Thank you for your question to the ADNI MRI Core.
MT1 is a magnetization (IR)-prepped T1-weighted image intended for structural analysis. For ADNI-1 this was always and MP-RAGE sequence. For ADNI-2 it may be an IR-SPGR on GE scanners. (Conversely, ST1 --which does not appear in ADNI data -- would be a "structural T1" with no IR-prep.)