A thallium-201 stress test is a type of radionuclide myocardial perfusion imaging used to assess blood flow through the heart muscle at rest and during stress. Thallium-201 is a radioactive isotope that behaves similarly to potassium and is taken up by living, metabolically active heart muscle cells in proportion to their blood supply.
During the test, a small dose of thallium-201 is injected into the bloodstream at peak exercise (or at the point of maximum pharmacological stress if the patient cannot exercise). A gamma camera then captures images of the heart, showing how much of the isotope has been taken up in each region. Areas with good blood flow absorb the tracer normally; areas deprived of blood flow (due to coronary artery disease or scarring from a previous heart attack) show reduced uptake (a perfusion defect). Repeat images taken at rest allow comparison: defects that appear on stress images but resolve at rest indicate reversible ischaemia, while defects present on both images suggest permanent scar tissue.
The thallium-201 test has largely been superseded in many UK centres by technetium-based tracers (such as sestamibi), which produce higher-quality images with a lower radiation dose. However, the principles and clinical applications are the same across all myocardial perfusion imaging techniques. These tests are used to identify and quantify significant coronary artery disease, guide decisions about coronary angiography or revascularisation, and assess prognosis after cardiac arrest or heart attack.
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