Interelectrode impedance or electric field potential measurements
are used to determine the relative orientation of one lead to other leads in the
spinal column or other body/tissue location. Interelectrode impedance is determined
by measuring impedance vectors. The value of the impedance vector is due primarily
to the electrode-electrolyte interface, and the bulk impedance between the electrodes.
The bulk impedance between the electrodes is, in turn, made up of (1) the impedance
of the tissue adjacent to the electrodes, and (2) the impedance of the tissue between
the electrodes. In one embodiment, the present invention makes both monopolar and
bipolar impedance measurements, and then corrects the bipolar impedance measurements
using the monopolar measurements to eliminate the effect of the impedance of the
tissue adjacent the electrodes. The orientation and position of the leads may be
inferred from the relative minima of the corrected bipolar impedance values. These
corrected impedance values may also be mapped and stored to facilitate a comparison
with subsequent corrected impedance measurement values. Such comparison allows
a determination to be made as to whether the lead position and/or orientation has
changed appreciably over time. In another embodiment, one or more electrodes are
stimulated and the resulting electric field potential on the non-stimulated electrodes
is measured. Such field potential measurements provide an indication of the relative
orientation of the electrodes. Once known, the relative orientation may be used
to track lead migration, to setup stimulation configurations and parameters for
nominal stimulation and/or navigation. Also, such measurements allow automatic
adjustment of stimulation energy to a previously-defined optimal potential field
in the case of lead migration or postural changes.