# Thallium-201

Source: https://nuclide.cc/isotopes/tl-201/  
Nuclide record `tl-201` (Isotope). Data CC BY-NC 4.0, attribute "Data from Nuclide (nuclide.cc)"; commercial use needs a licence.

## TL;DR

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.

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.

## Fields

- Kind: Isotope
- Status: established
- Last checked: 2026-09-22
- Also known as: 201Tl
- Tags: gamma emitter; cardiac perfusion; myocardial viability

## Sources

- Radiopaedia: thallium-201 scintigraphy: https://radiopaedia.org/articles/thallium-201-scintigraphy
- Health Canada product monograph: Thallous Chloride Tl 201 Injection: https://pdf.hres.ca/dpd_pm/00050840.PDF

## Connected records

- technologies: [Cyclotron isotope production (F-18, Ga-68, Cu-64, Zr-89, At-211)](https://nuclide.cc/technologies/cyclotron-isotope-production/), [SPECT & bone scan](https://nuclide.cc/technologies/spect/)
- indications: [Coronary artery disease](https://nuclide.cc/indications/coronary-artery-disease/)
- drugs: [Technetium Tc-99m sestamibi](https://nuclide.cc/drugs/tc-99m-sestamibi/), [Thallous chloride Tl-201](https://nuclide.cc/drugs/tl-201-chloride/)

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