It's also not easy for doctors to always know exactly what kind of dementia a person may have, because there hasn't been a test.
Researchers at Mayo Clinic are making a lot of progress in this field.
They've developed a way to look at the living brain and more accurately tell what type of dementia a person has.
And this may help get patients on the right treatment.
At a distance these scans look like typical pictures of a brain taken during an MRI.
But at closer look they reveal information researchers used to only get during an autopsy.
"We are mainly looking at the most common forms of dementia," says Mayo Clinic Dr. Prashanthi Vemuri.
KAAL by Axel Gumbel