EP0564530B1 - Method and device for placing a liquid between electrodes in a shock wave apparatus - Google Patents

Abstract

A method and a device for producing an electric discharge between two electrodes are described. The method is characterized in that the electric arc resistance is substantially reduced at least between the electrodes to a resistance value which is close to or even lower than the critical resistance. This is achieved by placing between the electrodes an electrically conductive liquid medium (32) held in a substantially sealed container (30) surrounding the electrodes, whereby the electrical discharge rate between two electrodes can be improved while completely or substantially completely eliminating the latency time.

Description

The invention essentially relates to a method and a device interposing an electrically conductive liquid between electrodes and shock wave apparatus with application thereof.

It is known from the document RIEBER US-A-2 559 227 a device for generating shock waves of high frequency, comprising a truncated ellipsoidal reflector 80 in which shock waves are generated by discharge or electric arc between two concurrent electrodes at the first focal point of the ellipsoid, so as to destroy a target placed at the second focal point of the ellipsoid, located outside of the truncated reflector 80 (see FIG. 3 and column 7, line 51, column 9, line 30) .

Electrodes 12 and 13 are made of highly conductive material such as copper or brass and are mounted on an insulator 26 which is pivotally supported by means of a device 11a, 11b, so as to adjust the spacing between them (see column 4, lines 42 to 53 and column 8, lines 40 to 47).

When using the RIEBER device or similar device, the discharge or electric arc between the electrodes is produced and, by the sudden discharge of a capacitor 11, by the closing of a high voltage switch (see FIG. 2b) . According to the RIEBER device, the circuit between the electrodes includes a capacitor, as well as an associated self-inductance. It has been observed that the discharge of the capacitor is of the damped oscillatory type. In other words, the capacitor will discharge and then recharge in the opposite direction at a lower voltage than the initial voltage, which is very high and of the order of 15,000 to 20,000 V, then again recharge in the direct direction until 'when the charges contained in the capacitor have been exhausted.

Simultaneously, an electric arc is established between the two electrodes, the current of which will also, consequently, be of the damped oscillatory type, as is well conceived with reference in Figure 1a, 1b and 1c. Thus, FIG. 1a represents the timing diagram of the voltages, while FIG. 1b represents the timing diagram of the currents established in the RIEBER type discharge circuit. It can be seen that when the circuit is closed at time t₁, the voltage across the electrodes rises sharply to the value of the voltage across the capacitor (see Figure 1a). A weak current is installed between the two electrodes (Figure 1b) because on the one hand, the liquid in which the electrodes are immersed, usually water, is always slightly electrically conductive, and on the other hand, for reasons of safety and ignition of the arc, a high resistance is arranged in parallel with the electrode supply capacitor.

After a certain time, that is to say at time t₂, which is called the latency time, the arc is established between the electrodes. At this instant, the current increases sharply by several KA as is clearly visible in FIG. 1b. We know that the arc consists of a plasma whose resistance is extremely low (of the order of 1/100 or 1/1000 ohm) and it is the low value of this resistance which explains the importance of current (Figure 1b) and voltage (Figure 1a) oscillations during the discharge of a capacitor in an RL type circuit.

The energy contained and dissipated by the arc contributes to the vaporization of the liquid in which the electrodes bathe, usually water, to the creation of a vapor bubble and consequently to the formation of the shock wave. The more quickly this energy is dissipated, the more effective the shock wave will be.

It can thus be seen that, due to the oscillatory nature of the current, as shown in FIG. 1b, the energy supplied to the external medium takes place gradually as is clearly illustrated in FIG. 1c.

It is thus understood that the faster the vaporization of the liquid, in particular of the water, the stronger the pressure wave and the shorter its rise time.

Thus, for a large quantity of liquid, in particular water, to be vaporized, it will be necessary to suddenly deliver a large energy.

However, almost all of the currently known devices result in discharges which are all of the damped oscillatory type, as shown in FIGS. 1a and 1b, resulting in a gradual dissipation of energy over time (FIG. 1c).

In the prior document EP-A-0 296 912 of the depositors, a first solution has been proposed for delivering suddenly or in a relatively short time most of the energy stored by the charge of the capacitor of the discharge circuit between two electrodes . To do this, it has been proposed to increase the electrical resistance to the passage of the electric arc at least between the electrodes by interposing an insulating element (32) with high resistance, between the electrodes 12, 14 generating the arc. . This solution is entirely satisfactory for generating shock waves whose initial pressure wave is substantially spherical.

However, this prior solution is difficult to achieve mechanically because of the small size of the electrodes and the mechanical resistance to shock waves. In addition, the problem of the latency time is not resolved since it only aims to improve the discharge regime once the electric arc is established which does not improve the reproducibility of the discharge and so of the shock wave.

Thus, the main object of the present invention is to solve the new technical problem consisting in providing a solution allowing the sudden delivery in a relatively short time of most of the energy stored by the charge of the capacitor of the discharge circuit. between two electrodes, completely or substantially completely eliminating the latency time usually required to generate an electrical discharge between the electrodes.

The present invention also aims to solve the new technical problem consisting in providing a solution making it possible to completely or substantially completely eliminate the latency time of generation of an electric discharge between two electrodes while considerably improving the reproducibility of the shock wave thanks to a significant improvement in the localization of the generation of the electric discharge.

The present invention also aims to solve the new technical problem consisting in providing a solution making it possible to completely or substantially completely eliminate the latency time of generation of the electric discharge between the electrodes, while carrying out an electric discharge of the type critical damping resulting in sudden or relatively short delivery of most of the energy stored by the charge of the discharge circuit capacitor between the electrodes.

The present invention also aims to solve the aforementioned new technical problems while providing a solution making it possible to reduce the wear of the electrodes, and by limiting the importance of the modifications to be carried out on previously existing devices.

The present invention also aims to solve the new technical problems stated above in an extremely simple manner, usable on an industrial scale, in particular in the context of extracorporeal lithotripsy.

All these new technical problems are solved for the first time by the present invention in a satisfactory, inexpensive manner, usable on an industrial scale, in particular in the context of extracorporeal lithotripsy.

Thus, according to the present invention, there is provided, according to a first aspect, a method for improving the electrical discharge regime produced in a liquid medium such as water, between at least two electrodes generating such a discharge, characterized in that that the resistance to the passage of the electric discharge is considerably reduced at least between the electrodes to bring it to a resistance value close to the critical resistance of the discharge circuit by interposing at least between the electrodes an electrically conductive liquid medium contained in an essentially closed reservoir surrounding the electrodes, the resistance of the electrically conductive liquid medium being less than or equal to 1/10 of the resistance value of normally ionized water, serving as a reference.

This reservoir is made of a material which does not substantially disturb the propagation of shock waves. Examples of such a material are a latex, a silicone, or a metallic strip, which is well known to those skilled in the art.

According to another advantageous embodiment, the electrodes support the reservoir, and are removable. They can therefore be supplied with the tank, the assembly therefore being consumable and disposable, which reduces maintenance costs compared to previous solutions.

According to a particularly advantageous embodiment, an electrically conductive liquid medium is used, the electrical resistance of which is less than or equal to 1/100 of the electrical resistance value of the normally ionized water serving as a reference. More preferably, the electrical resistance of the electrically conductive medium according to the invention, expressed in linear resistivity, is less than about 15 Ohm.cm. Electrically conductive liquid media can consist of an aqueous or non-aqueous electrolyte. As aqueous electrolyte, mention may be made of water charged with ionizable compounds, in particular salts, such as halide salts, for example NaCl, NH₄Cl, sulfates or nitrates with alkali or alkaline earth metals or transition metals. such as copper. A currently preferred electrically conductive aqueous liquid medium consists of salt water at 100 or 200 g / l having respectively a linear resistivity value of 10 and 5 Ohm.cm.

A more preferred electrically conductive aqueous liquid medium comprises approximately 10% by weight of NaCl (approximately 100 g / l) and between 0.5 and 2% by weight of phosphate salt, in particular disodium phosphate (Na₂HPO _4.12 H₂O) . The linear resistivity of such an electrically conductive medium is approximately 8 Ohm.cm. It is advantageously possible to add a dye, for example methylene blue, in a proportion of 2 mg / l, in order to make it possible to recognize a possible leak from the reservoir.

As non-aqueous conductive liquid medium, there may be mentioned conductive oils, made conductive by the addition of particles conductive, such as metal particles, which are well known to those skilled in the art.

According to a second aspect, the present invention also provides a device for improving the electrical discharge regime produced in a liquid medium such as water, between at least two electrodes generating such a discharge, characterized in that it comprises means for reducing the resistance to the passage of an electrical discharge at least between the electrodes to bring it to a resistance value close to the critical resistance comprising a substantially closed reservoir surrounding the electrodes filled with an electrically conductive liquid medium. The material of this reservoir is provided so as not to substantially disturb the propagation of shock waves. In particular, this reservoir can be made of latex, silicone, or a metallic strip. It may have the form of a membrane surrounding the electrodes.

According to a third aspect, the present invention also relates to an apparatus for generating shock waves by electrical discharge between at least two electrodes immersed in a liquid discharge medium, in particular of the extracorporeal type, characterized in that it comprises a device to improve the discharge regime as defined above. According to an advantageous embodiment, this device comprises a truncated ellipsoidal reflector comprising an internal focal point where the shock waves are generated by electrical discharge between at least two electrodes and a focal point external to the reflector where the shock waves are focused, said ellipsoidal reflector truncated being filled with a liquid coupling medium. In this case, the above-mentioned essentially closed reservoir is provided surrounding the electrodes and therefore the internal hearth, filled with electrically conductive liquid medium, while outside this reservoir, inside the truncated ellipsoidal reflector, another liquid medium is used, especially water.

Other characteristics of the electrically conductive medium according to the invention have been described in relation to the method and are obviously applicable to the device.

Thanks to the invention, the discharge takes place through an electrically conductive medium which completely or substantially completely eliminates the latency time. This also results in a considerable increase in the reproducibility of the shock wave generated between the electrodes. This is mainly due to the fact that in the classic case, the arc strikes randomly in time and space, inducing a vapor bubble not perfectly localized, which is not the case according to the present invention. Also, according to the invention, the presence of an oscillating current is eliminated, so that the discharge is of the critical damped type, which will appear clearly from the description given with reference to the accompanying drawings.

Also, thanks to the presence of the reservoir filled with electrically conductive liquid, the quantity of electrically conductive liquid used is greatly limited, that being not in contact with the patient. In addition, the electrical discharge takes place in a confined area, which limits the electrical risks.

It is thus understood that according to the invention; one obtains all the unexpected technical advantages, not obvious to a person skilled in the art, previously stated.

• les figures 1a, 1b et 1c représentent respectivement les courbes de tension, de courant et d'énergie lors de la décharge classique d'un arc électrique généré entre deux électrodes selon un circuit de décharge type RIEBER du brevet US-A-2 559 227 ;
• la figure 2 représente schématiquement, en coupe partielle, un appareil de génération d'ondes de choc, notamment pour la lithotritie extracorporelle, comportant un dispositif de décharge électrique selon la présente invention, qui comprend un réservoir sensiblement fermé rempli d'un milieu liquide électriquement conducteur dans lequel est générée la décharge électrique entre deux électrodes ; et
• les figures 3a, 3b, 3c représentent respectivement comme les figures 1a, 1b, 1c les courbes de tension, de courant et d'énergie obtenues selon la présente invention avec utilisation d'un milieu liquide électriquement conducteur interposé au moins entre les électrodes, conforme à la figure 2.

Other objects, characteristics and advantages of the invention will also appear to those skilled in the art in the light of the explanatory description which will follow made with reference to the appended drawings, in particular representing a currently preferred embodiment of the invention, given simply by way of illustration and which therefore cannot in any way limit the scope of the invention. In the drawings:

• Figures 1a, 1b and 1c respectively represent the voltage, current and energy curves during the conventional discharge of an electric arc generated between two electrodes according to a RIEBER type discharge circuit of US-A-2,559,227 ;
• FIG. 2 schematically represents, in partial section, an apparatus for generating shock waves, in particular for extracorporeal lithotripsy, comprising an electrical discharge device according to the present invention, which comprises a substantially closed reservoir filled with a liquid medium electrically conductive in which the electric discharge is generated between two electrodes; and
• Figures 3a, 3b, 3c respectively represent like Figures 1a, 1b, 1c the voltage, current and energy curves obtained according to the present invention with the use of an electrically conductive liquid medium interposed at least between the electrodes, conforming in Figure 2.

Referring to Figure 2, there is shown schematically an apparatus for generating shock waves, for example for extracorporeal lithotripsy, comprising a truncated ellipsoidal reflector generally referenced 10, which is of the type described in US patent RIEBER 2 559 227. This reflector 10 is provided with two discharge electrodes 12, 14 arranged facing one another, here in a cage structure as is known from document DE-A-26 35 635. These two discharge electrodes 12, 14 are concurrent with the internal focus symbolized by the reference F.

The second focal point of the ellipsoid is disposed outside the truncated ellipsoidal reflector 10 and it is at this second focal point that a target which is to be destroyed is made to coincide, as described at length in the US RIEBER patent. Obviously, this target can also be constituted by a concretion. The electrode 12 is for example connected to earth or ground as shown in FIG. 2 and to one side of a capacitor C. The other electrode 14 is connected to the capacitor C via a switch device I , for example a gas spark gap, which is intermittently closed by a command referenced 20 symbolically. In parallel with the capacitor C, a resistance R of high value or a choke is arranged. The capacitor is placed under high voltage of the order of 10,000 to 20,000 V by a power source as is for example described in FIG. 1 of document EP-A-0 296 912 of the applicants, this circuit not being not shown here.

According to the prior art, the ellipsoidal reflector 10 is filled with a shock wave transmission liquid usually consisting of water, the resistance of which to the passage of a current electric is not negligible. This electrical resistance value, of normally ionized water, expressed as a linear resistivity value, on average, is of the order of 1,500 Ohm.cm. In the case of oils, which are very insulating, as in the case of US Patent RIEBER 2,559,227, the value of linear resistivity is of the order of 3 to 5 M.Ohm.cm.

If an electrical discharge is carried out in such an anterior circuit, where the liquid medium between the electrodes 12, 14 consists of normally ionized water, a discharge timing diagram is obtained as shown in FIGS. 1a, 1b and 1c for which there is a significant latency time while the discharge regime is of the oscillatory type, which gradually delivers energy to the outside environment.

According to the present invention, a substantially closed reservoir 30 is used, filled with an electrically conductive liquid 32, which makes it possible to bring the resistance to the passage of the electric discharge between the electrodes 12, 14 in the vicinity or advantageously below the resistance critical, which constitutes a solution which goes against that which was recommended in document EP-A-0 296 912 by depositors, which on the contrary recommended considerably increasing the electrical resistance between the electrodes by interposing an element insulator between the electrodes.

This reservoir 30 is itself surrounded by a liquid coupling medium 34 filling the truncated ellipsoidal reflector 10, in particular water, which makes it possible to bring the skin of a patient into contact only with ordinary water .

This reservoir is made of a material which does not substantially disturb the shock waves generated by the electrical discharge between the electrodes 12, 14. Such materials are well known to those skilled in the art. In particular, there may be mentioned a latex, silicone, a metal strip. Practical embodiments are a membrane suitably attached, for example to the electrically conductive external electrode member 12a, as is well understood by those skilled in the art.

Advantageously, it is provided that the electrodes support the reservoir, and are removable, as shown in FIG. 2. They can therefore be supplied with the reservoir 30, all the electrodes and the reservoir therefore being consumable and disposable, which reduces costs maintenance compared to previous solutions.

According to an advantageous embodiment of the invention, the electrically conductive liquid medium 32, contained in the reservoir 30, has an electrical resistance which is less than or equal to 1/10 and preferably less than or equal to 1/100, the value electrical resistance of normally ionized water, serving as a reference and which is usually of the order, expressed in linear resistivity, of 1500 Ohm.cm. Preferably, the electrical resistance of the electrically conductive medium according to the invention, expressed in linear resistivity, is less than about 15 Ohm.cm.

As the electrically conductive liquid medium according to the invention, any aqueous or non-aqueous electrically conductive liquid can be used. As the aqueous electrically conductive liquid, an aqueous electrolyte can be used, made from pure water, to which are added soluble ionizable compounds such as salts such as halides, in particular chlorides, sulfates, nitrates. A particularly preferred aqueous electrolyte is water to which NaCl or NH₄Cl has been added. A preferred medium is salt water at 100 or 200 g / l, the respective linear resistivity of 10 and 5 Ohm.cm.

A more preferred electrically conductive aqueous medium comprises about 10% by weight of NaCl and from 0.5 to 2% by weight of disodium phosphate (Na₂HPO _4.12 H₂O) which has a linear resistivity of the order of 8 Ohm.cm to 25 ° C. The NaCl / phosphate proportion is not critical and allows the resistivity to be adjusted up to 10 Ohm.cm. It is preferred to maintain at least 0.5% by weight of phosphate. A dye can also be added to the electrically conductive medium, which makes it possible to observe any leakage in the sealing of the reservoir 30.

As non-aqueous electrolyte, there may be mentioned electrically conductive oils, that is to say oils which have been made conductive by the addition of electrically conductive particles, such as metallic particles.

With the invention, using an electrically conductive liquid medium, a discharge timing diagram is obtained as shown in Figures 3a, 3b, 3c. It can be seen that, as soon as the electrodes are energized at time t₁, the generation of the arc is almost instantaneous. Furthermore, this discharge is of the critical damped type, and is no longer of the oscillatory type. Energy is also delivered to the outside environment for a much shorter time than in the case of an oscillating regime, or in the case of previous regimes with lag time.

This leads to a considerable increase in the reproducibility of the shock wave thanks to the fact that the arc no longer starts randomly in time and in space, but on the contrary at time t₁ and induces a bubble of perfectly localized vapor. The timing diagram of FIG. 3 was obtained with the use of salt water at 200 g / l as an electrically conductive liquid medium bathing the electrodes 12, 14, using a capacitor having a capacitance of 100 nF, a spacing of the electrodes 0.4 mm, the discharge circuit of Figure 2 having a total internal choke L of 80 nH.

est sensiblement satisfaite.
formule dans laquelle L est la valeur de self interne du circuit de décharge du condensateur C, et C est la valeur de capacitance du condensateur.In the description and the claims, it will be recalled that the critical resistance is the value of the interelectrode resistance for which the relation: $${\text{R}}_{\text{vs}}\text{=}\sqrt{\frac{\text{L}}{\text{VS}}}$$

is substantially satisfied.
formula in which L is the internal self value of the discharge circuit of the capacitor C, and C is the capacitance value of the capacitor.

It will be observed that with the invention, using an electrically conductive liquid medium, an excellent reproducibility of the shock waves is obtained, the dispersion coefficient being less than 5%, in particular in the case of the use of salt water, while this standard deviation is of the order of 30% in the case of the use of normally ionized water. The invention therefore makes it possible to achieve all the unexpected, non-obvious technical advantages previously stated and therefore makes it possible to resolve the technical problems previously stated. The invention also makes it possible to implement the method previously stated.

Finally, the present invention also covers an apparatus for generating shock waves by generating an electric arc between two electrodes, characterized in that it uses a method or a device for improving the discharge regime as described above. . In particular, this shock wave generating apparatus is characterized in that it comprises a truncated ellipsoidal reflector comprising a reservoir filled with an electrically conductive liquid, as defined above, as well as another surrounding liquid coupling medium the tank and filling the reflector. A particular application relates to extracorporeal lithotripsy.

Claims (14)

1. Method for improving in particular the reproducibility of the rate of electric discharge produced in a liquid medium, such as water, between at least two electrodes generating such a discharge, characterized in that it consists in considerably reducing the resistance to the passage of the electric discharge at least between the electrodes to a resistance value approaching the critical resistance of the discharge circuit, by interposing at least between the electrodes an electrically conductive medium contained in an essentially closed reservoir (30) surrounding the electrodes, the resistance of the electrically conductive liquid medium being less than or equal to 1/10 of the resistance value of the normally ionised water, used as reference.
2. Method according to claim 1, characterized in that the resistance of said electrically conductive liquid medium is less than or equal to 1/100 of the value of the resistance of the normally ionised water, used as reference.
3. Method according to claim 2, characterized in that the electrical resistance of said electrically conductive liquid medium, as expressed in linear resistivity, is less than about 15 Ohm.cm.
4. Method according to one of claims 1 to 3, characterized in that the electrically conductive liquid medium is constituted by an aqueous or non-aqueous electrolyte, advantageously salted water.
5. Device for improving the rate of electric discharge produced in a liquid medium, such as water, between at least two electrodes generating such a discharge, characterized in that it comprises means for considerably reducing the resistance to the passage of the electric discharge at least between the electrodes so as to bring it close to the critical resistance of the discharge circuit, said means comprising an essentially closed reservoir (30) surrounding the electrodes and filled with an electrically conductive liquid medium (32) having a resistance less than or equal to 1/10 of the resistance value of the normally ionised water, used as reference.
6. Device according to claim 5, characterized in that the electrodes support the reservoir (30) and are removable, the assembly being therefore usable and disposable.
7. Device according to claim 5 or 6, characterized in that said electrically conductive liquid medium has an electrical resistance , measured in terms of linear resistivity, which is less than or equal to 1/100 of the resistance value of the normally ionised water, used as reference.
8. Device according to claim 7, characterized in that the electrically conductive medium is constituted by an aqueous or non-aqueous electrolyte.
9. Device according to one of claims 7 or 8, characterized in that the electrically conductive liquid medium is constituted by an aqueous electrolyte formed from pure water in which ionisable compounds have been added, notably salts such as halide salts, sulphates or nitrates.
10. Device according to one of claims 7 to 9, characterized in that the electrically conductive medium has an electrical resistance, expressed in terms of linear resistivity lower than about 15 Ohm.cm, for example salted at 100 or 200 g/l.
11. Device according to one of claims 7 to 10, characterized in that the electrically conductive liquid medium is constituted by an aqueous electrolyte formed from pure water in which about 10% by weight of sodium chloride and 0.5 to 2% by weight of sulphate, notably disodium sulphate, are added.
12. Apparatus for generating shockwaves by generating an electric arc between two electrodes, in particular of extracorporeal type, characterized in that it comprises a device such as described in any one of claims 5 to 11, or in that it uses the method such as defined in any one of claims 1 to 4.
13. Shockwave generating apparatus according to claim 12, characterized in that it comprises a truncated ellipsoidal reflector (10) comprising a reservoir (30) filled with an electrically conductive liquid (32), as previously defined, as well as another coupling liquid medium (34) surrounding the reservoir (30) and filling the reflector (10), said electrically conductive liquid (30) being as defined in any one of claims 7 to 11.
14. Apparatus according to claim 12 or 13, characterized in that it is an apparatus for extracorporeal lithotripsy.

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