EP0169123B1 - Electronic focusing device for ultrasonic waves - Google Patents

Abstract

A device for electronic focusing of ultrasonic waves in which the focusing means comprise a hierarchized assembly of elementary circuits each provided with a delay line connected in parallel with a direct line to a centralizing unit and in which the time-delay of the delay line in a circuit is a function of the relative time-delay which must exist between the two cells or the two circuits connected to said circuit. It is shown that, in the invention, a technological advance is achieved by thus minimizing the maximum time-delays to be established in the case of each delay line.

Description

The invention relates to a device for electronically focusing ultrasonic waves. Ultrasonic wave devices are particularly used in the medical field and in industrial control. Their purpose is to allow the measurement of physical parameters of an environment in which these waves are emitted and / or received. On transmission, an ultrasonic device comprises an emitter of electrical signals connected to a piezoelectric excitation probe. On reception, an ultrasonic device includes a piezoelectric probe connected to a receiver. Conventionally a duplexer allows the use of a single reversible probe for transmission and reception by sharing the time between each of these two functions.

The mechanical pressure wave that propagates from a probe into a medium is a spherical wave. Consequently the quantity of excitation energy applied to a particular point of the volume studied decreases with the square of the distance which separates this point from the probe. In addition, on reception the same phenomenon plays out: the backscattered signal coming from a particular point in the medium also propagates in a spherical wave. It is known, to increase the excitation power applied to a particular point in the medium at the time of transmission, and to increase the energy of the signal received from a particular point at the time of reception, to perform probes comprising an array of piezoelectric cells. In order for the expected effects to be effectively obtained, it is also necessary to organize delays between the electrical signals which attack the various cells of the network. These delays, which are intended to focus the wave on the point in question, are conventionally obtained by using delay lines connected in series to the cells. The delays of each delay line are fixed at given values for a given position of the point of the medium on which one wants to focus the emission wave or from which one wants to receive the transmitted wave. Each cell is associated with a particular delay line: there are as many delay lines as there are cells.

Furthermore, it is desired to be able to use the ultrasonic devices to point them at various regions of the environment. These regions are contained between two extreme depointing directions. It is then advisable to produce delay lines whose delay is variable and whose maximum delay corresponds to these extreme directions. For example, taking into account a deviation of 45 ° from a normal axis of propagation perpendicular to the network of cells, a propagation speed of 1,500 m per second in the medium, and a distance of approximately 100 mms between the target point and the network, it appears that a wave, which finds itself in phase on the point concerned, admits delay offsets between the different cells of the network. These offsets are such that in particular there is a maximum differential delay of approximately 10 microseconds between two extreme cells of a network measuring approximately 22 mm in length. To be able to explore all the points in the middle in the same way, by symmetry with respect to the normal at the center of the network, the delays of the delay lines concerning the cells situated at the ends of the network must be identical. However, the production of delay lines poses many technical and technological problems. It is advisable to reduce the number of these lines and especially the duration of their intrinsic delay. A solution approach is described in patent application FR-A-2 237 322.

The invention relates to a focusing device in which the delay of the delay lines is less than the maximum differential delay of the network and where, ultimately, the cumulative amount of delays of all the delay lines is much less than the total of the delays of all the delay lines of the cited state of the art.

The invention relates to an electronic focusing device for ultrasonic waves, in which an ultrasonic wave propagates, in one direction and / or another, between an array of piezoelectric cells and a focal point located inside a medium, in which the electrical signal corresponding to this wave is transmitted with different delays for each cell, delays which are dependent for each cell on the relative positions of the focal point and the cell in question, and in which means for creating the delays comprise delay circuits connected to the cells characterized in that these delay creation means comprise a hierarchical set of elementary circuits, each provided with a delay line connected, in parallel with a direct line, to a centralizer, and in that that the delay of the delay line in an elementary circuit is a function of the relative delay which must prevail between two cells or two circuits connected to this circuit t.

• - figure 1a : un dispositif à ultrasons de l'état de la technique,
• - figure 1b : un circuit élémentaire des moyens de retard ;
• - figure 1c : une représentation de l'organisation hiérarchique des circuits élémentaires ;
• - figure 2a : un perfectionnement du circuit élémentaire ;
• - figure 2b : une représentation schématique des moyens de retard de l'invention permettant la mise en évidence du gain quantitatif des retards à créer ;
• - figure 3a : un exemple de réalisation analogique d'un circuit élémentaire,
• - figure 3b : un exemple de réalisation numérique d'un circuit élémentaire ;
• - figure 4 : un exemple d'utilisation de l'invention.

The invention will be better understood on reading the description which follows and on examining the figures which accompany it. In these figures, the same references designate the same elements. They represent :

• - Figure 1a: an ultrasonic device of the prior art,
• - Figure 1b: an elementary circuit of the delay means;
• - Figure 1c: a representation of the hierarchical organization of elementary circuits;
• - Figure 2a: an improvement of the elementary circuit;
• - Figure 2b: a schematic representation of the delay means of the invention allowing the quantitative gain of the delays to be created to be highlighted;
• FIG. 3a: an example of an analog embodiment of an elementary circuit,
• - Figure 3b: an example of digital embodiment of an elementary circuit;
• - Figure 4: an example of use of the invention.

où d_i est la distance qui sépare une cellule Ci du point P et où c est la vitesse de propagation de l'onde 2 dans le milieu 3. Un additionneur 6 à multiples entrées reçoit les signaux S_i(t) après qu'ils ont subi leur retard respectif et les combine pour délivrer un signal S(t) représentatif de l'onde qui a été émise au point P.FIG. 1a represents an ultrasound device of the state of the art. It includes a piezoelectric probe 1 provided with a set of n cells C _i . An ultrasonic wave 2 propagates from a point P of a medium 3 in the direction of the probe. Wave 2 is spherical. The pressure wave is detected by the cells Ci and is translated into a set of electrical signals S _i (t). The cells are connected to delay means 4 comprising a set of identical delay lines such as 5. A multiplexing instruction M fixes, for each delay line, a specific delay T _i . Wave 2 which is received at different times by the different cells Ci is then extracted simultaneously from all of the delay lines. A delay T _i can be expressed as follows:

where d _i is the distance which separates a cell Ci from the point P and where c is the speed of propagation of the wave 2 in the medium 3. An adder 6 with multiple inputs receives the signals S _i (t) after they have undergone their respective delay and combine them to deliver a signal S (t) representative of the wave which was emitted at point P.

In the representation which is made in FIG. 1a, the ultrasonic device is used in reception. In use in transmission, the adder 6 is replaced by a current distribution point (quite simply an electrical connection point) and a signal E (t), arriving at this sampling point in the opposite direction to the signal S (t ), attacks all 4 of the delay lines 5. If the distribution of the delays in the different lines is identical to the previous one, the sound wave emanating from the probe 1 then focuses on point P. So from the point of view delays it is immaterial to examine the problem on transmission or reception. To simplify understanding, the following description is given in a reception configuration, it being understood that the focusing device of the invention can also be used for transmission. Similarly, we are not obliged to use reversible probes, but it is still more economical.

Figure 1b shows an elementary circuit, Figure 1c shows how a hierarchy is organized. The elementary circuit 7 of FIG. 1b essentially comprises a delay line 8 connected in parallel with a direct line 9 to a centralizer 10. In an application in transmission the centralizer is an electrical connection point. In the reception application, the centralizer 10 is an adder for adding the signals coming from the two lines 8 and 9. The circuit 7 has two inputs and one output. The direct line 9 of the circuit 7 receives a signal S _i (t) emanating from a sensor C _i . The delay line 8 of circuit 7 receives a signal S _{i + 1} (t) emanating from a sensor C _{i + 1} . The invention consists in observing that rather than assigning each of the signals S _i (t) and S _{i + 1} (t) of a delay T _i and T _{i + 1 respectively} it is possible to assign one of the two of a delay T _{i + 1} ―T _i . After the addition of these two signals, the result of a delay T i is affected. In the added signal the contribution of the signal S _i (t) is indeed affected by a delay T, and the contribution of the signal S _{i + 1} (t) is effectively affected by a delay T _{i + 1} (T _{i + 1} -T _i + T _i ). This is of course only possible if T _{i + 1} is greater than T _i since we do not know how to achieve a negative delay.

It can be seen by looking at FIG. 1 that the signal emanating from the circuit 7 must be affected by a delay T _i . Similarly in FIG. 1c, a circuit 11 adjacent to circuit 7 receives signals S _{i + 2} (t) and S _{i + 3} (t). Its output must be affected by a delay T _{i + 2} so that each of its signals cooperates with the final result with a fair contribution. By bringing the signal emitted by circuit 7 closer to the signal emitted by elementary circuit 11, these two signals can be introduced into a third elementary circuit 13 also comprising a direct line 14 in parallel with a delay line 15. The delay of the line with delay 15 must be equal to T _{i + 2} ―T _i . The signal emanating from the circuit 13 must then be assigned a delay T _i so that all the signals are assigned their respective delays T, T _{i + 1} , T _{i + 2} , and T _{i + 3} .

The structure of the elementary circuit of FIG. 1b implies three remarks. First the construction of the. The hierarchy of circuits can continue in a pyramidal manner according to the structure suggested in Figure 1c. On the other hand, given the complexity that there is in manufacturing delay lines whose maximum variable delay is high, it is advantageous to connect circuits 7 to adjacent cells and likewise circuits 13 to circuits adjacent elementaries. Indeed, the relative delay put by wave 2 to arrive on two geographically adjacent cells is low. It is for this reason that two adjacent cells, Ci and C _{i + 1} , are connected to the same circuit: the delay line 8 must be capable of a maximum delay which in the end is rather low. It is also for this reason that two adjacent circuits, 7 and 11 are connected to the same circuit 13. Two elementary circuits are thus said to be adjacent when, at their level of hierarchy, the signals which they produce exhibit a relative delay which is as low as possible. Thirdly, we notice that the delay T _i affects the signals received by all the cells. It is possible to do without this last delay line since it does not contribute to better focusing acting in an identical manner on all the signals.

The previous restriction requiring that T _{i + 1} be greater than T _i implicitly indicates that the point P can only be found to the right of the normal N to the network which rises at the right of the last cell C _n . Indeed, in this case the cells receive wave 2 earlier and earlier depending on whether their index increases (C _n receives it first). The delay T _n will therefore be greater than delay T _n-1 etc ... until delay T ₁ . To be able to focus the wave on points located to the left of cell C _i , it would be necessary to be able to produce delay lines T _{i + 1} ―T _i with negative delay. This apparently insoluble problem is solved by adding (fig. 2a) to the elementary circuit 7 a switching circuit 16 which makes it possible to choose which of the two signals S _i (t) or S _{i + 1} (t) will be affected by the delay T _{i + 1} ―T _i . Indeed, if T _{i + 1} - T _i is negative we obtain the correct result by delaying the other signal by the absolute value of T _{i + 1} ―T _i . The switch circuit 16 therefore has two inputs 17 and 18 and two outputs 19 and 20. It also has a control input 21 for receiving a switch signal A. In one example, the switch circuit 16 has 4 analog gates 22 to 25 receiving two by two (22, 24 and 23, 25) respectively the signals Si and S _i-1 . The order A or its complement A obtained from an inverter 26, is applied to the validation input of these analog doors so as to transmit at the output of these doors, on the outputs 19 and 20 of circuit 16, respectively the signals S _i (t) and S _{i + 1} (t) on the one hand or S _{i + 1} (t) and S _i (t) on the other hand. This ultimately makes it possible to delay only one of these two signals with respect to the other by the desired quantity.

However, it should still be noted that the position of the delay line, having regard to the distribution, S _i (t): S _i-1 (t), of the signals is not indifferent. By taking again the diagram of figure 1b and by having inverted the entries by means of the switch 16, of the fact that the delay T _{i + 1} ―T _i is negative, it is necessary well to realize that one delayed in this case the signal S _i (t) of a delay equal to T _i ―T _{i + 1} and that therefore consequently at the output of the circuit 7 after the adder 10 it is no longer a delay T _i that should be imposed but a delay T _{i + 1} . This remark is important because it makes it possible to combine with accuracy in a circuit 13 (FIG. 1c) the signals coming from two elementary circuits 7 and 11 when the inputs have been inverted. In practice this question of delay T _i changed to T _{i + 1} does not pose a problem since the delay imposed by the delay line is variable. We can therefore easily make it equal to T _{i + 1} - T _{i + 3} instead of T _{i + 2} ―T _i . A control signal M, in all respects comparable to the multiplexing signal of the prior art cited, controls the delay lines of the invention. Only the values that are displayed for the different delay lines change between the invention and the state of the art.

FIG. 2b makes it possible to grasp the technological gain of the accumulation of delays obtained with the delay means of the invention. The hierarchical organization has several floors. A first stage 30 combines in elementary circuits signals originating in each case from two adjacent cells. A second stage 31 combines the signals coming from two adjacent circuits of the stage 30. A third stage 32 combines the signals coming from two adjacent circuits of the stage 31. The circuit of FIG. 2b comprises for each elementary circuit a line with delay 33 in parallel with a direct line 34, both connected on the one hand to a centralizer 35 and to a routing circuit 36. What differs from one stage to another is that the maximum differential delays of delay lines of the floors are not equal to each other. Indeed, the maximum delay of a delay line 33 is imposed by the relative delay that may exist, when the probe is at the maximum depointing, between two adjacent cells (that is to say cells whose index n ' is different than one unit). Let ΔT be this maximum delay. The maximum delay of the delay lines 37 of the second stage is the delay which must exist between two cells whose index has a difference of two units: for example between Ci and C _{i + 2} . Consequently, the maximum delay of lines 37 is equal to twice ΔT. By extrapolation, the maximum delay of the delay line 38 of the third stage is 4ΔT. In the example shown to be able to reach all the desired depointing configurations, it is necessary to make 4 delay lines with a variable delay whose maximum value is ΔT, two variable delay lines whose maximum delay is 2AT, and one line with variable delay whose maximum delay is 4ΔT. In total, therefore, 12AT must be achieved. On the other hand, in the cited state of the art, the delay which may exist between the cell of index 1 and the cell of index 8 may be equal to 7AT, it would have been necessary to construct 8 variable delay lines with a maximum delay of 7ΔT each. : that is to say 56ΔT in total. It can therefore be seen that the invention brings about a quantitative reduction in the delays to be achieved. We can show by calculation that this reduction is equal to 1/2 log ₂ (n). In this expression n is the number of cells. Furthermore qualitatively we realize that the maximum variable delay for a delay line is 4AT in the invention. By cons it was 7ΔT in the state of the art cited. However, the making of delay lines is all the more difficult technically as the delay to be implemented is greater. So the quantitative reduction the invention also brings a qualitative improvement. We could show that it is the presence of switches 36, 39 and 40 in front of each elementary circuit which is at the origin of this qualitative improvement.

FIG. 3a represents an analog embodiment of an elementary circuit 41 according to the invention. This circuit 41 has two inputs 42 and 43 connected to the inputs of a switcher 44 whose outputs are connected to an analog delay line 45 on the one hand and to a direct line 46 on the other hand. The delay line 45 is for example of a type with localized constants. An adder 47 centralizes the signals from these two lines. The output 48 delivers the combined signal. The delay line 45 is provided with p intermediate sockets making it possible to obtain that a signal introduced at the input is delayed up to p times a minimum delay. A direct tap 27 also makes it possible to take the signal introduced into the delay line 45 when the latter presents a zero relative delay with respect to the signal on the line 46. If one calls 8T this minimum delay the product p times 8T equals ΔT which is the maximum delay of line 45 and therefore of circuit 41. A multiplexer 49 receives a multiplexing order M and selects a particular tap on delay line 45 to connect it at the input of the adder 47. This multiplexer 49 here plays the same role as in the state of the art cited. Its complexity is however less in the invention since the delay of the delay lines being less there the number of intermediate taps to be selected is less.

The instructions M of the invention are of course different from that of the state of the art cited. Note, however, that the number of multiplexing orders contained in a multiplexing instruction M for a given focus is substantially equal to the number of multiplexing orders that had to be given in the prior art. Indeed, with reference to FIG. 2b, it can be seen that for 8 cells there are 7 delay lines to be programmed: therefore at each multiplexing instruction there are seven multiplexing orders to be addressed to the delay lines. In a conventional configuration for the same 8 cells, there would be 8 delay lines, therefore 8 orders to be addressed. In the invention, there are therefore as many orders to be addressed as there are cells minus one unit. For the rest, the programming of the M orders is done as in the cited state of the art. It consists, for a given point in space on which we want to focus the waves, to calculate for each cell Ci the delay T _; corresponding, and to deduce, according to the hierarchical organization defined above, what delay T _{i + 1} ―T _i , T _{i + 2} ―T _i etc ... it is necessary to carry out for each of the elementary circuits. Then the multiplexing instruction relating to this position is established which contains all the multiplexing orders. Likewise, the switching orders A are established to be addressed to the different switches. All these orders are memorized. We repeat this operation as many times as there are different points to explore in the middle. The sequences of orders relating to each point are then sent to the delay means in good time at the time of the experiment.

FIG. 3b represents a digital embodiment of an elementary circuit 50 of the invention. Before this circuit 50, signals S _i (t) and S _{i + 1} (t) emanating from cells Ci and C _{i + 1} are each sent to an analog-digital converter 51 and 52 respectively. These converters are related to intermediate registers 53 and 54 contained in circuit 50. A switch 55 selectively directs the contents of these registers to a direct line 56 or to a set 57 of shift registers. The set 57 contains as many registers as there are bits for coding the sampled values of the signals S _i (t). If these signals are coded on q bits there are q shift registers. Each of these shift registers has p boxes and the set 57 is synchronized with the sampling frequency f _ech which drives the converters 51 and 52. A direct tap 28 allows the signal introduced into the delay line 57 to be taken without delay when the relative delay of this signal compared to the signal in the direct line is zero. At each sampling pulse the content of a given cell of a register is transmitted to the next cell of this register (from R _j to R _{j + 1} ). A multiplexer 58, receiving a multiplexing order M corresponding to a point P in the middle, extracts the contents of the corresponding boxes from the registers. The summer 59 receives quantized signals from the direct line 56 and from the delay line 57. The digital output of the summer 59 is coded on q + 1 bits. When the circuit 50 is placed in a higher stage of the hierarchy it receives quantized signals emanating from the preceding elementary circuits: it is no longer connected downstream of analog-digital converter.

There are cases of use of ultrasonic devices where an ultrasound antenna is produced in the form of a linear array of cells. On an antenna of n cells (for example n = 100) one can select a restricted group of contiguous cells (for example from 8 to 16) with which an ultrasonic beam is formed in a direction substantially perpendicular to the antenna, or possibly very slightly tilted from normal. With such probes we are not looking for significant depointing. On the contrary, we manage to focus only on points whose projection on the bar is always contained in the segment comprising the central cells of the group of contiguous cells. In this mode of exploration, it can be seen that two cells, symmetrical with respect to the center of the group of selected cells, must be affected by almost equal delays. So ultimately their relative delay is the lowest there is. The considerations which have prevailed so far for the adjacent cells teach that it is necessary to associate cells symmetrical with each other with respect to the center of the group.

FIG. 4 represents such an architecture for 8 cells. An elementary circuit 60 receives signals from two cells CB ₁ and CB ₈ of a strip 61. A second circuit 62 receives signals from cells CB ₂ and CB ₇ , a third circuit 63 receives signals from cells CB ₃ and CB ₆ , and the fourth circuit 64 receives it from CB ₄ and CB _S. Delays T _1-8 , T _2-7 , T ₃ . ₆ and T ₄ . ₅ of the delay lines of these elementary circuits are therefore minimum delays. In fact, the point P only moves very slightly away from the perpendicular bisector 65 of the group of cells. Ultimately, the delay line with the highest maximum delay is the delay line producing the delay T _1-8 . This delay T _1-8 corresponds to the difference in path of the wave when it propagates from point P towards the cell CB ₁ on the one hand and the cell CB ₈ on the other hand. For all other delay lines the relative delays are lower. Here again, it is noted that this delay T _1-8 which is the greatest, is much less than the delay T _4-8 which in the example would be the greatest. The latter is more than five times higher.

The circuits 60 and 62 to 64 are connected to two elementary circuits 66 and 67. The latter two are themselves connected to a last elementary circuit 68. In view of the small deviations Omitted in use, one could dispense with the presence of the circuits d 'switch (AIG) contained in each of the elementary circuits. The presence of these routing circuits lies in the scanning ability of the probe used. Indeed, the microangulations 8 designate points P, the projection of which on the bar 61 cannot be beyond one of the central cells (C ₄ , C ₅ ). As soon as the medium 3 has been explored between + 0 and ―θ, use is made of another cell, on an adequate side, by eliminating from the group of selected cells that which is on the other side. Step by step we realize that the delay between two cells is thus brought to change sign. This justifies the presence of the switching circuits.

In this application, the scanning of the medium to be studied results in a selection of the CB cells _; used. The multiplexer which performs this function is of a known type. If one wishes to carry out the invention in this application without modifying this multiplexer, one can choose to carry out the optimizations of the relative delays at the output of the first stage. The elementary circuits of the first stage then receive signals from geographically adjacent cells CB (CB _i ―CB _{i + 1} ). In a second stage (66-67) are introduced signals from elementary circuits, 60-62 and 63-64, which themselves receive signals from cells geographically symmetrical to each other with respect to the center from the group of selected contiguous cells.

The transformation of the switching circuits for the passage of a device from a reception function to a transmission function does not present any difficulty. It only requires the production of a second set of switches interposed between the different circuits or between the circuits and the cells. This second set receives the signals from the delay lines and applies them to the piezoelectric cells. It is oriented in the opposite direction to that described. It conforms by design.

Claims (10)

1. A device for the electronic focussing of ultrasonic waves wherein an ultrasonic wave (2) is propagated in one and/or the other direction, between a network (4) of piezoelectric cells (5) and a focal point (P) situated in the interior of a medium (3), wherein the electric signal Si(t) corresponding to this wave is transmitted with different delays (T_;) for each cell (C_i), such delays being dependent for each cell on the relative positions of the focal point and the cell in question, and wherein the means for creating the delays comprise delay circuits connected with the cells, such means for the creation of delays comprising a hierarchical system (figure 1c) of elementary circuits (7, 11 and 13) each furnished with a delay line (8) connected, in parallel with a direct line (9) to a centralizer (10) and in that the delay (T_i+1 -T_;) of the delay line in a circuit is a function of the relative delay having to obtain between two cells (C_; and C_i+1) or two circuits (7 and 11) connected with this circuit, characterized in that each elementary circuit (7) comprises a pointer (16 to 26) in order to invert the relative delay between the two cells or between the two circuits connected with such elementary circuit.

2. The device as claimed in claim 1, characterized in that the two cells or the two circuits connected with a circuit are mutually adjacent to each other (i and i+1).

3. The device as claimed in any one of the claims 1 and 2, characterized in that it comprises means in order to ensure that an ultrasonic wave is propagated from the cells towards the emitting focal point.

4. The device as claimed in any one of the claims 1 and 2, characterized in that it comprises means (10) in order to ensure that the wave is propagated from the focal point towards the receiving cells.

5. The device as claimed in claim 3 and in claim 4, characterized in that the cells are piezoelectric reversible cells.

6. The device as claimed in claim 3, characterized in that the centralizer is a point of electrical connection.

7. The device as claimed in claim 4, characterized in that the centralizer is an adder (10).

8. The device as claimed in any one of the claims 1 through 7, characterized in that the number of stages of the hierarchy of circuits is equal to the logarithm to the base 2 of the number of cells.

9. The device as claimed in claim 1, characterized in that two cells (CB1 and CB8) connected with a common elementary circuit (60) are placed symmetrically in relation to the center (65) of a group (CB1 to CB8) of cells of which they form a part.

10. The device as claimed in claim 1, characterized in that the two circuits (66 and 67) connected with a common elementary circuit (68) receive signals originating from the cells (CB1, CB2, CB7 and CB8) placed symmetrically in relation to the center (65) of a group of cells of which they form a part.

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