The former hypothesis that brain iron would play a role in the cause of AD is supported by combined QSM and BBB imaging studies, which revealed iron leakage owing to BBB disruption in CSVD using DCE-MRI (Mikati et al., 2014; Tariq et al., 2018; Uchida et al., 2020), and subtle BBB dysfunction in early stages of the AD continuum with the ɛ4 allele of the APOE gene using DP-pCASL (Uchida et al., 2022a). The gene discussed is APOE; the disease is Alzheimer disease.