1. A system for image processing to determine 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 the patient, the anatomical structure including at least a portion of a plurality of coronary arteries emanating from the patient's aorta; create a three-dimensional geometric model representing the anatomical structure using the patient-specific data; generate a one-dimensional blood flow model defining blood flow through the anatomical structure of the patient using the three-dimensional geometric model; generate a pressure model using the three-dimensional geometric model, wherein the pressure model defines blood pressure at two or more locations within the anatomical structure represented by the three-dimensional geometric model; and determine at least one blood flow characteristic within the anatomical structure using the pressure model and the one-dimensional blood flow model, wherein the blood flow characteristic is a ratio of pressures at two or more locations within the anatomical structure.