High resolution positron emission tomography demonstrates basal ganglia dysfunction in early Parkinson's disease

The Contribution of Type 2 Diabetes to Parkinson's Disease Aetiology

Type 2 diabetes (T2D) and Parkinson's disease (PD) are chronic disorders that have a significant health impact on a global scale. Epidemiological, preclinical, and clinical research underpins the assumption that insulin resistance and chronic inflammation contribute to the overlapping aetiologies of T2D and PD. This narrative review summarises the recent evidence on the contribution of T2D to the initiation and progression of PD brain pathology. It also briefly discusses the rationale and potential of alternative pharmacological interventions for PD treatment.

Glucose Metabolism Impairment in Parkinson's Disease

Impairments in systematic and regional glucose metabolism exist in patients with Parkinson's disease (PD) at every stage of the disease course, and such impairments are associated with the incidence, progression, and special phenotypes of PD, which affect each physiological process of glucose metabolism including glucose uptake, glycolysis, tricarboxylic acid cycle, oxidative phosphorylation, and pentose phosphate shunt pathway. These impairments may be attributed to various mechanisms, such as insulin resistance, oxidative stress, abnormal glycated modification, blood-brain-barrier dysfunction, and hyperglycemia-induced damages. These mechanisms could subsequently cause excessive methylglyoxal and reactive oxygen species production, neuroinflammation, abnormal aggregation of protein, mitochondrial dysfunction, and decreased dopamine, and finally result in energy supply insufficiency, neurotransmitter dysregulation, aggregation and phosphorylation of α-synuclein, and dopaminergic neuron loss. This review discusses the glucose metabolism impairment in PD and its pathophysiological mechanisms, and briefly summarized the currently-available therapies targeting glucose metabolism impairment in PD, including glucagon-likepeptide-1 (GLP-1) receptor agonists and dual GLP-1/gastric inhibitory peptide receptor agonists, metformin, and thiazoledinediones.

Small nuclei identification with a hemispherical brain PET

BACKGROUND

To confirm the performance of the first hemispherical positron emission tomography (PET) for the brain (Vrain) that we developed to visualise the small nuclei in the deep brain area, we compared ¹⁸F-fluorodeoxyglucose (FDG) brain images with whole-body PET images.

METHODS

Ten healthy male volunteers (aged 22-45 years) underwent a representative clinical whole-body PET, followed by Vrain each for 10 min. These two scans were initiated 30 min and 45 min after FDG injection (4.1 ± 0.5 MBq/kg), respectively. First, we visually identified the small nuclei and then compared their standardised uptake values (SUVs) with the participants' age. Next, the SUVs of each brain region, which were determined by applying a volume-of-interest template for anatomically normalised PET images, were compared between the brain images with the Vrain and those with the whole-body PET images.

RESULTS

Small nuclei, such as the inferior colliculus, red nucleus, and substantia nigra, were more clearly visualised in Vrain than in whole-body PET. The anterior nucleus and dorsomedial nucleus in the thalamus and raphe nucleus in the brainstem were identified in Vrain but not in whole-body PET. The SUVs of the inferior colliculus and dentate gyrus in the cerebellum positively correlated with age (Spearman's correlation coefficient r = 0.811, p = 0.004; r = 0.738, p = 0.015, respectively). The SUVs of Vrain were slightly higher in the mesial temporal and medial parietal lobes than those in whole-body PET.

CONCLUSIONS

This was the first time that the raphe nuclei, anterior nuclei, and dorsomedial nuclei were successfully visualised using the first hemispherical brain PET. TRIAL REGISTRATION  : Japan Registry of Clinical Trials, jRCTs032210086, Registered 13 May 2021, https://jrct.niph.go.jp/latest-detail/jRCTs032210086 .

Validation technique and improvements introduced in a new dedicated brain positron emission tomograph (CareMiBrain)

The goal of developing a PET dedicated to the brain (CareMiBrain) has evolved from its initial approach to diagnosis and monitoring of dementias, to the more ambitious of creating a revolutionary clinical pathway for the knowledge and personalized treatment of multiple neurological diseases. The main innovative feature of CareMiBrain is the use of detectors with continuous crystals, which allow a high resolution determination of the depth of annihilation photons interaction within the thickness of the scintillation crystal. The technical validation phase of the equipment consisted of a pilot, prospective and observational study whose objective was to obtain the first images (40 patients), analyze them and make adjustments in the acquisition, reconstruction and correction parameters, comparing the image quality of the CareMiBrain equipment with that of the whole-body PET-CT. Thanks to the team meetings and the joint analysis of the images, it was possible to detect its weak points and some of its causes. The calibration, acquisition and processing processes, as well as the reconstruction, were optimized, the number of iterations was set to achieve the best signal-to-noise ratio, the random correction was optimized and a post-processing algorithm was included in the reconstruction algorithm. The main technical improvements implemented in this phase of technical validation carried out through collaboration of the Services of Nuclear Medicine and Neurology of the Hospital Clínico San Carlos with the Spanish company Oncovision will be exposed in a project financed with funds from the European Union (Horizon 2020 innovation program, 713323).

Validation technique and improvements introduced in a new dedicated brain positron emission tomograph (CareMiBrain)

The goal of developing a PET dedicated to the brain (CareMiBrain) has evolved from its initial approach to diagnosis and monitoring of dementias, to the more ambitious of creating a revolutionary clinical pathway for the knowledge and personalized treatment of multiple neurological diseases. The main innovative feature of CareMiBrain is the use of detectors with continuous crystals, which allow a high resolution determination of the depth of annihilation photons interaction within the thickness of the scintillation crystal. The technical validation phase of the equipment consisted of a pilot, prospective and observational study whose objective was to obtain the first images (40 patients), analyze them and make adjustments in the acquisition, reconstruction and correction parameters, comparing the image quality of the CareMiBrain equipment with that of the whole-body PET/CT. Thanks to the team meetings and the joint analysis of the images, it was possible to detect its weak points and some of its causes. The calibration, acquisition and processing processes, as well as the reconstruction, were optimized, the number of iterations was set to achieve the best signal-to-noise ratio, the random correction was optimized and a post-processing algorithm was included in the reconstruction algorithm. The main technical improvements implemented in this phase of technical validation carried out through collaboration of the Services of Nuclear Medicine and Neurology of the Hospital Clínico San Carlos with the Spanish company Oncovision will be exposed in a project financed with funds from the European Union (Horizon 2020 innovation program, 713323).

Network degeneration in Parkinson’s disease: multimodal imaging of nigro-striato-cortical dysfunction

The spreading hypothesis of neurodegeneration assumes an expansion of neural pathologies along existing neural pathways. Multimodal neuroimaging studies have demonstrated distinct topographic patterns of cerebral pathologies in neurodegeneration. For Parkinson’s disease the hypothesis so far rests largely on histopathological evidence of α-synuclein spreading in a characteristic pattern and progressive nigrostriatal dopamine depletion. Functional consequences of nigrostriatal dysfunction on cortical activity remain to be elucidated. Our goal was to investigate multimodal imaging correlates of degenerative processes in Parkinson’s disease by assessing dopamine depletion and its potential effect on striatocortical connectivity networks and cortical metabolism in relation to parkinsonian symptoms. We combined 18F-DOPA-PET, 18F-fluorodeoxyglucose (FDG)-PET and resting state functional MRI to multimodally characterize network alterations in Parkinson’s disease. Forty-two patients with mild-to-moderate stage Parkinson’s disease and 14 age-matched healthy control subjects underwent a multimodal imaging protocol and comprehensive clinical examination. A voxel-wise group comparison of 18F-DOPA uptake identified the exact location and extent of putaminal dopamine depletion in patients. Resulting clusters were defined as seeds for a seed-to-voxel functional connectivity analysis. 18F-FDG metabolism was compared between groups at a whole-brain level and uptake values were extracted from regions with reduced putaminal connectivity. To unravel associations between dopaminergic activity, striatocortical connectivity, glucose metabolism and symptom severity, correlations between normalized uptake values, seed-to-cluster β-values and clinical parameters were tested while controlling for age and dopaminergic medication. Aside from cortical hypometabolism, 18F-FDG-PET data for the first time revealed a hypometabolic midbrain cluster in patients with Parkinson’s disease that comprised caudal parts of the bilateral substantia nigra pars compacta. Putaminal dopamine synthesis capacity was significantly reduced in the bilateral posterior putamen and correlated with ipsilateral nigral 18F-FDG uptake. Resting state functional MRI data indicated significantly reduced functional connectivity between the dopamine depleted putaminal seed and cortical areas primarily belonging to the sensorimotor network in patients with Parkinson’s disease. In the inferior parietal cortex, hypoconnectivity in patients was significantly correlated with lower metabolism (left P = 0.021, right P = 0.018). Of note, unilateral network alterations quantified with different modalities corresponded with contralateral motor impairments. In conclusion, our results support the hypothesis that degeneration of nigrostriatal fibres functionally impairs distinct striatocortical connections, disturbing the efficient interplay between motor processing areas and impairing motor control in patients with Parkinson’s disease. The present study is the first to reveal trimodal evidence for network-dependent degeneration in Parkinson’s disease by outlining the impact of functional nigrostriatal pathway impairment on striatocortical functional connectivity networks and cortical metabolism.

Overlapping and distinct neural metabolic patterns related to impulsivity and hypomania in Parkinson's disease

Impulsivity and hypomania are common non-motor features in Parkinson's disease (PD). The aim of this study was to find the overlapping and distinct neural correlates of these symptoms in PD. Symptoms of impulsivity and hypomania were assessed in 24 PD patients using the Barratt Impulsiveness Scale (BIS-11) and Self-Report Manic Inventory (SRMI), respectively. In addition, fluorodeoxyglucose positron emission tomography (FDG-PET) imaging for each individual was performed. We conducted two separate multiple regression analyses for BIS-11 and SRMI scores with FDG-PET data to identify the brain regions that are associated with both impulsivity and hypomania scores, as well as those exclusive to each symptom. Then, seed-based functional connectivity analyses on healthy subjects identified the areas connected to each of the exclusive regions and the overlapping region, used as seeds. We observed a positive association between BIS-11 and SRMI scores and neural metabolism only in the prefrontal areas. Conjunction analysis revealed an overlapping region in the middle frontal gyrus. Regions exclusive to impulsivity were found in the medial part of the right superior frontal gyrus and regions exclusive to hypomania were in the right superior frontal gyrus, right precentral gyrus and right paracentral lobule. Connectivity patterns of seeds exclusively related to impulsivity were different from those for hypomania in healthy brains. These results provide evidence of both overlapping and distinct regions linked with impulsivity and hypomania scores in PD. The exclusive regions for each characteristic are connected to specific intrinsic functional networks.

Development of Dedicated Brain PET Imaging Devices: Recent Advances and Future Perspectives

Whole-body PET scanners are not optimized for imaging small structures in the human brain. Several PET devices specifically designed for this task have been proposed either for stand-alone operation or as MR-compatible inserts. The main distinctive features of some of the most recent concepts and their performance characteristics, with a focus on spatial resolution and sensitivity, are reviewed. The trade-offs between the various performance characteristics, desired capabilities, and cost that need to be considered when designing a dedicated brain scanner are presented. Finally, the aspirational goals for future-generation scanners, some of the factors that have contributed to the current status, and how recent advances may affect future developments in dedicated brain PET instrumentation are briefly discussed.

Hybrid PET-MRI Applications in Movement Disorders

Even before the success of combined positron emission tomography and computed tomography (PET/CT), the neuroimaging community was conceiving the idea to integrate the positron emission tomography (PET), with very high molecular quantitative data but low spatial resolution, and magnetic resonance imaging (MRI), with high spatial resolution. Several technical limitations have delayed the use of a hybrid scanner in neuroimaging studies, including the full integration of the PET detector ring within the MRI system, the optimization of data acquisition, and the implementation of reliable methods for PET attenuation, motion correction, and joint image reconstruction. To be valid and useful in clinical and research settings, this instrument should be able to simultaneously acquire PET and MRI, and generate quantitative parametric PET images comparable to PET-CT. While post hoc co-registration of combined PET and MRI data acquired separately became the most reliable technique for the generation of "fused" PET-MRI images, only hybrid PET-MRI approach allows merging these measurements naturally and correlating them in a temporal manner. Furthermore, hybrid PET-MRI represents the most accurate tool to investigate in vivo the interplay between molecular and functional aspects of brain pathophysiology. Hybrid PET-MRI technology is still in the early stages in the movement disorders field, due to the limited availability of scanners with integrated optimized methodological models. This technology is ideally suited to investigate interactions between resting-state functional/arterial spin labeling MRI and [¹⁸F]FDG PET glucose metabolism in the evaluation of the brain "hubs" particularly vulnerable to neurodegeneration, areas with a high degree of connectivity and associated with an efficient synaptic neurotransmission. In Parkinson's disease, hybrid PET-MRI is also the ideal instrument to deeper explore the relationship between resting-state functional MRI and dopamine release at [¹¹C]raclopride PET challenge, in the identification of early drug-naïve Parkinson's disease patients at higher risk of motor complications and in the evaluation of the efficacy of novel neuroprotective treatment able to restore at the same time the altered resting state and the release of dopamine. In this chapter, we discuss the key methodological aspects of hybrid PET-MRI; the evidence in movement disorders of the key resting-state functional and perfusion MRI; [¹⁸F]FDG PET and [¹¹C]raclopride PET challenge studies; the potential advantages of using hybrid PET-MRI to investigate the pathophysiology of movement disorders and neurodegenerative diseases. Future directions of hybrid PET-MRI will be discussed alongside with up-to-date technological innovations on hybrid systems.

Functional neuroanatomical associations of working memory in early-onset Alzheimer's disease

OBJECTIVE

To characterize metabolic correlates of working memory impairment in clinically defined subtypes of early-onset Alzheimer's disease.

BACKGROUND

Established models of working memory suggest a key role for frontal lobe function, yet the association in Alzheimer's disease between working memory impairment and visuospatial and language symptoms suggests that temporoparietal neocortical dysfunction may be responsible.

METHODS

Twenty-four patients with predominantly early-onset Alzheimer's disease were clinically classified into groups with predominantly amnestic, multidomain or visual deficits. Patients underwent neuropsychological evaluation focused on the domains of episodic and working memory, T1-weighted magnetic resonance imaging and brain fluorodeoxyglucose positron emission tomography. Fluorodeoxyglucose positron emission tomography data were analysed by using a region-of-interest approach.

RESULTS

Patients with multidomain and visual presentations performed more poorly on tests of working memory compared with amnestic Alzheimer's disease. Working memory performance correlated with glucose metabolism in left-sided temporoparietal, but not frontal neocortex. Carriers of the apolipoprotein E4 gene showed poorer episodic memory and better working memory performance compared with noncarriers.

CONCLUSIONS

Our findings support the hypothesis that working memory changes in early-onset Alzheimer's disease are related to temporoparietal rather than frontal hypometabolism and show dissociation from episodic memory performance. They further support the concept of subtypes of Alzheimer's disease with distinct cognitive profiles due to prominent neocortical dysfunction early in the disease course. Copyright © 2017 John Wiley & Sons, Ltd.

Reproducibility of a Parkinsonism-related metabolic brain network in non-human primates: A descriptive pilot study with FDG PET

BACKGROUND

We have previously defined a parkinsonism-related metabolic brain network in rhesus macaques using a high-resolution research positron emission tomography camera. This brief article reports a descriptive pilot study to assess the reproducibility of network activity and regional glucose metabolism in independent parkinsonian macaques using a clinical positron emission tomography/CT camera.

METHODS

[(18)F]fluorodeoxyglucose PET scans were acquired longitudinally over 3 months in three drug-naïve parkinsonian and three healthy control cynomolgus macaques. Group difference and test-retest stability in network activity and regional glucose metabolism were evaluated graphically, using all brain images from these macaques.

RESULTS

Comparing the parkinsonian macaques with the controls, network activity was elevated and remained stable over 3 months. Normalized glucose metabolism increased in putamen/globus pallidus and sensorimotor regions but decreased in posterior parietal cortices.

CONCLUSIONS

Parkinsonism-related network activity can be reliably quantified in different macaques with a clinical positron emission tomography/CT scanner and is reproducible over a period typically employed in preclinical intervention studies. This measure can be a useful biomarker of disease process or drug effects in primate models of Parkinson's disease.

PET imaging in neurology: Alzheimer's and Parkinson's diseases

PET studies play an important role in the early detection of Alzheimer's and Parkinson's diseases (AD and PD). Fluorine-18 fluorodeoxyglucose (F-FDG) PET imaging of regional cerebral glucose metabolism and PET amyloid imaging are the two major PET studies for AD. F-FDG PET is highly sensitive for the early diagnosis of AD, in predicting conversion from mild cognitive impairment to AD, and in differentiating AD from other dementias. PET amyloid imaging is positive in the majority of patients with AD. Negative amyloid PET reduces the likelihood of AD. The main limitations of PET amyloid imaging is its high positivity in the normal elderly population and in other medical conditions with amyloid pathologies. Various PET tracers are available to assess motor and cognitive dysfunctions in PD. PET tracers targeting presynaptic dopaminergic function (F-DOPA, radiolabeled PET tracers assessing the availability of dopamine transporters and vesicular monoamine transporters) and postsynaptic dopamine receptors are used to assess motor dysfunction in PD. PET tracers, particularly dopamine transporters, are highly sensitive in the early diagnosis of PD. Uptake of PET tracers in the striatum is inversely correlated with disease severity. PET is valuable in differentiating PD from other movement disorders. PET studies, particularly F-FDG PET, help to evaluate cortical metabolism in PD patients with cognitive dysfunction and dementia.

Impulsivity is Associated with Increased Metabolism in the Fronto-Insular Network in Parkinson's Disease

Various neuroimaging studies demonstrated that the fronto-insular network is implicated in impulsive behavior. We compared glucose metabolism (as a proxy measure of neural activity) among 24 patients with Parkinson’s disease (PD) who presented with low or high levels of impulsivity based on the Barratt Impulsiveness Scale 11 (BIS) scores. Subjects underwent 18-fluorodeoxyglucose positron emission tomography (FDG-PET) and the voxel-wise group difference of FDG-metabolism was analyzed in Statistical Parametric Mapping (SPM8). Subsequently, we performed a partial correlation analysis between the FDG-metabolism and BIS scores, controlling for covariates (i.e., age, sex, severity of disease and levodopa equivalent daily doses). Voxel-wise group comparison revealed higher FDG-metabolism in the orbitofrontal cortex (OFC), anterior cingulate cortex (ACC), and right insula in patients with higher impulsivity scores. Moreover, there was a positive correlation between the FDG-metabolism and BIS scores. Our findings provide evidence that high impulsivity is associated with increased FDG-metabolism within the fronto-insular network in PD.

Dopaminergic correlates of metabolic network activity in Parkinson's disease

Parkinson's disease (PD) is associated with distinct metabolic covariance patterns that relate to the motor and cognitive manifestations of the disorder. It is not known, however, how the expression of these patterns relates to measurements of nigrostriatal dopaminergic activity from the same individuals. To explore these associations, we studied 106 PD subjects who underwent cerebral PET with both (18) F-fluorodeoxyglucose (FDG) and (18) F-fluoro-L-dopa (FDOPA). Expression values for the PD motor- and cognition-related metabolic patterns (PDRP and PDCP, respectively) were computed for each subject; these measures were correlated with FDOPA uptake on a voxel-by-voxel basis. To explore the relationship between dopaminergic function and local metabolic activity, caudate and putamen FDOPA PET signal was correlated voxel-wise with FDG uptake over the entire brain. PDRP expression correlated with FDOPA uptake in caudate and putamen (P < 0.001), while PDCP expression correlated with uptake in the anterior striatum (P < 0.001). While statistically significant, the correlations were only of modest size, accounting for less than 20% of the overall variation in these measures. After controlling for PDCP expression, PDRP correlations were significant only in the posterior putamen. Of note, voxel-wise correlations between caudate/putamen FDOPA uptake and whole-brain FDG uptake were significant almost exclusively in PDRP regions. Overall, the data indicate that PDRP and PDCP expression correlates significantly with PET indices of presynaptic dopaminergic functioning obtained in the same individuals. Even so, the modest size of these correlations suggests that in PD patients, individual differences in network activity cannot be explained solely by nigrostriatal dopamine loss.

Utility of magnetic resonance spectroscopic imaging for human epilepsy

This review discusses the potential utility of broad based use of magnetic resonance (MR) spectroscopic imaging for human epilepsy and seizure localization. The clinical challenges are well known to the epilepsy community, intrinsic in the variability of location, volumetric size and network extent of epileptogenic tissue in individual patients. The technical challenges are also evident, with high performance requirements in multiple steps, including magnet homogeneity, detector performance, sequence design, speed of acquisition in addition to large territory spectral processing. We consider how MR spectroscopy and spectroscopic imaging has been informative for epilepsy thus far, with specific attention to what is measured, the interpretation of such measurements and technical performance challenges. Examples are shown from medial temporal lobe and neocortical epilepsies are considered from 4T, 7T and most recently 3T.

Dopamine transporter single-photon emission computerized tomography supports diagnosis of akinetic crisis of parkinsonism and of neuroleptic malignant syndrome

Akinetic crisis (AC) is akin to neuroleptic malignant syndrome (NMS) and is the most severe and possibly lethal complication of parkinsonism. Diagnosis is today based only on clinical assessments yet is often marred by concomitant precipitating factors. Our purpose is to evidence that AC and NMS can be reliably evidenced by FP/CIT single-photon emission computerized tomography (SPECT) performed during the crisis. Prospective cohort evaluation in 6 patients. In 5 patients, affected by Parkinson disease or Lewy body dementia, the crisis was categorized as AC. One was diagnosed as having NMS because of exposure to risperidone. In all FP/CIT, SPECT was performed in the acute phase. SPECT was repeated 3 to 6 months after the acute event in 5 patients. Visual assessments and semiquantitative evaluations of binding potentials (BPs) were used. To exclude the interference of emergency treatments, FP/CIT BP was also evaluated in 4 patients currently treated with apomorphine. During AC or NMS, BP values in caudate and putamen were reduced by 95% to 80%, to noise level with a nearly complete loss of striatum dopamine transporter-binding, corresponding to the "burst striatum" pattern. The follow-up re-evaluation in surviving patients showed a recovery of values to the range expected for Parkinsonisms of same disease duration. No binding effects of apomorphine were observed. By showing the outstanding binding reduction, presynaptic dopamine transporter ligand can provide instrumental evidence of AC in Parkinsonism and NMS.

Brain 18F-FDG PET imaging in the differential diagnosis of parkinsonism

AIM

The aims in this study were to evaluate the role of brain F-FDG PET imaging in differential diagnosis of parkinsonism and to correlate brain metabolism findings with patients' clinical findings.

METHODS

Brain F-FDG PET images were evaluated both visually and quantitatively using the NeuroQ software in 21 parkinsonism patients in whom final clinical diagnoses were established.

RESULTS

Final clinical diagnoses were idiopathic Parkinson disease in 7, multisystem atrophy (MSA) in 7, progressive supranuclear palsy (PSP) in 4, corticobasal degeneration in 2, and Lewy body disease in 1 patient. Asymmetrical cortical hypometabolism was observed in most of the patients in frontal and parietotemporal regions. Fifteen of 21 patients had basal ganglia involvement, which was bilateral in patients with MSA and more frequently unilateral in patients with idiopathic Parkinson disease and PSP. Four patients with PSP and 1 patient with corticobasal degeneration had thalamic hypometabolism. Cerebellar hypometabolism was observed in 4 patients with MSA. The Unified Parkinson Disease Rating Scale motor and bradykinesia scores were higher in patients with basal ganglia involvement.

CONCLUSIONS

Brain F-FDG PET findings in subcortical nuclei and cerebellum were found to be useful in differential diagnosis of patients with parkinsonism. The extent of cerebral cortical and basal ganglia hypometabolism showed correlation with the presentation and severity of clinical findings.

Parkinson's disease subtypes show a specific link between dopaminergic and glucose metabolism in the striatum

BACKGROUND

Previous studies have shown different clinical and imaging pattern in tremordominant and akinetic-rigid Parkinson's disease (PD) subtypes. The association between dopaminergic and glucose metabolism has in contrast not been investigated yet. Therefore, this study compared PD subtypes with respect to clinical and imaging findings with the aim of establishing a relationship between clinical subtypes, dopamine and glucose metabolism.

METHODS

Two groups of a total of 64 idiopathic PD patients (42 male, 22 female, mean age 56 ± 10.9 years) were analysed: akinetic-rigid (AR, n = 32) and tremor-dominant (TD, n = 32) patients. Both were compared with respect to differential involvement of local striatal dopamine and glucose metabolism using [18F]-fluoro-L-dopa (F-dopa) and [18F]-fluorodeoxyglucose (FDG)-PET.

RESULTS

The analysis of PD subgroups showed significant differences in the F-dopa uptake in the anterior putamen. Using the results of the local striatal dopamine difference as a volume of interest for the FDG-analysis, analysis of AR patients revealed a significantly lower normalised cerebral metabolic rate of glucose (nCMRGlc) within the ventral striatum.

CONCLUSIONS

The dual tracer study illlustrates clear differences between TD and AR subtypes in the ventral striatum. In accordance with previous FP-CIT-SPECT studies, it discloses congruent results for the presynaptic dopaminergic system and extends the knowledge about an additional involvement of local metabolic activity in the caudate and anterior putamen. The findings corroborate the specific role of distinct PD subtypes within the cerebello-thalamo-cortical-circuits. Multitracer PET imaging may thus enhance the knowledge about the clinical segregation into subtypes.

The value of putaminal diffusion imaging versus 18-fluorodeoxyglucose positron emission tomography for the differential diagnosis of the Parkinson variant of multiple system atrophy

Differentiating the Parkinson variant of multiple system atrophy (MSA-P) from idiopathic Parkinson's disease (PD) and other forms of atypical parkinsonism can be difficult because symptoms overlap considerably. 18-Fluorodeoxyglucose positron emission tomography (FDG-PET) is a powerful imaging technique that can assist in the diagnosis of MSA-P via detection of putaminal and cerebellar hypometabolism. Recent studies suggest that diffusion-weighted imaging (DWI) might be of similar diagnostic value, as it can detect microstructural damage in the putamen by means of an increased mean diffusivity (MD). The aim of this study was a direct comparison of DWI and FDG-PET by using both methods on the same subject cohort. To this end, combined DWI and FDG-PET were employed in patients with MSA-P (n = 11), PD (n = 13), progressive supranuclear palsy (n = 8), and in 6 control subjects. MD values and FDG uptake ratios were derived from volumetric parcellations of the putamen and subjected to further analysis of covariance (ANCOVA) and receiver operating characteristics analyses. MSA-P was found to be associated with an increased posterior putaminal MD (P < 0.001 in all subgroup comparisons) that correlated strongly with local reductions in FDG uptake (r = -0.85, P = 0.002). DWI discriminated patients with MSA-P from other subgroups nearly as accurately as FDG-PET (area under the curve = 0.89 vs 0.95, P = 0.27 [pooled data]). Our data suggest a close association between the amount of putaminal microstructural damage and a reduced energy metabolism in patients with MSA-P. The clinical use of DWI for the differential diagnosis of MSA-P is encouraged.

Abnormal network topographies and changes in global activity: absence of a causal relationship

Changes in regional brain activity can be observed following global normalization procedures to reduce variability in the data. In particular, spurious regional differences may appear when scans from patients with low global activity are compared to those from healthy subjects. It has thus been suggested that the consistent increases in subcortical activity that characterize the abnormal Parkinson's disease-related metabolic covariance pattern (PDRP) are artifacts of global normalization, and that similar topographies can be identified in scans from healthy subjects with varying global activity. To address this issue, we examined the effects of experimental reductions in global metabolic activity on PDRP expression. Ten healthy subjects underwent ¹⁸F-fluorodeoxyglucose PET in wakefulness and following sleep induction. In all subjects, the global metabolic rate (GMR) declined with sleep (mean -34%, range: -17 to -56%), exceeding the test-retest differences of the measure (p<0.001). By contrast, sleep-wake differences in PDRP expression did not differ from test-retest differences, and did not correlate (R²=0.04) with concurrent declines in global metabolic activity. Indeed, despite significant GMR reductions in sleep, PDRP values remained within the normal range. Likewise, voxel weights on the principal component patterns resulting from combined analysis of the sleep and wake scans did not correlate (R²<0.07) with the corresponding regional loadings on the PDRP topography. In aggregate, the data demonstrate that abnormal PDRP expression is not induced by reductions in global activity. Moreover, significant declines in GMR are not associated with the appearance of PDRP-like spatial topographies.

Glucose metabolism in small subcortical structures in Parkinson's disease

OBJECTIVES

Evidence from experimental animal models of Parkinson's disease (PD) suggests a characteristic pattern of metabolic perturbation in discrete, very small basal ganglia structures. These structures are generally too small to allow valid investigation by conventional positron emission tomography (PET) cameras. However, the high-resolution research tomograph (HRRT) PET system has a resolution of 2 mm, sufficient for the investigation of important structures such as the pallidum and thalamic subnuclei.

MATERIALS AND METHODS

Using the HRRT, we performed [(18)F]-fluorodeoxyglucose (FDG) scans on 21 patients with PD and 11 age-matched controls. We employed three types of normalization: white matter, global mean, and data-driven normalization. We performed volume-of-interest analyses of small subcortical gray matter structures. Voxel-based comparisons were performed to investigate the extent of cortical hypometabolism.

RESULTS

The most significant level of relative subcortical hypermetabolism was detected in the external pallidum (GPe), irrespective of normalization strategy. Hypermetabolism was suggested also in the internal pallidum, thalamic subnuclei, and the putamen. Widespread cortical hypometabolism was seen in a pattern very similar to previously reported patterns in patients with PD.

CONCLUSION

The presence and extent of subcortical hypermetabolism in PD is dependent on type of normalization. However, the present findings suggest that PD, in addition to widespread cortical hypometabolism, is probably characterized by true hypermetabolism in the GPe. This finding was predicted by the animal 2-deoxyglucose autoradiography literature, in which high-magnitude hypermetabolism was also most robustly detected in the GPe.

Cerebral oxygen metabolism in patients with early Parkinson's disease

AIM

Decreased activity of the mitochondrial electron transport chain (ETC) has been implicated in the pathogenesis of Parkinson's disease (PD). This model would most likely predict a decrease in the rate of cerebral oxygen consumption (CMRO(2)). To test this hypothesis, we compared CMRO(2) and cerebral blood flow (CBF) PET scans from PD patients and healthy controls.

MATERIALS AND METHODS

Nine early-stage PD patients and 15 healthy age-matched controls underwent PET scans for quantitative mapping of CMRO(2) and CBF. Between-group differences were evaluated for absolute data and intensity-normalized values.

RESULTS

No group differences were detected in regional magnitudes of CMRO(2) or CBF. Upon normalization using the reference cluster method, significant relative CMRO(2) decreases were evident in widespread prefrontal, parieto-occipital, and lateral temporal regions. Sensory-motor and subcortical regions, brainstem, and the cerebellum were spared. A similar pattern was evident in normalized CBF data, as described previously.

CONCLUSION

While the data did not reveal substantially altered absolute CMRO(2) in brain of PD patients, employing data-driven intensity normalization revealed widespread relative CMRO(2) decreases in cerebral cortex. The detected pattern was very similar to that reported in earlier CBF and CMRglc studies of PD, and in the CBF images from the same subjects. Thus, the present results are consistent with the occurrence of parallel declines in CMRO(2), CBF, and CMRglc in spatially contiguous cortical regions in early PD, and support the hypothesis that ETC dysfunction could be a primary pathogenic mechanism in early PD.

The human nucleus accumbens suffers parkinsonism-related shrinkage: a novel finding

PURPOSE

The human nucleus accumbens (NA) plays an important role in motivation and emotional processes and is involved in some of the most disabling neuropsychiatric disorders such as Parkinson's disease (PD). The purpose of our study was to check out the potential existence of a statistically significant difference in NA size between parkinsonian and non-parkinsonian individuals, through studying brain magnetic resonance images (MRIs).

METHODS

For our study we used 52 NAs from 26 cerebral MRIs from neurosurgical patients. Of these MRIs, 15 were preoperative from patients with advanced PD who underwent bilateral deep brain stimulation of the subthalamic nucleus. The group of non-parkinsonian MRIs came from the rest 11 individuals. We measured the absolute and relative NA maximum transverse diameter (D (max)), and absolute and relative NA width at a specific transverse plane.

RESULTS

We found a statistically significant difference of the mean value of the D (max) (absolute and relative) between the two groups. The mean percentage reduction of the NA size was 11.77% represented by the relative D (max).

CONCLUSIONS

To our knowledge, this is the first report of parkinsonism-related shrinkage of the human NA. Further research is needed to identify whether a respective shrinkage is also observed in patients with early PD and whether this atrophy is correlated with dopaminergic neuropsychiatric symptoms (perhaps mediated by a malfunctioning NA) that occur in PD.

Typical cerebral metabolic patterns in neurodegenerative brain diseases

The differential diagnosis of neurodegenerative brain diseases on clinical grounds is difficult, especially at an early disease stage. Several studies have found specific regional differences of brain metabolism applying [(18)F]-fluoro-deoxyglucose positron emission tomography (FDG-PET), suggesting that this method can assist in early differential diagnosis of neurodegenerative brain diseases.We have studied patients who had an FDG-PET scan on clinical grounds at an early disease stage and included those with a retrospectively confirmed diagnosis according to strictly defined clinical research criteria. Ninety-six patients could be included of which 20 patients with Parkinson's disease (PD), 21 multiple system atrophy (MSA), 17 progressive supranuclear palsy (PSP), 10 corticobasal degeneration (CBD), 6 dementia with Lewy bodies (DLB), 15 Alzheimer's disease (AD), and 7 frontotemporal dementia (FTD). FDG PET images of each patient group were analyzed and compared to18 healthy controls using Statistical Parametric Mapping (SPM5).Disease-specific patterns of relatively decreased metabolic activity were found in PD (contralateral parietooccipital and frontal regions), MSA (bilateral putamen and cerebellar hemispheres), PSP (prefrontal cortex and caudate nucleus, thalamus, and mesencephalon), CBD (contralateral cortical regions), DLB (occipital and parietotemporal regions), AD (parietotemporal regions), and FTD (frontotemporal regions).The integrated method addressing a spectrum of various neurodegenerative brain diseases provided means to discriminate patient groups also at early disease stages. Clinical follow-up enabled appropriate patient inclusion. This implies that an early diagnosis in individual patients can be made by comparing each subject's metabolic findings with a complete database of specific disease related patterns.

Imaging of compensatory mechanisms in Parkinson's disease

PURPOSE OF REVIEW

The on-going quest for potentially disease-modifying therapies in Parkinson's disease has prompted the development of methods that can differentiate direct disease effects from compensatory processes.

RECENT FINDINGS

PET studies have suggested a number of changes at the synaptic level to maintain integrity of dopaminergic systems. Functional MRI studies support the long-held belief that relatively intact cerebellar circuits may compensate for impaired basal ganglia function. Altered connectivity and increased spatial extent of activation also appear to be mechanisms through which motor and cognitive performance can be maintained.

SUMMARY

Ascertaining which changes in brain activation in Parkinson's disease are, in fact, compensatory represents a serious challenge. Compensatory mechanisms have been demonstrated from the microscopic, synaptic level to the macroscopic, system level. Augmentation of compensatory mechanisms, in addition to ameliorating the loss of dopaminergic neurons, may represent a joint strategy for overall minimization of disability.