I will download the ADRC Clinical Data spreadsheet and use the subject ID together with the days-from-entry information to match the clinical assessments to the corresponding MRI/PET sessions. Since OASIS-3 is longitudinal, I will select the clinical assessment closest to each imaging session when assigning labels.

Thank you also for pointing me to the Imaging Data Dictionary and the ADRC Clinical Data documentation. This information is very useful for my study.