An apparatus for performing measurements in an artery includes a Doppler catheter, comprising a Doppler transducer and a wire connected to the Doppler transducer; and an elongated catheter body having a conduit therein for housing the wire of the Doppler catheter. The body has a proximal end and a distal end; wherein the wire is movable in the conduit relative to the catheter body so that the Doppler transducer and the wire are capable of being threaded into said conduit at the proximal end after the distal end of the catheter has been inserted into the artery, until the Doppler transducer emerges outside the conduit at the distal end of said body for performing ultrasound measurements in the artery. Another embodiment employs at least an additional side transducer for measuring the cross-sectional dimension of the artery useful for computing blood flow.