The Food and Drug Administration, worried about interpretations of the scans, has required something new: Doctors must take a test showing they can read them accurately before they begin doing them. So far, 700 doctors have qualified, according to Eli Lilly. Other kinds of diagnostic scans have no such requirement.

In another unusual feature, the F.D.A. requires that radiologists not be told anything about the patient. They are generally trained to incorporate clinical information into their interpretation of other types of scans, said Dr. R. Dwaine Rieves, director of the drug agency’s Division of Medical Imaging Products.

But in this case, clinical information may lead radiologists to inadvertently shade their reports to coincide with what doctors suspect is the underlying disease. With Alzheimer’s, Dr. Rieves said, “clinical impressions have been misleading.”

“This is a big change in the world of image interpretation,” he said.

Like some other Alzheimer’s experts, Dr. Fleisher used the amyloid scan for several years as part of a research study that led to its F.D.A. approval. Subjects were not told what the scans showed. Now, with the scan on the market, the rules have changed.

Dr. Fleisher’s first patient was Mrs. Jimenez. Her husband, the family breadwinner, had lost his job as a computer consultant when the couple moved from New York to Arizona to take care of Mrs. Jimenez’s mother. Paying several thousand dollars for a scan was out of the question. But Dr. Fleisher found a radiologist, Dr. Mantej Singh Sra of Sun Radiology, who was so eager to get into the business that he agreed to do Mrs. Jimenez’s scan free. His plan was to be the first in Arizona to do a scan, and advertise it.

After Dr. Sra did the scan, the Jimenezes returned to Dr. Fleisher to learn the result.

Dr. Fleisher, sad to see so much plaque in Mrs. Jimenez’s brain, referred her to a psychiatrist to help with anxiety and suggested she enter clinical trials of experimental drugs.

But Mr. Jimenez did not like that idea. He worried about unexpected side effects.

“Tempting as it is, where do you draw the line?” he asks. “At what point do you take a risk with a loved one?”