MAPT and Alzheimer disease: Recent in vivo PET imaging in Alzheimer’s disease using 11C-UCB-J as a marker of synaptic density and 18F-AV-1451 or 18F-MK-6240 PET as markers of tau pathology have shown decreased temporal lobe synaptic density with increasing pathological burden,21 but with individual variability depending on the severity of cortical pathology.22