The Ottawa Hospital is cancelling dozens of cardiac scanning appointments as it grapples with an unexpected shortage of the medical isotope thallium.

The hospital has been using the isotope for heart scans as a backup during a shortage of the nuclear isotope technetium-99.

Nuclear medicine experts said last week there would be a severe worldwide shortage of technetium-99, the isotope generally used for heart and bone scans, for a two- to three-week period starting Sunday.

That's because the three nuclear reactors that produce the radioactive substance molybdenum-99 — needed to generate technetium-99 — will be going down for maintenance and refuelling.

Guy Morency, the Ottawa Hospital's director of diagnostic imaging, said thallium is used in cardiac scans as an alternate to technetium-99.

He said hospital staff had not anticipated the thallium shortage, which he said was caused by "a production issue" and is expected to last until next week.

At that time, he said, the hospital will reschedule patients for their heart scans.

Meanwhile, the shortage of technetium-99 means scheduling for bone scans at the Ottawa Hospital is down 50 per cent. Morency said it's expected those scheduling for those scans will be back to normal in April.