U.S. patent application number 16/018568 was filed with the patent office on 2018-10-18 for apparatus and method for determining the relative position and orientation of neurostimulation leads.
The applicant listed for this patent is Boston Scientific Neuromodulation Corporation. Invention is credited to Kerry Bradley, Michael A. Moffitt, James R. Thacker.
Application Number | 20180296828 16/018568 |
Document ID | / |
Family ID | 46329814 |
Filed Date | 2018-10-18 |
United States Patent
Application |
20180296828 |
Kind Code |
A1 |
Bradley; Kerry ; et
al. |
October 18, 2018 |
APPARATUS AND METHOD FOR DETERMINING THE RELATIVE POSITION AND
ORIENTATION OF NEUROSTIMULATION LEADS
Abstract
A method for determining whether the relative position of
electrodes used by a neurostimulation system has changed within a
patient comprises determining the amplitude of a field potential at
each of at least one of the electrodes, determining if a change in
each of the determined electric field amplitudes has occurred, and
analyzing the change in each of the determined electric field
amplitudes to determine whether a change in the relative position
of the electrodes has occurred. Another method comprises measuring
a first monopolar impedance between a first electrode and a
reference electrode, measuring a second monopolar impedance between
second electrode and the reference electrode, measuring a bipolar
impedance between the first and second electrodes, and estimating
an amplitude of a field potential at the second electrode based on
the first and second monopolar impedances and the bipolar
impedance.
Inventors: |
Bradley; Kerry; (Glendale,
CA) ; Thacker; James R.; (Eureka, MO) ;
Moffitt; Michael A.; (Saugus, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Boston Scientific Neuromodulation Corporation |
Valencia |
CA |
US |
|
|
Family ID: |
46329814 |
Appl. No.: |
16/018568 |
Filed: |
June 26, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15459660 |
Mar 15, 2017 |
10022540 |
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16018568 |
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14933576 |
Nov 5, 2015 |
9610439 |
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15459660 |
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12856905 |
Aug 16, 2010 |
9192760 |
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14933576 |
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11938490 |
Nov 12, 2007 |
7853330 |
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12856905 |
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11343007 |
Jan 30, 2006 |
8682447 |
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11938490 |
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10310202 |
Dec 3, 2002 |
6993384 |
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11343007 |
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60338331 |
Dec 4, 2001 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61N 1/3605 20130101;
A61N 1/37247 20130101; A61B 5/053 20130101; A61N 1/0551
20130101 |
International
Class: |
A61N 1/08 20060101
A61N001/08; A61N 1/372 20060101 A61N001/372; A61N 1/36 20060101
A61N001/36; A61N 1/05 20060101 A61N001/05 |
Claims
1. (canceled)
2. A method comprising: determining a relative position of an
implanted lead with respect to at least one other implanted lead,
wherein the determining the relative position includes: delivering
electrical energy to at least one active electrode on one of the
leads in preparation to determine at least one electrical parameter
(e.g. impedance or field potential); determining the at least one
electrical parameter: and determining the relative position based
on the at least one electrical parameter.
3. The method of claim 2, wherein determining the at least one
electrical parameter includes measuring interelectrode
impedances.
4. The method of claim 2, wherein determining the at least one
electrical parameter includes measuring field potentials.
5. The method of claim 2, wherein determining the at least one
electrical parameter includes estimating field potentials.
6. The method of claim 2, further comprising displaying the
relative position.
7. The method of claim 2, further comprising determining if the
leads have migrated with respect to each other.
8. The method of claim 7, further comprising initiating corrective
action when the leads have migrated with respect to each other.
9. A system configured for use with implanted leads, comprising: a
pulse generator and a programmer configured to program the pulse
generator, wherein the system is configured to: determine a
relative position of one of the implanted leads with respect to at
least one other of the implanted leads, including: deliver
electrical energy to at least one active electrode on the one of
the leads in preparation to determine at least one electrical
parameter; determine the at least one electrical parameter; and
determine the relative position based on the at least one
electrical parameter.
10. The system of claim 9, wherein the at least one electrical
parameter is determined by measuring interelectrode impedances.
11. The system of claim 9, wherein the at least one electrical
parameter is determined by measuring field potentials.
12. The system of claim 9, wherein the at least one electrical
parameter is determined by estimating field potentials.
13. The system of claim 9, further composing a display configured
to display the relative position.
14. The system of claim 9, wherein the system is configured to
determine if the leads have migrated with respect to each
other.
15. The system of claim 14, wherein the system is configured to
initiate corrective action when the leads have migrated with
respect to each other.
16. A non-transitory machine-readable medium including
instructions, which when executed by a machine, cause the machine
to determine a relative position of an implanted lead with respect
to at least one other implanted lead, including: deliver electrical
energy to at least one active electrode on one of the leads in
preparation to determine at least one electrical parameter;
determine the at least one electrical parameter: and determine the
relative position based on the at least one electrical
parameter.
17. The non-transitory machine-readable medium of claim 16, wherein
the at least one electrical parameter is determined by measuring
interelectrode impedances.
18. The non-transitory machine-readable medium of claim 16, wherein
the at least one electrical parameter is determined by measuring
field potentials
19. The non-transitory machine-readable medium of claim 16, wherein
the at least one electrical parameter is determined by estimating
field potentials.
20. The non-transitory machine-readable medium of claim 16, wherein
the instructions, which when executed by a machine, cause the
machine to display the relative position
21. The non-transitory machine-readable medium of claim 16, wherein
the instructions, which when executed by a machine, cause the
machine to determine if the leads have migrated with respect to
each other.
Description
RELATED APPLICATIONS
[0001] The present application is a continuation-in-part of U.S.
patent application Ser. No. 11/343,007, , filed Jan. 30, 2006,
which is a continuation of U.S. patent application Ser. No.
10/310,202, filed Dec. 3, 2002 (now U.S. Pat. No. 6,993,384), which
application claims the benefit of U.S. Provisional Patent
Application Ser. No. 60/338,331, filed Dec. 4, 2001, each of which
is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to neurostimulation systems,
such as a spinal cord stimulation (SCS) system, and more
particularly to a method for determining the relative position and
orientation of electrodes on a neurostimulation lead or leads used
with such a system.
[0003] In SCS systems, positioning of the leads is critical to the
success of the therapy. During surgery, the physician places the
leads in a very careful manner in order to locate the electrodes
proximal to neural elements that are the target of the stimulation.
During and after placement, stimulation energy is delivered to
verify that the leads are indeed stimulating the appropriate neural
elements.
[0004] However, sf the leads happen to shift position, the targeted
neural elements may no longer he appropriately stimulated. At best,
this can require electrical reprogramming to restore therapy or, at
worst, surgical revision, where a second surgery is required and
the leads must be manually readjusted. In the first case, the
physician may have only a suspicion that a lead has shifted
position, based on patient reporting of paresthesia, which is not
foolproof. Also, attempting to re-program the leads based on
paresthesia locations can be challenging.
[0005] What is needed is a more objective technique for verifying
the position of the leads.
[0006] Prior art approaches for determining the lead position are
disclosed in U.S. Pat. Nos. 4,486,835; 4,539,640; and 5,184,624,
which patents are incorporated herein by reference.
SUMMARY OF THE INVENTION
[0007] The present invention addresses the above and other needs by
providing a cross-check technique for verifying the position of the
electrodes of the implanted leads. A first technique involves the
use of interelectrode impedance. A second technique involves
measured field potentials. A third technique involves estimated
field potentials. Any of these techniques advantageously allows the
relative orientation of one electrode on an implanted lead to other
electrodes on the implanted lead or adjacent implanted leads in the
spinal column or other body/tissue location to be readily
determined. Such techniques are useful not only for reprogramming,
but also to estimate if the shifted orientation of the electrodes
is sufficiently large so as to make electrical reprogramming a
waste of time, thereby suggesting that surgery may need to be
performed for repositioning.
[0008] At present, the correct lead position may only be determined
by X-ray or fluoroscopy. Disadvantageously, X-ray and fluoroscopy
require expensive equipment, significant time, and appropriate
medical facilities, most of which are not readily available. The
general process for fitting a neurostimulation patient, i.e., a
spinal cord stimulation patient, is described, e.g., in U.S. Pat.
Nos. 6,052,624; 6,393,325; in published international patent
application WO 02/09808 A1 (published 7 Feb. 2002); and in U.S.
patent applications (assigned to the same assignee as the present
application) Ser. No. 09/626,010, filed Jul. 26, 2000; and Ser. No.
09/740,339, filed Dec. 18, 2000, which patents, publication, and
applications am incorporated herein by reference.
[0009] As indicated in those documents, prior to fitting a patient
with the certain types of neurostimulation leads, the relative
orientation of the electrodes on the implanted leads should be
known in order to allow appropriate navigation of the stimulation
energy. At present, a determination of the relative orientation
typically requires that a fluoroscope or X-ray image of the
implanted leads be present at the time of patient setup with the
system programmer. Disadvantageously, however, such images may not
always be available. Moreover, between the time of implant and
follow-up visits, the leads may have shifted and the fluoroscope
image may no longer be valid. This can result in poor patient
outcomes due to inappropriate or unexpected stimulation effects
during fitting.
[0010] Hence, it is seen that there is a need for the cross-check
techniques provided by the present invention, which techniques can
be used to verify the position of the leads at numerous times
during the lifetime of the implanted leads, e.g., during initial
implantation and programming, during follow-up visits, throughout
the trial period, and during subsequent reprogramming sessions.
[0011] Other and further aspects and features of the invention will
be evident from reading the following detailed description of the
preferred embodiments, which are intended to illustrate, not limit,
the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The drawings illustrate the design and utility of preferred
embodiments of the present invention, in which similar elements are
referred to by common reference numerals. In order to better
appreciate how the above-recited and other advantages and objects
of the present inventions are obtained, a more particular
description of the present inventions briefly described above will
be rendered by reference to specific embodiments thereof, which are
illustrated in the accompanying drawings. Understanding that these
drawings depict only typical embodiments of the invention and are
not therefore to be considered limiting of its scope, the invention
will be described and explained with additional specificity and
detail through the use of the accompanying drawings in which;
[0013] FIG. 1 illustrates a neurostimulation system wherein two
leads, each having eight in-line electrodes thereon, are positioned
side-by-side, and wherein each lead is connected to an implantable
pulse generator (IPG), which IPG is, in turn, coupled to an
external programmer;
[0014] FIG. 2 shows a functional block diagram of an IPG that uses
multiple programmable current sources to activate selected
electrodes of the neurostimulation leads;
[0015] FIG. 3 shows a functional block diagram of an IPG that uses
multiple programmable voltage sources to activate selected
electrodes of the neurostimulation leads;
[0016] FIG. 4 is a table that contains impedance vector and
distance impedance data in accordance with one embodiment of the
invention;
[0017] FIG. 5 illustrates representative relative electrode
orientation in a patient having dual quadrapolar leads (two
side-by-side leads, each having four in-line electrodes
thereon);
[0018] FIG. 6 is an impedance map that illustrates application of
one embodiment of the invention to the electrode orientation shown
in FIG. 5;
[0019] FIG. 7 depicts a representative fluoroscopic image of dual
quadrapolar leads in a patient;
[0020] FIG. 8 illustrates, in accordance with another embodiment of
the invention, the measured electrode potential of non-activated
electrodes on the dual quadrapolar lead of FIG. 7 when the
activated electrode is activated through monopolar stimulation;
[0021] FIG. 9 illustrates the measured electrode potential of
non-activated electrodes on the dual quadrapolar lead of FIG. 7
when the activated electrodes are activated through tripolar
stimulation;
[0022] FIG. 10 illustrates an arrangement of two electrodes and a
return electrode, wherein monopolar impedance measurements are
taken between the two electrodes and the return electrodes, and a
bipolar impedance measurement is taken between the two
electrodes;
[0023] FIG. 11 is a field potential matrix created when the two
electrodes of FIG. 10 are sourcing current, while field potentials
are measured on the two electrodes;
[0024] FIG. 12 is a field potential matrix created when the three
electrodes are sourcing current while field potentials are measured
on the three electrodes;
[0025] FIGS. 13A-13P illustrate a comparison between the actual
measured field potential on leads of FIG. 1 and the estimated field
potential on the leads of FIG. 1; and
[0026] FIG. 14 is a flowchart that highlights the main steps used
with one embodiment of the present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0027] At the outset, it is noted that the present invention may be
used with an
[0028] implantable pulse generator (IPG), radio frequency (RF)
transmitter, or similar electrical stimulator, that may be used as
a component of numerous different types of stimulation systems. The
description that follows relates to a spinal cord stimulation (SCS)
system. However, it is to be understood that 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, a
peripheral nerve stimulator, or in any other neural stimulator
configured to treat urinary incontinence, sleep apnea, shoulder
sublaxation, etc.
[0029] The embodiments described herein use; (1) interelectrode
impedance; (2) actual field potentials; or (3) estimated field
potentials to determine the relative orientation of one electrode
on an implanted lead to other electrodes on the implanted lead or
adjacent implanted leads in the spinal column or other body/tissue
location,
[0030] Before describing the three techniques, any of which may be
used, it will be helpful to first briefly provide an overview of a
representative neurostimulation system of the type in which these
techniques may be used. A representative neurostimulation system is
illustrated in FIG. 1. Such system may include a first implantable
lead 20 and a second implantable lead 30. Each lead includes a
series of in-line electrodes thereon. For the example shown in FIG.
1, the first lead 20 contains eight in-line electrodes E1, E2, E3,
. . . E8. The second lead 30 also contains eight in-line electrodes
E9, E10, E11, . . . E16.
[0031] Each of the electrodes of each lead 20 or 30 are
electrically connected through respective wires, embedded or
carried within a body of the lead, to an implantable pulse
generator (IPG) 40. The wires connected to the electrodes E1, E2,
E3, . . . E8 of lead 20, for example, may be characterized as a
bundle of wires 22 that are electrically connected with the IPG 40.
Similarly, the wires connected to the electrodes E9, E10, E11, . .
. E16 of lead 30 may be characterized as a bundle of wires 32 that
are electrically connected with the IPG 40. Through these wires,
carried within the respective leads 20 or 30, the IPG is able to
direct electrical stimulation to selected electrodes of each
lead.
[0032] When a given electrode is selected to receive an electrical
stimulus, it is (for purposes of the present invention) said to be
"activated". When an electrode is not selected to receive an
electrical stimulus, it is said to be "non-activated". Electrical
stimulation must always occur between two or more electrodes (so
that the electrical current associated with the stimulus has a path
from the IPG to the tissue to be stimulated, and a return path from
the tissue to the IPG). The case of the IPG may function, in some
modes of operation, as a return electrode E.sub.R. Monopolar
stimulation occurs when a selected one or more of the electrodes of
one of the leads 20 or 30 is activated with a common polarity
(anode or cathode), and the return electrode E.sub.R is activated
at the opposite polarity. Bipolar stimulation occurs when two of
the electrodes of the leads 20 or 30 are activated, e.g., when
electrode E3 of lead 20 is activated as an anode at the same time
that electrode E11 of lead 30 is activated as a cathode. Tripolar
stimulation occurs when three of the electrodes of the leads 20 or
30 are activated, e.g., when electrodes E4 and E5 of lead 20 are
activated as an anode at the same time that electrode E13 of lead
30 is activated as a cathode. In general, multipolar stimulation
occurs when multiple electrodes of the leads 20 or 30 are
activated, but the IPG case is not used as a return electrode.
[0033] The IPG 40 is typically programmed, or controlled, through
the use of an external (non-implanted) programmer 60. The external
programmer 60 is coupled to the IPG 40 through a suitable
communications link, represented in FIG. 1 by the wavy arrow 50.
Such link 50 passes through the skin 18 of the patient.
Representative links that may be used to couple the programmer 60
with the IPG 40 include a radio frequency (RF) link, an inductive
link, an optical link, or a magnetic link. The programmer 60, or
other similar external device, may also be used to couple power
into the IPG for the purpose of operating the IPG or charging a
replenishable power source, e.g., a rechargeable battery, within
the IPG. Once the IPG 40 has been programmed, and its power source
has been fully charged or replenished, if may operate as programmed
without the need for the external programmer 60 to be present.
[0034] Turning next to FIG. 2, there is shown a representative
functional block diagram of one type of IPG 40 that may be used
with a neurostimulation system. As seen in FIG. 2, the IPG 40
therein depicted is made up of a multiplicity of dual current
sources 42. Each dual current source 42 includes a positive current
source, i.e., a current source that can function as an anode to
"source" current to a load, and a current source that can function
as a cathode to "sink" current from a load through the same node.
The "load" is the tissue that resides between the two or more
activated electrodes, and includes the wire (or other conductive
element) and a coupling capacitor C that connects the electrode to
the common node of the dual current source.
[0035] Thus, for example, and as depicted in FIG. 2, a first dual
current source connected to electrode E1 of a first lead through a
coupling capacitor C, may be programmed to produce a current of +I1
or -I1 through electrode E1, depending upon whether such dual
current source is configured to operate as a cathode or an anode,
when such first dual current source is turned on or enabled.
Similarly, a second current source, connected to electrode E2, when
turned on or enabled, may be programmed to produce a current of +I2
or -I2 through electrode E2. In a similar manner, a third current
source, when enabled, may be programmed to produce a current of +I3
or -I3 through electrode E3. An nth current source, where n
represents the number of electrodes on the first lead, is similarly
connected to electrode En, and may be programmed to produce a
current of +In or -In through electrode En when turned on or
enabled.
[0036] If a second lead, also having n electrodes, is positioned
adjacent the first lead, each, electrode is similarly connected to
a dual current source. For example, electrode E(n+1) is connected
to a dual current source that produces a current of +I(n+1) or
-I(n+1) through electrode E(n+1) when such (n+1)th current source
is enabled. In like manner, all of the electrodes of the second
lead are connected to respective dual current sources. There are
thus 2n dual current sources that are respectively connected to
each of the 2n electrodes of the first and second leads (n
electrodes on each lead). Alternative embodiments (not shown) may
employ less than 2n dual current sources connected to 2n electrodes
through a suitable multiplexer circuit.
[0037] A programmable current control circuit 44 is also provided
within the IPG 40 that controls, i.e., turns on or enables, at
specified times, a selected current source to operate as either a
cathode or an anode to source or sink a current having a desired
amplitude. The control circuit 44 also disables, or turns off,
selected current sources, as controlled by programmed control data
received from the external programmer, or otherwise resident within
the IPG. The control circuit 44 further includes the ability to
measure the electrode voltage, E.sub.V1, E.sub.V2, E.sub.V3, . . .
E.sub.Vn, . . . E.sub.V(2n), appearing at the output of each dual
current source 42, whether the electrode is activated or
non-activated. This effectively allows the electrode voltage, or
electric field at the electrode, to be measured, which in turn
facilitates impedance or field potential measurements to be made,
which measurements are used in carrying out various steps of the
invention as described below.
[0038] Thus, in operation, and as illustrated in FIG. 2, current
control circuit 44 may turn on current sources +I1 and +I2 at the
same time, i.e., during a time period T1, that current source
-I(n+2) is turned on. All other current sources are turned off, or
disabled, during the time T1. Such action causes electrodes E1 and
E2 to be activated as anodes at the same time that electrode E(n+2)
is activated as a cathode. That is, a current +I1 is "sourced" from
electrode E1 and a current +I2 is "sourced" from electrode E2 at
the same time that a current -I(n+2) is "sunk" into electrode
E(n+2). The amplitudes of the currents +I1 and +I2 may he any
programmed values, and the amplitude of the current -I(n+2) should
be equal to -(I1+I2). That is, the current that is sourced is equal
to the current that is sunk.
[0039] After the time period T1, it is common to switch the
polarities of the electrodes during a second-time period T2. During
T2, the electrodes E1 and E2 are activated as cathodes, so that
they both sink current, and electrode E(n+2) is activated as an
anode, so that it sources a current equal in amplitude to the
current that is sunk by electrodes E1 and E2. In this manner, a
biphasic stimulation pulse 46 is produced that is characterized by
a first pulse (during time period T1) of one polarity, followed by
a second pulse immediately or shortly thereafter (during time
period T2) of the opposite polarity. The electrical charge
associated with the first pulse is made so that it is equal to the
charge associated with the second pulse, thereby maintaining charge
balance during the stimulation. Maintaining charge balance when
stimulating living tissue is generally considered an important
component of a stimulation regime. Charge balance is commonly
achieved in a biphasic pulse 46 by making the amplitude of the
first pulse during time T1 equal to the amplitude of the second
pulse during time period T2, where T1 equals T2. However, charge
balance may also be achieved using other combinations of pulse
duration and amplitude, e.g., by making the amplitude of the second
pulse equal to 1/2 the amplitude of the first pulse, while making
the time period T2 equal to twice the time period T1.
[0040] Next, with respect to FIG. 3, a functional block diagram of
another type of IPG 40' that may be used in a neurostimulation
system is shown. The IPG 40' shown in FIG. 3, includes a
multiplicity of dual voltage sources 42', each being connected to
one of the electrodes E1, E2, E3, . . . En, of a first lead, or to
one of the electrodes E(n+1), E(n+2), . . . E(2n), of a second
lead. Each dual voltage source 42' applies a programmed voltage, of
one polarity or another, to its respective electrode, when enabled
or turned on. For the configuration shown in FIG. 3, a separate
dual voltage source 42' is connected to each electrode node through
a coupling capacitor C. Other embodiments, not shown, may use one
or two or more voltage sources that are selectively connected to
each electrode node through a multiplexer circuit
[0041] The control circuit 44', or other circuity within the IPG
40' further includes the ability to measure the electrode current,
E.sub.I1, E.sub.I2, E.sub.I3, . . . E.sub.In, . . . E.sub.I(2n),
flowing to or from its respective electrode, whether the electrode
is activated or non-activated, and the electrode voltage, E.sub.V1,
E.sub.V2, E.sub.V3, . . . E.sub.Vn, . . . E.sub.V(2n), appearing at
the output of each non-activated dual voltage source 42'. These
measurements facilitate impedance and electric field measurements
or calculations to be made, which measurements are used in carrying
out various steps of the invention as described below.
[0042] A programmable voltage control circuit 44' controls each of
the dual voltage sources 42', specifying the amplitude, polarity,
and duration of the voltage that is applied to its respective
terminal. Typically, stimulation is achieved by applying a diphasic
stimulation pulse 46' to the selected electrodes, wherein a voltage
of a first polarity and amplitude is applied during time period T3,
followed by a voltage of the opposite polarity and amplitude during
time period T4. The biphasic stimulation pulse 46' may be applied
between any two or more electrodes.
[0043] It should be noted that the functional block diagrams of
FIGS. 2 and 3 are functional diagrams only, and are 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 the voltage, or the
current, flowing through an activated or non-activated electrode.
Such measurements allow impedance to be determined (used with a
first embodiment of the invention), allow field potentials to be
measured (used with a second embodiment of the invention), or allow
field potentials to be estimated (used with a third embodiment of
the invention), as described in more detail below. A preferred IPG
is described in international patent application WO 02/09808 A1
(published 7 Feb. 2002); and in U.S. patent application Ser. No.
09/626,010, filed Jul. 26, 2000, which publication and application
have been previously referenced and are incorporated herein by
reference.
[0044] With the descriptions of FIGS. 1-3 thus providing background
information relative to a neurostimulation system, the embodiments
will next be described. As has been indicated, the embodiments
address the problem of determining the relative position between
electrodes once the leads on which the electrodes are carried have
been implanted. The embodiments use: (1) interelectrode impedances;
(2) actual field potentials; or (3) estimated Held potentials to
determine the relative orientation of one electrode on an implanted
lead to other electrodes on the implanted lead or adjacent
implanted leads in the spinal column or other body/tissue
location.
[0045] First, the interelectrode impedance technique for
determining relative electrode positions for multipolar leads of a
neurostimulation system will be explained in connection with FIGS.
4-6. The interelectrode impedance technique is performed by
measuring impedance vectors. A vector is defined as an impedance
value measured between two electrodes in the body. The value of the
impedance vector is due primarily to two physical entities: (1) the
electrode-electrolyte interface; and (2) the bulk impedance between
the electrodes. The impedance tomography technique of the present
invention relies upon the latter of the above two physical
entities, i.e., upon the bulk impedance between the electrodes. The
bulk impedance portion of the impedance vector may be further
broken up into two contributing factors; (a) the impedance of the
tissue adjacent to the electrodes; and (b) the impedance of the
tissue between the electrodes.
[0046] The first factor (part a) makes up the majority of the
measurement, due to the higher and non-uniform current densities
near the electrode surface. However, the second factor (part b),
where the current density is more uniform, has a roughly linear
relationship to distance between the two electrodes, due to the
definition of resistance. Resistance, R, is defined as
R=(resistivity).times.(distance)/cross-sectional area. The second
factor (part b) is used by the interelectrode impedance technique
embodiment of the invention to determine the relative spacing
between electrodes and to determine the relative orientation of the
leads.
[0047] By way of example, one first-order, simple embodiment of the
invention is as follows; if two multipolar leads are placed in the
spinal column, see FIG. 5, each having four electrodes (the
electrodes of one lead being designated as e1, e2, e3, and e4; and
the electrodes of the other lead being designated as E5, E6, E7 and
E8), their relative orientation may be inferred by making the
following measurements: (1) monopolar impedances for all
electrodes; and (2) bipolar impedances between a given electrode
and each electrode (one at a time) on opposing leads.
[0048] The monopolar impedances are used to "correct" the bipolar
impedances for the first factor of bulk impedance, the
strongly-weighted impedance near the electrode. The corrected
bipolar impedances are then used to develop an impedance "map"
between the electrodes. This map reveals the relative orientation
of the leads. To illustrate, a sample correction formula is as
follows: (distance between two electrodes e1 &
e2).apprxeq.(measured bipolar impedance between two electrodes e1
& e2)+(2*offset)-(monopolar Z for electrode e1)-(monopolar Z
for electrode e2), where offset=an estimate of the impedance in the
monopolar impedance measurement that is NOT due to the tissue near
the electrode.
[0049] After the bipolar impedances are corrected by the above
formula, the relative orientation of the leads may be inferred by
the relative minima of the impedance values. Where the corrected
bipolar impedance between two electrodes is a minimum relative to
other electrodes on an opposing array, those electrodes are
relatively adjacent. This information may then be loaded into a
programmer, which can then provide a graphic display of the assumed
relative lead positions. Such data and/or display might then be
compared with previously measured or entered and stored graphics,
indicating earlier orientations. Such comparison can thus help the
physician/clinician to track the lead orientation to determine
appropriate programming, reprogramming, or need for surgical
revision. Also, for some programming systems, the present invention
may be used to automatically setup the appropriate navigation
tables for steering multiple lead systems.
[0050] FIG. 4 illustrates data showing this simple embodiment
applied to data from a patient with dual quadrapolar leads, which
leads are oriented as depicted in FIG. 5. FIG. 6 shows the
impedance map resulting from the measurements of FIG. 4. It can be
seen that the impedance maps (FIG. 6) correlate well to the
orientation of the leads (FIG. 5).
[0051] The simple interelectrode impedance technique described
above may he enhanced by making more accurate corrections using the
appropriate field equations to calculate the monopolar and bipolar
impedance of the electrodes. Also, other geometric methods may be
employed using the improved "distance impedance" values to improve
the mapping of the electrode orientations.
[0052] Next, the actual field measurement technique for determining
relative electrode positions for multipolar leads of a
neurostimulation system will be explained in connection with FIGS.
7-9. Such a technique utilizes field potential measurements of the
implanted electrodes, and more particularly, field potential
measurements on non-active electrodes caused by activation of other
electrodes. In a preferred embodiment of this alternative
embodiment, a constant current is sourced (anodes) and sunk
(cathodes) from a predefined combination of electrodes. Such
electrodes thus comprise the activated electrodes. Then, the
resulting field potentials are measured at all other electrodes
(those not involved in sourcing or sinking current), i.e., the
non-activated electrodes. From these measured field potentials, the
relative orientation of the electrodes, and the leads on which the
electrodes are carried, may be determined. Advantageously, the use
of field potentials represents an improvement over the use of
impedance measurements, since the measured potential values are
less subject to the confounding effects of the tissue impedance
very close to the source/sink electrodes.
[0053] By way of example of this field potential measurement
technique, consider FIGS. 7, 8 and 9. FIG. 7 represents the
relative position of dual quadrapolar leads 21 and 31 after being
implanted in a patient, as obtained using a fluoroscopic imaging
device. In many instances, the necessary imaging equipment needed
to obtain a fluoroscopic image, such as is shown in FIG. 7, is not
readily available. Advantageously, the present field potential
measurement technique represents an alternative approach to
obtaining relative electrode position information rather than using
an expensive and cumbersome imaging device.
[0054] Two combinations of anodes/cathodes are used to deliver
current to the leads of the dual quadrapolar leads 21 and 31. The
first technique is monopolar; that is, current delivered or sourced
from one electrode (the cathode) and sunk to the return electrode
E.sub.R (the anode). Thus, for each active monopolar combination,
there are seven non-active electrodes on which the field potential
may be measured. The second technique is flanked tripolar
stimulation; that is, current delivered between two anodes and one
cathode, with the cathode being flanked on each side by an
anode.
[0055] In both the monopolar stimulation and the tripolar
stimulation, a constant current is delivered to each electrode
implanted in the patient's body while the field potential is
measured on all other electrodes NOT involved in sinking/sourcing
current. The constant current may be set to a subperception level,
or to another suitable level that is comfortable for the
patient.
[0056] The field potentials for the monopolar stimulation are
plotted on the same chart in FIG. 8. The vertical axis is
millivolts. As seen in FIG. 8, the electrodes closest to the source
electrode have a high field potential (note: all plots in FIG. 8
and FIG. 9 are "negative", i.e., more negative potentials results
in more positive measured values, as shown in the plots). Thus, for
example, consider electrode E8 (curve 71), which has its highest
field potential relative to electrode E4, and its lowest field
potential relative to electrodes E1 and E2, and an intermediate
potential relative to electrode E3. This corresponds to the actual
electrode positions shown in FIG. 7, where electrode E8 is closest
to electrode E4; somewhat further from electrode E3, and farthest
from electrodes E2 and E1. A similar analysis for the monopolar
stimulation fields of the other electrodes reveals a similar
relationship: the electrodes closest to the source electrode have
the higher potential.
[0057] The field potentials for the tripolar stimulation are
plotted on the same chart in FIG. 9. Again, the vertical axis is
millivolts. As seen in FIG. 9, a better relative orientation can be
obtained than can be obtained with the monopolar stimulation. Those
electrodes closest to the cathode have a high field potential while
those electrodes closest to the anode have a lower field potential
relative to the electrodes further away. For example, consider
curve 72, which shows the field potential of the non-active
electrodes relative to the tripolar stimulation of electrodes E2,
E3, E4, with E2 and E4 being anodes, and E3 being a cathode. As
seen in FIG. 9, curve 72 has a peak corresponding to electrode E7,
which means electrode E7 is closest to the cathode E3. Curve 72
further has lows or valleys corresponding to electrodes E6 and E8,
which means E6 and E8 are closest to anode electrodes E2 and E4.
The actual orientation of the electrodes shown in FIG. 7 reveals
that E6 is closest to E2, and E8 is closest to E4. Thus, it is seen
that those electrodes closest to the flanked cathodic electrode
have a high field potential while those electrodes closest to the
anodic electrodes, on either side of the cathodic electrode, have a
lower field potential relative to the electrodes further away.
[0058] Hence, it is seen that by measuring the field potential of
the non-active electrodes, when active electrodes are stimulated at
constant current levels, e.g., subperception levels, the relative
orientation of the neurostimulation leads may be determined. Once
known, the relative orientation may be used to perform any one or
more of a variety of corrective actions, as will be described in
further detail below.
[0059] Next, the estimated field potential technique for
determining relative electrode positions for multipolar leads of a
neurostimulation system will be explained in connection with FIGS.
10 and 11. Like the previous technique, this technique analyzes the
field potentials at the implanted electrodes. Unlike the previous
technique, however this technique estimates the field potentials at
the implanted electrodes based on measured electrical parameters,
and in particular, measured monopolar and bipolar impedances.
Notably, this technique has the advantage of minimizing the number
of actual measurements performed by the neurostimulation system in
the case where impedance measurements must already be taken to
effect another function, such as verifying contact continuity,
remaining battery charge estimation, detecting electrode shorts,
etc. That is, in the previous technique, actual field potential
measurements would have to be measured to determine the relative
orientation of the electrodes E1-E16 (shown in FIG. 1) in addition
to the impedance measurements measured to effect a function
unrelated to the relative electrode orientation determination.
[0060] As illustrated in FIG. 10, a first monopolar impedance
measurement is taken between a first one of the electrodes E1-E16
(in this case electrode E1) and the return electrode E.sub.R, a
second monopolar impedance measurement is taken between a second
one of the electrodes E1-E16 (in this case, electrode E2 ) and the
return electrode E.sub.R, and a bipolar impedance measurement is
taken between the first and second ones of the electrodes E1-E16
(in this case, between electrodes E1 and E2). As will be described
in further detail below, the field potential that would have been
created at electrode E2 had electrode E1 been activated in a
monopolar manner can then be estimated based on these impedance
measurements. The impedance measurements can be taken in the same
manner described above with respect to the first technique.
Notably, the estimate of the field potential at electrode E2 will
be approximately equal to the actual field potential measured at
electrode E2, as an inactive electrode, assuming that electrode E1
would have been activated. The monopolar and bipolar impedance
measurements can be performed on each pair of electrodes E1-E16 to
obtain field potential estimations for each electrode (i.e., 15
field potential estimations for each electrode corresponding to 15
assumed activations of the remaining electrodes).
[0061] Referring now to FIG. 11, the theory behind estimating field
potentials based on monopolar and bipolar impedance measurements
will now be explained. If electrode E1 sources current in a
monopolar fashion to electrode E.sub.R, a field potential
.PHI..sub.a is created at electrode E1. Similarly, if electrode E2
sources current in a monopolar fashion to electrode E.sub.R, a
field potential .PHI..sub.b is created at electrode E2. By
reciprocity, a field potential .PHI..sub.c is seen on electrode E1
when electrode E2 sources current, and on electrode E2 when
electrode E1 sources current. Assuming that the field potentials
.PHI..sub.a-.PHI..sub.c, are unknown, linear superposition can be
applied to estimate field potentials on electrodes E1, E2 when both
are used to source/sink current simultaneously. Note that the field
potential .PHI..sub.a is equal to the monopolar impedance of
electrode E1 if the source current is unit value, and the field
potential .PHI..sub.b is equal to the monopolar impedance of
electrode E2 if the source current is unit value. Solving for
bipolar impedance/field potentials, and letting electrode E1 source
unit current and electrode E2 sink unit current, the bipolar field
potential at electrode E1 will equal the monopolar field potential
.PHI..sub.a--the monopolar field potential .PHI..sub.c, and the
bipolar field potential at electrode E2 will equal the monopolar
field potential .PHI..sub.c--the monopolar field potential
.PHI..sub.b. Assuming that the bipolar impedance R.sub.bp between
electrodes E1 and E2 equals the voltage potential (.DELTA.V)
between electrodes E1 and E2 divided by the unit current (I)
between electrodes E1 and E2, then
R.sub.bp=((.PHI..sub.a-.PHI..sub.c)-(.PHI..sub.c-.PHI..sub.b))/1=.PHI..su-
b.a+.PHI..sub.b-2.PHI..sub.c. Because the monopolar field
potentials and .PHI..sub.a and .PHI..sub.b b at electrodes E1 and
E2 respectively equal the monopolar impedances R.sub.mp1 and
R.sub.mp2 at electrodes E1 and E2, assuming unit current, it
follows that .PHI..sub.c=-(R.sub.bp-R.sub.mp1-R.sub.mp2)/2. Thus,
the field potential at any electrode due to active passage of
current at other electrode(s) can be estimated based on measured
monopolar/bipolar impedances by solving for the electrical voltage
potential
[0062] Notably, while this equation has been presented herein to
estimate a field potential .PHI..sub.c for the purpose of
determining the migration of electrical leads or electrodes, any
one of the monopolar impedances between two electrodes and a return
electrode, the bipolar impedance between the two electrodes, and
the field potential at one of the two electrodes can be estimated
by actually measuring the remaining two of the monopolar
impedances, bipolar impedance, and field potential to solve the
equation for the estimated parameter. Thus, instead of measuring
ail three of these parameters, only two of them need to be actually
measured, while the remaining parameter can be estimated.
[0063] While estimation of field potentials from monopolar and
multipolar impedance measurements (or estimation of a multipolar
impedance using field potentials and monopolar impedance; or
estimation of a monopolar impedance using field potentials and
multipolar impedance) has been illustrated with a simple
two-contact (plus distant return for monopolar measurements)
system, it is noted that many monopolar and multipolar combinations
with two or more electrodes could be used to estimate field
potentials (or impedances) given the linearity and reciprocity
associated with the solution method.
[0064] For example, referring now to FIG. 12, if electrode E1
sources current in a monopolar fashion to electrode E.sub.R, a
field potential .PHI..sub.a is created at electrode E1; if
electrode E2 sources current in a monopolar fashion to electrode
E.sub.R, a field potential is created at electrode E2; and if
electrode E3 sources current in a monopolar fashion to electrode
E.sub.R, a field potential .PHI..sub.c is created at electrode E3.
By reciprocity, a field potential .PHI..sub.d is seen on electrode
E1 when electrode E2 sources current, and on electrode E2 when
electrode E1 sources current; a field potential .PHI..sub.e is seen
on electrode E2 when electrode E3 sources current, and on electrode
E3 when electrode E2 sources current; and a field potential
.PHI..sub.f is seen on electrode E1 when electrode E3 sources
current, and on electrode E3 when electrode E1 sources current.
[0065] Assuming that field potentials .PHI..sub.a-.PHI..sub.f are
initially unknown, there may be a variety of ways to estimate the
field potentials .PHI..sub.a-.PHI..sub.f using linear supposition.
For example, the field potentials .PHI..sub.a-.PHI..sub.f can he
estimated by configuring the electrodes E1-E3 in three
configuration (1 tripolar configuration, 1 bipolar configuration,
and 1 monopolar configuration).
[0066] Assume for the tripolar configuration that unit values of
+0.5, -1.0, and +0.5 are respectively applied to electrodes E1-E3,
then the respective monopolar field potentials m1-m3 on electrodes
E1-E3 will be will m1=0.5.PHI..sub.a+0.5.PHI..sub.f-.PHI..sub.d;
m2=.PHI..sub.b+0.5.PHI..sub.e+0.5.PHI..sub.d; nd
m3=0.5.PHI..sub.c+0.5.PHI..sub.f-.PHI..sub.e. Assume for the
bipolar configuration that unit values of +1.0 and -1.0 are
respectively applied to electrodes E1 and E2, then the respective
monopolar field potentials m4 and m5 on electrodes E1 and E2 will
be will m4=.PHI..sub.a-.PHI..sub.d; and m5=.PHI..sub.d-.PHI..sub.b.
Assume for the monopolar configuration that a unit value of -1.0 is
applied to electrode E3, then a monopolar field potential m6 on
electrode E3 will be -.PHI..sub.c.
[0067] Based on the field potential measurements m1-m6, the six
unknown field potentials can be solved using common algebraic
techniques or linear algebraic techniques, as follows:
.PHI..sub.a=2m1-4m2-2m3+4m5-m6
.PHI..sub.b=2m1-4m2-2m3-m4+3m5-m6
.PHI..sub.c=-m6
.PHI..sub.d=2m1-4m2-2m3-m4+4m5-m6
.PHI..sub.e=2m1-2m2-2m3-m4+2m5-m6
.PHI..sub.f=4m1-4m2-2m3-2m4+4m5-m6
The calculated field potentials .PHI..sub.a-.PHI..sub.f can be used
to estimate bipolar impedances and monopolar impedances and
potential differences for arbitrary electrode configurations. That
is assuming a unit current the monopolar impedances on electrodes
E1-E3 will be the field potentials .PHI..sub.a-.PHI..sub.c, and the
bipolar impedances on electrodes E1, E2 will be
.PHI..sub.a-.PHI..sub.b-.PHI..sub.d, bipolar impedances on
electrodes E2, E3 will be -.PHI..sub.b-.PHI..sub.c-2.PHI..sub.e,
and bipolar impedances on electrodes E1, E3 will be
.PHI..sub.a-.PHI..sub.c-2.PHI..sub.f.
[0068] Referring to FIGS. 13A-13P, it can be seen that the
estimated field potential of any electrode calculated in accordance
with this equation is approximately the same as the field potential
actually measured at the electrode in response to the monopolar
delivery of electrical current from another electrode to the return
electrode. In particular, two percutaneous leads carrying eight
electrodes (similar to the leads 20, 30 shown in FIG. 1) each were
introduced into a sausage loaf, which has been found to closely
simulate the introduction leads into the spinal column of a
patient. In each of the oases, current was sourced from a different
electrode in a monopolar manner and actual field potentials were
measured on each of the other electrodes. In each of the cases, a
monopolar impedance was also measured at a different electrode,
bipolar impedances were measured between the different electrode
and the other electrodes, and field potentials were estimated for
the other electrodes,
[0069] For each case, the actual and estimated field potentials
were then plotted together, as shown in FIGS. 13A-13P. The vertical
axis represents millivolts (in response to a 1 mA current sink),
and the horizontal axis represents the electrode designation
(m1-m16). The electrode having the highest field potential will be
the closest electrode to the electrode that is currently sourcing
the current (in the case where actual field potentials are
measured) or the closest electrode to the electrode at which the
monopolar and bipolar impedances are measured (in the case where
the field potentials are estimated. More significant to the
determination of the relative positions of the leads carrying the
electrodes, the electrode having the highest field potential on one
lead will typically he the closest electrode to the electrode that
is sinking the monopolar current from the other lead or the closest
electrode to the electrode at which the monopolar and bipolar
impedances are measured. For example, referring to FIG. 13A,
electrode E9 has the highest potential of the electrodes on the
second lead relative to the electrode E1 on the first lead, and
thus, is closest in proximity to the electrode E1,
[0070] Next, with reference to FIG. 14, a flowchart is shown that
illustrates the main steps that may be used to carry out and apply
the techniques described above. First, the amplitudes of the
pertinent electrical parameters are acquired at the electrodes
(block 82) For example, if the interelectrode impedance measurement
technique is used, the pertinent electrical parameters will be
impedance vectors, as described above with respect to FIGS. 4-7. If
the field potential measurement technique is used, the pertinent
electrical parameters will be actual field potentials, as described
above with respect to FIGS. 3-9. In this case, suitable stimuli,
e.g., subperception stimuli, are applied to different electrodes as
activated electrodes. While the stimulus is being applied to each
of the activated electrodes, field potentials at the non-activated
electrodes are measured. If the field potential estimation
technique is used, the electrical parameter will be an estimated
field potential, as described above with respect to FIG. 10-12. In
this case, the monopolar impedances between the electrodes and the
return electrode are measured, the bipolar impedances between the
electrodes are measured, and the field potentials at the electrodes
are estimated therefrom,
[0071] The amplitudes of the acquired electrical parameters are
then saved (block 86), and the stored electrical parameter data is
compared to previously-saved electrical parameter data for the same
electrodes (block 88). The previously-saved electrical parameter
data may have been obtained during initial implantation of the
leads, or during the last visit (several weeks or months ago) to
the doctor. Or, the previously-saved electrical parameter data may
have been obtained just a new hours or minutes ago at a time when
the patient's body had assumed a different posture position.
Regardless of when the previously-saved electrical parameter data
was obtained, the purpose of the comparison performed at block 88
is to determine if the relative position of the leads has changed,
which change in position would also have caused a relative change
in the position of the electrodes carried on the leads. Such
determination may be made by analyzing the electrical parameter
data (block 90) as described above in connection with FIGS. 4-7
(where the electrical parameter data are impedance vectors), in
connection with FIGS. 8-9 (where the electrical parameter data are
actual Held potentials), or in connection with FIGS. 10-12 (where
the electrical parameter data are estimated field potentials), to
determine whether the relative electrode orientation has
changed.
[0072] The magnitude of the difference in the compared electrical
parameter data may advantageously provide a relative measure of how
far the lead has shifted or moved since the last electrical
parameter data was obtained, if a determination has been made that
the leads (electrodes) have not shifted relative to each other
(block 90), the process returns to block 84. Advantageously, if a
determination has been made that the leads (electrodes) have
shifted relative to each other (block 90), appropriate correction
action may be taken, as needed (block 92). The corrective action
taken at block 92 may include, for example, simply tracking the
lead migration over time, so that other corrective action, e.g.,
surgery to reposition the leads, can be taken when necessary. Even
if new surgery to reposition the leads is not needed, simply
mapping the lead migration overtime will enable reprogramming of
the stimuli parameters as needed so that a desired effect can be
obtained regardless of the lead movement.
[0073] The corrective action may further include setting up
stimulation configurations and parameters for providing nominal
stimulation suitable for the electrodes in their new relative
positions. For example, the amplitude of the stimulus applied to
one electrode may be decreased if it is determined that the
electrode has migrated closer to another stimulating electrode of
the same polarity during stimulation, thereby preserving
approximately the same stimulation effect for the patient.
Alternatively, the amplitude of the stimulus applied to the
electrode may be increased if the electrode has migrated closer to
a stimulating electrode of the opposite polarity. Such amplitude
adjustments may be made manually or automatically, depending on the
mode of operation of the neurostimulation system.
[0074] Yet another corrective action that may be taken at block 92
is to adjust the distribution of the stimuli to a new location
through navigation. Navigation, as described in the previously
referenced patent documents, involves electronically shifting the
stimulus current from one group of electrodes to another so as to
shift or move the location where the patient feels the most
beneficial paresthesia, and/or receives the most benefit. Such
navigation allows the neurostimulation system to be quickly
"fitted" to a given patient. Fitting the neurostimulation system to
the patient is necessary after the system is first implanted, and
may also be necessary whenever the leads (electrodes) have moved.
The neurostimulation system provides a relatively easy way to
determine whether such lead movement has occurred, and thereby
whether a refitting is or may be necessary.
[0075] Yet additional corrective action that may be taken at block
92 in response to a determination that lead migration or postural
changes have occurred includes manually or automatically adjusting
the stimulation energy to a previously-defined optimal field
potential.
[0076] It is thus seen that the present invention uses a measure or
estimation of impedance or electric field to determine relative
lead positions for multipolar leads in a multi-lead configuration
of a neurostimulation system, e.g., a spinal cord stimulation
system. It is also seen that the neurostimulation system uses
impedance or electric field measurements or estimations to
determine relative lead positions, which impedance or electric
field measurement estimations may be used as an automated or
assistive method for setting up a programmer for navigation, other
programming, or diagnostic evaluations in spinal cord (or other
neural) stimulation. It is additionally seen that the
neurostimulation system may be directed to the storing of impedance
or electric field maps to chronically track relative lead positions
in a programmer linked to a database, along with other patient
data.
[0077] Although particular embodiments of the present inventions
have been shown and described, it will be understood that it is not
intended to limit the present inventions to the preferred
embodiments, and it will be obvious to those skilled in the art
that various changes and modifications may be made without
departing from the spirit and scope of the present inventions.
Thus, the present inventions are intended to cover alternatives,
modifications, and equivalents, which may be included within the
spirit and scope of the present inventions as defined by the
claims.
* * * * *