1. A computer-implemented method of simulating or optimizing hemodialysis access, the method comprising: receiving a patient-specific, three-dimensional anatomic model of a patient's vasculature; computing a pre-treatment hemodynamic characteristic of a pre-treatment geometry of a portion of the received three-dimensional anatomic model; modifying the geometry of the received three-dimensional anatomic model such that the geometry of the received three-dimensional anatomic model defines a post-treatment geometry of a vascular access in the portion of the received three-dimensional anatomic model, the post-treatment geometry being different from the pre-treatment geometry of the portion of the received three-dimensional anatomic model; determining a computational model of a hemodynamic characteristic of the modified three-dimensional anatomic model defining the post-treatment geometry of the vascular access; computing a post-treatment hemodynamic characteristic using the computational model associated with the modified three-dimensional anatomic model; and generating a representation of the post-treatment hemodynamic characteristic.