1. A computer-implemented method of modeling at least a portion of a patient's vasculature, using a computer system, the method comprising: obtaining an initial model of at least a portion of a patient's coronary vasculature; obtaining a first image of the patient's coronary vasculature using a first imaging modality; determining, from the first image, a first lumen boundary of a vessel of the patient's coronary vasculature; obtaining, using a second imaging modality different from the first imaging modality, a second image of the patient's coronary vasculature; determining, from the second image, a second lumen boundary of a vessel of the patient's coronary vasculature; updating the first lumen boundary based on the second lumen boundary; and generating an updated model of the patient's coronary vasculature based on the updated first lumen boundary.