Apparatus is provided for tracking an object in a body of a patient in the presence of an interfering article. A set of one or more radiators are adapted to generate an energy field at a fundamental frequency in a vicinity of the object. A position sensor, fixed to the object, is adapted to generate...http://www.google.com/patents/US7433728?utm_source=gb-gplus-sharePatent US7433728 - Dynamic metal immunity by hysteresis
