WASHINGTON (AP) — Does it really take an expensive brain scan to diagnose Alzheimer's? Not everybody needs one but new research suggests that for a surprising number of patients whose memory problems are hard to pin down, PET scans may lead to changes in treatment.

The findings, reported Wednesday, mark a first peek at a huge study under way to help determine if Medicare should start paying for specialized PET scans that find a hallmark of Alzheimer's — a sticky plaque called amyloid.

Alzheimer's is the most common form of dementia, and classic symptoms plus memory tests often are enough for a...