1. A computer-implemented method of assessing a risk of disease, the method comprising: obtaining an anatomic model of a patient's vasculature; determining a location of the anatomic model associated with a narrowing of the patient's vasculature; determining a first point of interest upstream of the location associated with the narrowing of the patient's vasculature; receiving a value of concentration of one or more virtual contrast agents at the determined first point of interest; determining a density of the patient's blood and a density of the one or more virtual contrast agents; determining a two-phase system for modeling the flow of the one or more virtual contrast agents and the patient's blood using the determined density of the patient's blood and determined density of the one or more virtual contrast agents; modeling, using a processor, an injection of the one or more virtual contrast agents into the anatomic model using the two-phase system; determining a second point of interest downstream of the location associated with the narrowing of the patient's vasculature; computing, based on the modeling, a value of concentration of the one or more virtual contrast agents at the determined second point of interest; and determining one or more diagnostic flow metrics by comparing the received value of concentration at the determined first point of interest to the computed value of concentration at the determined second point of interest.