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. Electrode pulse characteristics are adjusted in real time, based on the set of measured optical intensities, 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 of U.S. patent application Ser. No. 13/780,470, filed Feb. 28, 2013, which is a Continuation-in-Part Application of U.S. patent application Ser. No. 13/567,966, filed Aug. 6, 2012, which is continuation of U.S. patent application Ser. No. 12/925,231, filed Oct. 14, 2010, now U.S. Pat. No. 8,239,038. This application claims priority to U.S. Provisional Patent Application No. 61/867,413, filed Aug. 19, 2013. Each patent application identified above is incorporated here by reference in its entirety to provide continuity of disclosure. 
     
    
     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 parasthesias 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 vertebra  10 , a thoracic vertebra, 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 space within the spinal canal 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 parasthesias 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 parasthesias. 
         [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 with respect to 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 Publication 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 parasthesias 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. The optical fiber is further coupled to an active optical element which can be an optical emitter or an optical detector. 
         [0017]    Disclosed is a stimulator system having a stimulator lead encasing the optical fiber, a controller, an optical emitter operatively connected to the controller generating an emitted light beam into the optical fiber. An optical detector operatively connected to the controller, receives a set of reflected light beams from the optical fiber. 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. 
         [0018]    In a preferred embodiment of the stimulator system having two stimulator leads, the first stimulator lead encases an optical fiber coupled to an optical emitter and an optical element for emitting light into an epidural space. The second stimulator lead encases an optical fiber coupled to an optical detector and an optical element for collecting and detecting light from an epidural space. Both leads have a set of electrodes. 
         [0019]    In another embodiment of the stimulator system, a single stimulator lead encases an optical fiber which is coupled to an optical emitter and further coupled to an optical detector in the set of optical detectors. An optical circulator is operatively coupled to the optical emitter, the optical detector and the optical fiber. 
         [0020]    In an aspect of the system, the stimulator lead is an implantable lead encasing the optical fiber in a lumen wherein the implantable lead further comprises an EMI shield. In a related aspect, the implantable lead further comprises carbon nanotubes. 
         [0021]    In another 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. 
         [0022]    In another aspect of the system, the controller derives a set of current amplitudes based on time averaging of a set of historical current amplitudes. 
         [0023]    In yet another aspect of the system, the controller derives a current pulse width 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. 
         [0024]    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. 
         [0025]    In a preferred embodiment, the system further comprises a calibration and programming unit operatively connected to the controller for calibrating the set of current pulse amplitudes, pulse widths and pulse frequencies. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0026]    The following disclosure is understood best in association with the accompanying figures. Like components share like numbers. 
           [0027]      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; 
           [0028]      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; 
           [0029]      FIG. 3  shows a sagital cross section view of the human spine showing the approximate position of an electrode array for spinal cord stimulation; 
           [0030]      FIG. 4  shows a prior art electrode array for spinal cord stimulation; 
           [0031]      FIG. 5  shows the relative electric field produced by a preferred embodiment for the spinal cord in various positions within the spinal canal; 
           [0032]      FIGS. 6   a - 6   b  shows two perspective views of a stimulator lead for spinal cord stimulation incorporating an optical fiber. 
           [0033]      FIG. 6   c  shows a cross-section of a stimulator lead along line  6   c - 6   c  from  FIG. 6   a.    
           [0034]      FIG. 6   d  shows placement of a set of stimulator leads. 
           [0035]      FIGS. 7   a - 7   g  show various embodiments of a distal optical element. 
           [0036]      FIG. 8   a  shows a cross-sectional view of a single stimulator lead embodiment with a spinal cord at a forward position with respect to a stimulator lead. 
           [0037]      FIG. 8   b  shows a cross-sectional view of a single stimulator lead embodiment with a spinal cord at a rightward position with respect to a stimulator lead. 
           [0038]      FIG. 8   c  shows a cross-sectional view of a single stimulator lead embodiment with a spinal cord at posterior position with respect to a stimulator lead. 
           [0039]      FIG. 8   d  shows a cross-sectional view of a single stimulator lead embodiment with a spinal cord at leftward position with respect to a stimulator lead. 
           [0040]      FIG. 9   a  shows a dual stimulator lead embodiment having one optical fiber operating as an optical emitter and another optical fiber operating as an optical collector. 
           [0041]      FIG. 9   b  shows placement of two stimulator leads in a dual stimulator lead embodiment. 
           [0042]      FIG. 10   a  shows a cross-sectional view of a dual stimulator lead embodiment with two optical elements located in relation to a spinal cord at a forward position. 
           [0043]      FIG. 10   b  shows a cross-sectional view of a dual stimulator lead embodiment with two optical elements located in relation to a spinal cord at a rightward position. 
           [0044]      FIG. 10   c  shows a cross-sectional view of a dual stimulator lead embodiment with two optical elements located in relation to a spinal cord at a backward position. 
           [0045]      FIG. 10   d  shows a cross-sectional view of a dual stimulator lead embodiment with two optical elements located in relation to a spinal cord at a leftward position. 
           [0046]      FIG. 11  shows a schematic representation of a preferred embodiment of the positionally sensitive spinal cord stimulation system. 
           [0047]      FIG. 12  is a block diagram of the components of a preferred embodiment of a pulse generation and optical signal processing unit. 
           [0048]      FIG. 13  is a block diagram of the components of a preferred embodiment of an SCS controller. 
           [0049]      FIG. 14  is a block diagram of the components of a preferred embodiment of a calibration and programming unit. 
           [0050]      FIGS. 15   a  and  15   b  are flow diagrams of a method of operation of a preferred embodiment. 
           [0051]      FIG. 16  is a flow diagram of a preferred method of calibration. 
           [0052]      FIG. 17  is a flow diagram of a preferred method of calibration for a particular patient. 
           [0053]      FIG. 18  is a state diagram of a preferred embodiment of stimulator control system. 
           [0054]      FIG. 19  is a graphic representation of a calibration table for a single lead system with one optical emitter, one optical detector and a set of electrodes. 
           [0055]      FIG. 20  is a graphic representation of a calibration table for a dual lead system with one optical emitter, one optical detector and a set of electrodes. 
       
    
    
     DETAILED DESCRIPTION 
       [0056]    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. 
         [0057]    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. 
         [0058]    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,   (Eq. 1)
 
         [0059]    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 ).       
 
         [0063]    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 ).  (Eq. 2)
 
         [0064]    For deoxyhemoglobin and oxyhemoglobin, the extinction coefficient spectra are well known. 
         [0065]    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. 
         [0066]    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. 
         [0067]    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. 
         [0068]    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 ordinal body positions. 
         [0069]    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. 
         [0070]    The beam divergence of the light emitter relative to the incident and reflected rays will influence the detected light amplitude. 
         [0071]    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. With the patient in a prone position or bending forward (0° direction), the spinal cord moves anterior 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. In the right lateral position or bent to the right (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. In the supine position or bending backward (180° direction), the spinal cord moves dorsally within its orbit within the spinal canal. A decrease in electrode stimulation pulse amplitude bilaterally is required to maintain a constant electric field across the spinal cord. In the left lateral position or bent toward the left (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. 
         [0072]      FIG. 5  shows a plot  500  of relative electric field strength  502  required to be generated at a the electrodes, 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. The electric field strength at points A 1 -A 4  will be described in more detail below in relation to electrode current amplitude. 
         [0073]    Referring to  FIGS. 6   a  and  6   b , a preferred embodiment of a percutaneous stimulator lead is shown. Stimulator lead  600  includes lead cable  610  housing optical fiber  601  which is coupled to distal optical element  602  at a distal end  613  and coupled to optical fiber connector  603  at a proximal end  614 . Optical fiber connector  603  is further coupled to optical circulator  606 . Optical circulator  606  is connected to optical fiber  607  which is further coupled to optical emitter  625 . Optical circulator  606  is also connected to optical fiber  608  which is further coupled to optical detector  627 . Distal optical element  602  is configured as both an optical emitter and an optical collector. A set of electrodes  612 , near the distal end, is coupled to a current source  605  through a set of electrical leads  604  also housed in lead cable  610 . 
         [0074]    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. 
         [0075]    Distal optical element  602  extends into cap  609 . In a preferred embodiment, cap  609  is an extension of lead cable  610  which is sealed at the distal tip and bonded to lead cable  610  with adhesive at  611 . Cap  609  is a NIR-transparent hollow cylinder preferably comprised of glass or plastic and may contain an index matching fluid. 
         [0076]    In another embodiment, cap  609  is comprised of a solid cylinder formed in place around distal optical element  602 . In this embodiment, the cylinder is not hollow and is comprised of a transparent plastic such as Lexan™. In another embodiment, cap  609  is a continuation of the lead cable  610  which may be constructed of polyurethane or other suitable material and is sealed at the distal tip. 
         [0077]    Referring to  FIG. 6   c , a cross-section of stimulator lead  600  is shown. Stimulator lead  600  includes sheathed outer surface  615  which encapsulates a set of electrode leads  617 , lumen  616  in filler material  619 . Lumen  616  encloses optical fiber  601 . Lumen  616  also 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. Optical fiber  601  is inserted after removing the wire stylet from lumen  616 . 
         [0078]    In an alternate embodiment an additional lumen is included in the stimulator lead to provide a separate cavity for the wire stylet. 
         [0079]    In a preferred embodiment, sheathed outer surface  615  includes an EMI shield. Filler material  619  preferably includes a polyimide polymer. Filler material  619  can also include additional materials with physical properties that enhance the EMI shielding capability of lead cable  610 . 
         [0080]    In an alternate embodiment, filler material  619  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. 
         [0081]    Referring to  FIG. 6   d , where a vertebra  622  encloses a spinal cord  620 , a single stimulator lead  624  is placed in the epidural space  626  of vertebra  622  between the dura  621  and the walls of the spinal canal  629 . In a preferred single lead embodiment, stimulator lead  624  is configured with at least one optical fiber and with both an optical emitter and optical collector. Additional embodiments of a single stimulator lead system are possible which include multiple optical fibers in a single-lead assembly. 
         [0082]    Referring again to  FIG. 6   a , in use, probe light beam  661  is emitted from optical emitter  625  and propagates through first optical fiber  607 , through optical fiber  601 , and exits from optical element  602 . A responsive light beam  660  is collected by optical element  602  and propagates through optical fiber  601 , through second optical fiber  608  and detected by optical detector  627 . Optical circulator  606  allows responsive light beam  660  to propagate into second optical fiber  608  but not into first optical fiber  607 . Optical circulator  606  also allows probe light beam  661  to propagate into optical fiber  601  but not into second optical fiber  608 . 
         [0083]    Responsive light beam  660  is generated through interaction between probe light beam  661  and tissue within the spinal canal. 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. 
         [0084]      FIGS. 7   a - 7   g  show suitable optical configurations for an optical element disposed on an optical fiber at the distal end of a stimulator lead.  FIGS. 7   a - 7   g  are intended as examples and should not be interpreted as limiting to the invention. 
         [0085]    In  FIG. 7   a , distal optical element  701  includes optical fiber  708  encased in cap  691 . Optical fiber  708  includes optical axis  702  having core  704  surrounded by cladding  705  further surrounded by jacket  709 . Optical fiber  701  further includes negative axicon  706  etched at the distal end, centered on optical axis  702 , 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  704 . The complement of the critical angle is (90°−α). Jacket  709  is removed from optical fiber  708  for a distance  707  approximately the same as the depth of negative axicon  706 . When light travels through optical fiber  708  and out of the distal end, it will be emitted approximately perpendicular to the optical axis  702  near lateral line  703  in a radially symmetric 360 degree pattern. When used as an optical collector, optical fiber  708  will collect light through a 360 degree angle from directions near lateral line  703 . 
         [0086]    In  FIG. 7   b , distal optical element  710  comprises an optical fiber  711  covered by cap  692 . Optical fiber  710  includes optical axis  712  having core  714  surrounded by cladding  715  which is further surrounded by jacket  719 . Optical fiber  711  includes negative axicon  716  etched at the distal end, centered on optical axis  712 , and having an angular extent B. Angular extent B is approximately 90°. Jacket  719  is removed from optical fiber  711  for a distance  717  approximately the same as the depth of negative axicon  716 . Outer surface of negative axicon  716  is coated with a reflective coating  718 . When light travels through optical fiber  711  and out of the distal end, it will be emitted approximately perpendicular to the optical axis  712  near lateral line  713  in a uniform 360 degree pattern. When used as an optical collector, optical fiber  711  will collect light from through a 360 degree angle from directions near the lateral line  713 . 
         [0087]    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. 
         [0088]    In  FIG. 7   c , distal optical element  720  is enclosed in cap  693  and comprises optical fiber  721 . Optical fiber  721  includes optical axis  722  having core  724  surrounded by cladding  725  which is further surrounded by jacket  729 . Optical fiber  721  includes beveled surface  726  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  724 . Jacket  729  is removed from optical fiber  721  for a distance  727  approximately the same as the depth of beveled surface  726 . When light travels through optical fiber  721  and out of the distal end, it will be emitted approximately perpendicular to the optical axis  722  near lateral line  723  in an angular pattern determined by the position of the beveled surface. When used as an optical collector, optical fiber  721  will collect light in the approximate angular pattern from horizontal directions near the lateral line  723 . 
         [0089]    In  FIG. 7   d , distal optical element  730  is encased in transparent cap  694  and comprises optical fiber  731 . Optical fiber  731  includes optical axis  732  having core  734  surrounded by cladding  735  which is further surrounded by jacket  739 . Optical fiber  731  includes a beveled surface  736  etched at the distal end at an angle D where D is about 45°. Beveled surface  736  has a reflective coating  738 . Jacket  739  is removed from optical fiber  731  for a distance  737  approximately the same as the depth of beveled surface  736 . When light travels through optical fiber  731  and out of the distal end, it will be emitted approximately perpendicular to the optical axis  732  near lateral line  733  in an angular pattern determined by the position of beveled surface  736 . 
         [0090]    In  FIG. 7   e , distal optical element  740  is encased in transparent cap  695 . Distal optical element  740  includes optical fiber  741  with optical axis  742  having core  744 . Core  744  is surrounded by cladding  745  which is further surrounded by jacket  749 . Reflecting surface  746  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  741  and out of the distal end, it will be emitted approximately along the optical axis  742 , reflected from reflecting surface  746 , and further emitted in a horizontal range of directions near lateral line  743  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  741  will collect light in the approximate angular pattern from the horizontal range of direction near lateral line  743 . 
         [0091]      FIG. 7   f , distal optical element  750  is encased by transparent cap  696 . Distal optical element  750  includes optical fiber  751  with optical axis  752  and core  753 . Core  753  is surrounded by cladding  754  which is further surrounded by jacket  756 . Reflector  757  is positioned adjacent optical fiber  751  and coaxial with optical axis  752 . In a preferred embodiment, reflector  757  is conical, that includes silvered surface  758 . In use, light transmitted from the optical fiber is reflected in a 360° pattern, generally perpendicular to optical axis  752 . Similarly, reflector  757  collects light from a 360° axis and transmits it through optical fiber  751 , generally parallel to optical axis  752 . In a preferred embodiment, transparent cap  696  is filled with an optically transparent plastic matrix which supports and positions reflector  757  above optical fiber  751 . In an alternative embodiment, reflector  757  can be formed by a void in matrix  759  which is internally silvered on surface  758 . 
         [0092]      FIG. 7   g , distal optical element  760  is formed as a cap  697 . Distal optical element  760  includes optical fiber  761  with optical axis  762  having core  764 . Core  764  is surrounded by cladding  767  which is further surrounded by jacket  769 . One side of cap  697  includes a reflecting surface  768  which is positioned above the distal end of the optical fiber at an angle of about 45° from optical axis  763 . When light travels through optical fiber  761  and out of the distal end, it will be emitted approximately along the optical axis  762 , reflected from reflecting surface  768 , and further emitted in a horizontal range of directions near lateral line  763  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. The emitted light is collimated by lens  765 . When used as an optical collector, lens  765  focuses collected light as it enters cap  697 . The collected light is directed by reflecting surface  768  into optical fiber  761 . 
         [0093]    Referring to  FIGS. 8   a - 8   d , a single-lead embodiment is described in situ. Spinal cord  820  is shown in various respective positions in the spinal canal in relation to a lateral (coronal) axis  824  and a postero-anterior (sagittal) axis  825  which are perpendicular to one another. Forward direction is towards 0° parallel to the postero-anterior axis, rightward direction is toward 90° parallel to the lateral axis, backward direction is toward 180°, and leftward direction is toward 270°. A stimulator lead assembly, with electrode  801  and optical element  802 , is implanted outside dura  821 . Optical element  802  is optically coupled to optical emitter  865  and optical detector  867 . It should be understood that optical detector  867  will receive light originating from optical emitter  865  after reflection from spinal cord  820 . 
         [0094]    Electrode  801  and optical element  802  are positioned toward the dura and within an operational range of target cells  819 . Target cells  819  are positioned within spinal cord  820  in an arbitrary but constant position with respect to the spinal cord. 
         [0095]    In  FIG. 8   a , spinal cord  820  is in a forward position toward 0° along postero-anterior axis  825 . Path P 1  defines a light path from optical element  802  to reflection point R 1  and back to optical element  802 . The length of path P 1  is D 1 . Optical element  802  emits light from optical emitter  865  along path P 1  where it is reflected at point R 1  by the spinal cord surface after attenuation and scattering by intermediate tissue. Optical element  802  collects light from path P 1  after reflection at point R 1  and after attenuation and scattering by intermediate tissue. Light collected by optical element  802 , is detected by photodetector  867  and converted to photocurrent I 1 . 
         [0096]    In  FIG. 8   b , spinal cord  820  is in a rightward position with respect to optical element  802 , rotated by angle  828  from postero-anterior axis  825  where target cells  819  are shifted rightward toward 90° and parallel to lateral axis  824  by distance  827 . Path P 2  defines a light path from optical element  802  to reflection point R 2  and back to optical element  802 . The length of path P 2  is D 2  which is less than D 1 . Optical element  802  emits light from optical emitter  865  along path P 2  where it is reflected at point R 2  by the spinal cord surface after attenuation and scattering by intermediate tissue. Optical element  802  collects light from path P 2  after reflection at point R 2  and after attenuation and scattering by intermediate tissue. Light collected by optical element  802 , is detected by photodetector  867  and converted to photocurrent I 2 . 
         [0097]    In  FIG. 8   c , spinal cord  820  is in a posterior position shifted by a distance  826  towards optical element  802  along postero-anterior axis  825 . Path P 3  defines a light path from optical element  802  to reflection point R 3  and back to optical element  802 . The length of path P 3  is D 3  which is less than D 1  or D 2 . Optical element  802  emits light from optical emitter  865  along path P 3  where it is reflected at point R 3  by the spinal cord surface after attenuation and scattering by intermediate tissue. Optical element  802  collects light from path P 3  after reflection at point R 3  and after attenuation and scattering by intermediate tissue. Light collected by optical element  802 , is detected by photodetector  867  and converted to photocurrent I 3 . 
         [0098]    In  FIG. 8   d , spinal cord  820  is in a left position with respect to optical element  802 , rotated by angle  830  from postero-anterior axis  825  where target cells  819  are shifted leftward along lateral axis  824  by distance  829 . Path P 4  defines a light path from optical element  802  to reflection point R 4  and back to optical element  802 . The length of path P 4  is D 4  which is less than D 1 , but about the same as D 2 . Optical element  802  emits light from optical emitter  865  along path P 4  where it is reflected at point R 4  by the spinal cord surface after attenuation and scattering by intermediate tissue. Optical element  802  collects light from path P 4  after reflection at point R 4  and after attenuation and scattering by intermediate tissue. Light collected by optical element  802 , is detected by photodetector  867  and converted to photocurrent I 4 . 
         [0099]    Since D 2  and D 4  are less than D 1 , the photocurrents I 2  and I 4  are observed to be greater than I 1 . Since D 3  is less than D 1 , D 2  or D 4  the light is attenuated less, and the photocurrent I 3  is observed to be greater than I 1 , I 2  or I 4 . 
         [0100]    An electric field produced by the electrode  801  stimulates target cells  819  in the spinal cord  820 . Current amplitude is the average current supplied the set of electrodes, each having pulse width PW and pulse frequency PF. For the position of the spinal cord in  FIG. 8   a , the current amplitude has a value of about A 1 . For the rightward shifted position of the spinal cord in  FIG. 8   b , the current amplitude has a value of about A 2  which is about the same as A 1 . For the back shifted position of the spinal cord in  FIG. 8   c , the current amplitude has a value of about A 3  which is less than A 1 . For the leftward shifted position of the spinal cord in  FIG. 8   d , the current amplitude has a value of A 4  which is about the same as A 1 . Comparing the electrode currents for the positions of  FIGS. 8   a - d , A 3 &lt;(A 2 ≈A 4 )&lt;A 1  which is correspondingly displayed on the plot of  FIG. 5  and where electrode current is proportional to electric field strength. The foregoing results are tabulated in Table 1. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Photodetector 
                 Current 
               
               
                   
                 Position 
                 Signal, I 
                 Amplitude, A 
               
               
                   
                   
               
             
             
               
                   
                 1. Front 0° 
                 L 
                 H 
               
               
                   
                 2. Right 90° 
                 M 
                 M 
               
               
                   
                 3. Back 180° 
                 H 
                 L 
               
               
                   
                 4. Left 270° 
                 M 
                 M 
               
               
                   
                   
               
             
          
         
       
     
         [0101]    Referring to  FIG. 9   a , a preferred embodiment of a dual-lead configuration suitable for a stimulator lead system  900  is shown. Stimulator lead  930  includes optical fiber  902  coupled to optical element  932  at distal end  950  and coupled to optical detector  935  at the proximal end  951 . Optical element  932  is configured as an optical collector. A set of electrodes  931 , near the distal end, is coupled to a current source  955  through a set of leads  904  also included in stimulator lead  930 . 
         [0102]    Stimulator lead  940  includes optical fiber  901  coupled to optical element  942  at the distal end and coupled to optical emitter  945  at the proximal end. Optical element  942  is configured as an optical emitter. A set of electrodes  941 , near the distal end, is coupled to a current source  955  through a set of leads  903  also included in the stimulator lead  940 . 
         [0103]    Probe light beam  960  emitted from optical emitter  945  propagates through optical fiber  901  and exits from optical element  942 . A responsive light beam  961  collected by optical element  932 , propagates through optical fiber  902 , is detected by optical detector  935  and converted to a photocurrent signal. The photocurrent signal is processed to determine an amount of current to supply to electrodes  931  and  941 . 
         [0104]    Referring to  FIG. 9   b , where vertebra  922  houses spinal cord  920 , stimulator leads  930  and  940  are placed side by side in the epidural space  926  between the dura  921  and the walls of the spinal canal  929 . 
         [0105]    To operatively place the two stimulator leads, a first stimulator lead is positioned into the epidural space near the spinal cord using a wire stylus inserted in a lumen of the first stimulator lead. The wire stylus is withdrawn and an optical fiber assembly is inserted in the lumen. Then, a second stimulator lead is positioned in the epidural space near the spinal cord and to the side of the first stimulator lead using the wire stylus inserted in a lumen of the second stimulator lead. The wire stylus is withdrawn and an optical fiber assembly is inserted in the lumen. 
         [0106]    Referring to  FIGS. 10   a - 10   d , a dual-lead embodiment, utilizing the stimulator leads of  FIG. 9 , is described as in situ. Spinal cord  1020  is shown in various respective positions in the spinal canal in relation to a coronal axis  1024  which is centered through optical emitter  1041  and optical collector  1031 . A sagittal axis  1025  is perpendicular to the coronal axis and generally in the postero-anterior direction of the body encapsulating spinal cord  1020 . Forward direction is towards 0° parallel to the sagittal axis, rightward direction is toward 90° parallel to the coronal axis, backward direction is toward 180°, and leftward direction is toward 270°. 
         [0107]    Stimulator lead assembly  1010  is implanted outside dura  1021  having a left stimulator lead with electrode  1041  and optical element  1042  and having a right stimulator lead with electrode  1031  and optical element  1032 . Optical element  1042  is optically coupled to optical emitter  1045 . Optical element  1032  is optically coupled to optical detector  1035 . It should be understood that optical detector  1035  will receive light originating from optical emitter  1045 . In situ, the stimulator lead positions may be reversed where the stimulator lead with optical element  1032  and electrode  1031  is on the left and the stimulator lead with optical element  1042  and electrode  1041  is on the right. 
         [0108]    Electrodes  1031  and  1041  are positioned toward the dura and within an operational range of target cells  1019 . Target cells  1019  are positioned within spinal cord  1020  in an arbitrary but constant position with respect to the spinal cord. 
         [0109]    Referring to  FIG. 10   a , the spinal cord is positioned forward, path P 5  defines a light path from optical element  1042  to reflection point R 5  and then to optical element  1032 . The length of path P 5  is D 5 . Optical element  1042  emits light along path P 5  from optical emitter  1045  and optical element  1032  collects light from path P 5  after reflection at point R 5  from spinal cord  1020  and after attenuation and scattering by intermediate tissue. Light collected by optical element  1032  is detected by photodetector  1035  which produces a photocurrent of I 1  in response. 
         [0110]    An electric field produced by electrodes  1031  and  1041  stimulates target cells  1019 . Current amplitudes A R1  and A L1  are for the average currents supplied by electrode  1031  and electrode  1041 , 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. 
         [0111]    Referring to  FIG. 10   b , the spinal cord is rotated through angle  1028  and positioned rightward by a distance  1027  towards 90°, path P 6  defines a light path from optical element  1042  to reflection point R 6  and then to optical element  1032 . The length of path P 6  is D 6  which is less than the length D 5 . Optical element  1042  emits light along path P 6  from optical emitter  1045  and optical element  1032  collects light from path P 6  after reflection at point R 6  from spinal cord  1020  and after attenuation and scattering by intermediate tissue. Light collected by optical element  1032  is detected by photodetector  1035  which produces a photocurrent of I 2  in response where I 2  is greater than I 1 . 
         [0112]    An electric field produced by electrodes  1031  and  1041  stimulates target cells  1019 . Current amplitude A R2  is for the average current supplied by electrode  1031  and current amplitude A L2  is for the average current supplied by electrode  1041 , each having pulse widths PW 2  and pulse frequencies PF 2 . The current amplitudes A R2  and A L2  are greater than current amplitudes A RI  and A. These foregoing results are tabulated in Table 2, row 2. 
         [0113]    Referring to  FIG. 10   c , the spinal cord is positioned towards the back and displaced by a distance  1026  towards 180°, path P 7  defines a light path from optical element  1042  to reflection point R 7  and then to optical element  1032 . The length of path P 7  is D 7  which is shorter than length D 5  or D 6 . Optical element  1042  emits light along path P 7  from optical emitter  1045  and optical element  1032  collects light from path P 7  after reflection at point R 7  from spinal cord  1020  and after attenuation and scattering by intermediate tissue. Light collected by optical element  1032  is detected by photodetector  1035  which produces a photocurrent of I 3  in response, where I 3  is greater than I 1  and I 2 . 
         [0114]    An electric field produced by electrodes  1031  and  1041  stimulates target cells  1019 . Current amplitude A R3  is for the average current supplied by electrode  1031  and current amplitude A L3  is for the average current supplied by electrode  1041 , each having pulse widths PW 3  and pulse frequencies PF 3 . The current amplitudes A R3  and A L3  are less than the current amplitudes A R1 , A R2 , A L1  and A L2 . These foregoing results are tabulated in Table 2, row 3. 
         [0115]    Referring to  FIG. 10   d , the spinal cord is rotated through angle  1030  and positioned rightward by a distance  1029  towards 270°, path P 8  defines a light path from optical element  1042  to reflection point R 8  and then to optical element  1032 . The length of path P 8  is D 8  which is less than length D 5  but about the same as D 6 . Optical element  1042  emits light along path P 8  from optical emitter  1045  and optical element  1032  collects light from path P 8  after reflection at point R 8  from spinal cord  1020  and after attenuation and scattering by intermediate tissue. Light collected by optical element  1032  is detected by photodetector  1035  which produces a photocurrent of I 4  in response where I 4  is about the same as I 2 . 
         [0116]    An electric field produced by electrodes  1031  and  1041  stimulates target cells  1019 . Current amplitude A R4  is for the average current supplied by electrode  1031  and current amplitude A L4  is for the average current supplied by electrode  1041 , each having pulse widths PW 2  and pulse frequencies PF 2 . The current amplitudes A R4  and A L4  are about the same as the current amplitudes A R1  and A L1 . These foregoing results are tabulated in Table 2, row 4. 
         [0117]    The distances D 6  and D 8 , defining optical paths for the light emitted by the optical emitter and collected by the optical collector, are less than the distance D 5 . The distance D 7  is smaller than the distances D 5 , D 6  and D 8 . Comparing photocurrents of positions of  FIGS. 10   a  through  10   d , I 3 &gt;(I 2 ≈I 4 )&gt;I 1 . 
         [0118]    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 
               
               
                   
               
               
                   
                 Photodetector 
                 Current 
                 Current 
               
               
                 Position 
                 Signal, I 
                 Amplitude, A R   
                 Amplitude, A L   
               
               
                   
               
             
             
               
                 1. Front 0° 
                 L 
                 H 
                 H 
               
               
                 2. Right 90° 
                 M 
                 M 
                 M 
               
               
                 3. Back 180° 
                 H 
                 L 
                 L 
               
               
                 4. Left 270° 
                 M 
                 M 
                 M 
               
               
                   
               
             
          
         
       
     
         [0119]    Referring to  FIG. 11 , a preferred embodiment of the components of the system is shown. Stimulator lead assembly  1140  includes at least one stimulator lead with a set of electrodes. Positionally-sensitive spinal cord stimulator  1145  includes pulse generator and signal processor (PGSP unit)  1150  and is connected to stimulator lead assembly  1140 . PGSP unit  1150  provides power to the set of electrodes in stimulator lead assembly  1140  and houses electronic and opto-electronic components of the system. Stimulator lead assembly  1140  connects to PGSP unit  1150  further connecting the stimulator electrodes of each stimulator lead to a controllable current source. Stimulator lead assembly  1140  connects at least one IR emitter to at least one optical fiber through a first fiber optical connector and at least one photodetector to at least one optical fiber through additional fiber optic connectors. 
         [0120]    PGSP unit  1150  gathers and processes photodetector signals and makes adjustments to the stimulator electrode current (or voltage) based on the photodetector signals. PGSP unit  1150  is connected by wireless communication link  1152  across skin boundary  1156  to SCS controller  1153 . The SCS controller is configured to allow percutaneous activation of and adjustments to positionally-sensitive spinal cord stimulator  1145 . PGSP unit  1150  is also connected by wireless communication link  1155  to calibration and programming unit  1154 . Calibration and programming unit  1154  is programmed to accept patient input and transmit the patient input to PGSP  1150  during calibration. In an alternate embodiment, calibration and programming unit  1154  is incorporated into SCS controller  1153 . 
         [0121]    PGSP unit  1150  is preferably powered by batteries. In an alternate embodiment, PGSP unit  1150  derives power from capacitive or inductive coupling devices. Calibration may further calibrate the batteries, the capacitive devices, or inductive coupling in PGSP unit  1150 . Communication links  1152  or  1155  may further serve as a means of providing electrical charge for the batteries or capacitive devices of PGSP unit  1150 . 
         [0122]    Referring to  FIG. 12 , block diagram of PGSP unit  1150  is shown. PGSP unit  1150  includes CPU  1270  having onboard memory  1272 . CPU  1270  is connected to pulse modulator  1262  and pulse generator  1260 . Pulse modulator  1262  is connected to pulse generator  1260 . CPU  1270  is also operatively connected to optical modulator  1268  and optical signal processor  1264 . Optical modulator  1268  is connected to infrared emitter driver  1266 . Infrared emitter driver  1266  is connected to IR emitter  1279  and drives IR emitter  1279 . IR emitter  1279 , includes a fiber optic connector to effectively couple IR emitter  1279  to optical fiber  1281 . Optical fiber  1281  is connected to a distal optical emitter in a stimulator lead of the stimulator lead assembly. 
         [0123]    CPU  1270  is also connected to optical signal processor  1264 . Optical signal processor  1264  is connected to photodetector  1277  and receives an optical signal from the photodetector, filters the optical signal, and correlates the optical signal to electrode current amplitude, pulse width and frequency. Optical signal processor  1264  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. 
         [0124]    IR detector  1277  is connected to optical signal processor  1264  and optical fiber  1282 . IR detector  1277  translates incoming light pulses from optical fiber  1282  into electrical signals which are processed by optical signal processor  1264 . Optical fiber  1282  is coupled to a distal optical collector in a stimulator lead of the stimulator lead assembly. 
         [0125]    In a preferred embodiment, the photodetector is similar to that of Part No. OP501 from Optek Technology. 
         [0126]    CPU  1270  is connected to optical modulator  1268 . IR emitter driver  1266  is connected to both optical modulator  1268  and CPU  1270 . In operation, CPU  1270  activates optical modulator  1268  which generates a waveform and transmits the waveform to the IR emitter driver  1266 . The IR emitter driver then causes IR emitter  1279  to launch a pulse with the waveform into optical fiber  1281 . 
         [0127]    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 IR emitter is in a range from 800 nm to 870 nm. Typical output intensity of the IR emitter is 1 to 2 mW and a suitable part is Part No. VSMY1859 from Vishay Intertechnology, Inc. 
         [0128]    Pulse generator  1260  is connected to the set of electrodes in stimulator lead assembly  1140 . In order to generate a pulse to the electrodes, CPU  1270  consults a calibration table stored in onboard memory  1272  to determine pulse width PW, pulse frequency Pf and pulse amplitudes for the set of electrodes, respectively. The pulse width and frequency are transmitted to pulse modulator  1262  which creates a modified square wave signal. The modified square wave signal is passed to pulse generator  1260 . CPU  1270  passes the amplitudes for the set of electrodes to pulse generator  1260  in digital form. Pulse generator  1260  then amplifies the modified square waves according to the pulse amplitudes and transmits them to the set of electrodes. CPU  1270  is in transcutaneous communications, via RF transceiver  1271 , with calibration and programming unit  1154  and SCS controller  1153 . 
         [0129]    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 10,000 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. 
         [0130]    Referring to  FIG. 13 , SCS controller  1153  is shown. SCS controller  1153  includes processor  1300  connected to RF transceiver  1302 , to display  1304 , to input/output device  1306  and to memory  1308 . In the preferred embodiment, display  1304  is a low power liquid crystal display adapted to show the current operational state of the system. I/O device  1306  is a simple push button contact array which is constantly monitored by processor  1300 . In the preferred embodiment, RF transceiver  1302  is a low power transmitter/receiver combination. 
         [0131]    Referring to  FIG. 14 , calibration and programming unit  1154  will be described. Calibration and programming unit  1154  includes processor  1410  connected to onboard memory  1418 , to input/output devices  1416  and  1417 , to RF transceiver  1412  and to display  1414 . Display  1414 , in the preferred embodiment, is a low power liquid crystal display. Input/output device  1416  and input/output device  1417  are simple push button switches monitored continuously by the processor. RF transceiver  1412  is a low power transmitter/receiver combination. 
         [0132]    Referring to  FIGS. 15   a - 15   b , method  1500  of operation of the positionally-sensitive spinal cord stimulator of  FIG. 12  is shown. In the preferred embodiment, method  1500  takes the form of a computer program which is resident in memory  1272  of CPU  1270  of PGSP  1150 . When activated, the program forms a continuous cycle. 
         [0133]    Referring to  FIG. 15   a , at step  1531 , RF transceiver  1271  is continually polled for a change of operation code signal to be received from SCS controller  1153 . One of three options is always present, “start?”, “calibrate?” and “stop?” 
         [0134]    At step  1533 , if operation change code “start?” is received, the method moves to step  1542 . At step  1542 , CPU  1270  activates optical modulator  1268 , which in turn activates IR emitter driver  1266  to generate an optical pulse from the IR emitter. At step  1543 , a set of photocurrent levels for a photodetector [I] is measured by optical signal processor  1264  and passed to CPU  1270  for storage in memory. 
         [0135]    At step  1547 , the CPU determines a set of amplitudes [A] of a train of pulses to be sent to the set of electrodes, based on the photocurrent level and a calibration table. In step  1547 , the set of amplitudes are interpolated from the calibration table using the photocurrent level. At step  1549 , 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 set of electrodes. 
         [0136]    At step  1552 , the CPU activates the pulse modulator to create the waveforms of the pulse trains to be sent to the set of electrodes and then activates pulse generator  1260  to generate the pulse trains. At step  1554 , the CPU stores the values of [I], [A], P W  and P f  in a time series of data in memory for future retrieval. The method then returns to step  1531 . 
         [0137]    If at step  1533 , the operation change code is not “start?”, the method proceeds to step  1535 . At step  1535 , the CPU determines if the operation change code is “calibrate?” If so, the method moves to step  1537 . At step  1537 , the CPU transmits the time series of data to calibration and programming unit  1154 . At step  1539 , the CPU enters the calibration routine as will be described more fully below. The method then returns to step  1531 . 
         [0138]    If at step  1535 , the operation change code is not “calibrate?”, the method moves to step  1541 . At step  1541 , the CPU determines if the operation change code is “stop?”. If so, the method returns to step  1531 . If not, the method proceeds to step  1542  and continues as previously described. 
         [0139]    In the preferred embodiment, the pulse width and frequency is kept constant for a given patient and only the set of electrode amplitudes are varied. In another embodiment, step  1549  is performed whereby pulse width and pulse frequency are dynamically varied according to the calibration values stored in the calibration table for each electrode. 
         [0140]    Referring to  FIG. 15   b , an alternate embodiment of determining amplitude values, at step  1547  is shown. At step  1590 , the CPU performs interpolation to determine a predicted amplitude at time t from the photocurrent level. At step  1592 , the predicted amplitude is stored into a set of historical amplitudes which are predicted amplitudes for times t i &lt;t. At step  1594 , the CPU time averages historical amplitudes from the time series of data to determine a new set of electrode amplitudes. At step  1594 , the CPU also obtains a set of predetermined weighting factors w from memory. 
         [0141]    At step  1596 , the following equation is applied: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       A 
                       j 
                     
                      
                     
                       ( 
                       delivered 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           w 
                           k 
                         
                         · 
                         
                           
                             A 
                             j 
                           
                            
                           
                             ( 
                             k 
                             ) 
                           
                         
                       
                       + 
                       
                         
                           w 
                           
                             k 
                             - 
                             1 
                           
                         
                         · 
                         
                           
                             A 
                             j 
                           
                            
                           
                             ( 
                             
                               k 
                               - 
                               1 
                             
                             ) 
                           
                         
                       
                       + 
                       
                         
                           w 
                           
                             k 
                             - 
                             2 
                           
                         
                         · 
                         
                           
                             A 
                             j 
                           
                            
                           
                             ( 
                             
                               k 
                               - 
                               2 
                             
                             ) 
                           
                         
                       
                       + 
                       … 
                     
                     
                       
                         w 
                         k 
                       
                       + 
                       
                         w 
                         
                           k 
                           - 
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                         W 
                         
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                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
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         [0000]    where w k =predetermined weight for the values of A j  at the current time k and earlier times k−1, k−2, . . . , etc., and where A j =jth electrode amplitude. At step  1598 , if there are separate left and right electrode amplitudes, steps  1590 ,  1592 ,  1594  and  1596  are repeated for each electrode. 
         [0142]    Referring to  FIG. 16 , the processor is programmed to carry out steps of calibration method  1600  upon request by a calibration control program. At step  1615 , each current amplitude in a set of current amplitudes [A] are adjusted to an initial value, preferably the minimum value of a predetermined range. At step  1620 , the pulse generator is directed by the CPU to send a train of pulses to each electrode at the minimum values. At step  1625 , paresthesia feedback is solicited from the patient to determine a level of parasthesia. At step  1630 , it is determined if the level of parasthesia is sufficient and optimal for patient. 
         [0143]    If the level of parasthesia is not optimal according to the patient feedback, then the method moves to step  1633 . At step  1633 , the processor monitors the input/output device to determine if amplitude values need to be increased or decreased, or if the level of paresthesia is sufficient. If an amplitude value needs to be adjusted, then the amplitude value is correspondingly increased or decreased by a discrete amount. If the amplitude value reaches a maximum level or a minimum level and cannot be adjusted further, step  1634  is performed where an alert is indicated by the calibration and programming unit. The alert in step  1634  may be a visual indication, audio indication or both visual and audio indication. 
         [0144]    After adjustment of the amplitude values, step  1620  is repeated, and a train of pulses is delivered to each electrode at the new amplitude levels. At step  1625 , patient paresthesia feedback is again solicited. If, at step  1630 , the level of paresthesia is still not optimal according to the patient feedback, the method repeats steps  1633  and  1634  as required. If, at step  1630 , the level of paresthesia is sufficient according to patient feedback, the method moves to step  1635 . 
         [0145]    At step  1635 , the CPU stores the new amplitude levels for the electrodes. At step  1638 , the optical signal processor measures the photocurrent [I] for the photodetector and transfers the corresponding photocurrent value to the CPU. At step  1640 , the photocurrent [I] and amplitude levels [A] are recorded in a calibration table. At step  1642 , the calibration method steps complete by returning control to the calibration control program. 
         [0146]    Referring to  FIG. 17 , the processor of the calibration and programming unit is programmed to further carry out the following method steps for a calibration control program  1700  in cooperation with physical motion of the patient. 
         [0147]    At step  1750 , RF transceiver  1412  receives a signal indicative of a request to move the patient to a prone position and passes it to the calibration processor  1410 . At step  1752 , the patient is positioned in a prone position. At step  1754 , calibration method  1600 , is carried out to optimize the level of paresthesia experienced by the patient. 
         [0148]    At step  1760 , RF transceiver  1412  receives a signal indicative of a request to move the patient to a right lateral position and passes it to processor  1410 . At step  1762 , the patient is positioned in a right lateral position. At step  1764 , calibration method  1600  is then carried out to optimize the level of paresthesia experienced by the patient. 
         [0149]    At step  1770 , RF transceiver  1412  receives a signal indicative of a request to move the patient to a supine position and passes it to processor  1410 . At step  1772 , the patient is positioned in a supine position. At step  1774 , calibration method  1600  is then carried out to optimize the level of paresthesia experienced by the patient. 
         [0150]    At step  1780 , RF transceiver  1412  receives a signal indicative of a request to move the patient to a left lateral position and passes it to processor  1410 . At step  1782 , the patient is positioned in a left lateral position. At step  1784 , calibration method  1600  is then carried out to optimize the level of paresthesia experienced by the patient. 
         [0151]    After steps  1780 ,  1782  and  1784  are performed, the calibration program is complete. 
         [0152]    The order of patient positions in calibration program  1700  may be changed in alternative embodiments. Additional patient positions may be added to calibration program  1700  in alternative embodiments, for example, the patient may be rotated clockwise to calibrate a level of paresthesia required for a clockwise position. The result of carrying out a calibration using methods  1600  and  1700  is a calibration table with each record having a stored patient position, at least one photocurrent level and at least one corresponding electrode amplitude. 
         [0153]    Referring to  FIG. 18 , the various states of the SCS controller in operation will be described with the SCS controller apparatus. At wait state  1805 , SCS controller  1153  enters a waiting posture and continually polls I/O device  1306 . Upon receipt of a “run” signal from I/O device  1306 , processor  1300  enters “run” state  1807  and transmits a “run” signal to RF transceiver  1302 . RF transceiver  1302  then transmits the “run” signal to PGSP  1150  for further action. After transmission, the processor returns to wait state  1805 . 
         [0154]    If a “stop” signal is received from I/O device  1306 , at step  1809 , processor  1300  passes a “stop” signal to RF transceiver  1302 , which in turn sends the “stop” signal to PGSP  1150 . The processor then returns to wait state  1305 . 
         [0155]    If a “calibrate” signal is received from I/O device  1306 , at step  1811 , processor  1300  transmits a “calibrate” signal to RF transceiver  1302 , which in turn sends the “calibrate” signal to PGSP  1150 . Processor  1300  then returns to wait state  1805 . 
         [0156]      FIG. 19  shows a calibration table  1940  suitable for a single stimulator lead system, as shown in  FIGS. 6   a - d , with a single optical collector, a single optical emitter and a set of electrodes. Each row is a record for the optimal electrode settings for a patient position. Calibration table  1940  includes five columns for patient position identifier  1942 , photodetector value  1944  for photocurrent from light detected by the optical collector, electrode stimulation pulse amplitude  1946 , electrode stimulation pulse width  1948 , and electrode stimulation pulse frequency  1950 . 
         [0157]    Patient position identifier  1942  in a preferred embodiment includes four positions, forward (prone)—0°, right—90°, left—270°, back (supine—180°). Each row in calibration table  1940  is associated with one of the four patient positions. Electrode stimulation pulse amplitude  1946  includes values which are derived during calibration and recorded for different spinal cord positions, corresponding to the patient position. In the preferred embodiment, the electrode stimulation pulse amplitude  1946  prescribes a stimulation energy to neurons in the vicinity of spinal cord. 
         [0158]    To construct table  1940 , calibration methods  1600  and  1700  are performed to identify a set of stimulator lead values for the pulse amplitude, width and frequency with a set of photocurrent levels. 
         [0159]      FIG. 20  shows a calibration table  2040  suitable for a dual stimulator lead system, having one lead with a single optical emitter, having another lead with a single optical detector. Both leads have electrodes sharing the same current pulse width and frequency, but have different pulse amplitudes for each lead. Each row is a record for the optimal electrode settings for a patient position. Calibration table  2040  includes six columns for patient position identifier  2042 , photodetector value  2044  for photocurrent from light detected by the optical collector, electrode stimulation pulse amplitude  2046  for the left stimulation lead, electrode stimulation pulse amplitude  2048  for the right stimulation lead, electrode stimulation pulse width  2050 , and electrode stimulation pulse frequency  2052 . Patient position identifier  2042  includes four positions, forward (prone)—0°, right—90°, left—270°, back (supine—180°). Each row in calibration table  2040  is associated with one of the four patient positions. Electrode stimulation pulse amplitude  2046  for the left lead can be different from electrode stimulation pulse amplitude  2048  for the right lead according to values which are derived during calibration and recorded for different spinal cord positions, corresponding to the patient position. The electrode stimulation pulse amplitude  2046  prescribes a stimulation energy to nerves in the vicinity of the left side of spinal cord. The electrode stimulation pulse amplitude  2048  prescribes a stimulation energy to nerves in the vicinity of the right side of spinal cord. 
         [0160]    To construct table  2040 , calibration methods  1600  and  1700  are performed to identify a set of right stimulator lead values for a right electrode pulse amplitude, width and frequency with a set of photocurrent levels and to identify a set of left stimulator lead values for a left electrode pulse amplitude, width and frequency with the set of photocurrent levels. The set of left stimulator lead values can be different than the set of right stimulator lead values. 
         [0161]    In another embodiment, calibration methods  1600  and  1700  are performed where the electrode stimulation pulse amplitude for the left and right leads always have the same value. 
         [0162]    In an alternate embodiment, calibration is performed for additional physical positions such that additional rows are placed in calibration table  1940  or calibration table  2040 . 
         [0163]    In tables  1940  and  2040 , the electrode stimulation pulse width and electrode stimulation pulse frequency are shown as having constant values. However, in an alternate embodiment, the values of electrode stimulation pulse width and electrode stimulation pulse frequency are varied through a predetermined range during calibration and recorded for each patient position. 
         [0164]    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.