Abstract:
A spinal cord stimulation apparatus and method for automatic adjustments of SCS using near-infrared (NIR) reflectometry are provided. A positionally sensitive system for spinal cord stimulation including an electrode assembly with integrated optical components for sensing spinal cord position relative to a stimulating electrode array and an SCS controller for controlling electrode stimulation parameters is provided. The integrated optical components include an IR emitter and a pair of IR photodetectors. As light from the IR emitter reflects from the spinal cord, it is detected by each of the pair of IR photodetectors. As the spinal cord changes position so do the angles of incidence for detected light from the IR emitter, a ratio of optical intensities in combination with a total optical intensity is measured and used to interpolate a set of electrode stimulation settings from a calibration table. Electrode pulse characteristics are adjusted in real time to minimize changes in stimulation perceived by the patient during motion.

Description:
FIELD OF INVENTION 
     This invention relates generally to spinal cord stimulation (SCS) and technique for automatic adjustments of SCS using near-infrared (NIR) reflectometry. 
     BACKGROUND 
     Spinal cord stimulation is a technique which uses an implanted electrode array to control chronic pain. The electrode array is typically implanted in a fixed position within the epidural space near the spinal cord. A signal generator delivers current pulses to the spinal cord via the implanted electrode array. The current pulses induce parasthesiae which help block the perception of pain. 
     In  FIG. 1 , spinal column  1  is shown to have a number of vertebrae, categorized into four sections or types: lumbar vertebrae  2 , thoracic vertebrae  3 , cervical vertebrae  4  and sacral vertebrae  5 . Cervical vertebrae  4  include the 1st cervical vertebra (C1) through the 7th cervical vertebra (C7). Just below the 7th cervical vertebra is the first of twelve thoracic vertebrae  3  including the 1st thoracic vertebra (T1) through the 12th thoracic vertebra (T12). Just below the 12th thoracic vertebrae  3 , are five lumbar vertebrae  2  including the 1st lumbar vertebra (L1) through the 5th lumbar vertebra (L5), the 5th lumbar vertebra being attached to sacral vertebrae  5  (S1 to S5), sacral vertebrae  5  being naturally fused together in the adult. 
     In  FIG. 2 , representative thoracic vertebra  10  is shown to have a number of notable features which are in general shared with lumbar vertebrae  2  and cervical vertebrae  4 . The thick oval segment of bone forming the anterior aspect of vertebra  10  is vertebral body  12 . Vertebral body  12  is attached to bony vertebral arch  13  through which spinal nerves  19  run. Vertebral arch  13 , forming the posterior of vertebra  10 , is comprised of two pedicles  14 , which are short stout processes that extend from the sides of vertebral body  12  and bilateral laminae  15 . The broad flat plates that project from pedicles  14  and join in a triangle to form a hollow archway, spinal canal  16 . Spinous process  17  protrudes from the junction of bilateral laminae  15 . Transverse processes  18  project from the junction of pedicles  14  and bilateral laminae  15 . The structures of the vertebral arch protect spinal cord  20  and spinal nerves  19  that run through the spinal canal. 
     Surrounding spinal cord  20  is dura  21  that contains cerebrospinal fluid (CSF)  22 . Epidural space  24  is the outermost part of the spinal canal. It is the space within the spinal canal formed by the surrounding vertebrae lying outside the dura. 
     Referring to  FIGS. 1 ,  2  and  3 , the placement of an electrode array for spinal cord stimulation according to the prior art is shown. Electrode array  30  is positioned in epidural space  24  between dura  21  and the walls of spinal canal  16  towards the dorsal aspect of the spinal canal nearest bilateral laminae  15  and spinous process  17 . 
       FIG. 4  shows a prior art electrode array  30  including a set of electrode contacts  35  sealed into elastomeric housing  36 . Electrode array  30  has a set of electrode leads  31  which are connected to electrical pulse generator  32 . The electrical pulse generator may be outside of the body or it may be implanted subcutaneously. Each electrode contact has a separate electrical conductor in the set of electrode leads  31  so that the current to each contact may be independently conducted and controlled. 
     The anatomical distribution of parasthesiae is dependent upon the spatial relationship between a stimulating electric field generated by the electrode array and the neuronal pathways within the spinal cord. The distribution may be changed by altering the current across one or more electrodes of the electrode array. Changing anode and cathode configurations of the electrode array also alters the distribution and hence, the anatomical pattern of the induced parasthesiae. 
     Proper intensity of the current pulses is important. Excessive current produces an uncomfortable sensation. Insufficient current produces inadequate pain relief. Body motion, particularly bending and twisting, causes undesired and uncomfortable changes in stimulation due to motion of the spinal cord relative to the implanted electrode array. 
     There are methods and systems for controlling implanted devices within the human body. For example, Ecker et al, in U.S. Patent Application No. 2010/0105997, discloses an implantable medical device that includes a controller and a plurality of sensor modules. A sensor includes at least one light source that emits light at a particular wavelength, which scatters through blood-perfused tissue a detector senses the light reflected by blood mass of a patient. 
     U.S. Pat. No. 7,684,869 to Bradley, et al. discloses a system using an interelectrode impedance to determine the relative orientation of a lead other leads in the spinal column. Bradley et al. further disclose that interelectrode impedance may be used to adjust stimulation energy. 
     U.S. Patent Application No. 2009/0118787 to Moffitt, et al. discloses electrical energy conveyed between electrodes to create a stimulation region. Physiological information from the patient is acquired and analyzed to locate a locus of the stimulation region. The stimulation region is electronically displaced. 
     Deficiencies exist in the prior art related to accuracy of spinal cord stimulation in relieving pain under changing circumstances. The deficiencies are most pronounced while the patient is moving. The prior art does not provide a satisfactory way to automatically adjust spinal cord stimulation to compensate for motion between the electrodes and the spinal cord to maintain a constant level of pain relief during patient motion. 
     SUMMARY OF PREFERRED EMBODIMENTS 
     Embodiments of the present invention operate to automatically adjust spinal cord stimulation to compensate for patient movement. Automatic adjustment results in consistent parasthesiae and conservation of battery power. 
     A preferred embodiment includes an electrode array incorporating a centrally positioned infrared (IR) emitter bracketed by a set of electrodes. Adjacent the electrodes are a pair of photodetectors. The IR emitter and photodetectors are positioned facing the spinal cord. Light emitted from the IR emitter is reflected from the spinal cord and detected by the photodetectors. The first photodetector detects light at a first angle as measured from an optical axis of the IR emitter and generates a first current signal. The second photodetector detects light at a second angle as measured from the optical axis of the IR emitter and generates a second current signal. The detected current signals are used to vary the stimulation current delivered to the electrodes based on a calibration table. The varying current adjusts the induced electric field of each electrode to compensate for changes in the position of the spinal cord to achieve a constant electric field. 
     A method for calibration is also provided that creates the calibration table. The calibration table is used to store optimal current settings for the electrodes for several known physiological positions of the patient. 
     In another embodiment, provisions are made for remotely controlled operation of the stimulator. In this embodiment, a communications link is established between a remote calibration computer and the spinal cord stimulator to upload and download data. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The following disclosure is understood best in association with the accompanying figures. Like components share like numbers. 
         FIG. 1  shows a view of the human spine showing the various types of vertebrae and an approximate position of an electrode array for spinal cord stimulation; 
         FIG. 2  shows a transverse view of a thoracic vertebra indicating the position of the spinal cord and an electrode array for spinal cord stimulation; 
         FIG. 3  shows a sagital cross section view of the human spine showing the approximate position of an electrode array for spinal cord stimulation; 
         FIG. 4  shows a prior art electrode array for spinal cord stimulation; 
         FIGS. 5   a  and  5   b  show the relative electric field produced by a preferred embodiment as a spinal cord precesses about an orbit in the spinal canal; 
         FIG. 6   a  shows a cross-sectional view of an electrode array centrally located in relation to a spinal cord at 0° displacement; 
         FIG. 6   b  shows a cross-sectional view of an electrode array centrally located in relation to a spinal cord at 90° displacement; 
         FIG. 6   c  shows a cross-sectional view of an electrode array centrally located in relation to a spinal cord at 180° displacement; 
         FIG. 6   d  shows a cross-sectional view of an electrode array centrally located in relation to a spinal cord at 270° displacement; 
         FIG. 7  shows a schematic representation of a preferred embodiment of the positionally sensitive spinal cord stimulation system; 
         FIGS. 8   a  and  8   b  show two preferred embodiments of an electrode assembly; 
         FIG. 9  shows an alternate embodiment of an electrode assembly; 
         FIG. 10  shows an alternate embodiment of an electrode assembly; 
         FIG. 11   a  is a block diagram of the components of a preferred embodiment of a pulse generation and optical signal processing unit; 
         FIG. 11   b  is a block diagram of the components of a preferred embodiment of an SCS controller; 
         FIG. 11   c  is a block diagram of the components of a preferred embodiment of a calibration and programming unit; 
         FIGS. 12   a  through  12   d  are flow diagrams of a method of operation of a preferred embodiment; 
         FIGS. 13   a  through  13   c  are flow diagrams of a method of calibrating a preferred embodiment; 
         FIG. 14  is a graphic representation of a calibration table. 
     
    
    
     DETAILED DESCRIPTION 
     The distance between a stimulating electrode and the spinal cord surface may be inferred from a function dependent upon: 1) the optical path lengths of light between an IR emitter and a set of photodetectors, where the light is reflected from the spinal cord; 2) the spinal cord geometry; 3) the optical divergence of the IR emitter; and 4) the presence of chromophores in the optical path. 
     The dura surrounding the spinal cord itself is translucent to near infrared light. Near infrared light will be scattered by, and will reflect from the spinal cord. Cerebrospinal fluid will negligibly scatter near infrared light, but will not act as a significant reflector of near-infrared light. Light from the emitter passes through thin, relatively avascular, dura, to enter cerebrospinal fluid, CSF, which produces very little scattered light. Light incident on spinal cord experiences scatter resulting in a portion being reflected and another portion being absorbed by chromophores. 
     Optical absorption in fluid medium may be described by the Beer-Lambert Law (Beer&#39;s Law), which is reasonably accurate for a range of chromophores and concentrations. Beer&#39;s Law, states that the optical absorbance of a fluid with a chromophore concentration varies linearly with path length through the fluid and the chromophore concentration as:
 
 A   λ =ε λ   bc,   (1)
 
     where:
         ε λ =molar absorptivity or extinction coefficient of the chromophore at wavelength λ (the optical density of a 1-cm thick sample of a 1 M solution);   b=sample path length in centimeters; and   c=concentration of the compound in the sample, in molarity (mol L−1).       

     The absorbance (A λ ) at a wavelength λ is related to the ratio of light energy passing through the fluid, I, to the incident light energy, I 0 , in
 
 A   λ =−log( I/I   0 ).  (2)
 
     For deoxyhemoglobin and oxyhemoglobin, the extinction coefficient spectra are well known. 
     The path length within the spinal cord is dependent upon the geometry of the ellipsoid shaped spinal cord and its normal vector relative to the optical axes of the emitter and detector pair. 
     The optical path length within CSF is roughly equal to the nominal geometric path length as the scatter is small and the index of refraction does not vary considerably along the path. Light absorption of the CSF may be approximated by that of its primary constituent, H 2 0. Sensitivity of the system to CSF path length may be optimized using a light wavelength at a local maxima of the water extinction coefficient curve near 950-1000 nm. 
     When considering the emitter wavelength, one must also consider the extinction coefficients of the primary chromophores, deoxy- and oxy-hemoglobin. To minimize effects of blood flow changes within the spinal cord (although these are thought to be insignificant in the quasi-static sense), one may select the isosbestic wavelength of these chromophore species, preferably at about 805 nm. 
     The geometry of the emitter-detector pair relative to the spinal cord is the parameter most prone to variability. The variance results from factors such as dependence upon placement of the electrode within the spinal canal, canal diameter, spinal cord shape, spinal cord caliber, and presence of scoliotic or kyphotic curvature within the spine. Consequently, this geometric parameter is the primary reason that the system must be calibrated, in situ, in vivo. Spinal cord position may then be inferred through various methods from data obtained at extremes of body position. 
     The effects of geometry may be minimized by minimizing the angle between the emitter and detector optical axes relative to the spinal cord surface normal vector. 
     The beam divergence of the emitter relative to the incident and reflected rays will influence the detected light amplitude. 
     It is desirable to maintain a constant electric field at a group of target cells in the spinal cord as the spinal cord moves in order to consistently reduce the transmission of a pain sensation to the brain. As the patient bends forward to 0°, the spinal cord moves forward in its orbit in the spinal canal. An equal increase in stimulation pulse amplitude for each electrode pair is required to maintain the same electric field density. As the patient bends to the right 90°, the spinal cord moves to the right in its orbit in the spinal canal. A decrease in electrode stimulation pulse amplitude in the right electrode and an increase in electrode stimulation pulse amplitude in the left electrode of the electrode pair is required. As the patient bends backward to 180°, the spinal cord moves back in its orbit in the spinal canal. A decrease in electrode stimulation pulse amplitude is required to maintain a constant electric field across the spinal cord. As the patient bends to the left to 270°, the spinal cord moves to the left in its orbit. A decrease in electrode stimulation pulse amplitude in the left electrode and an increase in electrode stimulation pulse amplitude in the right electrode of the electrode pair is required. 
       FIGS. 5   a  and  5   b  show the relative electric field intensity required to be generated at a left and right electrode, respectively, for maintenance of a constant field at any point across in a horizontal cross section of the spinal cord as the spinal cord is moved through an orbit of 360° in the spinal canal. 
     Referring to  FIGS. 6   a  through  6   d , the positional relationship between the IR emitters, the photodetectors and the electrodes during spinal movement will be described. 
     Referring to  FIG. 6   a , spinal cord  20  is positioned forward at a 0° location in the spinal canal. Electrode array  40  is implanted outside dura  21 . IR emitter  42  is centrally positioned on optical axis  125 . Electrodes  41 L and  41 R are positioned toward the dura and within an operational range of target cells  19 . Photodetectors  43 L and  43 R are positioned within an operational range of spinal cord  20 . Target cells  19  are positioned within spinal cord  20  in an arbitrary but constant position with respect to the spinal cord. 
     In operation, IR emitter  42  produces light ray  48  which forms an angle  121  with optical axis  125 . Light ray  48  is reflected from spinal cord  20  and enters photodetector  43 R thereby producing an electrical current. IR emitter  42  also produces light ray  49  which forms angle  122  with optical axis  125 . Light ray  49  is reflected from spinal cord  20  and enters photodetector  43 L thereby producing an electric current. An electric field produced by electrode  41 R is produced reaching target cells  19 . Similarly, an electric field produced by electrode  41 L is produced reaching target cells  19 . Amplitudes A L  and A R  are the current to drive both the left and the right electrode, respectively. Both are relatively high. Light ray  48  traverses a distance D 1  between IR emitter  42  and photodetector  43 R. Light ray  49  traverses a distance of D 2  between IR emitter  42  and electrode  41 L. The distances D 1  and D 2  are roughly equal and both relatively high. The photocurrents produced by the photodetectors are roughly equal. 
     Referring to  FIG. 6   b , the spinal cord is shifted to the right 90° position by rotation through angle  128  and linear translation  127 . 
     In operation, IR emitter  42  produces light ray  48  which forms an angle  121  with optical axis  125 . Light ray  48  is reflected from spinal cord  20  and is received by photodetector  43 R. IR emitter  42  also produces light ray  49  which forms an angle  122  with optical axis  125 . Light ray  48  is reflected from spinal cord  20  and is received by photodetector  43 R. Light ray  49  is reflected from spinal cord  20  and is received by photodetector  43 L. The distance from electrode  41 L, to the target cells is relatively high compared to the distance from electrode  41 R. Hence, the current amplitude for electrode  43 L is relatively high compared to that of the electrode  43 R. The total distance traversed for light ray  48  is distance D 3 . The total distance traversed by light ray  49  is distance D 4 . It can be seen that distance D 3  is lesser than distance D 1  and distance D 2  and is relatively low. Distance D 4  is approximately equal to distance D 1  and distance D 2 . The photocurrent produced by photodetector  43 L is relatively low compared to the photocurrent produced by photodetector  43 R. 
     Referring to  FIG. 6   c , spinal cord  20  is positioned rearward at a 180° location in the spinal canal with a linear translation  126  with respect to the 0° location. 
     In operation, IR emitter  42  produces light ray  48  which forms an angle  121  with optical axis  125 . Light ray  48  is reflected from spinal cord  20  and enters photodetector  43 R. IR emitter  42  also produces light ray  49  which forms an angle  122  with optical axis  125 . Light ray  49  is reflected from spinal cord  20  and is received by photodetector  43 L. The distances from left electrode  41 L and right electrode  41 R to the target cells are both relatively low. Hence, the amplitudes of the current to the electrodes A L  and A R  are relatively low compared to  FIGS. 6   a  and  6   b . Light ray  48  traverses the distance D 5  between IR emitter  42  and photodetector  43 R. Light ray  49  traverses a distance D 6  between IR emitter  42  and photodetector  43 L. It can be seen that distances D 5  and D 6  are approximately equal. Further, distances D 5  and D 6  are less than distances D 1  and D 2 . The photocurrents produced by both photodetectors are relatively high compared to the photocurrents of  FIG. 6   a.    
     Referring to  6   d , the spinal cord is shifted in position by rotation through angle  130  and linear translation  129 . The 270° shifted position corresponds to a bend of the patient to left. 
     In operation, IR emitter  42  produces light ray  49  which forms an angle  122  with optical axis  125 . IR emitter  42  also produces light ray  48  which forms angle  121  with optical axis  125 . Light ray  49  is reflected from spinal cord  20  and is received by photodetector  43 L. Light ray  48  is reflected from spinal cord  20  and is received by photodetector  43 R. The distance from left electrode  41 L to the target cells is relatively low compared to the distance from the right electrode  41 R to the target cells. Hence, the current amplitude for electrode  41 L is relatively low compared to the current for right electrode  43 R. The distance traversed for light ray  49  is distance D 8 . The distance traversed for light ray  48  is D 7 . It can be seen that distance D 7  is greater than distance D 8 . It can also be seen that distance D 7  is approximately equal to distances D 1  and D 2 . It can further be seen that distance D 8  is approximately equal to distances D 6  and D 5 . The photocurrent produced by photodetector  43 L is relatively high compared to the photocurrent produced by photodetector  43 R. 
     The relative relationship between the received photocurrent signals, P L  and P R , (from photodetectors  43 L and  43 R, respectively) and the required current amplitudes of the current signals to the electrodes, A L  and A R , can be summarized in the following table for the four extreme positions of the spinal cord in the spinal canal. 
     
       
         
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Position 
                 P L   
                 P R   
                 A L   
                 A R   
               
               
                   
                   
               
             
             
               
                   
                 Front—0° 
                 L 
                 L 
                 H 
                 H 
               
               
                   
                 Right—90° 
                 L 
                 H 
                 H 
                 L 
               
               
                   
                 Back—180° 
                 H 
                 H 
                 L 
                 L 
               
               
                   
                 Left—270° 
                 H 
                 L 
                 L 
                 H 
               
               
                   
                   
               
             
          
         
       
     
     Optical ratios associated with each photodetector pair correlate to a function of spinal cord position as determined ratiometrically (for side-to-side movement) and proportionally (for front-to-back movement) to the detected light intensities. 
     The ratio of the current signals from photodetector  43 L and photodetector  43 R is representative of spinal position left to right. 
     
       
         
           
             
               
                 
                   r 
                   = 
                   
                     
                       P 
                       L 
                     
                     
                       P 
                       R 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     The intensity of the photocurrent signals is representative of spinal position front to back. The total intensity can be represented by:
 
 I=P   L   +P   R   (4)
 
     Referring to  FIG. 7 , a preferred embodiment of the implanted components of the system is described. Spinal cord stimulator  45  includes pulse generator and signal processor (PGSP)  50  and electrode assembly  40 . Main lead  51  connects electrode assembly  40  to PGSP unit  50 . PGSP unit  50  provides power to the electrodes and the IR emitter and houses the electrical components of the system. PGSP unit  50  gathers and processes photodetector signals and makes adjustments to the electrode current (or voltage) based on the photodetector signals. PGSP unit  50  is connected by wireless communication link  52  across skin boundary  56  to SCS controller  53 . The SCS controller is configured to allow activation of and adjustments to PS-SCS stimulator percutaneously. PSGP unit  50  is also connected by wireless communication link  55  to calibration unit  54 . Calibration unit  54  is programmed to accept patient feedback and transmit it to PGSP  50  during calibration. 
     Referring to  FIG. 8   a , a first preferred embodiment of electrode assembly  40   a  will be described. IR emitter  42  is centrally positioned in elastomeric housing  46   a . A vertical linear array of electrodes,  41 L and  41 R, are positioned to each side of the IR emitter. Photodetectors  43 L and  43 R are positioned to the left and right of the arrays of electrodes  41 L and  41 R, respectively, and in horizontal alignment with the IR emitter. Each electrode has a separate electrical conductor in a set of electrode leads contained in main lead  51  so that the current to each contact may be independently controlled. The components of the electrode assembly are sealed in elastomeric housing  46   a.    
     Referring to  FIG. 8   b , a second preferred embodiment of electrode assembly  40   a  will be described. A vertical linear array of electrodes,  41 L and  41 R, are positioned to each side of the IR emitter. An IR emitter and photodetector are packaged into a single integrated device as an IR emitter/detector pair. IR emitter/detector pair  45 L and IR emitter/detector pair  45 R are positioned to the left and right of the arrays of electrodes  41 L and  41 R, respectively. Each electrode has a separate electrical conductor in a set of electrode leads contained in main lead  51  so that the current to each contact may be independently controlled. The components of the electrode assembly are sealed in elastomeric housing  46   a .  FIG. 8B  indicates the physical locations of the IR emitter/detector pairs to be slightly outside of the array of electrodes. In alternate embodiments, the IR emitter detector pairs may be located in line with the electrode array or inside of the electrode array. In another alternate embodiment, a central IR emitter/detector pair may be situated in between the left and right IR emitter/detector pairs. 
     A suitable single integrated device comprising a photoemitter and a photodarlington detector is part number OPB707A from Optek Technology, Inc. 
       FIG. 9  shows an alternate embodiment of electrode assembly  40   b . Electrode assembly  40   b  includes two sets of electrodes  41 L and  41 R, a linear set of IR emitters  42   a  and two sets of IR photodetectors  44 L and  44 R. Set of IR emitters  42 A are preferably located in a vertical line near the center of the elastomeric housing. The sets of electrodes are positioned in vertical rows to each side of the IR emitters. The number of electrodes may vary depending on the dimensions of the elastomeric housing. IR photodetectors  44 L and  44 R positioned to each side of the two sets of electrodes. Each electrode has a separate electrical conductor in a set of electrode leads contained in main lead  51  so that the current to each contact may be independently controlled. The components of the electrode assembly are sealed into the elastomeric housing  46   b.    
       FIG. 10  illustrates an alternate embodiment of electrode assembly  440 . Electrode assembly  440  includes two sets of electrodes  441 L and  441 R, a set of optical lenses  442  for light delivery, optical lenses  443 L and optical lenses  443 R for light collection. Optical fibers are terminated in each of the lenses and routed into main lead  451 . Optical lenses  442  act to direct light from the optical fibers toward the spinal cord uniformly. In the preferred embodiment, the lenses are Fresnel lenses which reduce the profile depth of the elastomeric housing and diffuse light uniformly. Optical lenses  443 L and  443 R act as collectors to efficiently gather and collimate light received. Each electrode is provided a separate electrical conductor in main lead  451  so that the current to each electrode may be independently controlled. 
     PGSP unit  50  is preferably powered by batteries. In an alternate embodiment, PGSP unit  50  derives power from capacitive or inductive coupling devices. Calibration may further calibrate the batteries, the capacitive devices, or inductive coupling in PGSP unit  50 . Communication links  52  or  55  may further serve as a means of providing electrical charge for the batteries or capacitive devices of PGSP unit  50 . 
     Referring to  FIG. 11   a , PSGP unit  50  will be described. PSGP unit  50  comprises CPU  70  including onboard memory  72 . CPU  70  is connected to pulse modulator  62  and pulse generator  60 . Modulator  62  is also connected to pulse generator  60 . CPU  70  is also operatively connected to optical modulator  68  and optical signal processor  64 . Optical modulator  68  is connected to infrared emitter driver  66 . Infrared emitter driver  66  is connected to the IR emitter in the electrode assembly. 
     IR emitter driver  66  is also connected to IR emitter  79 . In embodiments which require fiber optic connection, infrared emitters  79  include appropriate lenses and connectors to effectively couple IR emitter  79  to fiber  81 . Fiber  81  is connected to light delivery lenses in the electrode array. 
     CPU is also connected to optical signal processor  64 . Optical signal processor  64  is connected to the set of photodetectors in electrode assembly  40 . Pulse generator  60  is connected to the set of electrodes in electrode assembly  40 . 
     In order to generate a pulse to the electrodes, the CPU consults a calibration table stored in onboard memory  72  to determine pulse width P W , pulse frequency P f  and pulse amplitudes A L  and A R  for the left and right electrodes, respectively. The pulse width and frequency are transmitted to pulse modulator  62  which creates a modified square wave signal. The modified square wave signal is passed to pulse generator  60 . CPU  70  passes the amplitude for the left and right electrodes to pulse generator  60  in digital form. Pulse generator  60  then amplifies the modified square wave according to A L  and A R  to form left and right modified square wares and transmits them to the left and right electrodes, respectively. 
     The stimulation waveform of the preferred embodiment is a modified square wave having an amplitude and duration (or width). Pulse widths varying from 20 to 1000 microseconds have been shown to be effective. The frequency of the pulse waveforms between 20 and 120 hertz have been shown to be effective. The output amplitude is preferably from 0(zero) to +/−20 mA or 0(zero) to +/−10 V but may vary beyond those ranges according to patient sensitivity. 
     Optical signal processor  64  receives signals from the set of photodetectors, filters the optical signals, and correlates the optical signals to the IR emitter amplitude, pulse width and frequency. Optical signal processor  64  may include a synchronized gated detection (e.g. lock-in amplifier type) function or other demodulation function to improve the signal to noise ratio of the detected light. 
     IR detector  77  is connected to optical signal processor  64 . IR detector  77  translates incoming light pulses from fiber  82  into electrical signals processed by optical signal processor  64 . IR detector  77  includes lenses to couple IR detector  77  to fiber  82 . Sensitivity of the set of IR photodetectors is similar to that of part APA3010P3Bt from Kingbright Corporation. 
     CPU  70  is connected to optical modulator  68 . Optical modulator  68  generates the IR emission waveform transmitted to the IR emitters according to parameters set and transmitted by CPU  70 . IR emitter driver  66  is connected to both optical modulator  68  and CPU  70 . In operation, to send an IR light pulse, the CPU activates the optical modulator to generate the appropriate waveform which is then transmitted to the IR emitter driver. The IR emitter driver transmits the waveform to the IR emitters. If IR emitter  79  is used, the pulse is launched into fiber  81 . 
     The IR emission waveform set by CPU  70  may take several forms. For example, IR emitter pulse width may be very short to minimize power consumption. A single IR emitter pulse may occur for a set of electrode stimulation pulses. Typical wavelength of the IR light from the set of IR emitters is 940 nm. Typical output intensity of the IR emitters is 1 to 2 mW and a suitable part is part # PDI-E900 from Advanced Photonix, Inc. 
     CPU  70  is in transcutaneous communications, via RF transceiver  71 , with calibration and programming unit  54  and SCS controller  53 . 
     Referring to  FIG. 11   b , SCS controller  53  will be described. SCS controller  53  includes processor  900  connected to RF transceiver  902 , display  904 , input/output device  906 , and memory  908 . In the preferred embodiment, display  904  is a low power liquid crystal display adapted to show the current operational state of the system. I/O device  906  is a simple push button contact array which is constantly monitored by processor  900 . Memory  908  is onboard memory connected to processor  900 . In the preferred embodiment, RF transceiver  902  is a low power transmitter/receiver combination. 
     Referring to  FIG. 11   c , calibration and programming unit  54  will be described. Calibration and programming unit  54  includes processor  1000  connected to onboard memory  1008 , input/output devices  1006  and  1007 , RF transceiver  1002  and display  1004 . Display  1004 , in the preferred embodiment, is a low power liquid crystal display. Input/output device  1006  and input/output device  1007  are simple push button switches monitored continuously by the processor. Memory  1008  is onboard processor  1000 . RF transceiver  1002  is a low power transmitter/receiver combination. 
     Referring to  FIGS. 12   a ,  11   a ,  11   b  and  11   c , method  80  of operation of the PS-SCS stimulator will be described. In the preferred embodiment, method  80  takes the form of a computer program which is resident in memory  72  of CPU  70  of PGSP  50 . When activated, the program forms a continuous cycle. At step  81 , RF transceiver  71  is continually polled for a change of operation code signal to be received from SCS controller  53 . One of three options is always present, “start?”, “calibrate?” and “stop?”. 
     At step  83 , if operation change code “start?” is received, the method moves to step  92 . At step  92 , CPU  70  activates optical modulator  68 , which in turn activates IR emitter driver  66  to generate a set of current pulses sent to the IR emitters. At step  93 , the resultant current levels at the photodetectors, PD L  and PD R , are measured by optical signal processor  64  and passed to CPU  70 . At step  95 , CPU estimates the amplitude A L  and A R  of the a train of pulses to be sent to the electrodes. At step  99 , optionally, the CPU sets the values of the pulse width P W  and frequency P f  of the pulse train to be sent to the electrodes. At step  152 , the CPU activates the pulse modulator to create the waveform of the pulse train to be sent to the electrodes and activates pulse generator  60  to generate the pulse train. At step  154 , the CPU stores the values of PD L , PD R , A L , A R , P W  and P f  in memory for future retrieval. The method then returns to step  81 . 
     If at step  83 , the operation change code is not “start?”, the method proceeds to step  85 . At step  85 , the CPU determines if the operation change code is “calibrate?”. If so, the method moves to step  87 . At step  87 , the CPU transmits the history log stored in memory to calibration unit  55 . At step  89 , the CPU enters the calibration routine as will be described more fully later. The method then returns to step  81 . 
     If at step  85 , the operation change code is not “calibrate?”, the method moves to step  91 . At step  91 , the CPU determines if the operation change code is “stop?”. If so, the method returns to step  81 . If not, the method proceeds to step  92  and continues as previously described. 
     In the preferred embodiment, the pulse width and frequency is kept constant for a given patient and only the left and right electrode amplitudes are varied. 
     Referring to  FIG. 12   b , an alternate embodiment of estimating amplitude values, at step  95  will be described. In this embodiment, the CPU time averages historical amplitudes A L  and A R  to arrive at the estimated electrode amplitudes. At step  96 , the CPU obtains a set of historical values for A L  and A R  and a predetermined weighting value from memory. 
     At step  97 , the following equation is applied: 
     
       
         
           
             
               
                 
                   
                     
                       A 
                       L 
                     
                     ⁡ 
                     
                       ( 
                       delivered 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           w 
                           k 
                         
                         · 
                         
                           
                             A 
                             L 
                           
                           ⁡ 
                           
                             ( 
                             k 
                             ) 
                           
                         
                       
                       + 
                       
                         
                           w 
                           
                             k 
                             - 
                             1 
                           
                         
                         · 
                         
                           
                             A 
                             L 
                           
                           ⁡ 
                           
                             ( 
                             
                               k 
                               - 
                               1 
                             
                             ) 
                           
                         
                       
                       + 
                       
                         
                           w 
                           
                             k 
                             - 
                             2 
                           
                         
                         · 
                         
                           
                             A 
                             L 
                           
                           ⁡ 
                           
                             ( 
                             
                               k 
                               - 
                               2 
                             
                             ) 
                           
                         
                       
                       + 
                       … 
                     
                     
                       
                         w 
                         k 
                       
                       + 
                       
                         w 
                         
                           k 
                           - 
                           1 
                         
                       
                       + 
                       
                         w 
                         
                           k 
                           - 
                           2 
                         
                       
                       + 
                       … 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     where:
         w k =predetermined weight for the values of A L  at the current time A L (k) and earlier times A L (k−1), A L (k−2), . . . . At time k;   A L =left electrode amplitude; and,       

     At step  98 , the following equation is applied: 
     
       
         
           
             
               
                 
                   
                     
                       A 
                       R 
                     
                     ⁡ 
                     
                       ( 
                       delivered 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           w 
                           k 
                         
                         · 
                         
                           
                             A 
                             R 
                           
                           ⁡ 
                           
                             ( 
                             k 
                             ) 
                           
                         
                       
                       + 
                       
                         
                           w 
                           
                             k 
                             - 
                             1 
                           
                         
                         · 
                         
                           
                             A 
                             R 
                           
                           ⁡ 
                           
                             ( 
                             
                               k 
                               - 
                               1 
                             
                             ) 
                           
                         
                       
                       + 
                       
                         
                           w 
                           
                             k 
                             - 
                             2 
                           
                         
                         · 
                         
                           
                             A 
                             R 
                           
                           ⁡ 
                           
                             ( 
                             
                               k 
                               - 
                               2 
                             
                             ) 
                           
                         
                       
                       + 
                       … 
                     
                     
                       
                         w 
                         k 
                       
                       + 
                       
                         w 
                         
                           k 
                           - 
                           1 
                         
                       
                       + 
                       
                         w 
                         
                           k 
                           - 
                           2 
                         
                       
                       + 
                       … 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     where:
         w k =predetermined weight for the values of A R  at the current time A R (k) and earlier times A R (k−1), A R (k−2), . . . . At time k;   A R =right electrode amplitude.       

     Referring to  FIG. 12   c , an alternate method of estimating amplitude values at step  95  will be described. 
     At step  100 , the CPU computes a distance factor dP according to the equation:
 
 dP =√{square root over (( PD   L   −P   L ) 2 +( PD   R   −P   R ) 2 )}{square root over (( PD   L   −P   L ) 2 +( PD   R   −P   R ) 2 )}  (7)
 
     where:
         PD L =measured value of left photodetector current;   PD R =measured value of right photodetector current;   P L =calibration table value of left photodetector current; and   P R =calibration table value of right photodetector current.       

     dP is calculated for each row corresponding to patient positions  1 - 4  of the calibration table. At step  102 , the values A L  and A R  are estimated as those that correspond to the row of the calibration table having the smallest distance factor dP. 
     Referring to  FIG. 12   d , an alternate method of estimating amplitude values, step  95 , will be described. 
     At step  105 , the CPU consults the calibration table to locate the closest pair of consecutive PD L  values that bracket the measured value P L , [PD L TOP , PD L BOTTOM ]. At step  110 , the CPU locates the pair of A L  values that correspond to the closest pair of PD L  values, [A L TOP , A L BOTTOM ]. At step  115 , the CPU applies the interpolation equation to locate the estimated value of A L , as follows: 
     
       
         
           
             
               
                 
                   
                     A 
                     L 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             ( 
                             
                               
                                 A 
                                 
                                   L 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   TOP 
                                 
                               
                               - 
                               
                                 A 
                                 
                                   L 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   BOTTOM 
                                 
                               
                             
                             ) 
                           
                           
                             ( 
                             
                               
                                 PD 
                                 
                                   L 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   TOP 
                                 
                               
                               - 
                               
                                 PD 
                                 
                                   L 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   BOTTOM 
                                 
                               
                             
                             ) 
                           
                         
                         ) 
                       
                       · 
                       
                         ( 
                         
                           
                             P 
                             L 
                           
                           - 
                           
                             PD 
                             
                               L 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               BOTTOM 
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       A 
                       
                         L 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         BOTTOM 
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     where:
         A L =estimated value of the left electrode pulse current;   PD L TOP =upper bracketed value of photodetector current from the calibration table;   PD L BOTTOM =lower bracketed value of the photodetector current from the calibration table;   A L TOP =upper value of the electrode pulse current from the calibration table corresponding to PD L TOP ;   A L BOTTOM =lower value of the pair of electrode amplitudes from the calibration table corresponding to PD L BOTTOM .       

     At step  117 , the CPU consults the calibration table to locate the closest pair of consecutive PD R  values that bracket the measured value P R , [PD R TOP , PD R BOTTOM ]. At step  119 , the CPU locates the pair of A R  values that correspond to the closest pair of PD R  values, [A R TOP , A R BOTTOM ]. At step  120 , the CPU applies the interpolation equation to locate the estimated value of A R , as follows: 
     
       
         
           
             
               
                 
                   
                     A 
                     R 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             ( 
                             
                               
                                 A 
                                 
                                   R 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   TOP 
                                 
                               
                               - 
                               
                                 A 
                                 
                                   R 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   BOTTOM 
                                 
                               
                             
                             ) 
                           
                           
                             ( 
                             
                               
                                 PD 
                                 
                                   R 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   TOP 
                                 
                               
                               - 
                               
                                 PD 
                                 
                                   R 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   BOTTOM 
                                 
                               
                             
                             ) 
                           
                         
                         ) 
                       
                       · 
                       
                         ( 
                         
                           
                             P 
                             R 
                           
                           - 
                           
                             PD 
                             
                               R 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               BOTTOM 
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       A 
                       
                         R 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         BOTTOM 
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     where:
         A R =estimated value of the right electrode pulse current;   PD R TOP =upper bracketed value of photodetector current from the calibration table;   PD R BOTTOM =lower bracketed value of photodetector current from the calibration table;   A R TOP =upper value of the electrode pulse current from the calibration table corresponding to PD R TOP ;   A R BOTTOM =lower value of the pair of electrode amplitudes from the calibration table corresponding to PD R BOTTOM .       

     Referring to  FIG. 12   a , in the preferred embodiment, pulse width and frequency is kept constant for a given patient and only the left and right electrode amplitudes are varied. In an alternate embodiment, step  150  is performed whereby pulse width and pulse frequency are varied according to the calibration values stored in the calibration table for each electrode. 
     Referring to  FIGS. 13   a  and  13   b , a method of calibration of the SCS stimulator will be described. 
     Referring to  FIGS. 13   a ,  11   a  and  11   c , the processor is programmed to carry out steps of calibration method  300  upon request by a calibration control program. At step  520 , the levels of A L  and A R  are set at the minimum value of a predetermined range for each. At step  525 , the pulse generator is directed by the CPU to send a train of pulses to each of the left and right electrodes at the minimum levels of A L  and A R , respectively. At step  530 , paresthesia perception feedback is solicited from the patient. 
     If the level of parasthesia is not optimal according to the patient feedback, then the method moves to step  532 . At step  532 , the processor monitors the input/output devices to determine if A L , A R  or both A L  and A R  need to be adjusted, or if the level of paresthesia is sufficient. If A L  needs to be increased or decreased from the current level, then the value of A L  is adjusted by a discrete amount in step  533 . If the level of A L  is at a maximum or a minimum level, an alert is made by the calibration and programming unit in step  534 . If A R  needs to be increased or decreased from the current level, then the value of A R  is adjusted by a discrete amount in step  535 . If the level of A R  is at a maximum or a minimum level, an alert is made by the calibration and programming unit in step  536 . The alert in step  534  and step  536  may be a visual indication, audio indication or both visual and audio indication. 
     After adjustment, the step  525  is repeated, and a train of pulses is delivered to each electrode at the new levels A L  and A R . At step  530 , patient paresthesia feedback is again solicited. If the level of paresthesia is still not optimal according to the patient feedback, the method repeats steps  533 ,  534 ,  535  and  536  as required. If the level of paresthesia is sufficient according to patient feedback at step  532 , the method moves to step  538 . 
     At step  538 , the CPU stores the value A L . At step  539 , the CPU stores the value of A R . At step  540 , the CPU measures the optical signal feedback from the optical signal processor representative of the current from the left photodetector P L . At step  550 , the CPU measures the optical feedback signal from the optical signal processor representative of the current from the right photodetector P R . At steps  560  and  565 , the CPU stores P L  and P R  in the calibration table. At step  570 , the calibration method steps complete by returning control to the calibration control program. 
     Referring to  FIGS. 13   b  and  11   c , the processor of calibration unit  54  is programmed to further carry out the following method steps for a calibration control program  400  in cooperation with physical motion of the patient. 
     At step  350 , RF transceiver  1002  receives a signal indicative of a request to move the patient to a prone position and passes it to processor  1000 . At step  352 , the patient is positioned in a prone position. Calibration method  300 , as described in  FIG. 13   a , is carried out to maximize the level of paresthesia experienced by the patient. 
     At step  360 , RF transceiver  1002  receives a signal indicative of a request to move the patient to a right lateral position and passes it to processor  1000 . At step  362 , the patient is positioned in a right lateral position. Calibration method  300  is then carried out to optimize the level of paresthesia experienced by the patient. 
     At step  370 , RF transceiver  1002  receives a signal indicative of a request to move the patient to a supine position and passes it to processor  1000 . At step  372 , the patient is positioned in a supine position. Calibration method  300  is then carried out to optimize the level of paresthesia experienced by the patient. 
     At step  380 , RF transceiver  1002  receives a signal indicative of a request to move the patient to a left lateral position and passes it to processor  1000 . At step  382 , the patient is positioned in a left lateral position. Calibration method  300  is then carried out to optimize the level of paresthesia experienced by the patient. 
     After steps  380 ,  382  and  300  finish, the calibration program is complete. 
     The order of patient positions in calibration program  400  may be changed in alternative embodiments. Additional patient positions may be added to calibration program  400  in alternative embodiments, for example, the patient may be rotated clockwise to calibrate a level of paresthesia required for a clockwise position. 
     Referring to  FIGS. 13   c  and  11   b , the various states of the SCS controller will be described. At state  505 , SCS controller  53  enters a waiting posture and continually polls I/O unit  906 . Upon receipt, processor  900  enters run state  507  and transmits a “run” signal to RF transceiver  902 . RF transceiver then transmits the “run” signal to PGSP  50  for further action. After transmission, the processor returns to wait state  505 . 
     If a “stop” signal is received from I/O device  906 , processor  900  passes a signal to RF transceiver  902 , which in turn sends the signal to PGSP  50 . The processor then returns to wait state  505 . 
     If a “calibrate” signal is received from I/O unit  906 , at step  511 , processor  900  transmits a “calibrate” signal to RF transceiver  902 , which in turn sends the signal to PGSP  50 . Processor  900  then returns to wait state  505 . 
       FIG. 14  shows a calibration table  1150  for the preferred embodiment. Each row is a record for the optimal electrode settings for a patient position for a specific pair of electrodes in the electrode assembly. Calibration table  1150  includes seven columns, patient position identifier  1152 , left photodetector value PD L    1154 , right photodetector value PD R    1156 , left electrode stimulation pulse amplitude A L    1158 , right electrode pulse amplitude A R    1160 , electrode stimulation pulse width P W    1161 , and electrode pulse frequency P f    1162 . 
     Patient position identifier  1152  in a preferred embodiment includes four positions, front (prone—0°), right—90°, back (supine—180°) and left—270°. Each row in Table  1150  is associated with one of the four patient positions. Left electrode stimulation pulse amplitude  1158  and right electrode stimulation pulse amplitude  1160  are values which are derived during calibration and recorded for different spinal cord positions, corresponding to the patient position. In the preferred embodiment, the left electrode stimulation pulse amplitude  1158  and right electrode stimulation pulse amplitude  1160  are directly proportional to the stimulation energy delivered to the respective electrodes. 
     In alternate embodiments, calibration may be performed for additional physical positions such that additional rows are placed in table  1150 . 
     Left photodetector value PD L    1154  is the measured intensity for the left photodetector. Right photodetector value  1156  is the measured intensity for the right photodetector. 
     Electrode stimulation pulse width  1161  and frequency  1162  are each constant. However, in an alternate embodiment, electrode stimulation pulse width  1161  and electrode pulse frequency  1162  are varied through a predetermined range during calibration and recorded for each patient position. 
     The method  80  of  FIG. 12   a  can be extended to those SCS electrode assemblies that contain more than one pair of photodetectors. Stimulation energy can be delivered in different regions of the spinal cord defined by sectors in the SCS electrode assembly. 
     There are various other embodiments in which to realize the present invention. The photoemitter may be an IR emitter diode embedded in the electrode array or alternatively, the IR emitter diode may be mounted in the generator device and coupled with the stimulator electrode array via a fiber optic cable. 
     While the present invention has been described in terms of specific embodiments thereof, it will be understood in view of the present disclosure, that numerous variations upon the invention are now enabled to those skilled in the art, which variations yet reside within the scope of the present teaching. Accordingly, the invention is to be broadly construed and limited only by the scope and spirit of the claims now appended hereto.