FR2596156A1 - Ultrasonic echograph made of an electrostrictive material in near Fresnel arrays - Google Patents

Abstract

Ultrasonic echograph in near Fresnel arrays comprising a focusing array of n ultrasonic transducers and in which the ultrasonic transducers (a, b, c,..., i, ..., n-1, n) are made of an electrostrictive material without residual polarisation, and the said polarisation is produced by a polarisation stage comprising: 1 a source 501 of p d.c. polarisation voltages, 2 a demultiplexing and switching circuit 502 receiving on the one hand the p polarisation voltages and on the other hand a combination of m bits selecting the said voltages in order to supply them to the transducers, 3 a circuit for determining the phase law 503 composed of a memory 521 intended to supply the n digital combinations relating to the n demultiplexers, and 4 a sequencer 520. Application: echographs used in medicines.

Description

"ULTRASONIC ECHOGRAPH IN ELECTROSTRICTIVE MATERIAL IN NETWORKS
FRESNEL APPROCHES ".

 The present invention relates to an ultrasound ultrasound system made of electrostrictive material in approximate Fresnel networks, intended in particular to be used in the medical field for the examination of biological tissues.

 The patent of the United States of America filed on May 24, 1954 and issued on February 24, 1959 under No. 2,875,355 in the name of the transferee company Gulton Industries describes, as it appears in particular from column 1, line 64, to column 4, line 13, a focusing ultrasonic transducer which, to enable said focusing. is composed of adjacent zones polarized in opposite directions. The dimensions of the zones are chosen so that the succession of polarizations of opposite sign effectively allows, according to the principle of Sauret zoned networks, an average shift of half a wavelength between the ultrasonic paths from two successive zones, the focusing thus obtained being of spherical type (see FIGS. 2 to 5 of the cited patent) or cylindrical type (see FIG. 15 of this same document).

 The phase law corresponding to this polarization mode is represented in FIG. 1 of the present application. This all-or-nothing phase law is therefore achieved by means of a sensitivity law with two states of opposite polarity.

 A drawback of such a device is to create an inhomogeneity in the focusing by obtaining harmonic foci with as a consequence a reinforcement of the ultrasonic background noise.

 The object of the invention is to propose an apparatus for examining environments which overcomes this drawback.

To this end, the invention relates to an ultrasound ultrasound system using approximate Fresnel networks comprising a focusing network of n ultrasonic transducers associated with a transmission stage for the repeated emission of ultrasonic waves towards the region to be explored and with a reception stage. for processing echoes received in return, characterized in that
(A) the ultrasonic transducers are made of an electrostrictive material with non-remanent polarization;
(B) said polarization is achieved by means of a polarization stage composed of the following circuits
(1) a source of p direct bias voltages;
(2) a demultiplexing and routing circuit, composed of n demultiplexers each receiving on the one hand the p DC bias voltages and on the other hand a combination of m bits selecting one of said voltages, and supplying said selected voltage , via a polarization resistor in series, to the ultrasonic transducer associated respectively with this demultiplexer;
(3) a circuit for determining the sensitivity law corresponding to two phase states and p / 2 or (p + l) / 2 amplitude states according to the parity of p, composed in particular of a memory, intended for providing n demultiplexers with n respective digital combinations and indicating to each demultiplexer that of the DC voltages which it must select for the polarization of the transducer which is associated with it;
(4) a sequencer essentially comprising a clock circuit intended to fix the duration of the transmission and reception periods.

 In the apparatus thus proposed, the use of transducers made of electrostrictive material and the polarization thereof using a polarization stage of original structure makes it possible to achieve very precise focusing using simplified electronic circuits. . In fact, the theoretical phase law to which the exact obtaining of zoned Fresnel networks would correspond, that is to say perfect focusing with the exact progressive progressiveness of the offsets between successive transducers, is represented in FIG. 2. use of the polarization stage according to the invention then makes it possible to impose on each transducer of the network, in each of its two possible states of opposite polarity, a discrete amplitude variation law making it possible to reduce the drawback mentioned above approximate focus to two phase states.

More precisely, the focusing thus obtained is all the more similar to that which is theoretically necessary as the number p of DC bias voltages, positive or negative, supplied by the DC voltage source to obtain these different amplitudes.

 In the case of an ultrasound ultrasound system according to the invention, of the sequential scanning type, constituted by a linear array of n ultrasonic transducers, said circuit for determining the law of sensitivity also includes a counter whose number of states corresponds the number of different parallel directions in which an examination with focusing of the ultrasonic waves is to be carried out, and said clock circuit fixes the rate of change of state of the counters.

 Still in the case of a sequential scanning ultrasound system, it is also expected that, the number of consecutive transducers participating in the same shot being q, the bias voltages of the n-q other transducers are equal to zero.

The features and advantages of the invention will now appear in more detail in the description which follows and in the appended drawings, given by way of nonlimiting examples, and in which
- Figures 1 and 2 are curves P = f (D) where
P represents the phase and D the distance considered from the central part of the network of transducers, respectively showing a simplified phase law which makes it possible to obtain zoned Sauret networks using a network of transducers with two states of polarization, and Fresnel's theoretical phase law which allows it to be obtained exactly.

The simplified phase law can be obtained by considering a sensitivity law S = f (D) with 2 states varying identically according to D,
FIG. 3 shows an embodiment of the device according to the invention,
- Figures 4a to 4d are time diagrams respectively showing the shape of the periodic signal (electrical voltage) defining the cadence of the emission-reception cycles, the shape of the delayed periodic signal actually supplied to the electrical excitation generator of the transducers , the shape of the periodic signal fixing the duration of the emission in the emission-reception cycle, and the shape of the electric voltage across one of the ultrasonic transducers in the case of a change in polarity and so phase at time said t3
- Figure 5 shows the shape of the sensitivity law which can be obtained if there are five possible DC bias voltages and which allows to realize a phase law with two states and an amplitude law with three states .

 - Figure 6 shows an example of a demultiplexer in this case of a sensitivity law with five possible polarization states.

The apparatus according to the invention comprises, with reference to FIG. 3, on the one hand a network of n ultrasonic transducers, for example 32 or 64, and made of an electrostrictive material with non-remanent (ferroelectric) polarization (this is ie a material whose transduction sensitivity is directly linked to the value of the bias voltage applied to it and cancels out at the same time as it), for example ceramics doped with lead niobate and magnesium such as those described in the article by
SJ Jang et al., IEEE Ferroelectrics Symposium, vol. 27, 1980, pp 31-34.

The apparatus also comprises an emission stage 100, composed of an electrical excitation generator 101 and a diode 102 in series with the latter for placing said emission stage under high impedance with respect to the transducer elements, n capacitors 200a to 200n in parallel interposed between the output of the transmission stage and each of the n transducers, and a reception stage 300, composed of an adder 301 receiving the n reception signals delivered by the n transducers and d a conventional CFC ultrasound signal processing circuit comprising here for example an amplifier, an automatic gain control circuit, and a storage and / or display device. Switches 400a to 400n placed between the output of the emission stage 100, the capacitors 200a to 200n and the n inputs of the summator 301 of the reception stage, allow the separation of the emission and reception stages, these switches toggling at the rate of the periodic signal of FIG. 4c described below, and in particular avoid the blindness of the reception stage by the transmission stage.

 The apparatus finally comprises a bias stage composed of a source of bias voltages 501, a demultiplexing and switching circuit 502, a circuit for determining the law of sensitivity 503, and a sequencer 520. The source 501 of polarization voltages is capable of delivering p separate continuous polarization voltages, positive or negative, corresponding to the p possible states of polarization which it is wished to impose on the n electrostrictive transducers. In a particular embodiment, assume that there are only two possible polarization states, corresponding for example to two polarization voltages of +500 volts and -500 volts, but the invention applies identically when a law is imposed on the n transducers sensitivity to p states, to allow more precise focusing as we saw above.

 The demultiplexing and switching circuit 502 comprises n demultiplexers 502a to 502n, which each receive on the one hand the p possible bias voltages (in the example described, p = 2, and the demultiplexers are then reduced to simple switches ) cores by the source 501 and on the other hand a combination of m bits fixing for the electrostrictive transducer concerned that of the p DC bias voltages that it will receive and consequently also fixing the sensitivity of this transducer. If p = 2, it suffices that m = 1 to express two possible states; if p = 3 or p = 4, m = 2 makes it possible to express the three, or four, corresponding states, and so on. The demultiplexing and switching circuit 502 provides at the output of the n demultiplexers 502a to 502n a number equal to resistors 512a to 512n at the input of the n branches leading to the n transducers (in the example described, the value of these n bias resistors has been chosen to be approximately 1000 ohms).

 The circuit 503 for determining the sensitivity law makes it possible to fix the combination of m bits selecting the bias voltage imposed separately on each of these transducers.

This digital combination associated with the corresponding sensitivity law is delivered by a memory 521, for example of the ROM or PROM type, previously loaded with the values corresponding to the best possible expression of said sensitivity law, taking into account the number of bias voltages. distinct sources provided by the source 501. This memory is connected to a counter 522 whose number of states is, in the case of a sequential scanning array, the number of different parallel directions desired and whose input is in turn controlled by a clock circuit 523, delivering the signal shown in FIG. 4a: the period dl of the periodic signal thus delivered corresponds to the duration of an ultrasonic transmission-reception cycle for a determined parallel direction.

 This clock circuit 523 is part of the sequencer 520 and controls not only the counter 522 but also, in the sequencer, delay circuits 601 and 602. The delay circuit 601, placed between the clock circuit 523 and the generator of electrical excitation 101, brings to the signal of FIG. 4a a delay T1 (see in FIG. 4b the new signal thus generated) which corresponds to the recovery time necessary to trigger the emission of ultrasonic waves when the polarization of the transducers is effective.

The delay circuit 602 fixes the duration d2 of operation in transmitter mode. The pulse of corresponding duration, represented in FIG. 4c, and of repetition period equal to that of the pulses of duration dl of FIG. 4a (for example 1 millisecond in the example considered) is delivered by a flip-flop 603 whose inputs are one directly connected to the output of the clock circuit 523, for example for coming to the upper level of the pulse, and the other connected to this same output but via the delay circuit 602 for reverse switching (the return to the lower level of the pulse in the example considered). It is this periodic signal of FIG. 4c which is sent to the switches 400a to 400n inserted between the transmit and receive stages 100 and 300, in order to ensure automatic control of the operation of the examination apparatus in transmission mode or in reception mode.

FIG. 4d shows the shape of the electrical voltage across a transducer, in the case of a two-state sensitivity law embodied by the existence of two possible bias voltages. We highlighted in this figure 4d the delay time
T1, corresponding to the recovery time required before the effective triggering of the emission of ultrasonic waves, the duration dl of a transmission-reception cycle, and the duration d2 of operation in transmitter mode, and we have shown on the electrical signal for excitation of the echoes coming from the structure examined.

 The invention, described for the sake of simplification for a two-state sensitivity law, is applicable in the completely general case of a p-state law, and this application will be succinctly presented below.

The theoretical phase law to which perfect focusing would correspond has been represented in FIG. 2 and its
2 analytical translation is given by the expression y = cosax2 or
= = r / (X .f) (1 being the wavelength associated with the frequency of the ultrasonic waves generated, ie with the working frequency of the transducers, and f being the focal distance). A two-state phase law constitutes an approximation all the better as the amplitude law which is associated with it comprises more quantization levels and therefore makes it possible to reduce the effect of the harmonic foci all the better. If we choose for example p = 5, that is to say five levels of sensitivity corresponding to a two-state phase law and a three-state amplitude law. This sensitivity law is represented by the curve of FIG. 5. The circuits of the reception stage are then naturally a little more complex than in the example previously described. On the one hand, the source of bias voltages must now deliver four DC voltages
V, V / 2, -V / 2, -V, the zero voltage to which the fifth polarization state corresponds being obtained by simple grounding. On the other hand the memory word corresponding to any state of polarization and transmitted from memory 521 to any 502i of the demultiplexers is longer, which implies a greater memory capacity.

We can for five states choose for example words of 3 bits 000 for V, 001 for V / 2, 010 for O volt, 011 for -V / 2, 100 for -V, the n words addressed respectively to each of the n demultiplexers 502a to 502n of the demultiplexing and switching circuit 502 being chosen from these five expressions, and the binary triplet ABC constituting these words being addressed to each demultiplexer according to the diagram in FIG. 6.

 When a linear array of transducers is used in sequential scanning, it is known that only a determined number q of transducers participate in the same firing, the n-q others being generally grounded. The scanning is then carried out by shifting the position of the q transducers by one. The device described has the advantage of avoiding the grounding of the nq unused transducers by deciding to apply to all the n transducers a polarization voltage, this being zero for said nq transducers whose state is determined by a particular predefined memory word.

 The invention cannot be limited to the single embodiments described in relation to a linear array of transducers. It is understood that it could be used for any other type of ultrasound ultrasound system, in particular annular ultrasound systems in which the transducers are in the form of circular, elliptical or other rings.

Claims (4)

 1. 'Ultrasound ultrasound system using Fresnel gratings  including a focusing network of n ultrasonic transducers  associated with an emission stage (100) for the repeated emission of waves  ultrasound to the region to be explored and to one reception stage (300)  for processing echoes received in return, characterized in that  (A) the ultrasonic transducers (a, b, c, ..., i, ...,  n-l, n) are made of an electrostrictive material with polarization  non-retentive;  (B) said polarization is achieved by means of a polarization stage composed of the following circuits  (1) a source (501) of p direct bias voltages;  (2) a demultiplexing and switching circuit  (502), composed of n demultiplexers (502a to 502n) each receiving on the one hand the p DC bias voltages and on the other hand a combination of m bits selecting one of said voltages, and  supplying said selected voltage, via a  polarization resistance in series (512a to 512n), to the ultrasonic transducer associated respectively with this demultiplexer;  (3) a circuit for determining the sensitivity law (503), corresponding to two phase states and to p / 2 or (p + l) / 2 amplitude states according to the parity of p, composed in particular of a memory (521), intended to supply to the n demultiplexers n respective digital combinations and indicating to each demultiplexer that of the DC voltages which it must select for the polarization of the transducer which is associated with it;  (4) a sequencer (520), essentially comprising a clock circuit (523) intended to fix the duration of the transmission and reception periods.  2. Ultrasound ultrasound system according to claim 1,  sequential scanning type, consisting of a linear array of n ultrasonic transducers, characterized in that said circuit for determining the law of sensitivity also includes a counter whose number of states corresponds to the number of different parallel directions in which it is desired to operate an examination with focusing of the ultrasonic waves, and in that the said clock circuit fixes the rate of change of state of the counter. 3. Ultrasonic ultrasound system according to claim 2, characterized in that, the number of consecutive transducers participating in the same shot being q, the bias voltage of the n-q other transducers are equal to zero. 4. Ultrasonic ultrasound system according to claim 1, characterized in that the n ultrasonic transducers are of annular type.