Apparatus and method for determining the relative position and orientation of neurostimulation leads

Abstract

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.

1 - 17 . (canceled) 
   
   
       18 . A method of correcting the relative migration between neurostimulation leads implanted within a patient, the neurostimulation leads carrying an array of electrodes programmed with stimulation parameters, the method comprising:
 conveying electrical current through a first group of electrodes in accordance with the stimulation parameters, thereby generating a nominal electrical field to stimulate tissue of the patient;   allowing the neurostimulation leads to migrate relative to each other, thereby changing a relative position or orientation between the neurostimulation leads;   conveying electrical energy to or from an electrode on one of the neurostimulation leads after the neurostimulation leads have migrated relative to each other;   measuring an electrical parameter at an electrode on another of the neurostimulation leads in response to the conveyance of the electrical energy;   analyzing the measured electrical parameter to determine the relative position or orientation between the migrated neurostimulation leads; and   reprogramming the electrodes with new stimulation parameters in response to the determination of the relative position or orientation between the neurostimulation leads.   
   
   
       19 . The method of  claim 18 , further comprising automatically computing the new stimulation parameters in response to determining the relative position or orientation between the neurostimulation leads. 
   
   
       20 . The method of  claim 18 , further comprising manually selecting the new stimulation parameters in response to determining the relative position or orientation between the neurostimulation leads. 
   
   
       21 . The method of  claim 18 , wherein the electrical parameter is an impedance. 
   
   
       22 . The method of  claim 18 , wherein the electrical parameter is a field potential. 
   
   
       23 . The method of  claim 18 , wherein the electrodes are reprogrammed with the new stimulation parameters to maintain the generated nominal electrical field. 
   
   
       24 . The method of  claim 18 , further comprising conveying electrical current through a second group of electrodes in accordance with the new stimulation parameters, wherein at least one of electrode in the second electrode group is not included in the first electrode group. 
   
   
       25 . The method of  claim 18 , wherein a first electrode in the first electrode group is carried by one of the neurostimulation leads, a second electrode in the first electrode group of the same polarity as the first electrode is carried by the other neurostimulation lead, the migration of the neurostimulation leads relative to each other causes the first electrode to be physically displaced closer to the second electrode, and the electrodes are reprogrammed with the new stimulation parameters to decrease the amplitude of electrical current flowing through the first electrode. 
   
   
       26 . The method of  claim 18 , wherein a first electrode in the first electrode group is carried by one of the neurostimulation leads, a second electrode in the first electrode group of the same polarity as the first electrode is carried by the other neurostimulation lead, the migration of the neurostimulation leads relative to each other causes the first electrode to be physically displaced further from the second electrode, and the electrodes are reprogrammed with the new stimulation parameters to increase the amplitude of electrical current flowing through the first electrode. 
   
   
       27 . The method of  claim 18 , further comprising:
 conveying electrical energy to or from the electrode on the one neurostimulation lead before the neurostimulation leads have migrated relative to each other;   measuring another electrical parameter at the electrode on the other neurostimulation lead in response to the conveyance of the electrical energy;   storing the other electrical parameter in memory; and   comparing the electrical parameter to the other electrical parameter to determine the relative position or orientation between the neurostimulation leads.   
   
   
       28 . The method of  claim 18 , further comprising displaying the determined relative position or orientation of the neurostimulation leads. 
   
   
       29 . A tissue stimulation system, comprising:
 implantable neurostimulation leads carrying an array of electrodes;   an implantable control device programmed with stimulation parameters to convey electrical current through a first group of electrodes, thereby generating a nominal electrical field to stimulate tissue of the patient, the implantable device configured for conveying electrical energy to or from an electrode on one of the neurostimulation lead, and measuring an electrical parameter at an electrode on another of the neurostimulation leads in response to the conveyance of the electrical energy; and   an external programmer configured for analyzing the measured electrical parameter to determine the relative position or orientation between the neurostimulation leads, and for reprogramming the implantable control device with new stimulation parameters in response to determining that the neurostimulation lead have migrated relative to each other.   
   
   
       30 . The system of  claim 29 , wherein the external programmer is further configured for automatically computing the new stimulation parameters in response to determining the relative position or orientation between the neurostimulation leads. 
   
   
       31 . The system of  claim 29 , wherein the external programmer is further configured for allowing a user to manually select the new stimulation parameters in response to determining the relative position or orientation between the neurostimulation leads. 
   
   
       32 . The system of  claim 29 , wherein the electrical parameter is an impedance. 
   
   
       33 . The system of  claim 29 , wherein the electrical parameter is a field potential. 
   
   
       34 . The system of  claim 29 , wherein the external programmer is configured for reprogramming the implantable control device with the new stimulation parameters to maintain the generated nominal electrical field if the neurostimulation leads have migrated relative to each other. 
   
   
       35 . The system of  claim 29 , wherein the external programmer is configured for reprogramming the implantable control device to convey electrical current through a second group of electrodes, wherein at least one of electrode in the second electrode group is not included in the first electrode group. 
   
   
       36 . The system of  claim 29 , wherein the external programmer is further configured for displaying the determined relative position or orientation of the migrated neurostimulation leads.