Where to draw the line on medical technology costs?

In medical circles, the nuclear magnetic resonance machine is considered something of a marvel. A cylindrical device the size of a small room, it uses magnetic and radio-frequency waves to take intricate ''pictures'' of the body in diagnosing disease.

But these snapshots don't come cheaply. The machine itself costs $1 million to $2 million. It takes upwards of another $1 million to enclose it in a ray-proof room. Result: a $700 to $800 charge every time the machine is turned on.

This is one small example of a vast set of issues facing health-care professionals, lawmakers, and the general public today. At the core of the debate:

* Can the nation afford all the costly medical innovations emerging from the lab?

* Will new medical technologies undermine efforts to reduce overall health-care costs?

* And, ultimately, how much is society willing to pay, or forgo, to maintain a health-care system?

The past two decades have been a ''golden age'' for medical know-how and technology, partly as a result of large investments on medical R&D in the postwar era. At the same time, however, health-care costs have been skyrocketing , although there has been a recent slowdown in their rate of growth. Last year Americans spent some 10.5 percent of the gross national product ($360 billion) on health care - twice the amount in 1960.

Inflation has been a major culprit. But medical technology has been a factor, too. As much as 30 to 40 percent of the annual increase in personal health-care spending and hospital outlays in the 1970s has been attributed to new technology (new drugs, equipment, procedures, and the systems in which they are used). Moreover, Congress's Office of Technology Assessment (OTA) noted in a recent report that medicare expenses rose an average of 19 percent per year between 1977 and 1982. Nearly one-third of that was pegged to technology, or ''intensity'' of services.

''It's the problem of success,'' says Arthur Caplan of the Hastings Center, a New York State research institute dealing with ethical problems in health care. ''You discover all this wonderful stuff, Bnd you have to decide what to do with it.''

What concerns many experts is that, if left unchecked, the new tools and techniques will either add substantially to the nation's health tab or lead to a ''rationing'' of health services. Others argue that the nation can cut costs by eliminating hospital inefficiencies and duplication of service, without limiting use of medical advances. They point out, too, that new tools will reduce costs in some areas by replacing outmoded methods.

Yet the trend is toward more costly and complex tools. ''There are indications that we are on the threshold of a major medical technology explosion ,'' says Dr. Seymour Perry, deputy director of Georgetown University's Institute for Health Policy Analysis. ''We have got to look at the costs and national implications of these technologies.''

The price tags of many of these innovations can be high. One rapidly emerging area, for example, is organ transplants. Last year some $300 million was spent in the US for transplants. If more organs were to become available - and federal legislation is being pushed to make this possible - that could jump to $3 billion annually within a few years. Heart transplants, now a $50 million- to $ 110 million-a-year business, is shortly expected to jump to $1.5 billion. A year's prescription for cyclosporin, an immunity-suppressing drug used in connection with transplants, runs about $5,600.

The problem isn't always cost. It is also, in some cases, the changing nature of these new tools - and their potential for wasteful use. Dr. William B. Schwartz, a medical professor at Tufts University and senior physician at the New England Medical Center, notes that many new diagnostic tools are riskless and painless to use. Because no medical risk is involved, some doctors may be inclined to use them despite the cost, even when it may not be medically necessary.

Experts also point to other pressures tending to add to the cost - or, in some cases, to an overreliance on new technologies:

* A payment system - particularly the federal medicare program - that, until recently, reimbursed hospitals and physicians on the basis of cost and gave little incentive for limiting expenditures on machines or medicines.

* A public clamor to use a technology, regardless of its cost-effectiveness, simply because it is available. This could affect the development of something like the artificial heart, for instance.

* An increased dependence on technology in general. ''There is a lot of public pressure to use technical solutions to health care,'' says Mr. Caplan of the Hastings Center.

* The continuing problem of malpractice suits, which encourage the practice of ''defensive medicine,'' in which doctors tend to overtest or overprescribe therapies to protect against being sued for medical mishaps or oversights.

But changes are afoot. Being closely watched, for instance, is a controversial medicare payment system that moves away from the old practice of reimbursing hospitals based on cost. The new system gives hospitals a fixed amount for each patient based on diagnosis at admission - regardless of the service provided. But a recent study in the New England Journal of Medicine said this could raise, not lower, hospital costs.

The OTA, in its study, suggests other changes to reduce costs and ensure more appropriate use of medical technologies. These include, among other things, altering the way medicare pays for new hospital equipment (capital costs continue to be paid on a cost basis) and adopting fee schedules or caps on physician payments. Another recurring idea is to set up a federal or private center to help evaluate emerging health-care technologies. Several proposals before Congress would do just that.

Regardless of the approaches adopted - and all have their detractors - medical technologies are likely to come under increasing scrutiny in the future. Many of the questions are economic: Is costly lifesaving technology justifiable in every case? How much tax money should be spent in applying these advances? Is one technology diverting resources from some simpler or equally worthy approach? But behind them are tougher social and ethical problems: Is it, for example, always worth the cost if the result is a life dragged out agonizingly?

That these questions are being asked at all reflects, perhaps, how far health care has evolved. Medical decisions are no longer made solely on the basis of medical outcomes: They now involve legal, ethical, and economic considerations that increasingly attract not only the attention of physicians and patients but also the public. ''We have entered a new arena in which a careful look is going to be taken at how many dollars are spent and where,'' says Marion Ein Lewin, a senior policy analyst at the American Enterprise Institute.