Role in CAD diagnosis
In routine practice, the diagnosis of coronary artery disease (CAD) often begins with an exercise ECG test (Exercise ECG test). However, this method has low sensitivity (about 60%) and moderate specificity (around 80%). A normal exercise ECG does not rule out CAD, as electrical changes occur at the very late stages of the "ischemic cascade".
Non-invasive imaging methods that evaluate myocardial function or perfusion operate at an earlier level. These include scintigraphy, stress echocardiography, stress CMR, PET, and hybrid SPECT/CT. They allow the detection of blood supply disorders in the heart muscle before ECG changes appear, which is particularly important for low-risk populations (e.g., women) where exercise ECG is uninformative.
Protocols and radiopharmaceuticals
The method is based on measuring the relative distribution of radiopharmaceuticals (RPs) in cardiac tissues. Two main isotopes are used:
- Technetium-99m sestamibi (MIBI). The protocol involves two separate injections: one at peak stress (exercise), and the second at rest. The order of administration may vary.
- Thallium-201 ($^{201}Tl$). The agent is administered once during stress. Rest evaluation is performed later (e.g., after 4 hours) due to the natural redistribution of the isotope in the myocardium.
Interpretation of results: perfusion defects
In areas with impaired blood flow, RP uptake decreases, forming a "defect" on tomograms. Its characteristics are used to assess the state of the coronary bed:
- Reversible defects (reversible defects). The hypoperfusion zone is visible on stress images and absent at rest. This is a marker of a hemodynamically significant arterial stenosis that limits blood flow.
- Fixed defects (fixed defects). Decreased uptake is present both under stress and at rest (during redistribution). It reflects the replacement of muscle tissue with connective tissue—a post-infarction scar (myocardial scarring).
The severity of the defect, i.e., the degree of decreased radiopharmaceutical uptake, correlates directly with the severity of stenosis. In turn, the size of the defect corresponds to the volume of myocardium supplied by the affected artery.
Limitations of the method
Despite its status as the "cornerstone" of nuclear medicine, SPECT has several significant drawbacks:
- Radiation dose. The patient receives a dose of ionizing radiation (8–20 mSv).
- Spatial resolution. The method may miss small or subendocardial myocardial infarctions (unlike MRI).
- False-positive results. Artifacts mimicking a defect can occur. To prevent them, attenuation correction (attenuation correction) and gated wall motion analysis (gated analysis) are required.
- Multivessel disease. Patients with severe multivessel CAD (multivessel CAD) may develop global myocardial hypoperfusion that masks regional abnormalities.
Clinical nuances and comparison of methods
In practice, SPECT results are often compared with stress cardiac perfusion MRI (perfusion CMR) and positron emission tomography (PET).
For example, in silent ischemia (common in diabetes mellitus), SPECT can accurately detect a reversible perfusion defect, which is confirmed by severe vascular lesions on coronary angiography (requiring CABG). However, in cases of small infarctions, the method may be inaccurate. Thus, in a small inferolateral infarction, SPECT can show a false reversible defect in the anterior wall. In such situations, stress MRI with first-pass contrast enhancement (first pass) and late gadolinium enhancement (LGE) is more reliable—it clearly visualizes focal transmural contrast enhancement in the zone of a true scar without signs of stress-induced ischemia in other territories.
Briefly: answers to topical questions
What are the absolute contraindications to stress testing during myocardial SPECT?
Absolute contraindications to stress testing during myocardial SPECT include acute and unstable conditions, as well as the inability to communicate with the patient. These include:
- Rhythm disorders — uncontrolled hemodynamically significant arrhythmias.
- Acute conditions — endocarditis, pericarditis, pulmonary embolism, embolism of other localization, deep vein thrombosis of the lower extremities, aortic dissection.
- Severe anemia — severe anemia.
- Hemodynamic disorders — resting parameters: heart rate $\ge$ 120 bpm, systolic blood pressure $\le$ 90 mm Hg.
- Psychoneurological factors — pronounced cognitive impairment, lack of productive contact with the patient, or patient refusal.
What pharmacological agents are used for pharmacological stress testing in myocardial SPECT?
Vasodilators and inotropic agents are used for pharmacological stress testing in myocardial SPECT. The choice of a specific drug depends on the clinical situation:
- Dobutamine — most justified in individuals with baseline left ventricular regional wall motion abnormalities.
- Adenosine phosphate — effective for diagnosing coronary artery stenoses and microvascular dysfunction.
- Dipyridamole (Dipyridamole) — a vasodilator that induces a "steal" phenomenon in ischemic areas due to the accumulation of endogenous adenosine.
What is the sensitivity and specificity of myocardial SPECT in the diagnosis of CAD?
Exact numerical values for the sensitivity and specificity of myocardial SPECT are not specified in the sources. The method has good sensitivity for diagnosing and assessing the severity of coronary artery disease. To prevent false-positive results and increase specificity, it is recommended to use attenuation correction and gated analysis of wall motion. In multivessel disease, global myocardial hypoperfusion can mask regional abnormalities.
How to remember the types of perfusion defects: REVERSIBLE (appears during stress, disappears at rest) = Ischemia (can be "reversed" by restoring blood flow). FIXED (always present) = Scar (tissue is dead, blood supply does not change at rest or under stress).
Class and exam questions
Why is SPECT ordered if an exercise ECG has already been performed?
Exercise ECG has low sensitivity and often yields false-negative results because ECG changes occur at late stages of the ischemic cascade. SPECT detects blood supply disorders at earlier stages.
What does "balanced ischemia" mean in multivessel disease?
In severe disease of multiple coronary arteries or the left main coronary artery, maximum blood flow decreases in all segments of the left ventricle. Due to global hypoperfusion, the relative distribution of the radiopharmaceutical may appear uniform, masking real local problems.
How is myocardial PET superior to SPECT?
PET has higher spatial resolution and allows the measurement of absolute rather than relative blood flow. This makes it particularly valuable in balanced ischemia and microvascular dysfunction, although the method is more expensive and less accessible.
What else to study on this topic
- Myocardial positron emission tomography (PET): isotopes and advantages.
- Stress cardiac perfusion CMR: first-pass technique and late enhancement.
- Pathophysiology of the "ischemic cascade" and the role of imaging methods.
- Multivessel coronary artery disease: the phenomenon of balanced ischemia.
- Methods for correcting false-positive artifacts in SPECT.
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