A guide wire (10) for a catheter carrying an implant includes alternating radio-opaque zones (16) and radio-transparent zones (18) formed from a granular material at a portion of the guide wire (10) just proximal to a flexible tip (12). The granular material is compressed within a sheath formed by the guide wire (10) and held in place by a bung (11). By regularly spacing the radio-opaque zones (16), the guide wire (10) enables in situ measurements to be taken within a patient's vasculature. The use of granular material to form the radio-opaque zones (16) and the radio-transparent zones (18) provides rigidity to the portion of the guide wire (10) proximal to the tip (12).