Acoustic imaging catheters formed by a disposable liquid-confining sheath supporting a high fidelity, flexible drive shaft which carries on its end an ultrasound transducer. The shaft and transducer rotate with sufficient speed and fidelity to produce real time images on a T.V. screen. Special features that contribute to the high fidelity of the drive shaft include the particular multifilar construction of concentric, oppositely wound, interfering coils, a preloaded torque condition on the coils enhancing their interfering contact, and dynamic loading of the distal end of the probe, preferrably with viscous drag. The coil rotating in the presence of liquid in the sheath is used to produce a desirable pressure in the region of the transducer. Numerous selectable catheter sheaths are shown including a sheath with an integral acoustically-transparent window, sheaths with end extensions that aid in positioning, a liquid injection-producing sheath, a sheath having its window section under tension employing an axially loaded bearing, a sheath carrying a dilatation or positioning balloon over the transducer, a sheath carrying a distal rotating surgical tool and a sheath used in conjunction with a side-viewing trocar.