Global production of the feedstock for the leading medical-imaging isotope is low and erratic, putting health care in jeopardy. Within the next two weeks, the vast majority of radioactive-imaging medical tests could be delayed or replaced by less desirable procedures. The reason: temporary shutdowns of Canadian and Dutch reactors that together normally provide some 70 percent of the world’s supplies of the isotope molybdenum-99 and at least 80 percent of North American supplies. Each week, U.S. doctors prescribe some 300,000 medical-imaging tests that rely on technetium-99m, a radioactive isotope produced from molybdenum-99. About half of those tests measure heart function....