Reduced blood oxygenation level dependent connectivity is related to hypoperfusion in Alzheimer's disease

Functional connectivity of blood oxygenation level dependent signal fluctuations (BOLD-FC) is decreased in Alzheimer's disease (AD), and suggested to reflect reduced coherence in neural population activity; however, as both neuronal and vascular-hemodynamic processes underlie BOLD signals, impaired perfusion might also contribute to reduced BOLD-FC; 42 AD patients and 27 controls underwent simultaneous PET/MR imaging. Resting-state functional MRI assessed BOLD co-activity to quantify BOLD-FC, pulsed arterial spin labeling (pASL) assessed cerebral blood flow (CBF) as proxy for vascular hemodynamics, and F-18-fluorodeoxyglucose PET assessed glucose metabolism (GluMet) to index neuronal activity. Patients' BOLD-FC, CBF, and GluMet were reduced within the same precuneal parietal regions. BOLD-FC was positively associated with mean CBF, specifically in patients and controlled for GluMet levels, suggesting that BOLD-FC reductions correlate with pASL-derived hypoperfusion in AD, independently from F-18-fluorodeoxyglucose PET-derived hypometabolism. Data indicate that impaired vascular hemodynamic processes contribute to reduced BOLD connectivity in AD

Within-patient correspondence of amyloid-beta and intrinsic network connectivity in Alzheimer's disease

There is striking overlap between the spatial distribution of amyloid-beta pathology in patients with Alzheimer's disease and the spatial distribution of high intrinsic functional connectivity in healthy persons. This overlap suggests a mechanistic link between amyloid-beta and intrinsic connectivity, and indeed there is evidence in patients for the detrimental effects of amyloid-beta plaque accumulation on intrinsic connectivity in areas of high connectivity in heteromodal hubs, and particularly in the default mode network. However, the observed spatial extent of amyloid-beta exceeds these tightly circumscribed areas, suggesting that previous studies may have underestimated the negative impact of amyloid-beta on intrinsic connectivity. We hypothesized that the known positive baseline correlation between patterns of amyloid-beta and intrinsic connectivity may mask the larger extent of the negative effects of amyloid-beta on connectivity. Crucially, a test of this hypothesis requires the within-patient comparison of intrinsic connectivity and amyloid-beta distributions. Here we compared spatial patterns of amyloid-beta-plaques (measured by Pittsburgh compound B positron emission tomography) and intrinsic functional connectivity (measured by resting-state functional magnetic resonance imaging) in patients with prodromal Alzheimer's disease via spatial correlations in intrinsic networks covering fronto-parietal heteromodal cortices. At the global network level, we found that amyloid-beta and intrinsic connectivity patterns were positively correlated in the default mode and several fronto-parietal attention networks, confirming that amyloid-beta aggregates in areas of high intrinsic connectivity on a within-network basis. Further, we saw an internetwork gradient of the magnitude of correlation that depended on network plaque-load. After accounting for this globally positive correlation, local amyloid-beta-plaque concentration in regions of high connectivity co-varied negatively with intrinsic connectivity, indicating that amyloid-beta pathology adversely reduces connectivity anywhere in an affected network as a function of local amyloid-beta-plaque concentration. The local negative association between amyloid-beta and intrinsic connectivity was much more pronounced than conventional group comparisons of intrinsic connectivity would suggest. Our findings indicate that the negative impact of amyloid-beta on intrinsic connectivity in heteromodal networks is underestimated by conventional analyses. Moreover, our results provide first within-patient evidence for correspondent patterns of amyloid-beta and intrinsic connectivity, with the distribution of amyloid-beta pathology following functional connectivity gradients within and across intrinsic networks