Systems enable a method of determining the rate of flow of a bodily fluid through a vessel in a live patient by:positioning at least two separate passive infrared sensors at two points along a length of the vesselsensing local temperature changes at the two points along the vessel to generate signals of the local temperature changes from each passive infrared sensora processor receiving the signals and quantifying the local temperature changes at the two points andthe processor executing code to determine blood flow rate in the vessel from the local temperature changes quantified by the processor at the two points.
