An apparatus adapted to perform measurements in an artery, comprising: a Doppler catheter, including a Doppler transducer and an electrical wire connected to the Doppler transducer, the Doppler catheter further comprising a first connector and a second connector, the first connector containing the electrical wire; an elongated catheter body having a conduit therein for housing the electrical wire of the Doppler catheter, the elongated catheter body having a proximal end and a distal end; a third connector attached to the proximal end of elongated catheter body, the third connector shaped to mate with the first connector to form a stable connection between the elongated catheter body and the electrical wire; and a sleeve connected to the first connector and the second connector, the sleeve enclosing the electrical wire and protecting the electrical wire from an environment when the electrical wire is withdrawn from the conduit in the elongated catheter body, and wherein the sleeve is shorter than the electrical wire such that when the electrical wire is withdrawn from the conduit in the elongated catheter body, the sleeve limits an extent by which the electrical wire is withdrawn from the conduit, through the stable connection; wherein the electrical wire is movable in the conduit relative to the elongated catheter body so that the Doppler transducer and the electrical wire are capable of being threaded into the conduit at the proximal end through the stable connection after the distal end of the Doppler catheter has been inserted into the artery, until the Doppler transducer emerges outside the conduit at the distal end of the elongated catheter body for performing ultrasound measurements in the artery. a Doppler catheter, including a Doppler transducer and an electrical wire connected to the Doppler transducer, the Doppler catheter further comprising a first connector and a second connector, the first connector containing the electrical wire; an elongated catheter body having a conduit therein for housing the electrical wire of the Doppler catheter, the elongated catheter body having a proximal end and a distal end; a third connector attached to the proximal end of elongated catheter body, the third connector shaped to mate with the first connector to form a stable connection between the elongated catheter body and the electrical wire; and a sleeve connected to the first connector and the second connector, the sleeve enclosing the electrical wire and protecting the electrical wire from an environment when the electrical wire is withdrawn from the conduit in the elongated catheter body, and wherein the sleeve is shorter than the electrical wire such that when the electrical wire is withdrawn from the conduit in the elongated catheter body, the sleeve limits an extent by which the electrical wire is withdrawn from the conduit, through the stable connection; wherein the electrical wire is movable in the conduit relative to the elongated catheter body so that the Doppler transducer and the electrical wire are capable of being threaded into the conduit at the proximal end through the stable connection after the distal end of the Doppler catheter has been inserted into the artery, until the Doppler transducer emerges outside the conduit at the distal end of the elongated catheter body for performing ultrasound measurements in the artery.