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BACKGROUND TO THE INVENTION
An rf (radio frequency) lesion electrode is a common instrument for use in
neurosurgery. It typically consists of a metal conductor shaft which is
insulated over its outer surface except for the surface of the electrode's
distal tip. FIG. 1 represents such a shaft 1 with insulating coating 2.
The tip of the electrode 1a is in conductive continuity with the shaft 1.
Such a shaft is introduced into nervous tissue to a target which is to be
destroyed by heat. This is done by attaching a radio frequency potential V
form an rf source 3 through the metal shaft, thereby raising the tip 1a to
rf potential. An indifferent electrode, or ground electrode, is usually
attached to the patient's body at another location, thereby completing the
current circuit from tip 1a to ground through the body of the patient. RF
current eminating from the tip 1a will therefore ohmically heat the tissue
around the tip, killing the tissue in a volume around the tip 1a which is
raised above some critical temperature. Thus we achieve a kill-zone around
the electrode's tip for the nervous tissue which surrounds the tip. The
radius of the lesion or destruction zone depends on the radius of the
electrode tip, on the temperature to which the tissue around the tip is
raised, and on the physiologic nature of the nervous tissue which
surrounds the tip. In this way localized destruction of tissue deep inside
the brain, or other nervous structures such as the spinal cord can be
achieved. This destruction often relieves pain and other nervous disorders
in a dramatic, relatively non-evasive, way.
Because temperature is the basic lesioning or destruction parameter,
temperature control or monitoring of the electrode's tip has become an
essential means for carefully grading the degree of destruction and
quantifying the lesion size. A rapid and faithful readout of tissue
temperature just outside the tip is often critical to safety and
successful results. Temperature monitoring lesion electrodes have existed
since the early 1960's. They have all involved an internally located
temperature sensor, illustrated by element 4 in FIG. 1, i.e., the sensor
has always been placed inside the electrodes tip. Usually the shaft and
tip, elements 1 and 1a in FIG. 1, are hermetically sealed stainless steel.
Temperature sensor 4 is usually of either a thermistor or a thermocouple
type, but other types are also possible. In the case that a thermistor is
used, a pair of lead wires 5 and 5a must be brought out to the hub of the
electrode 6 through electric contacts 5 prime and 5a prime. These, in
turn, are connected to a temperature measuring circuit 7 which reads out
the temperature. A cable would connect 7 to pins 5' and 5a'. A third pin
5a might be the contact for the rf source 3 to the conductive steel shaft
1. Sensing element 4 of FIG. 1 has also been of a thermocouple sensor
type. Important performance criteria for the critical temperature
measuring means is that it be accurate and fast-responding. Very often a
fraction of a degree can mean the differernce between desired and unwanted
differential tissue destruction. Speed of response can mean the difference
between detecting a boiling or charring condition and not. Therefore,
intimate thermal contact of the sensor 4 with the tip 1a is essential to
improve these characteristics.
Typical rf lesioning electrodes run between diameters of 0.3 mm and 0.7 mm
for lesioning in the brain. Lesioning in smaller neural-structures, such
as the spinal cord, requires commensurately smaller electrode size.
Temperature monitoring in the larger electrodes, roughly larger in size
than 0.5 mm, has been relatively easy to achieve. Thermistors are
available in small enough sizes and thermocouple junctions can be made
small enough to allow such temperature sensors to be placed relatively
easily inside tip geometries greater than about 0.5 mm in outer diameter.
Furthermore for electrodes with tip diameters greater than about 0.5 mm,
especially those with rounded hemispherical tips, as is used in brain
lesioning, then inaccuracies due to non-uniform heating of the tissue are
reduced. This is primarily because current densities for the larger
electrode with smooth radii are relatively small, and thus tissue heating
is rather uniform. This enables the tip to heat up in a uniform and
average fashion, and permits the temperature sensor located within the tip
to give a rather faithful representation of the overall tip temperature,
and thus the surrounding tissue.
Severe technical problems, however, have been encountered in constructing
electrodes less than tip diameters of about 0.5 mm with temperature
sensors in their tip. Electrodes of 0.5 mm or less are essential for
making lesions in the spinal cord, a procedure known as percutaneous
cordotomy, which is a very common neurosurgical procedure and which has
been performed for the last 20 years. All percutaneous cordotomy
electrodes are less than 0.5 mm in diameter, and until very recently, all
have been non-thermometric. The electrodes of Dr. Rosomoff, who initiated
the technique, were 0.5 mm in diameter and had a tip length of 2.5 mm and
a sharpened pointed tip. Dr. Mullan, also a pioneer in percutaneous
cordotomy, used electrodes which were 0.25 mm in tip diameter with a 1.5
to 2 mm exposed tip length and also a sharpened pointed tip. The rf lesion
electrodes that they used were solid stainless steel wires, and no
temperature sensors were built into them. In fact, it was commonly
believed, until recently, that temperature control for small electrodes of
the cordotomy type could not be made on a commercial basis. The
publications and advertisements of a major manufacturer of rf lesion
generating systems and electrodes, the OWL Instrument Co., Limited of
Canada, openly conceded that no manufacturer was able to make temperature
monitoring percutaneous cordotomy electrodes because of the difficulties
posed by the small size of the tip.
In the case of thermistor temperature sensors within the tip, the reason
for the difficulty was clear. Thermistors have a finite size which are not
easily available in dimensions of less than about 0.3 to 0.4 mm in
diameter. Thus this poses an immediate limitation on the outer diameter
shaft into which a thermistor can be installed. Thermocouple temperature
sensors in principal do not have such a limitation since they only require
the junction of two dissimilar metals. However, there are a variety of
difficult technical problems in both fabricating such a thermocouple
electrode and in making it suitable in accuracy and speed of thermometric
response to be usable for very small-gauge rf lesion electrodes. These
will be elaborated below after description of the construction of previous
thermocouple rf lesion electrodes.
FIGS. 2A and 2B show the ways in which thermocouple rf lesion electrodes
have been made by previous manufacturers. In FIG. 2A, a thermocouple rf
cordotomy electrode made by Radionics, Inc. is shown. Wires 5a and 5b are
dissimilar metals and their electrical junction 4 is the temperature
sensing thermocouple junction. A variety of materials are possible for 5a
and 5b such as: iron-constantan, copper-constantan, or other common
thermocouple metal pairs. In this case, junction 4 is actually contacting
electrically the metal stainless steel tip 1a on the interior surface of
the tip. it is also possible to insulate 4 from 1a, but this reduces
thermal conduction as well as speed and accuracy of temperature
measurement. The electrode in FIG. 2A has a sharpened point on tip 1a for
piercing the spinal cord, and this commercially available design, known as
the TCE Thermocouple Cordotomy Electrode, and made by Radionics, Inc. is
used in percutaneous cordotomies. Such an electrode has several technical
problems. First, it becomes difficult to make the diameter of 1 below
about 0.5 mm because the two insulated thermocouple wires must be placed
within the tube 1. Second, the sensor 4 is not at the extreme tip end of
the sharpened tip 1a, and this results in various sources of inaccuracies.
For a sharpened point, the rf current density, and thus the tissue
heating, is much greater at the very tip of the sharp point. Thus, the
sharp point may be dangerously hot, even boiling, and the rest of tip 1a
may be relatively cooler. Because the sensor 4 is placed internally in tip
1a, then it senses only the average tissue temperature around tip 1a, and
this may be significantly below that at the very tip. Such a situation can
produce dangerous inaccuracies in a critical procedure like cordotomies.
Another inaccuracy arises from finite mass and heat conduction effects in
the tip 1a itself. The metal tip of 1a takes a certain time to heat up
when tissue at the sharp point end is raised quickly, as it often is when
the tissue temperature is greater than 75.degree. to 85.degree. C. during
typical cordotomies. The walls of tube 1 also conduct heat away at a
finite rate, and this means that there is a temperature gradient between
the very sharp end of tip 1a and the portion of 1a further back up the
shaft, even in a thermal equilibrium or static thermal situation. Thus,
the sensor 4, when not exactly at the surface of the sharp point end of
1a, will never be at the temperature of the hottest, most critical region
near the very sharpest point of tip 1a. It is also true that when the
sensor 4 is internal to tip 1a, and particularly when it is removed from
the sharpest point of the surface tip 1a, then the sensor cannot respond
as quickly as desirable to the rapid temperature changes taking place at
the hottest region near the sharp point.
The above mentioned problems of thermal monitoring accuracy and speed of
sensing response become relatively more important when the size of the
electrode tip dimensions become smaller. The reasons for this are: (1)
That, as the tip becomes smaller, the rf current densities become high for
a given rf voltage, causing more unpredictable and variable spot-heating
at the region of the electrodes tip; (2) For cordotomy electrodes, with a
pointed tip and for which lesioning temperatures of 80.degree. C. and up
are common, the chance of unwanted run-away boiling at the tip becomes
more of a problem, and faithful sensing response becomes critical to
prevent disasterous damage to the patient. Often, the smaller the
electrode, then the higher required tip temperature, and the more critical
is the need for instantaneous temperature readout from the very tip end
point; (3) As the diameter of shaft 1 becomes smaller, the larger is the
ratio of the wall thickness of the shaft tubing and the diameter of the
tubing. This results in great inaccuracies caused by heat flow losses up
the shaft itself, i.e., the greater is the thermal gradient in the tip 1a
itself, and, thus, the greater the difference between the pointed tip-end
temperature and that at sensor 4.
There is a great need for temperature monitoring rf lesion electrodes of
very small dimensions, viz. from about 0.5 mm to about 0.2 mm for
cordotomies, and down to 0.1 mm or less for neurophysiolic research (<0.1
mm). The only such electrode, up to the time of the present invention, was
the Type TCE Thermocouple Cordotomy Electrode System of Radionics, Inc.,
and that was of the design shown in FIG. 2A with a tip diameter of 0.5 mm.
This was the situation despite the obvious need and the large number of
cordotomies done around the world each year. This history is testimony to
the difficulty in making a thermometric rf electrode of smaller size.
In passing, I note that, in FIG. 2A, the rf voltage source 3 activates the
shaft 1, and thus the tip 1a. This voltage source is usually an externally
located electronic circuit which attaches to the electrode via a cable.
The temperature measuring circuit 7 is just a microammeter circuit for
measuring the thermionic potential difference across the thermocouple
junction 4. RF filter 8 blocks the rf voltage from 3 from getting into the
delicate circuit 7.
Often in percutaneous cordotomies, the insulated lesion electrode
telescopes through an uninsulated guide needle which serves as the return,
or indifferent, electrode. Other voltages, such as for stimulation can be
supplied to the electrode via the rf voltage connection. Also, electronic
recording apparatus may be connected to the electrode prior or after rf
lesion making. These techniques are standard and will not be elaborated in
detail here.
It is worth noting, further, as background to this invention, that there
has been only three other reported thermocouple rf lesion electrode
systems. The system of VandenBerg, published in 1960 and commercialized as
the Coagrader System by Vitatron in the same year, utilizes a thermocouple
electrode design is shown in FIG. 2B. In it, the junction 4 is again
internal to the tip 1a, and is made between the stainless steel tip
material and the constantan metal wire 5. VandenBerg, et al shows a
blunt-ended electrode of 2 mm tip diameter and 2 mm tip length, and the
junction sensor 4 well inside the tip 1a. The objective of VandenBerg's
electrode was for relatively large-volume lesion making in the brain, not
for very small lesion making as required for example in the spinal cord.
For the purposes of the very small electrodes, that is smaller than about
0.5 mm, VandenBerg, et al's electrode and internal sensor construction
would be inadequate for the reasons cited above. Furthermore, the
internalized thermocouple junction location, as shown by VandenBerg, is
difficult to fabricate in small gauge electrodes. For sharp tip designs,
as discussed above and used in cordotomy applications, VandenBerg's
construction is especially disadvantageous since it accentuates the above
cited problems. It might be noted that the Coagrader System and electrode
of VandenBerg, et. al. survived only about four years on a commercial
basis from about 1968 until 1972, suggesting the practical difficulties
encountered by their system, as contrasted with the success of the
Radionics and OWL systems.
The two other systems are: the Riechert-Mundinger Lesion Generator System
as described by Mundinger et. al. and commercialized by F. L. Fischer; and
the Leksell System. Both these systems utilized electrodes of the type
shown in FIG. 2A, i.e. internalized thermocouple temperature sensors. It
is notable also, that both of these systems offered only stereotaxic
lesion electrodes for use in the brain, these electrodes having a minimum
diameter of 1.8 mm. Of the five manufacturers or rf lesion electrodes and
lesion generators, the companies OWL Instruments Ltd. Vitatron, F. L.
Fischer, and Leksell never offered a thermometric cordotomy lesion
electrode, and only the company Radionics, Inc. did offer such an
electrode recently, but it was limited in size to greater than 0.5 mm in
diameter, and in performance because it was of the design shown in FIG.
2A.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2A and 2B are views in partial section and block form of prior
art of lesion electrodes;
FIG. 3A is a comparable view in partial section and block form showing one
embodiment of the invention; and,
FIGS. 3B through 3F are views in partial section of the distal end portions
of other embodiments of the lesion electrode of the present invention.
DESCRIPTION OF THE INVENTION
The present invention relates to a thermocouple rf lesion electrode
construction design which overcomes the problems discussed above, and
which enables commercially viable production of practical, accurate, and
fast-acting electrode with very small tip dimensions, as is vitally needed
in percutaneous cordotomies, other micro neurosurgical procedures, and
neurophysiological research. The design has been used to produce a 0.25 mm
diameter cordotomy electrode with sharpened tip, and use of this electrode
in its first 20 clinical trials has recently been reported by Levin and
Cosman.
FIG. 3 illustrates sectional views of the new electrode design for a
variety of embodiments which might be used in different mini or
microelectrode applications. FIG. 3A again has a conductive metal tubular
shaft 1 which is insulated by material 2 except for bare tip 1a. Inside 1
is another metal conductor element 5 which is insulated from 1 by material
9 except for a thermocouple junction 4 of metals 5 and 1 at the extreme
sharp point of tip 1a. The junction is positioned and so made that
portions 5" and 1" of conductors 5 and 1, respectively, are part of the
external surface of the rf lesion tip 1a at the location of the junction
4. This might be referred to as an externalized thermocouple junction, in
contrast to the internalized sensors illustrated above in FIGS. 1 and 2.
The advantages of this externalized thermal sensor are manifold. By having
the two contacting thermocouple elements of the sensor 1" and 5" joined
electrically at a point 4 which is in part a portion of the active rf
surface 1a of the electrode, then the sensor is actually in contact
physically with the adjacent tissue that is being heated. Since the
junction 4 in FIG. 3A includes part of the external surface, then the
junction potential generated across it is a close representation of the
real temperature at the surface of the tip. This is very important for
very small lesion electrodes such as for cordotomies, since for them the
lesion temperatures are typically very high, even close to the boiling
100.degree. C. point. For example, as noted by Levin and Cosman, who used
the 0.25 mm electrode in many cordotomies, the tip is raised to 80.degree.
and 85.degree. routinely to achieve a proper lesion. At that point, any
non-uniform hot spots must be monitored to prevent runaway flash heating
to the boiling point of 100.degree. C.
Another advantage of the externalized sensor in FIG. 3A, in contrast to
previous internalized sensor constructions, is that the sensing junction 4
is located at the sharpest point of tip 1a; in fact, it is the very
pointed end itself. This means that it is sensing the temperature where
the heated tissue is hottest, i.e. where the rf current density is
greatest. This is the first location that boiling or charring will occur,
and thus, for safety reasons, it is the most important point to be
monitored. That one is measuring the temperature of the hottest point, and
not an average tip temperature, as is the case for internalized sensor
designs, means that one has again a more faithful measure of the lesion
process, and more control thereof.
Another advantage of the design of FIG. 3A is the fact that the mass of the
sensor junction 4 is very small so that the speed of response is
commensurately fast. The wall thickness and associated thermal conduction
up the shaft makes no difference to the accuracy of the temperature
readout, since there are no gradients between the tissue and the location
of the sensor.
Yet another advantage of the design of FIG. 3A is the simplicity of its
construction and its amenability to construction of very small diameter
electrodes. The central metal element 5 is easily telescoped into a tube
1, and its end is easily welded or soldered to the tube at the tip end.
The point may then be sharpened easily, if a point is desired. The
junction 4 being external, means that its integrity is easily seen,
compared to internalized designs. Electrodes of the type in FIG. 3A have
already been made down to 0.1 mm diameter, and smaller ones are possible.
Such sizes are difficult to make if one begins with a closed end tube, and
then inserts a thermocouple wire pair down from the open end to the
interior of the tip. Such an internalized design then has all the
disadvantages cited above.
We note that tubing 1 is usually stainless steel, and wire 5 is usually
constantan, but other thermocouple pair materials may be used. We also
note that we do not mean to exclude from our scope designs such as in FIG.
3A, but where a plating of metal, such as gold, is put on the surface of
the tip and over the junction 4. Such a minor interface does not alter the
design feature discussed here. The tip 1a may not be sharpened, but it may
be rounded or hemispherical as the application governs. The tip need not
be straight either. It may have a permanent hook in it, as is needed in
certain procedures, or it could even have a flexible tip portion 1a . Such
an electrode is shown in FIG. 3B. In the latter case, the end of shaft 1
may, for example, be a coiled spring 1c which has a permanent arc in it,
thus making it flexible. Wire 5 may also be coiled inside 1 to give it
flexibility. Such a coiled spring has been reported by Zervas, et. al. and
by Radionics, Inc., but, in both cases, these previous designs had an
internal thermocouple sensor within the tip. In FIG. 3B, the externalized
thermocouple junction 4, where the two thermocouple metals are
electrically fused, is part of the external surface of the electrode tip.
We note in FIG. 3B, that the same thermocouple readout system as shown in
FIG. 2B is employed.
FIG. 3C illustrates a variant of the externalized thermocouple rf electrode
tip. In this case, instead of a tube, element 1 is a solid elongated
element, perhaps a wire, which runs along side element 5 throughout the
length of the electrode. Metals 1 and 5 are fused or electrically bonded
at the distal end, forming junction 4. The portions 1" of 1 and 5" of 5
at the distal end of 1 and 5 form the tip of the electrode, and they form
a portion of the external surface of the tip also. The junction is thus
intimately and integrally associated with the tip and its surface.
Insulation 2 may insulate 5 and 1 from each other, and also serve as the
coating for the rf electrode itself, except for the exposed surface at the
tip. Wires 1 and 5 may be individually insulated and twisted together
along the length of the electrode for stability. The point may be
sharpened as shown, but may be other shapes as well.
FIG. 3D illustrates a design similar to FIG. 3A involving a tubular metal
element 1, but in this case the electrode tip has a blunt, rounded end.
The metal wire 5 comes straight out the end 1b of tube 1, and the junction
4 is made by soldering a solder volume 10 between 1" and the tip end 5" of
element 5. Again the distal surface of 5" is part of the external surface
of the electrode tip, and the solder 10 is also part of the external
surface. Thus the junction potential between 5" and 1" is derived from
those between 5" and 10 and 10 and 1". Because these junctions are all on
the surface of the electrode, we achieve a faithful and rapid
representation of adjacent tissue heating. In this case, junction 4
comprises the fusing of 5", 10", and 1", and it is still made between
surface-exposed elements.
FIG. 3E shows yet another variant of the externalized thermocouple rf
electrode. Element 1 may be a steel wire, providing stiffness and a hard,
sharp point. Insulation 2' between 1 and 5 may be a thin coating or
microscopic layer. Element 5 may be a thin wire of, say, constantan, and
it may be fused to the tip 1" of 1. The portion 5" of 5 which joins to 1"
may be a coating of, say, constantan which is ultra thin. The entire tip
may be micro-etched to make a point of micron dimension. Such an electrode
would be used in neurophysiology to thermally destroy perhaps individual
cells with thermal monitoring. This illustrates the very fine gauges of
and the smallness of rf electrodes made with the externalized thermocouple
junction concept. Thus, such an electrode as in FIG. 3E may be made
entirely by a series of insulation and metal electro-depositions or
evaporations to yield very uniform and simply constructed thermocouple
micro rf lesion electrodes; viz, materials, 1b, 2', 5, and 2 may be
entirely built up from substrate depositions, with appropriate maskings,
to yield the final electrode. When such an electrodes dimension reaches
the micron range, it can be used for recording cellular electrical
activity, as well as stimulating the cells electrically and making heat
lesions. Thus such an electrode can serve multiple functions which are
important in neurophysiology.
Finally FIG. 3F shows an externalized thermocouple rf electrode wherein the
thermocouple wires 5a and 5b are brought up to the electrode tip and are
of different material from the shaft 1 or the shaft tip material 1a. This
may be done to use, for example, iron and constantan for 5a and 5b,
respectively, and stainless steel for shaft 1 and tip 1a. However, the
concept of the externalized junction may still apply, as shown in FIG. 3F.
Note, that the very distal tip surfaces of 5a and 5b, 5a' and 5b',
respectively, are part of the external surface of the electrode tip 1a.
The thermocouple junction 4 is between 5a and 5b just near the surface
portions 5a' and 5b', and thus 4 is again in intimate thermal contact with
the adjacent tissue to be heated by the rf current. One way of
implementing such a construction is by drilling a hole in the tip 1a,
threading the wires 5a and 5b through the hole, then fusing them together
by welding or soldering techniques, and then grinding them flush with the
rest of tip 1a. Again, our invention includes the addition of a thin metal
coating over the entire tip 1a and junction areas 5a', 5b', and 4, so as
to protect these areas from corrosion or other effects. In this sense, we
may say that a portion of each of the thermocouple elements near their
junction substantially comprises at least a portion of the external
surface of the rf lesioning electrode tip.
Having described in detail various embodiments of my invention, it will now
be apparent to those skilled in the art that numerous modifications can be
made therein without departing from the scope of the invention as defined
in the following claims.
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