It is best to read TR from the DICOM image data (DICOM tag RepetitionTime). It varies depending on ADNI phase (ADNI-2 vs ADNI-3 and within ADNI-3 by protocol). Approximately TR=3s without multi-band (SMS); SMS=8 images will commonly have 607ms and some around 720ms. Best to read from the DICOM data directly for each series.