tokens: For this we train DKT on a combined dataset containing subjects with two distinct diseases and sizes of data available : 1 ) a larger , multimodal typical AD ( tAD ) dataset from the TADPOLE Challenge , and 2 ) a smaller unimodal Posterior Cortical Atrophy ( PCA ) dataset from the Dementia Research Centre ( DRC ) , for which only a limited number of Magnetic Resonance Imaging ( MRI ) scans are available .