Adenosine single-photon emission computed tomography (SPECT) thallium (Tl-201) imaging is a non-invasive myocardial perfusion imaging (MPI) test.
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. SPECT scan performed after the stress event will reveal the distribution of thallium and therefore the relative blood flow to the different parts of the ventricle. Images are also obtained at rest and compared. The 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 reflect myocardial perfusion and viability.
Currently, SPECT Tl-201 is used mainly for myocardial viability assessment when positron emission tomography (PET) or magnetic resonance imaging viability assessment is not feasible. American Society of Nuclear Cardiology (ASNC) recommend against using adenosine SPECT Tl-201/technetium 99m, dual-isotope (rest-stress), imaging for detecting myocardial ischemia because this protocol has high radiation exposure (up to 23 mSv) compared to other isotopes. Tl-201 is a potassium analog, a radioactive isotope of thallium with a half-life of 73 hours, which is up-taken by myocardial cells and detects an area with hypo-perfusion and myocardial infarction as a cold spot. It has many other medical applications such as renal medullary imaging and tumor detection. In clinical practice, technetium 99m agents (Tc-99m sestamibi and Tc-99m tetrofosmin) are more commonly used with SPECT imaging to detect myocardial ischemia because of low radiation exposure (4.2–6.3 mSv) compared to Tl-201.
Adenosine is a nucleoside that is composed of adenine and d-ribose, a potent coronary vasodilator through activation of A2A receptors in smooth muscles and endothelium. It is used as a continuous infusion in pharmacological SPECT stress test for patients who can not exercise to increase coronary blood flow and radioisotopes uptake by myocardial cells with normal coronary perfusion. Adenosine has several side effects that correlate with the activation of other receptors such as A1AR, A2B, and A3AR. These sides effects are hypotension, tachycardia, atrioventricular block, bronchospasm, peripheral vasodilatation, and gastrointestinal symptoms. Other vasodilator agents that are also usable for pharmacological SPECT stress test are dipyridamole and regadenoson. Regadenoson is an adenosine derivative and selective A2A receptor agonist. Compared to adenosine, regadenoson dosing is as one injection because of long half-life, and it has a more favorable side effect profile because of its selectivity to the A2A receptor. Therefore, regadenoson is the most common pharmacologic vasodilator that is currently used in pharmacological SPECT stress test (83%).