Why do i have symptoms of dementia but a ct scan showed nothing?

Not sensitive: Ct is insensitive and nonspecific for dementia. Mri and pet are better, but clinical evaluation by a neurologist is most important.

Anatomy vs function: A very advanced dementia would likely show cerebral cortical atrophy and enlarged ventricles. Earlier in the process you might not see anything at all, though -- because a ct scan registers only anatomic changes, not functional or physiological ones. These may go on for a long time, and could also result in symptoms long before anatomic changes show up on ct.

Nothing: Imaging cannot diagnose "functional" problems, such as dementia, etc. Imaging such as ct/ MRI can only assess anatomy. It is common to have a normal study with dementia.

PET scan: Newer imaging techniques and particularly physiologic imaging can show changes with dementia. Also a nuclear test called a cisternogram can help diagnose normal pressure hydrocephalus, a treatable cause of dementia. Also MRI can detect abnormalities ct cannot, so it may be useful.