Imaging brain iron and protein aggregation with MRI for assessing Alzheimer's disease pathology and progression

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Abstract Alzheimer’s disease (AD) affects over 5 million Americans and is expected to affect 2-3-fold more in the next few decades. AD is associated with aggregation of amyloid-beta (Ab) and phosphorylated tau proteins. Curiously, burden of Aβ, classically considered the most important AD pathological hallmark is not enough to indicate clinical decline or progression. For example, cognitive-normal elders may also carry high levels of Aβ and recent clinical trials aiming to reduce Aβ have generally failed to improve patients’ conditions. Therefore, developing biomarkers that can better predict clinical outcome and progression are needed. Confluent evidence shows that regional brain magnetic susceptibility measured by MRI differs between AD patients and healthy controls, and importantly such changes may predict cognitive decline. However, it is unclear what causes these susceptibility changes in AD. While iron deposition has been widely suspected as the underlying cause, our recent study has discovered that aggregation of Ab and tau by itself produces strong diamagnetic susceptibility, opposite of the paramagnetic susceptibility generated by iron deposition. The opposing magnetic susceptibility of iron and aggregated pathological proteins poses a significant challenge as current MRI-based magnetic susceptibility mapping algorithms cannot differentiate iron from other colocalizing diamagnetic susceptibility sources within the same voxel. Our goal is to develop a novel technique that can differentially quantify molecular sources of magnetic susceptibility and test whether the resulting susceptibility components can serve as markers of progressive AD pathology. We will test our techniques and hypothesis utilizing a unique capability that combines in cranio MRI at autopsy with histological examinations. We have developed innovative histological processing methods that allow voxel-to-voxel matching between MRI and histology in 3D, thus permitting the examination of the relationship between magnetic susceptibility components and the neuropathology underlying AD. If successful, our techniques and findings might ultimately allow the detection of AD-related neuropathology at much earlier stages, permit intervention before neurons become irretrievably damaged and non-invasively assess disease progression. These techniques, once standardized, will be highly cost-effective, widely accessible and readily implementable in non-specialized clinical imaging centers, thus better serving the growing population of AD patients.