# Thallous chloride Tl-201

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

## TL;DR

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.

## Summary

Thallous chloride Tl-201 is the oldest of the routine myocardial perfusion agents, taken up by the Na+/K+-ATPase pump with a first-pass extraction fraction of roughly 82-88%, the highest of the perfusion tracers and closest to a linear flow tracer. Its defining clinical property is redistribution: after stress injection, thallium washes out of normally perfused myocardium and re-enters previously ischaemic but viable segments over several hours, so a single injection with stress and delayed (redistribution) imaging can separate ischaemia from infarction/scar and assess viability, a capability the non-redistributing Tc-99m agents lack without a second dose.

The current FDA label indication is myocardial perfusion imaging (planar or SPECT) for diagnosis of coronary artery disease by localising non-reversible defects (infarction) and reversible defects (ischaemia, with exercise or pharmacologic stress), plus a separate indication for localising parathyroid hyperactivity pre- and post-operatively. Recommended adult doses are 37-74 MBq (1-2 mCi) for planar imaging and 74-111 MBq (2-3 mCi) for SPECT.

Because thallium's 80 keV and 167 keV photons are lower-energy than technetium's 140 keV, image quality and count statistics are inferior, and the longer 73-hour half-life gives a higher radiation dose per study; the Tc-99m agents and PET tracers have therefore displaced most of thallium's rest/stress workload, but it remains in use, particularly for viability assessment and as the low-dose rest half of dual-isotope protocols.

## Fields

- Kind: Agent
- Last checked: 2026-09-22
- Also known as: Thallium-201; Tl-201 chloride
- Tags: SPECT; myocardial perfusion; viability
- Modality: SPECT tracer
- Mechanism: Tl+ is a potassium analogue actively taken up by viable myocytes via the Na+/K+-ATPase pump, giving the highest first-pass myocardial extraction fraction of the routine perfusion agents (about 82-88%) and a near-linear relationship with blood flow across a wide range. Unlike the Tc-99m agents, thallium redistributes over hours as it re-equilibrates between myocardial and blood compartments, which is exploited clinically: a stress image followed by 3-4 hour (or 24 hour) redistribution imaging distinguishes reversible ischaemia (fills in on redistribution) from fixed scar (stays a defect), and can assess myocardial viability without a second injection.
- Approvals: US 1977: Myocardial perfusion imaging (planar or SPECT) for diagnosis of coronary artery disease by localisation of infarction and, with stress, ischaemia; US 1977: Localisation of sites of parathyroid hyperactivity pre- and post-operatively
- Dosing: Intravenous injection; Planar myocardial imaging 37-74 MBq (1-2 mCi); SPECT myocardial imaging 74-111 MBq (2-3 mCi); parathyroid localisation 75-130 MBq (2-3.5 mCi)

## Sources

- FDA label: Thallous Chloride Tl 201 Injection, 2025 revision: https://www.accessdata.fda.gov/drugsatfda_docs/label/2025/018150s029lbl.pdf
- First-pass extraction fraction comparison table (J Nucl Med Technol review): https://tech.snmjournals.org/content/jnmt/35/1/3.full.pdf

## Connected records

- drugs: [Technetium Tc-99m pyrophosphate](https://nuclide.cc/drugs/tc-99m-pyrophosphate/), [Technetium Tc-99m sestamibi](https://nuclide.cc/drugs/tc-99m-sestamibi/), [Technetium Tc-99m tetrofosmin](https://nuclide.cc/drugs/tc-99m-tetrofosmin/)
- isotopes: [Technetium-99m](https://nuclide.cc/isotopes/tc-99m/), [Thallium-201](https://nuclide.cc/isotopes/tl-201/)
- indications: [Coronary artery disease](https://nuclide.cc/indications/coronary-artery-disease/)

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