An American and a Briton who have never communicated with one another have been named to receive the 1979 Nobel Prize in medicine for their development of an X-ray process capable of creating clear images of the body's internal organs.

Allan McLeod Cormack, a physics professor at Tufts University in Boston who 16 years ago published an article on the theory of the process, will share the $191,000 prize with Godfrey Newbold Housfield, who a decade ago obtained a patent for the first paractical application of Cormack's work.

The machine developed by Hounsfield is a child of the marriage of X-ray and computer technology, using X-rays to measure the desnity of tissues within the patient's body and the computer to translate those density measurements into a picture on a screen.

Cormack, 55, the only full professor of physics at Tufts who does not have a doctorate and the 53rd American to receive a Nobel in medicine, expressed the traditional shock at winning. "I had no idea," he told reporters. "I'm astonished."

Hounsfield, 60, reached with the news at the Middlesex laboratory where he works, told a reporter, "You've knocked me sideways. I didn't expect anything like that."

The very name by which the computerized axial tomography scanner is commonly known -- the CAT scanner -- has become at once a symbol of the best that modern medical technology has to offer and of the phenomenal impact such technology has on the cost of health care.

There are now two basic types of scanner, for the head and for the whole body. Some modifications produce images in color.

Unlike conventional X-ray equipment, CAT scanners rotate 360 degrees around the patient, taking density measurement. The picture produced is, in effect, a slice of the organ being viewed -- be it the brain, heart, kidney or liver.

A series of such slices, taken a few inches apart, produces an image, which, for example allows physicians to distinguish tumors from normal tissue in a manner never before possible. Conventional X-rays are "confused" by the differing densities of bone and soft tissue.

In its citation of Cormack and Housfield, the Karolinska Institute, which awards the Nobel prizes, said "it is no exaggeration to state that no other method within X-ray diagnostics within such a short peroid of time has led to such remarkable advances in research and in a multitude of application as computer-assisted tomography."

It is also no exaggeration to say that no other single device has caused such headaches for health planners or such uproar in the debate on the control of health costs.

The price of each machine ranges from $500,000 of more than $1 million.

Since EMI, the british company for which Housfield works, marketed the first scanner, virtually every hospital in America has either purchased or coveted one or more of the devices.

Many health planners argue that the CAT scanner is a perfect example of the kind of technology that hospitals should share. California, they point out, has more of the divices than Britain, which has more than twice as many people.

When Capitol Hill Hospital and Greater Southeast Community Hospital here recently began tentative merger discussions, officials pointed to the presence of a CAT scanner at Greater Southeast as a merger plus from Capitol Hill's point of view.

Sources in Stockholm reported yesterday that the selection of Cormack and Hounsfield was the subject of as much debate as their device. The institute reportedly chose the two after rejecting the selection committee's recommendation of a different, unidentified, candidate.

Cormack, a South African native who, among other duties, teaches a freshman physics course at Tufts, defended the CAT scanner against its cost-conscious critics yesterday.

"You have to realize what this is replacing," he said. Before the CAT scaner you had major surgery, and a great deal of risk and cost" to the patient. "It's not just a matter of taking X-rays."