If you want to get MRI or fMRI images of AD patients best option is
Alzheimer's Disease Neuroimaging Initiative (ADNI - http://adni.loni.usc.edu/ ). For endoscopic video frames, I would suggest NBI-InfFrames dataset ( https://zenodo.org/record/1162784#.XkYNlWgzZPZ ). For X-ray data, you can find a huge dataset from the MIT Laboratory for Computational Physiology ( https://physionet.org/content/mimic-cxr/2.0.0/ )
For more details please refer to the following link.