Abstract:
A positionally sensitive spinal cord stimulation apparatus and method using near-infrared (NIR) reflectometry are provided for automatic adjustments of spinal cord stimulation. The system comprises an electrode assembly with an integrated optical fiber sensor for sensing spinal cord position. The integrated optical fiber sensor, comprising a set of optical elements for emitting light from a set of IR emitters and for collecting reflected light into a set of IR photodetectors, determines a set of measured optical intensities. As the spinal cord changes position, the angles of incidence for light from the IR emitter and the measured optical intensities change. A ratio of measured optical intensities in combination with a total measured optical intensity is 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:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation-in-Part application claiming priority benefit from U.S. patent application Ser. No. 13/567,966, filed on Aug. 6, 2012, which is continuation of U.S. patent application Ser. No. 12/925,231, filed on Oct. 14, 2010, now U.S. Pat. No. 8,239,038. 
     
    
     FIELD OF INVENTION 
       [0002]    This invention relates generally to spinal cord stimulation (SCS) and technique for automatic adjustments of SCS using near-infrared (NIR) reflectometry. 
       BACKGROUND 
       [0003]    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. 
         [0004]    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. 
         [0005]    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  11  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  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  11  that run through the spinal canal. 
         [0006]    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. 
         [0007]    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 . 
         [0008]      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  and controller  33 . 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. 
         [0009]    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. 
         [0010]    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. 
         [0011]    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. 
         [0012]    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. 
         [0013]    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. 
         [0014]    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 
       [0015]    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. 
         [0016]    The disclosure demonstrates a novel optical sensor, generally useful in many fields of endeavor, in which a probe light beam is emitted from the sensor and a responsive light beam is collected by the sensor, where the sensor comprises a negative axicon element coupled to an optical fiber. In a preferred embodiment, the negative axicon is embedded in the end of the optical fiber. 
         [0017]    The optical fiber is further coupled to an active optical element which can be an optical emitter or an optical detector. In a preferred embodiment, both an optical emitter and an optical detector are coupled to a single optical fiber with the negative axicon using an optical circulator. In another preferred embodiment, an optical isolator can be employed. 
         [0018]    Disclosed is a stimulator system comprising a controller, a set of optical emitters operatively connected to the controller, generating a set of emitted light beams. A set of optical detectors are operatively connected to the controller, receiving a set of reflected light beams. A set of optical elements are operatively coupled to the set of optical emitters and to the set of optical detectors, emitting the set of emitted light beams and collecting the set of reflected light beams. A set of electrodes are operatively connected to the controller and the controller directs a set of currents to the set of electrodes based on the set of reflected light beams. 
         [0019]    In an aspect of the system, the set of electrodes are adjacent the set of optical elements. 
         [0020]    In another aspect of the system, an optical fiber is coupled to an optical emitter and further coupled to an optical detector in the set of optical detectors. The optical fiber is further coupled to an optical element in the set of optical elements. 
         [0021]    In an embodiment of the system, the system comprises an implantable lead encasing the optical fiber and a lumen wherein the implantable lead further comprises an EMI shield. In a related aspect, the implantable lead further comprises carbon nanotubes. 
         [0022]    In another aspect of the system an optical circulator is operatively coupled to the optical emitter, the optical detector and the optical fiber. 
         [0023]    A preferred embodiment is conceived wherein an optical element in the set of optical elements further comprises a negative axicon. Further to the preferred embodiment, the negative axicon subtends an angle less than twice the complement of the critical angle for the light emitted from the optical fiber. In an alternate embodiment, a reflective coating is applied to the negative axicon. 
         [0024]    In an alternate embodiment, an optical element in the set of optical elements further comprises a beveled surface. The beveled surface can comprise a reflective surface and in another aspect the reflective surface is positioned at an angle less than the complement of the critical angle for the light emitted from the optical fiber. 
         [0025]    In another embodiment, an optical element in the set of optical elements further comprises a lens. 
         [0026]    In an aspect of the system, the controller derives a set of current amplitudes for the set of currents based on an interpolation of a set of calibrated current amplitudes. 
         [0027]    In another aspect of the system, the controller derives a set of current amplitudes for the set of currents based on time averaging of a set of historical current amplitudes. 
         [0028]    In yet another aspect of the system, the controller derives a set of current pulse widths for the set of currents based on at least one of the group consisting of time averaging a set of current pulse widths, time averaging a set of current amplitudes, interpolating the set of current pulse widths and interpolating the set of current amplitudes. 
         [0029]    In yet another aspect of the system, the controller derives a set of current pulse frequencies for the set of currents based on at least one of the group consisting of time averaging a set of current pulse frequencies, time averaging a set of current amplitudes, interpolating the set of current pulse frequencies and interpolating the set of current amplitudes. 
         [0030]    In a preferred embodiment, the system further comprises a calibration unit operatively connected to the controller for calibrating the set of current pulse amplitudes, pulse widths and pulse frequencies. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0031]    The following disclosure is understood best in association with the accompanying figures. Like components share like numbers. 
           [0032]      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; 
           [0033]      FIG. 2  shows an axial view of a thoracic vertebra indicating the position of the spinal cord and an electrode array for spinal cord stimulation; 
           [0034]      FIG. 3  shows a sagital cross section view of the human spine showing the approximate position of an electrode array for spinal cord stimulation; 
           [0035]      FIG. 4  shows a prior art electrode array for spinal cord stimulation; 
           [0036]      FIGS. 5   a  and  5   b  show the relative electric field produced by a preferred embodiment as the spinal cord precesses about an orbit within the spinal canal; 
           [0037]      FIGS. 6   a  and  6   b  show a stimulator lead for spinal cord stimulation incorporating an optical fiber. 
           [0038]      FIG. 6   c  shows a cross-section of a stimulator lead along line  6   c - 6   c  from  FIG. 6   a.    
           [0039]      FIG. 6   d  shows placement of a set of stimulator leads. 
           [0040]      FIGS. 7   a - 7   f  show various embodiments of a distal optical element. 
           [0041]      FIG. 8   a  shows a cross-sectional view of a first embodiment of a stimulator lead array centrally located in relation to a spinal cord at a forward position with 0° displacement. 
           [0042]      FIG. 8   b  shows a cross-sectional view of a first embodiment of a stimulator lead array located in relation to a spinal cord at a rightward position at 90° displacement. 
           [0043]      FIG. 8   c  shows a cross-sectional view of a first embodiment of a stimulator lead array centrally located in relation to a spinal cord at 180° displacement. 
           [0044]      FIG. 8   d  shows a cross-sectional view of a first embodiment of a stimulator lead array located in relation to a spinal cord at 270° displacement. 
           [0045]      FIG. 9   a  shows an alternate embodiment of a stimulator lead having a single optical fiber operating as an optical emitter and an optical collector. 
           [0046]      FIG. 9   b  shows a cross-section of an alternate embodiment of a stimulator lead along line  9   b - 9   b  from  FIG. 9   a.    
           [0047]      FIG. 9   c  shows placement of a set of stimulator leads. 
           [0048]      FIG. 10   a  shows a cross-sectional view of a second embodiment of a stimulator lead array centrally located in relation to a spinal cord at a forward position with 0° displacement. 
           [0049]      FIG. 10   b  shows a cross-sectional view of a second embodiment of a stimulator lead array located in relation to a spinal cord at a rightward position at 90° displacement. 
           [0050]      FIG. 10   c  shows a cross-sectional view of a second embodiment of a stimulator lead array centrally located in relation to a spinal cord at 180° displacement. 
           [0051]      FIG. 10   d  shows a cross-sectional view of a second embodiment of a stimulator lead array located in relation to a spinal cord at 270° displacement 
           [0052]      FIGS. 11   a - 11   c  show a distal end cap for multiple stimulator leads. 
           [0053]      FIG. 12  shows a distal end cap configuration with a flat reflective surface. 
           [0054]      FIGS. 13   a  and  13   b  show a distal end cap with a curved reflective surface. 
           [0055]      FIG. 14  shows a distal end cap incorporating a ball lens. 
           [0056]      FIG. 15  shows a schematic representation of a preferred embodiment of the positionally sensitive spinal cord stimulation system. 
           [0057]      FIGS. 16   a - 16   e  show a physical drawing of a pulse generation and optical signal processing unit. 
           [0058]      FIG. 17   a  is a block diagram of the components of a preferred embodiment of a pulse generation and optical signal processing unit. 
           [0059]      FIG. 17   b  is a block diagram of the components of a preferred embodiment of an SCS controller. 
           [0060]      FIG. 17   c  is a block diagram of the components of a preferred embodiment of a calibration and programming unit. 
           [0061]      FIGS. 18   a - 18   d  are flow diagrams of a method of operation of a preferred embodiment. 
           [0062]      FIGS. 19   a - 19   c  are flow diagrams of a method of calibrating a preferred embodiment. 
           [0063]      FIG. 20  is a graphic representation of a calibration table for one optical emitter and multiple optical detectors. 
           [0064]      FIG. 21  is a graphic representation of a calibration table for multiple optical emitters and multiple optical detectors. 
       
    
    
     DETAILED DESCRIPTION 
       [0065]    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 a near infrared light emitter and a set of optical detectors, where the light is reflected from the spinal cord; 2) the spinal cord geometry; 3) the optical divergence of the light emitter; and 4) the presence of chromophores in the optical path. 
         [0066]    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 (CSF) will negligibly scatter near infrared light and will not act as a significant reflector of near-infrared light. Light from the light emitter passes through the thin, relatively avascular dura to enter the CSF. Light incident on the spinal cord experiences scatter resulting in a portion being reflected and another portion being absorbed by chromophores. 
         [0067]    Optical absorption in a 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: 
         [0000]        A   λ =ε λ   bc,   (1)
 
         [0068]    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 ).       
 
         [0072]    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 
         [0000]        A   λ =−log( I/I   0 ).  (2)
 
         [0073]    For deoxyhemoglobin and oxyhemoglobin, the extinction coefficient spectra are well known. 
         [0074]    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. 
         [0075]    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 950-1100 nm. 
         [0076]    When considering the light 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. 
         [0077]    The geometry of the light emitter and detector aperture 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. 
         [0078]    The effects of geometry may be minimized by minimizing the angle between the light emitter and optical detector optical axes relative to the spinal cord surface normal vector. 
         [0079]    The beam divergence of the light emitter relative to the incident and reflected rays will influence the detected light amplitude. 
         [0080]    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 towards a 0° direction, the spinal cord moves forward within 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 towards a 90° direction, the spinal cord moves to the right within 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 towards a 180° direction, the spinal cord moves back within its orbit within 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 towards a 270° direction, the spinal cord moves to the left within 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. 
         [0081]      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. 
         [0082]    Referring to  FIGS. 6   a  and  6   b , a preferred embodiment stimulator lead  100  is shown. Stimulator lead  100  includes a set of stimulator electrodes  112  at a distal end electrically connected through lead cable  110  to a proximal set of electrode contacts  114  at a proximal end. The set of stimulator electrodes and the set of proximal electrode contacts are preferably annular and integrated into the lead cable. Stimulator lead  100  further includes optical fiber  101  having distal optical element  102  at the distal end and fiber optic connector  103  at the proximate end. Distal optical element  102  is configured as an optical emitter, an optical collector or as a combination of optical emitter and collector. Distal optical element  102  extends into cap  109 . In a preferred embodiment, cap  109  is an extension of lead cable  110  which is sealed at the distal tip. In an alternate embodiment, cap  109  is a transparent hollow cylinder and bonded to lead cable  110  with adhesive at  111 . Cap  109  is preferably comprised of glass or plastic and may contain an index matching fluid. 
         [0083]    In an alternate embodiment, cap  109  can be comprised of a solid cylinder formed in place around distal optical element  102 . In this embodiment, the cylinder is not hollow and is comprised of a transparent plastic such as Lexan™. 
         [0084]    Referring to  FIG. 6   c , a cross-section of lead cable  110  is shown. Lead cable  110  comprises a sheathed outer surface  113  which encapsulates electrode leads  117 , lumen  115  and lumen  116  in filler material  112 . Lumen  116  encloses optical fiber  101 . Lumen  115  provides a hollow cavity for a wire stylet to be inserted into the lead cable for the purpose of directing the position of the lead cable while being inserted into the epidural space of a patient. In a preferred embodiment lumen  116  is centrally located in the electrode lead while lumen  115  is positioned off axis. In alternate embodiments, lumen  115  is centrally located. In other alternate embodiments, lumen  116  and lumen  115  are incorporated into a single lumen in which the wire stylet is initially placed for insertion of the lead cable. The wire stylet is removed after insertion of the lead cable and optical fiber  101  is then threaded into the single lumen. 
         [0085]    In a preferred embodiment, sheathed outer surface  113  includes an EMI shield. Filler material  112  preferably includes a polyimide polymer. Filler material  112  can also include additional materials with physical properties that enhance the EMI shielding capability of lead cable  110 . 
         [0086]    In an alternate embodiment, filler material  112  may include a carbon nano-tube composite such as that disclosed in U.S. Pat. No. 7,413,474 to Liu, et al. The disclosure of U.S. Pat. No. 7,413,474 is incorporated herein by reference. 
         [0087]    Referring to  FIG. 6   d , placement of a set of stimulator leads is shown. The stimulator leads are positioned in the epidural space between the dura and the walls of the spinal canal. In a preferred embodiment, optical emitter  108  is situated between optical collectors  131  and  133 . 
         [0088]      FIGS. 7   a - 7   e  show suitable optical configurations for a distal optical element disposed on an optical fiber of a stimulator lead.  FIGS. 7   a - 7   e  are intended as examples and should not be interpreted as limiting to the invention 
         [0089]    In  FIG. 7   a , distal optical element  1000  includes optical fiber  1001  encased in cap  109 . Optical fiber  1001  includes optical axis  1002  having core  1004  surrounded by cladding  1005  further surrounded by jacket  1009 . Optical fiber  1001  includes negative axicon  1006  etched at the distal end, centered on optical axis  1002 , and having an angular extent A. Angular extent A is less than about 66° for typical glass. The maximum value of A is determined as twice the complement of the critical angle α for the optical material in core  1004 . The complement of the critical angle is (90°−α). Jacket  1009  is removed for a distance  1007  approximately the same as the depth of negative axicon  1006 . When light travels through optical fiber  1001  and out of the distal end, it will be emitted approximately perpendicular to the optical axis  1002  near lateral line  1003  in a uniform 360 degree pattern. When used as an optical collector, optical fiber  1001  will collect light through a 360 degree angle from directions near lateral line  1003 . 
         [0090]    In  FIG. 7   b , distal optical element  1010  comprises an optical fiber  1011  covered by cap  109 . Optical element  1010  includes optical axis  1012  having core  1014  surrounded by cladding  1015  which is further surrounded by jacket  1019 . Optical fiber  1011  includes negative axicon  1016  etched at the distal end, centered on optical axis  1012 , and having an angular extent B. Angular extent B is approximately 90°. Jacket  1019  is removed for a distance  1017  approximately the same as the depth of negative axicon  1016 . Outer surface of negative axicon  1016  is coated with a reflective coating  1018 . When light travels through optical fiber  1011  and out of the distal end, it will be emitted approximately perpendicular to the optical axis  1012  near lateral line  1013  in a uniform 360 degree pattern. When used as an optical collector, optical fiber  1011  will collect light from through a 360 degree angle from directions near the lateral line  1013 . 
         [0091]    A negative axicon can be fabricated in an optical fiber end by a chemical etching process using about a 50% solution of hydrofluoric acid with a buffer of NH 4 F in deionized water. Volume ratio of HF to buffer is varied to achieve varying negative axicon angles. 
         [0092]    In  FIG. 7   c , distal optical element  1020  is enclosed in cap  109  and comprises optical fiber  1021 . Optical fiber  1021  includes optical axis  1022  having core  1024  surrounded by cladding  1025  which is further surrounded by jacket  1029 . Optical fiber  1021  includes beveled surface  1026  etched at the distal end at an angle C. Angle C is less than about 34° for typical glass. The value of C is determined as the complement of the critical angle for the optical material in core  1024 . Jacket  1029  is removed for a distance  1027  approximately the same as the depth of beveled surface  1026 . When light travels through optical fiber  1021  and out of the distal end, it will be emitted approximately perpendicular to the optical axis  1022  near lateral line  1023  in an angular pattern determined by the position of the beveled surface. When used as an optical collector, optical fiber  1021  will collect light in the approximate angular pattern from horizontal directions near the lateral line  1023 . 
         [0093]    In  FIG. 7   d , distal optical element  1030  is encased in transparent cap  109  and comprises optical fiber  1031 . Optical fiber  1031  includes optical axis  1032  having core  1034  surrounded by cladding  1035  which is further surrounded by jacket  1039 . Optical fiber  1031  includes a beveled surface  1036  etched at the distal end at an angle D where D is about 45°. Beveled surface  1036  has a reflective coating  1038 . Jacket  1039  is removed for a distance  1037  approximately the same as the depth of beveled surface  1036 . When light travels through optical fiber  1031  and out of the distal end, it will be emitted approximately perpendicular to the optical axis  1032  near lateral line  1033  in an angular pattern determined by the position of the beveled surface. When used as an optical collector, optical fiber  1031  will collect light approximately in the angular pattern from horizontal directions near the lateral line  1033 . 
         [0094]    In  FIG. 7   e , distal optical element  1040  is encased by transparent cap  109 . Distal optical element  1040  includes optical fiber  1041  with optical axis  1042  having core  1044 . Core  1044  is surrounded by cladding  1045  which is further surrounded by jacket  1049 . Reflecting surface  1046  is positioned above the distal end of the optical fiber at an angle E where E is about 45°. When light travels through optical fiber  1041  and out of the distal end, it will be emitted approximately along the optical axis  1042 , reflected from reflecting surface  1046 , and further emitted in a horizontal range of directions near lateral line  1043  in an approximate angular pattern determined by the aperture of the optical fiber, the aperture of the reflecting surface and the wavelength of the emitted light. When used as an optical collector, optical fiber  1041  will collect light in the approximate angular pattern from the horizontal range of direction near lateral line  1043 . 
         [0095]      FIG. 7   f , distal optical element  1050  is encased by transparent cap  109 . Distal optical element  1050  includes optical fiber  1051  with optical axis  1052  and core  1053 . Core  1053  is surrounded by cladding  1054  which is further surrounded by jacket  1056 . Reflector  1057  is positioned adjacent optical fiber  1051  and coaxial with optical axis  1052 . In a preferred embodiment, reflector  1057  is conical, that includes silvered surface  1058 . In use, light transmitted from the optical fiber is reflected in a 360° pattern, generally perpendicular to optical axis  1052 . Similarly, reflector  1057  collects light from a 360° axis and transmits it through optical fiber  1051 , generally parallel to optical axis  1052 . In a preferred embodiment, transparent cap  109  is filled with an optically transparent plastic matrix which supports and positions reflector  1057  above optical fiber  1051 . In an alternative embodiment, reflector  1057  can be formed by a void in matrix  1059  which is internally silvered on surface  1058 . 
         [0096]    Referring to  FIGS. 8   a  through  8   d , a distal optical element configured as an optical emitter will be termed an optical emitter and a distal optical element configured as an optical collector will be termed an optical collector. The positional relationship between the optical emitters, the optical collectors and the stimulator electrodes during spinal movement will be described. 
         [0097]    Referring to  FIG. 8   a , spinal cord  20  is positioned forward towards a 0° direction in the spinal canal. Neurostimulator electrode assembly  40 , implanted outside dura  21 , includes central electrode  41 C and optical emitter  42 C on the distal end of a central stimulator lead; left electrode  41 L and left optical collector  43 L on the distal end of a left stimulator lead; and, right electrode  41 R and right optical collector  43 R on the distal end of a right stimulator lead. Optical emitter  42 C is centrally positioned on optical axis  125  and emits light from IR emitter  45 C coupled to the proximal end of the central stimulator lead. Electrodes  41 L and  41 R are positioned toward the dura and within an operational range of target cells  19 . Left optical collector  43 L is positioned within an operational range of spinal cord  20  and is coupled to photodetector  44 L at the proximal end of the left stimulator lead. Right optical collector  43 R is positioned within an operational range of the surface of spinal cord  20  and is coupled to photodetector  44 R at the proximal end of the right stimulator lead. Target cells  19  are positioned within spinal cord  20  in an arbitrary but constant position with respect to the spinal cord. 
         [0098]    In operation, optical emitter  42 C produces light ray  48  which forms an angle  121  with optical axis  125 . Light ray  48  is reflected from the surface of spinal cord  20 , enters optical collector  43 R, then collected by photodetector  44 R and converted into a photocurrent I R1  by photodetector  44 R. Optical emitter  42 C also produces light ray  49  which forms angle  122  with optical axis  125 . Light ray  49  is reflected from the surface of spinal cord  20 , enters optical collector  43 L, then collected by photodetector  44 L and converted into a photocurrent I L1  at photodetector  44 L. An electric field produced by electrode  41 R stimulates target cells  19 . Similarly, an electric field produced by electrode  41 L stimulates target cells  19 . Amplitudes A L1  and A R1  are the resulting currents to drive both the left and the right electrode, respectively. Light ray  48  traverses a distance D 1  between optical emitter  42 C and right optical collector  43 R. Light ray  49  traverses a distance of D 2  between optical emitter  42 C and electrode  41 L. The distances D 1  and D 2  are roughly equal and both relatively high. The photocurrents produced by the photodetectors  44 R and  44 L are roughly equal with a value of I R1  and I L1 , respectively. 
         [0099]    Referring to  FIG. 8   b , the spinal cord is shifted to the right towards a 90° direction through linear translation  127  and rotated through angle  128  with respect to the forward position of  FIG. 8   a.    
         [0100]    In operation, optical emitter  42 C produces light ray  48  which forms an angle  121  with optical axis  125 . Light ray  48  is reflected from the surface of spinal cord  20 , enters right optical collector  43 R, then collected by photodetector  44 R which produces a photocurrent I R2  in response. Optical emitter  42 C also produces light ray  49  which forms an angle  122  with optical axis  125 . Light ray  49  is reflected from the surface of spinal cord  20 , enters left optical collector  43 L, then collected by photodetector  44 L which produces a photocurrent I L2  in response. An electric field produced by electrode  41 R stimulates target cells  19 . Similarly, an electric field produced by electrode  41 L stimulates target cells  19 . The distance from electrode  41 L to target cells  19  is greater than the distance from electrode  41 R to target cells  19 . Hence, to maintain a constant electric field at target cells  19 , the current amplitude A L2  for electrode  41 L must be greater than the current amplitude A R2  of the electrode  41 R for the spinal cord position of  FIG. 8   b . 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 less than D 1  and D 2  and is relatively low. Distance D 4  is approximately equal to D 1  and D 2 . The photocurrent I L2  produced by photodetector  43 L is much less than the photocurrent I R2  produced by photodetector  43 R. 
         [0101]    Referring to  FIG. 8   c , spinal cord  20  is positioned rearward towards a 180° direction with linear translation  126  with respect to the forward position of  FIG. 8   a.    
         [0102]    In operation, optical emitter  42 C produces light ray  48  which forms an angle  121  with optical axis  125 . Light ray  48  is reflected from surface of spinal cord  20 , enters optical collector  43 R, is then collected by photodetector  44 R and converted into a photocurrent I R3  in response. Optical emitter  42 C also produces light ray  49  which forms an angle  122  with optical axis  125 . Light ray  49  is reflected from the surface of spinal cord  20 , enters left optical collector  43 L, is then collected by photodetector  44 L and converted into a photocurrent I L3  in response. An electric field produced by electrode  41 R stimulates target cells  19 . Similarly, an electric field produced by electrode  41 L stimulates target cells  19 . The distances from left electrode  41 L and right electrode  41 R to target cells  19  are both smaller than D 1  or D 2 . Hence, the current amplitude A R3  to right electrode  41 R and the current amplitude A L3  to left electrode  41 L are about the same, but relatively low compared to amplitudes A R1 , A R2 , A L1  and A L2 . Light ray  48  traverses the distance D 5  between optical emitter  42 C and right optical collector  43 R. Light ray  49  traverses a distance D 6  between optical emitter  42 C and left optical collector  43 L. It can be seen that distances D 5  and D 6  are approximately equal. Distances D 5  and D 6  are less than distances D 1  and D 2 . The photocurrent I R3  produced by photodetector  44 R and photocurrent I L3  produced by photodetector  44 L are about the same but both relatively high compared to the photocurrents I R1 , I R2 , I L1  and I L2 . 
         [0103]    Referring to  8   d , the spinal cord  20  is shifted leftward towards a 270° direction in position by linear translation  129  and rotated through angle  130  with respect to the forward position of  FIG. 8   a.    
         [0104]    In operation, optical emitter  42 C produces light ray  49  which forms an angle  122  with optical axis  125 . Optical emitter  42 C also produces light ray  48  which forms angle  121  with optical axis  125 . Light ray  49  is reflected from the surface of spinal cord  20 , enters left optical collector  43 L, is collected at photodetector  44 L and converted to a photocurrent I L4  in response. Light ray  48  is reflected from spinal cord  20 , enters right optical collector  43 R, is collected by photodetector  44 R and converted to a photocurrent I R4  in response. An electric field produced by electrode  41 R stimulates target cells  19 . Similarly, an electric field produced by electrode  41 L stimulates target cells  19 . The distance D 8  from left electrode  41 L to the surface of spinal cord  20  is relatively low compared to the distance D 7  from the right electrode  41 R to the surface of spinal cord  20 . Hence, the current amplitude A L4  for electrode  41 L is relatively low compared to the current amplitude A R4  for right electrode  41 R. The distance traversed for light ray  49  is 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 I L4  produced by photodetector  44 L is relatively high compared to the photocurrent I R4  produced by photodetector  44 R. 
         [0105]    The relative relationship between received photodetector currents, I L  and I R , (from photodetectors  44 L and  44 R, respectively) and 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. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Position 
                 I L   
                 I R   
                 A L   
                 A R   
               
               
                   
                   
               
             
             
               
                   
                 1. Front - 0° 
                 L 
                 L 
                 H 
                 H 
               
               
                   
                 2. Right - 90° 
                 L 
                 H 
                 H 
                 L 
               
               
                   
                 3. Back - 180° 
                 H 
                 H 
                 L 
                 L 
               
               
                   
                 4. Left - 270° 
                 H 
                 L 
                 L 
                 H 
               
               
                   
                   
               
             
          
         
       
     
         [0106]    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. 
         [0107]    The ratio of the photocurrent signals from photodetector  44 L and photodetector  44 R is representative of spinal position left to right, 
         [0000]        r=I   L   /I   R   (3)
 
         [0000]    In some embodiments, the ratio r is used to determine the proportional difference between left and right electrode current signals. 
         [0108]    The difference between the photocurrent signals from photodetector  44 L and photodetector  44 R is also representative of spinal position left to right. In an alternate embodiment, the difference 
         [0000]        i   diff   =I   R   −I   L   (4)
 
         [0000]    is used to determine the proportional difference between left and right electrode current signals. 
         [0109]    The total intensity of the photocurrent signals is representative of spinal position front to back. The total intensity can be represented by: 
         [0000]        I   total   =I   R   +I   L   (5)
 
         [0000]    In an alternate embodiment, the total intensity is used to set the magnitude of both the left and right electrode current signals. 
         [0110]    Referring to  FIG. 9   a , an alternate embodiment of a stimulator lead is shown. Stimulator lead  600  includes optical fiber  601  coupled to optical element  602  at a distal end and coupled to optical fiber connector  603  at a proximal end. Optical fiber connector  603  is further coupled to optical circulator  605 . Optical circulator  605  is connected to first optical fiber  607 , coupled to optical emitter  610 , and second optical fiber  608 , coupled to optical detector  611 . Optical element  602  is configured as both an optical emitter and an optical collector. 
         [0111]    A suitable optical circulator is the PIOC310P component from AC Photonics, Inc., of Santa Clara, Calif., operating at a wavelength of 1060 nm. Optical circulators of smaller size and operating at wavelengths longer than 1060 nm are also suited for these embodiments. Optical circulators of larger size and operating at wavelengths shorter than 1060 nm are also suited for these embodiments. 
         [0112]    In use, a probe light beam  618  emitted from optical emitter  610  propagates through first optical fiber  607 , through optical fiber  601 , and exits from optical element  602 . A responsive light beam  620  is collected by optical element  602  and propagates through optical fiber  601 , through second optical fiber  608  and detected by optical detector  611 . Optical circulator  605  allows responsive light beam  620  to propagate into second optical fiber  608  but not into first optical fiber  607 . Optical circulator  605  also allows probe light beam  618  to propagate into optical fiber  601  but not into second optical fiber  608 . 
         [0113]    Responsive light beam  620  is generated through interaction between probe light beam  618  and tissue within the spinal cord. For example, probe light beam propagates through spinal canal, experiences absorption, is reflected by components within the spinal canal, and then experiences additional absorption before being collected as a responsive light beam with a different intensity and a different spectral profile. 
         [0114]    Referring to  FIG. 9   b , a cross-section of stimulator lead  600  is shown. Stimulator lead  600  includes sheathed outer surface  613  which encapsulates a set of electrode leads  617 , lumen  615  and lumen  616  in filler material  612 . Lumen  616  encloses optical fiber  601 . Lumen  615  provides a hollow cavity for a wire stylet to be inserted into the lead cable for the purpose of directing the position of the lead cable while being inserted into the epidural space of a patient. In a preferred embodiment lumen  616  is centrally located in the electrode lead while lumen  615  is positioned off axis. In alternate embodiments, lumen  615  is centrally located. In a preferred embodiment, sheathed outer surface  613  includes an EMI shield. Filler material  612  preferably includes a polyimide polymer. Filler material  612  can also include additional materials with physical properties that enhance the EMI shielding capability. 
         [0115]    Referring to  FIG. 9   c , placement of a set of stimulator leads is shown. The stimulator leads are positioned in the epidural space between the dura and the walls of the spinal canal. In a preferred embodiment, a pair of optical emitter/collectors  631  and  633  is situated side by side. 
         [0116]    Multiple stimulator leads such as stimulator lead  600  can be assembled into a stimulator lead assembly. Referring to  FIGS. 10   a - 10   d , spinal cord  20  is shown in various respective positions in the spinal canal: forward towards the 0° direction, rightward toward the 90° direction and back, backward toward the 180° direction, and leftward toward the 270° direction and back. Neurostimulator electrode assembly  40  is implanted outside dura  21  having a left stimulator lead with left electrode  244  and left distal optical element  245  and having a right stimulator lead with right electrode  242  and right distal optical element  243 . Left distal optical element  245  is optically coupled to IR emitter E L  and photodetector PD L . Right distal optical element is optically coupled to IR emitter E R  and photodetector PD R . 
         [0117]    Electrodes  242  and  244  are positioned toward the dura and within an operational range of target cells  19 . Target cells  19  are positioned within spinal cord  20  in an arbitrary but constant position with respect to the spinal cord. 
         [0118]    It should be understood that photodetector PD L  will receive light originating from both IR emitters E L  and E R , and that photodetector PD R  will receive light originating from both IR emitters E L  and E R . Various techniques can be used to separate the photocurrents derived from the two IR emitters. For example, IR emitter E L  and IR emitter E R  are alternatively powered on and the left and right photocurrents are temporally separated. The photocurrents detected while IR emitter E L  is powered on are I LL  at PD L  and I LR  at PD R . The photocurrents detected while IR emitter E R  is powered on are I RR  at PD R  and I RL  at PD L . 
         [0119]    Referring to  FIG. 10   a , wherein the spinal cord is positioned forward, path P 1  defines a light path from right distal optical element  243  to reflection point R 1  and back to right distal optical element  243 . Path P 3  defines a light path from left distal optical element  245  to reflection point R 3  and back to left distal optical element  245 . Path P 2  defines a light path from right distal optical element  243  to reflection point R 2  and then to left distal optical element  245 . Path P 2′  defines a light path from left distal optical element  245  to reflection point R 2  and then to right distal optical element  243 . The length of path P 1  is D 1 ; the length of path P 3  is D 3 ; and, the lengths of paths P 2  and P 2′  is D 2 . Right distal optical element  243  emits light along paths P 1  and P 2  from right IR emitter E R  and left distal optical element  245  emits light P 2′  and P 3  from IR emitter E L . 
         [0120]    Left distal optical element  245  collects light from path P 2  after reflection at point R 2  from spinal cord  20  and after attenuation and scattering by intermediate epidural tissue. Left distal optical element  245  further collects light from path P 3  after reflection from spinal cord  20  at point R 3  and after attenuation and scattering by epidural tissue. Light collected by distal optical element  245 , is detected by photodetector PD L . 
         [0121]    Right distal optical element  243  collects light from path P 2′  after reflection from spinal cord  20  at point R 2  and after attenuation and scattering by intermediate epidural tissue. Right distal optical element  243  further collects light from path P 1  after reflection at point R 1  from spinal cord  20  and after attenuation and scattering by epidural tissue. Light collected by right distal optical element  243  is detected by photodetector PD R . 
         [0122]    The distances D 1 , D 2  and D 3  are roughly equal when the spinal cord is positioned as shown in  FIG. 10   a . The photocurrents produced by the photodetectors PD R  and PD L  due to light emitted from right distal optical element  243  are roughly equal with a value of I RR1  and I RL1 , respectively. The photocurrents produced by the photodetectors PD L  and PD R  due to light emitted from left distal optical element  244  are roughly equal with a value of I LL1  and I LR1 , respectively. 
         [0123]    An electric field produced by right electrode  242  stimulates target cells  19 . Similarly, an electric field produced by left electrode  244  stimulates target cells  19 . Current amplitudes A R1  and A L1  are for the average currents supplied by right electrode  242  and left electrode  244 , respectively having pulse widths PW 1  and pulse frequencies PF 1 . For the position of the spinal cord in  FIG. 10   a , given a fixed pulse width PW 1  and a fixed pulse frequency PF 1 , the current amplitudes A R1  and A L1  are approximately the same. These foregoing results are tabulated in Table 2, row 1. 
         [0124]    Referring to  FIG. 10   b , wherein the spinal cord is positioned rightward and back, path P 5  defines a light path from right distal optical element  243  to reflection point R 5  and back to right distal optical element  243 . Path P 7  defines a light path from left distal optical element  245  to reflection point R 7  and back to left distal optical element  245 . Path P 6  defines a light path from right distal optical element  243  to reflection point R 6  and then to left distal optical element  245 . Path P 6′  defines a light path from left distal optical element  245  to reflection point R 6  and then to right distal optical element  243 . The length of path P 5  is D 5 ; the length of path P 7  is D 7 ; and, the lengths of paths P 6  and P 6′  is D 6 . Right distal optical element  243  emits light along paths P 5  and P 6  from right IR emitter E R  and left distal optical element  245  emits light P 6′  and P 7  from IR emitter E L . 
         [0125]    Left distal optical element  245  collects light from path P 6  after reflection at point R 6  from spinal cord  20  and after attenuation and scattering by intermediate epidural tissue. Left distal optical element  245  further collects light from path P 7  after reflection from spinal cord  20  at point R 7  and after attenuation and scattering by epidural tissue. Light collected by distal optical element  245 , is detected by photodetector PD L . 
         [0126]    Right distal optical element  243  collects light from path P 6′  after reflection from spinal cord  20  at point R 6  and after attenuation and scattering by intermediate epidural tissue. Right distal optical element  243  further collects light from path P 5  after reflection at point R 5  from spinal cord  20  and after attenuation and scattering by epidural tissue. Light collected by right distal optical element  243  is detected by photodetector PD R . 
         [0127]    The distance D 5  is smaller than the distance D 7  and considerably smaller than the distance D 6 : D 5 &lt;D 7 &lt;D 6 . The photocurrents produced by the photodetectors PD R  and PD L  due to light emitted from right distal optical element  243  have values I RR2  and I RL2 , respectively, where I RR2 &gt;&gt;I RL2 . The photocurrents produced by the photodetectors PD L  and PD R  due to light emitted from left distal optical element  245  have values I LL2  and I LR2 , respectively, where I LL2 &gt;&gt;I LR2 . Also, I LR2  is approximately the same as I RL2 . Comparing photocurrents of the spinal cord positions of  FIGS. 10   a  and  10   b , I LL2 &gt;I LL1 , I LR2 &gt;I LR1 , I RL2 &gt;I RL1  and I RR2 &gt;&gt;I RR1 . 
         [0128]    An electric field produced by right electrode  242  stimulates target cells  19 . Similarly, an electric field produced by left electrode  244  stimulates target cells  19 . Current amplitudes A R2  and A L2  are the average currents supplied delivered by right electrode  242  and left electrode  244 , respectively having pulse widths PW 2  and pulse frequencies PF 2 . For the position of the spinal cord in  FIG. 10   b , given a fixed pulse width PW 2  and a fixed pulse frequency PF 2 , the current amplitude A R2  is less than current amplitude A L2 . Comparing the electrode current amplitudes of  FIGS. 10   a  and  10   b : A L2 ≈A L1  and A R2 &lt;A R1 . The foregoing results are tabulated in Table 2, row 2. 
         [0129]    Referring to  FIG. 10   c , wherein the spinal cord is positioned towards the back. Path P 8  defines a light path from right distal optical element  243  to reflection point R 8  and back to right distal optical element  243 . Path P 10  defines a light path from left distal optical element  245  to reflection point R 10  and back to left distal optical element  245 . Path P 9  defines a light path from right distal optical element  243  to reflection point R 9  and then to left distal optical element  245 . Path P 9′  defines a light path from left distal optical element  245  to reflection point R 9  and then to right distal optical element  243 . The length of path P 8  is D 8 ; the length of path P 10  is D 10 ; and, the lengths of paths P 9  and P 9′  is D 9 . Right distal optical element  243  emits light along paths P 8  and P 9  from right IR emitter E R  and left distal optical element  245  emits light P 9′  and P 10  from IR emitter E L . 
         [0130]    Left distal optical element  245  collects light from path P 9  after reflection at point R 9  from spinal cord  20  and after attenuation and scattering by intermediate epidural tissue. Left distal optical element  245  further collects light from path P 10  after reflection from spinal cord  20  at point R 10  and after attenuation and scattering by epidural tissue. Light collected by distal optical element  245 , is detected by photodetector PD L . 
         [0131]    Right distal optical element  243  collects light from path P 9′  after reflection from spinal cord  20  at point R 9  and after attenuation and scattering by intermediate epidural tissue. Right distal optical element  243  further collects light from path P 8  after reflection at point R 8  from spinal cord  20  and after attenuation and scattering by epidural tissue. Light collected by right distal optical element  243  is detected by photodetector PD R . 
         [0132]    The distances D 8 , D 9  and D 10  are roughly equal when the spinal cord is positioned backward. The photocurrents produced by the photodetectors PD R  and PD L  due to light emitted from right distal optical element  243  are roughly equal with a value of I RR3  and I RL3 , respectively. The photocurrents produced by the photodetectors PD L  and PD R  due to light emitted from left distal optical element  245  are roughly equal with a value of I LL3  and I LR3 , respectively. Comparing photocurrents for the positions of  FIGS. 10   a  and  10   c : I RR3 &gt;I RR1 , I RL3 &gt;I RL1 , I LL3 &gt;I LL1  and I LR3 &gt;I LR1 . 
         [0133]    An electric field produced by right electrode  242  stimulates target cells  19 . Similarly, an electric field produced by left electrode  244  stimulates target cells  19 . Current amplitudes A R3  and A L3  are the average currents supplied delivered by right electrode  242  and left electrode  244 , respectively having pulse widths PW 1  and pulse frequencies PF 1 . For the position of the spinal cord in  FIG. 10   c , given a fixed pulse width PW 1  and a fixed pulse frequency PF 1 , the current amplitudes A R3  and A L3  are approximately the same. Comparing the electrode currents for the positions of  FIGS. 10   a  and  10   c , A R1 &gt;A R3  and A L1 &gt;A L3 . The foregoing results are tabulated in Table 2, row 3. 
         [0134]    Referring to  FIG. 10   d , wherein the spinal cord is positioned leftward and back, path P 11  defines a light path from right distal optical element  243  to reflection point R 11  and back to right distal optical element  243 . Path P 13  defines a light path from left distal optical element  245  to reflection point R 13  and back to left distal optical element  245 . Path P 12  defines a light path from right distal optical element  243  to reflection point R 12  and then to left distal optical element  245 . Path P 12′  defines a light path from left distal optical element  245  to reflection point R 12  and then to right distal optical element  243 . The length of path P 11  is D 11 ; the length of path P 13  is D 13 ; and, the lengths of paths P 12  and P 12′  is D 12 . Right distal optical element  243  emits light along paths P 11  and P 12  from right IR emitter E R  and left distal optical element  245  emits light P 12′  and P 13  from IR emitter E L . 
         [0135]    Left distal optical element  245  collects light from path P 13  after reflection at point R 13  from spinal cord  20  and after attenuation and scattering by intermediate epidural tissue. Left distal optical element  245  further collects light from path P 12  after reflection from spinal cord  20  at point R 12  and after attenuation and scattering by epidural tissue. Light collected by distal optical element  245 , is detected by photodetector PD L . 
         [0136]    Right distal optical element  243  collects light from path P 12′  after reflection from spinal cord  20  at point R 12  and after attenuation and scattering by intermediate epidural tissue. Right distal optical element  243  further collects light from path P 11  after reflection at point R 11  from spinal cord  20  and after attenuation and scattering by epidural tissue. Light collected by right distal optical element  243  is detected by photodetector PD R . 
         [0137]    The distance D 13  is smaller than the distance D 11  and considerably smaller than the distance D 12 : D 13 &lt;D 11 &lt;D 12 . The photocurrents produced by the photodetectors PD R  and PD L  due to light emitted from right distal optical element  243  have values I RR4  and I RL4 , respectively where I RR4 &lt;&lt;I RL4 . The photocurrents produced by the photodetectors PD L  and PD R  due to light emitted from left distal optical element  245  have values I LL4  and I LR4 , respectively, where I RR4 &lt;I LL4 &lt;&lt;I LR4 . Also I LR4  is approximately the same as I RL4 . Comparing photocurrents of positions of  FIGS. 10   a  and  10   d , I LL4 &gt;&gt;I LL1 , I LR4 &gt;I LR1 , I RL4 &gt;I RL1  and I RR4 &gt;I RR1 . 
         [0138]    An electric field produced by right electrode  242  stimulates target cells  19 . Similarly, an electric field produced by left electrode  244  stimulates target cells  19 . Current amplitudes A R4  and A L4  are the average currents supplied delivered by right electrode  242  and left electrode  244 , respectively having pulse widths PW 4  and pulse frequencies PF 4 . For the position of the spinal cord in  FIG. 10   d , given a fixed pulse width PW 4  and a fixed pulse frequency PF 4 , the current amplitude A L4  is less than current amplitude A R4 . Comparing the electrode current amplitudes of  FIGS. 10   a  and  10   d : A L4 &lt;A L1  and A R4 ≈A R1 . The foregoing results are tabulated in Table 2, row 4. 
         [0139]    The relative relationship between received photodetector currents and 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 example positions of the spinal cord in the spinal canal. 
         [0000]    
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Emitter L 
                 Emitter L 
                 Emitter R 
                 Emitter R 
                   
                   
               
               
                 Position 
                 Detector L 
                 Detector R 
                 Detector R 
                 Detector L 
                 A L   
                 A R   
               
               
                   
               
             
             
               
                 1. Front 0° 
                 L 
                 L 
                 L 
                 L 
                 H 
                 H 
               
               
                 2. Back-right 
                 L 
                 M 
                 H 
                 M 
                 H 
                 L 
               
               
                 90° 
               
               
                 3. Back-180° 
                 H 
                 H 
                 H 
                 H 
                 L 
                 L 
               
               
                 4- Back-left 
                 H 
                 M 
                 L 
                 M 
                 L 
                 H 
               
               
                 270° 
               
               
                   
               
             
          
         
       
     
         [0140]    Optical ratios associated with each photodetector 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. 
         [0141]    The ratio of the total photocurrent signals from photodetector PD L  to the total current signals from photodetector PD R  is representative of spinal position left to right. 
         [0000]    
       
         
           
             
               
                 
                   r 
                   = 
                   
                     
                       ( 
                       
                         
                           I 
                           LL 
                         
                         + 
                         
                           I 
                           LR 
                         
                       
                       ) 
                     
                     
                       ( 
                       
                         
                           I 
                           RR 
                         
                         + 
                         
                           I 
                           RL 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0000]    In an alternate embodiment, the ratio r is used to determine the proportional difference between A L  and A R . 
         [0142]    The difference between the total photocurrent signals from photodetector PD L  to the total current signals from photodetector PD R  is also representative of spinal position left to right. 
         [0000]        I   diff =( I   LL   +I   LR )−( I   RR   +I   RL )≅ I   LL   −I   RR  
 
         [0000]    In an alternate embodiment, the photocurrent difference I diff  is used to determine the proportional difference between A L  and A R . 
         [0143]    The total photocurrent of all four photodetector current signals is representative of spinal position front to back, where the total photocurrent is: 
         [0000]        I   total   =I   RR   +I   RL   +I   LL   +I   LR   (8)
 
         [0000]    In an alternate embodiment, the total photocurrent I total  is used to determine the overall magnitude of A L  and A R . 
         [0144]    Referring to  FIGS. 11   a  and  11   b , an alternate embodiment of a stimulator lead assembly is shown. Stimulator lead assembly  300  includes a set of stimulator leads  310  having a distal end and a proximal end and incorporating a set of optical fibers  316 . The set of stimulator leads  310  further incorporate a set of electrical wires which terminate on the proximal end in set of electrode contacts  314  and at the distal end in set of electrode contacts  312 . The set of optical fibers  316  terminate on the proximal end in a set of fiber optic connectors  318  and at the distal end in a set of distal optical elements  325 . Electrode housing  320  has a set of electrode contacts  322  contacting the set of electrode contacts  312 . Electrode housing  320  further includes a set of cavities  321  containing the set of distal optical elements  325  and optically isolating the set of distal optical elements from each other. In the embodiment shown, set of electrode contacts  322  include a set of set screws  328  threaded into a set of threaded holes in electrode housing to electrically and mechanically attach the set of stimulator leads to the electrode housing. 
         [0145]    Referring to  FIG. 11   c , electrode housing  320  further comprises a set of holes  324  ductedly connected to set of cavities  321  and a set of threaded holes  326 . Set of set screws  328  thread into set of threaded holes  326 . 
         [0146]      FIG. 12  shows a side cross-section of electrode housing  320   a . Cavity  321   a  of set of cavities  321  includes reflective surface  330  above the distal end of optical fiber  316   a . Reflective surface  330  is polished approximately flat and at about a 45° angle to the optical axis of optical fiber  316   a . Stimulator lead  311   a  is held in place by set screw  328   a  threaded into threaded hole  326   a  and fastened against stimulator electrode  312   a.    
         [0147]      FIG. 13   a  shows a side view cross-section of electrode housing  320   b .  FIG. 13   b  shows a top view cross-section of electrode housing  320   b . Cavity  321   b  of set of cavities  321  includes a cylindrical reflective surface  331  surrounding the distal end of optical fiber  316   b . Optical fiber  316   b  includes a negative axicon at the distal end. Stimulator lead  311   b  including stimulator electrode  312   b  is held in place by set screw  328   b  threaded into threaded hole  326   b  and fastened against stimulator electrode  312   b.    
         [0148]    An alternate embodiment of electrode housing  320  is arranged as in  FIGS. 13   a  and  13   b , but does not include the cylindrical reflective surface  331 . The alternate embodiment utilizes one of either an optical fiber having a negative axicon at the distal end or an optical fiber having a beveled surface. 
         [0149]      FIG. 14  shows a side cross-section of electrode housing  320   c . Cavity  321   c  of set of cavities  321  includes ball lens  332  positioned at approximately a focal distance from the distal end of optical fiber  316   c . Distal end of optical fiber  316   c  includes one of either a negative axicon or a beveled surface. Stimulator lead  311   c  including stimulator electrode  312   c  is held in place by set screw  328   c  threaded into threaded hole  326   c  and fastened against stimulator electrode  312   c.    
         [0150]    Referring to  FIG. 15 , a preferred embodiment of the implanted components of the system is shown. Positionally-sensitive spinal cord stimulator  45  includes pulse generator and signal processor (PGSP)  50  and stimulator lead assembly  40 . PGSP unit  50  provides power to a set of electrodes in stimulator lead assembly  40  and houses electronic and opto-electronic components of the system. Stimulator lead assembly  40  connects to PGSP unit  50  further connecting the stimulator electrodes of each stimulator lead to a controllable current source. Stimulator lead assembly  40  connects at least one IR emitter to at least one optical fiber through a first fiber optical connector and at least two photodetectors to at least two optical fibers through additional fiber optic connectors. PGSP unit  50  gathers and processes photodetector signals and makes adjustments to the stimulator 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 percutaneous activation of and adjustments to positionally-sensitive spinal cord stimulator  45 . PGSP 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. In an alternate embodiment, calibration unit  54  is incorporated into SCS controller  53 . 
         [0151]    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 . 
         [0152]    Referring to  FIGS. 16   a - 16   c , mechanical form of PGSP unit is shown. PGSP unit comprises housing  200  having top surface  207  and recess  202  with front wall  203   a  and back wall  203   b . Cover  209  hermetically seals recess  202 . Housing  200  further includes front face  204  and cavity  201  below recess  202  and below top surface  207 . Cavity  201  contains and supports electronics board  212  and battery  216 . A set of horizontal holes  206  extend through front face  204  into housing  200  and through front wall  203   a . Each horizontal hole in the set of horizontal holes  206  includes a set of electrode contactors to match the set of proximal electrodes on the proximal end of a stimulator lead assembly. A set of slots  205  are cut from top surface  207  into the center of the set of horizontal holes  206 . A set of fiber optical connectors  210  are mounted into housing  200  from back wall  203   b  and coupled to a set of active optical components  218 . Each active optical component in set of active optical components  218  comprises at least one of the group consisting of an IR emitter and a photodetector. The set of fiber optic connectors  210  match the fiber optic connector on the proximal end of a stimulator lead assembly. Set of active optical components  218  are electrically coupled to and controlled by electronics board  212 . Set of electrode contactors are electrically coupled to and driven by electronics board  212 . Battery  216  powers electronics board  212 . 
         [0153]    Referring to  FIGS. 16   d  and  16   e , PGSP unit  50  is shown with a set of stimulator lead assemblies inserted and mechanically engaged. Lead cable  110  is inserted into a horizontal hole in set of horizontal holes  206  causing proximal set of electrode contacts  114  to come into electrical contact with set of electrode contactors  208  and further connecting proximal set of electrode contacts to electronics board  212 . Optical fiber  101  is brought through slot  205  and over recess  202  so that fiber optic connector  103  is inserted into set of fiber optic connectors  210  making a low loss optical connection between optical fiber  101  and active optical component  218 . Lead cable  110  and optical fiber  101  are mechanically engaged into place. 
         [0154]    Referring to  FIG. 17   a , block diagram of PGSP unit  50  is shown. PGSP unit  50  includes CPU  70  having onboard memory  72 . CPU  70  is connected to pulse modulator  62  and pulse generator  60 . Pulse modulator  62  is 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 IR emitter  79 . IR emitter  79 , in set of IR emitters  69  includes a fiber optic connector to effectively couple IR emitter  79  to fiber  81 . Fiber  81  is connected to a distal optical emitter in the stimulator lead. Other IR emitters in set of IR emitters  69  are similarly connected to a set of optical fibers and a set of distal optical emitters. 
         [0155]    CPU  70  is also connected to optical signal processor  64 . Optical signal processor  64  is connected to a set of photodetectors  67  and receives signals from the set of photodetectors, filters the optical signals, and correlates the optical signals to an 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 or to separate optical signals detected by one detector which was generated from multiple IR emitters. 
         [0156]    IR detector  77 , in set of photodetectors  67 , 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 fiber optic connector to fiber  82  which is coupled to a distal optical collector. Other photodetectors in the set of photodetectors are similarly connected to a set of fibers and a set of optical collectors. In a preferred embodiment, the photodetectors are similar to that of Part No. APA3010P3Bt from Kingbright Corporation of City of Industry, Calif. 
         [0157]    CPU  70  is connected to optical modulator  68 . Optical modulator  68  generates the IR emission waveform transmitted to the set of 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 an electrical waveform which is then transmitted to the IR emitter driver. The IR emitter driver transmits the waveform to IR emitter  79  and a pulse with the waveform is launched into fiber  81 . 
         [0158]    The optical waveform may take several forms. For example, the pulse width of the optical waveform may have a low duty cycle to minimize power consumption. A single optical pulse may occur for a set of electrode stimulation pulses. The optical waveform may include frequency, phase or amplitude modulation. 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 No. PDI-E900 from Advanced Photonix, Inc. of Ann Arbor, Mich. 
         [0159]    Set of IR emitters  69  are driven by IR emitter driver  66 . The IR emitter driver is programmably configured to drive the set of IR emitters such that in a first mode of operation, optical pulses are launched alternatively into multiple optical fibers. In a second mode of operation, a set of uniquely modulated optical waveforms are launched simultaneously in multiple optical fibers, each optical fiber carrying one uniquely modulated optical waveform. In a third mode of operation, optical pulses are launched simultaneously into multiple optical fibers, each optical pulse having the same waveform. The first, second and third modes of operation are operationally equivalent when there is only one IR emitter in the set of IR emitters. 
         [0160]    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. 
         [0161]    The modified square wave has 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 1000 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. 
         [0162]    CPU  70  is in transcutaneous communications, via RF transceiver  71 , with calibration and programming unit  54  and SCS controller  53 . Referring to  FIG. 17   b , SCS controller  53  is shown. 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. 
         [0163]    Referring to  FIG. 17   c , calibration and programming unit  54  will be described. Calibration and programming unit  54  includes processor  910  connected to onboard memory  918 , input/output devices  916  and  917 , RF transceiver  912  and display  914 . Display  914 , in the preferred embodiment, is a low power liquid crystal display. Input/output device  916  and input/output device  917  are simple push button switches monitored continuously by the processor. Memory  918  is onboard processor  910 . RF transceiver  912  is a low power transmitter/receiver combination. 
         [0164]    Referring to  FIGS. 18   a ,  18   b ,  18   c  and  FIG. 17   a , method  80  of operation of the positionally-sensitive spinal cord stimulator is shown. 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. 
         [0165]    Beginning with  FIG. 18   a , 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?”. 
         [0166]    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 an optical pulse from a first IR emitter in the set of IR emitters. At step  93 , a first pair of photocurrent levels at the photodetectors, I L [1] and I R [1], are measured by optical signal processor  64  and passed to CPU  70  for storage in memory. At step  94 , if additional IR emitters are utilized then the method repeats step  92  for the second IR emitter and repeats step  93  to measure a second pair of photocurrent levels I L [2] and I R [2] and store them in memory. Steps  92 ,  93  and  94  are repeated for n IR emitters, storing a set of photocurrent pairs (I L [1],I R [1]), (I L [2],I R [2]), . . . , (I L [n],I R [n]). 
         [0167]    At step  95 , CPU estimates the amplitude A L  and A R  of a train of pulses to be sent to the electrodes, based on the set of photocurrent pairs. 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 I L [1], A L [n], I R [1], . . . I R [1], A L , A R , P W  and P f  in memory for future retrieval. The method then returns to step  81 . 
         [0168]    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  54 . At step  89 , the CPU enters the calibration routine as will be described more fully later. The method then returns to step  81 . 
         [0169]    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. 
         [0170]    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. In another embodiment, step  99  is performed whereby pulse width and pulse frequency are dynamically varied according to the calibration values stored in the calibration table for each electrode. 
         [0171]    Referring to  FIG. 18   b , an alternate embodiment of estimating amplitude values, at step  95  is shown. 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. 
         [0172]    At step  97 , the following equation is applied: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       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 
                         
                       
                       + 
                       … 
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
         [0000]    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,       
 
         [0175]    At step  98 , the following equation is applied: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       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 
                         
                       
                       + 
                       … 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
         [0000]    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.       
 
         [0178]    Referring to  FIG. 18   c , another alternate method of estimating amplitude values at step  95  is shown. 
         [0179]    At step  1800 , the CPU computes a distance factor dP according to the equation: 
         [0000]    
       
         
           
             
               
                 
                   dP 
                   = 
                   
                     
                       
                         ∑ 
                         
                           i 
                           , 
                           j 
                         
                       
                        
                       
                         
                           ( 
                           
                             
                               PD 
                               ij 
                             
                             - 
                             
                               I 
                               ij 
                             
                           
                           ) 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where =measured value of jth photodetector current due to ith optical emitter, and PD ij =calibration table value of jth photodetector current generated by ith optical emitter. 
         [0180]    dP is calculated for each row corresponding to patient positions 1-4 of the calibration table. At step  1802 , 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. 
         [0181]    Referring to  FIG. 18   d , another alternate method of estimating amplitude values, step  95 , is shown. 
         [0182]    At step  1805 , the CPU consults the calibration table to locate the closest pair of consecutive PD iL  values that bracket the measured value I iL , [PD iL TOP , PD iL BOTTOM ]. At step  1810 , the CPU locates the pair of A L  values that correspond to the closest pair of PDi L  values, [A L TOP , A L BOTTOM ]. At step  1815 , the CPU applies the interpolation equation to locate the estimated value of A L , as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     A 
                     L 
                   
                   = 
                   
                     Average 
                      
                     
                         
                       
                         [ 
                         
                           
                             
                               ( 
                               
                                 
                                   ( 
                                   
                                     
                                       A 
                                       LTOP 
                                     
                                     - 
                                     
                                       A 
                                       LBOTTOM 
                                     
                                   
                                   ) 
                                 
                                 
                                   ( 
                                   
                                     
                                       PD 
                                       iLTOP 
                                     
                                     - 
                                     
                                       PD 
                                       iLBOTTOM 
                                     
                                   
                                 
                               
                               ) 
                             
                             · 
                             
                               ( 
                               
                                 
                                   I 
                                   iL 
                                 
                                 - 
                                 
                                   PD 
                                   iLBOTTOM 
                                 
                               
                               ) 
                             
                           
                           + 
                           
                             A 
                             LBOTTOM 
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where: 
         [0183]    A L =estimated value of the left electrode pulse current; 
         [0184]    I iL =measured value of photodetector current for the left photodetector and the ith optical emitter; 
         [0185]    PD iLTOP =upper bracketed value of photodetector current from the calibration table for the left photodetector and the ith optical emitter; 
         [0186]    PD iL BOTTOM =lower bracketed value of the photodetector current from the calibration table for the left photodetector and the ith optical emitter; 
         [0187]    A L TOP =upper value of the electrode pulse current from the calibration table corresponding to PDi L TOP ; 
         [0188]    A L BOTTOM =lower value of the pair of electrode amplitudes from the calibration table corresponding to PDi L BOTTOM ; and, 
         [0189]    the average is taken over all optical emitters i. 
         [0190]    At step  1817 , the CPU consults the calibration table to locate the closest pair of consecutive PD iR  values that bracket the measured value I iR , [PD iR TOP , PD iR BOTTOM ]. At step  1819 , the CPU locates the pair of A R  values that correspond to the closest pair of PD iR  values, [A R TOP , A R BOTTOM ]. At step  1820 , the CPU applies the interpolation equation to locate the estimated value of A R , as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     A 
                     R 
                   
                   = 
                   
                     Average 
                      
                     
                         
                       
                         [ 
                         
                           
                             
                               ( 
                               
                                 
                                   ( 
                                   
                                     
                                       A 
                                       RTOP 
                                     
                                     - 
                                     
                                       A 
                                       RBOTTOM 
                                     
                                   
                                   ) 
                                 
                                 
                                   ( 
                                   
                                     
                                       PD 
                                       iRTOP 
                                     
                                     - 
                                     
                                       PD 
                                       iRBOTTOM 
                                     
                                   
                                 
                               
                               ) 
                             
                             · 
                             
                               ( 
                               
                                 
                                   I 
                                   iR 
                                 
                                 - 
                                 
                                   PD 
                                   iRBOTTOM 
                                 
                               
                               ) 
                             
                           
                           + 
                           
                             A 
                             RBOTTOM 
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where: 
         [0191]    A R =estimated value of the right electrode pulse current; 
         [0192]    I iR =measured value of photodetector current for the right photodetector and the ith optical emitter; 
         [0193]    PD iR TOP =upper bracketed value of photodetector current from the calibration table for the ith optical emitter; 
         [0194]    PD iR BOTTOM =lower bracketed value of photodetector current from the calibration table for the ith optical emitter; 
         [0195]    A R TOP =upper value of the electrode pulse current from the calibration table corresponding to PD iR TOP ; 
         [0196]    A R BOTTOM =lower value of the pair of electrode amplitudes from the calibration table corresponding to PDi R BOTTOM ; and, 
         [0197]    the average is taken over all optical emitters i. 
         [0198]    Referring to  FIG. 19   a , the processor is programmed to carry out steps of calibration method  500  upon request by a calibration control program. At step  520 , the amplitudes A L  and A R  are set at the minimum value of a predetermined range. 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. 
         [0199]    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. 
         [0200]    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 . 
         [0201]    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 signals from the optical signal processor representative of the set of photocurrents I iL  from the left photodetector generated by the ith optical emitter. At step  550 , the CPU measures the optical signals from the optical signal processor representative of the set of photocurrents I iR  from the right photodetector generated by the ith optical emitter. At steps  560  and  565 , the CPU stores the sets of photocurrents I iL  and I iR  in the calibration table as calibrated values PD iL  and PD iR . At step  570 , the calibration method steps complete by returning control to the calibration control program. 
         [0202]    Referring to  FIG. 19   b , 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. 
         [0203]    At step  350 , RF transceiver  912  receives a signal indicative of a request to move the patient to a prone position and passes it to processor  910 . At step  352 , the patient is positioned in a prone position. At step  354 , calibration method  500 , as described in  FIG. 19   a , is carried out to maximize the level of paresthesia experienced by the patient. 
         [0204]    At step  360 , RF transceiver  912  receives a signal indicative of a request to move the patient to a right lateral position and passes it to processor  910 . At step  362 , the patient is positioned in a right lateral position. At step  364 , calibration method  500  is then carried out to optimize the level of paresthesia experienced by the patient. 
         [0205]    At step  370 , RF transceiver  912  receives a signal indicative of a request to move the patient to a supine position and passes it to processor  910 . At step  372 , the patient is positioned in a supine position. At step  374 , calibration method  500  is then carried out to optimize the level of paresthesia experienced by the patient. 
         [0206]    At step  380 , RF transceiver  912  receives a signal indicative of a request to move the patient to a left lateral position and passes it to processor  910 . At step  382 , the patient is positioned in a left lateral position. At step  384 , calibration method  500  is then carried out to optimize the level of paresthesia experienced by the patient. 
         [0207]    After steps  380 ,  382  and  384  are performed, the calibration program is complete. 
         [0208]    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. 
         [0209]    Referring to  FIG. 19   c , the various states of the SCS controller will be described. At wait state  505 , SCS controller  53  enters a waiting posture and continually polls I/O device  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 . 
         [0210]    If a “stop” signal is received from I/O device  906 , at step  509 , 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 . 
         [0211]    If a “calibrate” signal is received from I/O device  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 . 
         [0212]      FIG. 20  shows a calibration table  1150  for a first embodiment suitable for the arrangement of optical emitters, optical collectors and electrodes described in  FIGS. 8   a - 8   d . 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 current value PD L    1154 , right photodetector current 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 . 
         [0213]    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. 
         [0214]    In alternate embodiments, calibration may be performed for additional physical positions such that additional rows are placed in table  1150 . 
         [0215]    Electrode stimulation pulse width  1161  and frequency  1162  are shown as having constant values. However, in an alternate embodiment, the values of electrode stimulation pulse width  1161  and electrode pulse frequency  1162  are varied through a predetermined range during calibration and recorded for each patient position using interpolation means, such as those shown for pulse amplitude, to adjust the values while in operation. 
         [0216]      FIG. 21  shows a calibration table  1250  for a second embodiment suitable for the arrangement of optical emitters, optical collectors and electrodes described in  FIGS. 10   a - 10   d . 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  1250  includes nine columns, patient position identifier  1252 , left photodetector value PD LL    1253  for light detected from the left optical emitter, right photodetector value PD LR    1254  for light detected from the left optical emitter, right photodetector value PD RR    1255  for light detected from the right optical emitter, right photodetector value PD RL    1256  for light detected from the right optical emitter, left electrode stimulation pulse amplitude A L    1258 , right electrode pulse amplitude A R    1260 , electrode stimulation pulse width P W    1261 , and electrode pulse frequency P f    1262 . 
         [0217]    Patient position identifier  1252  in a preferred embodiment includes four positions, front (prone −0°), right −90°, back (supine −180°) and left −270°. Each row in Table  1250  is associated with one of the four patient positions. Left electrode stimulation pulse amplitude  1258  and right electrode stimulation pulse amplitude  1260  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  1258  and right electrode stimulation pulse amplitude  1260  are directly proportional to the stimulation energy delivered to the respective electrodes. 
         [0218]    In alternate embodiments, calibration may be performed for additional physical positions such that additional rows are placed in table  1250 . 
         [0219]    Electrode stimulation pulse width  1261  and frequency  1262  are shown as having constant values. However, in an alternate embodiment, the values of electrode stimulation pulse width  1261  and electrode pulse frequency  1262  are varied through a predetermined range during calibration and recorded for each patient position. Interpolation may be used to determine the pulse width and frequency, such as those shown for pulse amplitude, to adjust the values while in operation. 
         [0220]    The disclosure of Table  1250  is not intended to limit the invention, but show an example of multiple optical detectors with multiple optical emitters. Other calibration tables can be generated in a similar manner for more than two optical detectors and two optical emitters. 
         [0221]    The disclosure demonstrates a novel optical sensor, generally useful in many fields of endeavor, in which a probe light beam is emitted from the sensor and a responsive light beam is collected by the sensor, where the sensor comprises a negative axicon element coupled to an optical fiber. In a preferred embodiment, the negative axicon is embedded in the end of the optical fiber. 
         [0222]    The optical fiber is further coupled to an active optical element which can be an optical emitter or an optical detector. In a preferred embodiment, both an optical emitter and an optical detector are coupled to a single optical fiber with the negative axicon using an optical circulator. In an alternate embodiment, a set of optical fibers coupled to a set of negative axicons for emitting and detecting light are conceived. 
         [0223]    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.