Abstract: The invention is directed to apparatus, methods and systems including a sheath for use with intracorporeal optical imaging instruments such as imaging guidewires, catheters, or endoscopes. The invention provides a sheath suitable for guiding an enclosed instrument, that is effective to guide the placement within a patient's body and replacement to a distal position after retraction of the imaging instrument, as during an imaging scan. The sheaths may include at least a portion that is translucent to a desired wavelength of radiation. The translucent portion may have an index of refraction similar to the index of refraction of a bodily fluid such as blood plasma, or an artificial fluid suitable for introduction into a body lumen.