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%).
Indications for pharmacological SPECT stress test include patients who are unable to exercise or have disabling comorbidity for the following purposes:
Pharmacological SPECT stress test is appropriate for patients who are unable to exercise or have disabling comorbidity for the following purposes:
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.
All patients should be informed that myocardial perfusion imaging also increases exposure to significant radiation (more than a CT scan).
There is also a small risk of an allergic reaction to the radiocontrast.
All patients stressed with adenosine must abstain from caffeinated foods, cola beverages, and medications for at least 12 hours before the procedure. In addition, both theophylline and aminophylline mist be discontinued 24 hours prior to the test.
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 in clinical practice should be familiar with the SPECT thallium imaging study. It is a tool to assess myocardial viability when other tests are not available. A nuclear-cardiologist usually performs the test with a team of nurses and radiology technicians. One nurse is dedicated to the monitoring of the patient during the test and is responsible for ensuring that resuscitative equipment is present in the suite before the test is started. A pharmacist specializing in nuclear medicine should assist with proper dosing. At all times during the test, vital signs and oxygenation must be recorded every few minutes. If abnormalities are detected, the team should be apprised of the abnormality immediately. A team approach is vital to prevent complications from the SPECT thallium 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.
The nurse should ensure that the patient has not ingested any caffeinated beverages or colas for at least 12 hours prior to the procedure. In addition, the nurse must ensure that the patient did not take theophylline for 24 hours prior to the test. The labs should be checked for renal function and any allergy to the contrast dye should be documented.
Prior to injecting adenosine, the patient must be on a cardiac monitor and have an intravenous line. The vitals should be measured at baseline and every 2 minutes thereafter. A dedicated nurse must monitor the patient during the procedure. After the procedure, the patient's vitals are assessed for the next 45-90 minutes.
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