Fractional flow reserve as the reference standard for myocardial perfusion studies: fool's gold?

The evidence for the use of cardiovascular magnetic resonance (CMR) as a first-line diagnostic tool in patients with suspected coronary artery disease (CAD) has been solidified by the recent publication of large clinical studies and meta-analyses.1–3 Most of this evidence has been gathered against anatomical angiographic endpoints, despite the well-known fact that luminal stenosis correlates poorly with haemodynamic significance, particularly for coronary stenoses between 30 and 80% of luminal diameter.4,5 Multiple factors, such as entrance effects, friction, and turbulence, can contribute to coronary blood flow resistance and the loss of pressure across a stenosis, and therefore the haemodynamic significance of lesions can be underappreciated by two-dimensional angiographic images, particularly in the setting of eccentric or irregular stenoses.6 In clinical practice, coronary angiography is therefore often complemented by non-invasive imaging tests to determine the functional significance of a borderline stenosis and guide patient management. Recent clinical practice guidelines for revascularization recommend a combination of anatomical and ischaemia testing for most clinical scenarios.7 Invasive pressure-wire-derived fractional flow reserve (FFR) has become a popular alternative for the functional assessment of coronary stenosis that can be performed in the catheter laboratory. FFR is calculated as the ratio of maximal blood flow in a stenotic artery relative to maximal flow in the same artery in the theoretic absence of any stenosis.8–11 In recent years, …