Thallium-201
A radioactive potassium look-alike with a three-day lifetime, injected to see how well blood reaches, and heart muscle survives in, different parts of the heart on a SPECT scan.
Overview
Thallium-201 decays by electron capture to stable mercury-201 with a 73-hour (about 3-day) physical half-life, and is handled biologically as a potassium analogue, taken up by viable myocardium in proportion to regional blood flow and cell membrane integrity. Imaging exploits the mercury X-rays emitted at 69-83 keV (about 88 per 100 decays) rather than its weaker 135 and 167 keV gamma rays, using a low-energy collimator.
It is produced on a cyclotron by proton bombardment of enriched thallium-203, via 203Tl(p,3n)201Pb, and the resulting lead-201 (half-life about 9.4 hours) is left to decay for roughly a day before a second chemical separation yields carrier-free thallium-201.
Because its relatively long physical and biological half-lives allow flexible same-day or next-day rest/redistribution imaging without a dedicated on-site cyclotron, thallium-201 was the dominant myocardial perfusion SPECT agent for decades; it has since been substantially displaced by technetium-99m-based perfusion agents (which give a lower radiation dose and better image quality) but remains in use, particularly for myocardial viability assessment, where thallium's redistribution behaviour is diagnostically useful.
- Hg X-rays 69-83 keV (~88%)
- γ 135 keV (~3%)
- γ 167 keV (~10%)
A radioactive tracer injected into a vein that lodges inside heart muscle cells in proportion to blood flow, so a gamma camera picture taken after exercise or a stress drug shows which parts of the heart are not getting enough blood.
The original heart-blood-flow tracer: a radioactive form of the metal thallium that heart muscle cells pull in almost like potassium, so a scan straight after exercise and a second scan hours later can tell a starved-but-alive muscle from permanently dead muscle without a second injection.
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