1. A process to establish a standard range of brain viscosity and elasticity in normal subjects, Mild Cognitive Impairment (MCI) patients and Alzheimer's Disease (AD) patients comprising: positioning a patient within a magnetic resonance imaging (MRI) device; providing a pair of pneumatic drum drivers useful within said MRI device, each driven synchronously by a single wave generator; positioning said pair of pneumatic drum drivers in contact with the patient's skull, thereby simultaneously generating a shear wave, and a corresponding interference wave effective to neutralize attenuation of said shear wave, each wave propagating in the brain; elucidating brain stiffness data as a function of shear wavelength; and establishing a standard range of brain viscosity and elasticity in normal subjects, MCI patients and AD patients; and correlating changes in brain viscosity and elasticity between normal, MCI and AD patients.