A new machine that uses sound waves to examine a woman's breasts for cancer will be used by doctors at Columbia Hospital for Women beginning next week as an adjunct to X-ray tests.

The new technique, called ultrasound mammography, produces a visual image of the breast just as an X-ray mammogram does, but it does so without exposing the breast to radiation. Although the technique is not expected to replace X-rays as a screening test for cancer, the hospital's radiologists believe it will save some patients from undergoing surgery in cases when a lump is felt and an X-ray mammogram does not show it clearly.

The machine, which cost the hospital just under $150,000, is the first in the Washington area and one of a handful in the country. It was developed in the hope of providing a screening test for breast cancer that would eliminate the risk of radiation exposure.

But studies so far indicate that it is not as good as X-ray mammography for finding small cancers before they can be felt, according to Dr. Martin A. Thomas, director of the hospital's department of radiology.

However, he said, an ultrasound mammogram can be invaluable when a woman discovers a lump and an X-ray does not help doctors determine what it is. In such situations now, the next step is usually surgery to remove the lump for a biopsy.

"We feel this technique is good for the problem patient who has dense breasts, where X-ray shows nothing or is equivocal," he said.

Researchers who have studied the new machine feel it is especially well suited for examining breast lumps in younger women -- particularly those under 30 -- whose breasts often contain defense tissue full of glands and fiber, which interferes with the ability of X-ray to show a lump clearly.

At Indiana University Hospital, ultrasound mammorgraphy has proved so successful in differentiating cancer from benign lumps in such patients that it is routinely used there instead of X-ray mammography, according to Dr. Elizabeth Kelly-Fry, who has studied the new technique.

She said ultrasound was 100 percent successful in studies at Indiana University for identifying breast cysts and fibroadenomas -- the two most common causes of benign lumps. Besides its advantages over X-rays in young women, she believes it is excellent for examining women with fibrocystic breast disease, a condition that produces dense breasts with multiple cysts. Patients with the condition need periodic test because of an increased risk of breast cancer, and Kelly-Fry said ultrasound seems preferable to X-rays because it eliminates the risk of repeated radiation exposure.

To undergo an ultrasound examination on the new machine, a woman lies face down on a table so that her breasts are suspended in a tank of warm water. A transducer below the tank sends a beam of sound waves -- too high-pitched to be heard -- through the water so that they enter a cross-section of one of the breasts. The waves pass through the breast tissue, and some of them are reflected when they strike borders between one kind of tissue and another.

The machine uses the reflected sound waves to produce an image of that section of the breast. In less than half an hour it takes more than 120 cross-sectional picturs of each breast. Radiologists can examine the pictures for signs of cancer or other diseases.

Ultrasound mammography is so new that its ultimate role in breast cancer testing is unknown. Thomas emphasized that it will not be used at Columbia Hospital as a screening test, but only as an adjunct to physical examination and X-rays. Although most doctors believe the sound waves used in testing present no health risks, Dr. Jane Taylor of the National Cancer Institute's breast cancer program branch said the safety of ultrasound is not really proved.

"One thing we've been concerned about is, do we know enough about what ultrasound does to the cells?" she said.

Thomas recalled predictions in 1968, when the hospital bought a machine to perform ultrasound examinations of the pelvis in pregnant patients, that the investment would prove to be a waste of money."Now," he said, "we do more pelvic ultrasounds than anything else except chest X-rays."