privileged status as a fundamental metric of brain functioning. In doing so, we challenge the utility of studies of the resting state in a number of important domains of research.

Extracranial carotid Doppler ultrasound evaluation of cerebral blood flow volume in COPD patients

OBJECTIVE

Doppler ultrasound of extracranial internal carotid artery (ICA) and vertebral artery (VA) were performed and total cerebral blood flow volume (tCBFV) was evaluated in chronic obstructive pulmonary disease (COPD) patients. CBFV changes due to blood gas changes were also evaluated.

METHODS

Bilateral ICA and VA have been examined with 7.5 MHz linear array transducer in COPD patients. Angle-corrected time averaged flow velocity and cross-sectional areas of vessels have been measured. Flow volumes and tCBFV have been calculated. Flow velocities and waveform parameters have been measured.

RESULTS

tCBFV, anterior-posterior CBFVs, left-right ICA flow volumes, bilateral ICA and VA cross-sectional areas and left ICA peak-systolic velocity were significantly higher in COPD patients than control group. Among COPD patients tCBFVs were highest in hypoxemic-hypercapnic ones, and lowest in normocapnic ones. Bilateral VA flow volumes, bilateral ICA (except left ICA V(ps)) and VA flow velocities and waveform parameters were not different in COPD patients compared with control group. When compared among the subgroups of COPD patients, there were no significant differences for all parameters.

CONCLUSION

tCBFVs were found to be significantly higher in COPD patients. This increment which is probably due to balancing the oxygen deficit is low with hypoxemia and high with hypercapnia and hypoxemia. Particularly, bilateral ICA and VA cross-sectional area changes and increased left ICA V(ps) were considered as the main reason for increased tCBFV in COPD patients.

The effect of global cerebral vasodilation on focal activation hemodynamics

In view of the potential of global resting blood flow level to confound the interpretation of blood oxygenation level-dependent (BOLD) fMRI studies, we investigated the effect of pronounced elevation in baseline cerebral blood flow (CBF) on BOLD and CBF responses to functional activation. Twelve healthy volunteers performed bilateral finger apposition while attending to a radial yellow/blue checkerboard. Three levels of global CBF increase were achieved by inhaling 5, 7.5 or 10% CO2. CBF and BOLD signals were simultaneously quantified using interleaved multi-slice pulsed arterial spin labeling (PASL) and T2*-weighted gradient echo sequences. Increasing basal CBF produced a significant decrease in the activation-induced BOLD response, with the slope of the optimal linear fit of activation versus basal BOLD signal changes of -0.32 +/- 0.01%/% for motor and visual cortex regions of interest (ROIs). While the modulation in basal flow level also produced a statistically significant effect on the activation-induced CBF change, the degree of relative attenuation of the flow response was slight, with a slope of -0.18 +/- 0.02%/% in the motor and -0.13 +/- 0.01%/% in the visual cortex ROI. The current findings describe a strong attenuation of the BOLD response at significantly elevated basal flow levels and call for independent quantification of resting CBF in BOLD fMRI studies that involve subjects and/or conditions with markedly elevated global perfusion.

Cerebral vascular response to hypercapnia: Determination with perfusion MRI at 1.5 and 3.0 Tesla using a pulsed arterial spin labeling technique

PURPOSE

To compare the quantification of cerebral blood flow (CBF) at 1.5 and 3.0 Tesla, under normo- and hypercapnia, and to determine the cerebral vascular response (CVR) of gray matter (GM) to hypercapnia, a pulsed arterial spin labeling technique was used. Additionally, to improve GM CBF quantification a high-resolution GM-mask was applied.

MATERIALS AND METHODS

CBF was determined with the QUIPSS II with thin slice TI1 periodic saturation (Q2TIPS) sequence at 1.5 and 3.0 Tesla in the same group of eight subjects, both under normocapnia and hypercapnia. Absolute GM-CBF maps were calculated using a GM-mask obtained from a high-resolution structural scan by segmentation. The CVR to hypercapnia was derived from the quantitative GM-CBF maps.

RESULTS

For both field strengths, the GM-CBF was significantly higher under hypercapnia compared to normocapnia. For both conditions, there was no significant difference of GM-CBF for 1.5 and 3.0 Tesla; the same applies to the CVR, which was 4.3 and 4.5%/mmHg at 1.5 and 3.0 Tesla, respectively.

CONCLUSION

The method presented allows for the quantification of CBF and CVR in GM at the common clinical field strengths of 1.5 and 3.0 Tesla and could therefore be a useful tool to study these parameters under physiological and pathophysiological conditions.

Neuroimaging of emotion: empirical effects of proportional global signal scaling in fMRI data analysis

Global variations of BOLD-fMRI signal are often considered as nuisance effects. This unwanted source of variance is commonly eliminated using proportional global signal scaling (PGSS). However, application of PGSS relies on the assumption that global variations of BOLD signal and experimental conditions are uncorrelated. It has been shown for cognitive tasks that the unjustified application of PGSS might greatly distort statistical results. The present study examined this issue in the domain of emotion research. Specifically, fMRI data were obtained in a block-design, while 21 subjects passively viewed high and low emotionally arousing pleasant, unpleasant, and neutral pictures. Violations of the orthogonality assumption were found for analyses of emotional pictures high in arousal, causing dramatically different outcomes when compared to analyses performed without PGSS. Application of PGSS was associated with attenuated emotional activation in visual cortical areas, insensitivity to emotional activations in limbic and paralimbic regions, and widely distributed artificial deactivations. In contrast, the orthogonality assumption was not violated for low arousing emotional materials. Thus, the validity of using PGSS varied as a function of the emotional arousal of the stimuli. Taken together, the unwarranted use of PGSS might contribute to conflicting results in affective neuroscience fMRI studies, in particular with respect to limbic and paralimbic structures.

The cross-modal interaction between pain-related and saccade-related cerebral activation: a preliminary study by event-related functional magnetic resonance imaging

Pain-related cerebral activation in functional magnetic resonance imaging shows less consistent signals that decay earlier than in conventional task-related activation. This may result from pain's top-down inhibition mediated by cognitive or hemodynamic interaction that could affect activation by other modalities. Using event-related functional magnetic resonance imaging, we examined whether pain affects cerebral activation by a saccade task through such cross-modal interaction. Six right-handed volunteers underwent whole-brain echo-planar imaging on a 3.0 T magnetic resonance imaging scanner while they received thermal pain stimulus at 50 degrees C on the right forearm (P; n = 6), performed a visually guided saccade task (V; n = 6), and went through a simultaneous pain-plus-saccade paradigm (PV; n = 5). Averaged functional activation maps were synthesized and signal time courses were analyzed at activation clusters. P activated the bilateral secondary somatosensory cortex (S2). V activated the posterior, supplementary, frontal eye fields, and visual areas. PV enhanced the S2 activation and activated additional pain-related areas, including the bilateral premotor area, right insula, anterior, and posterior cingulate cortices. In contrast, V-related activation was attenuated in PV. We propose that pain caused cross-modal suppression on the oculomotor activity and that an oculomotor task enhanced pain-related activation by triggering attention toward pain.

IMPLICATIONS

Pain-related cerebral activation is enhanced by attention toward pain. It may involve top-down suppression over the unrelated neural networks of saccade.