EP2691948B1 - Method and apparatus for generating focused ultrasonic waves with surface modulation - Google Patents

Description

The present invention relates to the technical field of devices or devices comprising an ultrasonic probe formed by a plurality of ultrasonic transducer elements, adapted to emit high intensity focused ultrasound (HIFU).

The object of the present invention finds particularly advantageous applications in the field of therapeutic treatments by focussed ultrasonic waves.

It is known that focused ultrasound therapy can create biological lesions in tissues resulting from a combination of thermal effects and acoustic cavitation activity. The shape of these tissue lesions arises directly from the shape of the emission surface of the ultrasound probe used. For example, an ultrasound probe of spherical shape makes it possible to obtain a point focusing zone whereas a toroidal shape probe leads to obtaining a focusing zone in the form of a ring or crown.

At each point of the focusing zone, it should be noted that the distances traveled from the emission surface by the ultrasonic waves are identical and that the pressure is directly related to the convergence of the ultrasonic waves at this point. In practice, the ultrasonic waves pass between the emission surface and the focusing zone, various propagation media of different nature such as the water of a cooling circuit, the skin, the fat, the muscles, etc. However, these different media have different acoustic attenuation characteristics. It thus appears, for each of the paths traveled, an attenuation of the ultrasonic waves depending on the distance traveled in each medium through.

Moreover, after the emission in the propagation media, a focusing effect is observed due to the concavity of the emission surface. Ultrasonic waves will focus on the focus area (point or crown) giving rise to a progressive increase in pressure along the path of the ultrasonic wave.

In an attempt to overcome the drawbacks related to the acoustic heterogeneity of the fabrics, it is known, for example by the patent FR 2 642 640 to use a focussing device whose probe emission face is divided into a plurality of transducer elements to which activation signals obtained by reversing the time distribution are applied via control circuits. and the shape of the echo signals received back from an unfocused beam sent to the tissues to be treated. The transducer elements thus emit different acoustic powers depending on the attenuation and the focusing effect of the acoustic waves.

In practice, the transducer elements have identical emission surfaces so that each of them has the same electrical impedance. The control circuits of each of these transducer elements are also identical to facilitate the realization of such a device.

However, this solution has a major disadvantage. Indeed, the available electrical power for each of the transducer elements is limited by the control circuit electronics. Thus, as soon as one of the transducer elements operates at its maximum power to compensate for the attenuation and focusing difference of the ultrasonic waves, the other ultrasonic transducers must operate with a reduced electrical power and the control circuit electronics will be unable to provide the maximum power for which it was designed. In practice, the control circuit always operates below its maximum capacity.

He is also known by the patent US 4,888,746 a therapy transducer formed of a plurality of transducer elements that can be activated independently of each other by signals of varying amplitude and phase to modulate the shape of the ultrasonic wave at the focusing point with a view, in particular, to reducing the cavitation effects.

Similarly, the patent FR 2 903 616 discloses a torus-shaped therapy probe whose various transducer elements are sequentially activated to allow ultrasonic wave focusing in a ring.

The transducers described by these patents do not make it possible to homogenize the energy contributions made by the various ultrasonic transducer elements to a specific treatment area since the focusing and attenuation effects experienced by the ultrasonic waves on their path are not not taken into account.

In the field of imaging, the patent US 5,922,962 discloses an ultrasonic transducer having a series of transducer elements having identical lengths but different widths. The widths of the transducer elements are determined so as to maintain the same ultrasound beam profile, ie the same ultrasound resolution, regardless of the focusing distance.

This paper describes various beamforming techniques for dynamically focusing at different depths in transmit and receive modes as well as various apodization techniques to reduce side-lobe effects. These beamforming techniques do not take into account acoustic attenuations of the ultrasonic waves on the path between the target zone and the transducer elements in order to obtain in the target zone a substantially identical energy input of the ultrasonic waves emitted by each of the transducer elements.

Similarly, the documents US 5,165,414 , EP 0 689 187 and EP 0 401 027 describe imaging transducers with the same disadvantages as the transducer described by the patent US 5,922,962 . The transducers described by such documents are not intended to homogenize the energy contributions of the different transducer elements, insofar as it is not sought a supply of energy in a target area for therapy.

The present invention therefore aims to overcome the drawbacks of the state of the art by proposing a new focusing technique of ultrasonic waves for homogenizing energy contributions on a target area to obtain tissue biological lesions.

To achieve such an objective, the method of generating ultrasound waves focused on a focusing zone to ensure biological lesions, comprises activating a plurality of ultrasonic transducer elements distributed on a transmitting surface to emit respectively a a plurality of focused ultrasound waves in the focusing zone, passing through propagation media with different acoustic attenuation.

• on choisit une zone cible dans laquelle est souhaitée une homogénéisation des apports d'énergie des ondes ultrasonores émises par les éléments transducteurs ultrasonores,
• on détermine l'effet de focalisation ainsi que les atténuations acoustiques des ondes ultrasonores sur leur trajet entre la zone cible et les éléments transducteurs ultrasonores,
• on compense l'effet de focalisation et les atténuations acoustiques des ondes ultrasonores, avec des éléments transducteurs ultrasonores dont au moins certains d'entre eux présentent des surfaces d'émissions non identiques afin que dans la zone cible, l'apport d'énergie des ondes ultrasonores émises par les différents éléments transducteurs ultrasonores soit sensiblement identique.

According to the invention:

• a target zone is chosen in which it is desired to homogenize the energy inputs of the ultrasonic waves emitted by the ultrasonic transducer elements,
• the focusing effect and the acoustic attenuations of the ultrasound waves are determined on their path between the target zone and the ultrasonic transducer elements,
• the focusing effect and the acoustic attenuations of the ultrasonic waves are compensated with ultrasonic transducer elements, at least some of which have non-identical emission surfaces so that in the target zone the energy input of the Ultrasonic waves emitted by the different ultrasonic transducer elements are substantially identical.

• compenser les effets de focalisation et les atténuations acoustiques en affectant à chacun des éléments transducteurs ultrasonores un facteur de pondération surfacique dépendant de l'atténuation acoustique et de l'effet de focalisation subies par les ondes ultrasonores,
• déterminer les facteurs de pondération acoustiques, en prenant en compte la distance entre les éléments transducteurs ultrasonores et la zone de séparation des milieux de propagation,
• prendre en compte la distance entre les éléments transducteurs ultrasonores et la zone de séparation des milieux de propagation, en calculant cette distance en fonction de la configuration du milieu de propagation par rapport auxdits éléments transducteurs ultrasonores,
• prendre en compte la distance entre les éléments transducteurs ultrasonores et la zone de séparation des milieux de propagation, en mesurant les échos réfléchis à la suite de l'envoi d'un signal d'étalonnage par les éléments transducteurs ultrasonores,
• regrouper des éléments transducteurs ultrasonores de tailles élémentaires de manière à constituer des éléments transducteurs ultrasonores de surfaces d'émissions différentes configurables en fonction des atténuations acoustiques rencontrées,
• pour une pluralité d'éléments transducteurs ultrasonores répartis sur une surface d'émission concave de rayon de courbure Rc, à calculer l'aire Sn de chaque élément transducteur ultrasonore n telle que : $$\mathrm{Sn}=\left[{\mathrm{S}}_{\mathrm{totale}}\left(\mathrm{1}/\left(\mathrm{Fp}\left(\mathrm{n}\right)\mathrm{.}\mathrm{Z}\right)\right)\right]$$
  
• avec S_totale : la somme des surfaces des éléments transducteurs ultrasonores,
• Fp (n) = Max E(t) / Max E(n),

avec Max E(t), la valeur maximale de la contribution énergétique de l'élément transducteur t situé à la périphérie de la surface d'émission et Max E(n), la valeur maximale de la contribution énergétique de l'élément transducteur n dans la zone cible,In addition, the method according to the invention may additionally comprise in combination at least one and / or the following additional characteristics:

• compensating the focusing effects and the acoustic attenuations by assigning to each of the ultrasonic transducer elements a surface weighting factor dependent on the acoustic attenuation and focusing effect of the ultrasonic waves,
• determine the acoustic weighting factors, taking into account the distance between the ultrasonic transducer elements and the propagation medium separation zone,
• taking into account the distance between the ultrasonic transducer elements and the propagation medium separation zone, by calculating this distance as a function of the configuration of the propagation medium with respect to said ultrasonic transducer elements,
• taking into account the distance between the ultrasound transducer elements and the propagation medium separation zone, by measuring the reflected echoes following the sending of a calibration signal by the ultrasonic transducer elements,
• grouping ultrasonic transducer elements of elementary sizes so as to constitute ultrasound transducer elements of different emission surfaces configurable according to the acoustic attenuation encountered,
• for a plurality of ultrasonic transducer elements distributed over a concave radius of curvature emission surface Rc, calculating the area Sn of each ultrasonic transducer element n such that: $$\mathrm{Sn}=\left[{\mathrm{S}}_{\mathrm{Total}}\left(\mathrm{1}/\left(\mathrm{fp}\left(\mathrm{not}\right)\mathrm{.}\mathrm{Z}\right)\right)\right]$$
  
• with _total S: the sum of the surfaces of the ultrasonic transducer elements,
• Fp (n) = Max E (t) / Max E (n),

with Max E (t), the maximum value of the energy contribution of the transducer element t located at the periphery of the emission surface and Max E (n), the maximum value of the energy contribution of the transducer element n in the target area,

Z: sum of 1 / Fp for all transducer elements.

Another object of the invention is to propose a therapy apparatus for generating ultrasound waves focused on a focusing zone, comprising an ultrasonic probe formed by a plurality of ultrasonic transducer elements distributed on a transmission surface to emit a plurality of focused ultrasonic waves in the focusing zone, in passing through propagation media with different acoustic attenuations, the ultrasonic transducer elements being excited by control signals from a control circuit, characterized in that at least some of the ultrasonic transducer elements have non-identical transmitting surfaces for emitting focused ultrasonic waves which, in a target area, have substantially identical energy inputs.

• au moins certains des éléments transducteurs ultrasonores sont pilotés par des signaux d'excitation de valeurs sensiblement identiques,
• les éléments transducteurs ultrasonores sont répartis selon une surface d'émission concave tronquée ou non,
• les éléments transducteurs ultrasonores sont répartis selon des anneaux ou des segments d'anneaux concentriques les uns aux autres selon l'axe de focalisation en présentant des surfaces d'émission différentes,
• les éléments transducteurs ultrasonores sont répartis sur une surface plane.

In addition, the apparatus according to the invention may additionally have in combination at least one and / or the following additional characteristics:

• at least some of the ultrasonic transducer elements are driven by excitation signals of substantially identical values,
• the ultrasonic transducer elements are distributed according to a concave emission surface truncated or not,
• the ultrasonic transducer elements are distributed according to rings or ring segments concentric with each other along the axis of focus by having different emission surfaces,
• the ultrasonic transducer elements are distributed on a flat surface.

• La Figure 1 est une vue en perspective d'une première forme de réalisation d'une sonde de thérapie conforme à l'invention.
• La Figure 2 est une vue schématique d'une demie-coupe élévation de la sonde de thérapie illustrée à la Fig. 1 permettant de décrire l'objet de l'invention.
• Les Figures 3A à 3D sont des vues schématiques en demi-coupe élévation de la sonde de thérapie illustrée à la Fig. 1 et montrant respectivement, l'effet de focalisation, l'effet d'absorption acoustique, la combinaison des effets de focalisation et d'absorption, le rééquilibrage de l'apport énergétique dans une zone cible par application de l'invention.
• Les Figures 4 et 5 sont des schémas en demi-coupe élévation permettant d'expliciter une variante selon l'invention.
• La Figure 6 est une vue de dessus montrant sur la partie gauche, la répartition des éléments transducteurs ultrasonores de l'art antérieur et sur la partie droite, la répartition des éléments transducteurs ultrasonores conforme à l'invention.
• La Figure 7 montre un exemple de réalisation d'une sonde de thérapie conforme à l'invention du type plan.

Various other characteristics appear from the description given below with reference to the accompanying drawings which show, by way of non-limiting examples, embodiments of the subject of the invention.

• The Figure 1 is a perspective view of a first embodiment of a therapy probe according to the invention.
• The Figure 2 is a schematic view of a half-section elevation of the therapy probe illustrated in the Fig. 1 to describe the object of the invention.
• The Figures 3A to 3D are schematic views in half-section elevation of the therapy probe illustrated in the Fig. 1 and respectively showing the focusing effect, the acoustic absorption effect, the combination of the focusing and absorption effects, the rebalancing of the energy supply in a target area by application of the invention.
• The Figures 4 and 5 are diagrams half-section elevation for explaining a variant according to the invention.
• The Figure 6 is a top view showing on the left side, the distribution of ultrasonic transducer elements of the prior art and on the right side, the distribution of ultrasonic transducer elements according to the invention.
• The Figure 7 shows an exemplary embodiment of a therapy probe according to the invention of the planar type.

The Fig. 7A and 7B show another variant embodiment of the probe described in Fig. 7 , with the Fig. 7A illustrating the probe with elementary ultrasonic transducer elements of the same surface that at the Fig. 7B are electronically assembled to present a surface modulation identical to that illustrated in FIG. Fig. 7 .

The Figures 1 and 2 illustrate a first exemplary embodiment of an ultrasound therapy probe 1 forming part of a device for generating focused ultrasound waves. The ultrasonic probe 1 comprises a plurality of ultrasonic transducer elements 3 distributed along a transmission surface 4. The ultrasonic transducer elements 3 are excited by control signals coming from a control circuit (not shown but known per se and adapted from whereby the ultrasonic transducer elements 3 emit focused ultrasonic waves in a focusing zone 5 to ensure biological or tissue lesions. In the example shown in Fig. 1 and 2 , The ultrasonic transducer elements 3 are distributed in a concave emitting surface 4 and each have a ring-shaped or ring. The ultrasonic transducer elements 3 are therefore mounted concentrically with respect to each other and with respect to the axis of focus X.

According to the invention, at least some of the ultrasonic transducer elements 3 have non-identical emission surfaces for emitting focused ultrasound waves which in a target zone 7 have substantially identical energy inputs. In other words, the ultrasonic transducer elements 3 have surfaces of different values to compensate for differences in focus and acoustic attenuation experienced by the ultrasound waves during their path between the emission surface 4 and the target zone 7. This target zone 7 can thus be chosen, as will be shown in the following description at all locations from the transmission surface 4 and up to the focusing area 5, the latter being the target area 7 in an advantageous embodiment.

Indeed, it must be considered that the ultrasonic waves pass from the transmission surface 4 to the target zone 7, several propagation media E ₁ , E ₂ ... E _i ... E _k , each presenting acoustic attenuations respectively A ₁ , A ₂ ... A _i , ... A _k . For example, the Fig. 2 illustrates the interposition between the focusing zone 5 and the probe 1, of a first propagation medium E ₁ in contact with the emission surface 4, having an acoustic attenuation A ₁ = 0 and a second medium E ₂ located at a distance a from the plane tangent to the probe. The first propagation medium E ₁ and the second propagation medium E ₂ have a separation zone or an interface 6. This second medium E ₂ which has an acoustic attenuation A ₂ (with A ₂ ≠ A ₁ ) extends at least to the focus area 5. The target area 7 is a plane in the example shown in FIG. Fig. 2 In the second medium E ₂ between the focusing zone 5 and the interface 6.

During the path of the ultrasonic wave between the emission surface 4 and the focusing zone 5, two phenomena, from the point of view of pressure, come into play, namely the geometric focusing effect and the acoustic attenuation. . The focusing effect is due to the concavity of the emission surface 4 giving rise to a strong increase of the pressure along the path of the ultrasonic wave while the acoustic attenuation which represents the energy transfer of the The ultrasonic wave towards its propagation medium depends mainly on the absorbing properties of the propagation medium, resulting in a decrease in pressure during the path traveled.

• Ei : milieu de propagation avec i = 1 à k,
• D_i : distance parcourue dans le milieu de propagation Ei (m),
• P(r) : pression à la distance r de la surface d'émission (P_a),
• Rc : rayon de courbure de l'élément transducteur (m),
• P₀ : pression lors de l'émission (Pa),
• Ai : Absorption acoustique du milieu de propagation Ei (Np.m^-1)

The pressure of an ultrasonic wave between the target zone 7 and the probe 1 is a function of the distance traveled by the waves in each of the media E ₁ , E ₂ and has the following expression (1): $$P\left(r\right)=P_0\mathrm{.}{\displaystyle \underset{i=1}{\overset{i=k}{\Pi }}}\left(e^{-{\mathrm{AT}}_i\mathrm{.}{D}_i}\right)\mathrm{.}\frac{\mathit{rc}}{\mathit{rc}-r}$$

• Ei: propagation medium with i = 1 to k,
• D _i : distance traveled in the propagation medium Ei (m),
• P (r): pressure at the distance r from the emission surface (P _a ),
• Rc: radius of curvature of the transducer element (m),
• P ₀ : pressure during transmission (Pa),
• Ai: Acoustic absorption of the propagation medium Ei (Np.m ^-1 )

In order to calculate the pressure in the target zone 7, only the attenuation and the focusing effect were taken into account. It is of course possible to refine the model by considering any other effect involved in the ultrasonic emission, including diffraction with a Rayleigh model for example.

In the case where the ultrasonic wave passes through two media E ₁ , E ₂ between the emission surface 4 and the target zone 7, the expression is as follows: $$\mathrm{P}\left(\mathrm{r}\right)={\mathrm{P}}_{\mathrm{0}}\mathrm{.}{\mathrm{e}}^{-\mathrm{AT}\mathrm{1}*\mathrm{D}\mathrm{1}}\mathrm{.}{\mathrm{e}}^{-\mathrm{AT}\mathrm{2}*\mathrm{D}\mathrm{2}}\mathrm{.}\mathrm{rc}/\left(\mathrm{rc}-\mathrm{r}\right)$$

At target area 7, it should be noted as shown in Fig. 3A , an inequality of the energy contributions within this zone along the x axis since the focusing effect is stronger in the center of this zone and weaker on the periphery. In addition, this phenomenon is increased by acoustic attenuation as illustrated in Fig. 3B . In the case where the first medium E ₁ (water for example) has a zero acoustic attenuation, the ultrasonic waves are not attenuated in the medium E ₁ , so these ultrasonic waves all have the same intensity when they arrive at the interface 6 (that is to say the skin for example). Beyond the interface 6, the distances traveled are unequal so that the ultrasonic waves emitted by a transducer element located at the periphery the emission surface have a greater distance to travel than those emitted from the center of the emission surface and are therefore attenuated when moving away from the focusing axis x. In the end, the combination of these two phenomena gives rise to the pressure curve P ₁ illustrated in FIG. Fig. 3C . This pressure curve shows a pressure inequality at the target zone 7 (namely the skin in the example considered), this pressure inequality can lead to the creation of burns close to the axis of focus x.

Given that the focusing effect as well as the attenuation experienced by the ultrasound waves are different as a function of their place of emission on the probe 1, it follows an inequality, in the target zone 7, in terms of energy contribution provided by the different ultrasonic waves.

According to the invention, this inequality in terms of the energy contribution in the target zone 7 is compensated by assigning to the ultrasonic transducer elements 3, surfaces of different sizes or values. It should be noted that all the ultrasonic transducer elements 3 are driven by excitation signals of substantially identical values. In other words, the same power setpoint is applied to all the ultrasonic transducer elements 3 . It thus appears possible to use all the available power by the probe.

avec 0< F_s <1

• n : le numéro de l'élément transducteur 3 et variant de 1 à t dans le sens axe de focalisation X vers la périphérie de la surface d'émission 4,
• F_p : le facteur de puissance,
• Z : la somme sur les éléments transducteurs des 1/F_p.

The method according to the invention thus aims at determining a surface weighting factor F _s for each of the ultrasonic transducer elements 3, such that: $${\mathrm{F}}_{\mathrm{s}}\left(\mathrm{not}\right)=\mathrm{1}/\left[{\mathrm{F}}_{\mathrm{p}}\left(\mathrm{not}\right)\mathrm{.}\mathrm{Z}\right]$$

with 0 <F _s <1

• n: the number of the transducer element 3 and varying from 1 to t in the axis of focus direction X towards the periphery of the emission surface 4,
• F _p : the power factor,
• Z: the sum on the transducer elements of 1 / F _p .

The power factor F _p (n) is expressed as a function of the focusing effect and the acoustic attenuation on each transducer element ultrasound 3 between the transducer element and the target zone 7, during a cutting of the emission surface in equal areas (before modulation).

• Max E(t) : valeur maximale de la contribution énergétique de l'élément transducteur t situé à la périphérie de la surface d'émission 4,
• Max E(n) : valeur maximale de la contribution énergétique de l'élément transducteur n dans la zone cible 7.

The power factor F _p (n) can be expressed as follows: $${\mathrm{F}}_{\mathrm{p}}\left(\mathrm{not}\right)=\mathrm{Max\; E}\left(\mathrm{t}\right)/\mathrm{Max\; E}\left(\mathrm{not}\right),$$

• Max E (t): maximum value of the energy contribution of the transducer element t located at the periphery of the emission surface 4,
• Max E (n): maximum value of the energy contribution of the transducer element n in the target zone 7.

avec S_totale, la surface totale de la sonde.The area or area S (n) of each ultrasonic transducer element 3 of rank n is such that: $$\mathrm{S}\left(\mathrm{not}\right)={\mathrm{S}}_{\mathrm{Total}}{\mathrm{F}}_{\mathrm{s}}\left(\mathrm{not}\right)$$

with S _total , the total area of the probe.

From the above expressions, it can be seen that the transducer elements 3 close to the center of the probe (of the X axis of focus ) have a greater surface area with respect to the transducer elements 3 close to the periphery of the probe. Thus, the surface of the transducer elements 3 increases for the transducer elements 3 close to the center and inversely decreases for the transducer elements close to the periphery of the probe.

The application of these different surface weighting factors F _s for the ultrasonic transducer elements 3 causes a modification of the pressure field and thus makes it possible to rebalance the energy supply of each of the ultrasonic transducer elements 3 in the target zone 7. of the Fig. 3D , the energy input of the ultrasonic waves emitted by the different ultrasonic transducer elements 3 is substantially identical in the target zone 7 ( P ₂ curve ) despite the focusing effect and the acoustic attenuation experienced by the ultrasonic waves in their path.

In the example shown in Fig. 2 , the ultrasonic waves pass through two acoustic attenuation media whose interface 6 between the media is flat, parallel to the plane tangent to the probe. Of course, the number of acoustic attenuation media traversed by the ultrasonic waves can to be more important. Similarly, the shape of the interface 6 between the acoustic attenuation media may be different from a plane parallel to the plane tangent to the probe.

The Fig. 4 illustrates an example in which the interface 6 between the two acoustic attenuation media E ₁ , E ₂ is convex. Indeed, at the Fig. 4 , the volume of water (acoustic attenuation medium E ₁ ) is greater so that the focusing contrast and attenuation is greater. The contrast of the energy contributions is accentuated for an interface 6 of convex shape with respect to a plane interface.

On the contrary, an interface 6 of concave shape as illustrated in FIG. Fig. 5 leads to a rebalancing of energy contributions compared to the example illustrated in Fig. 2 . Of course, in the specific case where the interface 6 between the acoustic media and the target zone 7 has the same center of curvature as the emission face of the probe 1, then the energy contributions of the ultrasonic transducer elements are identical in the target area 7.

In general, it should be considered that the method according to the invention aims to choose a target zone 7 in which is desired a homogenization of the energy input of the ultrasonic waves emitted by the ultrasonic transducer elements 3. According to a first variant preferred embodiment, this target zone corresponds to the focusing zone. According to a second preferred embodiment, this target zone corresponds to a plane included in a propagation medium and in particular in the second propagation medium, corresponding to the tissues situated between the cooling water and the tissue to be treated.

The method according to the invention aims at determining the focusing effect as well as the acoustic attenuations of the ultrasonic waves on their path between this target zone 7 and the ultrasonic transducer elements 3. As explained above, this determination phase consists in taking the focusing effect and the acoustic attenuation of the various propagation media traversed and the distance between the ultrasonic transducer elements 3 and the interface or the interfaces between the media. This distance can be calculated according to the configuration of the propagation medium or media with respect to the ultrasonic transducer elements 3. It should be noted that the distance between the ultrasound transducer elements 3 and the interface of the media can be determined more precisely. by measuring the reflected echoes in A mode which consists in measuring the echoes reflected following the sending of a calibration signal by the ultrasonic transducer elements 3.

As a first approximation, from equation (1), it is possible to calculate the pressure in the target zone 7 for a multitude of ultrasonic waves coming from the emission surface making it possible to obtain the pressure curve P ₁ illustrated. to the Fig. 3C .

The transmission surface 4 is cut from the focusing axis x to its peripheral portion. In the case of a transmission surface 4 of revolution, the emission surface 4 is cut into concentric rings each contributing to a portion of the pressure curve P ₁ . For each ring, the maximum pressure value is determined and a surface weighting factor F _s is applied so that this maximum pressure value becomes the same on all the elements (curve P ₂ ).

The method according to the invention therefore makes it possible to modulate the emission surface of the ultrasonic transducer elements 3 in areas of different sizes but adapted so that the energy input of the ultrasonic waves is substantially identical in the target zone 7. Thus, the different transducer elements 3 are configured with emission surfaces of different values adapted for one or more given applications. It should be noted that the greater the number of ultrasonic transducer elements 3 , the more accurate and efficient the modulation.

The Fig. 6 illustrates the cutting of a focusing probe having transducer elements 3 in the form of rings. The left part of the Fig. 6 represents ultrasonic transducer elements of equal surfaces while the right part of the Fig. 6 represents ultrasonic transducer elements 3 with different surfaces modulated according to the method according to the invention.

Of course, the method according to the invention can be implemented for therapy probes of various shapes. In the example shown on the Fig. 1 The ultrasonic transducers 3 elements are distributed according to a complete concave emitting surface of revolution. For specific applications, this concave surface may be truncated on either side of a central plane of symmetry so that the ultrasonic transducer elements 3 are distributed according to ring segments concentric with each other. According to a preferred variant embodiment, this concave surface has the shape of a torus, that is to say that this concave surface is generated by the rotation of a concave curve segment of finite length around an axis of symmetry. which is at a non-zero distance from the center of curvature of the concave curve segment. Of course, this emission area of toric shape can be truncated on either side of a central plane of symmetry. According to another variant embodiment, the concave emission surface is derived from a cylindrical geometry generated by the translation of two concave curve segments of finite length, symmetrical with respect to a plane of symmetry, this translation being carried out along a length limited and in a direction perpendicular to the plane containing said concave curve segments. The Fig. 7 illustrates by way of example, a probe 1 of planar shape, the various ultrasonic transducer elements 3 have emission surfaces of different sizes.

Of course, in the case of a therapy probe 1 of planar shape, each ultrasound transducer element is powered by signals having phase shifts to obtain a focusing effect in the target area.

Another object of the invention is to be able to propose a technique for producing a probe configurable on demand depending on the configuration of the ultrasonic wave propagation media. As more specifically Fig. 7A , 7B This technique provides to choose an elementary size for all of the ultrasonic transducers elements 3 _1. So in the example shown in the Fig. 7A illustrating a planar emission surface, all elementary ultrasonic transducer elements 3 ₁ have the same emission surface. These elementary ultrasonic transducer elements 3 ₁ are then grouped so as to produce ultrasonic transducer elements 3 which have different surface sizes ( Fig. 7B ). Thus, this technique makes it possible to create, on demand, ultrasonic transducer elements 3 having different emission surfaces. It should be noted that in the case of a concave emission surface, the ultrasonic transducer elements 3 ₁ may have different elementary sizes, with an identical width for all the ultrasonic transducer elements 3 ₁ .

Claims (13)

1. A method for generating focused ultrasonic waves over a focal zone (5) to produce biological lesions, comprising the activation of a plurality of ultrasonic transducer elements (3) distributed over an emission surface (4) to respectively emit a plurality of focused ultrasonic waves in the focal zone (5), while crossing through propagation media (E_i ) having different acoustic attenuations, characterized in that:

   - a target zone (7) is chosen in which homogenization of the energy contributions of the ultrasonic waves emitted by the ultrasonic transducer elements is desired,

   - the focal effect and the acoustic attenuations of the ultrasonic waves on their paths between the target zone (7) and the ultrasonic transducer elements are determined (3),

   - the focal effect and the acoustic attenuations of the ultrasonic waves are compensated, with ultrasonic transducer elements (3), at least some of which have non-identical emission surfaces so that in the target zone (7), the energy contribution of the ultrasonic waves emitted by the various ultrasonic transducer elements (3) are substantially identical,

   - the ultrasonic transducer elements (3) are activated by excitation signals of substantially identical values so that in the target zone (7), the energy contribution of the ultrasonic waves emitted by the various ultrasonic transducers are substantially identical and produce biological lesions.
2. The method according to claim 1, characterized in that it consists of compensating the focal effects and the acoustic attenuations by assigning each of the ultrasonic transducer elements (3) a surface weight factor (F_s ) depending on the acoustic attenuation and the focal effect undergone by the ultrasonic waves.
3. The method according to claim 2, characterized in that it consists of determining the acoustic weight factors (F_s ), taking into account the distance between the ultrasonic transducer elements (3) and the separating zone (6) of the propagation media (E_i ).
4. The method according to claim 3, characterized in that it consists of taking into account the distance between the ultrasonic transducer elements and the separating zone (6) of the propagation media, calculating that distance as a function of the configuration of the propagation media (E_i ) relative to said ultrasonic transducer elements.
5. The method according to claim 3, characterized in that it consists of taking into account the distance between the ultrasonic transducer elements and the separating zone (6) of the propagation media, by measuring the echoes reflected following the sending of a calibration signal by the ultrasonic transducer elements (3).
6. The method according to one of claims 1 to 5, characterized in that it consists of grouping together ultrasonic transducer elements (3₁ ) with elementary sizes so as to form ultrasonic transducer elements (3) with different emission surfaces configurable based on the encountered acoustic attenuations.
7. The method according to one of claims 1 to 6, characterized in that it consists, for a plurality of ultrasonic transducer elements (3) distributed on a concave emission surface with a radius of curvature Rc, of calculating the area Sn of each ultrasonic transducer element n such that:

   Sn = [S_total (1/(Fp(n).Z))]

   - with S_total: the sum of the surfaces of the ultrasonic transducer elements,

   - Fp (n) = Max E(t) / Max E(n),

   with Max E(t), the maximum value of the energy contribution of the transducer element t situated at the periphery of the emission surface (4) and Max E(n), the maximum value of the energy contribution of the transducer element n in the target zone (7),

   Z: sum of the 1/Fp for all of the transducer elements.
8. A therapeutic apparatus for generating focused ultrasonic waves on a focal zone (5), including an ultrasonic probe (1) formed by a plurality of ultrasonic transducer elements (3) distributed on an emission surface (4) to emit a plurality of ultrasonic waves focused in the focal zone (5), crossing through the propagation media (E_i ) having different acoustic attenuations (A_i ), the ultrasonic transducer elements (3) being excited by control signals coming from a control circuit, characterized in that at least some of the ultrasonic transducer elements (3) have non-identical emission surfaces to emit focused ultrasonic waves which, in a target zone (7), have substantially identical energy contributions and in that the control circuit activates the ultrasonic transducer elements (3) by excitation signal of substantially identical values, so that in the target zone (7) the energy contribution of the ultrasonic waves emitted by the various transducer elements is substantially identical and produces biological lesions.
9. The apparatus according to claim 8, characterized in that at least some of the ultrasonic transducer elements (3) are controlled by excitation signals with substantially identical values.
10. The apparatus according to claim 8, characterized in that the ultrasonic transducer elements (3) are distributed according to a concave emission surface (4) that may or may not be truncated and in particular according to a concave emission surface of truncated toroïdal shape.
11. The apparatus according to claim 8, characterized in that the ultrasonic transducer elements (3) are distributed in rings or ring segments concentric to each other along the focal axis while having emissions surfaces with different values.
12. The apparatus according to claim 8, characterized in that the ultrasonic transducer elements (3) are distributed on a planar surface.
13. The apparatus according to claim 8, characterized in that the ultrasonic transducer elements (3) are distributed on a concave emission surface resulting from a cylindrical geometry created by translating two concave curve segments with a finite length, which are symmetrical relative to a plane of symmetry, the translation being done along a limited length and in a direction perpendicular to the plane containing said concave curve segments.

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