Despite all the medical advantages possible with new imaging devices, high tech is getting into trouble: It's starting to cost so much that many hospitals can't afford it.

When both Alexandria Hospital and Arlington Hospital recently decided to install the newest imaging device -- a magnetic resonance imager, which uses strong magnetic fields to see inside the body -- neither could afford the $2 million machine and the $1.5 million building needed to house it.

So the two hospitals formed a partnership that will include physician investors to buy the MRI and install it near Bailey's Crossroads, not at one of the hospitals. They hope to open the center next spring.

"The doctors would love to have their own machine," says Dennis Light, director of business and economic development for the Arlington Hospital Foundation, "but the economics of MRI require . . . the participation of a number of providers [hospitals and physicians] to support that unit."

Although Arlington and Alexandria hospitals are unique because of their cooperative approach, they are not alone in finding creative ways to buy expensive new technology.

"Free-standing imaging clinics are becoming more common than ever before," says Bob Moliter, head of goverment affairs for General Electric's Medical Products Group in the District. "It is partly driven by economics and partly driven by the technology which allows services to be delivered outside the hospital." Several free-standing facilities have opened or are under development in surburban Maryland.

"About 40 percent of the MRI units are outside of hospitals, at this time," says Jane Sisk, a senior associate in the congressional Office of Technology Assessment's health program.

"The push for developing free-standing imaging centers arises from the payment systems and the health planning rules," says Sisk. These changes, say industry experts, have led hospitals to hold onto older equipment longer and to purchase newer technology more slowly.

It is not clear, however, that even free-standing, cooperatively supported centers will be able to finance the next, and still most experimental, imaging system, positron emission tomography or PET.

Each PET scanner itself can cost $2.5 million, about the price of the most sophisticated magnetic resonance scanner. But unlike MRI, a PET cannot operate by itself. The proton-emitting particles injected into the patient's body to produce the images have such a short half-life, from a few minutes to an hour or so, that a cyclotron to make those particles must be right next door.

In the last two years, the National Institutes of Health spent $3.5 million to install two cyclotrons in a three-story, underground facility next to the Clinical Center. To completely outfit the new PET department, NIH has spent some $15 million in two years, says Paul Strudler, a nuclear chemist and one of the scientists overseeing the project.

Some observers of the field predict that PET's expense will severely limit its spread, though others disagree.

"I personally believe . . . that major hospitals and university hospitals will have PET," says Dr. Steven M. Larson, chief of nuclear medicine at NIH's Clinical Center.

But the original cost of the new technology is just part of the financial picture. It also costs money to run it, and one way or another, the costs of these tests, which range from $360 to $1,400 per scan for magnetic resonance, come out of everyone's pocket. PET may even be more expensive.

"The thing that frightens people who have some responsibility for reimbursement systems is that each time a technology is introduced, it is simply additive," says John Marshall, director of the National Center for Health Services Research and Health Care Technology Assessment. "It does not replace something that went before."

"MRI gives rise to the specter that people will use the traditional X-ray and then a CAT scan and then MRI. So that patients will get everything."

And that will raise the cost of care, he says, perhaps unnecessarily. There are no good studies, however, that show how much technology contributes to the inflation of the cost of health care, Marshall says.

But industry leaders do not think the explosion of imaging systems is either drastically or inappropriately adding to the cost.

"It is important to put imaging in perspective in terms of the cost to health care," says Walter L. Robb, General Electric's senior vice president for the Medical Systems Group. "In the early '70s, when there was only X-ray equipment, [the cost of buying] imaging equipment was only two thirds of 1 percent of the health care budget.

"Today, with CT, nuclear, MRI and X-ray, it's less than two-thirds of 1 percent of the total health care budget. We have actually decreased as a percent of the cost of health care while taking an expanding role." The total cost of all imaging technologies, including buying the equipment, film costs and other operating costs is 3 to 4 percent, Robb estimates.

Citing the end of exploratory surgery and the replacement of such procedures as some open-heart surgery with coronary angioplasty done under fluoroscopy, Robb argues that imaging systems actually have reduced the cost and pain of many medical procedures.

"Our feeling," says Robb, "is that improvements in this 3 percent is making savings in the other 97 percent of the total health care budget."