What is the purpose and procedure of a Myocardial Perfusion Imaging (MPI) test for patients with suspected or known coronary artery disease?

Myocardial Perfusion Imaging (MPI) Test

Purpose

MPI is a nuclear imaging test that evaluates blood flow to the heart muscle to detect coronary artery disease, assess the extent and severity of ischemia, and predict cardiac risk. 1

The test serves multiple critical functions:

• Diagnoses obstructive CAD in patients with intermediate to high pre-test likelihood (>15-85%) of disease 1
• Quantifies myocardial ischemia and/or scar tissue to determine the extent of coronary involvement (single-vessel versus multivessel disease) 2
• Risk stratifies patients by predicting major adverse cardiac events, with a normal scan indicating <1% annual risk of cardiac death or myocardial infarction 2
• Guides revascularization decisions by identifying which patients will benefit from percutaneous coronary intervention (PCI) or coronary artery bypass grafting (CABG) 2
• Evaluates patients after revascularization to assess for restenosis or progression of native coronary disease 2, 3

Procedure

Patient Preparation

• Patients must avoid all methylxanthine-containing products (caffeinated coffee, tea, caffeine-containing drugs, aminophylline, theophylline) for at least 12 hours before the test 4
• Cardioactive medications may need adjustment: beta-blockers, calcium channel blockers, and nitrates can affect test interpretation 4

Stress Component

The test requires inducing cardiac stress through one of two methods:

Exercise Stress (Preferred when possible):

• Patients capable of maximal exercise should undergo exercise MPI 2
• Exercise must achieve ≥85% maximum predicted heart rate and ≥5 METS for adequate stress 4

Pharmacological Stress (For patients unable to exercise adequately):

• Vasodilator agents (regadenoson, adenosine, or dipyridamole) are used for patients with functional disability 2, 4
• Regadenoson is administered as a single 0.4 mg IV bolus over 10 seconds, followed by saline flush 4
• If regadenoson is given after inadequate exercise, wait 1 hour (not 3 minutes) to minimize serious cardiac adverse reactions 4

Imaging Techniques

SPECT MPI (Most Common):

• Uses technetium-99m labeled agents (sestamibi or tetrofosmin) as the preferred radioisotopes over thallium-201 due to superior image resolution, higher count density, and better dosimetry 2, 1
• Achieves sensitivity of 87-89% and specificity of 73-75% for detecting angiographically significant CAD 2
• Acquires both rest and stress images to demonstrate reversibility (ischemia) versus irreversibility (infarction) of perfusion defects 2
• Uses electrocardiogram gating to assess left ventricular ejection fraction (LVEF) and regional wall motion 2

PET MPI (Superior but Less Available):

• Uses rubidium-82 (Rb-82) or nitrogen-13 ammonia (N-13 ammonia) as radioisotopes 2, 5
• Provides absolute myocardial blood flow quantification at rest and stress, allowing calculation of myocardial flow reserve (MFR) 2, 5
• MFR <2.0 indicates microvascular dysfunction, even without visible perfusion defects 5
• Offers approximately 20% higher diagnostic accuracy than SPECT (88% versus 67%, p=0.009) 5
• Particularly valuable for detecting microvascular disease through uniform reduction in MFR across multiple coronary territories 5

Hybrid Imaging (Enhanced Accuracy):

• SPECT/CCTA or PET/CT combines functional perfusion data with anatomical coronary information 2, 1
• Achieves sensitivity of 96% and specificity of 95%, superior to either modality alone 2, 1
• Shows 92% agreement on revascularization decisions compared to invasive angiography 2, 1

Image Acquisition Timing

• Stress images are acquired 60-90 minutes after radiotracer injection during peak stress 4
• Rest images are obtained on the same day or a separate day depending on protocol 2
• The 17-segment model is used to quantify reversible perfusion defects 4

Key Interpretation Parameters

For Standard SPECT/PET:

• Extent and severity of perfusion defects in coronary territories 2
• Reversibility (ischemia) versus fixed defects (scar) 2
• LVEF and regional wall motion abnormalities 2
• Transient ischemic dilation of the left ventricle (high-risk marker) 2

For PET with Flow Quantification:

• Myocardial flow reserve (MFR) calculated as stress/rest blood flow ratio, with normal ≥2.0 5
• MFR 1.8-2.0 suggests early microvascular dysfunction requiring medical therapy optimization 5
• MFR <1.8 indicates significant microvascular dysfunction 5
• LVEF reserve provides incremental prognostic value 5

Critical Pitfalls to Avoid

• Do not rely solely on visual perfusion assessment when evaluating for microvascular disease, as globally reduced MFR may not produce visible defects 5
• Breast tissue artifact decreases SPECT specificity in women (74% versus 94% in men); consider prone imaging or attenuation correction 2
• Left bundle branch block or ventricular pacing requires pharmacological stress (not exercise) to avoid false-positive septal defects 2
• Do not misinterpret globally reduced MFR as technical error—it may represent diffuse microvascular or balanced multivessel disease 5
• Patients with >20% ischemic myocardium or defects in multiple vascular territories have >3% annual mortality and require revascularization consideration 3

Specific Patient Populations

Women:

• Exercise ECG has reduced accuracy in women; MPI should be considered as first-line testing in symptomatic women with intermediate likelihood of CAD, abnormal resting ECG, diabetes, or reduced functional capacity 2
• Women have higher prevalence of microvascular dysfunction requiring MFR assessment 5

Post-Revascularization:

• MPI is indicated for symptomatic patients after CABG or PCI to detect restenosis or disease progression 2
• Can be performed early (3-6 months) after PCI for in-stent restenosis or late (≥5 years) after CABG 3