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requirement, study risks, ratio of drug to placebo, and study procedures. Frequency of visits and total study length were rated similarly by the groups.In response to open-ended questions, African Americans more frequently mentioned financial compensation (23% vs. 14%) and returning of research results (19% vs. 6%) as potential incentives than Caucasians. In structured questions, African Americans reported that receiving personal cognitive test results, personal genetic test results, personal estimates for AD risk, and overall study results had greater impact than receiving personal lab reports or financial incentives. Overall, however, Caucasians were more likely than African Americans to base their decisions on the potential incentives that were studied. Conclusions: African Americans are less likely than Caucasians to express a willingness to enroll in preclinical AD trials. Further research is needed to identify and test potential incentives to increase willingness to participate in underserved populations to ensure equitable benefits of research. Background: Atrophy measures derived from volumetric MRI have been proposed as surrogate outcome measures for early phase clinical trials because of their exquisite sensitivity to disease progression. This is especially promising for rare dementias such as primary progressive aphasia (PPA), where the small available subject pool limits our ability to perform meaningfully powered trials with traditional cognitive and functional outcome measures. Recently proposed composite atrophy indexes promise to optimize the efficiency of MRI-based measures. Methods: We investigated a composite atrophy index in 26 PPA participants with longitudinal MRIs separated by two years. Rogalski et al. [Neurology 2014;83:1184-1191 previously demonstrated that atrophy of the left perisylvian cortex is a highly sensitive measure of disease progression in this population and a promising endpoint for clinical trials. Using methods described by Ard et al. [Pharmaceutical Statistics 2015;14:418-426], we constructed a composite atrophy index composed of an optimally weighted linear combination of focal volumetric measures from 10 regions of interest within the left perisylvian cortex. All components of this composite measure were taken from the standard output measures returned by the FreeSurfer image processing software package. Sample size required to detect a fixed percentage slowing in atrophy in a two-year clinical trial with equal allocation of subjects across arms and 90% power was calculated for the two candidate surrogate biomarker endpoints. Results: The optimal composite endpoint required 38% less subjects to detect the same percent slowing in atrophy than required by the total cortical volume measure. For example, only 15 subjects per arm would be require to detect a 25% slowing in rate of progression in the composite outcome measure assuming the distribution of decline observed in our pilot study. Conclusions: We found that the optimal composite atrophy measure resulted in a mean...

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