Nuclear medicine in cerebrovascular disease

Macrophage Infiltration Reduces Neurodegeneration and Improves Stroke Recovery after Delayed Recanalization in Rats

Background

Recent cerebrovascular recanalization therapy clinical trials have validated delayed recanalization in patients outside of the conventional window. However, a paucity of information on the pathophysiology of delayed recanalization and favorable outcomes remains. Since macrophages are extensively studied in tissue repair, we anticipate that they may play a critical role in delayed recanalization after ischemic stroke.

Methods

In adult male Sprague-Dawley rats, two ischemic stroke groups were used: permanent middle cerebral artery occlusion (pMCAO) and delayed recanalization at 3 days following middle cerebral artery occlusion (rMCAO). To evaluate outcome, brain morphology, neurological function, macrophage infiltration, angiogenesis, and neurodegeneration were reported. Confirming the role of macrophages, after their depletion, we assessed angiogenesis and neurodegeneration after delayed recanalization.

Results

No significant difference was observed in the rate of hemorrhage or animal mortality among pMCAO and rMCAO groups. Delayed recanalization increased angiogenesis, reduced infarct volumes and neurodegeneration, and improved neurological outcomes compared to nonrecanalized groups. In rMCAO groups, macrophage infiltration contributed to increased angiogenesis, which was characterized by increased vascular endothelial growth factor A and platelet-derived growth factor B. Confirming these links, macrophage depletion reduced angiogenesis, inflammation, neuronal survival in the peri-infarct region, and favorable outcome following delayed recanalization.

Conclusion

If properly selected, delayed recanalization at day 3 postinfarct can significantly improve the neurological outcome after ischemic stroke. The sanguineous exposure of the infarct/peri-infarct to macrophages was essential for favorable outcomes after delayed recanalization at 3 days following ischemic stroke.

Normal cerebral blood vessels under ultrasound in SD rats of different ages

The objective of this study was to examine whether ultrasound can examine the development of cerebral vascular structure and cerebral blood flow in Sprague-Dawley (SD) rats by ultrasound in a noninvasive manner, which provides a reference for ultrasound research of SD rats. Thirty-nine SD rats (7-16 days old) were divided into seven groups according to age, and the number of SD rats in each group was, respectively, 7, 17, 1, 3, 2, 8, and 1. Ultrasound was used to detect cerebral blood vessels, cerebrovascular flow velocity, and heart rate in SD rats in the sagittal and coronal positions, and images were obtained in B-mode ultrasound. The cerebral vascular structure of 39 SD rats (7-16 days) was dynamically observed under B-ultrasound. We found that the cerebral vascular structure of the rats aged 7-10 days was clear and detectable. Rats aged 11-16 days of cerebral vascular structures became thinner and undetectable. Quantitative analysis of cerebrovascular flow rate and heart rate in rats found that there was no significant difference in cerebrovascular blood flow rate and heart rate between 7 and 8 days. Ultrasound can also be used in rat animal studies, that is, the cerebral blood flow in rats of different ages can be monitored in real-time by ultrasound in a noninvasive way.

The shifting role of the cerebellum in executive, emotional and social processing across the lifespan

The cerebellum's anatomical and functional organization and network interactions between the cerebellum and the cerebral cortex and subcortical structures are dynamic across the lifespan. Executive, emotional and social (EES) functions have likewise evolved during human development from contributing to primitive behaviors during infancy and childhood to being able to modulate complex actions in adults. In this review, we address how the importance of the cerebellum in the processing of EES functions might change across development. This evolution is driven by the macroscopic and microscopic modifications of the cerebellum that are occurring during development including its increasing connectivity with distant supra-tentorial cortical and sub-cortical regions. As a result of anatomical and functional changes, neuroimaging and clinical data indicate that the importance of the role of the cerebellum in human EES-related networks shifts from being crucial in newborns and young children to being only supportive later in life. In early life, given the immaturity of cortically mediated EES functions, EES functions and motor control and perception are more closely interrelated. At that time, the cerebellum due to its important role in motor control and sequencing makes EES functions more reliant on these computational properties that compute spatial distance, motor intent, and assist in the execution of sequences of behavior related to their developing EES expression. As the cortical brain matures, EES functions and decisions become less dependent upon these aspects of motor behavior and more dependent upon high-order cognitive and social conceptual processes. At that time, the cerebellum assumes a supportive role in these EES-related behaviors by computing their motor and sequential features. We suspect that this evolving role of the cerebellum has complicated the interpretation of its contribution to EES computational demands.

Changes in synaptic density in the subacute phase after ischemic stroke: A 11C-UCB-J PET/MR study

Functional alterations after ischemic stroke have been described with Magnetic Resonance Imaging (MRI) and perfusion Positron Emission Tomography (PET), but no data on in vivo synaptic changes exist. Recently, imaging of synaptic density became available by targeting synaptic vesicle protein 2 A, a protein ubiquitously expressed in all presynaptic nerve terminals. We hypothesized that in subacute ischemic stroke loss of synaptic density can be evaluated with ¹¹C-UCB-J PET in the ischemic tissue and that alterations in synaptic density can be present in brain regions beyond the ischemic core. We recruited ischemic stroke patients to undergo ¹¹C-UCB-J PET/MR imaging 21 ± 8 days after stroke onset to investigate regional ¹¹C-UCB-J SUVR (standardized uptake value ratio). There was a decrease (but residual signal) of ¹¹C-UCB-J SUVR within the lesion of 16 stroke patients compared to 40 healthy controls (ratio_{lesion/controls} = 0.67 ± 0.28, p = 0.00023). Moreover, ¹¹C-UCB-J SUVR was lower in the non-lesioned tissue of the affected hemisphere compared to the unaffected hemisphere (ΔSUVR = -0.17, p = 0.0035). The contralesional cerebellar hemisphere showed a lower ¹¹C-UCB-J SUVR compared to the ipsilesional cerebellar hemisphere (ΔSUVR = -0.14, p = 0.0048). In 8 out of 16 patients, the asymmetry index suggested crossed cerebellar diaschisis. Future research is required to longitudinally study these changes in synaptic density and their association with outcome.

Neuronal activity reorganization in motor cortex for successful locomotion after a lesion in the ventrolateral thalamus

Thalamic stroke leads to ataxia if the cerebellum-receiving ventrolateral thalamus (VL) is affected. The compensation mechanisms for this deficit are not well understood, particularly the roles that single neurons and specific neuronal subpopulations outside the thalamus play in recovery. The goal of this study was to clarify neuronal mechanisms of the motor cortex involved in mitigation of ataxia during locomotion when part of the VL is inactivated or lesioned. In freely ambulating cats, we recorded the activity of neurons in layer V of the motor cortex as the cats walked on a flat surface and horizontally placed ladder. We first reversibly inactivated ∼10% of the VL unilaterally using glutamatergic transmission antagonist CNQX and analyzed how the activity of motor cortex reorganized to support successful locomotion. We next lesioned 50%-75% of the VL bilaterally using kainic acid and analyzed how the activity of motor cortex reorganized when locomotion recovered. When a small part of the VL was inactivated, the discharge rates of motor cortex neurons decreased, but otherwise the activity was near normal, and the cats walked fairly well. Individual neurons retained their ability to respond to the demand for accuracy during ladder locomotion; however, most changed their response. When the VL was lesioned, the cat walked normally on the flat surface but was ataxic on the ladder for several days after lesion. When ladder locomotion normalized, neuronal discharge rates on the ladder were normal, and the shoulder-related group was preferentially active during the stride's swing phase.NEW & NOTEWORTHY This is the first analysis of reorganization of the activity of single neurons and subpopulations of neurons related to the shoulder, elbow, or wrist, as well as fast- and slow-conducting pyramidal tract neurons in the motor cortex of animals walking before and after inactivation or lesion in the thalamus. The results offer unique insights into the mechanisms of spontaneous recovery after thalamic stroke, potentially providing guidance for new strategies to alleviate locomotor deficits after stroke.

Crossed cerebellar diaschisis on 18F-FDG PET: Frequency across neurodegenerative syndromes and association with 11C-PIB and 18F-Flortaucipir

We used 18F-FDG-PET to investigate the frequency of crossed cerebellar diaschisis (CCD) in 197 patients with various syndromes associated with neurodegenerative diseases. In a subset of 117 patients, we studied relationships between CCD and cortical asymmetry of Alzheimer's pathology (β-amyloid (11C-PIB) and tau (18F-Flortaucipir)). PET images were processed using MRIs to derive parametric SUVR images and define regions of interest. Indices of asymmetry were calculated in the cerebral cortex, basal ganglia and cerebellar cortex. Across all patients, cerebellar 18F-FDG asymmetry was associated with reverse asymmetry of 18F-FDG in the cerebral cortex (especially frontal and parietal areas) and basal ganglia. Based on our operational definition (cerebellar asymmetry >3% with contralateral supratentorial hypometabolism), significant CCD was present in 47/197 (24%) patients and was most frequent in corticobasal syndrome and semantic and logopenic variants of primary progressive aphasia. In β-amyloid-positive patients, mediation analyses showed that 18F-Flortaucipir cortical asymmetry was associated with cerebellar 18F-FDG asymmetry, but that cortical 18F-FDG asymmetry mediated this relationship. Analysis of 18F-FDG-SUVR values suggested that CCD might also occur in the absence of frank cerebellar 18F-FDG asymmetry due to symmetrical supratentorial degeneration resulting in a bilateral diaschisis process.

PET Detection of Cerebral Necrosis Using an Infarct-Avid Agent 2-Deoxy-2-[18F]Fluoro-D-Glucaric Acid (FGA) in a Mouse Model of the Brain Stroke

PURPOSE

Ischemic stroke is a leading cause of disability worldwide. The volume of necrotic core in affected tissue plays a major role in selecting stroke patients for thrombolytic therapy or endovascular thrombectomy. In this study, we investigated a recently reported positron emission tomography (PET) agent 2-deoxy-2-[¹⁸F]fluoro-D-glucaric acid (FGA) to determine necrotic core in a model of transient middle cerebral artery occlusion (t-MCAO) in mice.

PROCEDURES

The radiopharmaceutical, FGA, was synthesized by controlled, rapid, and quantitative oxidation of clinical doses of 2-deoxy-2-[¹⁸F]fluoro-D-glucose (FDG) in a one-step reaction using a premade kit. Brain stroke was induced in the left cerebral hemisphere of CD-1 mice by occluding the middle cerebral artery for 1 h, and then allowing reperfusion by removing the occlusion. One day post-ictus, perfusion single-photon emission tomography (SPECT) was performed with ^99mTc-lableled hexamethylpropyleneamine oxime (HMPAO), followed by PET acquisition with FGA. Plasma and brain tissue homogenates were assayed for markers of inflammation and neurotrophins.

RESULTS

The kit-based synthesis was able to convert up to 2.2 GBq of FDG into FGA within 5 min. PET images showed 375 % more accumulation of FGA in the ipsilateral hemisphere than in the contralateral hemisphere. SPECT images showed that the ipsilateral HMPAO accumulation was reduced to 55 % of normal levels; there was a significant negative correlation between the ipsilateral accumulation of FGA and HMAPO (p < 0.05). FGA accumulation in stroke also correlated with IL-6 levels in the ipsilateral hemisphere. There was no change in IL-6 or TNFα in the plasma of stroke mice.

CONCLUSIONS

Accumulation of FGA correlated well with the perfusion defect and inflammatory injury. As a PET agent, FGA has potential to image infarcted core in the brain stroke injury with high sensitivity, resolution, and specificity.

Brain SPECT as a Biomarker of Neurodegeneration in Dementia in the Era of Molecular Imaging: Still a Valid Option?

Biomarkers are playing a progressively leading role in both clinical practice and scientific research in dementia. Although amyloid and tau biomarkers have gained ground in the clinical community in recent years, neurodegeneration biomarkers continue to play a key role due to their ability to identify different patterns of brain involvement that sign the transition between asymptomatic and symptomatic stages of the disease with high sensitivity and specificity. Both ¹⁸F-FDG positron emission tomography (PET) and perfusion single photon emission computed tomography (SPECT) have proved useful to reveal the functional alterations underlying various neurodegenerative diseases. Although the focus of nuclear neuroimaging has shifted to PET, the lower cost and wider availability of SPECT make it a still valid alternative for the study of patients with dementia. This review discusses the principles of both techniques, compares their diagnostic performance for the diagnosis of neurodegenerative diseases and highlights the role of SPECT to characterize patients from low- and middle-income countries, where special care of additional costs is particularly needed to meet the new recommendations for the diagnosis and characterization of patients with dementia.

A case of crossed cerebellar diaschisis on follow-up positron emission tomography/computed tomography with (18F) fluoro-D-glucose after treatment for glioblastoma

Crossed cerebellar diaschisis (CCD) represents the reduction of blood flow, metabolism, and oxygen consumption in the cerebellar hemisphere contralateral to a cerebral focal lesion. This phenomenon is the result of remote metabolic effects of cerebral lesions and it has been described since the first attempts for functional imaging of the brain, almost 40 years ago. Nevertheless, its clinical significance remains uncertain and new ways to use imaging of CCD for prognosis or assessment of novel therapies are being investigated. In this report, we present treatment for glioblastoma as a cause of CCD imaged on positron emission tomography/computed tomography with (¹⁸F) fluoro-D-glucose in our department.

Single-photon emission tomography

Single-photon emission computed tomography (SPECT) is a functional nuclear imaging technique that allows visualization and quantification of different in vivo physiologic and pathologic features of brain neurobiology. It has been used for many years in diagnosis of several neurologic and psychiatric disorders. In this chapter, we discuss the current state-of-the-art of SPECT imaging of brain perfusion and dopamine transporter (DAT) imaging. Brain perfusion SPECT imaging plays an important role in the localization of the seizure onset zone in patients with refractory epilepsy. In cerebrovascular disease, it can be useful in determining the cerebrovascular reserve. After traumatic brain injury, SPECT has shown perfusion abnormalities despite normal morphology. In the context of organ donation, the diagnosis of brain death can be made with high accuracy. In neurodegeneration, while amyloid or (18)F-fluorodeoxyglucose positron emission tomography (FDG-PET) are the nuclear diagnostic tools of preference for early and differential diagnosis of dementia, perfusion SPECT imaging can be useful, albeit with slightly lower accuracy. SPECT imaging of the dopamine transporter system is widely available in Europe and Asia, but since recently also in the USA, and has been accepted as an important diagnostic tool in the early and differential diagnosis of parkinsonism in patients with unclear clinical features. The combination of perfusion SPECT (or FDG-PET) and DAT imaging provides differential diagnosis between idiopathic Parkinson's disease, Parkinson-plus syndromes, dementia with Lewy bodies, and essential tremor.

Early-Phase 11C-PiB PET in Amyloid Angiopathy-Related Symptomatic Cerebral Hemorrhage: Potential Diagnostic Value?

Although late-phase (>35min post-administration) 11C-PiB-PET has good sensitivity in cerebral amyloid angiopathy (CAA), its specificity is poor due to frequently high uptake in healthy aged subjects. By detecting perfusion-like abnormalities, early-phase 11C-PiB-PET might add diagnostic value. Early-frame (1-6min) 11C-PiB-PET was obtained in 11 non-demented patients with probable CAA-related symptomatic lobar intracerebral haemorrhage (70±7yrs), 9 age-matched healthy controls (HCs) and 10 HCs <55yrs. There was a significant decrease in early-phase atrophy-corrected whole-cortex SUV relative to cerebellar vermis (SUVR) in the CAA vs age-matched HC group. None of the age-matched controls fell below the lower 95% confidence limit derived from the young HCs, while 6/11 CAA patients did (sensitivity = 55%, specificity = 100%). Combining both early- and late-phase 11C-PiB data did not change the sensitivity and specificity of late-phase PiB, but combined early- and late-phase positivity entails a very high suspicion of underlying Aβ-related clinical disorder, i.e., CAA or Alzheimer disease (AD). In order to clarify this ambiguity, we then show that the occipital/posterior cingulate ratio is markedly lower in CAA than in AD (N = 7). These pilot data suggest that early-phase 11C-PiB-PET may not only add to late-phase PiB-PET with respect to the unclear situation of late-phase positivity, but also help differentiate CAA from AD.

Diaschisis: past, present, future

After a century of false hopes, recent studies have placed the concept of diaschisis at the centre of the understanding of brain function. Originally, the term 'diaschisis' was coined by von Monakow in 1914 to describe the neurophysiological changes that occur distant to a focal brain lesion. In the following decades, this concept triggered widespread clinical interest in an attempt to describe symptoms and signs that the lesion could not fully explain. However, the first imaging studies, in the late 1970s, only partially confirmed the clinical significance of diaschisis. Focal cortical areas of diaschisis (i.e. focal diaschisis) contributed to the clinical deficits after subcortical but only rarely after cortical lesions. For this reason, the concept of diaschisis progressively disappeared from the mainstream of research in clinical neurosciences. Recent evidence has unexpectedly revitalized the notion. The development of new imaging techniques allows a better understanding of the complexity of brain organization. It is now possible to reliably investigate a new type of diaschisis defined as the changes of structural and functional connectivity between brain areas distant to the lesion (i.e. connectional diaschisis). As opposed to focal diaschisis, connectional diaschisis, focusing on determined networks, seems to relate more consistently to the clinical findings. This is particularly true after stroke in the motor and attentional networks. Furthermore, normalization of remote connectivity changes in these networks relates to a better recovery. In the future, to investigate the clinical role of diaschisis, a systematic approach has to be considered. First, emerging imaging and electrophysiological techniques should be used to precisely map and selectively model brain lesions in human and animals studies. Second, the concept of diaschisis must be applied to determine the impact of a focal lesion on new representations of the complexity of brain organization. As an example, the evaluation of remote changes in the structure of the connectome has so far mainly been tested by modelization of focal brain lesions. These changes could now be assessed in patients suffering from focal brain lesions (i.e. connectomal diaschisis). Finally, and of major significance, focal and non-focal neurophysiological changes distant to the lesion should be the target of therapeutic strategies. Neuromodulation using transcranial magnetic stimulation is one of the most promising techniques. It is when this last step will be successful that the concept of diaschisis will gain all the clinical respectability that could not be obtained in decades of research.

Parametric Cerebrovascular Reserve Images Using Acetazolamide (99m)Tc-HMPAO SPECT: A Feasibility Study of Quantitative Assessment

PURPOSE

Basal/acetazolamide stress (99m)Tc-HMPAO single-photon emission computed tomography (SPECT) has been widely used for evaluation of hemodynamics; however, qualitative and subjective visual assessment of cerebrovascular reserve (CVR) has been performed in clinical settings. The aim of this study was to generate parametric CVR images and evaluate its feasibility of quantification.

METHODS

Basal/acetazolamide stress (99m)Tc-HMPAO SPECT data from 17 patients who underwent bypass surgery or percutaneous transluminal angioplasty were used. Spatial normalization was performed and parametric CVR images were generated using relative CVR (rCVR) of each voxel proportional to CVR of the whole brain. Binary parametric maps to show area of relatively reduced CVR were generated also using threshold of rCVR < 90 %. We calculated rCVR of internal carotid artery (ICA) using the parametric CVR images and probabilistic maps for ICA territory. Pre- and postprocedural parametric CVR images were obtained and quantitative rCVRs were compared. The rCVRs were evaluated according to visual grades for regional decreased CVR.

RESULTS

Postprocedural rCVR obtained from parametric CVR images increased significantly from preprocedural rCVR. The rCVR was significantly correlated with visual grades of reduced CVR for each side of ICA territories.

CONCLUSIONS

We generated parametric CVR images for basal/acetazolamide stress (99m)Tc-HMPAO SPECT. As a quantitative measurement, rCVR obtained from the parametric image was feasibly assessed hemodynamic abnormalities with preserved anatomical information.