Enhanced risk stratification with noninvasive measurement of coronary flow reserve using positron emission tomography.

Major progress has been made in the development and clinical application of dynamic imaging and tracer kinetic models to accurately measure absolute blood flow and coronary flow reserve (CFR) using quantitative positron emission tomography (PET) myocardial perfusion imaging (MPI) in patients with known or suspected coronary artery disease (CAD).1–6 Assessing CFR provides significantly greater separation of lowand high-risk subsets of patients compared with conventional single photon emission tomography or PET, imaging in which only relative tracer uptake is evaluated after exercise or pharmacological stress. Certainly, the traditional semiquantitative techniques used for many years for conventional single photon emission tomography and PET MPI have provided valuable diagnostic and prognostic information, particularly when extensive focal defects are identified on poststress images, which normalize on resting studies. In addition, the annual cardiac death or infarction rate is reported to be quite low ( 1.0%) in patients undergoing stress MPI as derived from a pooled analysis of the literature.7 Nevertheless, multivessel disease is underestimated by using semiquantitative analysis of relative tracer uptake, and some patients with extensive 3-vessel disease or left main disease have normal or low-risk scintigraphic findings.8 This is attributable to what is often referred to as balanced ischemia, characterized by a rather uniform diminution in tracer activity throughout the myocardium. No one area of the left ventricle has a higher myocardial blood flow than other areas, so when a perfusion tracer such as Tc-99m sestamibi or Rb-82 is injected at peak vasodilator stress, no relative focal defects are identified. Occasionally, a single defect region associated with transient ischemic dilation of the left ventricle is observed in such patients with extensive epicardial CAD and balanced ischemia. This is attributed to extensive subendocardial hypoperfusion encompassing 1 coronary supply region. Another indication that multivessel or left main CAD may be present without extensive defects is presence of multiple wall motion abnormalities on single photon emission tomography or PET scans. Article see p 1858