A group of cardiologists recently had a proposition for Dr. Andrew Rosenblatt, who runs a busy heart clinic in San Francisco: Would he join them in buying a CT scanner, a $1 million machine that produces detailed images of the heart?

The scanner would give Dr. Rosenblatt a new way to look inside patients’ arteries, enable his clinic to market itself as having the latest medical technology and provide extra revenue.

Although tempted, Dr. Rosenblatt was reluctant. CT scans, which are typically billed at $500 to $1,500, have never been proved in large medical studies to be better than older or cheaper tests. And they expose patients to large doses of radiation, equivalent to at least several hundred X-rays, creating a small but real cancer risk.

Dr. Rosenblatt worried that he and other doctors in his clinic would feel pressure to give scans to people who might not need them in order to pay for the equipment, which uses a series of X-rays to produce a composite picture of a beating heart.

“If you have ownership of the machine,” he later recalled, “you’re going to want to utilize the machine.” He said no to the offer.

And yet, more than 1,000 other cardiologists and hospitals have installed CT scanners like the one Dr. Rosenblatt turned down. Many are promoting heart scans to patients with radio, Internet and newspaper ads. Time magazine and Oprah Winfrey have also extolled the scans, which were given to more than 150,000 people in this country last year at a cost exceeding $100 million. Their use is expected to soar through the next decade. But there is scant evidence that the scans benefit most patients.

Increasing use of the scans, formally known as CT angiograms, is part of a much larger trend in American medicine. A faith in innovation, often driven by financial incentives, encourages American doctors and hospitals to adopt new technologies even without proof that they work better than older techniques. Patient advocacy groups and some doctors are clamoring for such evidence. But the story of the CT angiogram is a sobering reminder of the forces that overwhelm such efforts, making it very difficult to rein in a new technology long enough to determine whether its benefits are worth its costs.

Some medical experts say the American devotion to the newest, most expensive technology is an important reason that the United States spends much more on health care than other industrialized nations — more than $2.2 trillion in 2007, an estimated $7,500 a person, about twice the average in other countries — without providing better care.

No one knows exactly how much money is spent on unnecessary care. But a Rand Corporation study estimated that one-third or more of the care that patients in this country receive could be of little value. If that is so, hundreds of billions of dollars each year are being wasted on superfluous treatments.

At a time when Americans are being forced to pay a growing share of their medical bills and when access to medical care has become a major political issue for states, Congress and the presidential candidates, health care experts say it will be far harder to hold down premiums and expand insurance coverage unless money is spent more wisely.

The problem is not that newer treatments never work. It is that once they become available, they are often used indiscriminately, in the absence of studies to determine which patients they will benefit.

Some new treatments, like the cancer drug Gleevec and implantable heart defibrillators, undoubtedly save lives, contributing to the United States’ reputation for medical breakthroughs. But others — like artificial spinal disks, which can cost tens of thousands of dollars to implant but have not been shown to reduce back pain in many patients, and Vytorin, a new cholesterol drug that costs 20 times as much as older medicines but has not been proved superior — have been criticized for not justifying their costs.

And sometimes, the new technologies prove harmful. Physicians were stunned, for example, when clinical trials showed last year that expensive anemia medicines might actually hasten death in kidney and cancer patients. Such drugs are used more widely in the United States than elsewhere.

“We have too many situations where we thought we knew what the answer was and it didn’t turn out like everyone thought,” said Dr. Mark Hlatky, a cardiologist and professor of health research and policy at Stanford University.

A Tool of Dubious Value

The problem of inadequate study is especially serious for medical devices and imaging equipment like scanners, which typically are not as strictly regulated as prescription drugs. Under Food and Drug Administration regulations, the makers of CT scanners — CT is short for computed tomography — do not have to conduct studies to prove that their products benefit patients, as drug makers do. The manufacturers must certify only that the scanners are safe and provide accurate images.

Once the F.D.A. approves a test or device, Medicare rarely demands evidence that it benefits patients before agreeing to pay for it. But last year, Medicare officials raised questions about the benefits of CT heart scans and said it would demand more studies before paying for them. But after heavy lobbying by cardiologists, Medicare backed down. Private insurers, while initially reluctant to pay for the tests, are also covering them.

Physicians in this country have a free hand in deciding when to use new technology like CT angiography. Some are conservative. But others, especially doctors in private practice who own their scanners, use the tests aggressively.

Douglas Ring, a 63-year-old Los Angeles real estate developer, said he received a CT heart scan in October 2005, on the advice of Dr. Ronald P. Karlsberg, a Beverly Hills cardiologist. “Ron has been my physician for 15 or 20 years, and he got this new toy in his office, and he said I should try it,” Mr. Ring said. He took the test despite having no symptoms of heart disease, like shortness of breath and chest pain. He was already taking cholesterol medicine, and a different test had shown no problems with his heart.

The CT heart scan by Dr. Karlsberg found a moderate buildup of plaque in one of Mr. Ring’s coronary arteries. The doctor increased Mr. Ring’s cholesterol medicines and encouraged him to diet and exercise.

Dr. Karlsberg said he considered the information from Mr. Ring’s CT scan extremely valuable. “Here’s a case of near-serious coronary disease that required medical management,” said Dr. Karlsberg, a partner at the Cardiovascular Medical Group of Southern California, which conducted about 1,400 CT heart scans last year.