Chapter 5: Quantitative Myocardial Blood Flow: Practical Clinical Perspective with Case Examples

INTRODUCTION

Measurement of blood flow to the myocardium represents a quantum leap forward for the field of nuclear cardiology. Quantification of myocardial blood flow as a routine part of rest/stress myocardial perfusion imaging is a unique attribute of cardiac PET. Although the quantification is actually measured in mL/min/g of myocardium, it is usually expressed as a ratio between peak hyperemia and rest. This ratio is termed myocardial blood flow reserve (MBFR) or coronary flow reserve (CFR). In this chapter we use the term MBFR to be in agreement with a recently published joint position paper of the American Society of Nuclear Cardiology (ASNC) and the Society of Nuclear Medicine and Molecular Imaging (SNMMI).¹ MBFR can be measured on a pixel-by-pixel basis if there are sufficient counts, but for clinical applications MBFR is reported globally (an average for the entire myocardium), segmentally, or by coronary territory. Some patients have high resting flows that can artifactually lower MBFR; therefore, it is important to also consider peak hyperemic myocardial blood flow (MBF) in some cases, especially in patients with low MBFR and high resting MBF. There has not been a consensus as to whether peak hyperemic MBF or MBFR is superior for assessing epicardial and microvascular health.^2,3 A recent study examined cardiovascular mortality in more than 4000 consecutive patients, and found that MBFR was a stronger predictor of outcomes than peak flow.⁴ In keeping with many studies in the literature, in this chapter we focus primarily on MBFR.

Why is myocardial perfusion PET and the ability to quantify MBFR such an important advance for nuclear cardiology? One of the important attributes of SPECT myocardial perfusion imaging is its ability to risk-stratify populations based on the size and severity of perfusion defects.^5,6 Those whose findings suggest moderate to high risk are often referred to coronary angiography with the expectation of revascularization.⁷ Unfortunately, clinicians and interventionists have recognized the shortfalls of traditional spatially relative interpretations of myocardial perfusion studies. When in the catheterization laboratory with an individual patient, decisions need to be made when the coronary angiography appearances deviate from those anticipated from the MPI reports. Publications from the nuclear cardiology community acknowledge the deficiencies of spatially relative interpretation, in which image appearances may suggest limited CAD in patients found to have multivessel CAD at angiography.^8,9 Other publications emphasize the false-positive potential of MPI due primarily to attenuation artifacts.^10,11 Hence, the issues with strong population-based evidence and yet management challenges with application of this population-based evidence to a significant number of individual patients. Cardiac PET MPI has several attributes that can address these problems. First, every study is attenuation corrected. Second, with ⁸²Rb, the gated stress images are acquired under peak hyperemia conditions, such that regional changes in wall thickening or motion indicate ischemia and hence highly narrowed coronaries. Under conditions of increased myocardial blood flow, global increases ...