{"entity":{"id":"tl-201","kind":"isotope","name":"Thallium-201","aka":["201Tl"],"tldr":"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.","summary":"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.\n\nIt 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.\n\nBecause 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.","status":"established","asOf":"2026-09-22","links":[{"label":"Radiopaedia: thallium-201 scintigraphy","url":"https://radiopaedia.org/articles/thallium-201-scintigraphy"},{"label":"Health Canada product monograph: Thallous Chloride Tl 201 Injection","url":"https://pdf.hres.ca/dpd_pm/00050840.PDF"}],"tags":["gamma emitter","cardiac perfusion","myocardial viability"],"related":[],"indications":[],"sections":[],"technologies":["spect","cyclotron-isotope-production"],"targets":[],"drugs":[],"companies":[],"institutions":[],"pathways":[],"terms":[],"trials":[],"people":[],"bottlenecks":[],"keyPapers":[],"journals":[],"dependsOn":[],"notes":[],"symbol":"201Tl","element":"Thallium","massNumber":201,"halfLife":"73 hours (3.04 days)","emissions":["Hg X-rays 69-83 keV (~88%)","γ 135 keV (~3%)","γ 167 keV (~10%)"],"emissionClass":"gamma","use":"imaging","production":"Cyclotron: 203Tl(p,3n)201Pb on enriched thallium-203, followed by decay of 201Pb (half-life ~9.4 hours) to 201Tl and a second chemical separation.","supply":"adequate","supplyNote":"Cyclotron-produced at a limited number of high-energy facilities; clinical demand has declined relative to Tc-99m myocardial perfusion agents but remains in routine use for viability studies.","chelators":[],"pairedWith":[],"since":1975},"route":"/isotopes/tl-201/","neighbours":{"technology":[{"id":"cyclotron-isotope-production","kind":"technology","name":"Cyclotron isotope production (F-18, Ga-68, Cu-64, Zr-89, At-211)","route":"/technologies/cyclotron-isotope-production/"},{"id":"spect","kind":"technology","name":"SPECT & bone scan","route":"/technologies/spect/"}],"indication":[{"id":"coronary-artery-disease","kind":"indication","name":"Coronary artery disease","route":"/indications/coronary-artery-disease/"}],"drug":[{"id":"tc-99m-sestamibi","kind":"drug","name":"Technetium Tc-99m sestamibi","route":"/drugs/tc-99m-sestamibi/"},{"id":"tl-201-chloride","kind":"drug","name":"Thallous chloride Tl-201","route":"/drugs/tl-201-chloride/"}]}}