An impedance reflector apparatus, systems, and methods are provided for detecting a marker implanted within tissue that includes a switch for changing a configuration of an antenna of the marker. The apparatus includes a set of transmit electrodes coupled to a signal generator for transmitting a drive current into tissue to generate an electromagnetic field around the marker, a set of receive electrodes configured to detect voltage signals within the tissue corresponding to the electromagnetic field, and a light source for delivering light pulses into the body to open and close the switch to change the configuration of the antenna of the marker. A processor coupled to the receive electrodes processes the detected voltage signals to identify changes in the electromagnetic field that are synchronized with the light pulses to determine whether the marker is operating properly.
