Comparison of global cerebral blood flow measured by phase-contrast mapping MRI with 15 O-H2 O positron emission tomography

Compartmental modeling for blood flow quantification from dynamic 15 O-water PET images of humans: a systematic review

Dynamic positron emission tomography (PET) can be used to non-invasively estimate the blood flow of different organs via compartmental modeling. Out of different PET tracers, water labeled with the radioactive15 O isotope of oxygen (half-life of 2.04 min) is freely diffusable, and therefore, very well-suited for blood flow quantification. While the earlier15 O-water PET research has primarily focused on cerebral or myocardial blood flow quantification, the recent emergence of total-body PET scanners has enabled greater application possibilities for both PET imaging in general and also15 O-water PET based blood flow quantification in particular. However, to validate new methods, it is necessary to compare them to earlier research. To help in this process, we systematically review 53 articles quantifying blood flow via compartmental modeling. We introduce the articles organized within subcategories of cerebral, myocardial, renal, pulmonary, pancreatic, hepatic, muscle, and tumor blood flow and summarize their results so that they can easily be evaluated in terms of population characteristics of the patients such as age or sex ratio and their potential diagnoses. We compare how both the compartment model used and the potential corrections for arterial blood volume, non-perfusable tissue, spill-over from the heart cavities, and time delay caused while the tracer travels between different areas of interest are generally implemented in the articles. We also analyze the differences in the data pre-processing techniques. According to our results, the estimates of cerebral and tumor blood flow vary considerably more between the articles than those of myocardial blood flow. This might be caused by differences in the model approaches or the study populations. We also note that the choice of the unit for these estimates is quite inconsistent as certain researchers seem to prefer mL/min/g over mL/min/mL even if no weight or density parameter is present in the modeling. We encourage more research on sex- and age-based differences in blood flow estimates and organ-specific blood flow quantification studies for kidneys, lungs, liver, and other important organs besides brain and heart.

Cerebral Metabolic Rate of Oxygen and Accelerometry-Based Fatigability in Community-Dwelling Older Adults

Alterations in energy metabolism may drive fatigue in older age, but prior research primarily focused on skeletal muscle energetics without assessing other systems, and utilized self-reported measures of fatigue. We tested the association between energy metabolism in the brain and an objective measure of fatigability in the Study of Muscle, Mobility and Aging (N=119, age 76.8±4.0 years, 59.7% women). Total brain cerebral metabolic rate of oxygen (CMRO 2 ) was measured using arterial spin labeling and T 2 -relaxation under spin tagging MRI protocols. Accelerometry-based fatigability status during a fast-paced 400m walk was determined using the Pittsburgh Fatigability Index (PPFI, higher=worse). Confounders included skeletal muscle energetics, measured in vivo using spectroscopy and ex vivo using respirometry, cardiorespiratory fitness (VO 2 peak), weight, medication count, and multimorbidity. Multivariable logistic regression models were used to estimate the association (odds ratio (OR)) of CMRO 2 with PPFI>0 compared to the referent group PPFI=0. Models were first adjusted for age and sex, and further adjusted for confounders. In this sample, 41.2% had PPFI>0 (median 3.3% [0.4-8.0%]). CMRO 2 was positively associated with PPFI>0 (age and sex adjusted OR=1.61, 95% CI: 1.06, 2.45, p=0.03); adjustment for confounders attenuated the association. The positive association of brain energetics and fatigability warrants further study in older adults.

The cerebral blood flow response to neuroactivation is reduced in cognitively normal men with β-amyloid accumulation

BACKGROUND

Accumulation of β-amyloid (Aβ) in the brain is a hallmark of Alzheimer's Disease (AD). Cerebral deposition of Aβ initiates deteriorating pathways which eventually can lead to AD. However, the exact mechanisms are not known. A possible pathway could be that Aβ affects the cerebral vessels, causing inadequate cerebrovascular function. In the present study, we examined if Aβ accumulation is associated with a reduced cerebral blood flow response (CBF) to neuronal activation by visual stimulation (ΔCBFVis.Act.) in cognitively normal subjects from the Metropolit Danish Male Birth Cohort.

METHODS

64 subjects participated in the present study. ΔCBFVis.Act. was measured using arterial spin labelling (ASL) combined with blood-oxygen-level-dependent (BOLD) MRI. Neuronal activation was obtained by visual stimulation by a flickering checkerboard presented on a screen in the MRI-scanner. Brain Aβ accumulation and cerebral glucose metabolism were assessed by PET imaging using the radiotracers [11C]Pittsburgh Compound-B (PiB) and [18F]Fluorodeoxyglucose (FDG), respectively. Cortical thickness was measured from structural MRI.

RESULTS

ΔCBFVis.Act. correlated negatively ( β = -32.1 [95% confidence interval (CI): -60.2; -4.1], r = -0.30, p = 0.025) with PiB standardized uptake value ratio (SUVr) in the brain regions activated by visual stimulation. ΔCBFVis.Act. did not correlate with FDG SUVr ( β = 1.9 [CI: -23.8; 27.6], r = 0.02, p = 0.88) or cortical thickness ( β = 10.3 [CI: -8.4; 29.0], r = 0.15, p = 0.27) in the activated brain regions. Resting CBF did not correlate with PiB SUVr neither in the regions activated by visual stimulation ( β = -17.8 [CI:-71.9; 36.2], r =- 0.09, p = 0.51) nor in the remaining cortex ( β = 5.2 [CI:-3.9; 14.2], r = 0.15, p = 0.26).

CONCLUSION

We found a correlation between high PiB SUVr and reduced CBF response to neuronal activation, indicating a link between Aβ accumulation and impaired cerebrovascular function. The impairment was not associated with cortical thinning or hypometabolism, suggesting that Aβ accumulation affecting brain vessel function could be a very early pathology leading to neurodegenerative disease.

Automated Quantitative Image-Derived Input Function for the Estimation of Cerebral Blood Flow Using Oxygen-15-Labelled Water on a Long-Axial Field-of-View PET/CT Scanner

The accurate estimation of the tracer arterial blood concentration is crucial for reliable quantitative kinetic analysis in PET. In the current work, we demonstrate the automatic extraction of an image-derived input function (IDIF) from a CT AI-based aorta segmentation subsequently resliced to a dynamic PET series acquired on a Siemens Vision Quadra long-axial field of view scanner in 10 human subjects scanned with [15O]H2O. We demonstrate that the extracted IDIF is quantitative and in excellent agreement with a delay- and dispersion-corrected sampled arterial input function (AIF). Perfusion maps in the brain are calculated and compared from the IDIF and AIF, respectively, showed a high degree of correlation. The results demonstrate the possibility of defining a quantitatively correct IDIF compared with AIFs from the new-generation high-sensitivity and high-time-resolution long-axial field-of-view PET/CT scanners.

Age-related decline in cerebral oxygen consumption in multiple sclerosis

Cerebral oxygen metabolism is altered in relapsing-remitting multiple sclerosis (RRMS), possibly a result of disease related cerebral atrophy with subsequent decreased oxygen demand. However, MS inflammation can also inhibit brain metabolism. Therefore, we measured cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) using MRI phase contrast mapping and susceptibility-based oximetry in 44 patients with early RRMS and 36 healthy controls. Cerebral atrophy and white matter lesion load were assessed from high-resolution structural MRI. Expanded Disability Status Scale (EDSS) scores were collected from medical records. The CMRO2 was significantly lower in patients (-15%, p = 0.002) and decreased significantly with age in patients relative to the controls (-1.35 µmol/100 g/min/year, p = 0.036). The lower CMRO2 in RRMS was primarily driven by a higher venous oxygen saturation in the sagittal sinus (p = 0.007) and not a reduction in CBF (p = 0.69). There was no difference in cerebral atrophy between the groups, and no correlation between CMRO2 and MS lesion volume or EDSS score. Therefore, the progressive CMRO2 decline observed before the occurrence of significant cerebral atrophy and despite adequate CBF supports emerging evidence of dysfunctional cellular respiration as a potential pathogenic mechanism and therapeutic target in RRMS.

Subclinical epileptiform discharges in Alzheimer's disease are associated with increased hippocampal blood flow

BACKGROUND

In epilepsy, the ictal phase leads to cerebral hyperperfusion while hypoperfusion is present in the interictal phases. Patients with Alzheimer's disease (AD) have an increased prevalence of epileptiform discharges and a study using intracranial electrodes have shown that these are very frequent in the hippocampus. However, it is not known whether there is an association between hippocampal hyperexcitability and regional cerebral blood flow (rCBF). The objective of the study was to investigate the association between rCBF in hippocampus and epileptiform discharges as measured with ear-EEG in patients with Alzheimer's disease. Our hypothesis was that increased spike frequency may be associated with increased rCBF in hippocampus.

METHODS

A total of 24 patients with AD, and 15 HC were included in the analysis. Using linear regression, we investigated the association between rCBF as measured with arterial spin-labelling MRI (ASL-MRI) in the hippocampus and the number of spikes/sharp waves per 24 h as assessed by ear-EEG.

RESULTS

No significant difference in hippocampal rCBF was found between AD and HC (p-value = 0.367). A significant linear association between spike frequency and normalized rCBF in the hippocampus was found for patients with AD (estimate: 0.109, t-value = 4.03, p-value < 0.001). Changes in areas that typically show group differences (temporal-parietal cortex) were found in patients with AD, compared to HC.

CONCLUSIONS

Increased spike frequency was accompanied by a hemodynamic response of increased blood flow in the hippocampus in patients with AD. This phenomenon has also been shown in patients with epilepsy and supports the hypothesis of hyperexcitability in patients with AD. The lack of a significant difference in hippocampal rCBF may be due to an increased frequency of epileptiform discharges in patients with AD.

TRIAL REGISTRATION

The study is registered at clinicaltrials.gov (NCT04436341).

Automated Quantification of Total Cerebral Blood Flow from Phase-Contrast MRI and Deep Learning

Knowledge of input blood to the brain, which is represented as total cerebral blood flow (tCBF), is important in evaluating brain health. Phase-contrast (PC) magnetic resonance imaging (MRI) enables blood velocity mapping, allowing for noninvasive measurements of tCBF. In the procedure, manual selection of brain-feeding arteries is an essential step, but is time-consuming and often subjective. Thus, the purpose of this work was to develop and validate a deep learning (DL)-based technique for automated tCBF quantifications. To enhance the DL segmentation performance on arterial blood vessels, in the preprocessing step magnitude and phase images of PC MRI were multiplied several times. Thereafter, a U-Net was trained on 218 images for three-class segmentation. Network performance was evaluated in terms of the Dice coefficient and the intersection-over-union (IoU) on 40 test images, and additionally, on externally acquired 20 datasets. Finally, tCBF was calculated from the DL-predicted vessel segmentation maps, and its accuracy was statistically assessed with the correlation of determination (R2), the intraclass correlation coefficient (ICC), paired t-tests, and Bland-Altman analysis, in comparison to manually derived values. Overall, the DL segmentation network provided accurate labeling of arterial blood vessels for both internal (Dice=0.92, IoU=0.86) and external (Dice=0.90, IoU=0.82) tests. Furthermore, statistical analyses for tCBF estimates revealed good agreement between automated versus manual quantifications in both internal (R2=0.85, ICC=0.91, p=0.52) and external (R2=0.88, ICC=0.93, p=0.88) test groups. The results suggest feasibility of a simple and automated protocol for quantifying tCBF from neck PC MRI and deep learning.

Early-Frame [18F]Florbetaben PET/MRI for Cerebral Blood Flow Quantification in Patients with Cognitive Impairment: Comparison to an [15O]Water Gold Standard

Cerebral blood flow (CBF) may be estimated from early-frame PET imaging of lipophilic tracers, such as amyloid agents, enabling measurement of this important biomarker in participants with dementia and memory decline. Although previous methods could map relative CBF, quantitative measurement in absolute units (mL/100 g/min) remained challenging and has not been evaluated against the gold standard method of [15O]water PET. The purpose of this study was to develop and validate a minimally invasive quantitative CBF imaging method combining early [18F]florbetaben (eFBB) with phase-contrast MRI using simultaneous PET/MRI. Methods: Twenty participants (11 men and 9 women; 8 cognitively normal, 9 with mild cognitive impairment, and 3 with dementia; 10 β-amyloid negative and 10 β-amyloid positive; 69 ± 9 y old) underwent [15O]water PET, phase-contract MRI, and eFBB imaging in a single session on a 3-T PET/MRI scanner. Quantitative CBF images were created from the first 2 min of brain activity after [18F]florbetaben injection combined with phase-contrast MRI measurement of total brain blood flow. These maps were compared with [15O]water CBF using concordance correlation (CC) and Bland-Altman statistics for gray matter, white matter, and individual regions derived from the automated anatomic labeling (AAL) atlas. Results: The 2 methods showed similar results in gray matter ([15O]water, 55.2 ± 14.7 mL/100 g/min; eFBB, 55.9 ± 14.2 mL/100 g/min; difference, 0.7 ± 2.4 mL/100 g/min; P = 0.2) and white matter ([15O]water, 21.4 ± 5.6 mL/100 g/min; eFBB, 21.2 ± 5.3 mL/100 g/min; difference, -0.2 ± 1.0 mL/100 g/min; P = 0.4). The intrasubject CC for AAL-derived regions was high (0.91 ± 0.04). Intersubject CC in different AAL-derived regions was similarly high, ranging from 0.86 for midfrontal regions to 0.98 for temporal regions. There were no significant differences in performance between the methods in the amyloid-positive and amyloid-negative groups as well as participants with different cognitive statuses. Conclusion: We conclude that eFBB PET/MRI can provide robust CBF measurements, highlighting the capability of simultaneous PET/MRI to provide measurements of both CBF and amyloid burden in a single imaging session in participants with memory disorders.

Patients with type 1 diabetes and albuminuria have a reduced brain glycolytic capability that is correlated with brain atrophy

Introduction

Patients with type 1 diabetes (T1D) demonstrate brain alterations, including white matter lesions and cerebral atrophy. In this case-control study, we investigated if a reason for this atrophy could be because of diabetes-related complications affecting cerebrovascular or cerebral glycolytic functions. Cerebral physiological dysfunction can lead to energy deficiencies and, consequently, neurodegeneration.

Methods

We examined 33 patients with T1D [18 females, mean age: 50.8 years (range: 26-72)] and 19 matched healthy controls [7 females, mean age: 45.0 years (range: 24-64)]. Eleven (33%) of the patients had albuminuria. Total brain volume, brain parenchymal fraction, gray matter volume and white matter volume were measured by anatomical MRI. Cerebral vascular and glycolytic functions were investigated by measuring global cerebral blood flow (CBF), cerebral metabolic rate of oxygen (CMRO2) and cerebral lactate concentration in response to the inhalation of hypoxic air (12-14% fractional oxygen) using phase-contrast MRI and magnetic resonance spectroscopy (MRS) techniques. The inspiration of hypoxic air challenges both cerebrovascular and cerebral glycolytic physiology, and an impaired response will reveal a physiologic dysfunction.

Results

Patients with T1D and albuminuria had lower total brain volume, brain parenchymal fraction, and gray matter volume than healthy controls and patients without albuminuria. The inhalation of hypoxic air increased CBF and lactate in all groups. Patients with albuminuria had a significantly (p = 0.032) lower lactate response compared to healthy controls. The CBF response was lower in patients with albuminuria compared to healthy controls, however not significantly (p = 0.24) different. CMRO2 was unaffected by the hypoxic challenge in all groups (p > 0.16). A low lactate response was associated with brain atrophy, characterized by reduced total brain volume (p = 0.003) and reduced gray matter volume (p = 0.013).

Discussion

We observed a reduced response of the lactate concentration as an indication of impaired glycolytic activity, which correlated with brain atrophy. Inadequacies in upregulating cerebral glycolytic activity, perhaps from reduced glucose transporters in the brain or hypoxia-inducible factor 1 pathway dysfunction, could be a complication in diabetes contributing to the development of neurodegeneration and declining brain health.

Reproducibility of cerebral blood flow, oxygen metabolism, and lactate and N-acetyl-aspartate concentrations measured using magnetic resonance imaging and spectroscopy

In humans, resting cerebral perfusion, oxygen consumption and energy metabolism demonstrate large intersubject variation regardless of methodology. Whether a similar large variation is also present longitudinally in individual subjects is much less studied, but knowing the time variance in reproducibility is important when designing and interpreting longitudinal follow-up studies examining brain physiology. Therefore, we examined the reproducibility of cerebral blood flow (CBF), global cerebral metabolic rate of oxygen (CMRO2), global arteriovenous oxygen saturation difference (A-V.O2), and cerebral lactate and N-acetyl-aspartate (NAA) concentrations measured using magnetic resonance imaging (MRI) and spectroscopy (MRS) techniques through repeated measurements at 6 h, 24 h, 7 days and several weeks after initial baseline measurements in young healthy adults (N = 26, 13 females, age range 18-35 years). Using this setup, we calculated the correlation, limit of agreement (LoA) and within-subject coefficient of variation (CoVWS) between baseline values and the subsequent repeated measurements to examine the longitudinal variation in individual cerebral physiology. CBF and CMRO2 correlated significantly between baseline and all subsequent measurements. The strength of the correlations (R2) and reproducibility metrics (LoA and CoVWS) demonstrated the best reproducibility for the within-day measurements and generally declined with longer time between measurements. Cerebral lactate and NAA concentrations also correlated significantly for all measurements, except between baseline and the 7-day measurement for lactate. Similar to CBF and CMRO2, lactate and NAA demonstrated the best reproducibility for within-day repeated measurements. The gradual decline in reproducibility over time should be considered when designing and interpreting studies on brain physiology, for example, in the evaluation of treatment efficacy.

A scoping review of the discrepancies in the measurement of cerebral blood flow in idiopathic intracranial hypertension: oligemia, euvolemia or hyperemia?

BACKGROUND

The literature regarding the global cerebral blood flow (CBF) in idiopathic intracranial hypertension (IIH) is divergent leading to skepticism about the significance of blood flow to the disease's underlying pathophysiology.

METHODS

The purpose of the current paper is to perform a PRISMA scoping review of the literature describing the CBF in IIH. The review investigated the PUBMED and Scopus data bases looking at case mix, technique and the methodologies employed by the studies selected.

DISCUSSION

Many studies indicate that the flow in IIH is normal but others show the flow to be altered. These later studies show a range of flows from a reduction of 20% to an increase of 50% compared to control values. Obesity is a common finding in IIH and is known to reduce CBF, anemia occurs in approximately 20% of IIH patients and is a potent cause of an increased CBF. Thus, variations in case mix may have a significant effect on the final outcome in those studies which are underpowered. The varying techniques which have been used to estimate CBF have differing strengths and weaknesses which may also have a bearing on the outcome. Some papers have significant confounding methodological issues.

CONCLUSIONS

This review suggests each of the variables investigated are responsible for the divergent CBF findings in IIH.

Capillary transit time heterogeneity inhibits cerebral oxygen metabolism in patients with reduced cerebrovascular reserve capacity from steno-occlusive disease

The healthy cerebral perfusion demonstrates a homogenous distribution of capillary transit times. A disruption of this homogeneity may inhibit the extraction of oxygen. A high degree of capillary transit time heterogeneity (CTH) describes that some capillaries have very low blood flows, while others have excessively high blood flows and consequently short transit times. Very short transit times could hinder the oxygen extraction due to insufficient time for diffusion of oxygen into the tissue. CTH could be a consequence of cerebral vessel disease. We examined whether patients with cerebral steno-occlusive vessel disease demonstrate high CTH and if elevation of cerebral blood flow (CBF) by administration of acetazolamide (ACZ) increases the cerebral metabolic rate of oxygen (CMRO₂), or if some patients demonstrate reduced CMRO₂ related to detrimental CTH. Thirty-four patients and thirty-one healthy controls participated. Global CBF and CMRO₂ were acquired using phase-contrast MRI. Regional brain maps of CTH were acquired using dynamic contrast-enhanced MRI. Patients with impaired cerebrovascular reserve capacity demonstrated elevated CTH and a significant reduction of CMRO₂ after administration of ACZ, which could be related to high CTH. Impaired oxygen extraction from CTH could be a contributing part of the declining brain health observed in patients with cerebral vessel disease.

Cerebrovascular Reactivity and Neurovascular Coupling in Multiple Sclerosis-A Systematic Review

The inflammatory processes observed in the central nervous system in multiple sclerosis (MS) could damage the endothelium of the cerebral vessels and lead to a dysfunctional regulation of vessel tonus and recruitment, potentially impairing cerebrovascular reactivity (CVR) and neurovascular coupling (NVC). Impaired CVR or NVC correlates with declining brain health and potentially plays a causal role in the development of neurodegenerative disease. Therefore, we examined studies on CVR or NVC in MS patients to evaluate the evidence for impaired cerebrovascular function as a contributing disease mechanism in MS. Twenty-three studies were included (12 examined CVR and 11 examined NVC). Six studies found no difference in CVR response between MS patients and healthy controls. Five studies observed reduced CVR in patients. This discrepancy can be because CVR is mainly affected after a long disease duration and therefore is not observed in all patients. All studies used CO₂ as a vasodilating stimulus. The studies on NVC demonstrated diverse results; hence a conclusion that describes all the published observations is difficult to find. Future studies using quantitative techniques and larger study samples are needed to elucidate the discrepancies in the reported results.

Subclinical cognitive deficits are associated with reduced cerebrovascular response to visual stimulation in mid-sixties men

Reduced cerebrovascular response to neuronal activation is observed in patients with neurodegenerative disease. In the present study, we examined the correlation between reduced cerebrovascular response to visual activation (ΔCBF_Vis.Act) and subclinical cognitive deficits in a human population of mid-sixties individuals without neurodegenerative disease. Such a correlation would suggest that impaired cerebrovascular function occurs before overt neurodegenerative disease. A total of 187 subjects (age 64-67 years) of the Metropolit Danish Male Birth Cohort participated in the study. ΔCBF_Vis.Act was measured using arterial spin labelling (ASL) MRI. ΔCBF_Vis.Act correlated positively with cognitive performance in: Global cognition (p = 0.046), paired associative memory (p = 0.025), spatial recognition (p = 0.026), planning (p = 0.016), simple processing speed (p < 0.01), and with highly significant correlations with current intelligence (p < 10^-5), and more complex processing speed (p < 10^-3), the latter two explaining approximately 11-13% of the variance. Reduced ΔCBF_Vis.Act was independent of brain atrophy. Our findings suggest that inhibited cerebrovascular response to neuronal activation is an early deficit in the ageing brain and associated with subclinical cognitive deficits. Cerebrovascular dysfunction could be an early sign of a trajectory pointing towards the development of neurodegenerative disease. Future efforts should elucidate if maintenance of a healthy cerebrovascular function can protect against the development of dementia.

Deep-learning-based attenuation correction in dynamic [15O]H2O studies using PET/MRI in healthy volunteers

Quantitative [¹⁵O]H₂O positron emission tomography (PET) is the accepted reference method for regional cerebral blood flow (rCBF) quantification. To perform reliable quantitative [¹⁵O]H₂O-PET studies in PET/MRI scanners, MRI-based attenuation-correction (MRAC) is required. Our aim was to compare two MRAC methods (RESOLUTE and DeepUTE) based on ultrashort echo-time with computed tomography-based reference standard AC (CTAC) in dynamic and static [¹⁵O]H₂O-PET. We compared rCBF from quantitative perfusion maps and activity concentration distribution from static images between AC methods in 25 resting [¹⁵O]H₂O-PET scans from 14 healthy men at whole-brain, regions of interest and voxel-wise levels. Average whole-brain CBF was 39.9 ± 6.0, 39.0 ± 5.8 and 40.0 ± 5.6 ml/100 g/min for CTAC, RESOLUTE and DeepUTE corrected studies respectively. RESOLUTE underestimated whole-brain CBF by 2.1 ± 1.50% and rCBF in all regions of interest (range -2.4%- -1%) compared to CTAC. DeepUTE showed significant rCBF overestimation only in the occipital lobe (0.6 ± 1.1%). Both MRAC methods showed excellent correlation on rCBF and activity concentration with CTAC, with slopes of linear regression lines between 0.97 and 1.01 and R² over 0.99. In conclusion, RESOLUTE and DeepUTE provide AC information comparable to CTAC in dynamic [¹⁵O]H₂O-PET but RESOLUTE is associated with a small but systematic underestimation.

Relationship between Cerebral Perfusion on Arterial Spin Labeling (ASL) MRI with Brain Volumetry and Cognitive Performance in Mild Cognitive Impairment and Dementia due to Alzheimer's Disease

Context

Cerebral blood flow (CBF) measurement using arterial spin labelling (ASL) MRI sequences has recently emerged as a prominent tool in dementia research.

Aims

To establish association between quantified regional cerebral perfusion and gray matter (GM) volumes with cognitive measures in mild cognitive impairment (MCI) and early Alzheimer's Dementia (AD), using three dimensional fast spin echo pseudo-continuous ASL MRI sequences.

Settings and Design

Hospital-based cross-sectional study.

Methods and Material

Three age-matched groups, i.e., 21 cognitively normal healthy controls (HC), 20 MCI and 19 early AD patients diagnosed using neuropsychological tests and who consented for multimodality 3T MRI were recruited for the study.

Statistical Analysis Used

Statistical parametric mapping and regions of interest (ROI) multivariate analysis of variance was used to ascertain differences between patients and controls on MRI-volumetry and ASL. Linear regression was used to assess relationship between CBF with GM atrophy and neuropsychological test measures.

Results

Compared to HC, patients with MCI and AD had significantly lower quantified perfusion in posterior cingulate and lingual gyri, over hippocampus in MCI, with no differences noted between MCI and AD. Atrophy over the middle temporal gyrus and hippocampus differentiated AD from MCI. No significant positive correlations were noted between perfusion and GM volumes in ROI with the exception of temporal neocortex. Significantly positive coefficient b-value (p < 0.01) were apparent between global cognition with CBF in precuneus, temporal neocortex and precuneus volume, with negative b-values noted between medial temporal CBF for global cognition and recall scores.

Conclusions

ROI-based CBF measurements differentiated MCI and AD from HC; volumetry of medial and neocortical temporal GM separates AD from MCI. Correlations between CBF and neuropsychology are variable and require further longitudinal studies to gauge its predictive utility on cognitive trajectory in MCI.

Human Cerebral Perfusion, Oxygen Consumption, and Lactate Production in Response to Hypoxic Exposure

Exposure to moderate hypoxia in humans leads to cerebral lactate production, which occurs even when the cerebral metabolic rate of oxygen (CMRO2) is unaffected. We searched for the mechanism of this lactate production by testing the hypothesis of upregulation of cerebral glycolysis mediated by hypoxic sensing. Describing the pathways counteracting brain hypoxia could help us understand brain diseases associated with hypoxia. A total of 65 subjects participated in this study: 30 subjects were exposed to poikilocapnic hypoxia, 14 were exposed to isocapnic hypoxia, and 21 were exposed to carbon monoxide (CO). Using this setup, we examined whether lactate production reacts to an overall reduction in arterial oxygen concentration or solely to reduced arterial oxygen partial pressure. We measured cerebral blood flow (CBF), CMRO2, and lactate concentrations by magnetic resonance imaging and spectroscopy. CBF increased (P < 10-4), whereas the CMRO2 remained unaffected (P > 0.076) in all groups, as expected. Lactate increased in groups inhaling hypoxic air (poikilocapnic hypoxia: $0.0136\ \frac{\mathrm{mmol}/\mathrm{L}}{\Delta{\mathrm{S}}_{\mathrm{a}}{\mathrm{O}}_2}$, P < 10-6; isocapnic hypoxia: $0.0142\ \frac{\mathrm{mmol}/\mathrm{L}}{\Delta{\mathrm{S}}_{\mathrm{a}}{\mathrm{O}}_2}$, P = 0.003) but was unaffected by CO (P = 0.36). Lactate production was not associated with reduced CMRO2. These results point toward a mechanism of lactate production by upregulation of glycolysis mediated by sensing a reduced arterial oxygen pressure. The released lactate may act as a signaling molecule engaged in vasodilation.

Cerebrovascular effects of glibenclamide investigated using high-resolution magnetic resonance imaging in healthy volunteers

Glibenclamide inhibits sulfonylurea receptor (SUR), which regulates several ion channels including SUR1-transient receptor potential melastatin 4 (SUR1-TRPM4) channel and ATP-sensitive potassium (K_ATP) channel. Stroke upregulates SURl-TRPM4 channel, which causes a rapid edema formation and brain swelling. Glibenclamide may antagonize the formation of cerebral edema during stroke. Preclinical studies showed that glibenclamide inhibits K_ATP channel-induced vasodilation without altering the basal vascular tone. The in vivo human cerebrovascular effects of glibenclamide have not previously been investigated.In a randomized, double-blind, placebo-controlled, three-way cross-over study, we used advanced 3 T MRI methods to investigate the effects of glibenclamide and K_ATP channel opener levcromakalim on mean global cerebral blood flow (CBF) and intra- and extracranial artery circumferences in 15 healthy volunteers. Glibenclamide administration did not alter the mean global CBF and the basal vascular tone. Following levcromakalim infusion, we observed a 14% increase of the mean global CBF and an 8% increase of middle cerebral artery (MCA) circumference, and glibenclamide did not attenuate levcromakalim-induced vascular changes. Collectively, the findings demonstrate the vital role of K_ATP channels in cerebrovascular hemodynamic and indicate that glibenclamide does not inhibit the protective effects of K_ATP channel activation during hypoxia and ischemia-induced brain injury.

Quantification of Regional Cerebral Blood Flow Using Diffusion Imaging With Phase Contrast

BACKGROUND

The perfusion-related diffusion coefficient obtained from triexponential diffusion analysis is closely correlated with regional cerebral blood flow (rCBF), as assessed by arterial spin labeling (ASL) methods. However, this provides only a semiquantitative measure of rCBF, thereby making absolute rCBF quantification challenging.

PURPOSE

To obtain rCBF in a noninvasive manner using a novel diffusion imaging method with phase contrast (DPC), in which the total CBF from phase-contrast (PC) MRI was utilized to convert perfusion-related diffusion coefficients to rCBF values.

STUDY TYPE

Prospective.

SUBJECTS

Eleven healthy volunteers (nine men and two women; mean age, 23.9 years) participated in this study.

FIELD STRENGTH/SEQUENCE

A 3.0 T, single-shot diffusion echo-planar imaging with multiple b-values (0-3000 s/mm² ), PC-MRI, pulsed continuous ASL, and 3D T₁ -weighted fast field echo.

ASSESSMENT

rCBF and its correlations in the gray matter (GM) and white matter (WM) were compared between DPC and ASL methods. rCBF in the GM and WM and the GM/WM ratio were compared with the literature values obtained using [¹⁵ O]-water positron emission tomography (¹⁵ O-H₂ O PET).

STATISTICAL TESTS

Spearman's correlation coefficient and Wilcoxon signed-rank test were used. Significance was set at P < 0.05.

RESULTS

A significant positive correlation between DPC and ASL in terms of rCBF was observed in GM (R = 0.9), whereas the correlation between the two methods was poor in WM (R = 0.09). The rCBF in GM and WM and the GM/WM ratio obtained using DPC were consistent with the literature values assessed using ¹⁵ O-H₂ O PET. The rCBF value obtained using DPC was significantly higher in the GM and WM than that using ASL.

DATA CONCLUSION

DPC enabled noninvasive quantification of rCBF.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 1.

Regional and interindividual relationships between cerebral perfusion and oxygen metabolism

Quantitative measurements of resting cerebral blood flow (CBF) and metabolic rate of oxygen (CMRO₂) show large between-subject and regional variability, but the relationships between CBF and CMRO₂ measurements regionally and globally are not fully established. Here, we investigated the between-subject and regional associations between CBF and CMRO₂ measures with independent and quantitative PET techniques. We included resting CBF and CMRO₂ measurements from 50 healthy volunteers (aged 22-81 yr), and calculated the regional and global values of oxygen delivery (Do₂) and oxygen extraction fraction (OEF). Linear mixed-model analysis showed that CBF and CMRO₂ measurements were closely associated regionally, but no significant between-subject association could be demonstrated, even when adjusting for arterial Pco₂ and hemoglobin concentration. The analysis also showed regional differences of OEF, reflecting variable relationship between Do₂ and CMRO₂, resulting in lower estimates of OEF in thalami, brainstem, and mesial temporal cortices and higher estimates of OEF in occipital cortex. In the present study, we demonstrated no between-subject association of quantitative measurements of CBF and CMRO₂ in healthy subjects. Thus, quantitative measurements of CBF did not reflect the underlying between-subject variability of oxygen metabolism measures, mainly because of large interindividual OEF variability not accounted for by Pco₂ and hemoglobin concentration.NEW & NOTEWORTHY Using quantitative PET-measurements in healthy human subjects, we confirmed a regional association of CBF and CMRO₂, but did not find an association of these values across subjects. This suggests that subjects have an individual coupling between perfusion and metabolism and shows that absolute perfusion measurements does not serve as a surrogate measure of individual measures of oxygen metabolism. The analysis further showed smaller, but significant regional differences of oxygen extraction fraction at rest.

Validation of kinetic modeling of [15O]H2O PET using an image derived input function on hybrid PET/MRI

In present study we aimed to validate the use of image-derived input functions (IDIF) in the kinetic modeling of cerebral blood flow (CBF) measured by [¹⁵O]H₂O PET by comparing with the accepted reference standard arterial input function (AIF). Additional comparisons were made to mean cohort AIF and CBF values acquired by methodologically independent phase-contrast mapping (PCM) MRI. Using hybrid PET/MRI an IDIF was generated by measuring the radiotracer concentration in the internal carotid arteries and correcting for partial volume effects using the intravascular volume measured from MRI-angiograms. Seven patients with carotid steno-occlusive disease and twelve healthy controls were examined at rest, after administration of acetazolamide, and, in the control group, during hyperventilation. Agreement between the techniques was examined by linear regression and Bland-Altman analysis. Global CBF values modeled using IDIF correlated with values from AIF across perfusion states in both patients (p<10^-6, R²=0.82, 95% limits of agreement (LoA)=[-11.3-9.9] ml/100 g/min) and controls (p<10^-6, R²=0.87, 95% LoA=[-17.1-13.7] ml/100 g/min). The reproducibility of gCBF using IDIF was identical to AIF (15.8%). Values from IDIF and AIF had equally good correlation to measurements by PCM MRI, R²=0.86 and R²=0.84, (p<10^-6), respectively. Mean cohort AIF performed substantially worse than individual IDIFs (p<10^-6, R²=0.63, LoA=[-12.8-25.3] ml/100 g/min). In the patient group, use of IDIF provided similar reactivity maps compared to AIF. In conclusion, global CBF values modeled using IDIF correlated with values modeled by AIF and similar perfusion deficits could be established in a patient group.

Glutamate levels and perfusion in pons during migraine attacks: A 3T MRI study using proton spectroscopy and arterial spin labeling

Migraine is a complex disorder, involving peripheral and central brain structures, where mechanisms and site of attack initiation are an unresolved puzzle. While abnormal pontine neuronal activation during migraine attacks has been reported, exact implication of this finding is unknown. Evidence suggests an important role of glutamate in migraine, implying a possible association of pontine hyperactivity to increased glutamate levels. Migraine without aura patients were scanned during attacks after calcitonin gene-related peptide and sildenafil in a double-blind, randomized, double-dummy, cross-over design, on two separate study days, by proton magnetic resonance spectroscopy and pseudo-continuous arterial spin labeling at 3T. Headache characteristics were recorded until 24 h after drug administrations. Twenty-six patients were scanned during migraine, yielding a total of 41 attacks. Cerebral blood flow increased in dorsolateral pons, ipsilateral to pain side during attacks, compared to outside attacks (13.6%, p = 0.009). Glutamate levels in the same area remained unchanged during attacks (p = 0.873), while total creatine levels increased (3.5%, p = 0.041). In conclusion, dorsolateral pontine activation during migraine was not associated with higher glutamate levels. However, the concurrently increased total creatine levels may suggest an altered energy metabolism, which should be investigated in future studies to elucidate the role of pons in acute migraine.

Predicting 15O-Water PET cerebral blood flow maps from multi-contrast MRI using a deep convolutional neural network with evaluation of training cohort bias

To improve the quality of MRI-based cerebral blood flow (CBF) measurements, a deep convolutional neural network (dCNN) was trained to combine single- and multi-delay arterial spin labeling (ASL) and structural images to predict gold-standard ¹⁵O-water PET CBF images obtained on a simultaneous PET/MRI scanner. The dCNN was trained and tested on 64 scans in 16 healthy controls (HC) and 16 cerebrovascular disease patients (PT) with 4-fold cross-validation. Fidelity to the PET CBF images and the effects of bias due to training on different cohorts were examined. The dCNN significantly improved CBF image quality compared with ASL alone (mean ± standard deviation): structural similarity index (0.854 ± 0.036 vs. 0.743 ± 0.045 [single-delay] and 0.732 ± 0.041 [multi-delay], P < 0.0001); normalized root mean squared error (0.209 ± 0.039 vs. 0.326 ± 0.050 [single-delay] and 0.344 ± 0.055 [multi-delay], P < 0.0001). The dCNN also yielded mean CBF with reduced estimation error in both HC and PT (P < 0.001), and demonstrated better correlation with PET. The dCNN trained with the mixed HC and PT cohort performed the best. The results also suggested that models should be trained on cases representative of the target population.

Validation of diffuse correlation spectroscopy against 15O-water PET for regional cerebral blood flow measurement in neonatal piglets

Diffuse correlation spectroscopy (DCS) can non-invasively and continuously asses regional cerebral blood flow (rCBF) at the cot-side by measuring a blood flow index (BFI) in non-traditional units of cm2/s. We have validated DCS against positron emission tomography using 15O-labeled water (15O-water PET) in a piglet model allowing us to derive a conversion formula for BFI to rCBF in conventional units (ml/100g/min). Neonatal piglets were continuously monitored by the BabyLux device integrating DCS and time resolved near infrared spectroscopy (TRS) while acquiring 15O-water PET scans at baseline, after injection of acetazolamide and during induced hypoxic episodes. BFI by DCS was highly correlated with rCBF (R = 0.94, p < 0.001) by PET. A scaling factor of 0.89 (limits of agreement for individual measurement: 0.56, 1.39)×109× (ml/100g/min)/(cm2/s) was used to derive baseline rCBF from baseline BFI measurements of another group of piglets and of healthy newborn infants showing an agreement with expected values. These results pave the way towards non-invasive, cot-side absolute CBF measurements by DCS on neonates.

Cerebrovascular effects of endothelin-1 investigated using high-resolution magnetic resonance imaging in healthy volunteers

Endothelin-1 (ET-1) is a highly potent vasoconstrictor peptide released from vascular endothelium. ET-1 plays a major role in cerebrovascular disorders and likely worsens the outcome of acute ischaemic stroke and aneurismal subarachnoid haemorrhage through vasoconstriction and cerebral blood flow (CBF) reduction. Disorders that increase the risk of stroke, including hypertension, diabetes mellitus, and acute myocardial infarction, are associated with increased plasma levels of ET-1. The in vivo human cerebrovascular effects of systemic ET-1 infusion have not previously been investigated. In a two-way crossover, randomized, double-blind design, we used advanced 3 tesla MRI methods to investigate the effects of high-dose intravenous ET-1 on intra- and extracranial artery circumferences, global and regional CBF, and cerebral metabolic rate of oxygen (CMRO₂) in 14 healthy volunteers. Following ET-1 infusion, we observed a 14% increase of mean arterial blood pressure, a 5% decrease of middle cerebral artery (MCA) circumference, but no effects on extracerebral arteries and no effects on CBF or CMRO₂. Collectively, the findings indicate MCA constriction secondarily to blood pressure increase and not due to a direct vasoconstrictor effect of ET-1. We suggest that, as opposed to ET-1 in the subarachnoid space, intravascular ET-1 does not exert direct cerebrovascular effects in humans.

Comparison of simultaneous arterial spin labeling MRI and 15O-H2O PET measurements of regional cerebral blood flow in rest and altered perfusion states

Arterial spin labelling (ASL) is a non-invasive magnetic resonance imaging (MRI) technique that may provide fully quantitative regional cerebral blood flow (rCBF) images. However, before its application in clinical routine, ASL needs to be validated against the clinical gold standard, ¹⁵O-H₂O positron emission tomography (PET). We aimed to compare the two techniques by performing simultaneous quantitative ASL-MRI and ¹⁵O-H₂O-PET examinations in a hybrid PET/MRI scanner. Duplicate rCBF measurements were performed in healthy young subjects (n = 14) in rest, during hyperventilation, and after acetazolamide (post-ACZ), yielding 63 combined PET/MRI datasets in total. Average global CBF by ASL-MRI and ¹⁵O-H₂O-PET was not significantly different in any state (40.0 ± 6.5 and 40.6 ± 4.1 mL/100 g/min, respectively in rest, 24.5 ± 5.1 and 23.4 ± 4.8 mL/100 g/min, respectively, during hyperventilation, and 59.1 ± 10.4 and 64.7 ± 10.0 mL/100 g/min, respectively, post-ACZ). Overall, strong correlation between the two methods was found across all states (slope = 1.01, R²= 0.82), while the correlations within individual states and of reactivity measures were weaker, in particular in rest (R²= 0.05, p = 0.03). Regional distribution was similar, although ASL yielded higher perfusion and absolute reactivity in highly vascularized areas. In conclusion, ASL-MRI and ¹⁵O-H₂O-PET measurements of rCBF are highly correlated across different perfusion states, but with variable correlation within and between hemodynamic states, and systematic differences in regional distribution.

Higher physiological vulnerability to hypoxic exposure with advancing age in the human brain

The aging brain is associated with atrophy along with functional and metabolic changes. In this study, we examined age-related changes in resting brain functions and the vulnerability of brain physiology to hypoxic exposure in humans in vivo. Brain functions were examined in 81 healthy humans (aged 18-62 years) by acquisitions of gray and white matter volumes, cerebral blood flow, cerebral oxygen consumption, and concentrations of lactate, N-acetylaspartate, and glutamate+glutamine using magnetic resonance imaging and spectroscopy. We observed impaired cerebral blood flow reactivity in response to inhalation of hypoxic air (p = 0.029) with advancing age along with decreased cerebral oxygen consumption (p = 0.036), and increased lactate concentration (p = 0.009), indicating tissue hypoxia and impaired metabolism. Diminished resilience to hypoxia and consequently increased vulnerability to metabolic stress could be a key part of declining brain health with age. Furthermore, we observed increased resting cerebral lactate concentration with advancing age (p = 0.007), which might reflect inhibited brain clearance of waste products.

Phase contrast mapping MRI measurements of global cerebral blood flow across different perfusion states - A direct comparison with 15O-H2O positron emission tomography using a hybrid PET/MR system

Phase-contrast mapping (PCM) magnetic resonance imaging (MRI) provides easy-access non-invasive quantification of global cerebral blood flow (gCBF) but its accuracy in altered perfusion states is not established. We aimed to compare paired PCM MRI and 15O-H2O positron emission tomography (PET) measurements of gCBF in different perfusion states in a single scanning session. Duplicate combined gCBF PCM-MRI and 15O-H2O PET measurements were performed in the resting condition, during hyperventilation and after acetazolamide administration (post-ACZ) using a 3T hybrid PET/MR system. A total of 62 paired gCBF measurements were acquired in 14 healthy young male volunteers. Average gCBF in resting state measured by PCM-MRI and 15O-H2O PET were 58.5 ± 10.7 and 38.6 ± 5.7 mL/100 g/min, respectively, during hyperventilation 33 ± 8.6 and 24.7 ± 5.8 mL/100 g/min, respectively, and post-ACZ 89.6 ± 27.1 and 57.3 ± 9.6 mL/100 g/min, respectively. On average, gCBF measured by PCM-MRI was 49% higher compared to 15O-H2O PET. A strong correlation between the two methods across all states was observed (R2 = 0.72, p < 0.001). Bland-Altman analysis suggested a perfusion dependent relative bias resulting in higher relative difference at higher CBF values. In conclusion, measurements of gCBF by PCM-MRI in healthy volunteers show a strong correlation with 15O-H2O PET, but are associated with a large and non-linear perfusion-dependent difference.

MRI-Driven PET Image Optimization for Neurological Applications

Positron emission tomography (PET) and magnetic resonance imaging (MRI) are established imaging modalities for the study of neurological disorders, such as epilepsy, dementia, psychiatric disorders and so on. Since these two available modalities vary in imaging principle and physical performance, each technique has its own advantages and disadvantages over the other. To acquire the mutual complementary information and reinforce each other, there is a need for the fusion of PET and MRI. This combined dual-modality (either sequential or simultaneous) could generate preferable soft tissue contrast of brain tissue, flexible acquisition parameters, and minimized exposure to radiation. The most unique superiority of PET/MRI is mainly manifested in MRI-based improvement for the inherent limitations of PET, such as motion artifacts, partial volume effect (PVE) and invasive procedure in quantitative analysis. Head motion during scanning significantly deteriorates the effective resolution of PET image, especially for the dynamic scan with lengthy time. Hybrid PET/MRI device can offer motion correction (MC) for PET data through MRI information acquired simultaneously. Regarding the PVE associated with limited spatial resolution, the process and reconstruction of PET data can be further optimized by using acquired MRI either sequentially or simultaneously. The quantitative analysis of dynamic PET data mainly relies upon an invasive arterial blood sampling procedure to acquire arterial input function (AIF). An image-derived input function (IDIF) method without the need of arterial cannulization, can serve as a potential alternative estimation of AIF. Compared with using PET data only, combining anatomical or functional information from MRI for improving the accuracy in IDIF approach has been demonstrated. Yet, due to the interference and inherent disparity between the two modalities, these methods for optimizing PET image based on MRI still have many technical challenges. This review discussed upon the most recent progress, current challenges and future directions of MRI-driven PET data optimization for neurological applications, with either sequential or simultaneous acquisition approach.

Cerebral metabolism and vascular reactivity during breath-hold and hypoxic challenge in freedivers and healthy controls

The goal of the present study was to examine the cerebral metabolism and vascular reactivity during extended breath-holds (ranging from 2 min 32 s to 7 min 0 s) and during a hypoxic challenge in freedivers and non-diver controls. Magnetic resonance imaging was used to measure the global cerebral blood flow (CBF) and metabolic rate of oxygen (CMRO₂), and magnetic resonance spectroscopy was used to measure the cerebral lactate, glutamate+glutamine, N-acetylaspartate and phosphocreatine+creatine concentrations in the occipital lobe. Fifteen freedivers and seventeen non-diver controls participated. The freedivers showed remarkable increases in CBF (107%) during the breath-holds, compensating for arterial desaturation, and sustained cerebral oxygen delivery (CDO₂). CMRO₂ was unaffected throughout the breath-holds. During the hypoxic challenge, the freedivers had larger increases in blood flow in the sagittal sinus than the non-divers, and could sustain normal CDO₂. No differences were found in lactate production, global CBF or CMRO₂. We conclude that the mechanism for sustaining brain function during breath-holding in freedivers involves an extraordinary increase in perfusion, and that freedivers present evidence for higher cerebrovascular reactivity, but not for higher lactate-producing glycolysis during a hypoxic challenge compared to controls.

Simultaneous phase-contrast MRI and PET for noninvasive quantification of cerebral blood flow and reactivity in healthy subjects and patients with cerebrovascular disease

BACKGROUND

H₂ ¹⁵ O-positron emission tomography (PET) is considered the reference standard for absolute cerebral blood flow (CBF). However, this technique requires an arterial input function measured through continuous sampling of arterial blood, which is invasive and has limitations with tracer delay and dispersion.

PURPOSE

To demonstrate a new noninvasive method to quantify absolute CBF with a PET/MRI hybrid scanner. This blood-free approach, called PC-PET, takes the spatial CBF distribution from a static H₂ ¹⁵ O-PET scan, and scales it to the whole-brain average CBF value measured by simultaneous phase-contrast MRI.

STUDY TYPE

Observational.

SUBJECTS

Twelve healthy controls (HC) and 13 patients with Moyamoya disease (MM) as a model of chronic ischemic disease.

FIELD STRENGTH/SEQUENCES

3T/2D cardiac-gated phase-contrast MRI and H₂ ¹⁵ O-PET.

ASSESSMENT

PC-PET CBF values from whole brain (WB), gray matter (GM), and white matter (WM) in HCs were compared with literature values since 2000. CBF and cerebrovascular reactivity (CVR), which is defined as the percent CBF change between baseline and post-acetazolamide (vasodilator) scans, were measured by PC-PET in MM patients and HCs within cortical regions corresponding to major vascular territories. Statistical Tests: Linear, mixed effects models were created to compare CBF and CVR, respectively, between patients and controls, and between different degrees of stenosis.

RESULTS

The mean CBF values in WB, GM, and WM in HC were 42 ± 7 ml/100 g/min, 50 ± 7 ml/100 g/min, and 23 ± 3 ml/100 g/min, respectively, which agree well with literature values. Compared with normal regions (57 ± 23%), patients showed significantly decreased CVR in areas with mild/moderate stenosis (47 ± 17%, P = 0.011) and in severe/occluded areas (40 ± 16%, P = 0.016). Data Conclusion: PC-PET identifies differences in cerebrovascular reactivity between healthy controls and cerebrovascular patients. PC-PET is suitable for CBF measurement when arterial blood sampling is not accessible, and warrants comparison to fully quantitative H₂ ¹⁵ O-PET in future studies.

LEVEL OF EVIDENCE

3 Technical Efficacy Stage: 2 J. Magn. Reson. Imaging 2019. J. Magn. Reson. Imaging 2020;51:183-194.

Effect of hypoxia on BOLD fMRI response and total cerebral blood flow in migraine with aura patients

Experimentally induced hypoxia triggers migraine and aura attacks in patients suffering from migraine with aura (MA). We investigated the blood oxygenation level-dependent (BOLD) signal response to visual stimulation during hypoxia in MA patients and healthy volunteers. In a randomized double-blind crossover study design, 15 MA patients were allocated to 180 min of normobaric poikilocapnic hypoxia (capillary oxygen saturation 70-75%) or sham (normoxia) on two separate days and 14 healthy volunteers were exposed to hypoxia. The BOLD functional MRI (fMRI) signal response to visual stimulation was measured in the visual cortex ROIs V1-V5. Total cerebral blood flow (CBF) was calculated by measuring the blood velocity in the internal carotid arteries and the basilar artery using phase-contrast mapping (PCM) MRI. Hypoxia induced a greater decrease in BOLD response to visual stimulation in V1-V4 in MA patients compared to controls. There was no group difference in hypoxia-induced total CBF increase. In conclusion, the study demonstrated a greater hypoxia-induced decrease in BOLD response to visual stimulation in MA patients. We suggest this may represent a hypoxia-induced change in neuronal excitability or abnormal vascular response to visual stimulation, which may explain the increased sensibility to hypoxia in these patients leading to migraine attacks.

Physiology of static breath holding in elite apneists

NEW FINDINGS

What is the topic of this review? This review provides an up-to-date assessment of the physiology involved with extreme static dry-land breath holding in trained apneists. What advances does it highlight? We specifically highlight the recent findings involved with the cardiovascular, cerebrovascular and metabolic function during a maximal breath hold in elite apneists.

ABSTRACT

Breath-hold-related activities have been performed for centuries, but only recently, within the last ∼30 years, has it emerged as an increasingly popular competitive sport. In apnoea sport, competition relates to underwater distances or simply maximal breath-hold duration, with the current (oxygen-unsupplemented) static breath-hold record at 11 min 35 s. Remarkably, many ultra-elite apneists are able to suppress respiratory urges to the point where consciousness fundamentally limits a breath-hold duration. Here, arterial oxygen saturations as low as ∼50% have been reported. In such cases, oxygen conservation to maintain cerebral functioning is critical, where responses ascribed to the mammalian dive reflex, e.g. sympathetically mediated peripheral vasoconstriction and vagally mediated bradycardia, are central. In defence of maintaining global cerebral oxygen delivery during prolonged breath holds, the cerebral blood flow may increase by ∼100% from resting values. Interestingly, near the termination of prolonged dry static breath holds, recent studies also indicate that reductions in the cerebral oxidative metabolism can occur, probably attributable to the extreme hypercapnia and irrespective of the hypoxaemia. In this review, we highlight and discuss the recent data on the cardiovascular, metabolic and, particularly, cerebrovascular function in competitive apneists performing maximal static breath holds. The physiological adaptation and maladaptation with regular breath-hold training are also summarized, and future research areas in this unique physiological field are highlighted; particularly, the need to determine the potential long-term health impacts of extreme breath holding.

Accuracy of blood flow assessment in cerebral arteries with 4D flow MRI: Evaluation with three segmentation methods

Background

Accelerated 4D flow MRI allows for high‐resolution velocity measurements with whole‐brain coverage. Such scans are increasingly used to calculate flow rates of individual arteries in the vascular tree, but detailed information about the accuracy and precision in relation to different postprocessing options is lacking.

Purpose

To evaluate and optimize three proposed segmentation methods and determine the accuracy of in vivo 4D flow MRI blood flow rate assessments in major cerebral arteries, with high‐resolution 2D PCMRI as a reference.

Study Type

Prospective.

Subjects

Thirty‐five subjects (20 women, 79 ± 5 years, range 70–91 years).

Field Strength/Sequence

4D flow MRI with PC‐VIPR and 2D PCMRI acquired with a 3 T scanner.

Assessment

We compared blood flow rates measured with 4D flow MRI, to the reference, in nine main cerebral arteries. Lumen segmentation in the 4D flow MRI was performed with k‐means clustering using four different input datasets, and with two types of thresholding methods. The threshold was defined as a percentage of the maximum intensity value in the complex difference image. Local and global thresholding approaches were used, with evaluated thresholds from 6–26%.

Statistical Tests

Paired t‐test, F‐test, linear correlation (P < 0.05 was considered significant) along with intraclass correlation (ICC).

Results

With the thresholding methods, the lowest average flow difference was obtained for 20% local (0.02 ± 15.0 ml/min, ICC = 0.97, n = 310) or 10% global (0.08 ± 17.3 ml/min, ICC = 0.97, n = 310) thresholding with a significant lower standard deviation for local (F‐test, P = 0.01). For all clustering methods, we found a large systematic underestimation of flow compared with 2D PCMRI (16.1–22.3 ml/min).

Data Conclusion

A locally adapted threshold value gives a more stable result compared with a globally fixed threshold. 4D flow with the proposed segmentation method has the potential to become a useful reliable clinical tool for assessment of blood flow in the major cerebral arteries.

Level of Evidence: 2

Technical Efficacy: Stage 2

J. Magn. Reson. Imaging 2019;50:511–518.

Cerebral blood flow, oxygen metabolism, and lactate during hypoxia in patients with obstructive sleep apnea

Study Objectives

Obstructive sleep apnea (OSA) is associated with increased risk of stroke but the underlying mechanism is poorly understood. We suspect that the normal cerebrovascular response to hypoxia is disturbed in patients with OSA.

Methods

Global cerebral blood flow (CBF), cerebral metabolic rate of oxygen (CMRO2), and lactate concentration during hypoxia were measured in patients with OSA and matched controls. Twenty-eight patients (82.1% males, mean age 52.3 ± 10.0 years) with moderate-to-severe OSA assessed by partial polysomnography were examined and compared with 19 controls (73.7% males, mean age 51.8 ± 10.1 years). Patients and controls underwent magnetic resonance imaging (MRI) during 35 min of normoxia followed by 35 min inhaling hypoxic air (10%-12% O2). After 3 months of continuous positive airway pressure (CPAP) treatment, 22 patients were rescanned.

Results

During hypoxia, CBF significantly increased with decreasing arterial blood oxygen concentration (4.53 mL (blood)/100 g/min per -1 mmol(O2)/L, p < 0.001) in the control group, but was unchanged (0.89 mL (blood)/100 g/min per -1 mmol(O2)/L, p = 0.289) in the patient group before CPAP treatment. The CBF response to hypoxia was significantly weaker in patients than in controls (p = 0.003). After 3 months of CPAP treatment the CBF response normalized, showing a significant increase during hypoxia (5.15 mL (blood)/100 g/min per -1 mmol(O2)/L, p < 0.001). There was no difference in CMRO2 or cerebral lactate concentration between patients and controls, and no effect of CPAP treatment.

Conclusions

Patients with OSA exhibit reduced CBF in response to hypoxia. CPAP treatment normalized these patterns.

Cerebrovascular Reactivity during Prolonged Breath-Hold in Experienced Freedivers

BACKGROUND AND PURPOSE

Experienced freedivers can endure prolonged breath-holds despite severe hypoxemia and are therefore ideal subjects to study apnea-induced cerebrovascular reactivity. This multiparametric study investigated CBF, the spatial coefficient of variation as a correlate of arterial transit time and brain metabolism, dynamics during prolonged apnea.

MATERIALS AND METHODS

Fifteen male freedivers (age range, 20-64 years; cumulative previous prolonged breath-holds >2 minutes and 30 seconds: 4-79,200) underwent repetitive 3T pseudocontinuous arterial spin-labeling and ³¹P-/¹H-MR spectroscopy before, during, and after a 5-minute breath-hold (split into early and late phases) and gave temporally matching venous blood gas samples. Correlation of temporal and regional cerebrovascular reactivity to blood gases and cumulative previous breath-holds of >2 minutes and 30 seconds in a lifetime was assessed.

RESULTS

The spatial coefficient of variation of CBF (by arterial spin-labeling) decreased during the early breath-hold phase (-30.0%, P = .002), whereas CBF remained almost stable during this phase and increased in the late phase (+51.8%, P = .001). CBF differed between the anterior and the posterior circulation during all phases (eg, during late breath-hold: MCA, 57.3 ± 14.2 versus posterior cerebral artery, 42.7 ± 10.8 mL/100 g/min; P = .001). There was an association between breath-hold experience and lower CBF (1000 previous breath-holds reduced WM CBF by 0.6 mL/100 g/min; 95% CI, 0.15-1.1 mL/100 g/min; P = .01). While breath-hold caused peripheral lactate rise (+18.5%) and hypoxemia (oxygen saturation, -24.0%), cerebral lactate and adenosine diphosphate remained within physiologic ranges despite early signs of oxidative stress [-6.4% phosphocreatine / (adenosine triphosphate + adenosine diphosphate); P = .02].

CONCLUSIONS

This study revealed that the cerebral energy metabolism of trained freedivers withstands severe hypoxic hypercarbia in prolonged breath-hold due to a complex cerebrovascular hemodynamic response.

Interindividual and regional relationship between cerebral blood flow and glucose metabolism in the resting brain

Studies of the resting brain measurements of cerebral blood flow (CBF) show large interindividual and regional variability, but the metabolic basis of this variability is not fully established. The aim of the present study was to reassess regional and interindividual relationships between cerebral perfusion and glucose metabolism in the resting brain. Regional quantitative measurements of CBF and cerebral metabolic rate of glucose (CMR_glc) were obtained in 24 healthy young men using dynamic [¹⁵O]H₂O and [¹⁸F]fluorodeoxyglucose positron emission tomography (PET). Magnetic resonance imaging measurements of global oxygen extraction fraction (gOEF) and metabolic rate of oxygen ([Formula: see text]) were obtained by combined susceptometry-based sagittal sinus oximetry and phase contrast mapping. No significant interindividual associations between global CBF, global CMR_glc, and [Formula: see text] were observed. Linear mixed-model analysis showed a highly significant association of CBF with CMR_glc regionally. Compared with neocortex significantly higher CBF values than explained by CMR_glc were demonstrated in infratentorial structures, thalami, and mesial temporal cortex, and lower values were found in the striatum and cerebral white matter. The present study shows that absolute quantitative global CBF measurements appear not to be a valid surrogate measure of global cerebral glucose or oxygen consumption, and further demonstrates regionally variable relationship between perfusion and glucose metabolism in the resting brain that could suggest regional differences in energy substrate metabolism. NEW & NOTEWORTHY Using method-independent techniques the study cannot confirm direct interindividual correlations of absolute global values of perfusion with oxygen or glucose metabolism in the resting brain, and absolute global perfusion measurements appear not to be valid surrogate measures of cerebral metabolism. The ratio of both perfusion and oxygen delivery to glucose metabolism varies regionally, also when accounting for known methodological regional bias in quantification of glucose metabolism.

Components of day-to-day variability of cerebral perfusion measurements - Analysis of phase contrast mapping magnetic resonance imaging measurements in healthy volunteers

Purpose

The aim of the study was to investigate the components of day-to-day variability of repeated phase contrast mapping (PCM) magnetic resonance imaging measurements of global cerebral blood flow (gCBF).

Materials and methods

Two dataset were analyzed. In Dataset 1 duplicated PCM measurements of total brain flow were performed in 11 healthy young volunteers on two separate days applying a strictly standardized setup. For comparison PCM measurements obtained from a previously published study (Dataset 2) were analyzed in order to assess long-term variability in an aged population in a less strictly controlled setup. Global CBF was calculated by normalizing total brain flow to brain volume. On each day measurements of hemoglobin, caffeine and glucose were obtained. Linear mixed models were applied to estimate coefficients of variation (CV) of total (CV_t), between-subject (CV_b), within-subject day-to-day (CV_w), and intra-session residual variability (CV_r).

Results

In Dataset 1 CV_t, CV_b, CV_w and CV_r were estimated to be 11%, 9.4%, 4% and 4.2%, respectively, and to 8.8%, 7.2%, 2.7% and 4.3%, respectively, when adjusting for hemoglobin and plasma caffeine. In Dataset 2 CV_t, CV_b and CV_w were estimated to be 25.4%, 19.2%, and 15.0%, respectively, and decreased to 16.6%, 8.2% and 12.5%, respectively, when adjusting for the same covariates.

Discussion

Our results suggest that short-term day-to-day variability of gCBF is relatively low compared to between-subject variability when studied in standardized conditions, whereas long-term variability in an aged population appears to be much larger when studied in less a standardized setup. The results further showed that from 20% to 35% of the total variability in gCBF can be attributed to the effects of hemoglobin and caffeine.

No evidence for direct effects of recombinant human erythropoietin on cerebral blood flow and metabolism in healthy humans

Erythropoietin (EPO) is expressed in human brain tissue, but its exact role is unknown. EPO may improve the efficiency of oxidative metabolism and has neuroprotective properties against hypoxic injuries in animal models. We aimed to investigate the effect of recombinant human EPO (rHuEPO) administration on healthy cerebral metabolism in humans during normoxia and during metabolic stress by inhalation of 10% O₂ hypoxic air. Twenty-four healthy men participated in a two-arm double-blind placebo-controlled trial. rHuEPO was administered as a low dose (5,000 IU) over 4 wk ( n = 12) or as a high dose (500 IU·kg body wt^-1·day^-1) for three consecutive days ( n = 12). Global cerebral blood flow (CBF) and metabolic rate of glucose (CMR_glc) were measured with positron emission tomography. CBF, metabolic rate of oxygen ([Formula: see text]), and cerebral lactate concentration were measured by magnetic resonance imaging and spectroscopy. Low-dose treatment increased hemoglobin and was associated with a near-significant decrease in CBF during baseline normoxia. High-dose treatment caused no change in CBF. Neither treatment had an effect on normoxia CMR_glc, [Formula: see text], or lactate concentration or an effect on the cerebral metabolic response to inhalation of hypoxic air. In conclusion, the study found no evidence for a direct effect of rHuEPO on cerebral metabolism. NEW & NOTEWORTHY We demonstrate with magnetic resonance imaging and positron emission tomography that administration of erythropoietin does not have a substantial direct effect on healthy human resting cerebral blood flow or effect on cerebral glucose and oxygen metabolism. Also, administration of erythropoietin did not have a direct effect on the metabolic response to acute hypoxic stress in healthy humans, and a suggested neuroprotective effect from erythropoietin is therefore likely not a direct effect of erythropoietin on cerebral metabolism.

Lower cerebral blood flow in subjects with Alzheimer's dementia, mild cognitive impairment, and subjective cognitive decline using two-dimensional phase-contrast magnetic resonance imaging

Introduction

In this cross-sectional study, we aimed to detect differences in cerebral blood flow (CBF) between subjects with Alzheimer's disease (AD), mild cognitive impairment (MCI), and subjective cognitive decline (SCD), using two-dimensional phase-contrast magnetic resonance imaging.

Methods

We included 74 AD patients (67 years, 51% female), 36 MCI patients (66 years, 33% female), and 62 patients with SCD (60 years, 32% female) from the Amsterdam Dementia Cohort. Patients with SCD are those who visited the memory clinic with subjective cognitive complaints without objective cognitive impairment. Whole-brain CBF (mL/100 g/min) was calculated using total volume flow measured with two-dimensional phase-contrast magnetic resonance imaging and normalized for brain volume.

Results

Mean CBF values (SD) were lower in AD compared to SCD (age and sex adjusted 70 ± 26 vs. 82 ± 24 mL/100 g/min, P < .05). Mean CBF values of MCI were comparable to AD. Across clinical groups, lower CBF was associated with lower scores on the Mini–Mental State Examination (age and sex adjusted stβ = 0.19 per mL/100 g/min; P = .02).

Discussion

Lower whole-brain CBF is seen in AD patients compared to SCD patients and is associated with worse cognitive function.

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The study consisted of a large sample of patients with AD, MCI, and controls.

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CBF measured with 2D PC MRI differed between AD patients and controls.

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Lower CBF was associated with worse cognitive function measured with MMSE.

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2D PC MRI may be used as a marker for disease severity in a memory clinic.

3D phase contrast MRI: Partial volume correction for robust blood flow quantification in small intracranial vessels

PURPOSE

Recent advances in 3D-PCMRI (phase contrast MRI) sequences allow for measuring the complex hemodynamics in cerebral arteries. However, the small size of these vessels vs spatial resolution can lead to non-negligible partial volume artifacts, which must be taken into account when computing blood flow rates. For this purpose, we combined the velocity information provided by 3D-PCMRI with vessel geometry measured with 3DTOF (time of flight MRI) or 3DRA (3D rotational angiography) to correct the partial volume effects in flow rate assessments.

METHODS

The proposed methodology was first tested in vitro on cylindrical and patient specific vessels subject to fully controlled pulsatile flows. Both 2D- and 3D-PCMRI measurements using various spatial resolutions ranging from 20 to 1.3 voxels per vessel diameter were analyzed and compared with flowmeter baseline. Second, 3DTOF, 2D- and 3D-PCMRI measurements were performed in vivo on 35 patients harboring internal carotid artery (ICA) aneurysms indicated for endovascular treatments requiring 3DRA imaging.

RESULTS

The in vitro 2D- and 3D-PCMRI mean flow rates assessed with partial volume correction showed very low sensitivity to the acquisition resolution above ≈2 voxels per vessel diameter while uncorrected flow rates deviated critically when decreasing the spatial resolution. 3D-PCMRI flow rates measured in vivo in ICA agreed very well with 2D-PCMRI data and a good flow conservation was observed at the C7 bifurcation. Globally, partial volume correction led to 10-15% lower flow rates than uncorrected values as those reported in most of the published studies on intracranial flows.

CONCLUSION

Partial volume correction may improve the accuracy of PCMRI flow rate measurements especially in small vessels such as intracranial arteries. Magn Reson Med 79:129-140, 2018. © 2017 International Society for Magnetic Resonance in Medicine.