Thallous chloride Tl-201

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.