 The central goal of this study is to determine and compare the similarities and differences in regional brain uptake of 18FT807 in patients with typical Alzheimers Disease AD, Posterior Cortical Atrophy PCA, and Logopenic Variant of Primary Progressive Aphasia lvPPA. The investigators will correlate patterns of 18FT807 binding with magnetic resonance imaging MRI-based regional volumetric and cortical thickness measures. If cerebral spinal fluid CSF samples are not available, patients may be asked to get an optional lumbar puncture LP for additional comparisons. The investigators will recruit 20 participants, 45-70 years old, with clinical evidence of young onset focal dementia. 