A rotatable thrombectomy wire for breaking up thrombus or other obstructive material comprising an inner core composed of a flexible material and an outer wire surrounding at least a portion of the inner core. A configuration of the superelastic alloy is provided with a geometric structural feature of the alloy that has an extent that is no greater than about 200 micrometers and that is no larger than the average grain size of the alloy. The photometer may measure the optical property of the background along the path between the start position and the end position.