{"entity":{"id":"tl-201-chloride","kind":"drug","name":"Thallous chloride Tl-201","aka":["Thallium-201","Tl-201 chloride"],"tldr":"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.\n\nThe 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.\n\nBecause 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.","asOf":"2026-09-22","links":[{"label":"FDA label: Thallous Chloride Tl 201 Injection, 2025 revision","url":"https://www.accessdata.fda.gov/drugsatfda_docs/label/2025/018150s029lbl.pdf"},{"label":"First-pass extraction fraction comparison table (J Nucl Med Technol review)","url":"https://tech.snmjournals.org/content/jnmt/35/1/3.full.pdf"}],"tags":["SPECT","myocardial perfusion","viability"],"related":["tc-99m-sestamibi","tc-99m-tetrofosmin","tc-99m","tl-201"],"indications":["coronary-artery-disease"],"sections":[],"technologies":[],"targets":[],"drugs":[],"companies":[],"institutions":[],"pathways":[],"terms":[],"trials":[],"people":[],"bottlenecks":[],"keyPapers":[],"journals":[],"dependsOn":[],"notes":[],"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":[{"region":"US","year":1977,"indication":"Myocardial perfusion imaging (planar or SPECT) for diagnosis of coronary artery disease by localisation of infarction and, with stress, ischaemia"},{"region":"US","year":1977,"indication":"Localisation of sites of parathyroid hyperactivity pre- and post-operatively"}],"mechanismSteps":[],"dosing":{"route":"Intravenous injection","schedule":"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)","source":"https://www.accessdata.fda.gov/drugsatfda_docs/label/2025/018150s029lbl.pdf"},"toxicity":[],"access":[],"regulatoryEvents":[]},"route":"/drugs/tl-201-chloride/","neighbours":{"drug":[{"id":"tc-99m-pyrophosphate","kind":"drug","name":"Technetium Tc-99m pyrophosphate","route":"/drugs/tc-99m-pyrophosphate/"},{"id":"tc-99m-sestamibi","kind":"drug","name":"Technetium Tc-99m sestamibi","route":"/drugs/tc-99m-sestamibi/"},{"id":"tc-99m-tetrofosmin","kind":"drug","name":"Technetium Tc-99m tetrofosmin","route":"/drugs/tc-99m-tetrofosmin/"}],"isotope":[{"id":"tc-99m","kind":"isotope","name":"Technetium-99m","route":"/isotopes/tc-99m/"},{"id":"tl-201","kind":"isotope","name":"Thallium-201","route":"/isotopes/tl-201/"}],"indication":[{"id":"coronary-artery-disease","kind":"indication","name":"Coronary artery disease","route":"/indications/coronary-artery-disease/"}]}}