In one aspect, the invention relates to system comprising: a processor configured to receive a first optical coherence tomography (OCT) data set obtained during a pullback of a data collection probe along a first length of a first blood vessel; determine a minimum lumen area disposed along the first length using the first OCT data set; and determine a first FFR value along the first length based on the minimum lumen area. In one embodiment, the first FFR value is an estimated FFR. In another aspect, the invention relates to a method that includes measuring, using OCT, the area of a lumen of a vessel for which the vessel's FFR is to be determined; and calculating, using a computer, A2m/(A2m+k) or YA2min/(YA2min+k) as a FFR value. In one embodiment, k is about 0.7 mm2 and γ is patient-specific variable that depends on the coronary branch in which the images were obtained.