This is consistent with findings in other modalities; functional MRI caudate activation is reduced during memory tasks in PD-MCI38 and cognitive decline in PD is linked to lower caudate uptake in dopamine transporter single-photon emission computed tomography (DAT-SPECT) and positron emission tomography imaging.39 We found QSM increases in basal forebrain with lower MoCA scores, although not in the substantia innominata (online supplementary figure 3). Here, SLC6A3 is linked to Parkinson disease.