How to assess a pet for microvascular disease using a Myocardial Perfusion Imaging (MPI) test?

Assessing PET MPI for Microvascular Disease

The most effective way to assess microvascular disease using PET MPI is to calculate the myocardial blood flow reserve (MBFR or MFR), with values <1.9-2.0 indicating microvascular dysfunction, even in the absence of visible perfusion defects. 1

Key Parameters to Evaluate

• Absolute Myocardial Blood Flow (MBF): Quantify both rest and stress (hyperemic) absolute myocardial blood flow values 2
• Myocardial Flow Reserve (MFR): Calculate the ratio of stress to rest absolute myocardial blood flow; normal MFR is ≥2.0, while values <2.0 suggest impaired coronary flow reserve 1, 2
• Regional Distribution: Assess for uniform reduction in MFR across multiple coronary territories, which is characteristic of microvascular disease rather than focal epicardial stenosis 1, 2
• LVEF Reserve: Evaluate the change in left ventricular ejection fraction from rest to stress, as abnormal LVEF reserve provides incremental value in predicting microvascular dysfunction 1

Advantages of PET for Microvascular Assessment

• PET offers superior spatial resolution compared to SPECT, improving diagnostic accuracy particularly in women and obese patients 1
• PET uniquely allows calculation of absolute blood flow across coronary vessels, which is essential for detecting microvascular dysfunction 1
• The overall improvement in diagnostic accuracy is approximately 20% for PET compared to SPECT MPI (88% versus 67%, p=0.009) 1
• PET can detect microvascular dysfunction even when perfusion images appear normal or show only low-risk findings 1, 3

Interpretation Guidelines

• Normal MFR (≥2.0): Indicates normal coronary vasodilatory capacity 2
• Mildly Reduced MFR (1.8-2.0): Suggests early microvascular dysfunction requiring optimization of medical therapy 2
• Moderately to Severely Reduced MFR (<1.8): Indicates significant microvascular dysfunction 2, 3
• Pattern Recognition: Diffuse reduction in MFR across multiple territories suggests microvascular disease, while territory-specific reductions may indicate epicardial disease 2, 3

Clinical Correlation

• Correlate MFR findings with patient symptoms, particularly in women who have higher prevalence of non-obstructive CAD and microvascular dysfunction 1
• Consider the presence of risk factors for microvascular disease (diabetes, hypertension, smoking) when interpreting borderline MFR values 2
• Evaluate for discordance between perfusion images and MFR values - normal perfusion with reduced MFR is a hallmark of isolated microvascular dysfunction 2, 3

Technical Considerations

• Rb-82 and N-13 ammonia are the most commonly used radioisotopes for PET MPI 1, 4
• Ensure adequate vasodilator stress response by assessing splenic switch-off (reduced splenic uptake during stress compared to rest) 5
• Consider hybrid PET/CT to evaluate both functional and anatomical aspects of coronary circulation 1

Pitfalls to Avoid

• Do not rely solely on visual assessment of perfusion defects, as microvascular disease may not produce visible perfusion abnormalities 1, 2
• Avoid misinterpreting globally reduced MFR as technical error; this may represent diffuse microvascular disease or balanced multi-vessel epicardial disease 1, 2
• Ensure adequate pharmacologic stress, as inadequate vasodilation can falsely reduce MFR values 5
• Don't overlook the prognostic value of reduced MFR, as it independently predicts adverse cardiovascular events even in patients with normal perfusion images 2, 3

By systematically evaluating these parameters, clinicians can effectively use PET MPI to detect and characterize microvascular disease, enabling appropriate management strategies for this often overlooked condition.