A method for determining patient-specific blood flow characteristics comprises: acquiring, for each of a plurality of individuals, a geometric model and blood flow characteristics of at least part of the individual's vascular system, and one or more physiological parameters of the individual； executing a machine learning algorithm by, for a plurality of points in the geometric model for each of the plurality of individuals, creating a feature vector of the points or the physiological parameters and associating the feature vector for each of the plurality of individuals with the blood flow characteristics of the corresponding individual； and using the machine learning algorithm to identify features predictive of blood flow characteristics. The method also includes acquiring a geometric model of at least part of a patient's vascular system and using the identified features to determine a blood flow characteristic of the patient.
