A signal processing unit (208) for determining a value of a blood flow quantity characterizing blood flow inside a blood vessel comprises a vibration sensor signal input, which is configured to receive a vibration sensor signal from an intravascular blood flow sensor, the vibration sensor signal comprising a vibration sensor signal component caused by vortex-generated blood flow oscillations of intravascular blood flow, and a blood flow determination unit which is configured to determine the vibration sensor signal component using the vibration sensor signal, to determine an oscillation frequency of the vortex-generated blood flow oscillations using the vibration sensor signal component and to determine and provide the value of the blood flow quantity using the determined oscillation frequency.
