Stress MIBI (Myocardial Perfusion Imaging) Test
A stress MIBI test is a nuclear medicine imaging procedure that uses technetium-99m sestamibi (MIBI) as a radioactive tracer to evaluate myocardial perfusion during stress and rest conditions, allowing for assessment of coronary artery disease by identifying areas of reduced blood flow to the heart muscle. 1
How the Test Works
Basic Principles
- The test involves intravenous injection of a small amount of radioactive tracer (technetium-99m sestamibi)
- MIBI is avidly extracted by cardiac myocytes, with initial myocardial distribution reflecting both myocyte distribution and perfusion
- Images are taken during both stress and rest conditions to compare blood flow differences 1
Imaging Process
- Single-photon emission computed tomography (SPECT) is the preferred imaging technique
- The camera rotates around the patient for 10-20 minutes
- Planar projection images are reconstructed into three-dimensional tomographic slices
- Images are displayed using a color scale for semi-quantitative assessment of regional tracer uptake 1
Stress Methods
Two main approaches are used to stress the heart:
Exercise Stress:
- Patient exercises on a treadmill to maximum capacity
- MIBI is injected at peak exercise
- Suitable for patients who can achieve adequate exercise levels
Pharmacological Stress:
- Used for patients unable to exercise adequately (30-50% of patients)
- Options include:
- Vasodilators (e.g., adenosine)
- Beta sympathetic agonists (e.g., dobutamine)
- MIBI is injected during pharmacological stress 1
Interpretation of Results
Normal Result
- Homogeneous myocardial uptake of tracer
- Indicates normal myocardium and perfusion
- Absence of clinically significant infarction or coronary stenosis
- Associated with excellent prognosis (annual cardiac death/MI risk <1%) 1, 2
Abnormal Results
Reversible defect: Area appears as a defect during stress but normalizes at rest
- Indicates inducible perfusion abnormality
- Corresponds to significant coronary stenosis
- Higher risk of cardiac events (annual rate ~6% or greater) 1
Fixed defect: Area appears as a defect in both stress and rest images
- Indicates loss of viable myocardium (e.g., after myocardial infarction)
- Represents scarred tissue 1
Clinical Value
Diagnostic Accuracy
- For women: Sensitivity 78-88%, specificity 64-91% with exercise MPI
- For women: Sensitivity 91%, specificity 86% with pharmacological stress MPI 1
- Overall high diagnostic accuracy for detecting coronary artery disease
Prognostic Value
- Powerful predictor of future cardiac events
- Normal scan associated with <1% annual risk of cardiac death or MI
- Abnormal scans, especially with reversible defects, associated with significantly higher risk
- The extent and severity of perfusion defects correlate with risk in a graded fashion 1, 3
Safety Profile
- Very safe procedure with low complication rates:
- 0.01% mortality risk
- 0.02% morbidity risk
- Radiation exposure: 12 mSv for a 1,000 MBq MIBI study
- Equivalent to 5 years of natural background radiation
- Similar to CT scan or coronary angiography exposure
- Associated with 1 in 1,800 risk of cancer 10-20 years after exposure 1
Key Advantages of MIBI Over Other Tracers
- Higher-energy isotope compared to thallium-201, resulting in:
- Better image quality
- Less attenuation artifact
- Less low-count artifact 4
- Allows for ECG-gated imaging to assess ventricular function simultaneously
- Enables assessment of both perfusion and function in a single test 5
Common Pitfalls and Limitations
- Breast tissue attenuation in women can cause false-positive results
- Small heart size (more common in women) may reduce accuracy
- Technical improvements to overcome these limitations include:
- ECG-gated SPECT imaging
- Attenuation correction techniques
- Prone imaging 1
Stress MIBI testing provides valuable diagnostic and prognostic information for patients with suspected coronary artery disease, allowing for non-invasive assessment of myocardial perfusion and function with high accuracy and safety.