Two nuclides on one page
Technetium 99m and technetium 99 are the same 43 protons and 56 neutrons. The m stands for metastable: the nucleus holds extra energy and sheds it as a gamma ray, becoming ordinary technetium 99 without changing element.
⚠️ The medical isotope is the 99m, and it has no page of its own here because its half-life of six hours falls below the threshold used to decide which nuclides get one. What it decays into does have a page, so both are covered together — which is also how they occur, since every 99m in a patient becomes 99 within a day.
An element that should not exist
Technetium has no stable isotope. Element 43 sits in the middle of the periodic table, surrounded on all sides by ordinary metals, and every one of its nuclides decays.
The reason is a quirk of nuclear structure known as the Mattauch isobar rule: an element with an odd proton count squeezed between two stable neighbours has nowhere to sit. Promethium, at 61, is the only other case.
So the gap in Mendeleev’s table stayed empty for seventy years, and the several claimed discoveries were all wrong. Carlo Perrier and Emilio Segrè finally made it in 1937 by bombarding molybdenum with deuterons in Berkeley — the first element produced artificially, which is what its name records.
⚠️ There is a little natural technetium, formed by spontaneous fission of uranium in ore, at concentrations of nanograms per kilogram. Not enough to have been found first.
Why it suits imaging so well
Technetium 99m emits a single gamma ray at 140 kiloelectronvolts and nothing else.
Every part of that is useful. The energy is high enough to escape the body and reach the camera, low enough to be stopped by a collimator and give a sharp image. There is no beta particle, so the patient absorbs no dose from radiation the camera cannot use.
The half-life of six hours matches the working day. Long enough to deliver, prepare, inject and scan; short enough that a day later the patient carries almost nothing.
⚠️ And technetium’s chemistry is unusually accommodating. It forms stable compounds in several oxidation states, so it can be attached to molecules that go to bone, kidney, heart muscle, lung or thyroid. One isotope, a dozen different scans, depending on what it is bound to.
The cow in the corner
Six hours is too short to ship. A dose made in a reactor on Monday would be gone before it reached a hospital on Wednesday.
The answer is to ship the parent instead. Molybdenum 99 has a half-life of 66 hours and decays to technetium 99m, so a generator — a shielded column of alumina with molybdenum bound to it — keeps producing fresh technetium as it sits.
A technician flushes saline through the column and the technetium washes out while the molybdenum stays behind. The device is called a cow and the procedure is called milking, and a hospital can milk one daily for about a week before replacing it.
⚠️ The elegance is that the difficult part happens in a reactor thousands of kilometres away and weeks earlier, while the isotope that reaches the patient was made an hour ago in a cupboard.
The five reactors
Molybdenum 99 is made by fissioning uranium targets in a research reactor. For decades, five reactors supplied most of the world: NRU in Canada, HFR in the Netherlands, BR2 in Belgium, OSIRIS in France, SAFARI-1 in South Africa.
All but one were built in the 1950s or 1960s.
In 2009 NRU shut down for repairs after a leak was found. HFR was already offline for its own maintenance. Between them they had supplied more than half the world, and the shortage lasted into 2010.
Scans were postponed and cancelled across Europe and North America. Cardiac and bone imaging were affected worst, because the alternatives are either less informative or much more expensive.
⚠️ The response has been slow. NRU closed for good in 2018, new production has come online in Australia and elsewhere, and several countries have converted from highly enriched to low enriched uranium targets on non-proliferation grounds. But the supply chain still runs through a handful of ageing facilities for an isotope used in tens of millions of procedures a year.
What is left afterwards
Technetium 99m decays to technetium 99, which has a half-life of 211,000 years.
For the patient this does not matter — the quantity is minute and it is excreted within days. For waste management it does. Technetium 99 is one of the longer-lived fission products found in spent fuel and in the effluent from reprocessing plants, and it is mobile in groundwater in its usual chemical form.
⚠️ So the isotope that is nearly ideal for medicine leaves behind one that is awkward for disposal. The quantities from hospitals are trivial next to those from reprocessing, but it is the same nuclide at both ends.