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.