Multiple studies have documented significant interobserver variability in the grading of coronary artery steno- with disease severity overestimated by visual assessment when coronary stenosis is 50%.21,22 Although quantitative coro- nary angiography provides a more accurate assessment of lesion severity than does visual assessment, it is rarely used in clinical practice because it does not accurately assess the physiological significance of lesions.23 Many stenoses considered to be severe by visual assessment of coronary angiograms (ie, 70% luminal narrowing) do not restrict coronary blood flow at rest or with maximal dilatation, whereas others considered to be insignifi- cant (ie, <70% luminal narrowing) are hemodynamically sig- Coronary angiography also cannot assess whether an atherosclerotic plaque is stable or vulnerable (ie, likely to rup- ture and cause an acute coronary syndrome). Intravascular ultrasound and optical coherence tomography provide more precise information about the severity of stenosis and plaque morphology than does coronary angiography and, in certain cases, can be useful adjunctive tests.9 These imaging pro- cedures are discussed in the 2011 PCI guideline.9 FFR can assess the hemodynamic significance of angiographically intermedi- ate or indeterminant lesions and allows one to decide when PCI may be beneficial or safely deferred.24,25 It has been sug- gested in several studies that a PCI strategy guided by FFR may be superior to a strategy guided by angiography alone.5,24,26,27 Invasive procedures may cause complications. Data from the ACC s National Cardiovascular Data Registry CathPCI Registry during the 2012 calendar year included a 1.5% incidence of pro- cedural complications of diagnostic angiography. 