Only a decade after CT scans debuted as the latest high-tech way to peek into the human body, a still newer technology, magnetic resonance imaging, is on a roll. Although MRI machines typically cost more than $1.5 million and must be placed in expensive housing, there are already an estimated 650 of them in the United States, more than 200 in California alone.

Last week, an expert panel assembled by the National Institutes of Health spent two days hearing from 20 speakers about MRI's capabilities. The next day, its chairman, Dr. Herbert L. Abrams of the University of California, reported that the panel had concluded that this "innovative technique . . . is an extraordinary addition to our diagnostic armamentarium."

But at the same time the panel cautioned that MRI scans are now of proven value only for diagnosing certain conditions and that the technology is so new that there is potential for error in interpreting results.

MRI has two important qualities that computerized tomography and conventional X-rays lack. It does not use cell-damaging radiation, and it discloses not only anatomical information about tissues, but some chemical information, too.

Also a plus for MRI, the panel found, is that scanning with it can be done without injecting dyes into the patient's bloodstream to make otherwise invisible tissues discernible to X-rays. About one in 40,000 patients has a reaction to these so-called contrast materials, and about 250 of them die.

To be sure, contrast materials for MRI have been developed that, as Abrams put it, "make its already superb images even better." Indeed, Food and Drug Administration approval of the first one -- gadolinium DPTA -- is expected in early 1988. But the new MRI contrast materials are thought to be far safer than those given for some X-ray scans.

Perhaps the best-known use of MRI scans is for the detection of tumors in the posterior fossa, a region at the back of the brain in the area below the ears. Largely because MRI can detect growths there that otherwise would be obscured by bone, the panel unanimously agreed that, for such tumors, MRI is "the diagnostic method of choice."

But the panel also said there are some brain tumors that CT and MRI are equally adept at identifying and still others for which CT is superior. The same is true of MRI scans of the lower back for the possible detection of a ruptured disc. Although one panel member said he preferred MRI for this purpose, the opinion of the panel as a whole was that CT is just as good.

CT scans typically cost $150 to $200; MRI scans usually are $500 to $1,000.

MRI also was called "useful" for diagnosis of head injuries but not those that are brand new. Injuries less than four days old, said the panel, "are more reliably demonstrated by CT."

The panel called MRI "the most sensitive imaging technique for the diagnosis of multiple sclerosis," but again with qualifications. As a speaker at the meeting explained, the same MRI-generated bright spots in the brain that in some people are indications of MS are in others evidence of AIDS or -- when the patient is 55 or older -- often just a normal consequence of aging.

Of the 1.5 million to 2 million MRI scans a year now being performed in this country, most are being used for conditions of the brain and spine, according to Dr. J. Sanford Schwartz of the University of Pennsylvania. Schwartz, a member of the panel, added that MRI scans of the muscles, tendons and cartilage -- especially in the knee -- are the next most popular.

The accuracy of MRI in diagnosing problems of the knee and the hinges of the jaw was endorsed by the panel. But other uses, including to examine the heart, need further study, the panel concluded.

Patients undergoing MRI are put in the hollow core of a large magnet. Protons in the patient's body -- the nuclei of hydrogen atoms -- change their orientation accordingly. Then a radio pulse is applied to create a second magnetic field perpendicular to that provided by the magnet, causing the protons to change their direction again. When the pulse is discontinued, a small electrical voltage is produced in a copper coil surrounding the patient that is detected and recorded by a computer. After repetitions of the process, the computer translates the information into two-dimensional images.

The technique cannot be used on people wearing heart pacemakers or who have shrapnel or metal surgical clips in their bodies and those on life-support systems with steel parts.

Other problems are that obese patients won't fit inside the magnet, while others are overcome by claustrophobia or so beset by the machine's noise they cannot lie still for the 45 minutes to an hour or more that examinations require.

Perhaps MRI's biggest problem, however, is that because of its apparent safety, it could be overused.

All but one of the speakers who addressed the panel were MRI enthusiasts. The exception -- Dr. Eric B. Larson, an internist at the University of Washington -- recently completed, with his colleague, Dr. Daniel Kent, a critical evaluation of the technology as used for the spine and brain.

Larson and Kent found that "over 90 percent of the {research} reports" they reviewed "probably inflate estimation of MRI's ability to detect disease." And what particularly worried them was the method's proclivity for "false positives" -- finding trouble where, in fact, none exists.

In one instance, a woman was urged to undergo a brain biopsy when MRI detected what seemed to be abnormalities there. An experienced neurologist, however, judged her to be normal, and the biopsy was canceled.

"I think in the long run MRI will replace much of what is done in radiology," Kent said, "but it's still very early to know to what extent this will be appropriate."Judith Randal is a Washington free-lance writer.