Alzheimer's disease (AD) is the most common form of dementia in the elderly. Classic symptoms of the disease include memory loss and confusion associated with the hallmark neuro-pathologic lesions of neurofibrillary tangles (NFT) and senile plaques (SP) and their sequelae, gray matter atrophy. Volumetric assessment methods measure tissue atrophy, which typically follows early biochemical changes. An alternate MRI contrast mechanism to visualize the early pathological changes is T1rho (or "T-1-rho"), the spin lattice relaxation time constant in the rotating frame, which determines the decay of the transverse magnetization in the presence of a "spin-lock" radio-frequency field. Macromolecular changes (in plaques and tangles) that accompany early AD are expected to alter bulk water T1rho relaxation times. In this work, we measure T1rho MRI on patients with clinically diagnosed AD, MCI and in age-matched cognitively normal control subjects in order to compare T1rho values with changes in brain volume in the same regions of the brain and demonstrate that T1rho can potentially constitute an important biomarker of AD.