1. A non-transitory computer readable medium for use on a computer system containing computer-executable programming instructions for performing a method for planning treatment for a patient, the method comprising: receiving patient-specific data regarding a geometry of an anatomical structure of the patient; creating a three-dimensional model representing at least a portion of the anatomical structure of the patient based on the patient-specific data; determining first information regarding a blood flow characteristic within the anatomical structure of the patient based on the three-dimensional model and a patient-specific physics-based model relating to the anatomical structure of the patient; creating a reduced-order model of the portion of the anatomical structure by using the first information to define a plurality of resistance values of the reduced-order model; receiving input from a user selecting a first plan of treatment, the received input including a selection of a desired location for widening of the anatomical structure; modifying one of the resistance values at a location corresponding to the desired location based on the received input; and determining second information regarding the blood flow characteristic within the anatomical structure of the patient based on the reduced-order model.