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Armin Scheurich

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6 papers

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YNICL Journal 2015 Journal Article

Altered whole-brain white matter networks in preclinical Alzheimer's disease

Florian Udo Fischer
Dominik Wolf
Armin Scheurich
Andreas Fellgiebel

Surrogates of whole-brain white matter (WM) networks reconstructed using diffusion tensor imaging (DTI) are novel markers of structural brain connectivity. Global connectivity of networks has been found impaired in clinical Alzheimer's disease (AD) compared to cognitively healthy aging. We hypothesized that network alterations are detectable already in preclinical AD and investigated major global WM network properties. Other structural markers of neurodegeneration typically affected in prodromal AD but seeming largely unimpaired in preclinical AD were also examined. 12 cognitively healthy elderly with preclinical AD as classified by florbetapir-PET (mean age 73.4 ± 4.9) and 31 age-matched controls without cerebral amyloidosis (mean age 73.1 ± 6.7) from the ADNI were included. WM networks were reconstructed from DTI using tractography and graph theory. Indices of network capacity and the established imaging markers of neurodegeneration hippocampal volume, and cerebral glucose utilization as measured by fludeoxyglucose-PET were compared between the two groups. Additionally, we measured surrogates of global WM integrity (fractional anisotropy, mean diffusivity, volume). We found an increase of shortest path length and a decrease of global efficiency in preclinical AD. These results remained largely unchanged when controlling for WM integrity. In contrast, neither markers of neurodegeneration nor WM integrity were altered in preclinical AD subjects. Our results suggest an impairment of WM networks in preclinical AD that is detectable while other structural imaging markers do not yet indicate incipient neurodegeneration. Moreover, these findings are specific to WM networks and cannot be explained by other surrogates of global WM integrity.

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YNIMG Journal 2013 Journal Article

Metabolic and structural connectivity within the default mode network relates to working memory performance in young healthy adults

Igor Yakushev
Gael Chételat
Florian U. Fischer
Brigitte Landeau
Christine Bastin
Armin Scheurich
Audrey Perrotin
Mohamed Ali Bahri

Studies of functional connectivity suggest that the default mode network (DMN) might be relevant for cognitive functions. Here, we examined metabolic and structural connectivity between major DMN nodes, the posterior cingulate (PCC) and medial prefrontal cortex (MPFC), in relation to normal working memory (WM). DMN was captured using independent component analysis of [18F]fluorodeoxyglucose positron emission tomography (FDG-PET) data from 35 young healthy adults (27. 1±5. 1years). Metabolic connectivity, a correlation between FDG uptake in PCC and MPFC, was examined in groups of subjects with (relative to median) low (n=18) and high (n=17) performance on digit span backward test as an index of verbal WM. In addition, fiber tractography based on PCC and MPFC nodes as way points was performed in a subset of subjects. FDG uptake in the DMN nodes did not differ between high and low performers. However, significantly (p=0. 01) lower metabolic connectivity was found in the group of low performers. Furthermore, as compared to high performers, low performers showed lower density of the left superior cingulate bundle. Verbal WM performance is related to metabolic and structural connectivity within the DMN in young healthy adults. Metabolic connectivity as quantified with FDG-PET might be a sensitive marker of the normal variability in some cognitive functions.

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YNIMG Journal 2013 Journal Article

Relationships between brain metabolism decrease in normal aging and changes in structural and functional connectivity

Gaël Chételat
Brigitte Landeau
Eric Salmon
Igor Yakushev
Mohamed Ali Bahri
Florence Mézenge
Audrey Perrotin
Christine Bastin

Normal aging is characterized by brain glucose metabolism decline predominantly in the prefrontal cortex. The goal of the present study was to assess whether this change was associated with age-related alteration of white matter (WM) structural integrity and/or functional connectivity. FDG-PET data from 40 young and 57 elderly healthy participants from two research centers (n=49/48 in Center 1/2) were analyzed. WM volume from T1-weighted MRI (Center 1), fractional anisotropy from diffusion-tensor imaging (Center 2), and resting-state fMRI data (Center 1) were also obtained. Group comparisons were performed within each imaging modality. Then, positive correlations were assessed, within the elderly, between metabolism in the most affected region and the other neuroimaging modalities. Metabolism decline in the elderly predominated in the left inferior frontal junction (LIFJ). LIFJ hypometabolism was significantly associated with macrostructural and microstructural WM disturbances in long association fronto-temporo-occipital fibers, while no relationship was found with functional connectivity. The findings offer new perspectives to understand normal aging processes and open avenues for future studies to explore causality between age-related metabolism and connectivity changes.

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YNIMG Journal 2012 Journal Article

Cognitive reserve impacts on inter-individual variability in resting-state cerebral metabolism in normal aging

Christine Bastin
Igor Yakushev
Mohamed Ali Bahri
Andreas Fellgiebel
Francis Eustache
Brigitte Landeau
Armin Scheurich
Dorothée Feyers

There is a great deal of heterogeneity in the impact of aging on cognition and cerebral functioning. One potential factor contributing to individual differences among the elderly is the cognitive reserve, which designates the partial protection from the deleterious effects of aging that lifetime experience provides. Neuroimaging studies examining task-related activation in elderly people suggested that cognitive reserve takes the form of more efficient use of brain networks and/or greater ability to recruit alternative networks to compensate for age-related cerebral changes. In this exploratory multi-center study, we examined the relationships between cognitive reserve, as measured by education and verbal intelligence, and cerebral metabolism at rest (FDG-PET) in a sample of 74 healthy older participants. Higher degree of education and verbal intelligence was associated with less metabolic activity in the right posterior temporoparietal cortex and the left anterior intraparietal sulcus. Functional connectivity analyses of resting-state fMRI images in a subset of 41 participants indicated that these regions belong to the default mode network and the dorsal attention network respectively. Lower metabolism in the temporoparietal cortex was also associated with better memory abilities. The findings provide evidence for an inverse relationship between cognitive reserve and resting-state activity in key regions of two functional networks respectively involved in internal mentation and goal-directed attention.

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YNIMG Journal 2009 Journal Article

SPM-based count normalization provides excellent discrimination of mild Alzheimer's disease and amnestic mild cognitive impairment from healthy aging

Igor Yakushev
Alexander Hammers
Andreas Fellgiebel
Irene Schmidtmann
Armin Scheurich
Hans-Georg Buchholz
Jürgen Peters
Peter Bartenstein

Statistical comparisons of [18F]FDG PET scans between healthy subjects and patients with Alzheimer's disease (AD) or amnestic mild cognitive impairment (aMCI) using Statistical Parametric Mapping (SPM) usually require normalization of regional tracer uptake via ROIs defined using additional software. Here, we validate a simple SPM-based method for count normalization. FDG PET scans of 21 mild, 15 very mild AD, 11 aMCI patients and 15 age-matched controls were analyzed. First, we obtained relative increases in the whole patient sample compared to controls (i. e. areas relatively preserved in patients) with proportional scaling to the cerebral global mean (CGM). Next, average absolute counts within the cluster with the highest t-value were extracted. Statistical comparisons of controls versus three patients groups were then performed using count normalization to CGM, sensorimotor cortex (SMC) as standard, and to the cluster-derived counts. Compared to controls, relative metabolism in aMCI patients was reduced by 15%, 20%, and 23% after normalization to CGM, SMC, and cluster-derived counts, respectively, and 11%, 21%, and 25% in mild AD patients. Logistic regression analyses based on normalized values extracted from AD-typical regions showed that the metabolic values obtained using CGM, SMC, and cluster normalization correctly classified 81%, 89% and 92% of aMCI and controls; classification accuracies for AD groups (very mild and mild) were 91%, 97%, and 100%. The proposed algorithm of fully SPM-based count normalization allows for a substantial increase of statistical power in detecting very early AD-associated hypometabolism, and very high accuracy in discriminating mild AD and aMCI from healthy aging.

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YNIMG Journal 2005 Journal Article

Functional implications of hippocampal volume and diffusivity in mild cognitive impairment

Matthias J. Müller
Dirk Greverus
Paulo Roberto Dellani
Carsten Weibrich
Paulo R. Wille
Armin Scheurich
Peter Stoeter
Andreas Fellgiebel

Hippocampal atrophy has been related to mild cognitive impairment (MCI) and early Alzheimer disease (AD), but the diagnostic significance of cross-sectionally determined hippocampal volumes is still ambiguous. Diffusion-Tensor-Imaging (DTI) in MCI patients revealed an association of microstructural changes in hippocampal areas with verbal memory decline. MRI volumetry and DTI were combined to investigate 18 MCI patients attending a memory clinic, and 18 carefully age- and gender-matched healthy controls. Neuropsychological testing, high resolution T1-weighted volume MRI scans, and DTI scans with regions-of-interest in hippocampal areas were applied. Left hippocampal volume was significantly lower (−11%, P = 0. 02) in MCI patients than in control subjects. No significant differences were found for the right hippocampus (−4%). Mean diffusivity (MD) was significantly elevated in MCI patients vs. controls in left (+ 10%, P = 0. 002) and right hippocampal areas (+ 13%, P = 0. 02). Hippocampal volume and MD values were not significantly correlated. Combining left hippocampal volume and MD measures showed that lower left hippocampal volumes were associated with poor verbal memory performance particularly when co-occurring with high MD values. No comparable associations could be found regarding the right hippocampal formation and with respect to non-verbal memory function. The results demonstrate that microstructural abnormalities as revealed by DTI are very sensitive early indicators of hippocampal dysfunction. The combination of macro- and microstructural parameters in hippocampal areas could be promising in early detection of neurodegenerative processes.

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