Comparison of regional cerebral blood flow responses to hypoglycemia using pulsed arterial spin labeling and positron emission tomography

Acute cerebral blood flow response to heavy cream ingestion in older adults: A non-randomized pilot study

BackgroundHypertension and the APOE4 allele are known risk factors for Alzheimer's disease (AD) and E4 carriers show different blood pressure (BP) and cognitive responses to high fat feeding.ObjectiveWe investigated the influence of these factors on global cerebral blood flow (CBF) and four regions of interest (ROIs) (angular gyrus, hippocampus, posterior cingulate, temporal lobe) using arterial spin labeling (ASL) MRI in fasting state and after ingestion of heavy cream in older adults.Methods29 adults (age in years 66.8 ± 4.1) underwent baseline and 1, 2, 3-h ASL MRI after ingestion of 100 mL heavy cream. We used pCASL MRI with background suppression to measure CBF in ml/100 g/min. Statistical analyses included mixed-effects modeling and Pearson correlation to ascertain whether CBF changed over time and how variables influenced results.ResultsGlobal CBF decreased at 1-, 2-, and 3-h post-heavy cream, compared to time 0 (overall change 7.11%, p < 0.01); recapitulated in 3 of 4 ROIs. Mean arterial pressure emerged as a predictive variable for both baseline and post-heavy cream CBF (β = -0.25, 95% CI = -0.39, -0.10, p = 0.002). Individuals with higher BP demonstrated reduced CBF, particularly in posterior cingulate and temporal lobe (β = -5.50, 95% CI = -9.9, -1.09; β = -6.28, 95% CI = -12.35, -0.21, respectively, both p < 0.05). Examination of correlations with BP and change scores revealed that this relationship was driven largely by E4 carriers.ConclusionsCBF decreased after ingestion of heavy cream, globally and in regions known to be important in AD, and this finding was driven by E4 carriers with higher BP.

Pathophysiology and management of hypoglycemia in diabetes

In the century since the discovery of insulin, diabetes has changed from an early death sentence to a manageable chronic disease. This change in longevity and duration of diabetes coupled with significant advances in therapeutic options for patients has fundamentally changed the landscape of diabetes management, particularly in patients with type 1 diabetes mellitus. However, hypoglycemia remains a major barrier to achieving optimal glycemic control. Current understanding of the mechanisms of hypoglycemia has expanded to include not only counter-regulatory hormonal responses but also direct changes in brain glucose, fuel sensing, and utilization, as well as changes in neural networks that modulate behavior, mood, and cognition. Different strategies to prevent and treat hypoglycemia have been developed, including educational strategies, new insulin formulations, delivery devices, novel technologies, and pharmacologic targets. This review article will discuss current literature contributing to our understanding of the myriad of factors that lead to the development of clinically meaningful hypoglycemia and review established and novel therapies for the prevention and treatment of hypoglycemia.

Altered functional connectivity during hypoglycaemia in type 1 diabetes

Behavioural responses to hypoglycaemia require coordinated recruitment of broadly distributed networks of interacting brain regions. We investigated hypoglycaemia-related changes in brain connectivity in people without diabetes (ND) and with type 1 diabetes with normal (NAH) or impaired (IAH) hypoglycaemia awareness. Two-step hyperinsulinaemic hypoglycaemic clamps were performed in 14 ND, 15 NAH and 22 IAH participants. BOLD timeseries were acquired at euglycaemia (5.0 mmol/L) and hypoglycaemia (2.6 mmol/L), with symptom and counter-regulatory hormone measurements. We investigated hypoglycaemia-related connectivity changes using established seed regions for the default mode (DMN), salience (SN) and central executive (CEN) networks and regions whose activity is modulated by hypoglycaemia: the thalamus and right inferior frontal gyrus (RIFG). Hypoglycaemia-induced changes in the DMN, SN and CEN were evident in NAH (all p < 0.05), with no changes in ND or IAH. However, in IAH there was a reduction in connectivity between regions within the RIFG (p = 0.001), not evident in the ND or NAH groups. We conclude that hypoglycaemia induces coordinated recruitment of the DMN and SN in diabetes with preserved hypoglycaemia awareness which is absent in IAH and ND. Changes in connectivity in the RIFG, a region associated with attentional modulation, may be key in impaired hypoglycaemia awareness.

Effect of lactate administration on cerebral blood flow during hypoglycemia in people with type 1 diabetes

INTRODUCTION

Impaired awareness of hypoglycemia, clinically reflected by the inability to timely detect hypoglycemia, affects approximately 25% of the people with type 1 diabetes. Both altered brain lactate handling and increased cerebral blood flow (CBF) during hypoglycemia appear to be involved in the pathogenesis of impaired awareness of hypoglycemia. Here we examine the effect of lactate on CBF during hypoglycemia.

RESEARCH DESIGN AND METHODS

Nine people with type 1 diabetes and normal awareness of hypoglycemia underwent two hyperinsulinemic euglycemic-hypoglycemic (3.0 mmol/L) glucose clamps in a 3T MR system, once with sodium lactate infusion and once with sodium chloride infusion. Global and regional changes in CBF were determined using pseudocontinuous arterial spin labeling.

RESULTS

Lactate (3.3±0.6 vs 0.9±0.2 mmol/L during lactate infusion vs placebo infusion, respectively) suppressed the counter-regulatory hormone responses to hypoglycemia. Global CBF increased considerably in response to intravenous lactate infusion but did not further increase during hypoglycemia. Lactate also blunted the hypoglycemia-induced regional redistribution of CBF towards the thalamus.

CONCLUSIONS

Elevated lactate levels enhance global CBF and blunt the thalamic CBF response during hypoglycemia in patients with type 1 diabetes, mimicking observations of impaired awareness of hypoglycemia. These findings suggest that alteration of CBF associated with lactate may play a role in some aspects of the development of impaired awareness of hypoglycemia.

TRIAL REGISTRATION NUMBER

NCT03730909.

Defining a Taxonomy of Intracranial Hypertension: Is ICP More Than Just a Number?

Intracranial pressure (ICP) monitoring and control is a cornerstone of neuroanesthesia and neurocritical care. However, because elevated ICP can be due to multiple pathophysiological processes, its interpretation is not straightforward. We propose a formal taxonomy of intracranial hypertension, which defines ICP elevations into 3 major pathophysiological subsets: increased cerebral blood volume, masses and edema, and hydrocephalus. (1) Increased cerebral blood volume increases ICP and arises secondary to arterial or venous hypervolemia. Arterial hypervolemia is produced by autoregulated or dysregulated vasodilation, both of which are importantly and disparately affected by systemic blood pressure. Dysregulated vasodilation tends to be worsened by arterial hypertension. In contrast, autoregulated vasodilation contributes to intracranial hypertension during decreases in cerebral perfusion pressure that occur within the normal range of cerebral autoregulation. Venous hypervolemia is produced by Starling resistor outflow obstruction, venous occlusion, and very high extracranial venous pressure. Starling resistor outflow obstruction tends to arise when cerebrospinal fluid pressure causes venous compression to thus increase tissue pressure and worsen tissue edema (and ICP elevation), producing a positive feedback ICP cycle. (2) Masses and edema are conditions that increase brain tissue volume and ICP, causing both vascular compression and decrease in cerebral perfusion pressure leading to oligemia. Brain edema is either vasogenic or cytotoxic, each with disparate causes and often linked to cerebral blood flow or blood volume abnormalities. Masses may arise from hematoma or neoplasia. (3) Hydrocephalus can also increase ICP, and is either communicating or noncommunicating. Further research is warranted to ascertain whether ICP therapy should be tailored to these physiological subsets of intracranial hypertension.

Dynamic response of cerebral blood flow to insulin-induced hypoglycemia

The dynamics of cerebral blood flow (CBF) at the onset of hypoglycemia may play a key role in hypoglycemia unawareness; however, there is currently a paucity of techniques that can monitor adult CBF with high temporal resolution. Herein, we investigated the use of diffuse correlation spectroscopy (DCS) to monitor the dynamics of CBF during insulin-induced hypoglycemia in adults. Plasma glucose concentrations, cortisol levels, and changes in CBF were measured before and during hypoglycemia in 8 healthy subjects. Cerebral blood flow increased by 42% following insulin injection with a delay of 17 ± 10 min, while the onset of hypoglycemia symptoms was delayed by 24 ± 11 min. The findings suggest that the onset of CBF increments precedes the appearance of hypoglycemia symptoms in nondiabetic subjects with normal awareness to hypoglycemia, and DCS could be a valuable tool for investigating the role of CBF in hypoglycemia unawareness.

Central nervous pathways of insulin action in the control of metabolism and food intake

Insulin acts on the CNS to modulate behaviour and systemic metabolism. Disturbances in brain insulin action represent a possible link between metabolic and cognitive health. Current findings from human research suggest that boosting central insulin action in the brain modulates peripheral metabolism, enhancing whole-body insulin sensitivity and suppressing endogenous glucose production. Moreover, central insulin action curbs food intake by reducing the salience of highly palatable food cues and increasing cognitive control. Animal models show that the mesocorticolimbic circuitry is finely tuned in response to insulin, driven mainly by the dopamine system. These mechanisms are impaired in people with obesity, which might increase their risk of developing type 2 diabetes and associated diseases. Overall, current findings highlight the role of insulin action in the brain and its consequences on peripheral metabolism and cognition. Hence, improving central insulin action could represent a therapeutic option for people at an increased risk of developing metabolic and cognitive diseases.

Hypoglycemic thalamic activation in type 1 diabetes is associated with preserved symptoms despite reduced epinephrine

Brain responses to low plasma glucose may be key to understanding the behaviors that prevent severe hypoglycemia in type 1 diabetes. This study investigated the impact of long duration, hypoglycemia aware type 1 diabetes on cerebral blood flow responses to hypoglycemia. Three-dimensional pseudo-continuous arterial spin labeling magnetic resonance imaging was performed in 15 individuals with type 1 diabetes and 15 non-diabetic controls during a two-step hyperinsulinemic glucose clamp. Symptom, hormone, global cerebral blood flow and regional cerebral blood flow responses to hypoglycemia were measured. Epinephrine release during hypoglycemia was attenuated in type 1 diabetes, but symptom score rose comparably in both groups. A rise in global cerebral blood flow did not differ between groups. Regional cerebral blood flow increased in the thalamus and fell in the hippocampus and temporal cortex in both groups. Type 1 diabetes demonstrated lesser anterior cingulate cortex activation; however, this difference did not survive correction for multiple comparisons. Thalamic cerebral blood flow change correlated with autonomic symptoms, and anterior cingulate cortex cerebral blood flow change correlated with epinephrine response across groups. The thalamus may thus be involved in symptom responses to hypoglycemia, independent of epinephrine action, while anterior cingulate cortex activation may be linked to counterregulation. Activation of these regions may have a role in hypoglycemia awareness and avoidance of problematic hypoglycemia.

Impaired Awareness of Hypoglycemia Disrupts Blood Flow to Brain Regions Involved in Arousal and Decision Making in Type 1 Diabetes

OBJECTIVE

Impaired awareness of hypoglycemia (IAH) affects one-quarter of adults with type 1 diabetes and significantly increases the risk of severe hypoglycemia. Differences in regional brain responses to hypoglycemia may contribute to the susceptibility of this group to problematic hypoglycemia. This study investigated brain responses to hypoglycemia in hypoglycemia aware (HA) and IAH adults with type 1 diabetes, using three-dimensional pseudo-continuous arterial spin labeling (3D pCASL) functional MRI to measure changes in regional cerebral blood flow (CBF).

RESEARCH DESIGN AND METHODS

Fifteen HA and 19 IAH individuals underwent 3D pCASL functional MRI during a two-step hyperinsulinemic glucose clamp. Symptom, hormone, global, and regional CBF responses to hypoglycemia (47 mg/dL [2.6 mmol/L]) were measured.

RESULTS

In response to hypoglycemia, total symptom score did not change in those with IAH (P = 0.25) but rose in HA participants (P < 0.001). Epinephrine, cortisol, and growth hormone responses to hypoglycemia were lower in the IAH group (P < 0.05). Hypoglycemia induced a rise in global CBF (HA P = 0.01, IAH P = 0.04) but was not different between groups (P = 0.99). IAH participants showed reduced regional CBF responses within the thalamus (P = 0.002), right lateral orbitofrontal cortex (OFC) (P = 0.002), and right dorsolateral prefrontal cortex (P = 0.036) and a lesser decrease of CBF in the left hippocampus (P = 0.023) compared with the HA group. Thalamic and right lateral OFC differences survived Bonferroni correction.

CONCLUSIONS

Responses to hypoglycemia of brain regions involved in arousal, decision making, and reward are altered in IAH. Changes in these pathways may disrupt IAH individuals' ability to recognize hypoglycemia, impairing their capacity to manage hypoglycemia effectively and benefit fully from conventional therapeutic pathways to restore awareness.

Cerebral blood flow response to hypoglycemia is altered in patients with type 1 diabetes and impaired awareness of hypoglycemia

It is unclear whether cerebral blood flow responses to hypoglycemia are altered in people with type 1 diabetes and impaired awareness of hypoglycemia. The aim of this study was to investigate the effect of hypoglycemia on both global and regional cerebral blood flow in type 1 diabetes patients with impaired awareness of hypoglycemia, type 1 diabetes patients with normal awareness of hypoglycemia and healthy controls ( n = 7 per group). The subjects underwent a hyperinsulinemic euglycemic-hypoglycemic glucose clamp in a 3 T MR system. Global and regional changes in cerebral blood flow were determined by arterial spin labeling magnetic resonance imaging, at the end of both glycemic phases. Hypoglycemia generated typical symptoms in patients with type 1 diabetes and normal awareness of hypoglycemia and healthy controls, but not in patients with impaired awareness of hypoglycemia. Conversely, hypoglycemia increased global cerebral blood flow in patients with impaired awareness of hypoglycemia, which was not observed in the other two groups. Regionally, hypoglycemia caused a redistribution of cerebral blood flow towards the thalamus of both patients with normal awareness of hypoglycemia and healthy controls, consistent with activation of brain regions associated with the autonomic response to hypoglycemia. No such redistribution was found in the patients with impaired awareness of hypoglycemia. An increase in global cerebral blood flow may enhance nutrient supply to the brain, hence suppressing symptomatic awareness of hypoglycemia. Altogether these results suggest that changes in cerebral blood flow during hypoglycemia contribute to impaired awareness of hypoglycemia.

Evaluation of Precision in Optoacoustic Tomography for Preclinical Imaging in Living Subjects

Optoacoustic tomography (OT) is now widely used in preclinical imaging; however, the precision (repeatability and reproducibility) of OT has yet to be determined. Methods: We used a commercial small-animal OT system. Measurements in stable phantoms were used to independently assess the impact of system variables on precision (using coefficient of variation, COV), including acquisition wavelength, rotational position, and frame averaging. Variables due to animal handling and physiology, such as anatomic placement and anesthesia conditions, were then assessed in healthy nude mice using the left kidney and spleen as reference organs. Temporal variation was assessed by repeated measurements over hours and days both in phantoms and in vivo. Sensitivity to small-molecule dyes was determined in phantoms and in vivo; precision was assessed in vivo using IRDye800CW. Results: OT COV in a stable phantom was less than 2.8% across all wavelengths over 30 d. The factors with the greatest impact on signal repeatability in phantoms were rotational position and user experience, both of which still resulted in a COV of less than 4% at 700 nm. Anatomic region-of-interest size showed the highest variation, at 12% and 18% COV in the kidney and spleen, respectively; however, functional SO2 measurements based on a standard operating procedure showed an exceptional reproducibility of less than 4% COV. COV for repeated injections of IRDye800CW was 6.6%. Sources of variability for in vivo data included respiration rate, degree of user experience, and animal placement. Conclusion: Data acquired with our small-animal OT system were highly repeatable and reproducible across subjects and over time. Therefore, longitudinal OT studies may be performed with high confidence when our standard operating procedure is followed.

The effects of hemodynamic lag on functional connectivity and behavior after stroke

Stroke disrupts the brain's vascular supply, not only within but also outside areas of infarction. We investigated temporal delays (lag) in resting state functional magnetic resonance imaging signals in 130 stroke patients scanned two weeks, three months and 12 months post stroke onset. Thirty controls were scanned twice at an interval of three months. Hemodynamic lag was determined using cross-correlation with the global gray matter signal. Behavioral performance in multiple domains was assessed in all patients. Regional cerebral blood flow and carotid patency were assessed in subsets of the cohort using arterial spin labeling and carotid Doppler ultrasonography. Significant hemodynamic lag was observed in 30% of stroke patients sub-acutely. Approximately 10% of patients showed lag at one-year post-stroke. Hemodynamic lag corresponded to gross aberrancy in functional connectivity measures, performance deficits in multiple domains and local and global perfusion deficits. Correcting for lag partially normalized abnormalities in measured functional connectivity. Yet post-stroke FC-behavior relationships in the motor and attention systems persisted even after hemodynamic delays were corrected. Resting state fMRI can reliably identify areas of hemodynamic delay following stroke. Our data reveal that hemodynamic delay is common sub-acutely, alters functional connectivity, and may be of clinical importance.

Brain Insulin Resistance at the Crossroads of Metabolic and Cognitive Disorders in Humans

Ever since the brain was identified as an insulin-sensitive organ, evidence has rapidly accumulated that insulin action in the brain produces multiple behavioral and metabolic effects, influencing eating behavior, peripheral metabolism, and cognition. Disturbances in brain insulin action can be observed in obesity and type 2 diabetes (T2D), as well as in aging and dementia. Decreases in insulin sensitivity of central nervous pathways, i.e., brain insulin resistance, may therefore constitute a joint pathological feature of metabolic and cognitive dysfunctions. Modern neuroimaging methods have provided new means of probing brain insulin action, revealing the influence of insulin on both global and regional brain function. In this review, we highlight recent findings on brain insulin action in humans and its impact on metabolism and cognition. Furthermore, we elaborate on the most prominent factors associated with brain insulin resistance, i.e., obesity, T2D, genes, maternal metabolism, normal aging, inflammation, and dementia, and on their roles regarding causes and consequences of brain insulin resistance. We also describe the beneficial effects of enhanced brain insulin signaling on human eating behavior and cognition and discuss potential applications in the treatment of metabolic and cognitive disorders.

Brain glucose metabolism during hypoglycemia in type 1 diabetes: insights from functional and metabolic neuroimaging studies

Hypoglycemia is the most frequent complication of insulin therapy in patients with type 1 diabetes. Since the brain is reliant on circulating glucose as its main source of energy, hypoglycemia poses a threat for normal brain function. Paradoxically, although hypoglycemia commonly induces immediate decline in cognitive function, long-lasting changes in brain structure and cognitive function are uncommon in patients with type 1 diabetes. In fact, recurrent hypoglycemia initiates a process of habituation that suppresses hormonal responses to and impairs awareness of subsequent hypoglycemia, which has been attributed to adaptations in the brain. These observations sparked great scientific interest into the brain's handling of glucose during (recurrent) hypoglycemia. Various neuroimaging techniques have been employed to study brain (glucose) metabolism, including PET, fMRI, MRS and ASL. This review discusses what is currently known about cerebral metabolism during hypoglycemia, and how findings obtained by functional and metabolic neuroimaging techniques contributed to this knowledge.

Positron emission tomography/magnetic resonance hybrid scanner imaging of cerebral blood flow using (15)O-water positron emission tomography and arterial spin labeling magnetic resonance imaging in newborn piglets

Abnormality in cerebral blood flow (CBF) distribution can lead to hypoxic-ischemic cerebral damage in newborn infants. The aim of the study was to investigate minimally invasive approaches to measure CBF by comparing simultaneous (15)O-water positron emission tomography (PET) and single TI pulsed arterial spin labeling (ASL) magnetic resonance imaging (MR) on a hybrid PET/MR in seven newborn piglets. Positron emission tomography was performed with IV injections of 20 MBq and 100 MBq (15)O-water to confirm CBF reliability at low activity. Cerebral blood flow was quantified using a one-tissue-compartment-model using two input functions: an arterial input function (AIF) or an image-derived input function (IDIF). The mean global CBF (95% CI) PET-AIF, PET-IDIF, and ASL at baseline were 27 (23; 32), 34 (31; 37), and 27 (22; 32) mL/100 g per minute, respectively. At acetazolamide stimulus, PET-AIF, PET-IDIF, and ASL were 64 (55; 74), 76 (70; 83) and 79 (67; 92) mL/100 g per minute, respectively. At baseline, differences between PET-AIF, PET-IDIF, and ASL were 22% (P<0.0001) and -0.7% (P=0.9). At acetazolamide, differences between PET-AIF, PET-IDIF, and ASL were 19% (P=0.001) and 24% (P=0.0003). In conclusion, PET-IDIF overestimated CBF. Injected activity of 20 MBq (15)O-water had acceptable concordance with 100 MBq, without compromising image quality. Single TI ASL was questionable for regional CBF measurements. Global ASL CBF and PET CBF were congruent during baseline but not during hyperperfusion.

Validation of [(11) C]ORM-13070 as a PET tracer for alpha2c -adrenoceptors in the human brain

This study explored the use of the α2C -adrenoceptor PET tracer [(11) C]ORM-13070 to monitor α2C -AR occupancy in the human brain. The subtype-nonselective α2 -AR antagonist atipamezole was administered to eight healthy volunteer subjects to determine its efficacy and potency (Emax and EC50 ) at inhibiting tracer uptake. We also explored whether the tracer could reveal changes in the synaptic concentrations of endogenous noradrenaline in the brain, in response to several pharmacological and sensory challenge conditions. We assessed occupancy from the bound-to-free ratio measured during 5-30 min post injection. Based on extrapolation of one-site binding, the maximal extent of inhibition of striatal [(11) C]ORM-13070 uptake (Emax ) achievable by atipamezole was 78% (95% CI 69-87%) in the caudate nucleus and 65% (53-77%) in the putamen. The EC50 estimates of atipamezole (1.6 and 2.5 ng/ml, respectively) were in agreement with the drug's affinity to α2C -ARs. These findings represent clear support for the use of [(11) C]ORM-13070 for monitoring drug occupancy of α2C -ARs in the living human brain. Three of the employed noradrenaline challenges were associated with small, approximately 10-16% average reductions in tracer uptake in the dorsal striatum (atomoxetine, ketamine, and the cold pressor test; P < 0.05 for all), but insulin-induced hypoglycemia did not affect tracer uptake. The tracer is suitable for studying central nervous system receptor occupancy by α2C -AR ligands in human subjects. [(11) C]ORM-13070 also holds potential as a tool for in vivo monitoring of synaptic concentrations of noradrenaline, but this remains to be further evaluated in future studies.

Effects of overnight fasting on working memory-related brain network: an fMRI study

Glucose metabolism serves as the central source of energy for the human brain. Little is known about the effects of blood glucose level (BGL) on higher-order cognitive functions within a physiological range (e.g., after overnight fasting). In this randomized, placebo-controlled, double blind study, we assessed the impact of overnight fasting (14 h) on brain activation during a working memory task. We sought to mimic BGLs that occur naturally in healthy humans after overnight fasting. After standardized periods of food restriction, 40 (20 male) healthy participants were randomly assigned to receive either glucagon to balance the BGL or placebo (NaCl). A parametric fMRI paradigm, including 2-back and 0-back tasks, was used. Subclinically low BGL following overnight fasting was found to be linked to reduced involvement of the bilateral dorsal midline thalamus and the bilateral basal ganglia, suggesting high sensitivity of those regions to minimal changes in BGLs. Our results indicate that overnight fasting leads to physiologically low levels of glucose, impacting brain activation during working memory tasks even when there are no differences in cognitive performance.

Comparison of cerebral blood flow acquired by simultaneous [15O]water positron emission tomography and arterial spin labeling magnetic resonance imaging

Until recently, no direct comparison between [(15)O]water positron emission tomography (PET) and arterial spin labeling (ASL) for measuring cerebral blood flow (CBF) was possible. With the introduction of integrated, hybrid magnetic resonance (MR)-PET scanners, such a comparison becomes feasible. This study presents results of CBF measurements recorded simultaneously with [(15)O]water and ASL. A 3T MR-BrainPET scanner was used for the simultaneous acquisition of pseudo-continuous ASL (pCASL) magnetic resonance imaging (MRI) and [(15)O]water PET. Quantitative CBF values were compared in 10 young healthy male volunteers at baseline conditions. A statistically significant (P<0.05) correlation was observed between the two modalities; the whole-brain CBF values determined with PET and pCASL were 43.3±6.1 mL and 51.9±7.1 mL per 100 g per minute, respectively. The gray/white matter (GM/WM) ratio of CBF was 3.0 for PET and 3.4 for pCASL. A paired t-test revealed differences in regional CBF between ASL and PET with higher ASL-CBF than PET-CBF values in cortical areas. Using an integrated, hybrid MR-PET a direct simultaneous comparison between ASL and [(15)O]water PET became possible for the first time so that temporal, physiologic, and functional variations were avoided. Regional and individual differences were found despite the overall similarity between ASL and PET, requiring further detailed investigations.

Glycemic extremes in youth with T1DM: the structural and functional integrity of the developing brain

The adult brain accounts for a disproportionally large percentage of the body’s total energy consumption (1). However, during brain development,energy demand is even higher, reaching the adult rate by age 2 and increasing to nearly twice the adult rate by age 10, followed by gradual reduction toward adult levels in the next decade (1,2). The dramatic changes in brain metabolism occurring over the first two decades of life coincide with the initial proliferation and then pruning of synapses to adult levels.The brain derives its energy almost exclusively from glucose and is largely driven by neuronal signaling, biosynthesis, and neuroprotection (3–6).Glucose homeostasis in the body is tightly regulated by a series of hormones and physiologic responses. As a result, hypoglycemia and hyperglycemia are rare occurrences in normal individuals, but they occur commonly inpatients with type 1 diabetes mellitus (T1DM) due to a dysfunction of peripheral glucose-insulin-glucagon responses and non-physiologic doses of exogenous insulin, which imperfectly mimic normal physiology. These extremes can occur more frequently in children and adolescents with T1DM due to the inadequacies of insulin replacement therapy, events leading to the diagnosis [prolonged untreated hyperglycemia and diabetic ketoacidosis (DKA)], and to behavioral factors interfering with optimal treatment. When faced with fluctuations in glucose supply the metabolism of the body and brain change dramatically, largely to conserve resources and, at a cost to other organs, to preserve brain function (7). However,if the normal physiological mechanisms that prevent these severe glucose fluctuations and maintain homeostasis are impaired, neuronal function and potentially viability can be affected (8–11).