Neurostimulation system and method for compounding current to minimize current sources

A neurostimulation system and method of providing therapy to a patient implanted with a plurality of electrodes using a plurality of electrical sources is provided. A source-electrode coupling configuration is determined from the electrical sources and electrodes. Electrical current is respectively conveyed between active ones of the plurality of electrical sources and active subsets of the plurality of electrodes in accordance with the determined source-electrode coupling configuration. The total number of the electrodes in the active electrode subsets is greater than the total number of the active electrical sources.

FIELD OF THE INVENTION

The present invention relates to tissue stimulation systems, and more particularly, to a system and method for compounding a discrete number of current sources to controllably generate a spectrum of stimulation currents to be passed to electrodes.

BACKGROUND OF THE INVENTION

Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems, such as the Freehand system by NeuroControl (Cleveland, Ohio), have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.

These implantable neurostimulation systems typically include one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator (e.g., an implantable pulse source (IPG)) implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via a lead extension. The neurostimulation system may further comprise an external control device to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters.

Electrical stimulation energy may be delivered from the neurostimulator to the electrodes in the form of an electrical pulsed waveform. Thus, stimulation energy may be controllably delivered to the electrodes to stimulate neural tissue. The combination of electrodes used to deliver electrical pulses to the targeted tissue constitutes an electrode combination, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), or left off (zero). In other words, an electrode combination represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied include the amplitude, width, and rate of the electrical pulses provided through the electrode array. Each electrode combination, along with the electrical pulse parameters, can be referred to as a “stimulation parameter set.”

With some neurostimulation systems, and in particular, those with independently controlled electrical energy sources, the distribution of the electrical energy conveyed to or from the electrodes (including the case of the neurostimulator, which may act as an electrode) may be varied such that the electrical energy is supplied via numerous different electrode configurations. In different configurations, the electrodes may provide current or voltage in different relative percentages of positive and negative current or voltage to create different electrical current distributions (i.e., fractionalized electrode configurations).

For example, with reference toFIG. 1, a neurostimulator may have an output current source1a(sometimes referred to as an “anodic current source”) and an output current sink1b(sometimes referred to as a “cathodic current source”) that are configured to supply/receive stimulating current to/from the electrodes Ex, Ey, and ultimately to/from tissue (represented by load5having a resistance R). The source1aand sink1bare sometimes respectively referred to as PDACs and NDACs, reflecting the fact that the source1ais typically formed of P-type transistors, while the sink1bis typically formed of N-type transistors. The use of transistors of these polarities is sensible given that the source1ais biased to a high voltage (V+), where P-type transistors are most logical, while the sink1bis biased to a low voltage (V−), where N-type transistors are most logical, as shown inFIG. 1. A suitable current source is disclosed in U.S. Pat. No. 6,181,969 (“the '969 patent”), which is expressly incorporated herein by reference in its entirety.

The output current source1aand output current sink1brespectively include current sources2a,2beach configured to generate a reference current Iref, and digital-to-analog converter (DAC) circuitry3a,3bconfigured for regulating/amplifying the reference current Irefprovided by the current sources2a,2b, and delivering output current Ioutto the load5(having a resistance R). Specifically, the relation between Ioutand Irefis determined in accordance with input bits arriving on busses4a,4b, which respectively give the output current source1aand output current sink1btheir digital-to-analog functionality. In accordance with the values of the various M bits on busses4a,4bany number of output stages (i.e., transistors M1, M2) are tied together in parallel such that Ioutcan range from Irefto 2M*Iref.

As shown inFIG. 1for simplicity, the current source1ais coupled to an electrode Ex, while the current sink1bis coupled to a different electrode Ey. However, each electrode may actually be hard-wired to both the current source1aand the current sink1b, only one (or neither) of which is activated at a particular time to allow the electrode to selectively be used as either a source or sink (or as neither).

This architecture is shown inFIG. 2, which shows four exemplary electrodes E1, E2, E3, and E4, each having its own dedicated and hard-wired current source1aand current sink1b. Thus, the output current source1amay be associated with electrode E2(e.g., EXofFIG. 1) at a particular point in time, while the output current sink1bmay be associated with electrode E3(e.g., EYofFIG. 1) at that time. At a later time, electrodes E2and E3could be switched, such that E2now operates as the sink, while electrode E3operates as the source, or new sources or sinks could be selected, etc.

Another architecture, shown inFIG. 3, uses a plurality of current sources1and sinks2, and further uses a low impedance switching matrix6that intervenes between the sources/sinks and the electrodes EX. Each source/sink pair is hard-wired together at common nodes7, such that the switching matrix6intervenes between the nodes7and the electrodes. Of course, only one of the source or the sink in each pair is activated at one time, and thus the node7in any pair will source or sink current at any particular time. Through appropriate control of the switching matrix6, any of the nodes7may be connected to any of the electrodes EXat any time. Because all of the available electrodes EXwill typically not be activated at one time, the use of the switching matrix6decreases the number of current sources needed to supply the electrical current to the activated electrodes EX. Because a relatively large capacitor is typically associated with each current source/sink, decreasing the number of current sources/sinks in any particular architecture is especially advantageous in that it substantially reduces the space needed in the implantable pulse source.

Further details discussing various architectures of current source/sink circuitry are provided in U.S. Patent Publication No. 2007/0100399, which is expressly incorporated herein by reference.

While the use of switching matrices or networks reduces the number of current sources/sinks needed in order to source/sink electrical current to the desired electrodes, a current source/sink is still utilized for each activated electrode. It is, thus, desirable to minimize the number of current sources/sinks needed, while still providing a requisite spectrum of currents to be distributed to the electrodes.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present inventions, a neurostimulation system comprises a plurality of electrical terminals configured for being coupled to a respective plurality of electrodes, a plurality of electrical sources (e.g., a current sources, which may be anodic and/or cathodic), and processing circuitry configured for determining a source-electrode coupling configuration from the electrical sources and electrodes. The neurostimulation system further comprises control circuitry configured for respectively conveying electrical current between active ones of the plurality of electrical sources and active subsets of the plurality of electrodes in accordance with the determined source-electrode coupling configuration. The electrical terminals and control circuitry may, e.g., be contained within an implantable device, and the processing circuitry may, e.g., be contained within the implantable device and/or an external programmer.

Any of the active electrical sources may have a fixed absolute output, which may simplify the architecture, or a variable absolute output, which may provide more flexibility in providing current values. At least two of the active electrode sets may be different or may be the same. In one embodiment, at least two of the active electrode subsets include at least one common electrode. In another embodiment, at least two of the active electrode subsets do not include a common electrode. In any event, the total number of the electrodes in the active electrode subsets is greater than the total number of the active electrical sources. As a result, the number of electrical sources (and thus, the number of capacitors that may be associated with the electrical sources) can be decreased relative to the number of electrodes.

In one embodiment, the processor is further configured for selecting electrical current values for the plurality of electrodes, and determining the source-electrode coupling configuration based on the selected electrical current values. For example, the processing circuitry may be configured for determining the source-electrode coupling configuration to best meet the selected electrical current values for the plurality of electrodes. In an optional embodiment, the neurostimulation system further comprises monitoring circuitry configured for measuring impedances adjacent the electrodes, in which case, the processing circuitry may be configured for determining the source-electrode coupling configuration based on the measured impedances. In another optional embodiment, the neurostimulation system further comprises a switching network coupled between the plurality of electrical sources and the plurality of electrical terminals, wherein the control circuitry is configured for operating the switching network to implement the determined source-electrode coupling configuration.

In accordance with a second aspect of the present inventions, a method of providing therapy to a patient implanted with a plurality of electrodes using a plurality of electrical sources (e.g., a current sources, which may be anodic and/or cathodic). The method comprises determining a source-electrode coupling configuration from the electrical sources and electrodes, and respectively conveying electrical current between active ones of the plurality of electrical sources and active subsets of the plurality of electrodes in accordance with the determined source-electrode coupling configuration.

Any of the active electrical sources may have a fixed absolute output, which may simplify the architecture, or a variable absolute output, which may provide more flexibility in providing current values. At least two of the active electrode sets may be different or may be the same. In one method, at least two of the active electrode subsets include at least one common electrode. In another method, at least two of the active electrode subsets do not include a common electrode. In any event, the total number of the electrodes in the active electrode subsets is greater than the total number of the active electrical sources. As a result, the number of electrical sources (and thus, the number of capacitors that may be associated with the electrical sources) can be decreased relative to the number of electrodes.

One method further comprises selecting electrical current values for the plurality of electrodes, and determining the source-electrode coupling configuration based on the selected electrical current values. For example, the source-electrode coupling configuration may be determined to best meet the selected electrical current values for the plurality of electrodes. Another method further comprises measuring impedances adjacent the electrodes, in which case, the source-electrode coupling configuration may be determined based on the measured impedances.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The description that follows relates to a spinal cord stimulation (SCS) system. However, it is to be understood that the while the invention lends itself well to applications in SCS, the invention, in its broadest aspects, may not be so limited. Rather, the invention may be used with any type of implantable electrical circuitry used to stimulate tissue. For example, the present invention may be used as part of a pacemaker, a defibrillator, a cochlear stimulator, a retinal stimulator, a stimulator configured to produce coordinated limb movement, a cortical stimulator, a deep brain stimulator, peripheral nerve stimulator, microstimulator, or in any other neural stimulator configured to treat urinary incontinence, sleep apnea, shoulder sublaxation, headache, etc.

Turning first toFIG. 4, an exemplary SCS system10generally includes one or more (in this case, two) implantable stimulation leads12, an implantable pulse source (IPG)14, an external remote controller RC16, a clinician's programmer (CP)18, an External Trial Stimulator (ETS)20, and an external charger22.

The IPG14is physically connected via one or more percutaneous lead extensions24to the stimulation leads12, which carry a plurality of electrodes26arranged in an array. In the illustrated embodiment, the stimulation leads12are percutaneous leads, and to this end, the electrodes26are arranged in-line along the stimulation leads12. In alternative embodiments, the electrodes26may be arranged in a two-dimensional pattern on a single paddle lead. As will be described in further detail below, the IPG14includes pulse generation circuitry that delivers electrical stimulation energy in the form of a pulsed electrical waveform (i.e., a temporal series of electrical pulses) to the electrode array26in accordance with a set of stimulation parameters.

The ETS20may also be physically connected via the percutaneous lead extensions28and external cable30to the stimulation leads12. The ETS20, which has similar pulse generation circuitry as that of the IPG14, also delivers electrical stimulation energy in the form of a pulsed electrical waveform to the electrode array26in accordance with a set of stimulation parameters. The major difference between the ETS20and the IPG14is that the ETS20is a non-implantable device that is used on a trial basis after the stimulation leads12have been implanted and prior to implantation of the IPG14, to test the responsiveness of the stimulation that is to be provided.

The RC16may be used to telemetrically control the ETS20via a bi-directional RF communications link32. Once the IPG14and stimulation leads12are implanted, the RC16may be used to telemetrically control the IPG14via a bi-directional RF communications link34. Such control allows the IPG14to be turned on or off and to be programmed with different stimulation parameter sets. The IPG14may also be operated to modify the programmed stimulation parameters to actively control the characteristics of the electrical stimulation energy output by the IPG14.

The CP18provides clinician detailed stimulation parameters for programming the IPG14and ETS20in the operating room and in follow-up sessions. The CP18may perform this function by indirectly communicating with the IPG14or ETS20, through the RC16, via an IR communications link36. Alternatively, the CP18may directly communicate with the IPG14or ETS20via an RF communications link (not shown). The clinician detailed stimulation parameters provided by the CP18are also used to program the RC16, so that the stimulation parameters can be subsequently modified by operation of the RC16in a stand-alone mode (i.e., without the assistance of the CP18). The external charger22is a portable device used to transcutaneously charge the IPG14via an inductive link38. Once the IPG14has been programmed, and its power source has been charged by the external charger22or otherwise replenished, the IPG14may function as programmed without the RC16or CP18being present.

For purposes of brevity, the details of the RC16, CP18, ETS20, and external charger22will not be described herein. Details of exemplary embodiments of these devices are disclosed in U.S. Pat. No. 6,895,280, which is expressly incorporated herein by reference.

As shown inFIG. 5, the electrode leads12are implanted within the spinal column42of a patient40. The preferred placement of the stimulation leads12is adjacent, i.e., resting near, or upon the dura, adjacent to the spinal cord area to be stimulated. Due to the lack of space near the location where the electrode leads12exit the spinal column42, the IPG14is generally implanted in a surgically-made pocket either in the abdomen or above the buttocks. The IPG14may, of course, also be implanted in other locations of the patient's body. The lead extension24facilitates locating the IPG14away from the exit point of the electrode leads12. As there shown, the CP18communicates with the IPG14via the RC16.

Referring now toFIG. 6, the external features of the stimulation leads12and the IPG14will be briefly described. One of the stimulation leads12(1) has eight electrodes26(labeled E1-E8), and the other stimulation lead12(2) has eight electrodes26(labeled E9-E16). The actual number and shape of leads and electrodes will, of course, vary according to the intended application. The IPG14comprises an outer case40for housing the electronic and other components (described in further detail below), and a connector42to which the proximal ends of the stimulation leads12(1) and12(2) mate in a manner that electrically couples the electrodes26to the electronics within the outer case40. The outer case40is composed of an electrically conductive, biocompatible material, such as titanium, and forms a hermetically sealed compartment wherein the internal electronics are protected from the body tissue and fluids. In some cases, the outer case40may serve as an electrode.

As will be described in further detail below, the IPG14includes pulse generation circuitry that provides electrical conditioning and stimulation energy in the form of a pulsed electrical waveform to the electrode array26in accordance with a set of stimulation parameters programmed into the IPG14. Such stimulation parameters may comprise electrode combinations, which define the electrodes that are activated as anodes (positive), cathodes (negative), and turned off (zero), percentage of stimulation energy assigned to each electrode (fractionalized electrode configurations), and electrical pulse parameters, which define the pulse amplitude (measured in milliamps or volts depending on whether the IPG14supplies constant current or constant voltage to the electrode array26), pulse width (measured in microseconds), pulse rate (measured in pulses per second), and burst rate (measured as the stimulation on duration X and stimulation off duration Y).

Electrical stimulation will occur between two (or more) activated electrodes, one of which may be the IPG case. Simulation energy may be transmitted to the tissue in a monopolar or multipolar (e.g., bipolar, tripolar, etc.) fashion. Monopolar stimulation occurs when a selected one of the lead electrodes26is activated along with the case of the IPG14, so that stimulation energy is transmitted between the selected electrode26and case. Bipolar stimulation occurs when two of the lead electrodes26are activated as anode and cathode, so that stimulation energy is transmitted between the selected electrodes26. For example, an electrode on one lead12may be activated as an anode at the same time that an electrode on the same lead or another lead12is activated as a cathode. Tripolar stimulation occurs when three of the lead electrodes26are activated, two as anodes and the remaining one as a cathode, or two as cathodes and the remaining one as an anode. For example, two electrodes on one lead12may be activated as anodes at the same time that an electrode on another lead12is activated as a cathode.

The stimulation energy may be delivered between electrodes as monophasic electrical energy or multiphasic electrical energy. Monophasic electrical energy includes a series of pulses that are either all positive (anodic) or all negative (cathodic). Multiphasic electrical energy includes a series of pulses that alternate between positive and negative. For example, multiphasic electrical energy may include a series of biphasic pulses, with each biphasic pulse including a cathodic (negative) stimulation pulse and an anodic (positive) recharge pulse that is generated after the stimulation pulse to prevent direct current charge transfer through the tissue, thereby avoiding electrode degradation and cell trauma. That is, charge is conveyed through the electrode-tissue interface via current at an electrode during a stimulation period (the length of the stimulation pulse), and then pulled back off the electrode-tissue interface via an oppositely polarized current at the same electrode during a recharge period (the length of the recharge pulse).

Turning next toFIG. 7, the main internal components of the IPG14will now be described. The IPG14includes stimulation output circuitry50configured for generating electrical stimulation energy in accordance with a defined pulsed waveform having a specified pulse amplitude, pulse rate, pulse width, and pulse shape under control of control logic52over data bus54. Control of the pulse rate and pulse width of the electrical waveform is facilitated by timer logic circuitry56, which may have a suitable resolution, e.g., 10 μs. The stimulation energy generated by the stimulation output circuitry50is output to electrical terminals58corresponding to electrodes E1-E16.

The analog output circuitry50may either comprise one or more independently controlled electrical sources, which take the form of current sources and/or current sinks, for providing stimulation pulses of a specified and known amperage to or from the electrodes26, or voltage sources and/or voltage sinks for providing stimulation pulses of a specified and known voltage at the electrodes26.

For example, in the illustrated embodiment, the stimulation output circuitry50comprises a plurality m independent current source pairs60capable of supplying stimulation energy to the electrical terminals58at a specified and known amperage. One current source62of each pair60functions as a positive (+) or anodic current source, while the other current source64of each pair60functions as a negative (−) or cathodic current source. The outputs of the anodic current source62and the cathodic current source64of each pair60are connected to a common node66.

In essence, each current source pair60takes the form of a reconfigurable current source whose polarity can be switched. That is, by activating the anodic current source62and deactivating the cathodic current source64, the current source pair60can be configured as an anodic current source, and by deactivating the anodic current source62and activating the cathodic current source64, the current source pair60can be configured as a cathodic current source. Alternatively, instead of a having current sources pairs60, each of which includes an anodic current source and a cathodic current source, the reconfigurable current source can have a current source that can be switched between the positive terminal and the positive terminal of an energy source to selectively reconfigure the current source between an anodic current source and a cathodic current source. For example, as illustrated inFIG. 8, the reconfigurable current source is coupled between an electrode and an energy source. Switches1and2are coupled between the respective positive and negative terminals of the energy source and the side of the current source opposite to the electrode. When the switch1is closed and the switch2is opened, the current source is coupled to the positive terminal of the energy source, thereby being configured as an anodic current source. In contrast, when the switch1is opened and the switch2is closed, the current source is coupled to the negative terminal of the energy source, thereby being configured as a cathodic current source.

Referring back toFIG. 7, the stimulation output circuitry50further comprises a low impedance switching matrix68through which the common node66of each current source pair60is connected to any of the electrical terminals58, and a capacitor70coupled between the common node66of each current source pair60and the switching matrix68. Significantly, as will be discussed in further detail below, the switching matrix68may be used to form source/electrode couplings (i.e., which active current sources64and active electrode(s)26are to be coupled together) that include more activated electrodes than activated current sources, thereby minimizing the number of current sources needed. As will also be discussed in further detail below, the current sources64may have a fixed absolute magnitude value or may have a variable absolute magnitude value, and preferably have different magnitude values or ranges relative to each other in order to increase the variability of the source/electrode couplings.

Hence, it is seen that each of the programmable electrical terminals58can be programmed to have a positive (sourcing current), a negative (sinking current), or off (no current) polarity. Further, the amplitude of the current pulse being sourced or sunk to or from a given electrode may be programmed to one of several discrete current levels, e.g., between 0 to 10 mA in steps of 100 μA, within the output voltage/current requirements of the IPG14. Additionally, in one embodiment, the total current output by a group of electrical terminals58can be up to ±20 mA (distributed among the electrodes included in the group). Also, the pulse width of the current pulses is preferably adjustable in convenient increments, e.g., from 0 to 1 milliseconds (ms) in increments of 10 microseconds (μs). Similarly, the pulse rate is preferably adjustable within acceptable limits, e.g., from 0 to 1000 pulses per second (pps). Other programmable features can include slow start/end ramping, burst stimulation cycling (on for X time, off for Y time), interphase (i.e., the duration between first and second phases of biphasic energy), and open or closed loop sensing modes. Moreover, it is seen that each of the electrical terminals58can operate in a multipolar mode, e.g., where two or more electrical terminals are grouped to source/sink current at the same time. Alternatively, each of the electrical terminals58can operate in a monopolar mode where, e.g., the electrical terminals58are configured as cathodes (negative), and case of the IPG14is configured as an anode (positive).

It can be appreciated that an electrical terminal58may be assigned an amplitude and included with any of up to k possible groups, where k is an integer corresponding to the number of channels, and in one embodiment, is equal to 4, and with each channel k having a defined pulse amplitude, pulse width, pulse rate, and pulse shape. Other channels may be realized in a similar manner. Thus, each channel identifies which electrical terminals58(and thus electrodes) are selected to synchronously source or sink current, the pulse amplitude at each of these electrical terminals, and the pulse width, pulse rate, and pulse shape. Amplitudes and polarities of electrodes on a channel may vary, e.g., as controlled by the RC16. External programming software in the CP18is typically used to set stimulation parameters including electrode polarity, amplitude, pulse rate and pulse width for the electrodes of a given channel, among other possible programmable features. The operation of this output stimulation circuitry, including alternative embodiments of suitable output circuitry for performing the same function of generating stimulation pulses of a prescribed amplitude and width, is described more fully in U.S. Pat. Nos. 6,516,227 and 6,993,384, which are expressly incorporated herein by reference.

It should be appreciated that, although the embodiment described in reference toFIG. 7uses current sources, alternative embodiments may utilize voltage sources. In such cases, current generated by a given voltage source may be maintained at a constant defined current utilizing techniques such as those described in U.S. Provisional Patent Application Ser. No. 61/083,491, entitled “System and Method for Maintaining a Distribution of Currents in an Electrode Array Using Independent Voltage Sources,” which is expressly incorporated herein by reference. The currents pertinent to several voltage sources may be combined with a switch in a similar manner as described herein in reference to combined current sources.

The IPG14further comprises monitoring circuitry72for monitoring the status of various nodes or other points74throughout the IPG14, e.g., power supply voltages, temperature, battery voltage, and the like. Notably, the electrodes26fit snugly within the epidural space of the spinal column, and because the tissue is conductive, electrical measurements can be taken between the electrodes26. Thus, the monitoring circuitry72is configured for taking such electrical measurements (e.g., current output magnitude, electrode impedance, field potential, evoked action potentials, etc.) for performing such functions as detecting fault conditions between the electrodes26and the analog output circuitry50, determining the coupling efficiency between the electrodes26and the tissue, facilitating lead migration detection, etc. In the case where voltage sources (instead of current sources) are used, the monitoring circuitry72can measure the impedances on the electrodes26in order to maintain a desired current distribution on the active electrodes26by adjusting the voltages on the active electrodes26. Furthermore, whether current sources or voltage sources are used, the monitoring circuitry72will be used to measure impedances for ensuring that the actual current values best match the desired current values on the electrodes, as will be discussed in further detail below.

Further details discussing the measurement of electrical parameter data, such as electrode impedance, field potential, and evoked action potentials, as well as physiological parameter data, such as pressure, translucence, reflectance and pH (which can alternatively be used) are set forth in U.S. patent application Ser. No. 10/364,436, entitled “Neural Stimulation System Providing Auto Adjustment of Stimulus Output as a Function of Sensed Impedance,” U.S. patent application Ser. No. 10/364,434, entitled “Neural Stimulation System Providing Auto Adjustment of Stimulus Output as a Function of Sensed Pressure Changes,” and U.S. patent application Ser. No. 11/096,483, entitled “Apparatus and Methods for Detecting Migration of Neurostimulation Leads,” which are expressly incorporated herein by reference.

The IPG14further comprises processing circuitry in the form of a microcontroller (μC)76that controls the control logic over data bus78, and obtains status data from the monitoring circuitry72via data bus80. The IPG14additionally controls the timer logic56and switching matrix68. The IPG14further comprises memory82and oscillator and clock circuitry84coupled to the microcontroller76. The microcontroller76, in combination with the memory82and oscillator and clock circuit84, thus comprise a microprocessor system that carries out a program function in accordance with a suitable program stored in the memory82. Alternatively, for some applications, the function provided by the microprocessor system may be carried out by a suitable state machine.

Thus, the microcontroller76generates the necessary control and status signals, which allow the microcontroller76to control the operation of the IPG14in accordance with a selected operating program and stimulation parameters. In controlling the operation of the IPG14, the microcontroller76is able to individually generate a train of stimulus pulses at the electrodes26using the analog output circuitry50, in combination with the control logic52and timer logic56, thereby activating selected ones of the electrodes26, including the monopolar case electrode. In accordance with stimulation parameters stored within the memory82, the microcontroller76may control the polarity, amplitude, rate, pulse width and channel through which the current stimulus pulses are provided. The microcontroller76also facilitates the storage of electrical parameter data (or other parameter data) measured by the monitoring circuitry72within memory82, and also provides any computational capability needed to analyze the raw electrical parameter data obtained from the monitoring circuitry72and compute numerical values from such raw electrical parameter data.

The IPG14further comprises an alternating current (AC) receiving coil86for receiving programming data (e.g., the operating program and/or stimulation parameters) from the RC16and/or CP18(shown inFIG. 5) in an appropriate modulated carrier signal, and charging and forward telemetry circuitry88for demodulating the carrier signal it receives through the AC receiving coil86to recover the programming data, which programming data is then stored within the memory82, or within other memory elements (not shown) distributed throughout the IPG14.

The IPG14further comprises back telemetry circuitry90and an alternating current (AC) transmission coil92for sending informational data sensed through the monitoring circuitry72to the RC16and/or CP18(shown inFIG. 5). The back telemetry features of the IPG14also allow its status to be checked. For example, when the RC16and/or CP18initiates a programming session with the IPG14, the capacity of the battery is telemetered, so that the external programmer can calculate the estimated time to recharge. Any changes made to the current stimulus parameters are confirmed through back telemetry, thereby assuring that such changes have been correctly received and implemented within the implant system. Moreover, upon interrogation by the RC16and/or CP18, all programmable settings stored within the IPG14may be uploaded to the RC16and/or CP18. The back telemetry features allow raw or processed electrical parameter data (or other parameter data) previously stored in the memory82to be downloaded from the IPG14to the RC16and/or CP18.

The IPG14further comprises a rechargeable power source94and power circuits96for providing the operating power to the IPG14. The rechargeable power source94may, e.g., comprise a lithium-ion or lithium-ion polymer battery. The rechargeable battery94provides an unregulated voltage to the power circuits96. The power circuits96, in turn, generate the various voltages98, some of which are regulated and some of which are not, as needed by the various circuits located within the IPG14. The rechargeable power source94is recharged using rectified AC power (or DC power converted from AC power through other means, e.g., efficient AC-to-DC converter circuits, also known as “inverter circuits”) received by the AC receiving coil86. To recharge the power source94, the external charger22(shown inFIG. 4), which generates the AC magnetic field, is placed against, or otherwise adjacent, to the patient's skin over the implanted IPG14. The AC magnetic field emitted by the external charger induces AC currents in the AC receiving coil86. The charging and forward telemetry circuitry88rectifies the AC current to produce DC current, which is used to charge the power source94. While the AC receiving coil86is described as being used for both wirelessly receiving communications (e.g., programming and control data) and charging energy from the external device, it should be appreciated that the AC receiving coil86can be arranged as a dedicated charging coil, while another coil can be used for bi-directional telemetry.

It should be noted that the diagram ofFIG. 7is functional only, and is not intended to be limiting. Those of skill in the art, given the descriptions presented herein, should be able to readily fashion numerous types of IPG circuits, or equivalent circuits, that carry out the functions indicated and described, which functions include not only producing a stimulus current or voltage on selected groups of electrodes, but also the ability to measure electrical parameter data at an activated or non-activated electrode.

Additional details concerning the above-described and other IPGs may be found in U.S. Pat. No. 6,516,227, U.S. Patent Publication No. 2003/0139781, and U.S. patent application Ser. No. 11/138,632, entitled “Low Power Loss Current Digital-to-Analog Converter Used in an Implantable Pulse Source,” which are expressly incorporated herein by reference. It should be noted that rather than an IPG, the SCS system10may alternatively utilize an implantable receiver-stimulator (not shown) connected to leads12. In this case, the power source, e.g., a battery, for powering the implanted receiver, as well as control circuitry to command the receiver-stimulator, will be contained in an external controller inductively coupled to the receiver-stimulator via an electromagnetic link. Data/power signals are transcutaneously coupled from a cable-connected transmission coil placed over the implanted receiver-stimulator. The implanted receiver-stimulator receives the signal and generates the stimulation in accordance with the control signals.

As briefly discussed above, the switching matrix68may be used to form source/electrode couplings in a manner that minimizes the number of current sources needed. In particular, processing circuitry (such as the microcontroller76contained in the IPG14, or alternatively, processing circuitry (not shown) contained within the RC16or CP18) determines a source-electrode coupling configuration from the available current sources64and electrodes26. Such processing circuitry may also select the electrical current values for the electrodes26(e.g., in response to programming functions performed at the RC16or CP18), in which case, the source-electrode coupling configuration can be based on the selected electrical current values. For example, the processing circuitry can determine the source-electrode coupling configuration that best meets the selected electrical current values for the electrodes26, as will be discussed in further detail below.

Once the source-electrode coupling configuration is determined, the control circuitry (including control logic52and timer logic56), as configured by the microcontroller76, conveys electrical current between active ones of the current sources64and active subsets (either one or a plurality) of the electrodes26in accordance with the determined source-electrode coupling configuration, such that the total number of electrodes26in the active electrode subsets is greater than the total number of the active current sources64. In essence, because the microcontroller76may determine the source-electrode coupling configuration in a manner that best matches the selected electrical current values, any requirement that the current sources independently convey electrical current to or from the electrodes (i.e., one-to-one correspondence between the current sources and electrodes) is obviated, and as such, the number of current sources64required to drive any particular combination of electrodes26may be decreased.

Before discussing the details of how the source-electrode coupling configuration is determined to minimize the number of activated sources required to drive the active electrodes, it will be first worthwhile to discuss how the outputs of the current sources can be combined to generate a variety of currents that will be assumed when determining the source-electrode couplings.

As a general rule, from a total number of N current sources, the output of two active sources can be compounded to create a total of 2×N2different outputs. For example, referring toFIG. 9, given two (N) different current sources (I1and I2), each having a different current amplitude102(±3 mA and ±5 mA), the current sources can be utilized alone, added together, or subtracted from each other to provide a total of four (N2) absolute current amplitudes104(2 mA, 3 mA, 5 mA, and 8 mA). Assuming that the current values can be both negative and positive, a total (2×N2) of eight current amplitudes (±2 mA, ±3 mA, ±5 mA, and ±8 mA) can be provided. Similarly, referring toFIG. 10, given three (N) different current sources (I1, I2, and I3), each having a different amplitude106(±1 mA, ±3 mA, and ±8 mA), the current sources can be utilized alone, added together, or subtracted from each other to provide a total (N2) of nine different absolute current amplitudes108(1 mA, 3 mA, 4 mA, 5 mA, 6 mA, 7 mA, 8 mA, 9 mA, and 13 mA). Assuming that the absolute current values can be both negative and positive, a total (2×N2) of eighteen current amplitudes (±1 mA, ±3 mA, ±4 mA, ±5 mA, ±6 mA, ±7 mA, ±8 mA, ±9 mA, and ±13 mA) can be provided. Each of the current sources can be reconfigurable between positive and negative polarities, such that they can be combined in an additive manner or a subtractive manner to produce the desired combined current.

It should be appreciated that although the examples inFIGS. 9 and 10illustrate the number of different outputs that can be provided by compounding two current sources at a time, more than two current sources can be compounded to provide even more current amplitudes. It should also be appreciated that, to the extent that current source pairs (an anodic source and a cathodic source) are used to implement both an adding function (use anodic source to add current) and a subtracting function (use cathodic source to subtract current), a theoretical total of N current sources will actually be 2*N current sources. However, as will be discussed in further detail below, the total number of active current sources may be less than the total number of active electrodes.

Notably, the many different current values provided by compounding current sources can be applied to active electrodes in a variety of manners to provide many more current values on the electrodes themselves. This can be accomplished by defining different source-electrode couplings, and conveying electrical current between active ones of the current sources and active subsets of the electrodes26in accordance with the determined source-electrode couplings, as briefly discussed above.

For example, with reference toFIGS. 11a-g, various source-electrode coupling configurations are shown as being defined for two current sources (S1, S2), and three electrodes (e1, e2, e3), with the first source S1having a nominal current output of 3 mA, and the second source S2having a nominal current output of 5 mA. The two current sources S1, S2may be arbitrarily assigned to any two of the reconfigurable current sources illustrated inFIG. 7, and the three electrodes e1, e2, e3are capable of being arbitrarily assigned to any three of the sixteen electrodes E1-E16and case electrode illustrated inFIG. 7. The different source-electrode coupling configurations can be made with an intervening switch, such as the switching matrix62shown inFIG. 7.

In the first source-electrode coupling configuration, the first source S1is coupled to all three electrodes e1, e2, e3, and the second source S2is not coupled to any of the electrodes e1, e2, e3, resulting in a current value of 1 mA for each of the electrodes e1, e2, e3(if the source S1is anodic) or a current value of −1 mA for each of the electrodes e1, e2, e3(if source S1is anodic) (FIG. 11a). In the second source-electrode coupling configuration, the first source S1is not coupled to any of the electrodes e1, e2, e3, and the second source S2is coupled to all three of the electrodes e1, e2, e3, resulting in a current value of 1⅔ mA for each of the electrodes e1, e2, e3(if the source S2is anodic) or a current value of −1⅔ mA for each of the electrodes e1, e2, e3(if the source S2is cathodic) (FIG. 11b).

In the third source-electrode coupling configuration, the first source S1is coupled to all three of the electrodes e1, e2, e3, and the second source S2is coupled to all three of the electrodes e1, e2, e3, resulting in a current value of 2⅔ mA for each of the electrodes e1, e2, e3(if both the sources S1, S2are anodic) or a current value of −2⅔ mA for each of the electrodes e1, e2, e3(if both the sources S1, S2are cathodic) (FIG. 11c). In the fourth source-electrode coupling configuration, the first source S1is coupled to all three of the electrodes e1, e2, e3, and the second source S1is coupled to all three electrodes e1, e2, e3, resulting in a current value of ⅔ mA for each of the electrodes e1, e2, e3(if the source S1is cathodic, and the source S2is anodic) or a current value of −⅔ mA for each of the electrodes e1, e2, e3(if the source S1is anodic, and the source S2is cathodic) (FIG. 11d).

In the fifth source-electrode coupling configuration, the first source S1is coupled to the first electrode e1, and the second source S2is coupled to the second and third electrodes e2, e3, resulting in a current value of 3 mA for the electrode e1and 2½ mA for each of the electrodes e2, e3(if the source S1is cathodic, and the source S2is anodic) or a current value of −3 mA for the electrode e1, and −2½ mA for each of the electrodes e2, e3(if the source S1is anodic, and the source S2is cathodic) (FIG. 11e). In the sixth source-electrode coupling configuration, the first source S1is coupled to the first and second electrodes e1, e2, and the second source S2is coupled to the third electrode e3, resulting in a current value of 1½ mA for each of the electrodes e1, e2and 5 mA for electrode e3(if the source S1is cathodic, and the source S2is anodic) or a current value of −1½ mA for each of the electrodes e1, e2, and −5 mA for the electrode e3(if the source S1is anodic, and the source S2is cathodic) (FIG. 11f).

In the seventh source-electrode coupling configuration, the first source S1is coupled to the first and second electrodes e1, e2, and the second source S2is coupled to the second and third electrodes e2, e3, resulting in a current value of 1½ mA for the electrode e1, 4 mA for the electrode e2, and 2½ mA for the electrode e3(if both the sources S1and S2are anodic), or a current value of −1½ mA for the electrode e1, −4 mA for the electrode e2, and −2½ mA for the electrode e3(if both the sources S1and S2are cathodic), or a current value of −1½®mA for the electrode e1, 1 mA for the electrode e2, and 2½ mA for the electrode e3(if the source S1is cathodic, and the source S2is anodic), or a current value of 1½ mA for the electrode e1, −1 mA for the electrode e2, and −2½ mA for the electrode e3(if the source S1is anodic, and the source S2is cathodic) (FIG. 11g).

It should be noted that the active electrode subsets associated with the respective sources may have at least one common electrode. For example, in the third and fourth source-electrode coupling configurations illustrated inFIGS. 11cand 11d, the electrode subsets respectively coupled to the first and second sources S1, S2include all three electrodes e1, e2, e3, and in the seventh source-electrode coupling configuration illustrated inFIG. 11g, the electrode subsets coupled to the first and second sources S1, S2include the second electrode e2. Or, the active electrode subsets associated with the respective source may not have a common electrode. For example, in the fifth source-electrode coupling configuration illustrated inFIG. 11e, the electrode subset coupled to the first source S1includes the electrode e1, whereas the electrode subset coupled to the second source S2includes the electrodes e2, e3, and in the sixth source-electrode coupling configuration illustrated inFIG. 11f, the electrode subset coupled to the first source S1includes the electrodes e1, e2, whereas the electrode subset coupled to the second source S2includes the electrode e3.

It should also be noted that the active electrode subsets associated with the respective sources can be different from each other. For example, in the fifth source-electrode coupling configuration illustrated inFIG. 11e, the electrode coupling subset coupled to the first source S1includes the electrode e1, whereas the electrode subset coupled to the second source S2includes the electrodes e2, e3. In the sixth source-electrode coupling configuration illustrated inFIG. 11f, the electrode coupling subset coupled to the first source S1includes the electrodes e1, e2, whereas the electrode subset coupled to the second source S2includes the electrode e3. In the seventh source-electrode coupling configuration illustrated inFIG. 11g, the electrode coupling subset coupled to the first source S1includes the electrodes e1, e2, whereas the electrode subset coupled to the second source S2includes the electrodes e2, e3. Or, the active electrode subsets associated with the respective sources can be the same as each other. For example, in the third and fourth source-electrode coupling configurations illustrated inFIGS. 11cand 11d, the electrode subsets respectively coupled to the first and second sources S1, S2include all three electrodes e1, e2, e3.

Although the current sources illustrated inFIGS. 11a-11gare described as having fixed magnitudes, it should be noted that one or more of these sources can have a variable magnitude, such that the current values on the electrodes e1, e2, e3can be better controlled. For example, the current magnitude of each of the sources S1, S2may be varied by ±1 mA, providing a relatively large current variability on the electrodes e1, e2, e3, as illustrated inFIGS. 12a-12g.

For example, the first source-electrode coupling configuration may result in a current range of ⅔ to 1⅓ mA for each of the electrodes e1, e2, e3(if both the sources S1, S2are anodic) or a current range of −⅔ to 1⅓ for each of the electrodes e1, e2, e3(if both the sources S1, S2are anodic) (FIG. 12a). The second source-electrode coupling configuration may result in a current range of 1⅓ to 2 mA for each of the electrodes e1, e2, e3(if both the sources S1, S2are anodic) or a current range of −1⅓ to −2 mA for each of the electrodes e1, e2, e3(if both the sources S1, S2are cathodic) (FIG. 12b). The third source-electrode coupling configuration may result in a current range of 2 to 3⅓ mA for each of the electrodes e1, e2, e3(if both the sources S1, S2are anodic) or a current range of −2 to −3⅓ mA for each of the electrodes e1, e2, e3(if both the sources S1, S2are cathodic) (FIG. 12c). The fourth source-electrode coupling configuration may result in a current range of 0 to 4/3 mA for each of the electrodes e1, e2, e3(if the source S1is cathodic, and the source S2is anodic) or a current range of 0 to − 4/3 mA for each of the electrodes e1, e2, e3(if the source S1is anodic, and the source S2is cathodic) (FIG. 12d). The fifth source-electrode coupling configuration may result in a current range of 2 to 4 mA for the electrode e1and 2 to 3 mA for each of the electrodes e2, e3(if the source S1is cathodic, and the source S2is anodic) or a current range of −2 to −4 mA for the electrode e1, and −2 to −3 mA for each of the electrodes e2, e3(if the source S1is anodic, and the source S2is cathodic) (FIG. 12e). The sixth source-electrode coupling configuration results in a current range of 1 to 2 mA for each of the electrodes e1, e2and 4 to 6 mA for electrode e3(if the source S1is cathodic, and the source S2is anodic) or a current range of −2 to −1 mA for each of the electrodes e1, e2, and −4 to −6 mA for the electrode e3(if the source S1is anodic, and the source S2is cathodic) (FIG. 12f). The seventh source-electrode coupling configuration may result in a current range of 1 to 2 mA for the electrode e1, 3 to 5 mA for the electrode e2, and 2 to 3 mA for the electrode e3(if both the sources S1and S2are anodic), or a current range of −1 to −2 mA for the electrode e1, −3 to −5 mA for the electrode e2, and −2 to −3 mA for the electrode e3(if both the sources S1and S2are cathodic), or a current range of −1 to −2 mA for the electrode e1, 0 to 2 mA for the electrode e2, and 2 to 3 mA for the electrode e3(if the source S1is cathodic, and the source S2is anodic), or a current range of 1 to 2 mA for the electrode e1, −2 to 0 mA for the electrode e2, and −2 to −3 mA for the electrode e3(if the source S1is anodic, and the source S2is cathodic) (FIG. 12g).

It should be noted that although the above-described embodiments illustrate and describe the current sources S1, S2as having different current values or different current ranges, which provides greater variability in the current values, the current sources S1, S2may have the same current values or the same current ranges. Furthermore, it should be noted that the above current values and ranges on the electrodes e1, e2, e3assume a uniform impedance at the electrodes e1, e2, e3. In actuality, the impedances at the electrodes e1, e2, e3will vary from each other due to the different tissue impedances. As such, the current values at the electrodes e1, e2, e3may vary from the theoretical current values. However, because the monitoring circuitry72can be utilized to measure the impedances at the electrodes e1, e2, e3, the processing circuitry (e.g., the microprocessor76), utilizing simple voltage models, can compute the actual current values or ranges on the electrodes e1, e2, e3from the known current values or ranges output by the sources S1, S2and the measured impedances at the electrodes e1, e2, e3. Furthermore, although only two sources S1, S2are described with respect toFIGS. 11 and 12, more than two sources can be coupled to subsets of the electrodes to provide many more different combinations of current values on larger sets of electrodes.

Referring toFIG. 13, a method of utilizing the SCS system10will now be described. First, the electrode leads12are implanted into the patient40, as shown inFIG. 5(block110). Then, the RC16and/or CP18may be operated to select the combination of the electrodes26to be activated and the electrical current values on the activated electrodes26(i.e., the electrode current distribution), and in this embodiment, to select the fractionalized current values for the electrodes26and to globally vary the current on the activated electrodes26to obtain the desired current values (block112). The source-electrode coupling configuration that best meets the desired current values can then be determined (block114). Such determination may be accomplished in various ways, including minimizing the total absolute current error on each selected electrode, minimizing the total current electrode in the cathodes (even if it means higher error in the anodes), finding a configuration that meets an error level that does not exceed a particular maximum error level, minimizing the maximum absolute current error on each electrode without exceeding a particular absolute (mA) or relative (%) error for any given electrode, etc.

Notably, the source-electrode coupling configuration that best meets the desired fractionalized current values may be determined, and then, the current to be output by the sources60may be globally scaled up or down to match the desired absolute current values on the electrodes26. Inputs to this process may be toleranced for matching the desired current values on the electrodes26. For example, the target current values may require that each of two electrodes convey cathodic current at 3 mA plus or minus 0.2 mA, and an additional three electrodes convey cathodic current at 2 mA plus or minus 0.4 mA. Notably, in the case where reconfigurable current sources are used, at least one of the current sources may be reconfigured from a second polarity to a first polarity (e.g., reconfigured from a cathodic source to an anodic source, or vice versa) in accordance with the source-electrode coupling configuration.

Subsequent to determination of the source-electrode coupling configuration, electrical current is conveyed between active ones of the current sources60and active subsets of the electrodes26in accordance with the determined source-electrode coupling configuration (block116). In some cases, portions of, or all, of the current conveyed from some of the sources60will be combined in an additive manner (if the sources are of the same polarity) and/or a subtractive manner (if the sources are of different polarities) to produce a combined electrical current that is conveyed to or from at least one of the electrodes, e.g., in the manner illustrated with respect to the electrodes e1, e2, e3in the source-electrode coupling configurations illustrated inFIGS. 11cand 11d, or the electrode e2in the source-electrode configuration illustrated inFIG. 11f. Significantly, in all cases illustrated inFIGS. 11a-11g, the total number of active electrodes (three) exceeds the total number of active sources (two), thereby minimizing the number of sources needed to drive the electrodes.

In making the source-electrode coupling configuration determination, the impedance at the electrodes to be activated may be assumed or measured and utilized to provide the best fit. Impedance measurements are especially significant when electrical current is to be conveyed from a source to multiple electrodes. Even if a current source is used in this case, the impedances on the electrodes coupled to the current source may be different, thereby creating a voltage divider network that will create an unequal split in the partial currents supplied to the electrodes. Using basic known voltage divider calculations, however, the magnitudes of the respective partial currents can be determined based on the magnitude of the current source output and the impedances.

Thus, the impedances can be subsequently measured (block118), and this information may be utilized to tune, or dynamically adjust, the source-electrode coupling configuration to optimize electrical stimulation treatment (block120). If reconfigurable current sources are used, at least one of the current sources may be reconfigured from the first polarity back to the second polarity in accordance with the new or adjusted source-electrode coupling configuration if necessary. In any event, all or portions of the current conveyed from some of the sources60may be combined in an additive manner and/or a subtractive manner to produce another combined electrical current that is conveyed to or from one of the electrodes. The sources used to generate the previously combined electrical current and the sources used to generate the subsequently combined electrical current can be associated can be the same or different, and the electrode(s) to or from which the previously combined electrical current is conveyed and the electrode(s) to or from which the subsequently combined electrical current is conveyed can be the same or different.

Control of the various aspects of the process described in reference toFIG. 13may be accomplished in a variety of manners and performed by either the IPG14in an “on-board processing” configuration, by an external control device (such as, e.g., the RC16and/or CP18) in an “off-board processing” configuration, or with combinations thereof. For example, in one off-board processing embodiment, the external control device may be configured to wirelessly transmit a command to the IPG14to measure the tissue resistances or impedances within the current branches associated with the activated electrodes26to assist with the selection of a desired stimulation schema. In such a configuration, the forward telemetry circuitry80receives the command, and the monitoring circuitry58, under control by the microcontroller76, measures the tissue resistances or impedances. The back telemetry circuitry76, under control by the microcontroller76, then wirelessly transmits the measured tissue resistances or impedances back to the external control device. Based on the measured values, the external control device may be configured to not only assist with selecting the electrodes to be activated and the desired current values on the activated electrodes, hardware configuration, and other input, but also to compute the source-electrode coupling configuration, or variation thereto, which would optimize the stimulation under the desired electrical stimulation schema. The external control device may then wirelessly transmit a control signal containing updated stimulation parameters, including the source-electrode coupling configuration, to the IPG14. The forward telemetry circuitry80, under control by the microcontroller76, receives the control signal, and the analog output circuitry50, including the switch90, under control of the microcontroller76, adjusts the source-electrode coupling to the desired configuration.

In another embodiment having at least some on-board processing for determining the source-electrode coupling configuration, the external control device wirelessly transmits a control signal containing the desired electrical stimulation parameters, including desired current values at the activated electrodes26, to the IPG14. The forward telemetry circuitry80, under control by the microcontroller76, receives the control signal, and the microcontroller76, based on measured tissue resistances or impedances (performed by the monitoring circuitry58either prior to or after receipt of the control signal), may determine the source-electrode coupling configuration needed to obtain the desired fractionalized currents at the activated electrodes26. In other words, the determination of the source-electrode coupling configuration is handled on-board with the microcontroller76. The analog output circuitry50, including the switch90, under control of the microcontroller76, then adjusts the source-electrode coupling to the desired configuration.

Alternatively, rather than determining the source-electrode coupling configuration through computational means, the monitoring circuitry58may measure the electrical current at the activated electrodes26, and the analog output circuitry50, under control of the microcontroller76, can modify the source-electrode coupling configuration until the measured electrical currents at the activated electrodes26match the desired electrical current values. In either case, the microcontroller76may either vary the source-electrode coupling configuration to achieve the desired electrical current distribution only in response to a command received by the external control device, or may periodically monitor the electrical currents at the activated electrodes26and adjust the source-electrode coupling configuration, if needed, to maintain the desired electrical current distribution at the activated electrodes102in a closed loop fashion.

It should be noted that the above techniques for using combined sources can be used in the ETS20. In this case, electrical current, under control of the ETS20and external control device, can be steered between the electrodes to determine one or more sets of stimulation parameters that provide effective therapy to the patient. The current distribution can either be measured in the ETS or estimated based on the effective resistances. Once the stimulation parameter sets, including the effective current distributions, are determined, they can be programmed into the IPG in the form of a source-electrode coupling configuration. This technique may be particularly advantageous when the IPG has minimal or no computer power, which may otherwise be needed to perform the techniques described herein.