An endo-luminal ultrasonic instrument has a motor with a rotatable drive shaft that oscillates back and forth. A flexible catheter sheath is coupled at one end to the motor to oscillate with the drive shaft. The sheath has an axis about which it oscillates and a diameter sized to fit inside a human blood vessel or other body lumen. An ultrasonic transducer is attached to the other end of the sheath to oscillate therewith. The transducer has an array of individually operable elements arranged parallel to the axis to form a rotatable beam perpendicular to the axis. Electronics near the motor operates the transducer elements. Wiring, passing through the catheter sheath connects electrically the transducer elements and the electronics. The electronics includes electronic beam focusing to enhance the axial resolution of the instrument.