Capping years of contentious scientific review, federal health officials said yesterday that they had made a preliminary decision to have Medicare pay for brain scans to help diagnose Alzheimer's disease in some patients who show symptoms of the ailment.

The decision had been aggressively sought by the brain-scanning industry, which could profit enormously from the assurance of coverage as the U.S. population ages. Health officials had previously rejected the idea of paying for the tests to help diagnose Alzheimer's because of a lack of definitive evidence of its usefulness for that disease.

Yesterday's decision, announced by the Centers for Medicare and Medicaid Services, a part of the Department of Health and Human Services,, was the result of "an exhaustive review of the evidence" undertaken after the agency received "a request to reconsider its previous non-coverage decision." The decision also came during a week of great attention to Alzheimer's disease after Ronald Reagan's death from the illness.

"This new Medicare coverage will improve care for Americans living with suspected Alzheimer's disease," HHS Secretary Tommy G. Thompson said. "It is one of the many ways we're working to help our Medicare beneficiaries with cognitive impairment get the best possible care based on the best scientific evidence."

Only an autopsy can confirm a diagnosis of Alzheimer's, but some experts have argued that earlier diagnosis could help patients, perhaps by allowing doctors to prescribe memory-enhancing drugs sooner. Others, however, have long countered that the practical value of such a test is marginal because doctors can prescribe memory drugs on the basis of cognitive tests alone and because the few available drugs for Alzheimer's are minimally effective.

PET -- or positron emission tomography, a 30-year-old imaging technology that can highlight areas of metabolic abnormalities in the body -- cannot make a diagnosis of Alzheimer's. Under the coverage plan, however, Medicare would pay for its use to aid in the diagnosis of suspected Alzheimer's disease when diagnosis "remains uncertain despite a thorough clinical evaluation." Officials provided no estimate of the percentage of patients suspected of having Alzheimer's that might be covered, or the potential cost to the nation. Tests typically cost about $2,500.

The high-powered campaign for Alzheimer's coverage was one of several such efforts by medical centers, dealers of radioactive PET tracer chemicals and others hoping to expand the number of uses for which PET can be reimbursed. The cause has attracted powerful personalities, notably Sen. Ted Stevens (R-Alaska), who is a friend of the technology's inventor, Michael Phelps of the University of California at Los Angeles .

A previous effort by Phelps and others to gain reimbursement for the use of PET to diagnose many diseases of aging, including 20 different kinds of cancer, was largely rejected after an outside review concluded that a UCLA analysis submitted to support the petition was "poorly conducted" and "exaggerated" the potential benefits.

In December 2001, an independent analysis of PET's value for Alzheimer's, conducted for the federal Agency for Healthcare Research and Quality, concluded that simply treating suspected Alzheimer's patients was "superior" to the option of performing a PET test. Federal health officials said new consultations with that agency, an expert panel and advocacy and medical professional organizations -- rather than new data -- were behind yesterday's reversal of the earlier decision.

The expert panel, assembled by the National Institute on Aging, found the value of PET for Alzheimer's limited and expressed concerns about potential overuse and misdiagnosis, unnecessary radiation exposure, and avoidable expenses. It recommended that the government pay for a study to see whether PET offers anything of practical value for Alzheimer's patients or their families. Officials said such a study was being planned.

The Medicare office will accept comments on its proposal for 30 days and will make a final decision within three months. Because Medicare is the nation's largest third-party payer of medical care, a positive decision would likely push other insurers to cover the procedure as well.