1. A system for determining cardiovascular information for a patient, the system comprising: at least one computer system configured to: receive patient-specific data regarding a geometry of an anatomical structure of a patient; create a model representing at least a portion of the anatomical structure of the patient based on the patient-specific data; create a physics-based model relating to a blood flow characteristic within the anatomical structure of the patient; determine a first blood flow rate within the anatomical structure of the patient at at least one point of interest of the model, based on a solution of the physics-based model; modify the model; determine a second blood flow rate at a point in the modified model corresponding to the at least one point of interest in the model; and determine a fractional flow reserve value as a ratio of the second blood flow rate in the modified model to the first blood flow rate in the model.