Just as the worldwide shortage of a radioactive isotope used in millions of medical procedures is about to get worse, officials say a new source for the substance has emerged: a nuclear reactor in Poland.

The isotope, technetium 99, is used to measure blood flow in the heart and to help diagnose bone and breast cancers. Almost two-thirds of the world’s supply comes from two reactors; one, in Ontario, has been shut for repairs for nine months and is not expected to reopen before April, and the other, in the Netherlands, will close for six months starting Friday.

Radiologists say that as a result of the shortage, their treatment of some patients has had to revert to inferior materials and techniques they stopped using 20 years ago.

The announcement of the new source for technetium 99 will come amid a flurry of activity around the United States to find new, more reliable ways to make the isotope and to avoid the use of bomb-grade uranium in the process.

General Electric has a plan to make technetium 99 using neutrons from power reactors owned by utilities, a neat trick because those reactors are usually sealed up and run for months at a time, while the medical isotope has to be removed within a few hours of its creation or it decays away. Babcock & Wilcox, a Virginia company that provides a variety of nuclear services, has a plan for a liquid-fueled reactor.