Regional differences of fMR signal changes induced by hyperventilation: comparison between SE-EPI and GE-EPI at 3-T

Cerebrovascular Reactivity Mapping Without Gas Challenges: A Methodological Guide

Cerebrovascular reactivity (CVR) is defined as the ability of vessels to alter their caliber in response to vasoactive factors, by means of dilating or constricting, in order to increase or decrease regional cerebral blood flow (CBF). Importantly, CVR may provide a sensitive biomarker for pathologies where vasculature is compromised. Furthermore, the spatiotemporal dynamics of CVR observed in healthy subjects, reflecting regional differences in cerebral vascular tone and response, may also be important in functional MRI studies based on neurovascular coupling mechanisms. Assessment of CVR is usually based on the use of a vasoactive stimulus combined with a CBF measurement technique. Although transcranial Doppler ultrasound has been frequently used to obtain global flow velocity measurements, MRI techniques are being increasingly employed for obtaining CBF maps. For the vasoactive stimulus, vasodilatory hypercapnia is usually induced through the manipulation of respiratory gases, including the inhalation of increased concentrations of carbon dioxide. However, most of these methods require an additional apparatus and complex setups, which not only may not be well-tolerated by some populations but are also not widely available. For these reasons, strategies based on voluntary breathing fluctuations without the need for external gas challenges have been proposed. These include the task-based methodologies of breath holding and paced deep breathing, as well as a new generation of methods based on spontaneous breathing fluctuations during resting-state. Despite the multitude of alternatives to gas challenges, existing literature lacks definitive conclusions regarding the best practices for the vasoactive modulation and associated analysis protocols. In this work, we perform an extensive review of CVR mapping techniques based on MRI and CO2 variations without gas challenges, focusing on the methodological aspects of the breathing protocols and corresponding data analysis. Finally, we outline a set of practical guidelines based on generally accepted practices and available data, extending previous reports and encouraging the wider application of CVR mapping methodologies in both clinical and academic MRI settings.

Cerebrovascular reactivity mapping using intermittent breath modulation

Cerebrovascular reactivity (CVR), an index of brain vessel's dilatory capacity, is typically measured using hypercapnic gas inhalation or breath-holding as a vasoactive challenge. However, these methods require considerable subject cooperation and could be challenging in clinical studies. More recently, there have been attempts to use resting-state BOLD data to map CVR by utilizing spontaneous changes in breathing pattern. However, in subjects who have small fluctuations in their spontaneous breathing pattern, the CVR results could be noisy and unreliable. In this study, we aim to develop a new method for CVR mapping that does not require gas-inhalation yet provides substantially higher sensitivity than resting-state CVR mapping. This new method is largely based on resting-state scan, but introduces intermittent modulation of breathing pattern in the subject to enhance fluctuations in their end-tidal CO2 (EtCO2) level. Here we examined the comfort level, sensitivity, and accuracy of this method in two studies. First, in 8 healthy young subjects, we developed the intermittent breath-modulation method using two different modulation frequencies, 6 ​s per breath and 12 ​s per breath, respectively, and compared the results to three existing CVR methods, specifically hypercapnic gas inhalation, breath-holding, and resting-state. Our results showed that the comfort level of the 6-s breath-modulation method was significantly higher than breath-holding (p ​= ​0.007) and CO2-inhalation (p ​= ​0.015) methods, while not different from the resting-state, i.e. free breathing method (p ​= ​0.52). When comparing the sensitivity of CVR methods, the breath-modulation methods revealed higher Z-statistics compared to the resting-state scan (p ​< ​0.008) and was comparable to breath-holding results. Next, we tested the feasibility of breath-modulation CVR mapping (6 ​s per breath) in 21 cognitively normal elderly participants and compared quantitative CVR values to that obtained with the CO2-inhalation method. Whole-brain CVR was found to be 0.150 ​± ​0.055 and 0.154 ​± ​0.032 %ΔBOLD/mmHg for the breath-modulation and CO2-inhalation method, respectively, with a significant correlation between them (y ​= ​0.97x, p ​= ​0.007). CVR mapping with intermittent breath modulation may be a useful method that combines the advantages of resting-state and CO2-inhalation based approaches.

The Impact of Echo Time Shifts and Temporal Signal Fluctuations on BOLD Sensitivity in Presurgical Planning at 7 T

OBJECTIVES

Gradients in the static magnetic field caused by tissues with differing magnetic susceptibilities lead to regional variations in the effective echo time, which modifies both image signal and BOLD sensitivity. Local echo time changes are not considered in the most commonly used metric for BOLD sensitivity, temporal signal-to-noise ratio (tSNR), but may be significant, particularly at ultrahigh field close to air cavities (such as the sinuses and ear canals) and near gross brain pathologies and postoperative sites.

MATERIALS AND METHODS

We have studied the effect of local variations in echo time and tSNR on BOLD sensitivity in 3 healthy volunteers and 11 patients with tumors, postoperative cavities, and venous malformations at 7 T. Temporal signal-to-noise ratio was estimated from a 5-minute run of resting state echo planar imaging with a nominal echo time of 22 milliseconds. Maps of local echo time were derived from the phase of a multiecho GE scan. One healthy volunteer performed 10 runs of a breath-hold task. The t-map from this experiment served as a criterion standard BOLD sensitivity measure. Two runs of a less demanding breath-hold paradigm were used for patients.

RESULTS

In all subjects, a strong reduction in the echo time (from 22 milliseconds to around 11 milliseconds) was found close to the ear canals and sinuses. These regions were characterized by high tSNR but low t-values in breath-hold t-maps. In some patients, regions of particular interest in presurgical planning were affected by reductions in the echo time to approximately 13-15 milliseconds. These included the primary motor cortex, Broca's area, and auditory cortex. These regions were characterized by high tSNR values (70 and above). Breath-hold results were corrupted by strong motion artifacts in all patients.

CONCLUSIONS

Criterion standard BOLD sensitivity estimation using hypercapnic experiments is challenging, especially in patient populations. Taking into consideration the tSNR, commonly used for BOLD sensitivity estimation, but ignoring local reductions in the echo time (eg, from 22 to 11 milliseconds), would erroneously suggest functional sensitivity sufficient to map BOLD signal changes. It is therefore important to consider both local variations in the echo time and temporal variations in signal, using the product metric of these two indices for instance. This should ensure a reliable estimation of BOLD sensitivity and to facilitate the identification of potential false-negative results. This is particularly true at high fields, such as 7 T and in patients with large pathologies and postoperative cavities.

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.

Respiratory challenge MRI: Practical aspects

Respiratory challenge MRI is the modification of arterial oxygen (PaO₂) and/or carbon dioxide (PaCO₂) concentration to induce a change in cerebral function or metabolism which is then measured by MRI. Alterations in arterial gas concentrations can lead to profound changes in cerebral haemodynamics which can be studied using a variety of MRI sequences. Whilst such experiments may provide a wealth of information, conducting them can be complex and challenging. In this paper we review the rationale for respiratory challenge MRI including the effects of oxygen and carbon dioxide on the cerebral circulation. We also discuss the planning, equipment, monitoring and techniques that have been used to undertake these experiments. We finally propose some recommendations in this evolving area for conducting these experiments to enhance data quality and comparison between techniques.

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Oxygen and carbon dioxide affect cerebral blood flow and metabolism.

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This can be imaged with various MRI sequences.

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The practicalities of these techniques are reviewed.

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Examples of how this has been used to understand disease mechanisms.

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.

Assessing Cerebrovascular Reactivity in Carotid Steno-Occlusive Disease Using MRI BOLD and ASL Techniques

Impaired cerebrovascular reactivity (CVR), a predictive factor of imminent stroke, has been shown to be associated with carotid steno-occlusive disease. Magnetic resonance imaging (MRI) techniques, such as blood oxygenation level-dependent (BOLD) and arterial spin labeling (ASL), have emerged as promising noninvasive tools to evaluate altered CVR with whole-brain coverage, when combined with a vasoactive stimulus, such as respiratory task or injection of acetazolamide. Under normal cerebrovascular conditions, CVR has been shown to be globally and homogenously distributed between hemispheres, but with differences among cerebral regions. Such differences can be explained by anatomical specificities and different biochemical mechanisms responsible for vascular regulation. In patients with carotid steno-occlusive disease, studies have shown that MRI techniques can detect impaired CVR in brain tissue supplied by the affected artery. Moreover, resulting CVR estimations have been well correlated to those obtained with more established techniques, indicating that BOLD and ASL are robust and reliable methods to assess CVR in patients with cerebrovascular diseases. Therefore, the present paper aims to review recent studies which use BOLD and ASL to evaluate CVR, in healthy individuals and in patients with carotid steno-occlusive disease, providing a source of information regarding the obtained results and the methodological difficulties.

Hypercapnia-induced effects on image contrast based on intermolecular double-quantum coherences

Intermolecular double-quantum coherences (iDQCs) are well known to be sensitive to magnetic-field perturbations inside tissues. However, the exact relation between iDQC contrast in magnetic resonance imaging (MRI) and the underlying physiology is less well understood. To investigate parameters that influence iDQC signal changes observed during neuronal activation, carbogen-inhalation experiments were performed to produce a pure hemodynamic response without affecting oxidative metabolism. Eight human volunteers were studied at 2.9 T using gradient-recalled echo (GRE) and spin-echo (SE) variants of a single-shot sequence selecting iDQCs. Results were compared with conventional recordings of the blood oxygen level-dependent (BOLD) effect. Maps of voxels responding to the carbogen challenge showed similar distributions for iDQC and conventional MRI after adjustment for different sensitivities. Strong diffusion weighting of iDQC sequences and transverse relaxation effects suggested quantitative suppression of intravascular signal contributions. A particular susceptibility to local gradients during the evolution period (in which iDQCs evolve at twice the Larmor frequency) plus a strong relaxation weighting during the detection period due to the use of a long echo time (for refocusing of the dipolar signal) produced iDQC signal changes up to 21.7% +/- 2.5%. These results agreed quantitatively with computations based on the balloon model of BOLD-weighted MRI without requiring further assumptions.

Selective reduction of blood flow to white matter during hypercapnia corresponds with leukoaraiosis

BACKGROUND AND PURPOSE

Age-related white matter disease (leukoaraiosis) clusters in bands in the centrum semiovale, about the occipital and frontal horns of the lateral ventricles, in the corpus callosum, and internal capsule. Cerebrovascular anatomy suggests that some of these locations represent border zones between arterial supply territories. We hypothesized that there are zones of reduced cerebrovascular reserve (susceptible to selective reductions in blood flow, ie, steal phenomenon) in the white matter of young, healthy subjects, the physiological correlate of these anatomically defined border zones. Furthermore, we hypothesized that these zones spatially correspond with the regions where the elderly develop leukoaraiosis.

METHODS

Twenty-eight healthy volunteers underwent functional MR mapping of the cerebrovascular response to hypercapnia. We studied 18 subjects by blood oxygen level-dependent MRI and 10 subjects by arterial spin labeling MRI. We controlled both end-tidal pCO(2) and pO(2). All functional data was registered in Montreal Neurological Institute space and generated composite blood oxygen level-dependent MR and arterial spin labeling MR maps of cerebrovascular reserve. We compared these maps with frequency maps of leukoaraiosis published previously.

RESULTS

Composite maps demonstrated significant (90% CI excluding the value zero) steal phenomenon in the white matter. This steal was induced by relatively small changes in end-tidal pCO(2). It occurred precisely in those locations where elderly patients develop leukoaraiosis.

CONCLUSIONS

This steal phenomenon likely represents the physiological correlate of the previously anatomically defined internal border zones. Spatial concordance with white matter changes in the elderly raises the possibility that this steal phenomenon may have a pathogenetic role.

Mapping cerebrovascular reactivity using blood oxygen level-dependent MRI in Patients with arterial steno-occlusive disease: comparison with arterial spin labeling MRI

BACKGROUND AND PURPOSE

Blood oxygen level-dependent MRI (BOLD MRI) of hypercapnia-induced changes in cerebral blood flow is an emerging technique for mapping cerebrovascular reactivity (CVR). BOLD MRI signal reflects cerebral blood flow, but also depends on cerebral blood volume, cerebral metabolic rate, arterial oxygenation, and hematocrit. The purpose of this study was to determine whether, in patients with stenoocclusive disease, the BOLD MRI signal response to hypercapnia is directly related to changes in cerebral blood flow.

METHODS

Thirty-eight patients with stenoocclusive disease underwent mapping of CVR by both BOLD MRI and arterial spin labeling MRI. The latter technique was used as a reference standard for measurement of cerebral blood flow changes.

RESULTS

Hemispheric CVR measured by BOLD MRI was significantly correlated with that measured by arterial spin labeling MRI for both gray matter (R=0.83, P<0.0001) and white matter (R=0.80, P<0.0001). Diagnostic accuracy (area under receiver operating characteristic curve) for BOLD MRI discrimination between normal and abnormal hemispheric CVR was 0.90 (95% CI=0.81 to 0.98; P<0.001) for gray matter and 0.82 (95% CI=0.70 to 0.94; P<0.001) for white matter. Regions of paradoxical CVR on BOLD MRI had a moderate predictive value (14 of 19 hemispheres) for spatially corresponding paradoxical CVR on arterial spin labeling MRI. Complete absence of paradoxical CVR on BOLD MRI had a high predictive value (31 of 31 hemispheres) for corresponding nonparadoxical CVR on arterial spin labeling MRI.

CONCLUSIONS

Arterial spin labeling MRI confirms that, even in patients with stenoocclusive disease, the BOLD MRI signal response to hypercapnia predominantly reflects changes in cerebral blood flow.

Regional impairment of cerebrovascular reactivity and BOLD signal in adults after stroke

BACKGROUND AND PURPOSE

Comparative studies across populations using functional magnetic resonance imaging (fMRI) rely on a similar relationship between blood oxygen level-dependent (BOLD) signal and neural activity. However, in elderly and patients with cerebrovascular disease, impaired cerebrovascular dynamics and neurovascular coupling may explain differences in BOLD contrast across populations and brain regions. The purpose of the study was to determine whether poststroke patients have regional heterogeneities of cerebrovascular reactivity (CVR) and their potential influence on voxel-wise motor-related BOLD signal.

METHODS

Using fMRI, 8 fully recovered patients from stroke in the frontal lobe without cortical lesion in the regions of interest located in the primary sensorimotor cortex (SMC), supplementary motor area (SMA), and cerebellum (CRB) were compared with 8 healthy subjects. Motor-related BOLD signal changes (%SC) were evaluated during simple unimanual and bimanual tasks, and CVR was evaluated during hyperventilation (HV). Analyses were performed using Lipsia software in SMC, SMA, and CRB.

RESULTS

In controls, amplitudes of BOLD signal were symmetrical in all regions of interest during all motor tasks and HV. In patients, %SC was decreased in SMC and SMA of the lesioned hemisphere despite their apparent anatomical integrity for all tasks. Impaired CVR was a predictor of impaired motor-related BOLD response in the SMC during contralateral movements (beta=-1.87; R=-0.75; P=0.03).

CONCLUSIONS

These preliminary findings suggest that CVR heterogeneities may account for task-related BOLD signal changes in patients after stroke.

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.

Whole-brain vascular reactivity measured by fMRI using hyperventilation and breath-holding tasks: efficacy of 3D prospective acquisition correction (3D-PACE) for head motion

Functional MR imaging (fMRI) study using hyperventilation and breath-holding task has been reported to be one of the non-invasive methods to examine whole-brain vascular reactivity. The purpose of this study was to evaluate the efficacy of a method for 3D prospective detection and correction of head motion (3D-PACE) in a study of whole-brain vascular reactivity using hyperventilation and breath-holding tasks. Eight healthy volunteers were scanned using an fMRI protocol of hyperventilation and breath-holding task blocks at 3 T in separate runs with and without 3D-PACE. In two subjects, two more runs with and without 3D-PACE were repeated. The mean total number of activated voxels +/- standard deviation was 26,405.3+/-1,822.2 in the run with 3D-PACE and 17,329.9+/-2,766.3 in the run without 3D-PACE ( P<0.05), although there is some intersubject variation regarding the effect of 3D-PACE. In the two subjects whose performed two more runs, the number of activated voxels were smaller in the run without 3D-PACE than even in the run with 3D-PACE performed later. We conclude that 3D-PACE is beneficial for fMRI studies of whole-brain vascular reactivity induced by hyperventilation and breath-holding.

Pre-surgical mapping of primary motor cortex by functional MRI at 3 T: effects of intravenous administration of Gd-DTPA

The functional magnetic resonance imaging (fMRI) is often performed at the end of a routine MRI examination during which, dependent on the clinical indication, contrast agent has been administered; however, the effects of Gd-DTPA injection on the results of blood oxygenation level dependent (BOLD)-fMRI remain unknown. The present study was conducted to investigate the effects of the intravenous administration of Gd-DTPA on the results of pre-surgical localization of the primary motor cortex by BOLD-fMRI at 3 T. Eight normal subjects were included in this study. After the anatomical scans, pre- and post-contrast fMRI scanning was performed. The number of significantly activated voxels and the mean percentage signal change were compared. The mean number of significantly activated voxels was 115.0+/-27.0 in pre-contrast runs and 90.8+/-27.1 in post-contrast runs (mean value of all 8 volunteers+/-standard deviation; p<0.05). The mean mean percentage signal change was 4.07+/-0.39 in pre-contrast runs and 3.86+/-1.91 in post-contrast runs ( p=0.16). Pre-surgical localization of the motor area by BOLD-fMRI should be performed before the administration of Gd contrast material.

Determination of cerebrovascular reactivity by means of FMRI signal changes in cerebral microangiopathy: a correlation with morphological abnormalities

BACKGROUND AND PURPOSE

A reduced cerebrovascular reactivity (CR) is a risk factor of cerebrovascular disease. In this study, we implemented a protocol to assess CR by means of functional MRI (fMRI) using hyperventilation.

SUBJECTS AND METHODS

In 5 patients with cerebral microangiopathy (CM/lacunar infarction and white matter degeneration), 6 healthy elderly subjects (age-matched control), and 6 young healthy subjects, the CR in response to hyperventilation was evaluated by fMRI using gradient echo-planar Imaging. The percentage signal change normalized by end-tidal CO(2) value was measured in various brain regions.

RESULTS

All subjects performed hyperventilation well without adverse reaction and significant gross motion. Patients with CM showed significant qualitative and quantitative differences (p < 0.05) as compared to controls. The volume of gray matter showing significant CR was significantly reduced in patients: by 40% in comparison to the age-matched elderly control group and by 60% when compared with the young controls. The CR impairment was most pronounced in the frontal cortices with a drastically reduced magnitude of the magnetic resonance (MR) signal change in the patients (-0.62 +/- 0.2% in patients versus -2.0 +/- 0.36% in age-matched controls, p < 0.0001). A strong relation was evident between the fMRI-based CR reduction in patients with CM and the individual severity of structural MR abnormalities (p = 0.002).

CONCLUSION

This study demonstrates that fMRI-based signal changes in response to hyperventilation reliably reflect cerebral vasoreactivity. The protocol is feasible in healthy young and elderly controls and patients with CM. Quantitative and qualitative assessment of the signal decrease in the T(2)-weighted MR sequence and coregistration with individual anatomical data allow the generation of an individual cerebral vasoreactivity map. Future research will address the effect of CR reduction on neuropsychological parameters in patients with CM.