Background: Patients with amnestic mild cognitive impairment (MCI) represent an important clinical group as they are at increased risk of developing Alzheimer disease (AD). ¹¹C-PIB PET is an in vivo marker of brain amyloid load.

Objective: To assess the rates of conversion of MCI to AD during a 3-year follow-up period and to compare levels of amyloid deposition between MCI converters and nonconverters.

Methods: Thirty-one subjects with MCI with baseline ¹¹C-PIB PET, MRI, and neuropsychometry have been clinically followed up for 1 to 3 years (2.68 ± 0.6 years). Raised cortical ¹¹C-PIB binding in subjects with MCI was detected with region of interest analysis and statistical parametric mapping.

Results: Seventeen of 31 (55%) subjects with MCI had increased ¹¹C-PIB retention at baseline and 14 of these 17 (82%) clinically converted to AD during follow-up. Only one of the 14 PIB-negative MCI cases converted to AD. Of the PIB-positive subjects with MCI, half (47%) converted to AD within 1 year of baseline PIB PET, these faster converters having higher tracer-retention values than slower converters in the anterior cingulate (p = 0.027) and frontal cortex (p = 0.031). Seven of 17 (41%) subjects with MCI with known APOE status were 4 allele carriers, this genotype being associated with faster conversion rates in PIB-positive subjects with MCI (p = 0.035).

Conclusions: PIB-positive subjects with mild cognitive impairment (MCI) are significantly more likely to convert to AD than PIB-negative patients, faster converters having higher PIB retention levels at baseline than slower converters. In vivo detection of amyloid deposition in MCI with PIB PET provides useful prognostic information.