Production Of Focal Brain Lesions By Inductive Heating

Abstract

A metallic pellet or seed is physically implanted into the brain in a position where a brain lesion is to be produced. The seed is composed of a metallic alloy of predetermined composition capable of being inductively heated by radio frequency energy. By virtue of the composition of the alloy, the seed reaches but does not exceed a predetermined temperature, and the desired size and degree of lesion is controlled as a function of time during which the radio frequency field is in operation.

Description

This invention relates generally to the known art of radio frequency thermo-coagulation of human tissue, and more particularly to improved means and methods of thermo-coagulative neurosurgery.

BACKGROUND OF THE INVENTION

With the advent of modern neurosurgery, the surgeon has gained access to the depths of the brain. In the past few decades, a great effort has been expended in the attempt to find a safe and effective means for producing controllable subcortical lesions. The need for this is as great as the spectrum of disease potentially amendable to such therapy. Such conditions include movement disorders, of which Parkinson's Disease is an example, intractable pain, focal epilepsy, various vascular malformations, and certain cancers, such as those arising from the breast and prostate, where palliation may be achieved by hypophysectomy.

Many imaginative lesion-producing techniques have been developed, but so far none has been entirely satisfactory. Most of the systems developed have necessitated physical connections from an external energy source to a probe or electrode, which could not remain in place over a period of time without the risk of infection. The major disadvantage of other systems, such as proton beam radiation, focused ultrasonics, lies in their destruction of normal brain tissue as well as that in the target area.

Induction heating is a well-established industrial method which permits the rapid and clean through-heating, melting, or precise surface hardening of metals by an external alternating electromagnetic field. This field induces currents in the metal eddy which are opposed by the resistance of the metal, resulting in the generation of internal heat. A second effect adding to the dissipation of energy as heat is the induction of hysteresis loops for the alternation of the magnetic poles within the metal. No physical contact between the source of electromagnetic energy and the metal is required.

In the present state of the art, it is known that if a small piece of appropriate metal is implanted into the brain, and the head then introduced into an electromagnetic induction field of radio frequency, the implanted metal can be heated at a frequency and power level innocuous to the body and brain tissue. In addition, it is known that the heating can be accomplished in increments over a period of time, as required to achieve the desired lesion.

A typical utilization of the known art involves the use of an industrial induction generator operating in the 300-400kc/s range with a plate power input of up to 23kw is modified for medical application, to include an elliptical single-turn one-fourth inch by one-half inch water-cooled copper coil, so that human heads can be accommodated. The induction coil current flow is approximately 500 amperes. Insulation of the coil is required to prevent radio frequency burns, and a suitable plastic resin compound is utilized for this purpose and applied in dip coats.

In the industrial use of induction heating, in which the mass of metal is large and lies in close approximation to the coil, the efficiency of the system approximates 60 percent, owing to a relatively efficient "coupling". In neurosurgical applications, the efficiency of the system ranges from 0.1 percent to 0.001 percent because of the small mass of the implant and its distance from the coil.

When an alternating current is induced in metal, the unique properties of induction heating apply. One of these properties is the skin-effect phenomenon where the current flow and heating are concentrated near the surface of the metal and fall off exponentially toward the center. This skin-effect is enhanced at higher frequencies. In a 300-400 kc/s range, an intense, fast, and localized heat pattern can be produced. This frequency, at the power level used, is innocuous to living tissue. In the evaluation of lesions in over 200 experimental animals and 12 human beings, there has been no observable effect of the RF induction field on anything other than the implant.

The ideal metallic implant, which can be termed an electroseed, must be composed of a material that will heat well, but neither react nor cause reaction in brain tissue, and be of suitable size and shape for surgical implantation. It is known that some intracerebral missile fragments tend to migrate through the brain over a period of time, but in the case of electroseeds, this behavior does not occur. Unheated electroseeds have been observed to remain in position after follow-up periods as long as two years. After the production of a lesion, pathologic examination reveals that the electroseeds are quite anchored by surrounding thermo-coagulated protein.

Subcortical implantation is a simple surgical procedure which is usually performed under local anesthesia through a small twist drill hole in the skull. A No. 14 gauge needle is guided "freehand" or by stereotaxis to the target area. The needle stylet extends beyond the distal end of the needle so that when it is withdrawn, a tissue defect is created. Needle position is verified by polaroid X-ray films or by a fluoroscopic image intensification unit. Air may be injected into the ventricular system to serve as a point of reference. The electroseed is then dropped into the needle and extended from the distal end thereof into the preformed tissue defect.

Following a survey of ferromagnetic metals, iron was found to be the most readily available material for an electroseed. Carbon steels have excellent heating properties, but caused too much tissue reaction. Although this reaction could be alleviated by coatings of teflon enamel or gold, the possibility remains of scratching the protective coating during the implantation. Even alloy steels with a chromium content of over 11 percent (stainless steels) undergo a typical rusting reaction if the oxide coat is disturbed or not allowed to form. The most satisfactory material known in the prior art is type 430 stainless steel having a chromium content of 17 percent. Heating properties have been enhanced by pot annealing the steel at a temperature of 1,600.degree. F. with very slow cooling. Because the electroseed is implanted by extrusion through a needle, it requires a cylindrical or spherical configuration. Experimentation has indicated that solid cylinders with length-to-diameter ratios greater than unity heat more rapidly than spheres, hollow cylinders, or cylinders composed of rolled foil.

In the determination of electroseed parameters related to maximal heating in a radio frequency induction field, the fundamental assumption was made that an electroseed heating in air would bear a constant relationship to one heating in-vivo. Prior to clinical experimentation on human beings, electroseeds have been tested in dummy heads constructed of plastic and filled with gelatin. Heating of the implanted seeds is accomplished by placing the head within the RF-induction coil.

It is known that brain neurons are quite heat sensitive, and that point of contact temperatures over 120.degree. F. destroy the cells. It is therefore essential to produce lesions at contact temperatures high enough to destroy the neurons and coagulate protein, but not so high as to cause liquification of brain tissue. Unfortunately, it is not possible in the present state of the art to accurately predict lesion size and shape, although it is possible to gradually increase the lesion size until the desired clinical result is obtained. In this fashion, patients with intractable pain and motion disorders have been successfully treated.

Several factors influence heat production in the electroseed. Theoretical calculations indicate that the rate of heat generation in electroseeds increases with the diameter of the same under all conditions investigated. The rate of heating increases linearly with the length of the electroseed in the practical range of from 0.5 to 1.0cm. There is a consistent increase in the rate of heat generation with increase in magnetic permeability of the metal comprising the electroseed. The rate of heating may increase or decrease with an increase in resistivity, depending upon the combination of other parameters, such as permeability, radius and frequency. Although one might expect that increasing hysteresis loop area would increase electroseed heating, it has been found that following pot annealing with its concomitant decreased hysteresis loop, heating is enhanced. Hysteresis heating is not as significant as that due to eddy currents.

Experimental in-air determinations of electroseed heating in regard to angular orientation of the plane of the coil and radial position in the coil, have been made. It has been determined that heating rate is maximum when the long axis of the electroseed is at an angle of 90.degree. to the plane of the coil, and degrades rapidly to 0.degree. at which time heat production is nil.

BRIEF DESCRIPTION OF THE PRESENT INVENTION

One of the principal problems encountered in the practice of the above described therapy is the obtaining of accurate control of the degree of and extent of the coagulation produced in tissue being treated. Since the degree of coagulation obtained is a function of the degree of heat produced by the electroseed, and the amount of time for which the heat is transmitted to the area to be coagulated, it is apparent that the electroseed ideally should be capable of maintaining a reasonably fixed temperature over a substantial period of time, once it has been initially heated from body temperature. Where this is not the case, the heat produced by the electroseed over a given period of time is not only a function of the radio frequency energy supplied and the amount of time over which the energy is supplied, but also a function of how long the seed is heated for a given exposure.

Type 430 stainless steel has been the material used in the past, being the most satisfactory material known. However, it continues to heat with increased power to approximately 2,000.degree. F. in air. The ideal temperature for producing coagulation varies between 150.degree. to 200.degree. F. Since the degree of "coupling" between the radio frequency generator and the thermoseed is less than 1 percent, it will be readily appreciated that controlling the heat of the thermoseed by regulating power supplied by the radio frequency generator is extremely difficult to do in practice.

It is, therefore, among the principal objects of the present invention to provide a thermoseed which will heat only to the desired range in a surplus of radio frequency energy supplied.

Another object of the invention lies in the provision of an improved thermoseed of the class described which will possess all of the advantages of known stainless steel thermoseeds, and which may be fabricated using known techniques.

A further object of the invention lies in the provision of an improved thermoseed of the class described formed from alloys having a predetermined Curie point within the desired range for effecting coagulation.

In the drawings,

FIG. 1 is a graph showing the heating with time of electroseeds formed from various alloys in air in the presence of excess radio frequency power.

FIG. 2 is a graph showing the heating of type 430 stainless steel in albumin.

FIG. 3 is a similar graph showing the heating of type 430 stainless steel in rat cortex.

FIG. 4 is a similar graph showing the heating of type 430 stainless steel in cat cortex with normal blood flow.

FIG. 5 is a graph showing the heating of a palladium nickel alloy in albumin.

FIG. 6 is a similar graph showing the heating of the same alloy in rat cortex with no blood flow.

FIG. 7 is a similar graph showing the same palladium-nickel alloy heated in cat cortex with normal blood flow.

FIG. 8 is a similar graph showing the heating of a copper nickel alloy in albumin.

FIG. 9 is a similar graph showing the heating of a nickel-chromium alloy in albumin.

FIG. 10 is a similar graph showing the heating of a nickel-iron alloy in albumin.

FIG. 11 is a similar graph showing a slightly different nickel-iron alloy heated in albumin.

Referring to FIG. 1 in the drawing, it will be understood that the illustrated examples are for purposes of illustration only, and not to be construed as limiting with respect to the extent of the present invention. It will be observed that when heated in air, the Curie points of each alloy are reached within a range of 60 to 200 seconds after the supply of power is initiated, and in each case is above 300.degree. F., or far in excess of the desired operative temperature of 150.degree. to 200.degree. F. Although the initial rise in temperature is relatively rapid, it will be apparent from a consideration of the graph that the total amount of heat transmitted for purposes of coagulation is much less during the first 20 seconds than during the second 20 seconds in each case. Excess operating temperature is reached in each case within the first 20 seconds. The highest temperature reached is 400.degree. F., with the copper-nickel alloy.

This performance can be contrasted with the showings in FIGS. 2 to 4 in the drawings. Type 430 stainless steel, the most widely used of the known materials in the art, heats almost instantaneously to 800.degree. F., and within a minute reaches 1,500.degree. in albumin, a protein material. Using the same material in rat cortex and cat cortex, heats as high as 1,900.degree. are obtained within 3 minutes.

FIG. 5 illustrates a similar experiment substituting a thermoseed formed of an alloy consisting of by weight 78.7 percent palladium, and the remainder nickel. When immersed in albumin, the Curie temperature of approximately 230.degree. F. is reached in 20 seconds, and the temperature remains substantially constant thereafter, until 2 minutes has elapsed.

In FIG. 6, placing a thermoseed of this alloy in rat cortex with no blood flow, results in a substantially constant Curie temperature of 206.degree. F., a very usable value.

As seen in FIG. 7, in cat cortex, with normal blood flow, a still more usable value of 203.degree. F. is obtained.

FIG. 8 illustrates the heating of a copper-nickel alloy thermoseed, the copper forming 25.8 percent by weight of the alloy. It will be observed that heating in this case to the Curie point requires 3 minutes, and a maximum of approximately 290.degree. is reached at that point. However, because of the lack of flatness in the heating curve, this alloy is less desirable than other alloys described herein.

FIG. 9 illustrates a nickel-chromium alloy, in which nickel comprises 93.75 percent by weight of the alloy. Here, the albumin test reveals that the thermocoagulation temperature is reached after approximately 1 minute, and the temperature continues to rise relatively slowly thereafter until approximately 10 minutes, at which point the temperature exhibits a marked drop.

FIG. 10 illustrates a nickel-iron alloy, nickel comprising 31.12 percent by weight of the alloy. Coagulation temperature is reached in approximately 30 seconds, and maximum temperature is approximately 300.degree., again in albumin, a sharp drop again being exhibited after 5 minutes.

FIG. 11 illustrates a slightly modified nickel-iron alloy, in which the nickel comprises 31.08 percent by weight. Here thermocoagulation temperature in albumin is reached almost instantaneously, and maximum temperature of approximately 330.degree. is reached after 9 minutes. However, the rise in temperature is erratic, exhibiting heat loss between 3 and 4 minutes, and between 6 and 8 minutes over somewhat similar patterns.

In the determination of electroseed parameters, related to maximal heating in an RF-induction field, the fundamental assumption was made that a given electroseed heating in air would bear a constant relationship to one heating in-vivo. Early comparative in-air testing was performed using a simple alcohol thermometer to record temperature. The more precise determinations of variation in heating in albumin and rat and cat cortex were performed using a 35 gauge spirally wound copper-constantan thermocouple connected to a grass model 5 polygraph with a Model 5 P5C dc preamplifier. The thermocouple itself was not heated by the RF field. Prior to clinical experimentation on human beings, electroseeds were tested in dummy heads constructed of plastic and filled with gelatin.

Thus, the pattern of heating of the alloys illustrated in FIGS. 8 through 11, inclusive, may be reasonably expected to be duplicated at substantially lower temperatures in cortex, with reasonable utility from the standpoint of obtaining relatively flat portions of curves in the desired thermocoagulation temperature range. With the temperature of the thermoseed maintained at a useable level, the control of the formation of the lesion is more closely regulated to the exposure time within the RF field, a value which can be simply determined by stop-watch, and controlled by interrupting the flow of current to the RF generator. Operating in this manner, the RF generator may be allowed to supply an excess of power to the thermoseed, and the presumption is made that the thermoseed will very quickly be operating at its Curie point.

For practical and convenient testing, I have found that operating the thermoseed in albumin maintained at 30.degree. C. (78.75.degree. F.), a very close approximation can be made of its behavior in brain cortex with normal blood flow. This may be conveniently done by immersing a basin containing the albumin in a temperature controlled bath. Comparing FIGS. 5, 6 and 7 in the drawing, using a palladium-nickel alloy, it will be observed that the Curie point in albumin is approximately 238.degree. F. The Curie point in rat cortex with no blood flow is 206.degree. F., and in cat cortex with normal blood flow is 203.degree. F. Thus, the actual difference is approximately 30.degree. F., which can be taken into account insofar as the predicted Curie temperature in brain cortex with normal blood flow is concerned. This relationship has been firmly established in multiple experiments and designates the albumin standard as a basis for accurate determination of thermoseed heating in brain tissue.

I wish it to be understood that I do not consider the invention limited to the precise details shown and set forth in this specification, for obvious modifications will occur to those skilled in the art to which the invention pertains.

Claims

I claim:

1. In the method of producing a lesion in living brain tissue by inserting a thermoseed into the tissue in the area where the lesion is required, and subjecting said thermoseed to an externally produced radio frequency field, the improvement comprising: implanting in living brain tissue an electroseed formed of a metallic alloy having a physiologic Curie point of approximately 238.degree. F. as measured in an albumin standard solution, and utilizing this implant to produce coagulative lesions in brain tissue in a temperature range of 110.degree. through 212.degree. F.

2. As a new article of manufacture, a brain implant for producing coagulative lesions composed of a metallic alloy, said alloy comprising at least 18 percent nickel by weight and the balance selected from the group consisting of chromium, palladium iron and copper and having a Curie point in air between 250.degree. and 300.degree. F.

3. A brain implant in accordance with claim 2, composed of an alloy comprising 18 to 24 percent by weight of nickel.

4. An electroseed in accordance with claim 2, composed of an alloy consisting of approximately 78.7 percent palladium by weight, and the balance nickel.