A medical instrument comprising a flexible, filamentous shaft slideably disposed in a sheath, the instrument including an electronic imaging system comprising an image sensor carried on a distal end portion of the instrument. The shaft may be used as a guidewire for a complementary guided device, or it may be used to carry a functional element for performing a procedure at a target site in a patient's body. In other embodiments, the present invention contemplates a flexible sheath, preferably having a simple tubular construction, with an electronic imaging system at its distal end. The sheath is adapted to slideably receive a shaft, preferably a filamentous shaft, that closely fits the sheath. The shaft carries functional element at its distal end. The instruments according to the present invention may include one or more filaments along their length for deflecting an insertable portion of the instrument.