A method of determining the distance between an electrode catheter disposed in a body fluid adjacent an internal body surface, and the internal body surface, the method comprising: applying an alternating voltage or an alternating current that alternates at between about 10 kHZ and about 100 kHz between at least one pair of electrodes on the electrode catheter； determining the impedance between at least one pair of electrodes on the electrode catheter； and determining the distance between the electrode catheter and the internal body surface based at least in part on the determined impedance.
