Adenosine single-photon emission computed tomography (SPECT) thallium (Tl-201) imaging is a non-invasive myocardial perfusion imaging (MPI) test. 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%).
The pharmacological vasodilators are contraindicated in the following cases:
The study should take place under the supervision of a board-certified nuclear cardiologist or nuclear radiologist.
Patients should avoid oral intake for three hours prior to the test except for medications with sips of water. Patients should avoid any medications that contain methylxanthines or caffeine and, food/beverages with caffeine for 12 hours because these products interfere with the action of pharmacological vasodilators and lead to poor image quality. Dipyridamole should also be avoided for 48 hours prior to the test because it can lead to severe hypotension. Patients should avoid wearing any metals or other potential attenuators (e.g., bras with under-wire) because these objects may lead to attenuation artifacts and poor image quality.
There are many protocols for pharmacological SPECT study. The following protocols are the two most common protocols that are used in the clinical practice:
1- Tl-201 stress/redistribution rest:
This test is a perfusion test and a viability test. The following are the steps of this protocol:
2- Tl-201 rest/redistribution:
This protocol used for a viability test only. The following are the steps of this protocol:
Most of the side effects of this test are related to pharmacological vasodilators and are usually self-limited. However, these drugs have correlations with a severe side effect such as myocardial infarction. The common side effects of these drugs are flushing, headache, chest discomfort, dyspnea, gastrointestinal discomfort, lightheadedness/dizziness, AV block, paresthesia, hypotension, nervousness, and arrhythmias. Aminophylline (50 to 250 mg) or caffeine should be used to reverse the effects of pharmacological vasodilator if a patient develops severe side effects such as hypotension with SBP less than 80 mmHg, 2nd or 3rd degree AV block, arrhythmia, wheezing, severe chest pain with ST-segment depression, or signs of poor perfusion.
SPECT stress test has been shown to have excellent diagnostic and prognostic values for IHD. The sensitivity, specificity, and accuracy of SPECT stress test for the diagnosis of coronary artery disease are 82%, 76%, and 83%, respectively. The risk of cardiac events (cardiac death or myocardial infarction) in patients with normal SPECT scan is less than 1% per year. The rate of cardiac events increases significantly with worsening in cardiac images scan findings.
Healthcare workers including nurse practitioners should be familiar with the SPECT thallium imaging study. It is a tool to assess for myocardial viability when other tests are not available. A nuclear-cardiologist usually performs the test. The SPECT stress test has been shown to have excellent diagnostic and prognostic values for IHD. The sensitivity, specificity, and accuracy of SPECT stress test for the diagnosis of coronary artery disease are 82%, 76%, and 83%, respectively.