1. A computer-implemented method of assessing tissue function, the method comprising: receiving a patient-specific tissue model generated from patient-specific imaging of at least an area or a volume of a patient's tissue; receiving a patient-specific vascular model generated from patient-specific imaging of at least a portion of a patient's vasculature; determining an association between a portion of the patient-specific vascular model and the area or the volume of the patient-specific tissue model; receiving an estimate of blood flow supplied from the portion of the patient-specific vascular model to the area or the volume of the patient-specific tissue model; calculating a value of perfusion to the area or the volume of the patient-specific tissue model using the received estimate of blood flow, one or more vessel outlets of the portion of the patient-specific vascular model, and a measure of the area or a measure of the volume of the patient-specific tissue model; calculating a patient-specific tissue characteristic value using the calculated value of perfusion to the area or the volume of the patient-specific tissue model; and outputting the calculated patient-specific tissue characteristic value.