Alzheimer's disease (AD) is a disorder with a pathological cascade that long precedes its clinical manifestations, so current research has shifted towards the concept of mild cognitive impairment due to AD and presymptomatic AD. Identification of presymptomatic individuals is essential, as the long preclinical phase of AD provides a critical opportunity for the development of new preventative treatment methods. The revised diagnostic criteria now focus largely on AD-specific protein biomarkers (beta amyloid Aβ and/or tau via spinal fluid or PET imaging), or on imaging biomarkers via large-scale structural imaging paradigms. In this study, we focus on the brain's white matter (WM), which consists primarily of myelinated axons that connect the brain's gray matter regions. While AD is typically considered a gray matter disease, postmortem studies have also provided evidence of pathological changes in WM occurring early in the course of AD, and MRI studies reported a good correlation between WM damage and disease severity. We propose now to investigate whether early WM pathological changes provide complimentary information in addition to established AD biomarkers including Aβ accumulation and structural MRI. The WM tract integrity will be assessed using diffusional kurtosis imaging (DKI), a recently developed clinically feasible MRI method. This technique allows for direct assessment of the microstructural integrity of axons and their surrounding myelin. We recently showed decreased WM integrity in subjects with amnestic mild cognitive impairment compared to normal controls, and a strong correlation with cognitive functions that are most relevant to WM integrity. In this study, we propose to compare WM integrity diffusion MRI measures with MRI structural markers and PET markers for Aβ accumulation using a state-of-the-art MR-PET scanner. Positive results will for the first time establish the role of WM integrity measures in the model of dynamic biomarkers for preclinical AD.