Positron imaging in ischemic stroke disease using compounds labeled with oxygen 15. Initial results of clinicophysiologic correlations

Predicting Penumbra Salvage and Infarct Growth in Acute Ischemic Stroke: A Multifactor Survival Game

BACKGROUND

Effective treatment of acute ischemic stroke requires reperfusion of salvageable tissue. We investigated the predictors of penumbra salvage (PS) and infarct growth (IG) in a large cohort of stroke patients.

METHODS

In the ASTRAL registry from 2003 to 2016, we selected middle cerebral artery strokes <24 h with a high-quality CT angiography and CT perfusion. PS and IG were correlated in multivariate analyses with clinical, biochemical and radiological variables, and with clinical outcomes.

RESULTS

Among 4090 patients, 551 were included in the study, 50.8% male, mean age (±SD) 66.3 ± 14.7 years, mean admission NIHSS (±SD 13.3 ± 7.1) and median onset-to-imaging-time (IQR) 170 (102 to 385) minutes. Increased PS was associated with the following: higher BMI and lower WBC; neglect; larger penumbra; absence of early ischemic changes, leukoaraiosis and other territory involvement; and higher clot burden score. Reduced IG was associated with the following: non-smokers; lower glycemia; larger infarct core; absence of early ischemic changes, chronic vascular brain lesions, other territory involvement, extracranial arterial pathology and hyperdense middle cerebral artery sign; and higher clot burden score. When adding subacute variables, recanalization was associated with increased PS and reduced IG, and the absence of haemorrhage with reduced IG. Collateral status was not significantly associated with IG nor with PS. Increased PS and reduced IG correlated with better 3- and 12-month outcomes.

CONCLUSION

In our comprehensive analysis, multiple factors were found to be responsible for PS or IG, the strongest being radiological features. These findings may help to better select patients, particularly for more aggressive or late acute stroke treatment.

Perfusion Status in Lacunar Stroke: A Pathophysiological Issue

The pathophysiology of lacunar infarction is an evolving and debated field, where relevant information comes from histopathology, old anatomical studies and animal models. Only in the last years, have neuroimaging techniques allowed a sufficient resolution to directly or indirectly assess the dynamic evolution of small vessel occlusion and to formulate hypotheses about the tissue status and the mechanisms of damage. The core-penumbra concept was extensively explored in large vessel occlusions (LVOs) both from the experimental and clinical point of view. Then, the perfusion thresholds on one side and the neuroimaging techniques studying the perfusion of brain tissue were focused and optimized for LVOs. The presence of a perfusion deficit in the territory of a single small perforating artery was negated for years until the recent proposal of the existence of a perfusion defect in a subgroup of lacunar infarcts by using magnetic resonance imaging (MRI). This last finding opens pathophysiological hypotheses and triggers a neurovascular multidisciplinary reasoning about how to image this perfusion deficit in the acute phase in particular. The aim of this review is to summarize the pathophysiological issues and the application of the core-penumbra hypothesis to lacunar stroke.

Penumbra Detection With Oxygen Extraction Fraction Using Magnetic Susceptibility in Patients With Acute Ischemic Stroke

Background

The oxygen extraction fraction (OEF) has been applied to identify ischemic penumbral tissue, but is difficult to use in an urgent care setting. This study aimed to investigate whether an OEF map generated via magnetic resonance quantitative susceptibility mapping (QSM) could help identify the ischemic penumbra in patients with acute ischemic stroke.

Materials and Methods

This prospective imaging study included 21 patients with large anterior circulation vessel occlusion who were admitted <24 h after stroke onset and 21 age-matched healthy controls. We identified the ischemic penumbra as the region with a Tmax of >6 s during dynamic susceptibility contrast-magnetic resonance imaging (DSC-MRI) and calculated the perfusion-core mismatch ratio between the ischemic penumbra and infarct core volumes. The OEF values were measured based on magnetic susceptibility differences between the venous structures and brain tissues using rapid QSM acquisition. Volumes with increased OEF values were compared to the ischemic penumbra volumes using an anatomical template.

Results

Eleven patients had a perfusion-core mismatch ratio of ≥1.8, and reperfusion therapy was recommended. In these patients, the volumes with increased OEF values of >51.5%, which was defined using the anterior circulation territory OEF values from the 21 healthy controls, were positively correlated with the ischemic penumbra volumes (r = 0.636, 95% CI: 0.059 to 0.895, P = 0.035) and inversely correlated with the 30-day change in the National Institutes of Health Stroke Scale scores (r = −0.624, 95% CI: −0.891 to −0.039, P = 0.041).

Conclusion

Tissue volumes with increased OEF values could predict ischemic penumbra volumes based on DSC-MRI, highlighting the potential of the QSM-derived OEF map as a penumbra biomarker to guide treatment selection in patients with acute ischemic stroke.

Ischemic Penumbra: A Personal View

The concept of the ischemic penumbra was defined over 40 years ago by Lindsay Symon and his group and is now an established principle of all acute ischemic stroke therapies. These reperfusion treatments rescue threatened, critically hypoperfused brain tissue and have been proven to improve clinical outcomes. We have been fortunate to have observed and played a small part in the penumbral story from its beginnings in the 1970s to its pivotal position today. Over this period, we have witnessed penumbral imaging evolve from positron emission tomography through to magnetic resonance imaging and now predominantly computed tomography perfusion, with the advent of automated imaging facilitating case selection for reperfusion therapies. We and others have conducted clinical trials using penumbral imaging to extend the time window for intravenous thrombolysis and select patients for thrombectomy. Together with the concept of fast- and slow-growing ischemic infarct patterns, this embeds the penumbral principle in everyday clinical management. The opportunity now exists to make penumbral imaging even more portable, affordable, and more widely available using mobile platforms, novel imaging techniques, digital linkage, and artificial intelligence.

Fifty Years of Acute Ischemic Stroke Treatment: A Personal History

BACKGROUND

It has been 50 years since the first explorations of the physiology of cerebral ischemia by measuring cerebral blood flow (CBF), and 25 years since the approval of tissue plasminogen activator for treating acute ischemic stroke. My personal career began and matured during those eras. Here, I provide my perspective on the evolution of acute stroke research and treatment from 1971 to the present, with some in-depth discussion of the National Institutes of Neurologic Disease and Stroke (NINDS) tissue-type plasminogen activator (tPA) stroke trial and development of mobile stroke units.

SUMMARY

Studies of CBF and metabolism in acute stroke patients revealed graded tissue injury that was dependent on the duration of ischemia. Subsequent animal research unraveled the biochemical cascade of events occurring at the cellular level after cerebral ischemia. After a decade of failed translation, the development of a relatively safe thrombolytic allowed us to achieve reperfusion and apply the lessons from earlier research to achieve positive clinical results. The successful conduct of the NINDS tPA stroke study coupled with positive outcomes from companion tPA studies around the world created the specialty of vascular neurology. This was followed by an avalanche of research in imaging, a focus on enhancing reperfusion through thrombectomy, and improving delivery of faster treatment culminating in mobile stroke units. Key Messages: The last half century has seen the birth and evolution of successful acute stroke treatment. More research is needed in developing new drugs and catheters to build on the advances we have already made with reperfusion and also in evolving our systems of care to get more patients treated more quickly in the prehospital setting. The history of stroke treatment over the last 50 years exemplifies that medical "science" is an evolving discipline worth an entire career's dedication. What was impossible 50 years ago is today's standard of care, what we claim as dogma today will be laughed at a decade from now, and what appears currently impossible will be tomorrow's realities.

Quantification of brain oxygen extraction and metabolism with [15O]-gas PET: A technical review in the era of PET/MRI

Oxygen extraction fraction (OEF) and the cerebral metabolic rate of oxygen (CMRO₂) are key cerebral physiological parameters to identify at-risk cerebrovascular patients and understand brain health and function. PET imaging with [¹⁵O]-oxygen tracers, either through continuous or bolus inhalation, provides non-invasive assessment of OEF and CMRO₂. Numerous tracer delivery, PET acquisition, and kinetic modeling approaches have been adopted to map brain oxygenation. The purpose of this technical review is to critically evaluate different methods for [¹⁵O]-gas PET and its impact on the accuracy and reproducibility of OEF and CMRO₂ measurements. We perform a meta-analysis of brain oxygenation PET studies in healthy volunteers and compare between continuous and bolus inhalation techniques. We also describe OEF metrics that have been used to detect hemodynamic impairment in cerebrovascular disease. For these patients, advanced techniques to accelerate the PET scans and potential synthesis with MRI to avoid arterial blood sampling would facilitate broader use of [¹⁵O]-oxygen PET for brain physiological assessment.

Penumbra detection in acute stroke with perfusion magnetic resonance imaging: Validation with 15 O-positron emission tomography

Objective

Accurate identification of the ischemic penumbra, the therapeutic target in acute clinical stroke, is of critical importance to identify patients who might benefit from reperfusion therapies beyond the established time windows. Therefore, we aimed to validate magnetic resonance imaging (MRI) mismatch–based penumbra detection against full quantitative positron emission tomography (¹⁵O‐PET), the gold standard for penumbra detection in acute ischemic stroke.

Methods

Ten patients (group A) with acute and subacute ischemic stroke underwent perfusion‐weighted (PW)/diffusion‐weighted MRI and consecutive full quantitative ¹⁵O‐PET within 48 hours of stroke onset. Penumbra as defined by ¹⁵O‐PET cerebral blood flow (CBF), oxygen extraction fraction, and oxygen metabolism was used to validate a wide range of established PW measures (eg, time‐to‐maximum [Tmax]) to optimize penumbral tissue detection. Validation was carried out using a voxel‐based receiver‐operating‐characteristic curve analysis. The same validation based on penumbra as defined by quantitative ¹⁵O‐PET CBF was performed for comparative reasons in 23 patients measured within 48 hours of stroke onset (group B).

Results

The PW map Tmax (area‐under‐the‐curve = 0.88) performed best in detecting penumbral tissue up to 48 hours after stroke onset. The optimal threshold to discriminate penumbra from oligemia was Tmax >5.6 seconds with a sensitivity and specificity of >80%.

Interpretation

The performance of the best PW measure Tmax to detect the upper penumbral flow threshold in ischemic stroke is excellent. Tmax >5.6 seconds–based penumbra detection is reliable to guide treatment decisions up to 48 hours after stroke onset and might help to expand reperfusion treatment beyond the current time windows. ANN NEUROL 2019;85:875–886.

Extension of therapeutic window in ischemic stroke by selective mismatch imaging

The concept of the ischemic penumbra was formulated on the basis of animal experiments showing functional impairment and electrophysiologic disturbances with decreasing flow to the brain below defined values (the threshold for function) and irreversible tissue damage with blood supply further decreased (the threshold for infarction). The perfusion range between these thresholds was termed the “penumbra,” and restitution of flow above the functional threshold was able to reverse the deficits without permanent damage. In further experiments, the dependency of the development of irreversible lesions on the interaction of the severity and the duration of critically reduced blood flow was established, proving that the lower the flow, the shorter the time for efficient reperfusion. As a consequence, infarction develops from the core of ischemia to the areas of less severe hypoperfusion. The translation of this experimental concept as the basis for the efficient treatment of stroke requires methods by which regional flow and energy metabolism can be repeatedly investigated to demonstrate penumbra tissue, which can benefit from therapeutic interventions. Positron emission tomography allows the quantification of regional cerebral blood flow, the regional oxygen extraction fraction, and the regional metabolic rate for oxygen. With these variables, clear definitions of irreversible tissue damage and of critically hypoperfused but potentially salvageable tissue (i.e. the penumbra) in stroke patients can be achieved. However, positron emission tomography is a research tool, and its complex logistics limit clinical routine applications. Perfusion-weighted or diffusion-weighted magnetic resonance imaging is a widely applicable clinical tool, and the “mismatch” between perfusion-weighted and diffusion-weighted abnormalities serves as an indicator of the penumbra. Also computed tomography angiography and computed tomography perfusion imaging can be used to detect areas suspicious of penumbra. The findings with both methods should be validated by positron emission tomography measurements. Several studies included the selection of patients for intravenous thrombolysis on the basis of a perfusion-weighted imaging–diffusion-weighted imaging mismatch or computed tomography perfusion studies. A meta-analysis of several mismatch-based thrombolysis studies of delayed treatment from the DIAS, DIAS-2, DEDAS, EPITHET, and DEFUSE trials revealed increased recanalization. However, this analysis did not confirm an improvement in clinical outcome with delayed thrombolysis. Randomized controlled trials that did enroll patients based on the presence of a target mismatch on multimodal imaging demonstrated a higher benefit of revascularization treatment by comparison with those who did not and demonstrated for the first time that revascularization treatment for occlusion of an internal carotid artery (ICA) or a proximal middle cerebral artery (MCA) was still beneficial from 6 to 24 h after onset among patients in whom the clinical examination and the multimodal brain imaging indicate a persistent penumbra. On this background, the yield of imaging for the selection of patients for a revascularization therapy will be discussed.

Prospects for investigating brain oxygenation in acute stroke: Experience with a non-contrast quantitative BOLD based approach

Metabolic markers of baseline brain oxygenation and tissue perfusion have an important role to play in the early identification of ischaemic tissue in acute stroke. Although well established MRI techniques exist for mapping brain perfusion, quantitative imaging of brain oxygenation is poorly served. Streamlined-qBOLD (sqBOLD) is a recently developed technique for mapping oxygenation that is well suited to the challenge of investigating acute stroke. In this study a noninvasive serial imaging protocol was implemented, incorporating sqBOLD and arterial spin labelling to map blood oxygenation and perfusion, respectively. The utility of these parameters was investigated using imaging based definitions of tissue outcome (ischaemic core, infarct growth and contralateral tissue). Voxel wise analysis revealed significant differences between all tissue outcomes using pairwise comparisons for the transverse reversible relaxation rate (R ₂ '), deoxygenated blood volume (DBV) and deoxyghaemoglobin concentration ([dHb]; p < 0.01 in all cases). At the patient level (n = 9), a significant difference was observed for [dHb] between ischaemic core and contralateral tissue. Furthermore, serial analysis at the patient level (n = 6) revealed significant changes in R ₂ ' between the presentation and 1 week scans for both ischaemic core (p < 0.01) and infarct growth (p < 0.05). In conclusion, this study presents evidence supporting the potential of sqBOLD for imaging oxygenation in stroke.

Validation of MRI Determination of the Penumbra by PET Measurements in Ischemic Stroke

The concept of the ischemic penumbra was formulated on the basis of animal experiments showing functional impairment and electrophysiologic disturbances with decreasing flow to the brain below defined values (the threshold for function) and irreversible tissue damage with blood supply further decreased (the threshold for infarction). The perfusion range between these thresholds was termed the "penumbra," and restitution of flow above the functional threshold was able to reverse the deficits without permanent damage. In further experiments, the dependency of the development of irreversible lesions on the interaction of the severity and the duration of critically reduced blood flow was established, proving that the lower the flow, the shorter the time for efficient reperfusion. As a consequence, infarction develops from the core of ischemia to the areas of less severe hypoperfusion. The translation of this experimental concept as the basis for the efficient treatment of stroke requires noninvasive methods with which regional flow and energy metabolism can be repeatedly investigated to demonstrate penumbra tissue, which can benefit from therapeutic interventions. PET allows the quantification of regional cerebral blood flow, the regional oxygen extraction fraction, and the regional metabolic rate for oxygen. With these variables, clear definitions of irreversible tissue damage and of critically hypoperfused but potentially salvageable tissue (i.e., the penumbra) in stroke patients can be achieved. However, PET is a research tool, and its complex logistics limit clinical routine applications. Perfusion-weighted or diffusion-weighted MRI is a widely applicable clinical tool, and the "mismatch" between perfusion-weighted and diffusion-weighted abnormalities serves as an indicator of the penumbra. However, comparative studies of perfusion-weighted or diffusion-weighted MRI and PET have indicated overestimation of the core of irreversible infarction as well as of the penumbra by the MRI modalities. Some of these discrepancies can be explained by the nonselective application of relative perfusion thresholds, which might be improved by more complex analytic procedures. The heterogeneity of the MRI signatures used for the definition of the mismatch are also responsible for disappointing results in the application of perfusion-weighted or diffusion-weighted MRI to the selection of patients for clinical trials. As long as validation of the mismatch selection paradigm is lacking, the use of this paradigm as a surrogate marker of outcome is limited.

PET imaging in ischemic cerebrovascular disease: current status and future directions

Cerebrovascular diseases are caused by interruption or significant impairment of the blood supply to the brain, which leads to a cascade of metabolic and molecular alterations resulting in functional disturbance and morphological damage. These pathophysiological changes can be assessed by positron emission tomography (PET), which permits the regional measurement of physiological parameters and imaging of the distribution of molecular markers. PET has broadened our understanding of the flow and metabolic thresholds critical for the maintenance of brain function and morphology: in this application, PET has been essential in the transfer of the concept of the penumbra (tissue with perfusion below the functional threshold but above the threshold for the preservation of morphology) to clinical stroke and thereby has had great impact on developing treatment strategies. Radioligands for receptors can be used as early markers of irreversible neuronal damage and thereby can predict the size of the final infarcts; this is also important for decisions concerning invasive therapy in large ("malignant") infarctions. With PET investigations, the reserve capacity of blood supply to the brain can be tested in obstructive arteriosclerosis of the supplying arteries, and this again is essential for planning interventions. The effect of a stroke on the surrounding and contralateral primarily unaffected tissue can be investigated, and these results help to understand the symptoms caused by disturbances in functional networks. Chronic cerebrovascular disease causes vascular cognitive disorders, including vascular dementia. PET permits the detection of the metabolic disturbances responsible for cognitive impairment and dementia, and can differentiate vascular dementia from degenerative diseases. It may also help to understand the importance of neuroinflammation after stroke and its interaction with amyloid deposition in the development of dementia. Although the clinical application of PET investigations is limited, this technology had and still has a great impact on research into cerebrovascular diseases.

Paradoxical reduction of cerebral blood flow after acetazolamide loading: a hemodynamic and metabolic study with (15)O PET

Paradoxical reduction of cerebral blood flow (CBF) after administration of the vasodilator acetazolamide is the most severe stage of cerebrovascular reactivity failure and is often associated with an increased oxygen extraction fraction (OEF). In this study, we aimed to reveal the mechanism underlying this phenomenon by focusing on the ratio of CBF to cerebral blood volume (CBV) as a marker of regional cerebral perfusion pressure (CPP). In 37 patients with unilateral internal carotid or middle cerebral arterial (MCA) steno-occlusive disease and 8 normal controls, the baseline CBF (CBF(b)), CBV, OEF, cerebral oxygen metabolic rate (CMRO2), and CBF after acetazolamide loading in the anterior and posterior MCA territories were measured by (15)O positron emission tomography. Paradoxical CBF reduction was found in 28 of 74 regions (18 of 37 patients) in the ipsilateral hemisphere. High CBF(b) (> 47.6 mL/100 mL/min, n = 7) was associated with normal CBF(b)/CBV, increased CBV, decreased OEF, and normal CMRO2. Low CBF(b) (< 31.8 mL/100 mL/min, n = 9) was associated with decreased CBF(b)/CBV, increased CBV, increased OEF, and decreased CMRO2. These findings demonstrated that paradoxical CBF reduction is not always associated with reduction of CPP, but partly includes high-CBF(b) regions with normal CPP, which has not been described in previous studies.

Reliability of cerebral blood volume maps as a substitute for diffusion-weighted imaging in acute ischemic stroke

PURPOSE

To assess the reliability of cerebral blood volume (CBV) maps as a substitute for diffusion-weighted MRI (DWI) in acute ischemic stroke. In acute stroke, DWI is often used to identify irreversibly injured "core" tissue. Some propose using perfusion imaging, specifically CBV maps, in place of DWI. We examined whether CBV maps can reliably subsitute for DWI, and assessed the effect of scan duration on calculated CBV.

MATERIALS AND METHODS

We retrospectively identified 58 patients who underwent DWI and MR perfusion imaging within 12 h of stroke onset. CBV in each DWI lesion's center was divided by CBV in the normal-appearing contralateral hemisphere to yield relative regional CBV (rrCBV). The proportion of lesions with decreased rrCBV was calculated. After using the full scan duration (110 s after contrast injection), rrCBV was recalculated using simulated shorter scans. The effect of scan duration on rrCBV was tested with linear regression.

RESULTS

Using the full scan duration (110 s), rrCBV was increased in most DWI lesions (62%; 95% confidence interval, 48-74%). rrCBV increased with increasing scan duration (P < 0.001). Even with the shortest duration (39.5 s) rrCBV was increased in 33% of lesions.

CONCLUSION

Because DWI lesions may have elevated or decreased CBV, CBV maps cannot reliably substitute for DWI in identifying the infarct core.

The development, past achievements, and future directions of brain PET

The early developments of brain positron emission tomography (PET), including the methodological advances that have driven progress, are outlined. The considerable past achievements of brain PET have been summarized in collaboration with contributing experts in specific clinical applications including cerebrovascular disease, movement disorders, dementia, epilepsy, schizophrenia, addiction, depression and anxiety, brain tumors, drug development, and the normal healthy brain. Despite a history of improving methodology and considerable achievements, brain PET research activity is not growing and appears to have diminished. Assessments of the reasons for decline are presented and strategies proposed for reinvigorating brain PET research. Central to this is widening the access to advanced PET procedures through the introduction of lower cost cyclotron and radiochemistry technologies. The support and expertize of the existing major PET centers, and the recruitment of new biologists, bio-mathematicians and chemists to the field would be important for such a revival. New future applications need to be identified, the scope of targets imaged broadened, and the developed expertize exploited in other areas of medical research. Such reinvigoration of the field would enable PET to continue making significant contributions to advance the understanding of the normal and diseased brain and support the development of advanced treatments.

Perfusion/Diffusion mismatch is valid and should be used for selecting delayed interventions

The mismatch between a larger perfusion lesion and smaller diffusion lesion on magnetic resonance imaging is a validated signal of the ischemic penumbra, namely the region at risk in acute ischemic stroke that is critically hypoperfused and the target of reperfusion therapies. Clinical trials have shown strong correlations between reperfusion in mismatch patients and improved clinical outcomes. Attenuation of infarct growth is associated with reperfusion and corresponding clinical gains. Using computed tomography perfusion, the mismatch between relative cerebral blood flow or cerebral blood volume and perfusion delay is a comparable penumbral marker. Automated techniques allow rapid quantitative assessment of mismatch with thresholding to exclude benign oligemia. The penumbra is often present beyond the current 4.5-h time window, defined for the use of intravenous tPA. Treatment beyond this time point remains investigational. Although the efficacy of thrombolysis in mismatch patients requires further validation in randomized trials, there is now sufficient evidence to recommend that advanced neuroimaging of mismatch should be used for selection of delayed therapies in phase 3 trials.

Multimodal MRI of experimental stroke

Stroke is the fourth leading cause of death and the leading cause of long-term disability in USA. Brain imaging data from experimental stroke models and stroke patients have shown that there is often a gradual progression of potentially reversible ischemic injury toward infarction. Reestablishing tissue perfusion and/or treating with neuroprotective drugs in a timely fashion are expected to salvage some ischemic tissues. Diffusion-weighted imaging based on magnetic resonance imaging (MRI) in which contrast is based on water motion can detect ischemic injury within minutes after onsets, whereas computed tomography and other imaging modalities fail to detect stroke injury for at least a few hours. Along with quantitative perfusion imaging, the perfusion-diffusion mismatch which approximates the ischemic penumbra could be imaged noninvasively. This review describes recent progresses in the development and application of multimodal MRI and image analysis techniques to study ischemic tissue at risk in experimental stroke in rats.

Early venous drainage after successful endovascular recanalization in ischemic stroke -- a predictor for final infarct volume?

INTRODUCTION

Peri-ischemic early venous filling (PEVD) has been reported to occur at certain stages of brain infarction and has previously been termed as "luxury perfusion". We report on the significance of PEVD after a successful endovascular recanalization.

METHODS

We retrospectively evaluated all patients who underwent endovascular stroke treatment from February 2006 to April 2010 in two centers. PEVD was rated as present or absent. Infarction was evaluated on computed tomography (CT) ≥ 18 h post-treatment. Localization of the PEVD and the infarction was noted for the anterior and posterior circulation; for the anterior circulation, also deep and superficial veins/brain regions were defined.

RESULTS

A total of 151 of the 175 patients developed an infarct. Of these 151 patients, 118 had PEVD (sensitivity 78.1%); meanwhile, 20 of 24 patients without an infarction had no PEVD (specificity 83.3%). Consistent localization of the PEVD and the infarct was seen in 107/151 patients (70.9%); in 28 of these 107 cases, the territory of PEVD was smaller than the infarct (26.2%) and exceeded it in 7/107 patients (5.6%). Territorial congruency of the PEVD and the final infarct was 57.6-75% for deep/superficial brain regions of the anterior, but only 16.7% for the posterior circulation. Separate evaluation for the anterior circulation resulted in a 94.9% sensitivity and an 81.0% specificity.

CONCLUSION

PEVD is a potential angiographic predictor for irreversible regional tissue damage and subsequent infarction despite successful recanalization. This finding deserves further studies and may influence therapeutic decisions such as post-treatment anticoagulative medication. It may also be considered in potential refined classifications of angiographic reperfusion success in the future.

Spatiotemporal characteristics of postischemic hyperperfusion with respect to changes in T1, T2, diffusion, angiography, and blood-brain barrier permeability

The spatiotemporal dynamics of postischemic hyperperfusion (HP) remains incompletely understood. Diffusion, perfusion, T2, T1, angiographic, dynamic susceptibility-contrast magnetic resonance imaging (MRI) and magnetic resonance angiography were acquired longitudinally at multiple time points up to 7 days after stroke in rats subjected to 30-, 60-, and 90-minutes middle cerebral artery occlusion (MCAO). The spatiotemporal dynamics of postischemic HP was analyzed and compared with T1, T2 and blood-brain barrier (BBB) changes. No early HP within 3 hours after recanalization was observed. Late (12 hours) HP was present in all animals of the 30-minute MCAO group (N=20), half of the animals in the 60-minute MCAO group (N=8), and absent in the 90-minute MCAO group (N=9). Dynamic susceptibility-contrast MRI and magnetic resonance angiography corroborated HP. Hyperperfusion preceded T2 increase in some animals, but HP and T2 changes temporally coincided in others. T2 peaked first at 24 hours whereas HP peaked at 48 hours after occlusion, and HP resolved by day 7 in most animals at which point the arteries became tortuous. Pixel-by-pixel tracking analysis showed that tissue did not infarct (migrated from core or mismatch at 30 minutes to normal at 48 hours) showed normal cerebral blood flow (CBF), whereas infarct tissue (migrated from core or mismatch at 30 minutes to infarct at 48 hours) showed exaggerated CBF, indicating that HP was associated with poor outcome.

The ischemic penumbra: correlates in imaging and implications for treatment of ischemic stroke. The Johann Jacob Wepfer award 2011

The concept of the ischemic penumbra was formulated 30 years ago based on experiments in animal models showing functional impairment and electrophysiological disturbances with decreasing flow to the brain below defined values (the threshold for function) and irreversible tissue damage with the blood supply further decreased (the threshold for infarction). The perfusion range between these thresholds was termed 'penumbra', and restitution of flow above the functional threshold was able to reverse the deficits without permanent damage. However, in further experiments, the dependency of the development of irreversible lesions on the interaction of the severity and duration of critically reduced blood flow was established - proving that the lower the flow, the shorter the time for efficient reperfusion. Therefore, infarction develops from the core of ischemia to the areas of less severe hypoperfusion. The propagation of irreversible tissue damage is characterized by a complex cascade of interconnected electrophysiological, molecular, metabolic and perfusional disturbances. Waves of depolarizations, the peri-infarct spreading depression-like depolarizations, inducing activation of ion pumps and liberation of excitatory transmitters, have dramatic consequences as drastically increased metabolic demand cannot be satisfied in regions with critically reduced blood supply. The translation of experimental concept into the basis for efficient treatment of stroke requires non-invasive methods by which regional flow and energy metabolism can be repeatedly investigated to demonstrate penumbra tissue that can benefit from therapeutic interventions. Positron emission tomography (PET) allows the quantification of regional cerebral blood flow, the regional metabolic rate for oxygen and the regional oxygen extraction fraction. From these variables, clear definitions of irreversible tissue damage and critically perfused but potentially salvageable tissue (i.e. the penumbra) can be achieved in animal models and stroke patients. Additionally, further tracers can be used for early detection of irreversible tissue damage, e.g. by the central benzodiazepine receptor ligand flumazenil. However, PET is a research tool and its complex logistics limit clinical routine applications. As a widely applicable clinical tool, perfusion/diffusion-weighted (PW/DW) MRI is used, and the 'mismatch' between the PW and the DW abnormalities serve as an indicator of the penumbra. However, comparative studies of PW/DW-MRI and PET have pointed to an overestimation of the core of irreversible infarction as well as of the penumbra by MRI modalities. Some of these discrepancies can be explained by unselective application of relative perfusion thresholds, which might be improved by more complex analytical procedures. Heterogeneity of the MRI signatures used for the definition of the mismatch are also responsible for disappointing results in the application of PW/DW-MRI for the selection of patients for clinical trials. As long as a validation of the mismatch selection paradigm is lacking, its use as a surrogate marker of outcome is limited.

Heterogeneity in the penumbra

Original experimental studies in nonhuman primate models of focal ischemia showed flow-related changes in evoked potentials that suggested a circumferential zone of low regional cerebral blood flow with normal K(+) homeostasis, around a core of permanent injury in the striatum or the cortex. This became the basis for the definition of the ischemic penumbra. Imaging techniques of the time suggested a homogeneous core of injury, while positing a surrounding 'penumbral' region that could be salvaged. However, both molecular studies and observations of vascular integrity indicate a more complex and dynamic situation in the ischemic core that also changes with time. The microvascular, cellular, and molecular events in the acute setting are compatible with heterogeneity of the injury within the injury center, which at early time points can be described as multiple 'mini-cores' associated with multiple 'mini-penumbras'. These observations suggest the progression of injury from many small foci to a homogeneous defect over time after the onset of ischemia. Recent observations with updated imaging techniques and data processing support these dynamic changes within the core and the penumbra in humans following focal ischemia.

The concept of the penumbra: can it be translated to stroke management?

The 'penumbra' is a concept coined in animal experiments suggesting that functionally impaired tissue can survive and recover if sufficient reperfusion is re-established within a limited time period, which depends on the level of residual flow. In an ischaemic territory, irreversible damage progresses over time from the centre of the most severe flow reduction to the periphery with less disturbed perfusion. This centrifugal progression of irreversible tissue damage is characterised by a complex cascade of interconnected electrophysiological, molecular, metabolic and perfusion disturbances. Waves of depolarisations, the peri infarct spreading depressions, inducing activation of ion pumps and liberation of excitatory transmitters play an important role in the drastically increased metabolic demand during reduced oxygen supply causing hypoxic tissue changes and lactacidosis, which further damage the tissue. Positron emission tomography allows the quantification of regional cerebral blood flow, the regional metabolic rate for oxygen and the regional oxygen extraction fraction, which can be used to identify regions with a critical reduction in these physiologic variables as indicators of penumbra and irreversible damage within ischaemic territories in animal models and patients with stroke. These positron emission tomography methods require arterial blood sampling and due to the complex logistics involved, are limited for routine application. Therefore, newer tracers were developed for the noninvasive detection of irreversible tissue damage (flumazenil) and of hypoxic tissue changes (fluoromisonidazole). As a widely applicable clinical tool, diffusion/perfusion-weighted magnetic resonance imaging is used; the 'mismatch' between perfusion and diffusion changes serves as a surrogate marker of the penumbra. However, in comparative studies of magnetic resonance imaging and positron emission tomography, diffusion-weighted imaging showed a high false-positive rate of irreversible damage, and the perfusion-weighted-diffusion-weighted mismatch overestimated the penumbra as defined by positron emission tomography. Advanced analytical procedures of magnetic resonance imaging data may improve the reliability of these surrogate markers but should be validated with quantitative procedures.

Multimodal magnetic resonance imaging for assessing evolution of ischemic penumbra: a key translational medicine strategy to manage the risk of developing novel therapies for acute ischemic stroke

The implicit aim of neuroprotection is to rescue neurons within distressed but still viable tissue, thereby promoting functional recovery upon neuronal salvage. The clinical failure of this approach suggests that previous efforts to develop stroke therapies lacked means to predict success or futility in pre-clinical and early clinical studies. A key translational medicine strategy that can improve predictability relies on imaging methodologies to map the spatiotemporal evolution of the ischemic penumbra. This could serve as a biomarker indicative of neuroprotective potential and could increase likelihood of success in clinical studies by allowing selection of patients who are most likely to respond to therapy.

Imaging the penumbra - strategies to detect tissue at risk after ischemic stroke

The aim of thrombolytic therapy after acute ischemic stroke is salvage of the ischemic penumbra. Several imaging techniques have been used to identify the penumbra in patients who may benefit from reperfusion beyond the currently narrow 3-hour time-window for thrombolysis. We discuss the advantages and disadvantages of positron emission tomography (PET), single photon emission computed tomography (SPECT), MRI and CT scans. We comment on concepts of clinical-imaging mismatch models and we explore the implications for clinical trials.

Positron emission tomography scans obtained for the evaluation of cognitive dysfunction

The degree of intactness of human cognitive functioning for a given individual spans a wide spectrum, ranging from normal to severely demented. The differential diagnosis for the causes of impairment along that spectrum is also wide, and often difficult to distinguish clinically, which has led to an increasing role for neuroimaging tools in that evaluation. The most frequent causes of dementia are neurodegenerative disorders, Alzheimer's disease being the most prevalent among them, and they produce significant alterations in brain metabolism, with devastating neuropathologic, clinical, social, and economic consequences. These alterations are detectable through positron emission tomography (PET), even in their earliest stages. The most commonly performed PET studies of the brain are performed with (18)F-fluorodeoxyglucose as the imaged radiopharmaceutical. Such scans have demonstrated diagnostic and prognostic utility for clinicians evaluating patients with cognitive impairment and in distinguishing among primary neurodegenerative disorders and other etiologies contributing to cognitive decline. In addition to focusing on the effects on cerebral metabolism examined with (18)F-fluorodeoxyglucose PET, some other changes occurring in the brains of cognitively impaired patients assessable with other radiotracers will be considered. As preventive and disease-modifying treatments are developed, early detection of accurately diagnosed disease processes facilitated by the use of PET has the potential to substantially impact on the enormous human toll exacted by these diseases.

Stroke Research in the Modern Era: Images versus Dogmas

Recovery of function following ischaemic stroke is a fascinating clinical observation. It comprises several modes, e.g. spectacular recovery in a matter of hours or days and gradual recovery over months or even years. That a non-functioning neural system can regain its function, even partially so, is challenging because of the obvious therapeutic implications. Until the mid-70s, however, dogmas largely prevailed which underpinned the then nihilistic approach to stroke patients. Proving these dogmas wrong has been a major achievement of modern stroke research. Thanks particularly to physiological imaging, key observations from the basic neurosciences have translated into the clinical realm in ways immediately understandable to the clinician, allowing the emergence of pathophysiology-based management.

PET imaging in the assessment of normal and impaired cognitive function

PET has been used to directly quantify several processes relevant to the status of cerebral health and function, including cerebral blood flow, cerebral blood volume, cerebral rate of oxygen metabolism, and cerebral glucose use. Clinically, the most commonly performed PET studies of the brain are performed with fluorine-18-fluorodeoxyglucose as the imaged radiopharmaceutical. Such scans have demonstrated diagnostic and prognostic use in evaluating patients who have cognitive impairment, and in distinguishing among primary neurodegenerative dementias and other causes of cognitive decline. In certain pathologic circumstances, the normal coupling between blood flow and metabolic needs may be disturbed, and changes in oxygen extraction fraction can have significant prognostic value.

Pharmacokinetics of a triarylmethyl-type paramagnetic spin probe used in EPR oximetry

The paramagnetic spin probe Oxo63 is used in oximetric imaging studies based on electron paramagnetic resonance (EPR) methods by monitoring the oxygen-dependent linewidth while minimizing the contributions from self-broadening seen at high probe concentrations. Therefore, it is necessary to determine a suitable dose of Oxo63 for EPR-based oxygen mapping where the self-broadening effects are minimized while signal intensity adequate for imaging can be realized. A constant tissue concentration of spin probe would be useful to image a subject and assess changes in pO2 over time; accumulation or elimination of the compound in specific anatomical regions could translate to and be mistaken for changes in local pO2, especially in OMRI-based oximetry. The in vivo pharmacokinetics of the spin probe, Oxo63, after bolus and/or continuous intravenous infusion was investigated in mice using a novel approach with X-band EPR spectroscopy. The results show that the half-life in blood was 17-21 min and the clearance by excretion was 0.033-0.040 min(-1). Continuous infusion following a bolus injection of the probe was found to be effective to obtain stable plasma concentration as well as image intensity to permit reliable pO2 estimates.

Perfusion-weighted MRI as a marker of response to treatment in acute and subacute stroke

We carried out baseline and short-term follow-up MRI, including perfusion-weighted imaging (PWI) and tests of neurologic and cognitive function on 15 consecutive patients with large-vessel ischemic stroke who showed a persistent large perfusion-diffusion mismatch at enrollment up to seven days after the onset of symptoms. Of these, ten underwent induced blood pressure elevation with phenylephrine and oral medications (in eight) or intravenous fluids (in two) with the goal of improving perfusion; five had no such treatment. Significant functional improvement was defined by a reduction of 3 or more points on the NIH stroke scale (NIHSS). Significant improvement in perfusion was defined by a reduction in the volume of hypoperfused brain by 30 cc on PWI using time-to-peak (TTP) maps, without enlargement of the infarct. There was a strong, statistically significant association between improved function and improved perfusion: six (75%) of eight patients who improved in function, but none of the seven who did not, showed a reduction in volume of hypoperfused brain. All six patients who met the perfusion goal, and only two (22%) of nine who did not showed significant functional improvement (Fisher's exact: P < 0.01). There were no differences between patients who improved functionally and those who did not with respect to age, initial volume of abnormality on DWI or PWI, initial NIHSS, or changes on DWI. These findings indicate that reduction in volume of hypoperfused brain on PWI is a marker of response to treatment to improve perfusion even in subacute stroke and that partial reperfusion of regions of salvageable but dysfunctional tissue is a mechanism of improved function associated with induced blood pressure elevation.

Stroke: imaging and differential diagnosis

Structural and vascular imaging helps to differentiate haemorrhagic from acute ischemic stroke (AIS) and rule out non-stroke causes, as well as identify specific subtypes of stroke such as carotid dissection and venous thrombosis. However, it is negative in most AIS patients within 3-6 hrs of onset and thus does not allow efficient patient classification for management purposes. Physiologic neuroimaging with PET, SPECT and combined diffusion- and perfusion-weighted MR gives access to tissue perfusion and cell function/homeostasis. It has near 100% sensitivity in AIS, even in small cortical or brainstem strokes. In middle-cerebral artery (MCA) stroke, physiologic imaging also allows pathophysiological differentiation into four tissue subtypes: i) already irreversibly damaged ("core"); ii) severely hypoperfused ("penumbra"), which represents the main target for therapy; iii) mildly hypoperfused ("oligaemia"), not at risk of infarction unless secondary complications arise; and iv) reperfused/hyperperfused. PET studies have evidenced the penumbra in man, shown its largely cortical topography, documented its anticipated impact on both acute-stage neurological deficit and recovery therefrom, and shown its persistence up to 16 hrs after stroke onset in some patients. However, some patients acutely exhibit extensive irreversible damage, which places them at considerable risk of malignant MCA infarction, and others early spontaneous reperfusion, which is almost invariably associated with rapid and complete recovery. Thrombolytics and/or neuroprotective agents would therefore be expected to benefit, and hence should ideally be reserved to, only those patients in whom a substantial penumbra is documented by physiologic neuroimaging, even perhaps beyond the 3 to 6 hrs rule. In addition, excluding from thrombolytic therapy those patients with substantial necrotic core should avoid many instances of symptomatic haemorrhagic transformations. Finally, patients with extensive core might benefit from early decompressive surgery, and those with early extensive reperfusion from anti-inflammatory agents. Overall, therefore, the pathophysiologic heterogeneity underlying AIS may account for both the complications from thrombolysis and the limited success of clinical trials of neuroprotective agents, despite apparent benefit in the laboratory. Pathophysiological diagnosis as afforded by neuroimaging should now be incorporated in the design of clinical trials as well as in the routine management of stroke.

The 5 Ps of acute ischemic stroke treatment: parenchyma, pipes, perfusion, penumbra, and prevention of complications

Stroke is a treatable disease. Despite the therapeutic nihilism of the past, the advent of thrombolysis has changed the way stroke is approached. Acute ischemic stroke is a challenging and heterogeneous disease. Treatment needs to be based on an understanding of the underlying pathophysiology of ischemia. Interventions are designed to improve neuronal salvage and outcome. The underlying tenets of stroke therapy focus on the brain parenchyma, arterial flow (pipes), perfusion, the ischemic milieu or penumbra, and prevention of complications. This article focuses on the practical issues of ischemic stroke care, with a brief review of supporting literature.

Ischaemic penumbra: highlights

The ischaemic penumbra was described for the first time in the late 1970s as a ring of hypoperfused zone surrounding the region of complete infarction. The penumbral zone is a functionally silent tissue which is able to regain its function if promptly reperfused. This implies that the ischaemic penumbra is not a static but a "dynamic" and "time-dependent" concept. In this paper we describe the role of neuroimmaging tecniques such as single photon emission tomography (SPET), positron emission tomography (PET), and diffusion-weighted and perfusion-weighted magnetic resonance imaging (DWI and PWI) in the study of ischaemic penumbra. These functional imaging techniques have the advantage of giving "in vivo" quantitative estimate of cerebral blood flow (CBF) as well as information on how the ischaemic tissue metabolic changes develop. It follows that, as therapeutic options for treating acute stroke evolve, neuroimaging strategies are assuming an increasingly important role in the initial evaluation and management of the acute ischaemic patient. In this regard, a wide range of therapeutic approaches have been investigated for either ameliorating the perfusion, or interfering with the pathobiochemical cascade leading to ischaemic neuronal damage, or improving endogenous neuroprotection pathways. The "time windows" required for these treatments to be effective varies being rather short for reperfusion and longer for neuroprotection. Salvaging more penumbra would enhance recovery and thereby allow the most appropriate candidate for therapeutic trials to be selected.

Determination of regional cerebral function with FDG-PET imaging in neuropsychiatric disorders

Functional brain imaging using 18F fluorodeoxyglucose (FDG) and positron emission tomography (PET) has greatly enhanced our understanding of brain function both in normal conditions as well as in a wide variety of neuropsychiatric disorders. We review the uses of FDG PET in the diagnosis, management, and follow-up of patients with neuropsychiatric disorders. This article will also explore what FDG-PET imaging has revealed in these neuropsychiatric disorders and how these findings relate to both research and clinical applications.

Ischemic penumbra: evidence from functional imaging in man

The ischemic penumbra is defined as tissue with flow within the thresholds for maintenance of function and of morphologic integrity. Penumbra tissue has the potential for recovery and therefore is the target for interventional therapy in acute ischemic stroke. The identification of the penumbra necessitates measuring flow reduced less than the functional threshold and differentiating between morphologic integrity and damage. This can be achieved by multitracer positron emission tomography (PET) and perfusion-weighted (PW) and diffusion-weighted magnetic resonance imaging (DW-MRI) in experimental models, in which the recovery of critically perfused tissue or its conversion to infarction was documented in repeat studies. Neuroimaging modalities applied in patients with acute ischemic stroke--multitracer PET, PW- and DW-MRI, single photon emission computed tomography (SPECT), perfusion, and Xe-enhanced computed tomography (CT)-- often cannot reliably identify penumbra tissue: multitracer studies for the assessment of flow and irreversible metabolic damage usually cannot be performed in the clinical setting; CT and MRI do not reliably detect irreversible damage in the first hours after stroke, and even DW-MRI may be misleading in some cases: determinations of perfusion alone yield a poor estimate of the state of the tissue as long as the time course of changes is not known in individual cases. Therefore, the range of flow values in ischemic tissue found later, either within or outside the infarct, was rather broad. New tracers--for example, receptor ligands or hypoxia markers--might improve the identification of penumbra tissue in the future. Despite these methodologic limitations, the validity of the concept of the penumbra was proven in several therapeutic studies in which thrombolytic treatment reversed critical ischemia and decreased the volume of final infarcts. Such neuroimaging findings might serve as surrogate targets in the selection of other therapeutic strategies for large clinical trials.

Early postischemic hyperperfusion: pathophysiologic insights from positron emission tomography

Early postischemic hyperperfusion (EPIH) has long been documented in animal stroke models and is the hallmark of efficient recanalization of the occluded artery with subsequent reperfusion of the tissue (although occasionally it may be seen in areas bordering the hypoperfused area during arterial occlusion). In experimental stroke, early reperfusion has been reported to both prevent infarct growth and aggravate edema formation and hemorrhage, depending on the severity and duration of prior ischemia and the efficiency of reperfusion, whereas neuronal damage with or without enlarged infarction also may result from reperfusion (so-called "reperfusion injury"). In humans, focal hyperperfusion in the subacute stage (i.e., more than 48 hours after onset) has been associated with tissue necrosis in most instances, but regarding the acute stage, its occurrence, its relations with tissue metabolism and viability, and its clinical prognostic value were poorly understood before the advent of positron emission tomography (PET), in part because of methodologic issues. By measuring both CBF and metabolism, PET is an ideal imaging modality to study the pathophysiologic mechanism of EPIH. Although only a few PET studies have been performed in the acute stage that have systematically assessed tissue and clinical outcome in relation to EPIH, they have provided important insights. In one study, about one third of the patients with first-ever middle cerebral artery (MCA) territory stroke studied within 5 to 18 hours after symptom onset exhibited EPIH. In most cases, EPIH affected large parts of the cortical MCA territory in a patchy fashion, together with abnormal vasodilation (increased cerebral blood volume), "luxury perfusion" (decreased oxygen extraction fraction), and mildly increased CMRO2, which was interpreted as postischemic rebound of cellular metabolism in structurally preserved tissue. In that study, the spontaneous outcome of the tissue exhibiting EPIH was good, with late structural imaging not showing infarction. This observation was supported by another PET study, which showed, in a few patients, that previously hypoperfused tissue that later exhibited hyperperfusion after thrombolysis did not undergo frank infarction at follow-up. In both studies, clinical outcome was excellent in all patients showing EPIH except one, but in this case the hyperperfused area coexisted with an extensive area of severe hypoperfusion and hypometabolism. These findings from human studies therefore suggest that EPIH is not detrimental for the tissue, which contradicts the experimental concept of "reperfusion injury" but is consistent with the apparent clinical benefit from thrombolysis. However, PET studies performed in the cat have shown that although hyperperfusion was associated with prolonged survival and lack of histologic infarction when following brief (30-minute) MCA occlusion, it often was associated with poor outcome and extensive infarction when associated with longer (60-minute) MCA occlusion. It is unclear whether this discrepancy with human studies reflects a shorter window for tissue survival after stroke in cats, points to the cat being more prone to reperfusion injury, or indicates that EPIH tends not to develop in humans after severe or prolonged ischemia because of a greater tendency for the no-reflow phenomenon, for example. Nevertheless, the fact that the degree of hyperperfusion in these cat studies was related to the severity of prior flow reduction suggests that hyperperfusion is not detrimental per se. Preliminary observations in temporary MCA occlusion in baboons suggest that hyperperfusion developing even after 6 hours of occlusion is mainly cortical and associated with no frank infarction, as in humans. Overall, therefore, PET studies in both humans and the experimental animal, including the baboon, suggest that hyperperfusion is not a key factor in the development of tissue infarction and that it may be a harmless phenomenon

Visualization of pressure-dependent luxury perfusion in a patient with subacute cerebral infarction

Luxury perfusion characterized by depressed metabolism compared with CBF might be changed by decreasing cerebral perfusion pressure during the sitting position. A 77-yr-old man with subacute cerebral infarction was studied with brain X-ray computed tomography (CT), raise-up test with 99mTc-d,1-hexamethylpropyleneamine oxime (HMPAO) brain single photon emission tomography (SPECT) and positron emission tomography (PET). Brain X-ray CT revealed a low-density area in the left middle cerebral artery (MCA) anterior area. Raise-up 99mTc-HMPAO brain SPECT revealed decreased uptake in the left MCA anterior area in the sitting position and subsequent supine 99mTc-HMPAO brain SPECT revealed hot accumulation there. PET study in the supine position demonstrated some differences between CBF and the cerebral metabolic rate for oxygen in the left MCA anterior area, indicating luxury perfusion. CBF in the area of luxury perfusion might be decreased during the sitting or standing position and increased during the supine position by dysautoregulation of the cerebral vessels in the luxury perfusion during the subacute infarct.

Clinical brain radionuclide imaging studies

A recent survey of the knowledge and practice of both positron-emission tomography (PET) and single-photon emission computed tomography (SPECT) of the brain among referring physicians in Europe (neurologists and psychiatrists) showed a disquieting lack of knowledge of the potential of these methodologies in the investigation and management of patients of their own specialities. The need to bring the knowledge of the potential of these techniques to the practicing physicians is paramount. It is imperative that the methodologies and concepts that preside over the application of these techniques in neurology and psychiatry must become more uniform if an impact is to be felt at a clinical level. There is clear improvement in the instrumentation available with the new state-of-the-art tomographic devices and with the development of new technetium-based radiopharmaceuticals for the study of cerebral perfusion. The constant progress made with ligands that permit the study of neurotransmission, tumor metabolism, and turnover do expand our capability to improve the knowledge concerning neurophysiology, neuropathology, and neuropharmacology of a variety of disease states. PET and SPECT will be progressively included in protocols aimed at stratifying patients with dementia, monitoring therapeutic trials, and improving our ability to determine outcome. Clinical usefulness of PET and SPECT begin to emerge in cerebral vascular disease, in the identification of cerebral death, in epilepsy, in cerebral trauma, in the investigation of HIV-positive patients with cerebral involvement, and in the monitoring of tumor recurrence and postirradiation damage. This review article outlines a current perspective of SPECT and PET as practiced in Europe, its potential, and its limitations.

Effect of intracarotid administration of 6-aminonicotinamide on cerebral blood flow in cats

BACKGROUND AND PURPOSE

We evaluated the effects of an adenosine triphosphate blocker, 6-aminonicotinamide (6-ANA), on the cerebral blood flow (CBF), cerebral metabolism, and electroencephalogram of cats.

METHODS

Catheters were inserted into the common carotid artery of 16 adult cats anesthetized with ketamine via the lingual artery. We measured CBF in the infused area by the inhaled hydrogen gas clearance method and analyzed the electroencephalogram frequency. Cerebral metabolism was estimated by oxygen extraction (vol/%) and glucose utilization (millimoles) using data arterial (aorta) and sagittal sinus blood samplings. A solution of 6-ANA (6.0 mg/mL) (n = 8) or saline (n = 8) was infused via catheter at 2.0 mL/min for 3 minutes followed by a 60-minute observation of CBF, cerebral metabolism, vascular resistance, and the electroencephalogram components, alpha-2 ratio [= alpha-2/(alpha-1+alpha-2)]. The effect of 6-ANA on capillaries was evaluated by extravasation of Evans blue dye and electron microscopic findings.

RESULTS

Moderate reductions in CBF, cerebral metabolism, and the alpha-2 ratio were observed during the infusion of 6-ANA versus saline infusion (P < .05 by paired t test and ANOVA). Vascular resistance was significantly increased (P < .05). No abnormalities were observed in the capillaries of the infused hemisphere.

CONCLUSIONS

Results indicated that 6-ANA produced a downregulation of cerebral blood flow in cats.

Surgically induced angiogenesis to compensate for hemodynamic cerebral ischemia

BACKGROUND AND PURPOSE

The ischemic brain may stimulate angiogenesis to compensate for impaired circulation. We examined the conditions promoting such angiogenesis to provide the basis for surgical treatment.

METHODS

The degree of cerebral hemodynamic stress was studied in patients with moyamoya disease using the stable xenon-enhanced computed tomographic acetazolamide tolerance test and positron emission tomography. Patients were subjected to surgery in which scalp arteries were placed on the cerebral cortex without vessel-to-vessel anastomosis. Formation of the newly vascularized collateral network connecting the implanted artery to cortical arteries was assessed angiographically 12 to 17 months after surgery.

RESULTS

Preoperative average resting cerebral blood flow for cortex that developed revascularization of cortical arteries was not significantly different from that for cortex that did not. However, cortex that developed revascularization had an average preoperative increase of blood flow by acetazolamide treatment of -3.29 +/- 4.6 mL/min per 100 cm3 (n = 20), which was significantly less (P = .0034) than that of cortex that did not show revascularization (20.7 +/- 4.3 mL/min per 100 cm3; n = 9). Good revascularization developed when the cortex showed increase of blood flow by acetazolamide treatment of less than 0 (steal phenomenon). Preoperative positron emission tomography data indicated that revascularization developed when the cortex was under "misery perfusion." Postoperative hemodynamics were ameliorated by revascularization.

CONCLUSIONS

Angiogenesis to connect the implanted scalp arteries to the cerebral cortical arteries was selectively initiated when ischemia of hemodynamic origin existed.