1. A method of estimating a blood flow characteristic in a patient, comprising: providing a first probability distribution for an actual value of a function of the blood flow characteristic based on a previous value of the function; providing a second probability distribution describing a probability that observations of the blood flow characteristic were made given that the blood flow characteristic took a certain value; and combining the first and second probability distributions to facilitate selection of a most likely value of the function for posting as the value of the estimated blood flow characteristic.