What is myocardial perfusion imaging (MPI)?

What is Myocardial Perfusion Imaging?

Myocardial perfusion imaging (MPI) is a nuclear medicine technique that uses radioactive tracers injected during cardiovascular stress to assess blood flow to the heart muscle, allowing detection of coronary artery disease, evaluation of myocardial viability, and risk stratification for cardiac events. 1

Technical Methodology

Radiotracer Administration

• The procedure involves intravenous injection of small amounts of radioactive tracer during cardiovascular stress 1
• Three commercially available tracers are used:
  • Thallium-201 thallous chloride (18 mSv radiation exposure) 1
  • Technetium-99m sestamibi (MIBI) (12 mSv radiation exposure) 1
  • Technetium-99m tetrofosmin (10 mSv radiation exposure) 1
• These tracers are avidly extracted by cardiac myocytes, with initial myocardial distribution reflecting both myocyte distribution and perfusion 1

Stress Testing Options

• Exercise stress is the preferred method when patients can achieve adequate cardiovascular stress on a treadmill 1
• Pharmacological stress is used for patients unable to exercise adequately, relevant to one-third to one-half of patients 1:
  • Vasodilators such as adenosine or regadenoson 1, 2
  • Beta-sympathetic agonists such as dobutamine 1

Imaging Technique

• Single-photon emission computed tomography (SPET/SPECT) is the preferred imaging method 1
• The camera rotates around the patient over 10-20 minutes, creating planar projection images reconstructed into three-dimensional tomographic slices 1
• Images are displayed using a color scale for semi-quantitative assessment of regional tracer uptake 1
• Stress and rest images are typically separated by 3-4 hours, with total patient contact time approximately 45 minutes 1

Image Interpretation

Normal Findings

• Homogeneous myocardial tracer uptake indicates normal myocardium and perfusion, excluding clinically significant infarction or coronary stenosis 1

Abnormal Patterns

• Reversible defect: A defect present on stress images that normalizes on rest images indicates inducible perfusion abnormality, typically corresponding to significant coronary stenosis 1
• Fixed defect: A defect present on both stress and rest images indicates loss of viable myocardium, such as after myocardial infarction 1
• The site, extent, and depth of abnormalities are readily assessed 1

Clinical Applications

Primary Indications

• Diagnosis of coronary heart disease in patients with suspected CAD 1
• Prognostication and risk stratification 1
• Selection for revascularization procedures 1
• Assessment of acute coronary syndromes 1

Special Patient Populations

• MPI has particular advantages in women, the elderly, and patients with diabetes 1
• It provides functional assessment complementary to anatomical coronary angiography 1

Safety Profile

Procedural Safety

• MPI is non-invasive and safe compared to invasive coronary angiography 1
• Complication rates are extremely low:
  • Exercise stress: ≤0.01% deaths and ≤0.02% morbidity 1
  • Pharmacological stress: Similar safety profile 1
• These rates are at least two orders of magnitude less than the risk of underlying coronary disease 1

Radiation Exposure

• Radiation exposure is comparable to CT angiography and coronary angiography 1
• The exposure produces a 1 in 1,800 risk of cancer 10-20 years after exposure 1
• This risk is small compared to the lifetime cancer risk in the general population (1 in 3) and is of lesser importance in elderly patients with known coronary disease 1

Absolute Contraindications

• High-risk unstable angina or acute coronary syndrome within 2 days 3
• Significant arrhythmias (ventricular tachycardia, second- or third-degree AV block) 3
• Significant hypotension (systolic BP <90 mmHg) 3
• Uncontrolled heart failure 3
• Sinus bradycardia <45 bpm 3
• Vasodilator-specific: Known bronchoconstrictive or bronchospastic disease when using adenosine, dipyridamole, or regadenoson 3

Common Pitfalls

• Normal perfusion does not exclude non-obstructive coronary disease, but such disease is unlikely to be symptomatic or prognostically important 1
• Complete accuracy in predicting coronary angiography findings is neither expected nor necessary for clinical management, as MPI assesses physiological significance rather than pure anatomical stenosis 1
• Small fixed defects (<20% of myocardial segments) carry minimal perioperative risk and predict long-term rather than acute cardiac events 4