Adenosine SPECT Thallium Imaging

Excerpt

Adenosine single-photon emission computed tomography (SPECT) thallium-201 (Tl-201) imaging is a noninvasive myocardial perfusion imaging (MPI) test using thallium as a radioisotope. The underlying principle of the test is that when the myocardium is under stress, the diseased ventricle receives less blood flow than the normal heart muscle. The SPECT scan performed after the stress event reveals the distribution of thallium and the relative blood flow to the different parts of the ventricle. Images are also obtained at rest and compared. Thallium is injected and taken up by the myocardial cells so that the initial distribution of the tracer reflects viable myocardium. Images are then taken during stress (induced by adenosine) and at rest to reflect myocardial perfusion and viability.

SPECT Tl-201 is used mainly for myocardial viability assessment when positron emission tomography (PET) or magnetic resonance imaging is not feasible. The American Society of Nuclear Cardiology recommends against using adenosine SPECT Tl-201/technetium 99m (Tc-99m), dual-isotope (rest-stress) imaging for detecting myocardial ischemia, as this protocol exposes patients to greater amounts of radiation (up to 23 mSv) than other isotopes.

Tl-201 is a potassium analog, a radioactive isotope of thallium with a half-life of 73 hours. Tl-201 is uptaken by myocardial cells and detects an area with hypoperfusion and myocardial infarction as a cold spot. This isotope has many other medical applications, including renal medullary imaging and tumor detection. In clinical practice, Tc-99m agents (Tc-99m Sestamibi and Tc-99m Tetrofosmin) are more commonly employed during SPECT imaging for myocardial ischemia detection due to lower radiation exposure (4.2–6.3 mSv) than Tl-201.