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).

Brain perfusion estimation by Tikhonov model-free deconvolution in a long axial field of view PET/CT scanner exploring five different PET tracers

PURPOSE

New total-body PET scanners with a long axial field of view (LAFOV) allow for higher temporal resolution due to higher sensitivity, which facilitates perfusion estimation by model-free deconvolution. Fundamental tracer kinetic theory predicts that perfusion can be estimated for all tracers despite their different fates given sufficiently high temporal resolution of 1 s or better, bypassing the need for compartment modelling. The aim of this study was to investigate whether brain perfusion could be estimated using model-free Tikhonov generalized deconvolution for five different PET tracers, [15O]H2O, [11C]PIB, [18F]FE-PE2I, [18F]FDG and [18F]FET. To our knowledge, this is the first example of a general model-free approach to estimate cerebral blood flow (CBF) from PET data.

METHODS

Twenty-five patients underwent dynamic LAFOV PET scanning (Siemens, Quadra). PET images were reconstructed with an isotropic voxel resolution of 1.65 mm3. Time framing was 40 × 1 s during bolus passage followed by increasing framing up to 60 min. AIF was obtained from the descending aorta. Both voxel- and region-based calculations of perfusion in the thalamus were performed using the Tikhonov method. The residue impulse response function was used to estimate the extraction fraction of tracer leakage across the blood-brain barrier.

RESULTS

CBF ranged from 37 to 69 mL blood min-1 100 mL of tissue-1 in the thalamus. Voxelwise calculation of CBF resulted in CBF maps in the physiologically normal range. The extraction fractions of [15O]H2O, [18F]FE-PE2I, [11C]PIB, [18F]FDG and [18F]FET in the thalamus were 0.95, 0.78, 0.62, 0.19 and 0.03, respectively.

CONCLUSION

The high temporal resolution and sensitivity associated with LAFOV PET scanners allow for noninvasive perfusion estimation of multiple tracers. The method provides an estimation of the residue impulse response function, from which the fate of the tracer can be studied, including the extraction fraction, influx constant, volume of distribution and transit time distribution, providing detailed physiological insight into normal and pathologic tissue.

Early cerebral amyloid-β accumulation and hypermetabolism are associated with subtle cognitive deficits before accelerated cerebral atrophy

AIMS

Alzheimer's disease (AD) is characterized by the accumulation of amyloid beta (Aβ) in the brain. The deposition of Aβ is believed to initiate a detrimental cascade, including cerebral hypometabolism, accelerated brain atrophy, and cognitive problems-ultimately resulting in AD. However, the timing and causality of the cascade resulting in AD are not yet fully established. Therefore, we examined whether early Aβ accumulation affects cerebral glucose metabolism, atrophy rate, and age-related cognitive decline before the onset of neurodegenerative disease.

METHODS

Participants from the Metropolit 1953 Danish Male Birth Cohort underwent brain positron emission tomography (PET) imaging using the radiotracers [11C]Pittsburgh Compound-B (PiB) (N = 70) and [18F]Fluorodeoxyglucose (FDG) (N = 76) to assess cerebral Aβ accumulation and glucose metabolism, respectively. The atrophy rate was calculated from anatomical magnetic resonance imaging (MRI) scans conducted presently and 10 years ago. Cognitive decline was examined from neurophysiological tests conducted presently and ten or 5 years ago.

RESULTS

Higher Aβ accumulation in AD-critical brain regions correlated with greater visual memory decline (p = 0.023). Aβ accumulation did not correlate with brain atrophy rates. Increased cerebral glucose metabolism in AD-susceptible regions correlated with worse verbal memory performance (p = 0.040).

CONCLUSIONS

Aβ accumulation in known AD-related areas was associated with subtle cognitive deficits. The association was observed before hypometabolism or accelerated brain atrophy, suggesting that Aβ accumulation is involved early in age-related cognitive dysfunction. The association between hypermetabolism and worse memory performance may be due to early compensatory mechanisms adapting for malfunctioning neurons by increasing metabolism.

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.

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 and Optimal Arterial Input Function Selection in Dynamic Contrast-Enhanced Perfusion MRI in the Healthy Brain

BACKGROUND

Dynamic contrast-enhanced MRI (DCE-MRI) has seen increasing use for quantification of low level of blood-brain barrier (BBB) leakage in various pathological disease states and correlations with clinical outcomes. However, currently there exists limited studies on reproducibility in healthy controls, which is important for the establishment of a normality threshold for future research.

PURPOSE

To investigate the reproducibility of DCE-MRI and to evaluate the effect of arterial input function (AIF) selection and manual region of interests (ROI) delineation vs. automated global segmentation.

STUDY TYPE

Prospective.

POPULATION

A total of 16 healthy controls; 11 females; mean age 28.7 years (SD 10.1).

FIELD STRENGTH/SEQUENCE

A 3T; GE DCE; 3D TFE T1WI. 2D TSE T2.

ASSESSMENT

The influx constant K_i , a measure of BBB permeability, and V_p , the blood plasma volume, was calculated using the Patlak model. Cerebral blood flow (CBF) was calculated using Tikhonov model free deconvolution. Manual tissue ROIs, drawn by H.J.S. (30+ years of experience), were compared to automatic tissue segmentation.

STATISTICAL TESTS

Intraclass correlation coefficient (ICC) and repeatability coefficient (RC) was used to assess reproducibility. Bland-Altman plots were used to evaluate agreement between measurements day 1 vs. day 2, and manual vs. segmentation method.

RESULTS

K_i showed excellent reproducibility in both white and gray matter with an ICC between 0.79 and 0.82 and excellent agreement between manual ROI and automatic segmentation, with an ICC of 0.89 for K_i in WM. Furthermore, K_i values in gray and white matter conforms with histological tissue characteristics, where gray matter generally has a 2-fold higher vessel density. The highest reproducibility measures of K_i (ICC = 0.83), CBF (ICC = 0.77) and V_d (ICC = 0.83) was obtained with the AIF sampled in the internal carotid artery (ICA).

DATA CONCLUSION

DCE-MRI shows excellent reproducibility of pharmacokinetic variables derived from healthy controls.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 2.

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.

Changes in hippocampal volume during a preceding 10-year period do not correlate with cognitive performance and hippocampal blood‒brain barrier permeability in cognitively normal late-middle-aged men

Hippocampal blood-brain barrier (BBB) permeability may increase in normal healthy ageing and contribute to neurodegenerative disease. To examine this hypothesis, we investigated the correlation between blood-brain barrier (BBB) permeability, regional brain volume, memory functions and health and lifestyle factors in The Metropolit 1953 Danish Male Birth Cohort. We used dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) with a gadolinium-based contrast agent to assess BBB permeability in 77 participants in the cohort. BBB permeability was measured as K_i values in the hippocampus, thalamus and white matter. Over a 10-year period, we observed progressive atrophy of both the left and right hippocampus (p = 0.001). There was no significant correlation between current BBB permeability and hippocampal volume, prior atrophy or cognition. The hippocampus volume ratio was associated with better visual and verbal memory scores (p < 0.01). Regional BBB differences revealed higher K_i values in the hippocampus and white matter than in the thalamus (p < 0.001). Participants diagnosed with type II diabetes had significantly higher BBB permeability in the white matter (p = 0.015) and thalamus (p = 0.016), which was associated with a higher Fazekas score (p = 0.024). We do not find evidence that BBB integrity is correlated with age-related hippocampal atrophy or cognitive functions. The association between diabetes, white matter hyperintensities and increased BBB permeability is consistent with the idea that cerebrovascular disease compromises BBB integrity. Our findings suggest that the hippocampus is particularly prone to age-related atrophy, which may explain some of the cognitive changes that accompany older age, but this prior atrophy is not correlated with current BBB permeability.

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.

Interictal pontine metabolism in migraine without aura patients: A 3 Tesla proton magnetic resonance spectroscopy study

In the pons, glutamatergic mechanisms are involved in regulating inhibitory descending pain modulation, serotoninergic neurotransmission as well as modulating the sensory transmission of the trigeminovascular system. Migraine involves altered pontine activation and structural changes, while biochemical, genetic and clinical evidence suggests that altered interictal pontine glutamate levels may be an important pathophysiological feature of migraine abetting to attack initiation. Migraine without aura patients were scanned outside attacks using a proton magnetic resonance spectroscopy protocol optimized for the pons at 3 Tesla. The measurements were performed on two separate days to increase accuracy and compared to similar repeated measurements in healthy controls. We found that interictal glutamate (i.e. Glx) levels in the pons of migraine patients (n = 33) were not different from healthy controls (n = 16) (p = 0.098), while total creatine levels were markedly increased in patients (9%, p = 0.009). There was no correlation of glutamate or total creatine levels to migraine frequency, days since the last attack, usual pain intensity of attacks or disease duration. In conclusion, migraine is not associated with altered interictal pontine glutamate levels. However, the novel finding of increased total creatine levels suggests that disequilibrium in the pontine energy metabolism could be an important feature of migraine pathophysiology.

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.

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.

Feasibility of Glutamate and GABA Detection in Pons and Thalamus at 3T and 7T by Proton Magnetic Resonance Spectroscopy

Glutamate detection in pons and thalamus using proton magnetic resonance spectroscopy (¹H-MRS) after an intervention is of interest for studying various brain disorders. However, ¹H-MRS in these brain regions is challenging and time-consuming, especially in longitudinal study designs. ¹H-MRS of more cortical structures at the ultrahigh magnetic field strength of 7T yields an improved spectral output, including separation of the glutamate signal from the glutamine signal, in a shorter and more feasible scan time, as compared to conventional clinical field strengths. For this purpose, we compared the feasibility of ¹H-MRS at 3T and 7T in pons and thalamus by applying a longitudinal study design of repeated measures on same day and three separate days at both field strength in five healthy participants. Total ¹H-MRS acquisition time was reduced by a factor 3.75 for pons and by a factor 3 for thalamus at 7T as compared to 3T. We found higher spectral signal-to-noise ratio (SNR) (p < 0.001), lower linewidth (p = 0.001) and lower Cramér–Rao lower bounds (CRLB) (p < 0.001) for the combined glutamate and glutamine signal (Glx) in thalamus at 7T as compared to 3T. In pons, CRLB of Glx and SNR were lower at 7T (p = 0.002 and p = 0.006), with no differences in linewidth compared to 3T. Mean within-subject variability of Glx concentration estimates was lower at 7T compared to 3T for both pons and thalamus. At 7T, it was possible to assess glutamate and γ-aminobutyric acid (GABA) simultaneously in pons and thalamus. In conclusion, ¹H-MRS at 7T resulted in improved spectral quality while allowing shorter scan times than at 3T as well as estimation of the pure glutamate signal in pons and thalamus. This opens up the opportunity for multimodal study designs and multiregional subcortical ¹H-MRS research. Glutamate and GABA measurement at 7T in pons and thalamus is advantageous for future investigations of excitatory–inhibitory mechanisms in brain disorders.

PET imaging of meningioma with 18F-FLT: a predictor of tumour progression

We have previously reported that PET with 3′-deoxy-3′-18F-fluorothymidine (18F-FLT) provides a non-invasive assessment of cell proliferation in vivo in meningiomas. The purpose of this prospective study was to evaluate the potential of 18F-FLT PET in predicting subsequent tumour progression in asymptomatic meningiomas. Forty-three adult patients harbouring 46 MRI-presumed (n = 40) and residual meningiomas from previous surgery (n = 6) underwent a 60-min dynamic 18F-FLT PET scan prior to radiological surveillance. Maximum and mean tumour-to-blood ratios (TBRmax, TBRmean) of tracer radioactivity were calculated. Tumour progression was defined according to the latest published trial end-point criteria for bidimensional (2D) and corresponding yet exploratory volumetric measurements from the Response Assessment of Neuro-Oncology (RANO) workgroup. Independent-sample t-test, Pearson correlation coefficient, Cox regression, and receiver operating characteristic (ROC) curve analyses were used whenever appropriate. The median follow-up time after 18F-FLT PET imaging was 18 months (range 5–33.5 months). A high concordance rate (91%) was found with regard to disease progression using 2D-RANO (n = 11) versus volumetric criteria (n = 10). Using 2D-RANO criteria, 18F-FLT uptake was significantly increased in patients with progressive disease, compared to patients with stable disease (TBRmax, 5.5 ± 1.3 versus 3.6 ± 1.1, P &lt; 0.0001; TBRmean, 3.5 ± 0.8 versus 2.4 ± 0.7, P &lt; 0.0001). ROC analysis yielded optimal thresholds of 4.4 for TBRmax [sensitivity 82%, specificity 77%, accuracy 78%, and area under curve (AUC) 0.871; P &lt; 0.0001] and 2.8 for TBRmean (sensitivity 82%, specificity 77%, accuracy 78%, AUC 0.848; P = 0.001) for early differentiation of patients with progressive disease from patients with stable disease. Upon excluding patients with residual meningioma or patients with stable disease with less than 12 months follow-up, the thresholds remained unchanged with similar diagnostic accuracies. Moreover, positive correlations were found between absolute and relative tumour growth rates and 18F-FLT uptake (r &lt; 0.513, P &lt; 0.015) that remained similar when excluding patients with residual meningioma or patients with stable disease and shorter follow-up period. Diagnostic accuracies were slightly inferior at 76% when assessing disease progression using volumetric criteria, while the thresholds remained unchanged. Multivariate analysis revealed that TBRmax was the only independent predictor of tumour progression (P &lt; 0.046), while age, gender, baseline tumour size, tumour location, peritumoural oedema, and residual meningioma had no influence. The study reveals that 18F-FLT PET is a promising surrogate imaging biomarker for predicting subsequent tumour progression in treatment-naïve and asymptomatic residual meningiomas.

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.

Plasma Glucose Levels Increase During Spontaneous Attacks of Migraine With and Without Aura

OBJECTIVE

To investigate plasma glucose changes during the ictal state of migraine compared to the interictal state.

BACKGROUND

Previous studies suggest abnormal glucose metabolism in migraine patients during and outside of attacks. It is not known if plasma glucose levels change during spontaneous migraine attacks.

METHODS

Plasma glucose levels were measured during and outside of spontaneous migraine attacks with and without aura. Plasma glucose values were corrected for diurnal variation of plasma glucose by subtracting the difference between the moving average (intervals of 2 hours) and overall mean from the plasma glucose values.

RESULTS

This was a sub-study of a larger study conducted at Rigshospitalet Glostrup in the Capital Region of Denmark. Thirty-one patients (24 F, 7 M, 13 with aura, 18 without aura) were included in the study. Mean time from attack onset to blood sampling was 7.6 hours. Mean pain at the time of investigation was 6 on a 0-10 verbal rating scale. Plasma glucose was higher ictally compared to the interictal phase (interictal mean: 88.63 mg/dL, SD 11.70 mg/dL; ictal mean: 98.83 mg/dL, SD 13.16 mg/dL, difference 10.20 mg/dL, 95% CI = [4.30; 16.10]), P = .0014). The ictal increase was highest in patients investigated early during attacks and decreased linearly with time from onset of migraine (-1.57 mg/dL/hour from onset of attack, P = .020). The attack-related increase in blood glucose was not affected by pain intensity or presence of aura symptoms.

CONCLUSIONS

We demonstrated higher plasma glucose values during spontaneous migraine attacks, independent of the presence of aura symptoms and not related to pain intensity, peaking in the early phase of attacks. Additional studies are necessary to confirm our findings and explore the possible underlying mechanisms.

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.

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.

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.

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.

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.

Brain capillary transit time heterogeneity in healthy volunteers measured by dynamic contrast-enhanced T1 -weighted perfusion MRI

Purpose

Capillary transit time heterogeneity, measured as CTH, may set the upper limit for extraction of substances in brain tissue, e.g., oxygen. The purpose of this study was to investigate the feasibility of dynamic contrast‐enhanced T₁ weighted MRI (DCE‐MRI) at 3 Tesla (T), in estimating CTH based on a gamma‐variate model of the capillary transit time distribution. In addition, we wanted to investigate if a subtle increase of the blood–brain barrier permeability can be incorporated into the model, still allowing estimation of CTH.

Materials and Methods

Twenty‐three healthy subjects were scanned at 3.0T MRI system applying DCE‐MRI and using a gamma‐variate model to estimate CTH as well as cerebral blood flow (CBF), cerebral blood volume (CBV), and permeability of the blood–brain barrier, measured as the influx constant K_i. For proof of principle we also investigated three patients with recent thromboembolic events and a patient with a high grade brain tumor.

Results

In the healthy subjects, we found a narrow symmetric delta‐like capillary transit time distribution in basal ganglia gray matter with median CTH of 0.93 s and interquartile range of 1.33 s. The corresponding residue impulse response function was compatible with the adiabatic tissue homogeneity model. In two patients with complete occlusion of the internal carotid artery and in the patient with a brain tumor CTH was increased with values up to 6 s in the affected brain tissue, with an exponential like residue impulse response function.

Conclusion

Our results open the possibility of characterizing brain perfusion by the capillary transit time distribution using DCE‐MRI, theoretically a determinant of efficient blood to brain transport of important substances.

Level of Evidence: 2

J. MAGN. RESON. IMAGING 2017;45:1809–1820

Acute hypoxia increases the cerebral metabolic rate - a magnetic resonance imaging study

The aim of the present study was to examine changes in cerebral metabolism by magnetic resonance imaging of healthy subjects during inhalation of 10% O2 hypoxic air. Hypoxic exposure elevates cerebral perfusion, but its effect on energy metabolism has been less investigated. Magnetic resonance imaging techniques were used to measure global cerebral blood flow and the venous oxygen saturation in the sagittal sinus. Global cerebral metabolic rate of oxygen was quantified from cerebral blood flow and arteriovenous oxygen saturation difference. Concentrations of lactate, glutamate, N-acetylaspartate, creatine and phosphocreatine were measured in the visual cortex by magnetic resonance spectroscopy. Twenty-three young healthy males were scanned for 60 min during normoxia, followed by 40 min of breathing hypoxic air. Inhalation of hypoxic air resulted in an increase in cerebral blood flow of 15.5% (p = 0.058), and an increase in cerebral metabolic rate of oxygen of 8.5% (p = 0.035). Cerebral lactate concentration increased by 180.3% ([Formula: see text]), glutamate increased by 4.7% ([Formula: see text]) and creatine and phosphocreatine decreased by 15.2% (p[Formula: see text]). The N-acetylaspartate concentration was unchanged (p = 0.36). In conclusion, acute hypoxia in healthy subjects increased perfusion and metabolic rate, which could represent an increase in neuronal activity. We conclude that marked changes in brain homeostasis occur in the healthy human brain during exposure to acute hypoxia.