Abstract: A pulse-echo ultrasonic transducer probe includes a substantially centrally disposed axial bore running therethrough and a radial slot extending from the bore to the periphery of its housing. A slotted cap dimensioned to rotatably fit over the top of the housing is provided with a hollow semi-circular stem extending into the bore, the slot of the cap being aligned with the longitudinal opening of the stem. The slot of the cap and that of the transducer, when passing a surgical instrument through the bore into an opaque body, are rotatably adjusted so that they are out of registry with one another for supporting the surgical instrument and for maintaining the instrument in the center of an ultrasonic search beam. Subsequently, after the instrument has reached the desired position in the body the slots are brought into registry for providing removal of the transducer probe from around the instrument.