ACHE and Cognitive impairment: Thirty-two patients with amnestic mild cognitive impairment or mild dementia due to AD underwent positron emission tomography (PET) with the tracer N-methyl-4-piperidyl-acetate (MP4A) to estimate acetylcholinesterase (AChE) activity, neurological examinations, cerebral magnetic resonance imaging (MRI) and neuropsychological assessment.