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 an anatomic model representing at least a portion of the anatomical structure of the patient based on the patient-specific data; identify at least one point of interest within the anatomical structure of the patient in the anatomic model; determine a first blood flow rate within the anatomical structure of the patient at the at least one point of interest in the anatomic model; modify a lumen diameter of the anatomic model at the point of interest; compute a second blood flow rate based on the modified lumen diameter of the anatomic model at the point of interest; and determine a fractional flow reserve value as a ratio of the second blood flow rate to the first blood flow rate in the model.