1. A system for planning treatment 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; create a three-dimensional model representing at least a portion of the anatomical structure of the patient based on the patient-specific data; determine a first blood flow characteristic within the anatomical structure of the patient based on the three-dimensional model and a physics-based model relating to the anatomical structure of the patient, the physics-based model having fewer than three dimensions; modify the three-dimensional model; and determine a second blood flow characteristic within the anatomical structure of the patient based on the modified three-dimensional model, wherein each of the first and second blood flow characteristics is chosen from blood pressure, blood velocity, blood flow rate, and fractional flow reserve within at least the portion of the anatomic structure.