Apparatus for detecting a surgical implement in human or animal tissue comprises a detector responsive to the presence, within an interrogation zone, of a surgical implement to which a marker is secured. The marker is adapted to produce identifying signal characteristics within a frequency band generated...http://www.google.com.hk/patents/US5057095?utm_source=gb-gplus-share專利 US5057095 - Surgical implement detector utilizing a resonant marker
