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IBM is adding medical images to the health data its Watson artificial-intelligence can mine to help doctors make diagnoses.

The big technology company announced on Thursday morning that it was buying Merge Healthcare, a medical-imaging software company, for $1 billion. When IBM set up its Watson health business in April, it began with a couple of smaller medical data acquisitions and industry partnerships with Apple, Johnson & Johnson and Medtronic. Last week, IBM announced a partnership with CVS Health, the large pharmacy chain, to develop data-driven services to help people with chronic ailments like diabetes and heart disease better manage their health.

But the purchase of Merge Healthcare is both a sizable investment and a new resource for IBM’s new Watson health unit. “We’re bringing Watson and analytics to the largest data set in health care — images,” John Kelly, IBM’s senior vice president of research who oversees the Watson business, said in an interview.

Images like CAT scans, X-rays and mammograms, IBM researchers estimate, represent about 90 percent of all medical data today. The images and a patient’s electronic health records are typically separate. So, for example, a radiologist might examine thousands of patient images a day, but only looking for abnormalities on the images themselves rather than also taking into account a person’s medical history, treatments and drug regimens.

“Watson will be able to understand both,” Mr. Kelly said.

The Watson artificial-intelligence technology has mainly been applied to analyzing text in documents and on the web. It employed this natural-language processing capability initially in its high-profile demonstration project — beating human champions in the question-and-answer game, Jeopardy!

But for the past two years, Mr. Kelly said, IBM researchers at its labs in Yorktown Heights, N.Y., and in San Jose, Calif., have been training Watson’s AI engine on image recognition. “We’ve been giving Watson eyes, so to speak,” he said.

Merge Healthcare, based in Chicago, specializes in software for storing, viewing and sharing medical images. Its technology is used by a wide array of health care providers and imaging-equipment makers, and its rights to use the archived images varies according to customer requirements, and state and federal health-privacy rules.

Merge Healthcare is the third medical data company IBM has acquired since setting up the Watson health business. In April, it agreed to buy two start-ups: Explorys, a spin-off from the Cleveland Clinic whose data on 50 million patients is used to spot patterns in diseases, treatments and outcomes; and Phytel, a Dallas maker of software to manage patient care and reduce readmission rates to hospitals. The financial details of those smaller deals, both with private companies, were not disclosed.

The Watson technology, sold as a cloud service, has been used in applications for IBM customers to help them spot patterns from the data gathered in their businesses. But health care is the first field where IBM is building a comprehensive offering for an entire industry.

In the past, automated decision-support systems in medicine have often been greeted with initial optimism yet have proved to be disappointingly limited in practice. But IBM is betting that its Watson technology can deliver a genuine breakthrough in the next few years.

The company is investing not only money but also some of its corporate reputation on the belief that it can be a technological leader in improving health care, with better outcomes for patients and more efficient spending for providers, insurers and patients.

In an interview on the “Charlie Rose” program on PBS in April, Virginia M. Rometty, IBM’s chief executive, spoke of the company’s role over the years in supplying technology for big projects, from computerizing census statistics to putting astronauts on the moon.

“Our moonshot,” Ms. Rometty said, “will be the impact we will have on health care.”