A SINGLE EVENT can suddenly reveal a trend that has been in the making for some time. The introduction and spread of the so-called CAT scanner -- a computer-assisted X-ray machine that can take detailed pictures of internal organs a conventional X-ray can't -- casts a new light on the increasing federal role in the management and regulation of medical technology.

The scanner, according to its more enthusiastic proponents, represents as great an advance in diagnostic medicine as the introduction of the X-ray itself did. This group presumably includes the Nobel Prize committee that awarded last year's prize to its inventors. To others, it is a device of questionable value -- lifesaving in some limited applications and capable of reducing the need for more painful and invasive procedures in others, but subject to heavy overuse and too likely to inflict another sharp upward push on skyrocketing health care costs.

CAT scanners were first introduced in 1973. By 1977, 400 machines had been installed in hospitals at a cost of about half a million dollars each. Today about three times that number have been sold, and already a second generation has been developed that makes the earlier devices obsolete. The new machines cost upward of $800,000.

The enormous cost and the hot competition among hospitals for the machines, led federal and state agencies to try to regulate their introduction to ensure that each machine would be fully used. But so far, it appears that the controls have met with little if any success. Hospitals need the latest technologies to attract the best doctors and, of course, patients. The threat of malpractice suits leads doctors to order every available diagnostic test, despite the diminishing returns in new information that redundant tests provide. In general, new technologies do not replace earlier tests; they are simply added to the available list. And insurance reimbursements have removed any incentive, on the part of doctors or patients, to economize.

A new trend -- one that may be an ironic consequence of government's effort to control hospital purchases -- is the purchase of scanners by unregulated physicians in private practice. One manufacturer reports that 70 percent of the machines it now has on order are for such use. Since it is unlikely that these machines will be in use two shifts a day, seven days a week, as they often are in hospitals, and since the purchasers must meet monthly payments on the order of $20,000 while quickly amortizing the machine before it becomes out of date, it is likely that the cost of a scan per patient will rise even higher.

None of these developments is limited to the CAT scanner: dozens of other new high-technology devices, from electronic fetal monitors to artificial kidneys, raise the same kinds of issues. But so far, a widening federal involvement does not seem to have improved or corrected the undeniable shortcomings of the unregulated medical marketplace. As in so many other areas of regulation, new ways of weighing usefulness, social values and cost are badly needed. And as in so many other areas, the new ideas are not yet on the horizon.