Abstract: The invention falls within the art field of electronic detection and measuring instruments for medical use. Connected to the audio output of a conventional Doppler device, the instrument disclosed processes the Doppler signal (P.sub.t) and produces an output signal (R.sub.t) in the form of a quantity per unit of time which is proportional to the radius of a circular cross-section equivalent in area to the effective cross-section of the blood vessel under examination. The instrument provides a video image of this `relative` cross-section as well as producing a numerical read-out proportional to the maximum and the minimum radius of the relative cross-section which register during heartbeat.