Assuming that 41% of operated patients develop POCD, and a standard deviation of the hippocampal volume change of 0.45 in both patients with POCD and without POCD, a total sample size of 56 subjects may detect a difference of 0.4 mm3 in hippocampal volume with a power of 90% at a two-sided significance level of 5%. To compensate for the high loss to follow-up which is unfortunately typical for studies on POCD, and estimated at 20%, we will recruit 70 patients in the surgery group.