<qa>  What is the primary efficacy measure?The primary efficacy measure is defined as the percent change from baseline to termination Month 36 or Early Termination in normalized brain volume Brain Atrophy measured according to the SIENA Structural Imaging Evaluation using Normalization of Atrophy method. However, since not many patients had completed the entire study at the time of futility analysis, it was the sponsors decision that the futility analysis be performed on patients with MRI scans at months 24 or 36 or at early termination visits - the latest available. 