Beta-amyloid burden in the temporal neocortex is related to hippocampal atrophy in elderly subjects without dementia

Recent Progress in Alzheimer's Disease Research, Part 3: Diagnosis and Treatment

The field of Alzheimer's disease (AD) research has grown exponentially over the past few decades, especially since the isolation and identification of amyloid-β from postmortem examination of the brains of AD patients. Recently, the Journal of Alzheimer's Disease (JAD) put forth approximately 300 research reports which were deemed to be the most influential research reports in the field of AD since 2010. JAD readers were asked to vote on these most influential reports. In this 3-part review, we review the results of the 300 most influential AD research reports to provide JAD readers with a readily accessible, yet comprehensive review of the state of contemporary research. Notably, this multi-part review identifies the "hottest" fields of AD research providing guidance for both senior investigators as well as investigators new to the field on what is the most pressing fields within AD research. Part 1 of this review covers pathogenesis, both on a molecular and macro scale. Part 2 review genetics and epidemiology, and part 3 covers diagnosis and treatment. This part of the review, diagnosis and treatment, reviews the latest diagnostic criteria, biomarkers, imaging, and treatments in AD.

In vivo visualization of tau deposits in corticobasal syndrome by 18F-THK5351 PET

OBJECTIVE

To determine whether ¹⁸F-THK5351 PET can be used to visualize tau deposits in brain lesions in live patients with corticobasal syndrome (CBS).

METHODS

We evaluated the in vitro binding of ³H-THK5351 in postmortem brain tissues from a patient with corticobasal degeneration (CBD). In clinical PET studies, ¹⁸F-THK5351 retention in 5 patients with CBS was compared to that in 8 age-matched normal controls and 8 patients with Alzheimer disease (AD).

RESULTS

³H-THK5351 was able to bind to tau deposits in the postmortem brain with CBD. In clinical PET studies, the 5 patients with CBS showed significantly higher ¹⁸F-THK5351 retention in the frontal, parietal, and globus pallidus than the 8 age-matched normal controls and patients with AD. Higher ¹⁸F-THK5351 retention was observed contralaterally to the side associated with greater cortical dysfunction and parkinsonism.

CONCLUSIONS

¹⁸F-THK5351 PET demonstrated high tracer signal in sites susceptible to tau deposition in patients with CBS. ¹⁸F-THK5351 should be considered as a promising candidate radiotracer for the in vivo imaging of tau deposits in CBS.

Relationship of Hippocampal Volume to Amyloid Burden across Diagnostic Stages of Alzheimer's Disease

AIMS

To assess how hippocampal volume (HV) from volumetric magnetic resonance imaging (vMRI) is related to the amyloid status at different stages of Alzheimer's disease (AD) and its relevance to patient care.

METHODS

We evaluated the ability of HV to predict the florbetapir positron emission tomography (PET) amyloid positive/negative status by group in healthy controls (HC, n = 170) and early/late mild cognitive impairment (EMCI, n = 252; LMCI, n = 136), and AD dementia (n = 75) subjects from the Alzheimer's Disease Neuroimaging Initiative Grand Opportunity (ADNI-GO) and ADNI2. Logistic regression analyses, including elastic net classification modeling with 10-fold cross-validation, were used with age and education as covariates.

RESULTS

HV predicted amyloid status only in LMCI using either logistic regression [area under the curve (AUC) = 0.71, p < 0.001] or elastic net classification modeling [positive predictive value (PPV) = 72.7%]. In EMCI, age (AUC = 0.70, p < 0.0001) and age and/or education (PPV = 63.1%), but not HV, predicted amyloid status.

CONCLUSION

Using clinical neuroimaging, HV predicted amyloid status only in LMCI, suggesting that HV is not a biomarker surrogate for amyloid PET in clinical applications across the full diagnostic spectrum.

In vivo assessment of iron content of the cerebral cortex in healthy aging using 7-Tesla T2*-weighted phase imaging

Accumulation of brain iron has been suggested as a biomarker of neurodegeneration. Increased iron has been seen in the cerebral cortex in postmortem studies of neurodegenerative diseases and healthy aging. Until recently, the diminutive thickness of the cortex and its relatively low iron content have hampered in vivo study of cortical iron accumulation. Using phase images of a T2*-weighted sequence at ultrahigh field strength (7 Tesla), we examined the iron content of 22 cortical regions in 70 healthy subjects aged 22-80 years. The cortex was automatically segmented and parcellated, and phase shift was analyzed using an in-house developed method. We found a significant increase in phase shift with age in 20 of 22 cortical regions, concurrent with current understanding of cortical iron accumulation. Our findings suggest that increased cortical iron content can be assessed in healthy aging in vivo. The high spatial resolution and sensitivity to iron of our method make it a potentially useful tool for studying cortical iron accumulation in healthy aging and neurodegenerative diseases.

Neuroimaging biomarkers in Alzheimer's disease and other dementias

In vivo imaging of β-amyloid (Aβ) has transformed the assessment of Aβ pathology and its changes over time, extending our insight into Aβ deposition in the brain by providing highly accurate, reliable, and reproducible quantitative statements of regional or global Aβ burden in the brain. This knowledge is essential for therapeutic trial recruitment and for the evaluation of anti-Aβ treatments. Although cross sectional evaluation of Aβ burden does not strongly correlate with cognitive impairment, it does correlate with cognitive (especially memory) decline and with a higher risk for conversion to AD in the aging population and MCI subjects. This suggests that Aβ deposition is a protracted pathological process starting well before the onset of symptoms. Longitudinal observations, coupled with different disease-specific biomarkers to assess potential downstream effects of Aβ are required to confirm this hypothesis and further elucidate the role of Aβ deposition in the course of Alzheimer's disease.

Surface-Based fMRI-Driven Diffusion Tractography in the Presence of Significant Brain Pathology: A Study Linking Structure and Function in Cerebral Palsy

Diffusion MRI (dMRI) tractography analyses are difficult to perform in the presence of brain pathology. Automated methods that rely on cortical parcellation for structural connectivity studies often fail, while manually defining regions is extremely time consuming and can introduce human error. Both methods also make assumptions about structure-function relationships that may not hold after cortical reorganisation. Seeding tractography with functional-MRI (fMRI) activation is an emerging method that reduces these confounds, but inherent smoothing of fMRI signal may result in the inclusion of irrelevant pathways. This paper describes a novel fMRI-seeded dMRI-analysis pipeline based on surface-meshes that reduces these issues and utilises machine-learning to generate task specific white matter pathways, minimising the requirement for manually-drawn ROIs. We directly compared this new strategy to a standard voxelwise fMRI-dMRI approach, by investigating correlations between clinical scores and dMRI metrics of thalamocortical and corticomotor tracts in 31 children with unilateral cerebral palsy. The surface-based approach successfully processed more participants (87%) than the voxel-based approach (65%), and provided significantly more-coherent tractography. Significant correlations between dMRI metrics and five clinical scores of function were found for the more superior regions of these tracts. These significant correlations were stronger and more frequently found with the surface-based method (15/20 investigated were significant; R² = 0.43–0.73) than the voxelwise analysis (2 sig. correlations; 0.38 & 0.49). More restricted fMRI signal, better-constrained tractography, and the novel track-classification method all appeared to contribute toward these differences.

High amyloid-β deposition related to depressive symptoms in older individuals with normal cognition: a pilot study

OBJECTIVE

Previous studies have reported depressive symptoms in the preclinical stages of Alzheimer's disease (AD). The objective of this study was to determine whether depressive symptoms are associated with cortical amyloid burden. In order to do this, we measured cortical amyloid via (11) C-labeled Pittsburgh Compound B ([(11) C]PIB) uptake using positron emission tomography (PET) in cognitively normal subjects.

METHODS

We performed [(11) C]PIB-PET in 29 cognitively normal, older participants. Depressive symptoms were assessed using the 15-item Geriatric Depression Scale (GDS). Aβ deposition was quantified by binding potential (BPND ), and the association between cortical mean BPND values and GDS scores was evaluated. Analysis of parametric BPND images was performed to examine the relationship between regional BPND and GDS scores.

RESULTS

We found a positive correlation between depressive symptoms and mean cortical PIB-BPND in groups of subjects with middle to high PIB-BPND . There was little change in GDS-depression score between subjects with low and middle PIB-BPND levels, while an increase in GDS was shown in the high PIB-BPND group. The main BPND increase was localized to the precuneus/posterior cingulate cortex (PCu/PCC) in subjects with high PIB-BPND , and we found a significant positive relationship between PIB-BPND in this area and depressive symptoms.

CONCLUSIONS

Emotional dysregulation because of Aβ neuropathology in the PCu/PCC may relate to depressive symptoms. More specifically, we found that older, cognitively normal patients with depressive episodes were more likely to have underlying AD pathology. Thus, depressive symptoms may increase the predictive ability of the identification of future AD cases. Copyright © 2016 John Wiley & Sons, Ltd.

Hippocampal Amyloid Burden with Downstream Fusiform Gyrus Atrophy Correlate with Face Matching Task Scores in Early Stage Alzheimer's Disease

Purpose: Neuronal activity during face matching shows co-activation of the fusiform gyrus (FG) and areas along the ventral visual network. To elucidate the mechanisms related to the facial discrimination deficits in Alzheimer’s disease (AD), the study evaluates the relationships between β-amyloid (Aβ) load and gray matter (GM) atrophy within the ventral visual network.

Methods: Comprehensive cognitive assessments and GM volumetry using 3-dimentional T1-weighted images and AV-45 positron emission tomography (PET) were studied in 44 patients with AD. We used AV-45 PET to measure regional Aβ to analyze the correlations between the regional neocortical AV-45 retention and atrophy in patients with AD.

Results: FG volume was positively correlated with the para-hippocampus (β = 0.565, P < 0.001), posterior cingulate cortex (PCC; β = 0.402, P < 0.001), and hippocampus volumes (β = 0.209, P = 0.044). After carefully confounded all possible factors simultaneously, the hippocampus standardized uptake value (SUV) ratio was independently associated with FG volume (β = −0.151, P = 0.017). Furthermore, volumes of the hippocampus (r = 0.473, P = 0.003), para-hippocampus (r = 0.515, P = 0.001), and FG (r = 0.383, P = 0.018) were associated with Benton’s facial recognition test (BFRT).

Conclusions: In conclusion, our study indicated that amyloid burden within the hippocampus might contribute to FG cortical hub GM atrophy. While the face matching task scores were related to the FG, hippocampus, and para-hippocampus volumes, concordant changes of the aforementioned three structures suggested the importance of the three ventral visual network hubs in AD.

Amyloid status imputed from a multimodal classifier including structural MRI distinguishes progressors from nonprogressors in a mild Alzheimer's disease clinical trial cohort

INTRODUCTION

Mild-Alzheimer's disease (AD) subjects without significant Aβ pathology represent a confounding finding for clinical trials because they may not progress clinically on the expected trajectory, adding variance into analyses where slowing of progression is being measured.

METHODS

A prediction model based on structural magnetic resonance imaging (MRI) in combination with baseline demographics and clinical measurements was used to impute Aβ status of a placebo-treated mild-AD sub-cohort (N = 385) of patients participating in global phase 3 trials. The clinical trajectories of this cohort were evaluated over 18 months duration of the trial, stratified by imputed Aβ status within a mixed-model repeated measures statistical framework.

RESULTS

In the imputed Aβ-positive cohort, both cognitive (ADAS-Cog14 and MMSE) and functional (ADCS-iADL) measures declined more rapidly than in the undifferentiated population.

DISCUSSION

Our results demonstrate imputing Aβ status from MRI scans in mild-AD subjects may be a useful screening tool in global clinical trials if amyloid measurement is not available.

Amyloid burden in cognitively normal elderly is associated with preferential hippocampal subfield volume loss

The earliest sites of brain atrophy in Alzheimer's disease are in the medial temporal lobe, following widespread cerebral cortical amyloid deposition. We assessed 74 cognitively normal participants with clinical measurements, amyloid-β-PET imaging, MRI, and a newly developed technique for MRI-based hippocampal subfield segmentation to determine the differential association of amyloid deposition and hippocampal subfield volume. Compared to amyloid-negative participants, amyloid-positive participants had significantly smaller hippocampal tail, presubiculum, subiculum, and total hippocampal gray matter volumes. We conclude that, prior to the development of cognitive impairment, atrophy in particular hippocampal subfields occurs preferentially with amyloid-β accumulation.

Prefrontal Lobe Brain Reserve Capacity with Resistance to Higher Global Amyloid Load and White Matter Hyperintensity Burden in Mild Stage Alzheimer's Disease

BACKGROUND

Amyloid deposition and white matter lesions (WMLs) in Alzheimer's disease (AD) are both considered clinically significant while a larger brain volume is thought to provide greater brain reserve (BR) against these pathological effects. This study identified the topography showing BR in patients with mild AD and explored the clinical balances among BR, amyloid, and WMLs burden.

METHODS

Thirty patients with AD were enrolled, and AV-45 positron emission tomography was conducted to measure the regional standardized uptake value ratio (SUVr) in 8 cortical volumes-of- interests (VOIs). The quantitative WMLs burden was measured from magnetic resonance imaging while the normalized VOIs volumes represented BR in this study. The cognitive test represented major clinical correlates.

RESULTS

Significant correlations between the prefrontal volume and global (r = 0.470, p = 0.024), but not regional (r = 0.264, p = 0.223) AV-45 SUVr were found. AD patients having larger regional volume in the superior- (r = 0.572, p = 0.004), superior medial- (r = 0.443, p = 0.034), and middle-prefrontal (r = 0.448, p = 0.032) regions had higher global AV-45 SUVr. For global WML loads, the prefrontal (r = -0.458, p = 0.019) and hippocampal volume (r = -0.469, p = 0.016) showed significant correlations while the prefrontal (r = -0.417, p = 0.043) or hippocampal volume (r = -0.422, p = 0.04) also predicted better composite memory scores. There were no interactions between amyloid SUVr and WML loads on the prefrontal volume.

CONCLUSIONS

BR of the prefrontal region might modulate the adverse global pathological burden caused by amyloid deposition. While prefrontal volume positively associated with hippocampal volume, WMLs had an adverse impact on the hippocampal volume that predicts memory performance in mild stage AD.

Role of Hybrid Brain Imaging in Neuropsychiatric Disorders

This is a focused review of imaging literature to scope the utility of hybrid brain imaging in neuropsychiatric disorders. The review focuses on brain imaging modalities that utilize hybrid (fusion) techniques to characterize abnormal brain molecular signals in combination with structural and functional changes that have been observed in neuropsychiatric disorders. An overview of clinical hybrid brain imaging technologies for human use is followed by a selective review of the literature that conceptualizes the use of these technologies in understanding basic mechanisms of major neuropsychiatric disorders and their therapeutics. Neuronal network abnormalities are highlighted throughout this review to scope the utility of hybrid imaging as a potential biomarker for each disorder.

Aβ-related hyperactivation in frontoparietal control regions in cognitively normal elderly

The accumulation of amyloid-beta (Aβ) peptides, a pathologic hallmark of Alzheimer's disease, has been associated with functional alterations in cognitively normal elderly, most often in the context of episodic memory with a particular emphasis on the medial temporal lobes. The topography of Aβ deposition, however, highly overlaps with frontoparietal control (FPC) regions implicated in cognitive control/working memory. To examine Aβ-related functional alternations in the FPC regions during a working memory task, we imaged 42 young and 57 cognitively normal elderly using functional magnetic resonance imaging during a letter Sternberg task with varying load. Based on (18)F-florbetaben-positron emission tomography scan, we determined older subjects' amyloid positivity (Aβ+) status. Within brain regions commonly recruited by all subject groups during the delay period, age and Aβ deposition were independently associated with load-dependent frontoparietal hyperactivation, whereas additional compensatory Aβ-related hyperactivity was found beyond the FPC regions. The present results suggest that Aβ-related hyperactivation is not specific to the episodic memory system but occurs in the PFC regions as well.

Is amyloid-β harmful to the brain? Insights from human imaging studies

Although the amyloid-β protein associated with the Alzheimer's disease plaque has been detectable in living people for over a decade, its importance in the pathogenesis of Alzheimer's disease is still debated. The frequent presence of amyloid-β in the brains of cognitively healthy older people has been interpreted as evidence against a causative role. If amyloid-β is crucial to the development of Alzheimer's disease, it should be associated with other Alzheimer's disease-like neurological changes. This review examines whether amyloid-β is associated with other biomarkers indicative of early Alzheimer's disease in normal older people. The preponderance of evidence links amyloid-β to functional change, progressive brain atrophy, and cognitive decline. Individuals at greatest risk of decline seem to be those with evidence of both amyloid-β and findings suggestive of neurodegeneration. The crucial question is thus how amyloid-β is related to brain degeneration and how these two processes interact to cause cognitive decline and dementia.

Mapping 3-year changes in gray matter and metabolism in Aβ-positive nondemented subjects

Gray matter (GM) atrophy and brain glucose hypometabolism are already detected in the predementia stages of Alzheimer's disease (AD), but the regional and longitudinal associations between the two are not well understood. Here, we analyzed the patterns of longitudinal atrophy (magnetic resonance imaging [MRI]) and (18)F-Fluorodeoxyglucose-positron emission tomography ([18F]FDG-PET) metabolism decline in 40 cognitively healthy control (HC) and 52 mildly impaired (mild cognitive impairment [MCI]) subjects during 3 years. Based on cerebrospinal fluid and brain amyloid-PET, the subjects were divided into amyloid-beta (Aβ)- and Aβ+ subgroups. In voxel-based and region of interest analyses, we compared the 3-year rates of change in GM and glucose metabolism between Aβ-subgroups, within each diagnostic group. In joint-independent component analyses, we assessed the patterns of covariation between longitudinal change in GM volume and glucose metabolism. MCI-Aβ+ showed faster atrophy than MCI-Aβ- within the temporal, medial temporal, and medial parietal lobes. HC-Aβ+ showed faster atrophy within the precuneus than HC-Aβ-. For FDG-PET metabolism, MCI-Aβ+ exhibited faster decline than MCI-Aβ- in temporoparietal regions, whereas no differences between HC subgroups were observed. Joint-independent component analysis showed that accelerated atrophy and metabolism decline correlated across distant brain regions for MCI-Aβ+. In conclusion, abnormally increased levels of Aβ in nondemented subjects were associated with accelerated decline in both GM and glucose metabolism, where both types of neurodegeneration progress in spatially divergent patterns.

The transitional association between β-amyloid pathology and regional brain atrophy

INTRODUCTION

Alzheimer's disease (AD) is characterized by the accumulation of β-amyloid (Aβ) associated with brain atrophy and cognitive decline. The functional form to model the association between Aβ and regional brain atrophy has not been well defined. To determine the relationship between Aβ and atrophy, we compared the performance of the usual dichotomization of cerebrospinal fluid (CSF) Aβ to identify subjects as Aβ+ and Aβ- with a trilinear spline model of CSF Aβ.

METHODS

One hundred and eighty-three subjects with mild cognitive impairment and 108 cognitively normal controls with baseline CSF Aβ and up to 4 years of longitudinal magnetic resonance imaging data from the Alzheimer's Disease Neuroimaging Initiative were analyzed using mixed-effects regression. Piecewise-linear splines were used to evaluate the nonlinear nature of the association between CSF Aβ and regional atrophy and to identify points of acceleration of atrophy with respect to Aβ. Several parameterizations of CSF Aβ were compared using likelihood ratio tests and the Akaike information criterion. Periods of acceleration of atrophy in which subjects transition from CSF Aβ negativity to CSF Aβ positivity were estimated from the spline models and tested for significance.

RESULTS

Spline models resulted in better fits for many temporal and parietal regions compared with the dichotomous models. The trilinear model showed that periods of acceleration of atrophy varied greatly by region with early changes seen in the insula, amygdala, precuneus, hippocampus, and other temporal regions, occurring before the clinical threshold for CSF Aβ positivity.

DISCUSSION

The use of piecewise-linear splines provides an improved model of the nonlinear association between CSF Aβ and regional atrophy in regions implicated in the progression of AD. The important biological finding of this work is that some brain regions show periods of accelerated volume loss well before the CSF Aβ42 threshold. This implies that signs of brain atrophy develop before the current conventional definition of "preclinical AD".

Basal Forebrain Atrophy Contributes to Allocentric Navigation Impairment in Alzheimer's Disease Patients

The basal forebrain degenerates in Alzheimer’s disease (AD) and this process is believed to contribute to the cognitive decline observed in AD patients. Impairment in spatial navigation is an early feature of the disease but whether basal forebrain dysfunction in AD is responsible for the impaired navigation skills of AD patients is not known. Our objective was to investigate the relationship between basal forebrain volume and performance in real space as well as computer-based navigation paradigms in an elderly cohort comprising cognitively normal controls, subjects with amnestic mild cognitive impairment and those with AD. We also tested whether basal forebrain volume could predict the participants’ ability to perform allocentric- vs. egocentric-based navigation tasks. The basal forebrain volume was calculated from 1.5 T magnetic resonance imaging (MRI) scans, and navigation skills were assessed using the human analog of the Morris water maze employing allocentric, egocentric, and mixed allo/egocentric real space as well as computerized tests. When considering the entire sample, we found that basal forebrain volume correlated with spatial accuracy in allocentric (cued) and mixed allo/egocentric navigation tasks but not the egocentric (uncued) task, demonstrating an important role of the basal forebrain in mediating cue-based spatial navigation capacity. Regression analysis revealed that, although hippocampal volume reflected navigation performance across the entire sample, basal forebrain volume contributed to mixed allo/egocentric navigation performance in the AD group, whereas hippocampal volume did not. This suggests that atrophy of the basal forebrain contributes to aspects of navigation impairment in AD that are independent of hippocampal atrophy.

Nonlinear cerebral atrophy patterns across the Alzheimer's disease continuum: impact of APOE4 genotype

The progression of Alzheimer's disease (AD) is characterized by complex trajectories of cerebral atrophy that are affected by interactions with age and apolipoprotein E allele ε4 (APOE4) status. In this article, we report the nonlinear volumetric changes in gray matter across the full biological spectrum of the disease, represented by the AD-cerebrospinal fluid (CSF) index. This index reflects the subject's level of pathology and position along the AD continuum. We also evaluated the associated impact of the APOE4 genotype. The atrophy pattern associated with the AD-CSF index was highly symmetrical and corresponded with the typical AD signature. Medial temporal structures showed different atrophy dynamics along the progression of the disease. The bilateral parahippocampal cortices and a parietotemporal region extending from the middle temporal to the supramarginal gyrus presented an initial increase in volume which later reverted. Similarly, a portion of the precuneus presented a rather linear inverse association with the AD-CSF index whereas some other clusters did not show significant atrophy until index values corresponded to positive CSF tau values. APOE4 carriers showed steeper hippocampal volume reductions with AD progression. Overall, the reported atrophy patterns are in close agreement with those mentioned in previous findings. However, the detected nonlinearities suggest that there may be different pathological processes taking place at specific moments during AD progression and reveal the impact of the APOE4 allele.

Association of plasma β-amyloid with MRI markers of structural brain aging the 3-City Dijon study

Cerebral β-amyloid (Aβ) deposition and atrophy are central features of Alzheimer disease. Studies of Alzheimer disease biomarkers have largely focused on Aβ in cerebrospinal fluid (CSF), and there is uncertainty as to what plasma Aβ may be a marker. We examined the association of Aβ levels in the plasma with magnetic resonance imaging (MRI)-markers of brain aging, including longitudinal changes in global and regional brain volumes, in dementia-free persons. We studied 1530 participants of the Three-City-Dijon cohort, aged 65-80 years. Plasma Aβ measurement and magnetic resonance imaging were performed at baseline and after a 4-year follow up. Total brain, gray matter, and hippocampal volume were estimated using voxel-based morphometry, and annualized change in brain volumes was calculated. Increased plasma Aβ1-40 was associated with lower baseline hippocampal volume. Although baseline plasma Aβ levels were not associated with longitudinal change in brain volumes, consistently high plasma Aβ1-40 levels were associated with faster total brain atrophy and consistently low plasma Aβ1-42/Aβ1-40 ratio, with increased total brain atrophy and gray matter atrophy. In dementia-free older adults, high plasma Aβ1-40 and low plasma Aβ1-42/Aβ1-40 ratio were associated with smaller hippocampal volume and accelerated global and regional brain atrophy respectively.

Joint effects of gray matter atrophy and altered functional connectivity on cognitive deficits in amnestic mild cognitive impairment patients

BACKGROUND

Gray matter (GM) atrophy and disrupted intrinsic functional connectivity (IFC) are often present in patients with amnestic mild cognitive impairment (aMCI), which shows high risk of developing into Alzheimer's disease. Little is known, however, about the relationship between GM atrophy and altered IFC, and whether they are related to cognitive decline.

METHOD

A total of 30 aMCI and 26 cognitively normal (CN) subjects were recruited for this study. Optimized voxel-based morphometric and resting-state functional connectivity magnetic resonance imaging approaches were performed to measure the GM volumes (GMVs) and atrophy-related IFC, respectively. Multivariate linear regression analysis was used to examine the effects of GM atrophy and IFC on cognitive performance across subjects, after controlling for the effects of age, education, gender and group.

RESULTS

Compared with CN subjects, aMCI subjects showed significantly reduced GMVs and decreased IFC in the frontal-parietal and medial temporal lobe systems. Multivariate regression analysis further demonstrated that the GMVs and decreased IFC simultaneously affected the cognitive function. Specifically, GMVs were positively correlated with cognitive performances, including global cognition and episodic memory, and showed a strong trend in correlation between GMVs and non-episodic memory, whilst IFC was positively correlated with the above three cognitive measures, across all subjects. In addition, significant correlation was found between GMVs and altered IFC strength across all subjects.

CONCLUSIONS

Our findings demonstrated that GMVs and IFC jointly contribute to cognitive performance, and combining quantitative information about GMVs and the strength of functional connectivity may serve as an indicator of cognitive deficits in non-demented elderly.

Increased levels of plasma amyloid-beta are related to cortical thinning and cognitive decline in cognitively normal elderly subjects

Plasma levels of circulating amyloid-beta (Aβ) peptides are of particular interest in Alzheimer' disease, but little is known about cognitive and cortical correlates of peripheral Aβ levels in normal aging. Here, we compared cognitive functioning, vascular risk factors, and patterns of cortical thickness between cognitively intact elderly subjects with low (N = 60) and high (N = 60) plasma Aβ levels (cutoffs: 225 pg/mL and 23 pg/mL for Aβ1-40 and Aβ1-42, respectively). Overall, subjects with high Aβ levels showed lower cognitive performance and thinner cortex than those with low Aβ levels. More specifically, subjects with high Aβ1-40 showed bilateral thinning of the prefrontal cortex, poorer objective memory, slower processing speed, and lower nonverbal reasoning skills, whereas subjects with high Aβ1-42 had thinner temporal lobe, poorer everyday memory, and increased levels of homocysteine. Overall, these results suggest that high plasma Aβ levels in normal elderly subjects are associated with subclinical markers of vulnerable aging, which may be helpful at predicting different trajectories of aging in cognitively intact older adults.

Amyloid burden in the hippocampus and default mode network: relationships with gray matter volume and cognitive performance in mild stage Alzheimer disease

Amyloid load, as measured by florbetapir positron emission tomography (PET) standardized uptake value ratio (SUVr), has high specificity in the diagnosis of Alzheimer disease (AD). As the posterior cingulate cortex (PCC) represents densely amyloid-affected regions early in AD, we hypothesized that amyloid load within the key hubs of the default mode networks (DMN) may result in local or distant interconnected gray matter (GM) volume atrophy, thereby affecting cognitive performance. Thirty AD patients with a clinical dementia rating sum of box score ≤2 were enrolled and underwent cognitive evaluation, 3-dimensional T1-weighted imaging and florbetapir PET. Volumes of interest (VOIs) included the hippocampus, lateral temporal region, and key hubs of the DMN [anterior cingulate cortex (ACC), PCC, posterior parietal, and precuneus]. The SUVr was calculated by florbetapir standard uptake value (SUV) within the T1-weighted image segmented GM VOIs divided by the cerebellar GM SUV. Our results suggested inverse correlations between ACC (ρ = -0.444, P = 0.016) and PCC SUVr (ρ = -0.443, P = 0.016) with PCC GM volume. In stepwise regression, the orientation scores were associated with PCC SUVr (β = 2.584, P = 0.02) and posterior parietal volume (β = -0.446, P = 0.04), whereas the word recall score was related to hippocampal volume (β = -0.391, P = 0.04). After removing the patients with a hippocampal VOI below the lowest tertile and adjusting for age, an inverse correlation was found between hippocampal volume and SUVr in the ACC (partial σ = -0.639, P = 0.002), precuneus (partial σ = -0.692, P = 0.002), and lateral temporal SUVr (partial σ = -0.604, P = 0.005). Our results suggest that amyloid burden within the key DMN regions may contribute to local and distant GM atrophy, and that this may explain the cognitive scores.

APOE ε4 genotype predicts memory for everyday activities

The apolipoprotein E (ApOE) ε4 allele is associated with neuropathological buildup of amyloid in the brain, and with lower performance on some laboratory measures of memory in some populations. In two studies, we tested whether ApOE genotype affects memory for everyday activities. In Study 1, participants aged 20-79 years old (n = 188) watched movies of actors engaged in daily activities and completed memory tests for the activities in the movies. In Study 2, cognitively healthy and demented older adults (n = 97) watched and remembered similar movies, and also underwent structural MRI scanning. All participants provided saliva samples for genetic analysis. In both samples we found that, in older adults, ApOE ε4 carriers demonstrated worse everyday memory performance than did ε4 noncarriers. In Study 2, ApOE ε4 carriers had smaller medial temporal lobes (MTL) volumes, and MTL volume mediated the relationship between ApOE genotype and everyday memory performance. These everyday memory tasks measure genetically determined cognitive decline that can occur prior to a clinical diagnosis of dementia. Further, these tasks are easily administered and may be a useful clinical tool in identifying ε4 carriers who may be at risk for MTL atrophy and further cognitive decline that is a common characteristic of the earliest stages of Alzheimer's disease.

Differences in functional brain connectivity alterations associated with cerebral amyloid deposition in amnestic mild cognitive impairment

Despite potential implications for the early detection of impending Alzheimer’s disease (AD), very little is known about the differences of large-scale brain networks between amnestic mild cognitive impairment (aMCI) with high cerebral amyloid-beta protein (Aβ) deposition (i.e., aMCI+) and aMCI with no or very little Aβ deposition (i.e., aMCI−). We first aimed to extend the current literature on altering intrinsic functional connectivity (FC) of the default mode network (DMN) and salience network (SN) from cognitively normal (CN) to AD dementia. Second, we further examined the differences of the DMN and the SN between aMCI−, aMCI+, and CN. Forty-three older adult (12 CN, 10 aMCI+, 10 aMCI−, and 11 AD dementia) subjects were included. All participants received comprehensive clinical and neuropsychological assessment, resting-state functional magnetic resonance imaging, structural MRI, and Pittsburgh compound-B-PET scans. FC data were preprocessed using multivariate exploratory linear optimized decomposition into independent components of FMRIB’s Software Library. Group comparisons were carried out using the “dual-regression” approach. In addition, to verify presence of gray matter volume changes with intrinsic functional network alterations, voxel-based morphometry was performed on the acquired T1-weighted data. As expected, AD dementia participants exhibited decreased FC in the DMN compared to CN (particularly in the precuneus and cingulate gyrus). The degree of alteration in the DMN in aMCI+ compared to CN was intermediate to that of AD. In contrast, aMCI− exhibited increased FC in the DMN compared to CN (primarily in the precuneus) as well as aMCI+. In terms of the SN, aMCI− exhibited decreased FC compared to both CN and aMCI+ particularly in the inferior frontal gyrus. FC within the SN in aMCI+ and AD did not differ from CN. Compared to CN, aMCI− showed atrophy in bilateral superior temporal gyri whereas aMCI+ showed atrophy in right precuneus. The results indicate that despite the similarity in cross-sectional cognitive features, aMCI− has quite different functional brain connectivity compared to aMCI+.

Brain structure and function as mediators of the effects of amyloid on memory

OBJECTIVE

The objective of this study was to test whether effects of β-amyloid (Aβ) pathology on episodic memory were mediated by metabolism and gray matter volume in the early stages of Alzheimer disease.

METHODS

This was a prospective cohort study. We measured baseline Aβ (using florbetapir-PET), brain function (using fluorodeoxyglucose-PET), and brain structure (using MRI). A mediation analysis was performed to test whether statistical effects of Aβ positivity on cross-sectional and longitudinal episodic memory were mediated by hypometabolism or regional gray matter volume in cognitively healthy controls (CN, n = 280) and mild cognitive impairment (MCI, n = 463).

RESULTS

Lower memory scores were associated with Aβ positivity (CN, mildly; MCI, strongly), smaller gray matter volumes (CN, few regions, including hippocampus; MCI, widespread), and hypometabolism. Smaller volumes and hypometabolism mediated effects of Aβ in MCI but not in CN. The strongest individual regions mediated up to approximately 25%. A combination of brain structure and function mediated up to approximately 40%. In several regions, gray matter atrophy and hypometabolism predicted episodic memory without being associated (at p < 0.05) with Aβ positivity.

CONCLUSIONS

Changes in brain structure and function appear to be, in part, downstream events from Aβ pathology, ultimately resulting in episodic memory deficits. However, Aβ pathology is also strongly related to memory deficits through mechanisms that are not quantified by these imaging measurements, and episodic memory decline is partly caused by Alzheimer disease-like brain changes independently of Aβ pathology.

Basal forebrain atrophy correlates with amyloid β burden in Alzheimer's disease

The brains of patients suffering from Alzheimer's disease (AD) have three classical pathological hallmarks: amyloid-beta (Aβ) plaques, tau tangles, and neurodegeneration, including that of cholinergic neurons of the basal forebrain. However the relationship between Aβ burden and basal forebrain degeneration has not been extensively studied. To investigate this association, basal forebrain volumes were determined from magnetic resonance images of controls, subjects with amnestic mild cognitive impairment (aMCI) and AD patients enrolled in the longitudinal Alzheimer's Disease Neuroimaging Initiative (ADNI) and Australian Imaging, Biomarkers and Lifestyle (AIBL) studies. In the AIBL cohort, these volumes were correlated within groups to neocortical gray matter retention of Pittsburgh compound B (PiB) from positron emission tomography images as a measure of Aβ load. The basal forebrain volumes of AD and aMCI subjects were significantly reduced compared to those of control subjects. Anterior basal forebrain volume was significantly correlated to neocortical PiB retention in AD subjects and aMCI subjects with high Aβ burden, whereas posterior basal forebrain volume was significantly correlated to neocortical PiB retention in control subjects with high Aβ burden. Therefore this study provides new evidence for a correlation between neocortical Aβ accumulation and basal forebrain degeneration. In addition, cluster analysis showed that subjects with a whole basal forebrain volume below a determined cut-off value had a 7 times higher risk of having a worse diagnosis within ~18 months.

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The link between amyloid (Aβ) and basal forebrain degeneration in AD is unclear.

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We find that basal forebrain volumes are correlated with neocortical Aβ burden.

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Basal forebrain volume correlates with Aβ burden in at-risk control subjects.

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Basal forebrain atrophy delineates subjects at increased risk of progressing to AD.

Advances in MRI biomarkers for the diagnosis of Alzheimer's disease

With the prevalence of Alzheimer's disease (AD) predicted to increase substantially over the coming decades, the development of effective biomarkers for the early detection of the disease is paramount. In this short review, the main neuroimaging techniques which have shown potential as biomarkers for AD are introduced, with a focus on MRI. Structural MRI measures of the hippocampus and medial temporal lobe are still the most clinically validated biomarkers for AD, but newer techniques such as functional MRI and diffusion tensor imaging offer great scope in tracking changes in the brain, particularly in functional and structural connectivity, which may precede gray matter atrophy. These new advances in neuroimaging methods require further development and crucially, standardization; however, before they are used as biomarkers to aid in the diagnosis of AD.

Non-Alzheimer's disease-related memory impairment and dementia

Although Alzheimer's disease (AD) is a common cause of memory impairment and dementia in the elderly disturbed memory function is a widespread subjective and/or objective symptom in a variety of medical conditions. The early detection and correct distinction of AD from non-AD memory impairment is critically important to detect possibly treatable and reversible underlying causes. In the context of clinical research, it is crucial to correctly distinguish between AD or non-AD memory impairment in order to build homogenous study populations for the assessment of new therapeutic possibilities. The distinction of AD from non-AD memory impairment may be difficult, especially in mildly affected patients, due to an overlap of clinical symptoms and biomarker alterations between AD and certain non-AD conditions. This review aims to describe recent aspects of the differential diagnosis of AD and non-AD related memory impairment and how these may be considered in the presence of memory deficits.

Emerging β-amyloid pathology and accelerated cortical atrophy

IMPORTANCE

The effect of β-amyloid (Aβ) accumulation on regional structural brain changes in early stages of Alzheimer disease (AD) is not well understood.

OBJECTIVE

To test the hypothesis that the development of Aβ pathology is related to increased regional atrophy in the brains of cognitively normal (CN) persons.

DESIGN, SETTING, AND PARTICIPANTS

Longitudinal clinicobiomarker cohort study involving 47 CN control subjects and 15 patients with AD dementia. All participants underwent repeated cerebrospinal fluid Aβ42 and structural magnetic resonance imaging measurements for up to 4 years. Cognitively normal controls were classified using the longitudinal cerebrospinal fluid Aβ42 data and included 13 stable Aβ negative (normal baseline Aβ42 levels, with less than the median reduction over time), 13 declining Aβ negative (normal baseline Aβ42 levels, with greater than the median reduction over time), and 21 Aβ positive (pathologic baseline Aβ42 levels). All 15 patients with AD dementia were Aβ positive.

MAIN OUTCOMES AND MEASURES

Group effects on regional gray matter volumes at baseline and over time, tested by linear mixed-effects models.

RESULTS

Baseline gray matter volumes were similar among the CN Aβ groups, but atrophy rates were increased in frontoparietal regions in the declining Aβ-negative and Aβ-positive groups and in amygdala and temporal regions in the Aβ-positive group. Aβ-positive patients with AD dementia had further increased atrophy rates in hippocampus and temporal and cingulate regions.

CONCLUSIONS AND RELEVANCE

Emerging Aβ pathology is coupled to increased frontoparietal (but not temporal) atrophy rates. Atrophy rates peak early in frontoparietal regions but accelerate in hippocampus, temporal, and cingulate regions as the disease progresses to dementia. Early-stage Aβ pathology may have mild effects on local frontoparietal cortical integrity while effects in temporal regions appear later and accelerate, leading to the atrophy pattern typically seen in AD.

Astrocytosis measured by ¹¹C-deprenyl PET correlates with decrease in gray matter density in the parahippocampus of prodromal Alzheimer's patients

PURPOSE

The Alzheimer's disease (AD) pathology is characterized by fibrillar amyloid deposits and neurofibrillary tangles, as well as the activation of astrocytosis, microglia activation, atrophy, dysfunctional synapse, and cognitive impairments. The aim of this study was to test the hypothesis that astrocytosis is correlated with reduced gray matter density in prodromal AD.

METHODS

Twenty patients with AD or mild cognitive impairment (MCI) underwent multi-tracer positron emission tomography (PET) studies with (11)C-Pittsburgh compound B ((11)C-PIB), (18) F-Fluorodeoxyglucose ((18) F-FDG), and (11)C-deuterium-L-deprenyl ((11)C-DED) PET imaging, as well as magnetic resonance imaging (MRI) scanning, cerebrospinal fluid (CSF) biomarker analysis, and neuropsychological assessments. The parahippocampus was selected as a region of interest, and each value was calculated for four different imaging modalities. Correlation analysis was applied between DED slope values and gray matter (GM) densities by MRI. To further explore possible relationships, correlation analyses were performed between the different variables, including the CSF biomarker.

RESULTS

A significant negative correlation was obtained between DED slope values and GM density in the parahippocampus in PIB-positive (PIB + ve) MCI patients (p = 0.025) (prodromal AD). Furthermore, in exploratory analyses, a positive correlation was observed between PIB-PET retention and DED binding in AD patients (p = 0.014), and a negative correlation was observed between PIB retention and CSF Aβ42 levels in MCI patients (p = 0.021), while the GM density and CSF total tau levels were negatively correlated in both PIB + ve MCI (p = 0.002) and MCI patients (p = 0.001). No significant correlation was observed with FDG-PET and with any of the other PET, MRI, or CSF biomarkers.

CONCLUSIONS

High astrocytosis levels in the parahippocampus of PIB + ve MCI (prodromal AD) patients suggest an early preclinical influence on cellular tissue loss. The lack of correlation between astrocytosis and CSF tau levels, and a positive correlation between astrocytosis and fibrillar amyloid deposition in clinical demented AD together indicate that parahippocampal astrocytosis might have some causality within the amyloid pathology.

Amyloid imaging in Alzheimer's disease: a literature review

Therapies targeting amyloid-β peptide currently represent approximately 50% of drugs now being developed for Alzheimer's disease. Some, including active and passive anti-Aβ immunotherapy, directly target the amyloid plaques. The new amyloid tracers are increasingly being included in the proposed updated diagnostic criteria, and may allow earlier diagnosis. Those targeting amyloid-β peptide allow identification of amyloid plaques in vivo. We need to gain insight into all aspects of their application. As florbetapir (Amyvid™) and flutemetamol (Vizamyl™) have received marketing authorization, clinicians require deeper knowledge to be rationally used in diagnosis. In this paper, we review both completed and ongoing observational, longitudinal and interventional studies of these tracers, our main objective being to show the performance of the four most commonly used tracers and their validation.

β-Amyloid is associated with aberrant metabolic connectivity in subjects with mild cognitive impairment

Positron emission tomography (PET) studies using [18F]2-fluoro-2-deoxyglucose (FDG) have identified a well-defined pattern of glucose hypometabolism in Alzheimer's disease (AD). The assessment of the metabolic relationship among brain regions has the potential to provide unique information regarding the disease process. Previous studies of metabolic correlation patterns have demonstrated alterations in AD subjects relative to age-matched, healthy control subjects. The objective of this study was to examine the associations between β-amyloid, apolipoprotein E ɛ4 (APOE ɛ4) genotype, and metabolic correlations patterns in subjects diagnosed with mild cognitive impairment (MCI). Mild cognitive impairment subjects from the Alzheimer's Disease Neuroimaging Initiative (ADNI) study were categorized into β-amyloid-low and β-amyloid-high groups, based on quantitative analysis of [18F]florbetapir PET scans, and APOE ɛ4 non-carriers and carriers based on genotyping. We generated voxel-wise metabolic correlation strength maps across the entire cerebral cortex for each group, and, subsequently, performed a seed-based analysis. We found that the APOE ɛ4 genotype was closely related to regional glucose hypometabolism, while elevated, fibrillar β-amyloid burden was associated with specific derangements of the metabolic correlation patterns.

A blood-based predictor for neocortical Aβ burden in Alzheimer's disease: results from the AIBL study

Dementia is a global epidemic with Alzheimer's disease (AD) being the leading cause. Early identification of patients at risk of developing AD is now becoming an international priority. Neocortical Aβ (extracellular β-amyloid) burden (NAB), as assessed by positron emission tomography (PET), represents one such marker for early identification. These scans are expensive and are not widely available, thus, there is a need for cheaper and more widely accessible alternatives. Addressing this need, a blood biomarker-based signature having efficacy for the prediction of NAB and which can be easily adapted for population screening is described. Blood data (176 analytes measured in plasma) and Pittsburgh Compound B (PiB)-PET measurements from 273 participants from the Australian Imaging, Biomarkers and Lifestyle (AIBL) study were utilised. Univariate analysis was conducted to assess the difference of plasma measures between high and low NAB groups, and cross-validated machine-learning models were generated for predicting NAB. These models were applied to 817 non-imaged AIBL subjects and 82 subjects from the Alzheimer's Disease Neuroimaging Initiative (ADNI) for validation. Five analytes showed significant difference between subjects with high compared to low NAB. A machine-learning model (based on nine markers) achieved sensitivity and specificity of 80 and 82%, respectively, for predicting NAB. Validation using the ADNI cohort yielded similar results (sensitivity 79% and specificity 76%). These results show that a panel of blood-based biomarkers is able to accurately predict NAB, supporting the hypothesis for a relationship between a blood-based signature and Aβ accumulation, therefore, providing a platform for developing a population-based screen.

Accelerating cortical thinning: unique to dementia or universal in aging?

Does accelerated cortical atrophy in aging, especially in areas vulnerable to early Alzheimer's disease (AD), unequivocally signify neurodegenerative disease or can it be part of normal aging? We addressed this in 3 ways. First, age trajectories of cortical thickness were delineated cross-sectionally (n = 1100) and longitudinally (n = 207). Second, effects of undetected AD on the age trajectories were simulated by mixing the sample with a sample of patients with very mild to moderate AD. Third, atrophy in AD-vulnerable regions was examined in older adults with very low probability of incipient AD based on 2-year neuropsychological stability, CSF Aβ(1-42) levels, and apolipoprotein ε4 negativity. Steady decline was seen in most regions, but accelerated cortical thinning in entorhinal cortex was observed across groups. Very low-risk older adults had longitudinal entorhinal atrophy rates similar to other healthy older adults, and this atrophy was predictive of memory change. While steady decline in cortical thickness is the norm in aging, acceleration in AD-prone regions does not uniquely signify neurodegenerative illness but can be part of healthy aging. The relationship between the entorhinal changes and changes in memory performance suggests that non-AD mechanisms in AD-prone areas may still be causative for cognitive reductions.

Parallel ICA of FDG-PET and PiB-PET in three conditions with underlying Alzheimer's pathology

The relationships between clinical phenotype, β-amyloid (Aβ) deposition and neurodegeneration in Alzheimer's disease (AD) are incompletely understood yet have important ramifications for future therapy. The goal of this study was to utilize multimodality positron emission tomography (PET) data from a clinically heterogeneous population of patients with probable AD in order to: (1) identify spatial patterns of Aβ deposition measured by ((11)C)-labeled Pittsburgh Compound B (PiB-PET) and glucose metabolism measured by FDG-PET that correlate with specific clinical presentation and (2) explore associations between spatial patterns of Aβ deposition and glucose metabolism across the AD population. We included all patients meeting the criteria for probable AD (NIA-AA) who had undergone MRI, PiB and FDG-PET at our center (N = 46, mean age 63.0 ± 7.7, Mini-Mental State Examination 22.0 ± 4.8). Patients were subclassified based on their cognitive profiles into an amnestic/dysexecutive group (AD-memory; n = 27), a language-predominant group (AD-language; n = 10) and a visuospatial-predominant group (AD-visuospatial; n = 9). All patients were required to have evidence of amyloid deposition on PiB-PET. To capture the spatial distribution of Aβ deposition and glucose metabolism, we employed parallel independent component analysis (pICA), a method that enables joint analyses of multimodal imaging data. The relationships between PET components and clinical group were examined using a Receiver Operator Characteristic approach, including age, gender, education and apolipoprotein E ε4 allele carrier status as covariates. Results of the first set of analyses independently examining the relationship between components from each modality and clinical group showed three significant components for FDG: a left inferior frontal and temporoparietal component associated with AD-language (area under the curve [AUC] 0.82, p = 0.011), and two components associated with AD-visuospatial (bilateral occipito-parieto-temporal [AUC 0.85, p = 0.009] and right posterior cingulate cortex [PCC]/precuneus and right lateral parietal [AUC 0.69, p = 0.045]). The AD-memory associated component included predominantly bilateral inferior frontal, cuneus and inferior temporal, and right inferior parietal hypometabolism but did not reach significance (AUC 0.65, p = 0.062). None of the PiB components correlated with clinical group. Joint analysis of PiB and FDG with pICA revealed a correlated component pair, in which increased frontal and decreased PCC/precuneus PiB correlated with decreased FDG in the frontal, occipital and temporal regions (partial r = 0.75, p < 0.0001). Using multivariate data analysis, this study reinforced the notion that clinical phenotype in AD is tightly linked to patterns of glucose hypometabolism but not amyloid deposition. These findings are strikingly similar to those of univariate paradigms and provide additional support in favor of specific involvement of the language network, higher-order visual network, and default mode network in clinical variants of AD. The inverse relationship between Aβ deposition and glucose metabolism in partially overlapping brain regions suggests that Aβ may exert both local and remote effects on brain metabolism. Applying multivariate approaches such as pICA to multimodal imaging data is a promising approach for unraveling the complex relationships between different elements of AD pathophysiology.

Multimodal MRI-based Imputation of the Aβ+ in Early Mild Cognitive Impairment

Objective

The primary goal of this study was to identify brain atrophy from structural MRI (magnetic resonance imaging) and cerebral blood flow (CBF) patterns from arterial spin labeling perfusion MRI that are best predictors of the Aβ-burden, measured as composite ¹⁸F-AV45-PET (positron emission tomography) uptake, in individuals with early mild cognitive impairment (MCI). Furthermore, another objective was to assess the relative importance of imaging modalities in classification of Aβ+/Aβ− early MCI.

Methods

Sixty-seven Alzheimer's Disease Neuroimaging Initiative (ADNI)-GO/2 participants with early MCI were included. Voxel-wise anatomical shape variation measures were computed by estimating the initial diffeomorphic mapping momenta from an unbiased control template. CBF measures normalized to average motor cortex CBF were mapped onto the template space. Using partial least squares regression, we identified the structural and CBF signatures of Aβ after accounting for normal cofounding effects of age, gender, and education.

Results

¹⁸F-AV45-positive early MCIs could be identified with 83% classification accuracy, 87% positive predictive value, and 84% negative predictive value by multidisciplinary classifiers combining demographics data, ApoE ε4-genotype, and a multimodal MRI-based Aβ score.

Interpretation

Multimodal MRI can be used to predict the amyloid status of early-MCI individuals. MRI is a very attractive candidate for the identification of inexpensive and noninvasive surrogate biomarkers of Aβ deposition. Our approach is expected to have value for the identification of individuals likely to be Aβ+ in circumstances where cost or logistical problems prevent Aβ detection using cerebrospinal fluid analysis or Aβ-PET. This can also be used in clinical settings and clinical trials, aiding subject recruitment and evaluation of treatment efficacy. Imputation of the Aβ-positivity status could also complement Aβ-PET by identifying individuals who would benefit the most from this assessment.

What is normal in normal aging? Effects of aging, amyloid and Alzheimer's disease on the cerebral cortex and the hippocampus

What can be expected in normal aging, and where does normal aging stop and pathological neurodegeneration begin? With the slow progression of age-related dementias such as Alzheimer's disease (AD), it is difficult to distinguish age-related changes from effects of undetected disease. We review recent research on changes of the cerebral cortex and the hippocampus in aging and the borders between normal aging and AD. We argue that prominent cortical reductions are evident in fronto-temporal regions in elderly even with low probability of AD, including regions overlapping the default mode network. Importantly, these regions show high levels of amyloid deposition in AD, and are both structurally and functionally vulnerable early in the disease. This normalcy-pathology homology is critical to understand, since aging itself is the major risk factor for sporadic AD. Thus, rather than necessarily reflecting early signs of disease, these changes may be part of normal aging, and may inform on why the aging brain is so much more susceptible to AD than is the younger brain. We suggest that regions characterized by a high degree of life-long plasticity are vulnerable to detrimental effects of normal aging, and that this age-vulnerability renders them more susceptible to additional, pathological AD-related changes. We conclude that it will be difficult to understand AD without understanding why it preferably affects older brains, and that we need a model that accounts for age-related changes in AD-vulnerable regions independently of AD-pathology.

Immunotherapy for the treatment of Alzheimer's disease: amyloid-β or tau, which is the right target?

Alzheimer's disease (AD) is characterized by the presence of amyloid plaques composed mainly of amyloid-β (Aβ) protein. Overproduction or slow clearance of Aβ initiates a cascade of pathologic events that may lead to formation of neurofibrillary tangles, neuronal cell death, and dementia. Although immunotherapy in animal models has been demonstrated to be successful at removing plaques or prefibrillar forms of Aβ, clinical trials have yielded disappointing results. The lack of substantial cognitive improvement obtained by targeting Aβ raises the question of whether or not this is the correct target. Another important pathologic process in the AD brain is tau aggregation, which seems to become independent once initiated. Recent studies targeting tau in AD mouse models have displayed evidence of cognitive improvement, providing a novel therapeutic approach for the treatment of AD. In this review, we describe new advances in immunotherapy targeting Aβ peptide and tau protein, as well as future directions.

Cortical surface-based analysis reduces bias and variance in kinetic modeling of brain PET data

Exploratory (i.e., voxelwise) spatial methods are commonly used in neuroimaging to identify areas that show an effect when a region-of-interest (ROI) analysis cannot be performed because no strong a priori anatomical hypothesis exists. However, noise at a single voxel is much higher than noise in a ROI making noise management critical to successful exploratory analysis. This work explores how preprocessing choices affect the bias and variability of voxelwise kinetic modeling analysis of brain positron emission tomography (PET) data. These choices include the use of volume- or cortical surface-based smoothing, level of smoothing, use of voxelwise partial volume correction (PVC), and PVC masking threshold. PVC was implemented using the Muller-Gartner method with the masking out of voxels with low gray matter (GM) partial volume fraction. Dynamic PET scans of an antagonist serotonin-4 receptor radioligand ([(11)C]SB207145) were collected on sixteen healthy subjects using a Siemens HRRT PET scanner. Kinetic modeling was used to compute maps of non-displaceable binding potential (BPND) after preprocessing. The results showed a complicated interaction between smoothing, PVC, and masking on BPND estimates. Volume-based smoothing resulted in large bias and intersubject variance because it smears signal across tissue types. In some cases, PVC with volume smoothing paradoxically caused the estimated BPND to be less than when no PVC was used at all. When applied in the absence of PVC, cortical surface-based smoothing resulted in dramatically less bias and the least variance of the methods tested for smoothing levels 5mm and higher. When used in combination with PVC, surface-based smoothing minimized the bias without significantly increasing the variance. Surface-based smoothing resulted in 2-4 times less intersubject variance than when volume smoothing was used. This translates into more than 4 times fewer subjects needed in a group analysis to achieve similarly powered statistical tests. Surface-based smoothing has less bias and variance because it respects cortical geometry by smoothing the PET data only along the cortical ribbon and so does not contaminate the GM signal with that of white matter and cerebrospinal fluid. The use of surface-based analysis in PET should result in substantial improvements in the reliability and detectability of effects in exploratory PET analysis, with or without PVC.

Contemplating Alzheimer's disease and the contribution of white matter hyperintensities

As the older adult segment of the population increases, Alzheimer's disease (AD) has emerged as a significant public health epidemic. Over the past 3 decades, advances in the understanding of the biology of AD have led to a somewhat unified hypothesis of disease pathogenesis that emphasizes the precipitating role of beta amyloid protein. However, several lines of evidence suggest that multiple pathologies are necessary for clinical manifestation of the disease. Our focus over the past several years has been on the contribution of small vessel cerebrovascular disease, visualized as white matter hyperintensities (WMH) on magnetic resonance imaging, to AD. White matter hyperintensity volume, particularly in parietal regions, is elevated among individuals with and at risk for AD, predicts future diagnosis of AD, predicts the rate of progression of cognitive symptoms among individuals with AD, and increases over time among individuals destined to develop AD. White matter hyperintensities may represent an independent source of impairment and/or may interact more fundamentally with "primary" AD pathology. Future work should focus on more inclusive models of that better define "normal" vs "pathological" aging.

Enabling a multidisciplinary approach to the study of ageing and Alzheimer's disease: an update from the Australian Imaging Biomarkers and Lifestyle (AIBL) study

The Australian Imaging Biomarkers and Lifestyle (AIBL) study is a longitudinal study of 1,112 volunteers from healthy, mild cognitive impairment (MCI) and Alzheimer's disease (AD) populations who are assessed at 18-month intervals in order to enable prospective research into ageing and AD. Using a multidisciplinary battery, AIBL assessments comprise the extensive study of clinical factors and cognitive function, collection of blood and cerebrospinal fluid (CSF) samples for biomarker discovery, structural and β-amyloid (Aβ) neuroimaging, and obtaining information on diet and physical activity patterns of the cohort. Now in its seventh year, AIBL is part of a substantial international effort to prospectively study the relationships between clinical characteristics and putative AD biomarkers in groups who carry different risk factors for AD. The identification of biomarkers would provide a window of opportunity to assess AD risk in individuals prior to the onset of advanced clinical symptoms, in addition to facilitating testing of therapeutic and lifestyle interventions likely to emerge within the next decade that prevent or delay symptom emergence in those at high risk for developing AD. In this paper, we present key findings from the AIBL study and discuss how they contribute to our understanding of AD pathogenesis and diagnosis.

Selective worsening of brain injury biomarker abnormalities in cognitively normal elderly persons with β-amyloidosis

IMPORTANCE

The appearance of β-amyloidosis and brain injury biomarkers in cognitively normal (CN) persons is thought to define risk for the future development of cognitive impairment due to Alzheimer disease (AD), but their interaction is poorly understood.

OBJECTIVE

To test the hypothesis that the joint presence of β-amyloidosis and brain injury biomarkers would lead to more rapid neurodegeneration. DESIGN Longitudinal cohort study.

SETTING

Population-based Mayo Clinic Study of Aging.

PARTICIPANTS

One hundred ninety-one CN persons (median age, 77 years; range, 71-93 years) in the Mayo Clinic Study of Aging who underwent magnetic resonance, fludeoxyglucose F 18 (FDG) positron emission tomography (PET), and Pittsburgh Compound B (PiB) PET imaging at least twice 15 months apart. Participants were grouped according to the recommendations of the National Institute on Aging-Alzheimer Association preclinical AD criteria based on the presence of β-amyloidosis, defined as a PiB PET standardized uptake value ratio (SUVr) greater than 1.5, alone (stage 1) or with brain injury (stage 2 + 3), defined as hippocampal atrophy or FDG hypometabolism. We also studied a group of patients with mild cognitive impairment (n = 17) or dementia (n = 9) from the Mayo Clinic Study of Aging or the Mayo Alzheimer Center with similar follow-up times who had undergone comparable imaging and had a PiB PET SUVr greater than 1.5.

MAIN OUTCOMES AND MEASURES

Rate of change of cortical volume on volumetric magnetic resonance images and rate of change of glucose metabolism on FDG PET scan results.

RESULTS

There were 25 CN participants with both high PiB retention and low hippocampal volume or FDG hypometabolism at baseline (preclinical AD stages 2 + 3). On follow-up scans, the preclinical AD stage 2 + 3 participants had greater loss of medial temporal lobe volume and greater glucose hypometabolism in the medial temporal lobe compared with the other CN groups. The changes were similar to those in the cognitively impaired participants. Extratemporal regions did not show similar changes.

CONCLUSIONS AND RELEVANCE

Higher rates of medial temporal neurodegeneration occur in CN individuals who, on their initial scans, had abnormal levels of both β-amyloid and brain injury biomarkers. Although preclinical AD is currently only a research topic, the description of its brain structural changes will be critical for trials designed to prevent or forestall dementia due to AD.

STroke imAging pRevention and Treatment (START): A Longitudinal Stroke Cohort Study: Clinical Trials Protocol

Rationale

Stroke and poststroke depression are common and have a profound and ongoing impact on an individual's quality of life. However, reliable biological correlates of poststroke depression and functional outcome have not been well established in humans.

Aims

Our aim is to identify biological factors, molecular and imaging, associated with poststroke depression and recovery that may be used to guide more targeted interventions.

Design

In a longitudinal cohort study of 200 stroke survivors, the START – STroke imAging pRevention and Treatment cohort, we will examine the relationship between gene expression, regulator proteins, depression, and functional outcome. Stroke survivors will be investigated at baseline, 24 h, three-days, three-months, and 12 months poststroke for blood-based biological associates and at days 3–7, three-months, and 12 months for depression and functional outcomes. A sub-group ( n = 100), the PrePARE: Prediction and Prevention to Achieve optimal Recovery Endpoints after stroke cohort, will also be investigated for functional and structural changes in putative depression-related brain networks and for additional cognition and activity participation outcomes. Stroke severity, diet, and lifestyle factors that may influence depression will be monitored. The impact of depression on stroke outcomes and participation in previous life activities will be quantified.

Study Outcomes

Clinical significance lies in the identification of biological factors associated with functional outcome to guide prevention and inform personalized and targeted treatments. Evidence of associations between depression, gene expression and regulator proteins, functional and structural brain changes, lifestyle and functional outcome will provide new insights for mechanism-based models of poststroke depression.

Disclosure of amyloid imaging results to research participants: has the time come?

Amyloid imaging is becoming an increasingly popular tool in clinical research on Alzheimer's disease. In early studies, questions of whether to disclose amyloid imaging results were largely mooted by the immature state of the science. Lack of clarity as to what constituted a positive scan and what a positive scan meant from a prognostic or diagnostic perspective required investigators to advise research participants that, because their scans could not be meaningfully interpreted in a clinical sense, individual research scan results would not be disclosed. With a focus on the most widely used research tracer, Pittsburgh Compound B (PiB), we consider how advances in amyloid imaging are raising new questions about the appropriateness of withholding research results from study participants. We conclude that, although it remains advisable to withhold amyloid scan results from cognitively normal participants, it is no longer reasonable to uphold policies that unilaterally advise research participants with documented cognitive impairment that their PiB scans are uninterpretable. We outline circumstances that we believe compel investigators to provide research participants with the option of receiving their PiB scan results in a carefully managed fashion. Our findings can potentially be generalized to research involving all validated amyloid tracers.

Cholinergic basal forebrain atrophy predicts amyloid burden in Alzheimer's disease

We compared accuracy of hippocampus and basal forebrain cholinergic system (BFCS) atrophy to predict cortical amyloid burden in 179 cognitively normal subjects (CN), 269 subjects with early stages of mild cognitive impairment (MCI), 136 subjects with late stages of MCI, and 86 subjects with Alzheimer's disease (AD) dementia retrieved from the Alzheimer's Disease Neuroimaging Initiative database. Hippocampus and BFCS volumes were determined from structural magnetic resonance imaging scans at 3 Tesla, and cortical amyloid load from AV45 (florbetapir) positron emission tomography scans. In receiver operating characteristics analyses, BFCS volume provided significantly more accurate classification into amyloid-negative and -positive categories than hippocampus volume. In contrast, hippocampus volume more accurately identified the diagnostic categories of AD, late and early MCI, and CN compared with whole and anterior BFCS volume, whereas posterior BFCS and hippocampus volumes yielded similar diagnostic accuracy. In logistic regression analysis, hippocampus and posterior BFCS volumes contributed significantly to discriminate MCI and AD from CN, but only BFCS volume predicted amyloid status. Our findings suggest that BFCS atrophy is more closely associated with cortical amyloid burden than hippocampus atrophy in predementia AD.