A blood flow rate sensor has at least one transmitter for emitting waves into a blood vessel, the propagation of which is deflected by cellular blood components, and at least two receiver units for receiving waves emitted by the transmitter. The receiver units are spaced from each other in the direction of blood flow, and are situated such that each receives waves from a different path through the blood. The output signal of each receiver unit is filtered or otherwise processed to obtain a noise component, and the noise components from the receiver units are cross-correlated to determine a time offset between the output signals. The time offset is inversely proportional to the blood flow rate.
