The VETSA1 MRI study began in year three of the primary VETSA project and data was conducted from 2005 to 2007. Upon initial contact, 6% of the sample declined to participate. Participants were excluded from the study for reasons such as metal in the body (7%), claustrophobia (3%), inability to travel to the testing site (5%), problems with the scanner (8%), the exclusion of their co-twin (9%), or other reasons (3%). Participants traveled to the University of California, San Diego Medical Center (UCSD) or Boston University for the VETSA project and were scanned the next day at UCSD or Massachusetts General Hospital (MGH). Informed consent was obtained from all participants and institutional review boards at both sites approved the scanning protocol. The present analyses were performed on data from 474 participants (110 monozygotic (MZ) pairs, 92 dizygotic (DZ) pairs, 68 unpaired individuals) ranging in age from 51 to 59 years (mean = 55.8, SD = 2.6) for whom suitable imaging data were available. A combination of DNA testing, previously obtained questionnaire and blood group methods was used to determine zygosity (Kremen et al. 2010; Eisen et al. 1989; Nichols and Bilbro 1966; Peeters et al. 1998).
