IL98564A

IL98564A - Process for monitoring the efficiency of pressure waves emitted by a pressure wave generator processes for adjustment applying same and an apparatus for monitoring the efficiency of pressure waves for carrying it out

Info

Publication number: IL98564A
Application number: IL9856491A
Authority: IL
Inventors: Paul Dancer; Maurice Bourlion; Jacques Colombier
Original assignee: Technomed Int Sa; Inst Nat Sante Rech Med
Priority date: 1990-06-20
Filing date: 1991-06-19
Publication date: 1996-01-19
Also published as: EP0535103A1; IL98564A0; FR2663531A1; EP0535103B1; WO1991019459A1; DE69123351T2; DE69123351D1

Description

1 1 0 6 "PROCESS FOR MONITORINS THE EFFICIENCY OF PRESSURE WAVES EMITTED BY A PRESSURE WAVE GENERATOR, PROCESSES FOR ADJUSTMENT APPLYING SAME, AND AN APPARATUS FOR MONITORING THE EFFICIENCY OF PRESSURE WAVES FOR CARRYING IT OUT ipnni riTDUi ΕΠΝΊΙ 'j' arr? π πη , γπ·_»α D'"JX -nurux " i na^uin QUI*? θ '*7λπ ym ηιΐ'ϊ' *7ui ΊΠΙ!Ρ_Ϊ7 THE APPLICANTS; 1 . TECHNOMED INTERNATIONAL - French Citizenship Societe Anonyme organized under the laws of FRANCE of Le Ponant 1, 11, Rue Leblanc 75015 PARIS, FRANCE. 8. INSERM (Institut National dela Sante et de la Reshi Medicale) - French Citizenship Etablissement public-lOl, Rue de tolbiac 75654 PARIS CEDE 13, FRANCE.

THE INVENTORS: 1. Paul DANCER 85, Cours Fauriel 42100 ST ETIENNE, FRANCE.

S. Maurice BQURLION. 22, Boulevard Delay 2 00 ST CHAMOND, FRANCE. 3. Jacques COLOMBIER 133, Rue Challemel Lacour 69008 LYON, FRANCE.

FIELD OF THE INVENTION The present invention essentially relates to a process for monitoring the efficiency of compression waves emitted by a pressure wave generator, to processes of adjustment applying same, and to an apparatus for monitoring the efficiency of pressure waves, for carrying it out.

BACKGROUND OF THE INVENTION Pressure wave generators have been well known to the man skilled in the art since Patent US-A-2 559 227 to RIEBER. These pressure waves, also called shock waves, are used for therapeutic treatment inside the body, in extra-corporal manner.

The efficiency of the pressure wave at the point of treatment depends on several important parameters which are a function of the characteristics of the compression generator, which is for example of the electro-hydraulic type as described in RIEBER. One of these parameters is constituted by the point of generation of the pressure wave. Within the framework of a truncated ellipsoidal generator, it is very important that the pressure wave be generated exactly at the inner focus of the truncated ellipsoid in order to obtain a good focussing at the second, outer focus.

Another parameter resides in the value of the distance between the discharge electrodes in order to generate pressure waves of correct intensity.

Applicants have proposed in documents FR-A-2 593 382, FR-A-2 598 074 and US-A-4 730 614, mechanical devices for adjusting the distance between the electrodes .

However, none of these devices enables the position of the electrodes to be monitored in real time.

Furthermore, no known device allows monitoring of the presssure generated at each shot during the treatment, nor any monitoring of the point of generation of the pressure wave.

It is therefore an object of the present invention to solve the novel technical problem of providing a solution for efficiently monitoring the pressure waves emitted by a pressure wave generator, preferably in real time, i.e. in the course of treatment without stopping the treatment, in order to be able possibly to correct the characteristics of the compression wave generator.

It is another object of the present invention to solve the novel technical problem of providing a solution for monitoring the efficiency of pressure waves in particularly simple, inexpensive manner, applicable on an industrial scale.

A further object of the present invention is to solve the novel technical problem of providing a process for adjusting the point of generation of the pressure waves, preferably in real time, i.e. in the course of treatment without stopping treatment, in particularly simple, reliable and inexpensive manner .

The invention has for a further object to solve the novel problem of providing a process for adjusting the pressure wave pressure value, in particular focussed at a focussing focus, particularly an outer focus .

The present invention also has for an object to solve the novel technical problem of providing a process for adjusting the volume of the focal spot, preferably in real time, i.e. in the course of treatment without stopping the treatment, in simple, reliable and inexpensive manner. yet another object of the present invention is to solve the novel technical problem of providing a process for adjusting the distance between the electrodes in a pressure wave generator of electro-hydraulic type by electrical discharge between the two electrodes.

All these technical problems are solved for the first time by the present invention in particularly simple, inexpensive, reliable manner, usable on an industrial scale.

SUMMARY OF THE INVENTION According to a first aspect, the present invention thus provides a process for monitoring the efficiency of pressure waves, preferably in real time, emitted by a pressure wave generator of the focused type, characterized in that: - at least one sensor of a characteristic parameter of a non-focused direct pressure wave, for example the pressure is provided, capable of picking up said parameter of said non-focused pressure wave; - this sensor is disposed on the path of said non-focused direct pressure wave, of fixed position with respect to said generator; - the signal emitted by said sensor is processed during reception of a non-focused direct pressure wave coming from a point of pressure wave generation in order to know the efficiency of the pressure wave, as a function of the characteristics of the generator, and - a correction is possibly made of the characteristics of the pressure wave generator.

According to an advantageous variant of this process, the said sensor receives each pressure wave, and the signal emitted for each pressure wave which is processed in order to know in real time the efficiency of the pressure wave and possibly to proceed in real time with a correction of the characteristics of the generator, is registered.

According to another advantageous feature of the process of the invention, the signal is processed to determine the value of the pressure of the pres- sure wave. Advantageously, within the framework of a generator focussing at a focal point, for example an outer focus, the value of the pressure of the pressure wave is used for calculating the value of the pressure focussed at the focal point.

According to another advantageous embodiment of the process of the invention, within the framework of a focussed generator comprising a truncated ellipsoid, having an inner focus where pressure waves are generated by electrical discharge between two electrodes, and an outer focus where the pressure waves are focussed, the process is characterized in that the value of the pressure wave is used for adjusting the initial distance between the electrodes .

According to another advantageous embodiment of the process of the invention, the time of passage of the pressure wave from the point of generation to the sensor is detected and this time of passage is compared with a reference time of passage for a wave emitted at the theoretical point of generation. According to a variant embodiment, the time of passage from the point of generation to two different, known positions of the sensor is preferably detected, and the difference in time Delta. t taken by each wave to arrive at each of the two known positions of the sensor is calculated, and the value of the time difference Delta. t is used for determining the real position of the point of generation of the pressure wave.

According to an advantageous variant embodiment, this time difference value is compared with a reference time difference value resulting from a pressure wave at the theoretical point of generation. In particular, the theoretical point of generation is constituted by the inner focus of a truncated ellipsoidal generator. In that case, the two known positions of the sensor are preferably disposed at equi-distance from the inner focus and preferably symmetrically with respect to the major axis of the truncated ellipsoidal reflector.

According to an advantageous embodiment, two pressure sensors are used, which are disposed in a longitudinal plane of symmetry of the generator, in particular passing through the electrodes in the case of a pressure wave generator employing electrical discharge between electrodes.

According to another advantageous embodiment of the process of the invention, the position of the real point of pressure wave generation is determined from the following mathematical formula: d = k x Delta. t in which k = D : (LO x v).

With D = the distance between the two different, known positions of the sensor, LO is the distance between the theoretical or ideal point of pressure wave generation and the known position of the sensor, v is the speed of propagation of sound in the coupling liquid, for example water (1480 m/s at 20°C), d is the shift value between the theoretical or ideal point of pressure wave generation and the real point of pressure wave generation; Delta. t = the time difference of passage measured. It should be noted that this mathematical formula is valid for low shift values d with respect to the value of the distance D separating the different positions of measurement of the sensor, which, in practice, is always the case.

According to another advantageous variant embodiment of the process of the invention, the position of the real point with respect to the theoretical point is determined by comparing the value of Delta. t with a calibrated curve giving the shift d as a function of Delta. t.

According to another advantageous feature of the process of the invention, the variation of the dispersion of the time difference Delta. t is determined and the variation of the dispersion is advantageously used for maintaining the distance between the electrodes constant. The distance between the electrodes is advantageously determined from a calibrated curve giving the distance between the electrodes as a function of the dispersion, preferably the standard deviation, of the time difference Delta. t.

According to a second aspect, the present inven-tion provides a process for adjusting the point of generation of the pressure waves, characterized in that the shift between the real point of generation of the pressure waves and the theoretical point of generation of the pressure waves is determined and the value of shift is corrected as a function of the desired position of the point of generation of the pressure waves.

According to another aspect, the present invention further provides a process for adjusting the volume of the focal spot, characterized in that a predetermined shift is made of the real point of generation of . pressure waves, with respect to the theoretical point of generation of pressure waves, as a function of the desired volume of the focal spot. According to a particular embodiment, within the framework of a truncated ellipsoidal generator comprising an inner focus and an outer focus, the theoretical point of generation of pressure waves is constituted by the inner focus, the focal spot being positioned at the outer focus.

The present invention also relates to a process for adjusting the value of the pressure of the compression waves focussed at the outer focus of a truncated ellipsoidal generator, characterized in that a predeter-mined shift is effected of the real point of generation of pressure waves with respect to the theoretical point of generation of pressure waves and/or a spaced apart relationship of the electrodes, as a as a function of the value of the desired pressure at the outer focus.

Finally, the present invention relates to a process for adjusting ,the distance between the electrodes of a truncated ellipsoidal pressure wave generator filled with a coupling liquid, characterized in that the value of pressure of the pressure waves emitted is detected, the electrodes are advanced, a sudden reduction in the pressure is detected, indicating the contacting of the electrodes, and a predetermined withdrawal of the electrodes is effected so as to arrive at a precise spaced apart relationship of the electrodes.

It is thus readily appreciated that, with the invention, all the decisive technical advantages set forth beforehand are obtained, making it possible to monitor the efficiency of the pressure waves emitted ^y a pressure wave generator, in particular by determining the real position of the point of generation of the pressure waves, the value of the pressure generated, the value of the distance between the electrodes, or the time taken by the pressure wave to cover a known or predetermined distance between the point of generation of the pressure waves and the surface of a pressure sensor.

The processes according to the invention therefore make it possible to effect a perfect centering of the electrodes on the theoretical point of generation of the pressure waves, in particular exactly at the inner focus of a truncated ellipsoidal reflector, and this in real time.

Furthermore, the present invention makes it possible to adjust as desired the point of generation of the pressure waves, the distance between the electrodes, the volume of the focal spot, or the value of pressure of the pressure waves focussed in particular at the outer focus.

This makes it possible to improve the safety and monitoring of correct functioning of the pressure wave generator, which is very important within the framework of therapeutic treatment. In fact, it is highly important that the practician be rapidly aware of a poor output of a generator in order to avoid treating patients inefficiently.

Furthermore, by the possibility of voluntarily defocussing the pressure waves, it is possible to obtain at the point of focussing, for example the second outer focus of a truncated ellipsoidal generator, lower pressures and larger focal spots, in order to reduce pain and adapt the treatment to the size of the concretion or lithiasis to be destroyed. By using larger focal spots, this also makes it possible to strike the calculi in movement due to respiration, more often. Such adjustment is automatic and extremely simple.

Finally, according to. another aspect, the present invention also relates to an apparatus for monitoring the efficiency of pressure waved emitted by a pressure wave generator of the focussed type, chracterized in that it comprises at least one sensor of a characteristic parameter of a non-focussed direct pressure wave, for example the pressure , capabl e of picking up said parameter of said non-focused direct pressure wave, of fixed position with respect to said generator, and means for processing the signal emitted by the sensor during reception of the non-focussed direct pressure wave as a fuction of the characteristics of the generator and possibly means for correcting the characteristics of the pressure wave generator.

According to an advantageous variant embodiment, the said signal processing means comprises an electronic signal processing device well-known to to a person skilled in the art, as well as calculating means, for example comprising a computer or micro-computer, which is advantageously capable of ordering corrections of the characteristics of the pressure wave generator, thus simultaneously constituting the said correction means. Such corrections may be a different distance between the electrodes, a voluntary shift of thereal point of generation of the pressure wavesby a voluntary dissymetry of positioning of the electrodes with respect to the theoretical point of generation of the pressure waves, a combination of the two, or a different value of the pressure of the pressure wave generated, for example by adjusting the value of the voltage of the high voltage source, or, finally a perfect recentering on the theoretical point of generation of the pressure waves, in particular the inner focus of a truncated ellipsoid.

According to another variant of the invent ion, each sensor is incorporated in the wall of the ellipsoidal reflector.

BRIEF DESCRIPTION OF THE INVENTION The invention will be more readily understood on reading the following description with reference to the accompanying drawings, in which: Figure 1 schematically shows an apparatus for monitoring the efficiency of pressure waves emitted by a pressure wave generator device, according to a preferred embodiment of the present invention, including the essential means for effecting such monitoring.

Figure 2 schematically shows, in longitudinal axial section, partially, the pressure wave generator device comprising a truncated ellipsoidal reflector filled with a coupling liquid, the electrodes comprising a shift d of the point of emission of the pressure waves with respect to the inner focus Fl, with the use of two sensors disposed at equi-distance from the inner focus Fl and symmetrically with respect to the major axis of the truncated ellipsoidal reflector .

Figure 3 shows the recordal of the pressure curves of each of the two sensors of Figures 1 and 2, as a function of time.

Figure 4 shows the calibration curve obtained fjom the shift or decentering d expressed in millimetres (mm), as a function of the time difference Delta. t expressed in nanoseconds (ns).

Figure 5 shows the pressure amplitude curve of the direct or incident pressure waves expressed in bars, on the y-axis, as a function of the distance in millimetres (mm) between the electrodes, on the x-axis.

Figure 6 shows the calibration curve of the standard deviation on the time difference Delta. t expressed in nanoseconds (ns), on the y-axis, as a function of the distance in millimetres (mm) between -li¬ the electrodes, on the x-axis.

DETAILED DESCRIPTION OF THE DRAWINGS Referring now to the drawings and firstly to Figures 1 and 2, an apparatus for monitoring the efficiency of pressure waves according to the present invention is represented by general reference numeral 10. This monitoring apparatus 10 comprises at least one pressure sensor 12, 13 capable of picking up the pressures of the pressure waves emitted by the generator device 14, comprising a truncated ellipsoidal reflector 16 comprising an inner focus Fl and an outer focus F2. The truncated ellipsoidal reflector 16 is usually filled with a coupling liquid 18, particularly water or an aqueous solution. The pressure waves are emitted at focus Fl, in the example shown, by electrical discharge between two electrodes 20, 22 preferably mounted to be movable in axial translation and in rotation so as to be able to be advanced or moved back independently, as desired, along the axis of translation passing through the focus Fl and the electrodes, as is described in detail in Applicants* prior Patent US-A-4 730 614, Figure 3.

According to the present invention, the monitoring apparatus 10 comprises at least one pressure sensor 12, 13 disposed on the path of the pressure wave emitted at focus Fl and preferably to receive the incident pressure wave. This apparatus 10 also comprises means 24 for processing the signal emitted by each sensor 12, 13 during reception of a pressure wave, in order to know the efficiency of the pressure wave as a function of the characteristics of the generator 14. These signal processing means 24 are for example constituted by conventional electronic processing means well known to the man skilled in the art.

Means 24 are advantageously coupled to calculating means 30 which make the various calculations on the results obtained by the processing means 24 from the signal emitted by the sensors 12, 13, these calcula ting means 30 comprising for example a computer or microcomputer. The calculation means 30 advantageously constitute means for automatically controlling the predetermined, individual advance or withdrawal of each of the electrodes 20, 22.

According to an advantageous embodiment, as shown, two pressure sensors 12, 13 are used, disposed at equi-distance from the inner focus Fl and symmetrically with respect to the major axis of the ellipsoidal reflector 16. In this way, the sensors 12, 13 are disposed in a longitinal plane of symmetry of the generator 14 passing through the electrodes, i.e. the plane of section of Figures 1 and 2.

It should be noted that, according to a particular variant embodiment, it is possible to use one single sensor 12 or 13, particularly when the geometrical parameters of the ellipsoidal reflector 16 have been determined in precise manner by prior measurements.

According to another variant embodiment of the monitoring apparatus according to the invention, at least three pressure sensors are used, disposed at equi-distance from focus Fl. These sensors may be disposed outside the plane of the electrodes.

According to an advantageous embodiment, the or each sensor 12, 13 is mounted on a positioning element 32 adapted to be fixed temporarily at the apex of the ellipsoidal reflector 16, as is clearly visible in Figure 2. This positioning element 32 may present a part 33 curved radially towards the inside, comprising an orifice in which is inserted sensor 12, 13, making it possible to orient the sensor precisely in the direction of the focus PI, therefore of the point of generation E of the "pressure waves. In this way, an extremely precise positioning of sensors 12, 13 relatively to focus Fl is obtained in extremely simple manner. It will be observed that, when two sensors 12, 13 are used, they are preferably positioned in diametrically opposite manner on the positioning element 32 which presents an annular shape. The positioning element 32 may comprise an annular radial shoulder 34 facilitating temporary fixation on the apex of the ellipsoidal reflector 16.

Functioning of the monitoring apparatus according to the invention is clear from the foregoing description and is as follows: When a pressure wave or shock wave is emitted at focus Fl, or in the vicinity thereof at a point E when the electrodes are offset with respect to focus Fl, for example by a distance d, as shown in Figure 2, the pressure wave emitted is reflected on the truncated ellipsoidal reflector 16 and is focussed at the outer focus F2 where it is possible to bring a target into register in order to treat said target, for example surgically. It will be observed that, from the construction of the monitoring apparatus according to the invention, the incident wave emitted is received by each sensor 12, 13. It will be observed that each sensor 12, 13 is at distance LO from focus Fl, which is the optimum theoretical point of generation of the pressure waves, whilst, when the electrodes are shifted with respect to the inner focus Fl, for example disposed symmetrically with respect to point E, the distance separating sensor 12 from point E is Ll and the distance separating sensor 13 from point E is L2.

Sensors 12 and 13 are separated from each other by distance D and are located in a horizontal plane distant by a height h from focus Fl. v is the speed of propagation of sound in the coupling liquid/ for example water (1480 m/s at 20°C). d is the value of shift between the inner focus Fl and the origin E of the pressure wave. Let Delta. t be the difference of time of passage measured between sensors 12 and 13.

The pressure signals obtained at sensors 12 and 13 are recorded in Figure 3 and enable the value of Delta. t to be calculated in extremely simple manner.

When sensors 12 and 13 are in the plane of the electrodes 20, 22, the following equations are obtained: LO2 = h2 + (D/2)2 LI2 = h2 + (D/2-d)2 L22 = h2 + (D/2+d)2 Delta. t = ( L2-L1 )/v Or: Delta. t = 1/v x [[h2 + (D/2+d)2]°\5 - [h2 + (D/2-d)2]0*5 ) This equation gives a relation between d and Delta. t.

This relation is linear for low values of d before D and may be simplified and be written as follows: d = k x Delta. t with k = D/(L0 x v) It is observed that k is a constant which depends on the geometry of the reflector.

In a particular embodiment, D may be equal to 186 mm, h equal to 162 mm, which gives a coefficient of 672 ns per mm of shift whilst this simplified relation is valid for d less than 65 mm.

The Delta. t curve as a function of the shift or decentering per mm is given in Figure 4.

It is thus observed that the invention makes it possible to determine in extremely simple manner the shift of the point of emission of the pressure waves E with respect to focus Fl.

This method may therefore be used for effecting a voluntary shift of the point of emission E of the pressure waves with respect to focus Fl, or inversely for recentering the point of emission E with respect to focus Fl, by corresponding displacement of elec-trodes 20, 22.

Figure 5 shows the pressure amplitude curve of the direct or incident pressure wave obtained with one or the other of sensors 12 or 13, as a function of the distance in mm between electrodes 20, 22.

This curve makes it possible to adjust the initial distance between electrodes 20, 22, using the fact that the incident pressure varies suddenly when the two electrodes come into contact, the pressure wave created no longer having any efficiency.

It is thus possible to produce a predetermined distance of electrodes 20, 22 by the detection of the pressure of the incident wave detected by the sensor, by effecting an approach of the electrodes until the pressure of the incident wave drops suddenly, which corresponds to a zero distance, then it is possible to proceed with a withdrawal by a predetermined distance of each electrode in order to obtain the initial distance desired.

The value of amplitude of the incident pressure wave also makes it possible to calculate the value of the pressure applied to the second outer focus F2. This calculation may be made by taking into account the real point of emission E of the pressure wave measured by the time difference Delta. t of passage mentioned above and the measured value of the incident pressure.

The invention also makes it possible to monitor the distance between the electrodes by the variation of the dispersion on the measurement of time difference Delta. t.

In fact, it is observed that the dispersion depends directly on the distance between the electrodes. It has been observed that, if the electrodes are brought closer to each other, the dispersion on the time difference decreases, whilst, if the electrodes move apart, the dispersion increases.

In this way, a correct initial distance may be maintained by watching the dispersion on the time diffe- rence, maintaining it constant.

All measurements of dispersion may be used, viz. range, mean deviation, semi-interquartile range, standard deviation. For example, Figure 6 shows the standard deviation of the measurement of the time difference Delta. t (on the y-axis) as a function of the distance in mm between the electrodes (on the x-axis). In this way, a calibration curve of the standard deviation on Delta. t relatively to the distance is obtained which shows at any instant whether the distance between the electrodes has changed, and therefore makes it possible efficiently to monitor in real time the values of the pressure waves.

In practice, in order to adjust the distance between the electrodes, one may begin by moving the electrodes back, for example by 0.15 mm in order to be sure of being in the flat part of the amplitude curve of Figure 5, then they may be advanced by proceeding with an emission of pressure wave after each advance, by steps of 0.05 mm until about 50% of inci-dent pressure is lost, which indicates that the two electrodes are in contact (cf. Figure 5).

It then suffices to move each of the electrodes back by a known distance in order to obtain the desired spaced apart relationship.

Subsequently, in order to maintain the distance between the electrodes constant, it suffices to maintain the standard deviation of the time difference between the two sensors constant, by automatically displacing the electrodes (cf. Figure 6). it may therefore be seen from the foregoing that the present invention produces decisive technical advantages which are particularly unexpected for the man skilled in the art.

Claims

- 18 - 98564/2

1. C L A IM S : - 1. A process for monitoring the efficiency of pressure waves emitted by a pressure wave generator of the focused type, characterised in that: - at least one sensor (12, 13) of a characteristic parameter of a non-focused direct pressure wave, for example the pressure is provided, capable of picking up said parameter of said non focused pressure wave; this sensor 12, 13 is disposed on the path of said non focused direct pressure wave so that the sensor has a fixed position with respect to the pressure wave generator ; - the signal emitted by said sensor 12, 13 is processed during reception of a non-focused direct pressure wave coming from a point E of pressure wave generation, in order to know the efficiency of the pressure wave, as a function of the characteristics of the generator 14; and - a correction is possibly made of the characteristics of the pressure wave generator 14.

2. The process according to Claim 1, characterized in that said sensor 12, 13 receives each pressure wave, and the signal emitted for each wave is processed in order to know in real time the efficiency of the pressure wave and possibly to proceed in real time with a correction of the characteristics of the generator 14, is registered.

3. The process according to Claims 1 or 2, characterized in that the signal is processed to determine the value of the pressure of the pressure wave.

4. The process according to Claim 3, characterized in that, within the framework of a generator focussing at a focal point F2, for example an outer focus, the value of the pressure of the pressure wave is used for calculating the value of the pressure focussed at the focal point F2. 98564/3

5. The process according to Claims 1 to 4, characterized in that, within the framework of a focussed generator 14 comprising a truncated ellipsoidal reflector 16, having an inner focus F1 where pressure waves are generated by electrical discharge between two electrodes 20, 22 and an outer focus F2 where the pressure waves are focussed, the value of the pressure wave is used for adjusting the initial distance between the electrodes 20 and 22.

6. The process according to Claims 1 to 5, characterized in that the time of passage of the pressure wave from the point of generation E to the sensor 12, 13 is detected and this time of passage is compared with a reference time of passage for a wave emitted at the theoretical point of generation F1.

7. The process according to Claim 6, characterized in that the time of passage from the point of generation E to the different, known positions of the sensor 12, 13 is detected, and the difference in time Delta.t taken by each wave to arrive at each of the two known positions of the sensor 12, 13 is calculated, and the value of the time difference Delta.t is used for determining the real position E of the point of generation of the pressure wave.

8. The process according to Claim 7, characterized in that this time difference value Delta.t is compared with a reference time difference value resulting from a pressure wave at the theoretical point of generation F1.

9. The process according to Claim 7, characteri ed in that the theoretical point of generation F1 is constituted by the inner focus of a truncated ellipsoidal reflector 16. - 20 - 98564/2

10. The process according to Claim 9, characterized in that the two known positions of the sensor 12, 13 are disposed at equi-distance from the inner focus F1 and preferably symmetrically with respect to the major axis of the truncated ellipsoidal reflector 16.

11. The process according to Claim 1, characterized in that two pressure sensors 1, 13 are used, which are disposed in a longitudinal plane of symmetry of the generator 14, in particular passing through the electrodes 20, 22 in the case of a pressure wave generator employing electrical discharge between two electrodes.

12. The process according to Claim 7, Characterized 1n that the position of the real point E of pressure wave generation is determined from the following mathematical formula: d = k X Delta.t wherein k = D : (LO X v); D, being the distance between the different, known positions of the sensor 12, 13,; LO, is the distance between the theoretical or ideal point F1 of pressure wave generation and the known position of the sensor 12, 13; v, is the speed of propagation of sound in the coupling liquid 18, for example water (1480 m/s); d, is the shift value between the theoretical or ideal point F1 of presssure wve generation and the real point E of pressure wave generation, and Delta.t is the time difference of passage measured.

13. The process according to Claim 7, characterized in that the position of the real point E with respect to the theoretical point F1 is determined by comparing the value of Delta.t with a calibrated curve giving the shift d as a function of Delta.t. - 21 - 98564/2

14. The process according to Claim 7, characterized in that the variation of the dispersion of the time dif fence Delta.t is determined and the variation of the dispersion is advantageously used for maintaining the distance between the electrodes 20, 22 constant.

15. The process according to Claim 14, characterized in that the distance between the electrodes 20, 22 is determined from a calibrated curve giving the distance between the electrodes as a function of the dispersion, preferably the standard deviation, of the time difference Delta.t.

16. The process according to Claims 1 to 15, characterized in that said correction of the characteristics of the generator consists in adjusting the real point of generation of the pressure waves, such adjustment comprising the steps of determining the shift between the real point E of generation of the pressure waves, and of correcting the value of shift d as a function of the desired position of the point of generation of the pressure waves.

17. The process according to Claims 1 to 15, characterized in that said correction of the characteristics of the pressure wave generator consists in- adjusting the volume of the focal spot, such adjustment comprising the step of affecting a predetermined shift D of the real point E of generation of pressure waves, with respect to the theoretical point F1 of generation of pressure waves, as a function of the desired volume of the focal spot.

18. The process according to Claim 17, characterized in that, within the framework of a truncated ellipsoidal generator 16 comprising an inner focus F1 and an outer focus F2, the theoretical point of pressure wave generation is constitute^ by the inner focus F1 , the focal spot being positioned at the outer focus F2. - 22 - 98564/2

19. The process according to Claims 1 to 18, characterized in that said correction of the characteristics of the pressure wave generator consists in an adjustment of the value of of pressure of the pressure waves focussed at the outer focus F2 of a truncated ellipsoidal generator, such adjustment comprising the step of effecting a predetermined shift of the real point E of pressure wave generation and/or a spacing apart of the electrodes, as a function of the values of the desired pressure of the oute focus F2.

20. The process according to Claim 1, characterized in that said correction of the characteristics of the pressure wave generator consists in adjusting the distance between the electrodes of said generator 14 which comprises a truncated ellipsoidal receiver 16 filled with a coupling liquid 18, such adjustment comprising the steps of detecting the pressure value of the pressure waves emitted of advancing the electrodes 20, 22, of detecting a sudden reduction in the pressure, indicating the correcting of the electrodes 20, 22, and of proceeding with a , ^redetermined withdrawal of the electrodes 20, 22 so as to arrive at a precise spaced apart relationship of the electrodes.

21. An apparatus for monitoring the efficiency of pressure waves, preferably in real time, emitted by a pressure wave generator 14 of the focused type, characterized in that it comprises at least one sensor 12, 13 of a characteristic parameter of a non-focussed direct pressure wave, for example the pressure capable of sensing said parameter of the non focussed direct pressure wave of fixed position with respect to said generator, and signal processing means 24 for processing the signal emitted by the sensors 12, 13, during reception of the non-focussed direct pressure wave by the sensors 12, 13, in order to know the efficiency of the pressure wave as a function of - 23 - 98564/2 the characteristics of the generator 14, and possibly corrections means 30 for correcting the characteristics of the pressure wave generator.

22. The apparatus according to Claim 21, characterized in that said signal processing means 24 for processing the signal, as well as calculating means 30, for example comprising a computer or micro-computer, which is advantageously capable of ordering corrections of the characteristics of the pressure wave generator, thus simultaneously constituting said corrections means.

23. The apparatus according to Claims 21 and 22, characterized in that the corrections are a different distance between the electrodes, a voluntary shift of the real point of pressure wave generation by a voluntary dissymetry of positioning of the electrodes 20, 22 with respect to the theoretical point E of pressure wave generation, a combination of the two, a different value of the pressure of the voltage of the high voltage source, or finally a perfect recentering on the theoretical point F1 of generation of the pressure waves, in particular the inner focus F1 of a truncated ellipsoid 16.

24. The apparatus according to Claims 21 to 23, characterized in that it comprises two pressure sensors 12, 13 disposed at equi -distance from the inner focus F1 and symmetrically with respect to to the major axis of the ellipsoidal reflector 16, preferably in a longitudinal plane of symmetry of the generator 14, passing through the electrodes 20, 21. ' For the Applicant, S mon Lav e Patent Attorney