Striatal dopamine distribution in parkinsonian patients during life

An automatic analysis framework for FDOPA PET neuroimaging

In this study we evaluate the performance of a fully automated analytical framework for FDOPA PET neuroimaging data, and its sensitivity to demographic and experimental variables and processing parameters. An instance of XNAT imaging platform was used to store the King's College London institutional brain FDOPA PET imaging archive, alongside individual demographics and clinical information. By re-engineering the historical Matlab-based scripts for FDOPA PET analysis, a fully automated analysis pipeline for imaging processing and data quantification was implemented in Python and integrated in XNAT. The final data repository includes 892 FDOPA PET scans organized from 23 different studies. We found good reproducibility of the data analysis by the automated pipeline (in the striatum for the Kicer: for the controls ICC = 0.71, for the psychotic patients ICC = 0.88). From the demographic and experimental variables assessed, gender was found to most influence striatal dopamine synthesis capacity (F = 10.7, p < 0.001), with women showing greater dopamine synthesis capacity than men. Our automated analysis pipeline represents a valid resourse for standardised and robust quantification of dopamine synthesis capacity using FDOPA PET data. Combining information from different neuroimaging studies has allowed us to test it comprehensively and to validate its replicability and reproducibility performances on a large sample size.

Dopamine and vesicular monoamine transport loss supports incidental Lewy body disease as preclinical idiopathic Parkinson

Incidental Lewy body disease (ILBD) is a neuropathological diagnosis of brains with Lewy bodies without clinical neuropsychiatric symptoms. Dopaminergic deficits suggest a relationship to preclinical Parkinson's disease (PD). We now report a subregional pattern of striatal dopamine loss in ILBD cases, with dopamine found significantly decreased in the putamen (-52%) and only to a lower extent in the caudate (-38%, not statistically significant); this is similar to the pattern in idiopathic PD in various neurochemical and in vivo imaging studies. We aimed to find out if our recently reported impaired storage of dopamine in striatal synaptic vesicles prepared from striatal tissue of cases with idiopathic PD might be an early or even causative event. We undertook parallel measurements of [3H]dopamine uptake and vesicular monoamine transporter (VMAT)2 binding sites by the specific label [3H]dihydrotetrabenazine on vesicular preparation from caudate and putamen in ILBD. Neither specific uptake of dopamine and binding of [3H]dihydrotetrabenazine, nor mean values of the calculated ratios of dopamine uptake and VMAT2 binding, a measure of uptake rate per transport site, were significantly different between ILBD and controls. ATP-dependence of [3H]dopamine uptake revealed significantly higher rates in putamen than in caudate at saturating concentrations of ATP in controls, a subregional difference lost in ILBD. Our findings support a loss of the normally higher VMAT2 activity in putamen as a contributing factor to the higher susceptibility of the putamen to dopamine depletion in idiopathic PD. Moreover, we suggest ILBD postmortem tissue as a valuable source for testing hypotheses on processes in idiopathic PD.

In vivo18F-DOPA PET imaging identifies a dopaminergic deficit in a rat model with a G51D α-synuclein mutation

Parkinson's disease (PD) is a neurodegenerative condition with several major hallmarks, including loss of substantia nigra neurons, reduction in striatal dopaminergic function, and formation of α-synuclein-rich Lewy bodies. Mutations in SNCA, encoding for α-synuclein, are a known cause of familial PD, and the G51D mutation causes a particularly aggressive form of the condition. CRISPR/Cas9 technology was used to introduce the G51D mutation into the endogenous rat SNCA gene. SNCA^G51D/+ and SNCA^G51D/G51D rats were born in Mendelian ratios and did not exhibit any severe behavourial defects. L-3,4-dihydroxy-6-¹⁸F-fluorophenylalanine (¹⁸F-DOPA) positron emission tomography (PET) imaging was used to investigate this novel rat model. Wild-type (WT), SNCA^G51D/+ and SNCA^G51D/G51D rats were characterized over the course of ageing (5, 11, and 16 months old) using ¹⁸F-DOPA PET imaging and kinetic modelling. We measured the influx rate constant (K_i) and effective distribution volume ratio (EDVR) of ¹⁸F-DOPA in the striatum relative to the cerebellum in WT, SNCA^G51D/+ and SNCA^G51D/G51D rats. A significant reduction in EDVR was observed in SNCA^G51D/G51D rats at 16 months of age indicative of increased dopamine turnover. Furthermore, we observed a significant asymmetry in EDVR between the left and right striatum in aged SNCA^G51D/G51D rats. The increased and asymmetric dopamine turnover observed in the striatum of aged SNCA^G51D/G51D rats reflects one aspect of prodromal PD, and suggests the presence of compensatory mechanisms. SNCA^G51D rats represent a novel genetic model of PD, and kinetic modelling of ¹⁸F-DOPA PET data has identified a highly relevant early disease phenotype.

Intercomparison of MR-informed PET image reconstruction methods

PURPOSE

Numerous image reconstruction methodologies for positron emission tomography (PET) have been developed that incorporate magnetic resonance (MR) imaging structural information, producing reconstructed images with improved suppression of noise and reduced partial volume effects. However, the influence of MR structural information also increases the possibility of suppression or bias of structures present only in the PET data (PET-unique regions). To address this, further developments for MR-informed methods have been proposed, for example, through inclusion of the current reconstructed PET image, alongside the MR image, in the iterative reconstruction process. In this present work, a number of kernel and maximum a posteriori (MAP) methodologies are compared, with the aim of identifying methods that enable a favorable trade-off between the suppression of noise and the retention of unique features present in the PET data.

METHODS

The reconstruction methods investigated were: the MR-informed conventional and spatially compact kernel methods, referred to as KEM and KEM largest value sparsification (LVS) respectively; the MR-informed Bowsher and Gaussian MR-guided MAP methods; and the PET-MR-informed hybrid kernel and anato-functional MAP methods. The trade-off between improving the reconstruction of the whole brain region and the PET-unique regions was investigated for all methods in comparison with postsmoothed maximum likelihood expectation maximization (MLEM), evaluated in terms of structural similarity index (SSIM), normalized root mean square error (NRMSE), bias, and standard deviation. Both simulated BrainWeb (10 noise realizations) and real [18 F] fluorodeoxyglucose (FDG) three-dimensional datasets were used. The real [18 F]FDG dataset was augmented with simulated tumors to allow comparison of the reconstruction methodologies for the case of known regions of PET-MR discrepancy and evaluated at full counts (100%) and at a reduced (10%) count level.

RESULTS

For the high-count simulated and real data studies, the anato-functional MAP method performed better than the other methods under investigation (MR-informed, PET-MR-informed and postsmoothed MLEM), in terms of achieving the best trade-off for the reconstruction of the whole brain and PET-unique regions, assessed in terms of the SSIM, NRMSE, and bias vs standard deviation. The inclusion of PET information in the anato-functional MAP method enables the reconstruction of PET-unique regions to attain similarly low levels of bias as unsmoothed MLEM, while moderately improving the whole brain image quality for low levels of regularization. However, for low count simulated datasets the anato-functional MAP method performs poorly, due to the inclusion of noisy PET information in the regularization term. For the low counts simulated dataset, KEM LVS and to a lesser extent, HKEM performed better than the other methods under investigation in terms of achieving the best trade-off for the reconstruction of the whole brain and PET-unique regions, assessed in terms of the SSIM, NRMSE, and bias vs standard deviation.

CONCLUSION

For the reconstruction of noisy data, multiple MR-informed methods produce favorable whole brain vs PET-unique region trade-off in terms of the image quality metrics of SSIM and NRMSE, comfortably outperforming the whole image denoising of postsmoothed MLEM.

Enhancement of Partial Volume Correction in MR-Guided PET Image Reconstruction by Using MRI Voxel Sizes

Positron emission tomography (PET) suffers from poor spatial resolution which results in quantitative bias when evaluating the radiotracer uptake in small anatomical regions, such as the striatum in the brain which is of importance in this paper of neurodegenerative diseases. These partial volume effects need to be compensated for by employing partial volume correction (PVC) methods in order to achieve quantitatively accurate images. Two important PVC methods applied during the reconstruction are resolution modeling, which suffers from Gibbs artifacts, and penalized likelihood using anatomical priors. The introduction of clinical simultaneous PET-MR scanners has attracted new attention for the latter methods and brought new opportunities to use MRI information to assist PET image reconstruction in order to improve image quality. In this context, MR images are usually down-sampled to the PET resolution before being used in MR-guided PET reconstruction. However, the reconstruction of PET images using the MRI voxel size could achieve a better utilization of the high resolution anatomical information and improve the PVC obtained with these methods. In this paper, we evaluate the importance of the use of MRI voxel sizes when reconstructing PET images with MR-guided maximum a posteriori (MAP) methods, specifically the modified Bowsher method. We also propose a method to avoid the artifacts that arise when PET reconstructions are performed in a higher resolution matrix than the standard for a given scanner. The MR-guided MAP reconstructions were implemented with a modified Lange prior that included anatomical information with the Bowsher method. The methods were evaluated with and without resolution modeling for simulated and real brain data. We show that the use of the MRI voxel sizes when reconstructing PET images with MR-guided MAP enhances PVC by improving the contrast and reducing the bias in six different regions of the brain using regional metrics for a single simulated data set and ensemble metrics for ten noise realizations. Similar results were obtained for real data, where a good enhancement of the contrast was achieved. The combination of MR-guided MAP reconstruction with point-spread function modeling and MRI voxel sizes proved to be an attractive method to achieve considerable enhancement of PVC, while reducing and controlling the noise level and Gibbs artifacts.

Neural Stem Cells Implanted into MPTP-Treated Monkeys Increase the Size of Endogenous Tyrosine Hydroxylase-Positive Cells Found in the Striatum: A Return to Control Measures

Neural stem cells (NSC) have been shown to migrate towards damaged areas, produce trophic factors, and replace lost cells in ways that might be therapeutic for Parkinson's disease (PD). However, there is very little information on the effects of NSC on endogenous cell populations. In the current study, effects of implanted human NSC (hNSC) on endogenous tyrosine hydroxylase-positive cells (TH+ cells) after treatment with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were explored in nonhuman primates. After MPTP damage and in PD, the primate brain is characterized by decreased numbers of dopamine neurons in the substantia nigra (SN) and an increase in neurons expressing TH in the caudate nucleus. To determine how implanted NSC might affect these cell populations, 11 St. Kitts African green monkeys were treated with the selective dopaminergic neurotoxin, MPTP. Human NSC were implanted into the left and right caudate nucleus and the right SN of eight of the MPTP-treated monkeys. At either 4 or 7 months after NSC implants, the brains were removed and the size and number of TH+ cells in the target areas were assessed. The results were compared to data obtained from normal untreated control monkeys and to the three unimplanted MPTP-treated monkeys. The majority of hNSC were found bilaterally along the nigrostriatal pathway and in the substantia nigra, while relatively few were found in the caudate. In the presence of NSC, the number and size of caudate TH+ cells returned to non-MPTP-treated control levels. MPTP-induced and hNSC-induced changes in the putamen were less apparent. We conclude that after MPTP treatment in the primate, hNSC prevent the MPTP-induced upregulation of TH+ cells in the caudate and putamen, indicating that hNSC may be beneficial to maintaining a normal striatal environment.

CNS Assessment Using Functional Neuro-PET Imaging

Disorders of the brain manifest in a variety of manners ranging from feeling or thought abnormalities to total paralysis. Until recently, most imaging methods of the brain have been limited to anatomic considerations, with little information about actual function of the brain except that deduced from clinical examination and physical and cognitive assessment testing. With the advent of positron emission tomography (PET) and enhanced computer and scintigraphic image detection systems, there is keen interest in applying this imaging technique to better understand brain physiology and pathophysiology. Potential applications of PET in CNS assessment is expanding avenues for improved diagnosis and staging of disease, as well as monitoring therapeutic interventions. A general review of the radiopharmaceuticals used for neuro-PET imaging, as well as their application in situations such as cerebrovascular disease, brain activation studies, various movement disorders and dementias, depression, epilepsy, obsessive-compulsive disorder, schizophrenia, and neuropharmacology (including cerebral receptor studies) will be presented.

Dopaminergic Dysfunction in Parkinsonism: New Lessons from Imaging

In recent decades, identification of the dopaminergic deficit in Parkinson's disease has spawned an explo sion of research in the molecular neurobiology of the basal ganglia. In vivo imaging has provided a tool to bridge developments in basic neuroscience and clinicial neurology. Imaging studies have opened a unique window on the neurochemical pathophysiology of Parkinson's disease and more specifically on the onset, progression and physiology of the degenerative process. As we are poised on the brink of new protective and restorative therapies for Parkinson's disease, the potential of imaging to teach us about in vivo brain neurochemistry offers both promise and challenge. NEUROSCIENTIST 5:333-340, 1999

Molecular imaging of movement disorders

Positron emission tomography measures the activity of radioactively labeled compounds which distribute and accumulate in central nervous system regions in proportion to their metabolic rate or blood flow. Specific circuits such as the dopaminergic nigrostriatal projection can be studied with ligands that bind to the pre-synaptic dopamine transporter or post-synaptic dopamine receptors (D1 and D2). Single photon emission computerized tomography (SPECT) measures the activity of similar tracers labeled with heavy radioactive species such as technetium and iodine. In essential tremor, there is cerebellar hypermetabolism and abnormal GABAergic function in premotor cortices, dentate nuclei and ventral thalami, without significant abnormalities in dopaminergic transmission. In Huntington’s disease, there is hypometabolism in the striatum, frontal and temporal cortices. Disease progression is accompanied by reduction in striatal D1 and D2 binding that correlate with trinucleotide repeat length, disease duration and severity. In dystonia, there is hypermetabolism in the basal ganglia, supplementary motor areas and cerebellum at rest. Thalamic and cerebellar hypermetabolism is seen during dystonic movements, which can be modulated by globus pallidus deep brain stimulation (DBS). Additionally, GABA-A receptor activity is reduced in motor, premotor and somatosensory cortices. In Tourette’s syndrome, there is hypermetabolism in premotor and sensorimotor cortices, as well as hypometabolism in the striatum, thalamus and limbic regions at rest. During tics, multiple areas related to cognitive, sensory and motor functions become hypermetabolic. Also, there is abnormal serotoninergic transmission in prefrontal cortices and bilateral thalami, as well as hyperactivity in the striatal dopaminergic system which can be modulated with thalamic DBS. In Parkinson’s disease (PD), there is asymmetric progressive decline in striatal dopaminergic tracer accumulation, which follows a caudal-to-rostral direction. Uptake declines prior to symptom presentation and progresses from contralateral to the most symptomatic side to bilateral, correlating with symptom severity. In progressive supranuclear palsy (PSP) and multiple system atrophy (MSA), striatal activity is symmetrically and diffusely decreased. The caudal-to-rostral pattern is lost in PSP, but could be present in MSA. In corticobasal degeneration (CBD), there is asymmetric, diffuse reduction of striatal activity, contralateral to the most symptomatic side. Additionally, there is hypometabolism in contralateral parieto-occipital and frontal cortices in PD; bilateral putamen and cerebellum in MSA; caudate, thalamus, midbrain, mesial frontal and prefrontal cortices in PSP; and contralateral cortices in CBD. Finally, cardiac sympathetic SPECT signal is decreased in PD. The capacity of molecular imaging to provide in vivo time courses of gene expression, protein synthesis, receptor and transporter binding, could facilitate the development and evaluation of novel medical, surgical and genetic therapies in movement disorders.

Motor Sequence Learning and Consolidation in Unilateral De Novo Patients with Parkinson's Disease

Previous research investigating motor sequence learning (MSL) and consolidation in patients with Parkinson’s disease (PD) has predominantly included heterogeneous participant samples with early and advanced disease stages; thus, little is known about the onset of potential behavioral impairments. We employed a multisession MSL paradigm to investigate whether behavioral deficits in learning and consolidation appear immediately after or prior to the detection of clinical symptoms in the tested (left) hand. Specifically, our patient sample was limited to recently diagnosed patients with pure unilateral PD. The left hand symptomatic (LH-S) patients provided an assessment of performance following the onset of clinical symptoms in the tested hand. Conversely, right hand affected (left hand asymptomatic, LH-A) patients served to investigate whether MSL impairments appear before symptoms in the tested hand. LH-S patients demonstrated impaired learning during the initial training session and both LH-S and LH-A patients demonstrated decreased performance compared to controls during the next-day retest. Critically, the impairments in later learning stages in the LH-A patients were evident even before the appearance of traditional clinical symptoms in the tested hand. Results may be explained by the progression of disease-related alterations in relevant corticostriatal networks.

An ERP study of vocal emotion processing in asymmetric Parkinson's disease

Parkinson's disease (PD) has been related to impaired processing of emotional speech intonation (emotional prosody). One distinctive feature of idiopathic PD is motor symptom asymmetry, with striatal dysfunction being strongest in the hemisphere contralateral to the most affected body side. It is still unclear whether this asymmetry may affect vocal emotion perception. Here, we tested 22 PD patients (10 with predominantly left-sided [LPD] and 12 with predominantly right-sided motor symptoms) and 22 healthy controls in an event-related potential study. Sentences conveying different emotional intonations were presented in lexical and pseudo-speech versions. Task varied between an explicit and an implicit instruction. Of specific interest was emotional salience detection from prosody, reflected in the P200 component. We predicted that patients with predominantly right-striatal dysfunction (LPD) would exhibit P200 alterations. Our results support this assumption. LPD patients showed enhanced P200 amplitudes, and specific deficits were observed for disgust prosody, explicit anger processing and implicit processing of happy prosody. Lexical speech was predominantly affected while the processing of pseudo-speech was largely intact. P200 amplitude in patients correlated significantly with left motor scores and asymmetry indices. The data suggest that emotional salience detection from prosody is affected by asymmetric neuronal degeneration in PD.

Meditation and neurodegenerative diseases

Neurodegenerative diseases pose a significant problem for the healthcare system, doctors, and patients. With an aging population, more and more individuals are developing neurodegenerative diseases and there are few treatment options at the present time. Meditation techniques present an interesting potential adjuvant treatment for patients with neurodegenerative diseases and have the advantage of being inexpensive, and easy to teach and perform. There is increasing research evidence to support the application of meditation techniques to help improve cognition and memory in patients with neurodegenerative diseases. This review discusses the current data on meditation, memory, and attention, and the potential applications of meditation techniques in patients with neurodegenerative diseases.

Revisiting the effects of Parkinson's disease and frontal lobe lesions on task switching: the role of rule reconfiguration

This study investigated the hypothesis that rule reconfiguration in task switching can isolate aspects of intact and impaired control at different stages of Parkinson's disease (PD) by comparing switches between concrete naming rules pertaining to stimulus selection, to switches between abstract rules which allocate categorization responses to these stimuli. Based on previous findings, it was hypothesized that attentional switches, where task set competition emerges at the stimulus but not response set level, highlights striatal dopaminergic function. Conversely, increasing the degree of task set competition to encompass reconfiguration of response set when switching between abstract rules, represents a condition which engages the prefrontal cortex (PFC) and renders this manipulation sensitive to frontal damage. To this end, we investigated task switching with concrete and abstract rules in unilaterally (Hoehn & Yahr stage I) and bilaterally (Hoehn & Yahr stage II) affected PD patients, as well as striatally intact frontal lesion patients. As predicted, frontal lesion patients demonstrated switching deficits only with abstract categorization rules, where switching engendered complete task set reconfiguration and a new response, as did stage II PD patients with presumed frontal cortical pathology. Replicating previous findings, stage I PD patients with relatively circumscribed striatal pathology demonstrated no such impairment. Disease severity also impacted on attentional switching indexed by naming rules, since medicated stage II but not stage I patients demonstrated switching deficits emerging from stimulus set reconfiguration, suggesting that the ameliorative efficacy of dopaminergic medication is inversely related to the severity of the striatal deficit. These findings illustrate that the nature of the rules that are switched, and its implication in terms of reconfiguring different task set elements, highlights different neural characters of cognitive flexibility. These manipulations may help decipher the differential effects of progressive neurodegeneration on parkinsonian cognition, and provide a framework in which to conceptualize the contributions of cortical and subcortical regions to cognitive control.

Use of [18F]FDOPA-PET for in vivo evaluation of dopaminergic dysfunction in unilaterally 6-OHDA-lesioned rats

Background

We evaluated the utility of L-3,4-dihydroxy-6-[¹⁸F]fluoro-phenylalanine ([¹⁸F]FDOPA) positron emission tomography (PET) as a method for assessing the severity of dopaminergic dysfunction in unilaterally 6-hydroxydopamine (6-OHDA)-lesioned rats by comparing it with quantitative biochemical, immunohistochemical, and behavioral measurements.

Methods

Different doses of 6-OHDA (0, 7, 14, and 28 μg) were unilaterally injected into the right striatum of male Sprague-Dawley rats. Dopaminergic functional activity in the striatum was assessed by [¹⁸F]FDOPA-PET, measurement of striatal dopamine (DA) and DA metabolite levels, tyrosine hydroxylase (TH) immunostaining, and methamphetamine-induced rotational testing.

Results

Accumulation of [¹⁸F]FDOPA in the bilateral striatum was observed in rats pretreated with both aromatic L-amino acid decarboxylase and catechol-O-methyltransferase (COMT) inhibitors. Unilateral intrastriatal injection of 6-OHDA produced a significant site-specific reduction in [¹⁸F]FDOPA accumulation. The topological distribution pattern of [¹⁸F]FDOPA accumulation in the ipsilateral striatum agreed well with the pattern in TH-stained corresponding sections. A significant positive relationship was found between Patlak plot K_i values and striatal levels of DA and its metabolites (r = 0.958). A significant negative correlation was found between both K_i values (r = -0.639) and levels of DA and its metabolites (r = -0.719) and the number of methamphetamine-induced rotations.

Conclusions

K_i values determined using [¹⁸F]FDOPA-PET correlated significantly with the severity of dopaminergic dysfunction. [¹⁸F]FDOPA-PET makes it possible to perform longitudinal evaluation of dopaminergic function in 6-OHDA-lesioned rats, which is useful in the development of new drugs and therapies for Parkinson's disease (PD).

Automated striatal uptake analysis of ¹⁸F-FDOPA PET images applied to Parkinson's disease patients

OBJECTIVE

6-[(18)F]Fluoro-L: -DOPA (FDOPA) is a radiopharmaceutical valuable for assessing the presynaptic dopaminergic function when used with positron emission tomography (PET). More specifically, the striatal-to-occipital ratio (SOR) of FDOPA uptake images has been extensively used as a quantitative parameter in these PET studies. Our aim was to develop an easy, automated method capable of performing objective analysis of SOR in FDOPA PET images of Parkinson's disease (PD) patients.

METHODS

Brain images from FDOPA PET studies of 21 patients with PD and 6 healthy subjects were included in our automated striatal analyses. Images of each individual were spatially normalized into an FDOPA template. Subsequently, the image slice with the highest level of basal ganglia activity was chosen among the series of normalized images. Also, the immediate preceding and following slices of the chosen image were then selected. Finally, the summation of these three images was used to quantify and calculate the SOR values. The results obtained by automated analysis were compared with manual analysis by a trained and experienced image processing technologist.

RESULTS

The SOR values obtained from the automated analysis had a good agreement and high correlation with manual analysis. The differences in caudate, putamen, and striatum were -0.023, -0.029, and -0.025, respectively; correlation coefficients 0.961, 0.957, and 0.972, respectively.

CONCLUSIONS

We have successfully developed a method for automated striatal uptake analysis of FDOPA PET images. There was no significant difference between the SOR values obtained from this method and using manual analysis. Yet it is an unbiased time-saving and cost-effective program and easy to implement on a personal computer.

Molecular imaging and the neuropathologies of Parkinson's disease

The main motor symptoms of Parkinson's disease (PD) are linked to degeneration of the nigrostriatal dopamine (DA) fibers, especially those innervating the putamen. This degeneration can be assessed in molecular imaging studies with presynaptic tracers such as [(18)F]-fluoro-L-DOPA (FDOPA) and ligands for DA transporter ligands. However, the pathologies of PD are by no means limited to nigrostriatal loss. Results of post mortem and molecular imaging studies reveal parallel degenerations of cortical noradrenaline (NA) and serotonin (5-HT) innervations, which may contribute to affective and cognitive changes of PD. Especially in advanced PD, cognitive impairment can come to resemble that seen in Alzheimer's dementia, as can the degeneration of acetylcholine innervations arising in the basal forebrain. The density of striatal DA D(2) receptors increases in early untreated PD, consistent with denervation upregulation, but there is an accelerated rate of DA receptor loss as the disease advances. Animal studies and post mortem investigations reveal changes in brain opioid peptide systems, but these are poorly documented in imaging studies of PD. Relatively minor changes in the binding sites for GABA are reported in cortex and striatum of PD patients. There remains some controversy about the expression of the 18 kDa translocator protein (TSPO) in activated microglia as an indicator of an active inflammatory component of neurodegeneration in PD. A wide variety of autonomic disturbances contribute to the clinical syndrome of PD; the degeneration of myocardial sympathetic innervation can be revealed in SPECT studies of PD patients with autonomic failure. Considerable emphasis has been placed on investigations of cerebral blood flow and energy metabolism in PD. Due to the high variance of these physiological estimates, researchers have often employed normalization procedures for the sensitive detection of perturbations in relatively small patient groups. However, a widely used normalization to the global mean must be used with caution, as it can result in spurious findings of relative hypermetabolic changes in subcortical structures. A meta-analysis of the quantitative studies to date shows that there is in fact widespread hypometabolism and cerebral blood flow in the cerebral cortex, especially in frontal cortex and parietal association areas. These changes can bias the use of global mean normalization, and probably represent the pathophysiological basis of the cognitive impairment of PD.

Hypertension increases cerebral 6-18F-fluorodopa-derived radioactivity

6-(18)F-fluorodopa PET depicts the striatal dopaminergic lesion characterizing Parkinson disease (PD); however, striatal uptake of 6-(18)F-fluorodopa-derived radioactivity can be normal. Supine hypertension (SH) might increase 6-(18)F-fluorodopa uptake.

METHODS

We measured putamen, caudate, and occipital cortex 6-(18)F-fluorodopa-derived radioactivity and supine blood pressure in patients with PD + SH (systolic pressure >/= 180 mm Hg, n = 8), patients with PD without SH (PD - SH, n = 19), patients with pure autonomic failure (n = 8), and controls (n = 16).

RESULTS

Peak putamen radioactivity correlated with supine systolic pressure across all subjects and among PD patients and was higher in PD + SH than in PD - SH (P = 0.01). Both subgroups had rapid fractional declines in radioactivity between the peak and late values (P < 0.0001, compared with controls). Arterial 6-(18)F-fluorodopa concentrations were similar in the compared groups.

CONCLUSION

In PD, SH is associated with augmented striatal 6-(18)F-fluorodopa-derived radioactivity. Regardless of SH, retention of 6-(18)F-fluorodopa-derived radioactivity is markedly reduced. A model-independent approach can identify striatal dopaminergic denervation in PD.

Simple ratio analysis of 18F-fluorodopa uptake in striatal subregions separates patients with early Parkinson disease from healthy controls

6-(18)F-fluoro-l-dopa ((18)F-FDOPA) is widely used to investigate dopaminergic hypofunction, for instance, in Parkinson disease (PD). Conventionally, a 90-min scan with either a graphical or a metabolite-purified plasma input approach has been used for quantification. In the clinical setting, to increase compliance, especially in patients with more advanced disease, and to increase the efficacy of tracer and scanner time use, a shorter acquisition and a simple quantitative analysis are desirable. Taking into account the asymmetry of clinical symptoms and the uneven distribution of striatal dopaminergic hypofunction may also improve the use of (18)F-FDOPA PET in early disease detection. Therefore, we compared subregional striatal (18)F-FDOPA PET data from a large group of nonmedicated patients with early PD and a set of healthy elderly volunteers to find out whether a simple ratio approach would reliably separate PD patients from healthy controls.

METHODS

A total of 89 nonmedicated patients with early PD and 21 healthy volunteers were studied with (18)F-FDOPA PET, and both a region-to-reference (striatal-to-occipital) ratio (SOR) calculated from 75 to 90 min after injection and a graphical analysis of data calculated from 15 to 90 min after (18)F-FDOPA injection (yielding the influx constant [K(i)(ref)]) were used.

RESULTS

Both SOR and K(i)(ref) values in the PD patients were lowest, relative to those in the healthy controls, in the posterior putamen contralateral to the side with predominant clinical symptoms. The contralateral posterior putamen showed the largest areas under the receiver operating characteristic (ROC) curve-0.994 for SOR and 0.998 for K(i)(ref)-indicating excellent separation of the PD and control groups. The caudate nucleus and the ventral striatum were less impressive in this respect.

CONCLUSION

A single 15-min scan 75 min after tracer injection seems to be sufficient for separating patients with PD from healthy controls in a clinical research environment. This method represents a powerful and economical alternative for research on the disease mechanism and differential diagnosis.

Positron emission tomography for neurologists

This short review focuses on practical, present day, clinical application of FDG PET, a technology available to practicing neurologists for managing their patients. Indications in the disease states of dementia, neuro-oncology, epilepsy, parkinsonism, and other less common settings are reviewed. Many third-party payers currently make reimbursements based on these indications. By measuring an aspect of brain function, PET provides information that often is unobtainable from other sources, thus facilitating more rationale and cost-effective management, which can only benefit the patient, the referring physician, and the health care system as a whole.

Imaging of cholinergic and monoaminergic neurochemical changes in neurodegenerative disorders

Positron emission tomography (PET) or single photon emission computer tomography (SPECT) imaging provides the means to study neurochemical processes in vivo. These methods have been applied to examine monoaminergic and cholinergic changes in neurodegenerative disorders. These investigations have provided important insights into disorders, such as Alzheimer's disease (AD) and Parkinson's disease (PD). The most intensely studied monoaminergic transmitter is dopamine. The extent of presynaptic nigrostriatal dopaminergic denervation can be quantified in PD and may serve as a diagnostic biomarker. Dopaminergic receptor imaging may help to distinguish idiopathic PD from atypical parkinsonian disorders. Cholinergic denervation has been identified not only in AD but also in PD and more severely in parkinsonian dementia. PET or SPECT can also provide biomarkers to follow progression of disease or evaluate the effects of therapeutic interventions. Cholinergic receptor imaging is expected to play a major role in new drug development for dementing disorders.

Transcranial color-coded sonography helps differentiation between idiopathic Parkinson's disease and vascular parkinsonism

BACKGROUND

Recently, transcranial color-coded sonography (TCCS) has been found to have a diagnostic value in patients with idiopathic Parkinson's disease (IPD), which displays increased hyperechogenicity at the substantia nigra (SN).

OBJECTIVE

To use TCCS, to assess the difference in SN hyperechogenicity and intracranial hemodynamics among subjects with IPD, vascular parkinsonism (VP) and controls.

METHODS

Eighty IPD and 30 VP patients, and 60 controls were recruited into this study. The hyperechogenicity area at the SN and midbrain were calculated by encircling the outer circumference from the ipsilateral temporal window, using TCCS in each subject. The hemodynamics of intracranial large arteries, including flow velocity and pulsatility index (PI), were also measured.

RESULTS

The presence of SN hyperechogenicity was significantly higher in the IPD patients than in the VP patients and controls (84% vs. 20% & 5%, respectively, p < 0.001). In IPD patients, the SN hyperechogenicity was correlated with the neurological severity and disease duration. Twenty-five (66.7%) VP patients had obvious vascular abnormality, as seen in TCCS study. The mean PI was significantly more elevated in the VP patients than those in the IPD patients and controls (all p < 0.05), but there was no significant difference of flow velocities among the VP, IPD patients and controls.

CONCLUSION

TCCS, combining B-mode imaging for SN echogenicity and trancranial Doppler for intracranial hemodynamics, is a useful diagnostic tool in the differentiation between IPD and VP. These findings also suggest that multiple subcortical vascular lesions may damage the basal ganglia and thalamocortical circuit and result in parkinsonism features in VP patients.

Applications of Clinical Dopamine Imaging

Recent technologic advances make it increasingly possible to image neurotransmitter systems in living human brain, The dopamine system has been most intensively studied owing to its involvement in several brain disorders such as Parkinson's disease and Huntington's disease, as well as psychiatric disorders such as schizophrenia, depression, and compulsive behavioral disorders of multiple types. A variety of aspects of dopamine receptor density, function, and dopaminergic terminal status can now be assessed using the minimally invasive neuroimaging techniques of positron emission tomography and single-photon emission computed tomography. Although these techniques are currently used most often in the context of research, clinical applications are rapidly emerging.

Role of [18F]-dopa-PET imaging in assessing movement disorders

FD-PET has proved to be an extremely useful technique for the noninvasive evaluation of nigrostriatal pathophysiology in patients with PD and other movement disorders. The development of ratio methods for image analysis has greatly reduced the complexity of these PET studies and has facilitated data analysis. With the recent advances in cyclotron targetry and automated synthesis modules FD-PET will soon become an important component of the clinical armamentarium.

The role of PET imaging in the management of patients with central nervous system disorders

PET will continue to play a critical role in both clinical and research applications with regard to CNS disorders. PET is useful in the initial diagnosis of patients presenting with CNS symptoms and can help clinicians determine the best course of therapy. PET studies can also be useful for studying the response to therapy. From the research perspective, the various neurotransmitter and other molecular tracers currently available or in development will provide substantial information about pathophysiologic process in the brain. As such applications become more widely tested, their introduction into the clinical arena will further advance the use of PET imaging in the evaluation and management of CNS disorders.

Cognitive- and motor-related regions in Parkinson's disease: FDOPA and FDG PET studies

OBJECTIVE

Using 6-[(18)F]fluoro-L-dopa (FDOPA) and [(18)F]fluorodeoxyglucoce (FDG) positron emission tomography (PET), multiple regression analyses were performed to determine the specific brain regions that are related to cognitive and motor symptoms in nondemented patients with Parkinson's disease.

METHODS

Spatially normalized images of FDOPA influx rate constant (Ki) values and relative regional cerebral metabolic rates for glucose (rrCMRglc) were created. Raven's Coloured Progressive Matrices (RCPM) scores and the Unified Parkinson's Disease Rating Scale (UPDRS) motor scores were used to determine the patients' cognitive and motor functions, respectively. Multiple correlation analyses between the FDOPA and FDG images and the cognitive and motor scores were performed for each voxel.

RESULTS

RCPM score was significantly positively correlated with the FDOPA Ki in the left hippocampus and with the rrCMRglc in the left middle frontal gyrus and right retrosplenial cortex. Motor function was significantly positively correlated with the FDOPA Ki in the bilateral striatum and with the rrCMRglc in association areas and primary visual cortex. The level of motor function was significantly inversely correlated with the FDOPA Ki in the anterior cingulate gyrus and with the rrCMRglc in bilateral primary motor cortex and right putamen.

CONCLUSIONS

Changes of striatal FDOPA uptake and rrCMRglc in the primary motor cortex likely represent dysfunction in the motor system involving the corticobasal ganglia-thalamocortical loop. Change of FDOPA uptake in the anterior cingulate gyrus may be related to up-regulation of dopamine synthesis in surviving dopamine neurons. The regions where correlation with cognitive function was observed belong to a cognitive frontoparietal-hippocampal network.

An Event-related fMRI Study of Artificial Grammar Learning in a Balanced Chunk Strength Design

Artificial grammar learning (Reber, 1967) is a form of implicit learning in which cognitive, rather than motor, implicit learning has been found. After viewing a series of letter strings formed according to a finite state rule system, people are able to classify new letter strings as to whether or not they are formed according to these grammatical rules despite little conscious insight into the rule structure. Previous research has shown that these classification judgments are based on knowledge of abstract rules as well as superficial similarity (“chunk strength”) to training strings. Here we used event-related fMRI to identify neural regions involved in using both sources of information as test stimuli were designed to unconfound chunk strength from rule use. Using functional connectivity analyses, the extent to which the sources of information are complementary or competitive was also assessed. Activation in the right caudate was associated with rule adherence, whereas medial temporal lobe activations were associated with chunk strength. Additionally, functional connectivity analyses revealed caudate and medial temporal lobe activations to be strongly negatively correlated (r = −88) with one another during the performance of this task.

Surrogate endpoints in Parkinson's disease research

Biomarkers are important tools in understanding the underlying mechanisms of causation, progression, and treatment effects in Parkinson's disease (PD). In addition, these biomarkers may be utilized as surrogate endpoints that, when used appropriately, can lead to important advances in therapeutics in a timely and cost-effective manner. This paper outlines the definition, role, validity process, and risks associated with surrogate endpoints. The use of biomarkers in recent PD clinical trials is discussed and potential shortcomings and unanswered questions related to interpreting these outcomes are reviewed. Finally, the significant challenges that lie ahead for validating and interpreting surrogate endpoints in PD are addressed.

Striatal size, glucose metabolic rate, and verbal learning in normal aging

The striatum has recently been implicated as an area that may mediate age-associated cognitive decline because of diminution of volume and functional activity. We used 18F-fluorodeoxyglucose (FDG) with positron emission tomography (PET) and high-resolution magnetic resonance imaging (MRI) to examine the effects of age on striatal glucose metabolic rate and size in 70 healthy, normal subjects. During the FDG tracer uptake period, subjects performed a serial verbal learning task, based on the California Verbal Learning Test. PET images were co-registered to the MR images. The interrelations among striatal glucose metabolic rate, size, and performance on the verbal learning task were examined with repeated-measures analysis of variance and correlational analysis. As age increased, relative glucose metabolic rate (GMR) increased in the putamen and decreased in the caudate. Female subjects had lower relative GMR than male subjects in the caudate, but equal in the putamen. Striatal size remained relatively constant across the lifespan in men but was lower in women aged 50-70 than in men. While there were significant associations between striatal activity and performance on the uptake task, these findings were mostly accounted for by age. The findings are consistent with our earlier report on the same cohort that demonstrated an age-related shift from anterior to posterior cortical metabolism, as the putamen receives primarily posterior cortical input and the caudate receives relatively more anterior cortical input. Findings of significant involvement of striatal functioning in verbal learning are most likely accounted for by age and suggest an age-related shift from anterior to posterior circuitry in the human telencephalon.

Endogenous dopamine release after pharmacological challenges in Parkinson's disease

Using (11)C-raclopride positron emission tomography after methamphetamine challenge, we have evaluated regional brain changes in synaptic dopamine (DA) levels in six volunteers and six advanced Parkinson's disease (PD) patients. The pharmacological challenge induced significant release of endogenous DA in putamen not only in the normal subjects, as reflected by a 25.2% reduction in (11)C-raclopride binding potential as compared with placebo, but also in the PD patients (6.8%). In individual PD patients, we found a correlation between putamen DA release and DA storage, as measured by (18)F-dopa uptake. Localization of significant changes in (11)C-raclopride binding after methamphetamine at a voxel level with statistical parametric mapping identified striatal and prefrontal DA release in both cohorts. Statistical comparisons between normal subjects and PD confirmed significantly reduced DA release in striatal areas in PD, but normal levels of prefrontal DA release. In conclusion, significant endogenous DA release can still be induced by pharmacological challenges in the putamen of advanced PD patients, and this release correlates with residual DA storage capacity. Our data also show that the capacity to release normal DA levels in prefrontal areas after a pharmacological challenge is preserved in severe stages of the disease.

[Clinical impact of 18F-fluorodeoxyglucose positron emission tomography in the diagnosis of brain diseases]

In this review it will be discussed in which neurological disorders positron emission tomography can yield important diagnostic information. Because positron emission tomography is an expensive method indications have to be clearly defined. One important question concerns the differentiation of tumor recurrence and scar due to radiation therapy or an operation. The grading of brain tumors is another application. In HIV patients fluorodeoxyglucose positron emission tomography can separate lymphoma and toxoplasmosis. In the evaluation of dementia positron emission tomography can help to clarify the differential diagnosis. Another important area is the presurgical evaluation of epilepsy patients and patients with cerebrovascular disease in whom a surgical revascularisation procedure is planned. In extrapyramidal disorders, positron emission tomography can often help to establish the final diagnosis.

Positron emission tomography of dopamine pathways in familial Parkinsonian syndromes

Positron emission tomography (PET) scan is considered to be the most useful tool with which to assess the integrity of nigrostriatal function in the living brain. Recently, different genetic defects have been associated with a variety of familial parkinsonian syndromes, the clinical phenotypes of which have varying degrees of similarities to idiopathic parkinsonism (IP), (sporadic Parkinson's disease). This review summarizes: (1) the PET scan findings (fluorodopa uptake and raclopride binding) in both familial parkinsonian syndromes and IP; and (2) the similarities and differences of the clinical and PET features between familial parkinsonian syndromes and IP. This analysis demonstrates that more similarities than differences exist in PET scan findings in the different familial parkinsonian syndromes with the exception of pallido-ponto-nigral degeneration (PPND), that is perhaps best considered as multisystem degeneration. As a result of this analysis, we believe that while different genetic defects may underlie different mechanisms of nigrostriatal degeneration, the final pattern of nigrostriatal dysfunction is essentially similar to that of IP. 'Parkinson's disease', therefore, may not represent a single disease entity, but rather the final manifestation of different pathogenetic mechanisms-mediated by genetic or environmental factors, or an interaction of genetic and environmental factors.

[123I]beta-CIT SPECT demonstrates decreased brain dopamine and serotonin transporter levels in untreated parkinsonian patients

Striatal dopamine transporters (DATs) and serotonin transporters (SERTs) were evaluated in untreated patients with Parkinson's disease (PD) and controls using single-photon emission computed tomography (SPECT) with 2beta-carboxymethoxy-3beta-(4-iodophenyl)tropane ([123I]beta-CIT). The striatal DAT specific to non-displaceable uptake ratios of 29, and the SERT uptake measurements of 27, PD patients were compared with those of 21 and 16 controls, respectively. The results were correlated with Unified Parkinson's Disease Rating Scale (UPDRS) scores, the Hoehn & Yahr stage, age, duration of the disease, and the major PD signs. The specific DAT binding in the caudate, the putamen and the caudate/putamen ratio were measured. In all of the PD patients the striatal uptake values were bilaterally reduced, being 36.9% (P < 0.001) lower than those of the controls. In the hemiparkinsonian patients the reduction was greater on the side contralateral to the initial symptoms (33.3% vs. 27.8%) and the uptake ratios indicated a more pronounced deficit in the putamen (39.1%) than in the caudate (27.9%). The DAT uptake correlated with the UPDRS total score and activities of daily living (ADL) and motor subscores, the Hoehn & Yahr stage, and rigidity score. PD patients had significantly higher caudate to putamen ratios than the controls. In the PD patients the SERT values were lower in the thalamic and frontal regions. The SERT uptake ratio of the frontal area correlated with the UPDRS subscore I. [123I]beta-CIT SPECT provides a useful method for confirming the clinical diagnosis of PD with correlation to disease severity. Additionally, this technique allows the simultaneous measurement of SERT uptake and shows that PD patients, interestingly, seem to have decreased SERT availability in the thalamic and frontal areas.

Striatal dopaminergic transmission and neocortical glucose utilization in Alzheimer's disease: a triple-tracer positron emission tomography study

The central dopaminergic neuron system is related to cognitive function and behavior. For Alzheimer's disease (AD), using PET and the ligands of dopa, D2 receptor antagonist, and deoxyglucose, we previously found that dopa uptake into the striatum (the Ki value) was correlated with the Mini-Mental State (MMS) score. AD patients exhibiting wandering had an increased level of D2 receptor (the k3/k4 value). The Ki value was correlated with glucose metabolism (CMRglc) of the frontal and temporal lobes, those of wanderers being lower than nonwanderers. We investigated the relationships between the Ki value and the k3/k4 value with reference to cognitive impairment and wandering. Using the three-tracer technique for the single subject, the Ki value, the k3/k4 value, and CMRglc in 12 AD patients were measured. For the wanderers, there was a reverse correlation between the Ki and the k3/k4 values. Multiple regression analysis showed that the score on the verbal fluency test was explained by the Ki value and CMRglc of the frontal and temporal lobes, while MMS score was related to more global areas.

Intuition: a social cognitive neuroscience approach

This review proposes that implicit learning processes are the cognitive substrate of social intuition. This hypothesis is supported by (a) the conceptual correspondence between implicit learning and social intuition (nonverbal communication) and (b) a review of relevant neuropsychological (Huntington's and Parkinson's disease), neuroimaging, neurophysiological, and neuroanatomical data. It is concluded that the caudate and putamen, in the basal ganglia, are central components of both intuition and implicit learning, supporting the proposed relationship. Parallel, but distinct, processes of judgment and action are demonstrated at each of the social, cognitive, and neural levels of analysis. Additionally, explicit attempts to learn a sequence can interfere with implicit learning. The possible relevance of the computations of the basal ganglia to emotional appraisal, automatic evaluation, script processing, and decision making are discussed.

Presynaptic and postsynaptic dopaminergic binding densities in the nigrostriatal and mesocortical systems in early Parkinson's disease: a double-tracer positron emission tomography study

To investigate changes in the relation between presynaptic and postsynaptic dopaminergic functions in vivo in both nigrostriatal and mesocortical systems in Parkinson's disease (PD), 10 drug-naive early PD patients were studied twice using positron emission tomography with [11C]CFT (dopamine transporter probe) followed by [11C]SCH 23390 (D1 receptor probe). Regional binding potentials (k3/k4) of [11C]CFT and [11C]SCH 23390 in the striatum (nigrostriatal system) and the orbitofrontal cortex (mesocortical system) were estimated by compartment analyses. Levels of [11C]CFT k3/k4 in the two projection areas were shown to be significantly lower in PD, whereas [11C]SCH 23390 levels remained unchanged. Regression analysis showed that estimates of CFT k3/k4 were positively correlated with those of SCH 23390 k3/k4 in the striatum in normal control, whereas the two binding estimates were less positively correlated in the caudate and inversely correlated in the putamen in PD. No significant correlation was observed in the orbitofrontal cortex in both groups. These results indicated that dopamine transporters and D1 receptors change in parallel in the normal striatal synapses, but the association becomes asymmetrical because of reduction in presynaptic and relative elevation in postsynaptic markers in PD. Alterations in synaptic parallel regulation in the nigrostriatal system might reflect early pathophysiology in the parkinsonian brain.

Single-photon emission computed tomography of the dopamine transporter in parkinsonism

The known dopaminergic abnormalities in Parkinson's disease have facilitated the development of radiolabeled biomarkers for diagnostic and research applications in humans. Presynaptic, intrasynaptic, and postsynaptic imaging now is possible using single-photon emission computed tomography. In particular, the development of new radiotracers that target the dopamine transporter located on degenerating dopamine neurons in Parkinson's disease and related disorders is directly relevant to improved clinical diagnosis, disease monitoring, and assessment of putative neuroprotective strategies in patients. In addition, the ability to characterize in vivo neuronal degeneration in these disorders provides a powerful research tool to better understand the natural course of these disorders and could provide clues to etiology.

Increased expression of dopamine receptors on lymphocytes in Parkinson's disease

Dopamine D1-like and D2-like receptors on peripheral blood lymphocytes (PBL) were assayed in 50 de novo patients with idiopathic Parkinson's disease (PD), in 36 neurologic control subjects (multiple-system atrophy, n = 16; essential tremor, n = 10; other neurodegenerative diseases, n = 10), and in 26 healthy control subjects by radioligand binding assay techniques using [3H]SCH 23390 and [3H]7OH-DPAT as ligands. Patients with PD revealed a higher density (Bmax) of dopamine D1-like (p <0.001) and D2-like (p <0.00001) receptors on PBL than either neurologic or healthy control subjects, whereas no differences in Bmax were observed among patients affected by other neurologic diseases and healthy control subjects. The affinity (Kd) of both radioligands was similar in the groups investigated. The pharmacologic profile of [3H]SCH 23390 and [3H]7OH-DPAT binding was consistent with the labeling of dopamine D5 and D3 receptor subtypes, respectively. Twenty-five of the 50 patients with PD were retested after 3 months of therapy with levodopa or bromocriptine. Both treatments reduced the density of D1-like (p <0.001) and D2-like (p <0.001) receptors on PBL to values comparable to those of control subjects. The increased density of D1-like and D2-like receptors on PBL in de novo PD patients may represent an upregulation mechanism resulting from the diffuse impairment of the dopaminergic system in PD.