Potential of Early [ 18 F]-2-Fluoro-2-Deoxy-D-Glucose Positron Emission Tomography for Identifying Hypoperfusion and Predicting Fate of Tissue in a Rat Embolic Stroke Model

[15O]H2O PET: Potential or Essential for Molecular Imaging?

Imaging water pathways in the human body provides an excellent way of measuring accurately the blood flow directed to different organs. This makes it a powerful diagnostic tool for a wide range of diseases that are related to perfusion and oxygenation. Although water PET has a long history, its true potential has not made it into regular clinical practice. The article highlights the potential of water PET in molecular imaging and suggests its prospective role in becoming an essential tool for the 21st century precision medicine in different domains ranging from preclinical to clinical research and practice. The recent technical advances in high-sensitivity PET imaging can play a key accelerating role in empowering this technique, though there are still several challenges to overcome.

In vivo Imaging of Cannabinoid Type 2 Receptors: Functional and Structural Alterations in Mouse Model of Cerebral Ischemia by PET and MRI

PURPOSE

Stroke is one of the most prevalent vascular diseases. Non-invasive molecular imaging methods have the potential to provide critical insights into the temporal dynamics and follow alterations of receptor expression and metabolism in ischemic stroke. The aim of this study was to assess the cannabinoid type 2 receptor (CB2R) levels in transient middle cerebral artery occlusion (tMCAO) mouse models at subacute stage using positron emission tomography (PET) with our novel tracer [18F]RoSMA-18-d6 and structural imaging by magnetic resonance imaging (MRI).

PROCEDURES

Our recently developed CB2R PET tracer [18F]RoSMA-18-d6 was used for imaging neuroinflammation at 24 h after reperfusion in tMCAO mice. The RNA expression levels of CB2R and other inflammatory markers were analyzed by quantitative real-time polymerase chain reaction using brain tissues from tMCAO (1 h occlusion) and sham-operated mice. [18F]fluorodeoxyglucose (FDG) was included for evaluation of the cerebral metabolic rate of glucose (CMRglc). In addition, diffusion-weighted imaging and T2-weighted imaging were performed for anatomical reference and delineating the lesion in tMCAO mice.

RESULTS

mRNA expressions of inflammatory markers TNF-α, Iba1, MMP9 and GFAP, CNR2 were increased to 1.3-2.5 fold at 24 h after reperfusion in the ipsilateral compared to contralateral hemisphere of tMCAO mice, while mRNA expression of the neuronal marker MAP-2 was markedly reduced to ca. 50 %. Reduced [18F]FDG uptake was observed in the ischemic striatum of tMCAO mouse brain at 24 h after reperfusion. Although higher activity of [18F]RoSMA-18-d6 in ex vivo biodistribution studies and higher standard uptake value ratio (SUVR) were detected in the ischemic ipsilateral compared to contralateral striatum in tMCAO mice, the in vivo specificity of [18F]RoSMA-18-d6 was confirmed only in the CB2R-rich spleen.

CONCLUSIONS

This study revealed an increased [18F]RoSMA-18-d6 measure of CB2R and a reduced [18F]FDG measure of CMRglc in the ischemic striatum of tMCAO mice at subacute stage. [18F]RoSMA-18-d6 might be a promising PET tracer for detecting CB2R alterations in animal models of neuroinflammation without neuronal loss.

Increased Accumulation of 18F-FDG Incidentally Observed in Hyperacute Cerebral Infarction

ABSTRACT

A 75-year-old man with lung cancer undertook an 18F-FDG PET/CT for staging. He presented neurological symptoms immediately after the 30-minute scan. An emergent MRI study revealed hyperacute cerebral infarction with occlusion of a left MCA branch. At PET/CT, an increased 18F-FDG uptake was observed in the corresponding areas of infarction. In literature, acceleration of compensatory anaerobic glycolysis has been proposed as 1 of the causes of increased uptake in the penumbra of acute cerebral infarction, and a similar process was hypothesized in this case. In addition, a decreased 18F-FDG uptake in the ipsilateral thalamus was noted on the PET/CT images.

Endogenous Oleoylethanolamide Crystals Loaded Lipid Nanoparticles with Enhanced Hydrophobic Drug Loading Capacity for Efficient Stroke Therapy

Introduction

Although the preparation of lipid nanoparticles (LNPs) achieves great success, their retention of highly hydrophobic drugs is still problematic.

Methods

Herein, we report a novel strategy for efficiently loading hydrophobic drugs to LNPs for stroke therapy. Oleoylethanolamide (OEA), an endogenous highly hydrophobic molecule with outstanding neuroprotective effect, was successfully loaded to OEA-SPC&DSPE-PEG lipid nanoparticles (OSDP LNPs) with a drug loading of 15.9 ± 1.2 wt%. Efficient retention in OSDP LNPs greatly improved the pharmaceutical property and enhanced the neuroprotective effect of OEA.

Results

Through the data of positron emission tomography (PET) and TTC-stained brain slices, it could be clearly visualized that the acute ischemic brain tissues were preserved as penumbral tissues and bounced back with reperfusion. The in vivo experiments stated that OSDP LNPs could significantly improve the survival rate, the behavioral score, the cerebral infarct volume, the edema degree, the spatial learning and memory ability of the MCAO (middle cerebral artery occlusion) rats.

Discussion

These results suggest that the OSDP LNPs have a great chance to develop hydrophobic OEA into a potential anti-stroke formulation.

Value of pre-intervention CT perfusion imaging in acute ischemic stroke prognosis

Noninvasive imaging plays an important role in acute stroke towards diagnosis and ongoing management of patients. Systemic thrombolysis and endovascular thrombectomy (EVT) are proven treatments currently used in standards of care in acute stroke settings. The role of computed tomography angiography (CTA) in selecting patients with large vessel occlusion for EVT is well established. However, the value of CT perfusion (CTP) imaging in predicting outcomes after stroke remains ambiguous. This article critically evaluates the value of multimodal CT imaging in early diagnosis and prognosis of acute ischemic stroke with a focus on the role of CTP in delineating tissue characteristics, patient selection, and outcomes after reperfusion therapy. Insights on various technical and clinical considerations relevant to CTP applications in acute ischemic stroke, recommendations for existing workflow, and future areas of research are discussed.

Multiparametric cardiac18F-FDG PET in humans: pilot comparison of FDG delivery rate with82Rb myocardial blood flow

Myocardial blood flow (MBF) and flow reserve are usually quantified in the clinic with positron emission tomography (PET) using a perfusion-specific radiotracer (e.g.⁸²Rb-chloride). However, the clinical accessibility of existing perfusion tracers remains limited. Meanwhile,¹⁸F-fluorodeoxyglucose (FDG) is a commonly used radiotracer for PET metabolic imaging without similar limitations. In this paper, we explore the potential of¹⁸F-FDG for myocardial perfusion imaging by comparing the myocardial FDG delivery rateK₁with MBF as determined by dynamic⁸²Rb PET in fourteen human subjects with heart disease. Two sets of FDGK₁were derived from one-hour dynamic FDG scans. One was the original FDGK₁estimates and the other was the correspondingK₁values that were linearly normalized for blood glucose levels. A generalized Renkin-Crone model was used to fit FDGK₁with Rb MBF, which then allowed for a nonlinear extraction fraction correction for converting FDGK₁to MBF. The linear correlation between FDG-derived MBF and Rb MBF was moderate (r= 0.79) before the glucose normalization and became much improved (r> 0.9) after glucose normalization. The extraction fraction of FDG was also similar to that of Rb-chloride in the myocardium. The results from this pilot study suggest that dynamic cardiac FDG-PET with tracer kinetic modeling has the potential to provide MBF in addition to its conventional use for metabolic imaging.

Glucose metabolic crosstalk and regulation in brain function and diseases

Brain glucose metabolism, including glycolysis, the pentose phosphate pathway, and glycogen turnover, produces ATP for energetic support and provides the precursors for the synthesis of biological macromolecules. Although glucose metabolism in neurons and astrocytes has been extensively studied, the glucose metabolism of microglia and oligodendrocytes, and their interactions with neurons and astrocytes, remain critical to understand brain function. Brain regions with heterogeneous cell composition and cell-type-specific profiles of glucose metabolism suggest that metabolic networks within the brain are complex. Signal transduction proteins including those in the Wnt, GSK-3β, PI3K-AKT, and AMPK pathways are involved in regulating these networks. Additionally, glycolytic enzymes and metabolites, such as hexokinase 2, acetyl-CoA, and enolase 2, are implicated in the modulation of cellular function, microglial activation, glycation, and acetylation of biomolecules. Given these extensive networks, glucose metabolism dysfunction in the whole brain or specific cell types is strongly associated with neurologic pathology including ischemic brain injury and neurodegenerative disorders. This review characterizes the glucose metabolism networks of the brain based on molecular signaling and cellular and regional interactions, and elucidates glucose metabolism-based mechanisms of neurological diseases and therapeutic approaches that may ameliorate metabolic abnormalities in those diseases.

PET Parametric Imaging: Past, Present, and Future

Positron emission tomography (PET) is actively used in a diverse range of applications in oncology, cardiology, and neurology. The use of PET in the clinical setting focuses on static (single time frame) imaging at a specific time-point post radiotracer injection and is typically considered as semi-quantitative; e.g. standardized uptake value (SUV) measures. In contrast, dynamic PET imaging requires increased acquisition times but has the advantage that it measures the full spatiotemporal distribution of a radiotracer and, in combination with tracer kinetic modeling, enables the generation of multiparametric images that more directly quantify underlying biological parameters of interest, such as blood flow, glucose metabolism, and receptor binding. Parametric images have the potential for improved detection and for more accurate and earlier therapeutic response assessment. Parametric imaging with dynamic PET has witnessed extensive research in the past four decades. In this paper, we provide an overview of past and present activities and discuss emerging opportunities in the field of parametric imaging for the future.

Positron Emission Tomography After Ischemic Brain Injury: Current Challenges and Future Developments

Positron emission tomography (PET) is widely used in clinical and animal studies, along with the development of diverse tracers. The biochemical characteristics of PET tracers may help uncover the pathophysiological consequences of cardiac arrest (CA) and ischemic stroke, which include cerebral ischemia and reperfusion, depletion of oxygen and glucose, and neuroinflammation. PubMed was searched for studies of the application of PET for "cardiac arrest," "ischemic stroke," and "targeted temperature management." Available studies were included and classified according to the biochemical properties involved and metabolic processes of PET tracers, and were summarized. The mechanisms of ischemic brain injuries were investigated by PET with various tracers to elucidate the pathological process from the initial decrease of cerebral blood flow (CBF) to the subsequent abnormalities in energy and oxygen metabolism, to the monitoring of inflammation. In general, the trends of cerebral blood flow and oxygen metabolism after ischemic attack are not unidirectional but closely related to the time point of injury and recovery. Glucose metabolism after injury showed significant differences in different brain regions whereas global cerebral metabolic rate of glucose (CMRglc) declined. PET monitoring of neuroinflammation shows comparable efficacy to immunostaining. The technology of PET targeting in brain metabolism and the development of tracers provide new tools to track and evaluate the brain's pathological changes after ischemic brain injury. Despite no existing evidence for an available PET-based prediction method, discoveries of new tracers are expected to provide more possibilities for the whole field.

Significantly Increased Accumulation of 18F-FDG Throughout the Left Middle Cerebral Artery Territory Corresponding to Acute-Phase Infarction

A 70-year-old woman had spontaneous resolution of an embolism in her right middle cerebral artery (MCA) (day 1); another embolism occurred in her left MCA (day 3), which was promptly removed. On day 5, F-FDG PET/CT performed for staging mediastinal lymphoma showed marked FDG accumulation in the left MCA territory, whereas a defect was seen in the right insular region. Eventually, bilateral lesions developed irreversible infarction. Anaerobic metabolism and/or inflammation in acute-phase infarction were the supposed mechanism for the increased accumulation of FDG in her left MCA territory.

Stroke detection with 3 different PET tracers

Stroke is a common cause of patient morbidity and mortality, being the fifth leading cause of death in the United States. Positron emission tomography (PET) is a proven tool for oncology patients, and may have utility in patients with stroke. We demonstrate findings of stroke incidentally detected on oncologic PET/CTs using ¹⁸F-FDG, ¹¹C-Choline, and ⁶⁸Ga-DOTATATE radiotracers. Specifically, focal ¹¹C-Choline or ⁶⁸Ga-DOTATATE uptakes localized in areas of MRI confirmed ischemia, and paradoxically increased ¹⁸F-FDG activity was visualized surrounding a region of hemorrhage, in different patients. These cases demonstrate that PET may have utility in evaluating patients with stroke based on flow dynamics, metabolic activity, and receptor expression.

Chemical exchange-sensitive spin-lock MRI of glucose analog 3-O-methyl-d-glucose in normal and ischemic brain

Glucose transport is important for understanding brain glucose metabolism. We studied glucose transport with a presumably non-toxic and non-metabolizable glucose analog, 3-O-methyl-d-glucose, using a chemical exchange-sensitive spin-lock MRI technique at 9.4 Tesla. 3-O-methyl-d-glucose showed comparable chemical exchange properties with d-glucose and 2-deoxy-d-glucose in phantoms, and higher and lower chemical exchange-sensitive spin-lock sensitivity than Glc and 2-deoxy-d-glucose in in vivo experiments, respectively. The changes of the spin-lattice relaxation rate in the rotating frame (Δ R₁ρ) in normal rat brain peaked at ∼15 min after the intravenous injection of 1 g/kg 3-O-methyl-d-glucose and almost maintained a plateau for >1 h. Doses up to 4 g/kg 3-O-methyl-d-glucose were linearly correlated with Δ R₁ρ. In rats with focal ischemic stroke, chemical exchange-sensitive spin-lock with 3-O-methyl-d-glucose injection at 1 h after stroke onset showed reduced Δ R₁ρ in the ischemic core but higher Δ R₁ρ in the peri-core region compared to normal tissue, which progressed into the ischemic core at 3 h after stroke onset. This suggests that the hyper-chemical exchange-sensitive spin-lock region observed at 1 h is the ischemic penumbra at-risk of infarct. In summary, 3-O-methyl-d-glucose-chemical exchange-sensitive spin-lock can be a sensitive MRI technique to probe the glucose transport in normal and ischemic brains.

Roles of electro-acupuncture in glucose metabolism as assessed by 18F-FDG/PET imaging and AMPKα phosphorylation in rats with ischemic stroke

Targeted energy metabolism balance contributes to neural survival during ischemic stroke. Herein, we tested the hypothesis that electro‑acupuncture (EA) can enhance cerebral glucose metabolism assessed by 18F‑fluorodeoxyglucose/positron emission tomography (18F‑FDG/PET) imaging to prevent propagation of tissue damage and improve neurological outcome in rats subjected to ischemia and reperfusion injury. Rats underwent middle cerebral artery occlusion (MCAO) and received EA treatment at the LI11 and ST36 acupoints or non‑acupoint treatment once a day for 7 days. After EA treatment, a significant reduction in the infarct volume was determined by T2‑weighted imaging, accompanied by the functional recovery in CatWalk and Rota-rod performance. Moreover, EA promoted higher glucose metabolism in the caudate putamen (CPu), motor cortex (MCTX), somatosensory cortex (SCTX) regions as assessed by animal 18F‑FDG/PET imaging, suggesting that three‑brain regional neural activity was enhanced by EA. In addition, the AMP‑activated protein kinase α (AMPKα) in the CPu, MCTX and SCTX regions was phosphorylated at threonine 172 (Thr172) after ischemic injury; however, phosphorylation of AMPK was further increased by EA. These results indicate that EA could promote AMPKα phosphorylation of the CPu, MCTX and SCTX regions to enhance neural activity and motor functional recovery after ischemic stroke.

Evolutional Characterization of Photochemically Induced Stroke in Rats: a Multimodality Imaging and Molecular Biological Study

Photochemically induced cerebral ischemia is an easy-manipulated, reproducible, relatively noninvasive, and lesion controllable model for translational study of ischemic stroke. In order to longitudinally investigate the characterization of the model, magnetic resonance imaging, ¹⁸F-2-deoxy-glucose positron emission tomography, fluorescence, and bioluminescence imaging system were performed in correlation with triphenyl tetrazolium chloride (TTC), hematoxylin-eosin staining, and immunohistochemistry examinations of glial fibrillary acidic protein, CD68, NeuN, von willebrand factor, and α-smooth muscle actin in the infarct zone. The results suggested that the number of inflammatory cells, astrocytes, and neovascularization significantly elevated in peri-infarct region from day 7 and a belt of macrophage/microglial and astrocytes was formed surrounding infarct lesion at day 14. Both vasogenic and cytotoxic edema, as well as blood brain-barrier leakage, occurred since day 1 after stroke induction and gradually attenuated with time. Numerous cells other than neuronal cells infiltrated into infarct lesion, which resulted in no visible TTC negative regional existence at day 14. Furthermore, recovery of cerebral blood flow and glucose utilization in peri-infarct zone were noted and more remarkably than that in infarct core following the stroke progression. In conclusion, these characterizations may be highly beneficial to the development of therapeutic strategies for ischemic stroke.

Glucose consumption of inflammatory cells masks metabolic deficits in the brain

Inflammatory cells such as microglia need energy to exert their functions and to maintain their cellular integrity and membrane potential. Subsequent to cerebral ischemia, inflammatory cells infiltrate tissue with limited blood flow where neurons and astrocytes died due to insufficient supply with oxygen and glucose. Using dual tracer positron emission tomography (PET), we found that concomitant with the presence of inflammatory cells, transport and consumption of glucose increased up to normal levels but returned to pathological levels as soon as inflammatory cells disappeared. Thus, inflammatory cells established sufficient glucose supply to satisfy their energy demands even in regions with insufficient supply for neurons and astrocytes to survive. Our data suggest that neurons and astrocytes died from oxygen deficiency and inflammatory cells metabolized glucose non-oxidatively in regions with residual availability. As a consequence, glucose metabolism of inflammatory cells can mask metabolic deficits in neurodegenerative diseases. We further found that the PET tracer did not bind to inflammatory cells in severely hypoperfused regions and thus only a part of the inflammation was detected. We conclude that glucose consumption of inflammatory cells should be taken into account when analyzing disease-related alterations of local cerebral metabolism.

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Inflammatory cells consume high amounts of glucose in supply-limited brain regions.

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Glucose metabolism of inflammatory cells masks metabolic deficits in the brain.

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In vivo markers only reach inflammatory cells in regions with residual blood supply.

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Measuring inflammation and metabolism provide complementary information.

The macrosphere model-an embolic stroke model for studying the pathophysiology of focal cerebral ischemia in a translational approach

The main challenge of stroke research is to translate promising experimental findings from the bench to the bedside. Many suggestions have been made how to achieve this goal, identifying the need for appropriate experimental animal models as one key issue. We here discuss the macrosphere model of focal cerebral ischemia in the rat, which closely resembles the pathophysiology of human stroke both in its acute and chronic phase. Key pathophysiological processes such as brain edema, cortical spreading depolarizations (CSD), neuroinflammation, and stem cell-mediated regeneration are observed in this stroke model, following characteristic temporo-spatial patterns. Non-invasive in vivo imaging allows studying the macrosphere model from the very onset of ischemia up to late remodeling processes in an intraindividual and longitudinal fashion. Such a design of pre-clinical stroke studies provides the basis for a successful translation into the clinic.

In-vivo detection of inflammation and neurodegeneration in the chronic phase after permanent embolic stroke in rats

Neuroinflammation with microglia activation (MA) constitutes a key tissue response in acute stroke. Until now, its course in the chronic stage is less well defined. Here, we investigated (i) neuroinflammation in the chronic stage of a rat model of embolic stroke (n=6), and (ii) whether this process can be visualized in vivo by multimodal imaging using Magnetic Resonance Imaging (MRI) and Positron-Emission-Tomography (PET). Imaging data were verified using histology and immunohistochemistry. Repetitive PET studies until week 6 after stroke reveal poststroke inflammation as a dynamic process that involved the infarct, the surrounding tissue and secondary degenerating areas in a complex fashion. At the end, 7 months after stroke, neuroinflammation had almost completely vanished at the lesion side. In contrast, remote from the primarily infarcted areas, a marked T2(*)- hypointensity was detected in the ipsilateral thalamus. In the corresponding area, [(11)C]PK11195-PET detected microglia activation. Immunohistochemistry confirmed activated microglia in the ipsilateral thalamus with signs of extensive phagocytosis and iron deposition around plaque-like amyloid deposition. Neuronal staining (NeuN) revealed pronounced neuronal loss as an endpoint of neurodegeneration in these areas. In conclusion, the data demonstrate not only ongoing thalamic neuroinflammation but also marked neurodegeneration remote from the lesion site in the chronic phase after stroke in rats. Both, neuroinflammation and neurodegeneration were accessible to (immuno-) histochemical methods as well as to in vivo methods using [(11)C]PK11195-PET and T2(*)-weighted MRI. Although the functional roles of these dynamic processes remain to be elucidated, ongoing destruction of neuronal tissue is conceivable. Its inhibition using anti-inflammatory substances may be beneficial in chronic post-stroke conditions, while multimodal imaging can be used to evaluate putative therapeutic effects in vivo.

Dynamic metabolic changes after permanent cerebral ischemia in rats with/without post-stroke exercise: a positron emission tomography (PET) study

OBJECTIVES

Recent studies have suggested that rehabilitation therapy can accelerate functional recovery after a stroke. Although often overlooked, the cortical hemisphere contralateral to an infarction plays an important role. This study investigates alterations in metabolism of both the damaged ('ipsilateral') as well as the undamaged ('contralateral') hemisphere using (18)F-fluorodeoxyglucose (FDG)-micro-positron emission tomography (PET) in a rat permanent stroke model (with or without post-injury exercise) in order to elucidate the relative importance of either hemisphere to the recovery process following stroke.

METHODS

Thirty-six adult, male Sprague-Dawley rats were divided into four groups before subsequent surgery: sham controls with or without exercise, and ischemic ('stroke') groups with or without exercise. Fluorodeoxyglucose micro-PET imaging was performed at 7, 14, and 21 days after the designated procedure according to group assignment. The imaging data was analyzed by ANOVA using SPMratIHEP software.

RESULTS

Both exercise and ischemia have measurable effects on the motor cortex as well as on the striatum, the effects of which notably include the contralateral hemisphere. To that end, regions of the contralateral motor cortex and striatum have been found to be in a hypermetabolic state following exercise. We further observed that exercise reversed the hypometabolism caused by ischemia back to control levels from day 7 through day 21 on the ipsilateral side. Its effect on the contralateral hemisphere, notably, bolsters an already vigorous response observed after ischemic insult. Thus, the beneficial effect of exercise, as inferred by an increase in metabolic activity, is evident in both hemispheres.

DISCUSSION

These findings suggest that the contralateral hemisphere can compensate for the damaged cortex by remodeling neuronal activity. Thus, clinical treatments specifically targeted to the 'intact' hemisphere following stroke may provide a complimentary strategy for promoting recovery of functional deficits and for improving quality of life in stroke patients.

Device for simultaneous positron emission tomography, laser speckle imaging and RGB reflectometry: validation and application to cortical spreading depression and brain ischemia in rats

Brain function critically relies on the supply with energy substrates (oxygen and glucose) via blood flow. Alterations in energy demand as during neuronal activation induce dynamic changes in substrate fluxes and blood flow. To study the complex system that regulates cerebral metabolism requires the combination of methods for the simultaneous assessment of multiple parameters. We developed a multimodal imaging device to combine positron emission tomography (PET) with laser speckle imaging (LSI) and RGB reflectometry (RGBR). Depending on the radiotracer, PET provides 3-dimensional quantitative information of specific molecular processes, while LSI and RGBR measure cerebral blood flow (CBF) and hemoglobin oxygenation at high temporal and spatial resolution. We first tested the functional capability of each modality within our system and showed that interference between the modalities is negligible. We then cross-calibrated the system by simultaneously measuring absolute CBF using (15)O-H2O PET (CBF(PET)) and the inverse correlation time (ICT), the LSI surrogate for CBF. ICT and CBF(PET) correlated in multiple measurements in individuals as well as across different animals (R(2)=0.87, n=44 measurements) indicating that ICT can be used for absolute quantitative assessment of CBF. To demonstrate the potential of the combined system, we applied it to cortical spreading depression (CSD), a wave of transient cellular depolarization that served here as a model system for neurovascular and neurometabolic coupling. We analyzed time courses of hemoglobin oxygenation and CBF alterations coupled to CSD, and simultaneously measured regional uptake of (18)F-2-fluoro-2-deoxy-D-glucose ((18)F-FDG) used as a radiotracer for regional glucose metabolism, in response to a single CSD and to a cluster of CSD waves. With this unique combination, we characterized the changes in cerebral metabolic rate of oxygen (CMRO2) in real-time and showed a correlation between (18)F-FDG uptake and the number of CSD waves that passed the local tissue. Finally, we examined CSD spontaneously occurring during focal ischemia also referred to as peri-infarct depolarization (PID). In the vicinity of the ischemic territory, we observed PIDs that were characterized by reduced CMRO2 and increased oxygen extraction fraction (OEF), indicating a limitation of oxygen supply. Simultaneously measured PET showed an increased (18)F-FDG uptake in these regions. Our combined system proved to be a novel tool for the simultaneous study of dynamic spatiotemporal alterations of cortical blood flow, oxygen metabolism and glucose consumption under normal and pathologic conditions.

Selective intra-arterial administration of ¹⁸F-FDG to the rat brain -- effects on hemispheric uptake

Introduction

The purpose of this study was to investigate the radioligand uptake and iodine contrast distribution in the intra- and extracranial circulation of the rat, after intra-arterial injections to the common carotid artery and different parts of the internal carotid artery.

Methods

All animal experiments were carried out in accordance with Karolinska Institutet’s guidelines and were approved by the local laboratory animal ethics committee. We used clinical neurointerventional systems to place microcatheters in the extra- or intracranial carotid artery of 15 Sprague–Dawley rats. Here, injection dynamics of iodine contrast was assessed using digital subtraction angiography. Maintaining the catheter position, the animals were placed in a micro PET and small-animal positron emission tomography (PET) was used to analyze injections [2-¹⁸F]-2-fluoro-2-deoxy-d-glucose (¹⁸F-FDG).

Results

Microcatheters had to be placed in the intracranial carotid artery (iICA) for the infusate to distribute to the brain. Selective injection via the iICA resulted in a 9-fold higher uptake of ¹⁸F-FDG in the injected hemisphere (p < 0.005) compared to both intravenous and more proximal carotid artery injections. Furthermore, selective injection gave a dramatically improved contrast between the brain and extracranial tissue.

Conclusion

Intra-arterial injection increases the cerebral uptake of a radiotracer dramatically compared to systemic injection. This technique has potential applications for endovascular treatment of malignancies allowing intra-interventional modifications of injection strategy, based on information on tumor perfusion and risk to surrounding normal parenchyma. Furthermore the technique may increase diagnostic sensitivity and avoid problems due to peripheral pharmacological barriers and first passage metabolism of labile tracers.

FDG-PET imaging in mild traumatic brain injury: a critical review

Traumatic brain injury (TBI) affects an estimated 1.7 million people in the United States and is a contributing factor to one third of all injury related deaths annually. According to the CDC, approximately 75% of all reported TBIs are concussions or considered mild in form, although the number of unreported mild TBIs (mTBI) and patients not seeking medical attention is unknown. Currently, classification of mTBI or concussion is a clinical assessment since diagnostic imaging is typically inconclusive due to subtle, obscure, or absent changes in anatomical or physiological parameters measured using standard magnetic resonance (MR) or computed tomography (CT) imaging protocols. Molecular imaging techniques that examine functional processes within the brain, such as measurement of glucose uptake and metabolism using [¹⁸F]fluorodeoxyglucose and positron emission tomography (FDG-PET), have the ability to detect changes after mTBI. Recent technological improvements in the resolution of PET systems, the integration of PET with magnetic resonance imaging (MRI), and the availability of normal healthy human databases and commercial image analysis software contribute to the growing use of molecular imaging in basic science research and advances in clinical imaging. This review will discuss the technological considerations and limitations of FDG-PET, including differentiation between glucose uptake and glucose metabolism and the significance of these measurements. In addition, the current state of FDG-PET imaging in assessing mTBI in clinical and preclinical research will be considered. Finally, this review will provide insight into potential critical data elements and recommended standardization to improve the application of FDG-PET to mTBI research and clinical practice.

Flat-panel volumetric computed tomography in cerebral perfusion: evaluation of three rat stroke models

Flat-panel volumetric computed tomography (fpVCT) is a non-invasive approach to three-dimensional small animal imaging. The capability of volumetric scanning and a high resolution in time and space enables whole organ perfusion studies. We aimed to assess feasibility and validity of fpVCT in cerebral perfusion measurement with impaired hemodynamics by evaluation of three well-established rat stroke models for temporary and permanent middle cerebral artery occlusion (MCAO). Male Wistar rats were randomly assigned to temporary (group I: suture model) and permanent (group II: suture model; III: macrosphere model) MCAO and to a control group. Perfusion scans with respect to cerebral blood flow (CBF) and volume (CBV) were performed 24h post intervention by fpVCT, using a Gantry rotation time of 1s and a total scanning time of 30s. Postmortem analysis included infarct-size calculation by 2,3,5-triphenyltetrazolium chloride (TTC) staining. Infarct volumes did not differ significantly throughout intervention groups. After permanent MCAO, CBF significantly decreased in subcortical regions to 78.2% (group II, p=0.005) and 79.9% (group III, p=0.012) and in total hemisphere to 77.4% (group II, p=0.010) and 82.0% (group III, p=0.049). CBF was less impaired with temporary vessel occlusion. CBV measurement revealed no significant differences. Results demonstrate feasibility of cerebral perfusion quantification in rats with the fpVCT, which can be a useful tool for non-invasive dynamic imaging of cerebral perfusion in rodent stroke models. In addition to methodological advantages, CBF data confirm the macrosphere model as a useful alternative to the suture model for permanent experimental MCAO.

Spatiotemporal uptake characteristics of [18]F-2-fluoro-2-deoxy-D-glucose in a rat middle cerebral artery occlusion model

BACKGROUND AND PURPOSE

Alterations of cerebral glucose metabolism are well anticipated during cerebral ischemia. However, detailed spatiotemporal characteristics of disturbed cerebral glucose metabolism during acute ischemia remain largely elusive. This study aims to delineate spatiotemporal distributions of [18]F-2-fluoro-2-deoxy-D-glucose (FDG) uptake using positron emission tomography imaging, particularly at the peri-ischemic zone, and its correlation with tissue outcome.

METHODS

The intraluminal suture middle cerebral artery occlusion model was used to induce focal cerebral ischemia in rats (n=48). All animals underwent sequential MRI and FDG positron emission tomography imaging at different times (30-150 minutes) after middle cerebral artery occlusion. MR and positron emission tomography images were coregistered. FDG uptake in the peri-ischemic zone was assessed in relation to middle cerebral artery occlusion duration, cerebral blood flow, apparent diffusion coefficient, and 24-hour T2 lesions.

RESULTS

Elevated FDG uptake was consistently observed at the peri-ischemic zone surrounding the presumed ischemic core with low FDG uptake. Both the spatial volume and the uptake level of the hyper-uptake region were inversely correlated with the duration of middle cerebral artery occlusion. The hyper-uptake regions exhibited a mild reduction of cerebral blood flow (28.2±3.2%) and apparent diffusion coefficient (9.1±1.4%) when compared with that in the contralateral hemisphere. Colocalization analysis revealed that, with reperfusion, an average of 12.1±1.7% of the hyper-uptake volume was recruited into final infarction.

CONCLUSIONS

Elevated FDG uptake at the peri-ischemic zone is consistently observed during acute cerebral ischemia. The region with elevated FDG uptake likely reflects viable tissues that can be salvaged with reperfusion. Therefore, acute FDG positron emission tomography imaging might hold promise in the management of patients with acute stroke.

The potential roles of 18F-FDG-PET in management of acute stroke patients

Extensive efforts have recently been devoted to developing noninvasive imaging tools capable of delineating brain tissue viability (penumbra) during acute ischemic stroke. These efforts could have profound clinical implications for identifying patients who may benefit from tPA beyond the currently approved therapeutic time window and/or patients undergoing neuroendovascular treatments. To date, the DWI/PWI MRI and perfusion CT have received the most attention for identifying ischemic penumbra. However, their routine use in clinical settings remains limited. Preclinical and clinical PET studies with [¹⁸F]-fluoro-2-deoxy-D-glucose (¹⁸F-FDG) have consistently revealed a decreased ¹⁸F-FDG uptake in regions of presumed ischemic core. More importantly, an elevated ¹⁸F-FDG uptake in the peri-ischemic regions has been reported, potentially reflecting viable tissues. To this end, this paper provides a comprehensive review of the literature on the utilization of ¹⁴C-2-DG and ¹⁸F-FDG-PET in experimental as well as human stroke studies. Possible cellular mechanisms and physiological underpinnings attributed to the reported temporal and spatial uptake patterns of ¹⁸F-FDG are addressed. Given the wide availability of ¹⁸F-FDG in routine clinical settings, ¹⁸F-FDG PET may serve as an alternative, non-invasive tool to MRI and CT for the management of acute stroke patients.

Applications of positron emission tomography in animal models of neurological and neuropsychiatric disorders

Positron emission tomography (PET) provides dynamic images of the biodistribution of radioactive tracers in the brain. Through application of the principles of compartmental analysis, tracer uptake can be quantified in terms of specific physiological processes such as cerebral blood flow, cerebral metabolic rate, and the availability of receptors in brain. Whereas early PET studies in animal models of brain diseases were hampered by the limited spatial resolution of PET instruments, dedicated small-animal instruments now provide molecular images of rodent brain with resolution approaching 1mm, the theoretic limit of the method. Major applications of PET for brain research have consisted of studies of animal models of neurological disorders, notably Parkinson's disease (PD), Alzheimer's disease (AD), and Huntington's disease (HD), stroke, epilepsy and traumatic brain injury; these studies have particularly benefited from selective neurochemical lesion models (PD), and also transgenic rodent models (AD, HD). Due to their complex and uncertain pathophysiologies, corresponding models of neuropsychiatric disorders have proven more difficult to establish. Historically, there has been an emphasis on PET studies of dopamine transmission, as assessed with a range of tracers targeting dopamine synthesis, plasma membrane transporters, and receptor binding sites. However, notable recent breakthroughs in molecular imaging include the development of greatly improved tracers for subtypes of serotonin, cannabinoid, and metabotropic glutamate receptors, as well as noradrenaline transporters, amyloid-β and neuroinflammatory changes. This article reviews the considerable recent progress in preclinical PET and discusses applications relevant to a number of neurological and neuropsychiatric disorders in humans.