Conventionally, coronary flow reserve is measured during invasive coronary angiography by the fractional flow reserve using the flow wire which comes with intravascular ultrasound systems. In this pressure proximal and distal to the stenosis before and after vasodilatation with intravenous adenosine is measured and the ratio used to assess the coronary flow reserve. Murthy VL and colleagues from the Brigham & Women’s Hospital, Boston evaluated around two thousand and eight hundred patients referred for rest/stress positron emission tomography (PET) and followed them up for a median of 1.4 years. Rest and stress myocardial blood flows were calculated. Myocardial blood flow during stress divided by resting flow was taken as the coronary flow reserve. They found that the lowest tertile of coronary flow reserve was associated with a 5.6 fold increase in risk of cardiac death. The authors concluded that non invasive quantitative assessment of coronary flow reserve by PET is powerful and independent predictor of cardiac mortality in those with known or suspected coronary artery disease. PET provides incremental risk stratification over and above that provided by clinical criteria and gated myocardial perfusion imaging.